file which includes the original illustrations4 ,see #eacbf-;h4htm or #eacbf-;h4zip3 7https3_/_/www4gutenberg4org_/cache_/epu b_/#eacbf_/pg#eacbf-;images4html7 or 7https3_/_/www4gutenberg4org_/files_/#ea cbf_/eacbf-;h4zip7 ,images of the original pages are available through ,internet ,archive4 ,see https3_/_/archive4org_/details_/cu#caibd jabcfgdda ,transcriber's note3 ,text enclosed by underscores is in italics 7italics74 ,a carat character is used to denote superscription4 ,a single character following the carat is superscripted 7example3 #jjagagt#b74 ,multiple superscripted characters are enclosed by curly brackets 7example3 ,v#j7a-,kt7-#a74 ,the ligature oe has been transcribed as oe4 ,the dagger sign has been transcribed as 4 ,the infinity sign has been transcribed as oo4 ,,the ,,principles ,,of ,,chemistry by ,d4 ,,mendel,,eff ,translated from the ,russian 7,sixth ,edition7 by ,george ,kamensky1 ,a4,r4,s4,m4 of the ,imperial ,mint1 ,st ,petersburg3 ,member of the ,russian ,physico-,chemical ,society ,edited by ,t4 ,a4 ,lawson1 ,b4,sc4 ,,ph4,d4 ,examiner in ,coal-,tar ,products to the ,city and ,guilds of ,london ,institute ,fellow of the ,institute of ,chemistry ,in ,two ,volumes ,,volume ,i4 ,longmans1 ,green1 and ,co #ci ,paternoster ,row1 ,london ,new ,york and ,bombay #ahig ,all rights reserved ,,preface ,,to ,,the ,,english ,,translation ,the first ,english edition of this work was published in #ahia1 and that a second edition is now called for is1 we think1 a sufficient proof that the enthusiasm of the author for his science1 and the philosophical method of his teaching1 have been duly appreciated by ,english chemists4 ,in the scientific work to which ,professor ,mendeleff's life has been devoted1 his continual endeavour has been to bring the scattered facts of chemistry within the domain of law1 and accordingly in his teaching he endeavours to impress upon the student the principles of the science1 the generalisations1 so far as they have been discovered1 under which the facts naturally group themselves4 ,of those generalisations the periodic law is perhaps the most important that has been put forward since the establishment of the atomic theory4 ,it is therefore interesting to note that ,professor ,mendeleff was led to its discovery in preparing the first ,russian edition of this book4 ,it is natural1 too1 that the further application and development of that generalisation should be the principal feature of this1 the latest edition4 ,there are special difficulties in rendering the ,russian language into good ,english1 and we are conscious that these have not been entirely overcome4 ,doubtless also there are errors of statement which have escaped correction1 but we believe that the present edition will be found better in both respects than its predecessor4 ,we have thought it our duty as translators to give as far as possible a faithful reproduction of ,professor ,mendeleff's work--the sixth ,russian edition--without amplifying or modifying his statements1 and in this we have the author's approval4 ,although other duties have prevented ,mr4 ,greenaway from undertaking the care of the present edition1 he has been kind enough to give us the benefit of his suggestions on several points4 ,we also wish to thank the ,managers of the ,royal ,institution for permission to reprint the lecture delivered at the ,royal ,institution by ,professor ,mendeleff 7,appendix ,i471 and to the ,council of the ,chemical ,society for permission to reprint the ,faraday lecture which forms ,appendix ,,ii4 ,in conclusion1 we are indebted to ,mr4 ,f4 ,evershed1 who has given us much valuable assistance in revising the sheets for the press4 ,g4 ,k4 ,t4 ,a4 ,l4 ,august #ahig ,,author',s ,,preface ,,to ,,the ,,sixth ,,russian ,,edition ,this work was written during the years #ahfh-ahgj1 its object being to acquaint the student not only with the methods of observation1 the experimental facts1 and the laws of chemistry1 but also with the insight given by this science into the unchangeable substratum underlying the varying forms of matter4 ,if statements of fact themselves depend upon the person who observes them1 how much more distinct is the reflection of the personality of him who gives an account of methods and of philosophical speculations which form the essence of science6 ,for this reason there will inevitably be much that is subjective in every objective exposition of science4 ,and as an individual production is only significant in virtue of that which has preceded and that which is contemporary with it1 it resembles a mirror which in reflecting exaggerates the size and clearness of neighbouring objects1 and causes a person near it to see reflected most plainly those objects which are on the side to which it is directed4 ,although ,i have endeavoured to make my book a true mirror directed towards the whole domain of chemical changes1 yet involuntarily those influences near to me have been the most clearly reflected1 the most brightly illuminated1 and have tinted the entire work with their colouring4 ,in this way the chief peculiarity of the book has been determined4 ,experimental and practical data occupy their place1 but the philosophical principles of our science form the chief theme of the work4 ,in former times sciences1 like bridges1 could only be built up by supporting them on a few broad buttresses and long girders4 ,in addition to the exposition of the principles of chemistry1 it has been my desire to show how science has now been built up like a suspension bridge1 supported by the united strength of a number of slender1 but firmly-fixed1 chains1 which individually are of little strength1 and has thus been carried over difficulties which before appeared insuperable4 ,in comparing the science of the past1 the present1 and the future1 in placing the particulars of its restricted experiments side by side with its aspirations after unbounded and infinite truth1 and in restraining myself from yielding to a bias towards the most attractive path1 ,i have endeavoured to incite in the reader a spirit of inquiry1 which1 dissatisfied with speculative reasonings alone1 should subject every idea to experiment1 encourage the habit of stubborn work1 and excite a search for fresh chains of evidence to complete the bridge over the bottomless unknown4 ,history proves that it is possible by this means to avoid two equally pernicious extremes1 the ,utopian--a visionary contemplation which proceeds from a current of thought only--and the stagnant realism which is content with bare facts4 ,sciences like chemistry1 which deal with ideas as well as with material substances1 and create a possibility of immediately verifying that which has been or may be discovered or assumed1 demonstrate at every step that the work of the past has availed much1 and that without it it would be impossible to advance into the ocean of the unknown4 ,they also show the possibility of becoming acquainted with fresh portions of this unknown1 and compel us1 while duly respecting the teachings of history1 to cast aside classical illusions1 and to engage in a work which not only gives mental satisfaction but is also practically useful to all our fellow-creatures4#a #a ,chemistry1 like every other science1 is at once a means and an end4 ,it is a means of attaining certain practical results4 ,thus1 by its assistance1 the obtaining of matter in its various forms is facilitated2 it shows new possibilities of availing ourselves of the forces of nature1 indicates the methods of preparing many substances1 points out their properties1 c4 ,in this sense chemistry is closely connected with the work of the manufacturer and the artisan1 its sphere is active1 and is a means of promoting general welfare4 ,besides this honourable vocation1 chemistry has another4 ,with it1 as with every other elaborated science1 there are many lofty aspirations1 the contemplation of which serves to inspire its workers and adherents4 ,this contemplation comprises not only the principal data of the science1 but also the generally-accepted deductions1 and also hypotheses which refer to phenomena as yet but imperfectly known4 ,in this latter sense scientific contemplation varies much with times and persons1 it bears the stamp of creative power1 and embraces the highest forms of scientific progress4 ,in that pure enjoyment experienced on approaching to the ideal1 in that eagerness to draw aside the veil from the hidden truth1 and even in that discord which exists between the various workers1 we ought to see the surest pledges of further scientific progress4 ,science thus advances1 discovering new truths1 and at the same time obtaining practical results4 ,the edifice of science not only requires material1 but also a plan1 and necessitates the work of preparing the materials1 putting them together1 working out the plans and the symmetrical proportions of the various parts4 ,to conceive1 understand1 and grasp the whole symmetry of the scientific edifice1 including its unfinished portions1 is equivalent to tasting that enjoyment only conveyed by the highest forms of beauty and truth4 ,without the material1 the plan alone is but a castle in the air--a mere possibility2 whilst the material without a plan is but useless matter4 ,all depends on the concordance of the materials with the plan and execution1 and the general harmony thereby attained4 ,in the work of science1 the artisan1 architect1 and creator are very often one and the same individual2 but sometimes1 as in other walks of life1 there is a difference between them2 sometimes the plan is preconceived1 sometimes it follows the preparation and accumulation of the raw material4 ,free access to the edifice of science is not only allowed to those who devised the plan1 worked out the detailed drawings1 prepared the materials1 or piled up the brickwork1 but also to all those who are desirous of making a close acquaintance with the plan1 and wish to avoid dwelling in the vaults or in the garrets where the useless lumber is stored4 ,knowing how contented1 free1 and joyful is life in the realm of science1 one fervently wishes that many would enter its portals4 ,on this account many pages of this treatise are unwittingly stamped with the earnest desire that the habits of chemical contemplation which ,i have endeavoured to instil into the minds of my readers will incite them to the further study of science4 ,science will then flourish in them and by them1 on a fuller acquaintance not only with that little which is enclosed within the narrow limits of my work1 but with the further learning which they must imbibe in order to make themselves masters of our science and partakers in its further advancement4 ,those who enlist in the cause of science have no reason to fear when they remember the urgent need for practical workers in the spheres of agriculture1 arts1 and manufacture4 ,by summoning adherents to the work of theoretical chemistry1 ,i am confident that ,i call them to a most useful labour1 to the habit of dealing correctly with nature and its laws1 and to the possibility of becoming truly practical men4 ,in order to become actual chemists1 it is necessary for beginners to be well and closely acquainted with three important branches of chemistry--analytical1 organic1 and theoretical4 ,that part of chemistry which is dealt with in this treatise is only the groundwork of the edifice4 ,for the learning and development of chemistry in its truest and fullest sense1 beginners ought1 in the first place1 to turn their attention to the practical work of analytical chemistry2 in the second place1 to practical and theoretical acquaintance with some special chemical question1 studying the original treatises of the investigators of the subject 7at first1 under the direction of experienced teachers71 because in working out particular facts the faculty of judgment and of correct criticism becomes sharpened2 in the third place1 to a knowledge of current scientific questions through the special chemical journals and papers1 and by intercourse with other chemists4 ,the time has come to turn aside from visionary contemplation1 from platonic aspirations1 and from classical verbosity1 and to enter the regions of actual labour for the common weal1 to prove that the study of science is not only air excellent education for youth1 but that it instils the virtues of industry and veracity1 and creates solid national wealth1 material and mental1 which without it would be unattainable4 ,science1 which deals with the infinite1 is itself without bounds4 ,thus the desire to direct those thirsting for truth to the pure source of the science of the forces acting throughout nature forms the first and most important aim of this book4 ,the time has arrived when a knowledge of physics and chemistry forms as important a part of education as that of the classics did two centuries ago4 ,in those days the nations which excelled in classical learning stood foremost1 just as now the most advanced are those which are superior in the knowledge of the natural sciences1 for they form the strength and characteristic of our times4 ,in following the above and chief aim1 ,i set myself a second object3 to furnish a text-book for an elementary knowledge of chemistry and so satisfy a want which undoubtedly exists among students and those who have recourse to chemistry either as a source of truth or welfare4#b ,hence1 although the fundamental object of this work was to express and embrace the general chemical teaching of the present day from a personal point of view1 ,i have nevertheless striven throughout to maintain such a level as would render the ',principles of ,chemistry' accessible to the beginner4 ,many aspects of this work are determined by this combination of requirements which frequently differ widely4 ,an issue was only possible under one condition1 i4e4 not to be carried away by what appears to be a plausible theory in explaining individual facts and to always endeavour to transmit the simple truth of a given fact1 extracting it from the vast store of the literature of the subject and from tried personal experience4 ,in publishing a new edition of this work ,i have striven to add any facts of importance recently discovered#c and to revise the former edition in the above spirit4 ,with this object ,i have entirely gone over this edition1 and a comparison of it with the former one will show that the additions and alterations have cost as much labour as many chapters of the work4 ,i also wished to show in an elementary treatise on chemistry the striking advantages gained by the application of the periodic law1 which ,i first saw in its entirety in the year #ahfi when ,i was engaged in writing the first edition of this book1 in which1 indeed1 the law was first enunciated4 ,at that time1 however1 this law was not established so firmly as now1 when so many of its consequences have been verified by the researches of numerous chemists1 and especially by ,roscoe1 ,lecoq de ,boisbaudran1 ,nilson1 ,brauner1 ,thorpe1 ,carnelley1 ,laurie1 ,winkler1 and others4 ,the1 to me1 unexpectedly rapid success with which the teaching of the periodicity of the elements has spread in our science1 and perhaps also1 the perseverance with which ,i collected in this work1 and upon a new plan1 the most important data respecting the elements and their mutual relations1 explained sufficiently the fact that the former 7#eth1 #ahhi7 edition of my work has been translated into ,english#d and ,german#e and is being translated into ,french4#f ,deeply touched by the favourable opinions expressed by ,english men of science upon my book1 ,i ascribe them chiefly to the periodic law placed at the basis of my treatise and especially of the second part of the book1 which contains a large amount of data having a special and sometimes quite unexpected1 bearing from the point of view of this law4 ,as the entire scheme of this work is subordinated to the law of periodicity1 which may be illustrated in a tabular form by placing the elements in series1 groups1 and periods1 two such tables are given at the end of this preface4 #b ,i recommend those who are commencing the study of chemistry with my book to first read only what is printed in the large type1 because in that part ,i have endeavoured to concentrate all the fundamental1 indispensable knowledge required for that study4 ,in the footnotes1 printed in small type 7which should be read only after the large text has been mastered71 certain details are discussed2 they are either further examples1 or debatable questions on existing ideas which ,i thought useful to lay before those entering into the sphere of science1 or certain historical and technical details which might be withdrawn from the fundamental portion of the book4 ,without intending to attain in my treatise to the completeness of a work of reference1 ,i have still endeavoured to express the principal developments of science as they concern the chemical elements viewed in that aspect in which they appeared to me after long continued study of the subject and participation in the contemporary advance of knowledge4 ,i have also placed my personal views1 suppositions1 and arguments in the footnotes1 which are chiefly designed for details and references4 ,but ,i have endeavoured to avoid here1 as in the text1 not only all that ,i consider doubtful1 but also those details which belong either to special branches of chemistry 7for instance1 to analytical1 organic1 physical1 theoretical1 physiological1 agricultural1 or technical chemistry71 or to different branches of natural science which are more and more coming into closer and closer contact with chemistry4 ,chemistry1 ,i am convinced1 must occupy a place among the natural sciences side by side with mechanics2 for mechanics treats of matter as a system of ponderable points having scarcely any individuality and only standing in a certain state of mobile equilibrium4 ,for chemistry1 matter is an entire world of life1 with an infinite variety of individuality both in the elements and in their combinations4 ,in studying the general uniformity from a mechanical point of view1 ,i think that the highest point of knowledge of nature cannot be attained without taking into account the individuality of things in which chemistry is set to seek for general higher laws4 ,mechanics may be likened to the science of statesmanship1 chemistry to the sciences of jurisprudence and sociology4 ,the general universe could not be built up without the particular individual universe1 and would be a dry abstract were it not enlivened by the real variety of the individual world4 ,mechanics forms the classical basis of natural philosophy1 while chemistry1 as a comparatively new and still young science1 already strives to--and will1 in the future introduce a new1 living aspect into the philosophy of nature2 all the more as chemistry alone is never at rest or anywhere dead--its vital action has universal sway1 and inevitably determines the general aspect of the universe4 ,just as the microscope and telescope enlarge the scope of vision1 and discover life in seeming immobility1 so chemistry1 in discovering and striving to discern the life of the invisible world of atoms and molecules and their ultimate limit of divisibility1 will clearly introduce new and important problems into our conception of nature4 ,and ,i think that its rle1 which is now considerable1 will increase more and more in the future2 that is1 ,i think that in its further development it will occupy a place side by side with mechanics for the comprehension of the secrets of nature4 ,but here we require some second ,newton2 and ,i have no doubt that he will soon appear4 #c ,i was much helped in gathering data from the various chemical journals of the last five years by the abstracts made for me by ,mr4 ,y4 ,v4 ,kouriloff1 to whom ,i tender my best thanks4 #d ,the ,english translation was made by ,g4 ,kamensky1 and edited by ,a4 ,j4 ,greenaway2 published by ,longmans1 ,green ,co4 #e ,the ,german translation was made by ,l4 ,jawein and ,a4 ,thillot2 published by ,ricker 7,st4 ,petersburg74 #f ,the ,french translation has been commenced by ,e4 ,achkinasi and ,h4 ,carrion from the fifth edition1 and is published by ,tignol 7,paris74 ,in this the sixth edition ,i have not altered any essential feature of the original work1 and have retained those alterations which were introduced into the fifth edition4#g ,i have1 however1 added many newly discovered facts1 and in this respect it is necessary to say a few words4 ,although all aspects of the simplest chemical relations are as far as possible equally developed in this book1 yet on looking back ,i see that ,i have1 nevertheless1 given most attention to the so-called indefinite compounds examples of which may be seen in solutions4 ,i recur repeatedly to them1 and to all the latest data respecting them1 for in them ,i see a starting point for the future progress of our science and to them ,i affiliate numerous instances of definite compounds1 beginning with alloys and silicates and ending with complex acids4 ,there are two reasons for this4 ,in the first place1 this subject has deeply interested me from my youth2 ,i have devoted a portion of my own researches to it1 and therefore it occupied an important position even in the first edition of my book2 besides which all that has been subsequently accomplished in our science1 especially during the last five or six years1 shows that at the present day an interest in these questions plays an important part in the minds of a large circle of contemporary workers in chemistry4 ,this personal attachment1 if ,i may so call it1 to the question of solutions and such indefinite compounds1 must involuntarily have impressed itself upon my work1 and in the later editions ,i have even had to strive not to give this subject a greater development than previously1 so great was the material accumulated1 which however does not yet give us the right to consider even the most elementary questions respecting solutions as solved4 ,thus1 we cannot yet say what a solution really is4 ,my own view is that a solution is a homogeneous liquid system of unstable dissociating compounds of the solvent with the substance dissolved4 ,but although such a theory explains much to me1 ,i cannot consider my opinion as proved1 and therefore give it with some reserve as one of several hypotheses4#h ,as a subject yet far from solved1 ,i might naturally have ignored it1 or only mentioned it cursorily1 but such a treatment of solutions1 although usual in elementary treatises on chemistry1 would not have answered my views upon the subject of our science1 and ,i wished that the reader might find in my book beyond everything an expression of all that a study of the subject built up for me4 ,if in solutions ,i see and can frequently prove distinct evidences of the existence of those definite compounds which form the more generalised province of chemical data1 ,i could not refrain from going into certain details respecting solutions2 otherwise1 there would have remained no trace of that general idea1 that in them we have only a certain instance of ordinary definite or atomic compounds1 subject to ,dalton's laws4 ,having long had this idea1 ,i wished to impress it upon the reader of my book1 and it is this desire which forms the second of those chief reasons why ,i recur so frequently to solutions in this work4 ,at present1 my ideas respecting solutions are shared by few1 but ,i trust that by degrees the instances ,i give will pave the way for their general recognition1 and it is my hope that they may find adherents among those of my readers who are in a position to work out by experiment this difficult but highly interesting problem4 #g ,the fifth edition was not only considerably enlarged1 compared with the preceding1 but also the foundations of the periodic system of the elements were placed far more firmly in it than in the former editions4 ,the subject-matter was also divided into text and footnotes1 which contained details unnecessary for a first acquaintance with chemistry4 ,the fifth edition sold out sooner than ,i expected1 so that instead of issuing supplements 7containing the latest discoveries in chemistry71 as ,i had proposed1 ,i was obliged to publish the present entirely new edition of the work4 #h ,this hypothesis is not only mentioned in different parts of this book1 but is partly 7from the aspect of the specific gravity of solutions7 developed in my work1 ,the ,investigation of ,solutions from their ,specific ,gravity1 #ahhg4 ,in conclusion1 ,i desire to record my thanks to ,v4 ,d4 ,sapogenikoff1 who has corrected the proofs of the whole of this edition and compiled the indexes which greatly facilitate the search for those details which are scattered throughout the work4 ,d4 ,,mendel,,eff4 ,,table ,i ,distribution of the ,elements in ,groups and ,series ---------------------------------------- --------------- ,group ,i4 ,,ii4 ,,iii4 ,,iv4 ,v4 ---------------------------------------- --------------- ,series #a ,h -- -- -- -- #b ,li ,be ,b ,c ,n #c ,na ,mg ,al ,si ,p #d ,k ,ca ,sc ,ti ,v #e 7,cu7 ,zn ,ga ,ge ,as #f ,rb ,sr ,y ,zr ,nb #g 7,ag7 ,cd ,in ,sn ,sb #h ,cs ,ba ,la ,ce ,di8 #i -- -- -- -- -- #aj -- -- ,yb -- ,ta #aa 7,au7 ,hg ,tl ,pb ,bi #ab -- -- -- ,th -- ---------------------------------------- --------------- ,r#b,o ,r#b,o#b,r#b,o#c,r#b,o#d,r#b,o#e -- ,,ro -- ,,ro#b -- -- -- -- ,,rh#d ,,rh#c ---------------------------------------- --------------- ---------------------------------------- --------- ,group ,,vi4 ,,vii4 ,,viii4 ---------------------------------------- --------- ,series #a -- -- #b ,o ,f #c ,s ,cl #d ,cr ,mn ,fe ,co ,ni ,cu #e ,se ,br #f ,mo -- ,ru ,rh ,pd ,ag #g ,te ,i #h -- -- -- -- -- -- #i -- -- #aj ,w -- ,os ,ir ,pt ,au #aa -- -- #ab ,u -- ---------------------------------------- --------- ,r#b,o#f,r#b,o#g ,higher oxides ,,ro#c -- ,,ro#d ,,rh#b ,,rh ,hydrogen compounds ---------------------------------------- --------- ,,table ,,ii ,periodic ,system and ,atomic ,weights of the ,elements 7,giving the pages on which they are described7 ---------------------------------------- ----------------------- #bnd ,series1 ,typical #dth #fth elements ,series ,series ---------------------------------------- ---------------------- ,i4 ,li #g ,k #ci ,rb #hf vol4 i4 #egd vol4 i4 #eeh vol4 i4 #egf ,,ii4 ,be #i ,ca #dj ,sr #hh ----- vol4 i4 #fah vol4 i4 #eij vol4 i4 #fad ,,iii4 ,b #aa ,sc #dd ,y #hi ---- vol4 ii4 #fj vol4 ii4 #id vol4 ii4 #ic ,,iv4 ,c #ab ,ti #dh ,zr #ia vol4 i4 #cch vol4 ii4 #add vol4 ii4 #ic ,v4 ,n #ad ,v #ea ,nb #id vol4 i4 #bbc vol4 ii4 #aid vol4 ii4 #aig ,,vi4 ,o #af ,cr #eb ,mo #if ---- ----- vol4 i4 #aee vol4 ii4 #bgf vol4 ii4 #bij ,,vii4 ,f #ai ,mn #ee 8 #ii vol4 i4 #dhi vol4 ii4 #cjc ,fe #ef ,ru #ajb vol4 ii4 #cag vol4 ii4 #cfi ,,viii4 ,co #ei ,rh #ajc vol4 ii4 #cec vol4 ii4 #cfi ,ni #eie ,pd #ajf vol4 ii4 #cec vol4 ii4 #cfi #crd ,series #eth ,series #gth ,series ,i4 ,h #a ,na #bc ,cu #fd ,ag #ajh ---- vol4 i4 #abi vol4 i4 #ecc vol4 ii4 #cih vol4 ii4 #dae ,,ii4 ,mg #bd ,zn #fe ,cd #aab vol4 i4 #eij vol4 ii4 #ci vol4 ii4 #dg ,,iii4 ,al #bg ,ga #gj ,in #aad vol4 ii4 #gj vol4 ii4 #ij vol4 ii4 #ia ,,iv4 ,si #bh ,ge #gb ,sn #aai vol4 ii4 #ii vol4 ii4 #abd vol4 ii4 #abe ,v4 ,p #ca ,as #ge ,sb #abj ----- vol4 ii4 #adi vol4 ii4 #agi vol4 ii4 #ahf ,,vi4 ,s #cb ,se #gi ,te #abe vol4 ii4 #bjj vol4 ii4 #bgj vol4 ii4 #bgj ,,vii4 ,cl #cee ,br #hj ,i #abg ----- vol4 i4 #dei vol4 i4 #did vol4 i4 #dif ---------------------------------------- ---------------------- ---------------------------------------- -------- #hth #ajth #abth ,series ,series ,series ---- ---------------------------------------- --- ,i4 ,cs #acc -- -- vol4 i4 #egf ,,ii4 ,ba #acg -- -- ------ vol4 i4 #fad ,,iii4 ,la #ach ,yb #agc -- vol4 ii4 #ic vol4 ii4 #ic ,,iv4 ,ce #adj 8 #agh ,th #bcb vol4 ii4 #ic vol4 ii4 #adh ,v4 8 ,di #adb ,ta #ahc -- vol4 ii4 #ic vol4 ii4 #aig ,,vi4 -- ,w #ahd ,u #bci vol4 ii4 #bij vol4 ii4 #big ,,vii4 -- -- -- -- ,os #aib vol4 ii4 #cfi ,,viii4 -- ,ir #aic vol4 ii4 #cfi -- ,pt #aif ------ vol4 ii4 #cfi #ith #aath ,series ,series ,i4 -- ,au #aig vol4 ii4 #ddb ,,ii4 -- ,hg #bjj vol4 ii4 #dh ,,iii4 -- ,tl #bjd vol4 ii4 #ia ,,iv4 -- ,pb #bjg vol4 ii4 #acd ,v4 -- ,bi #bji vol4 ii4 #ahi ,,vi4 -- -- ,,vii4 -- -- ---------------------------------------- -------- ,note4--,two lines under the elements indicate those which are very widely distributed in nature2 one line indicates those which1 although not so frequently met with1 are of general use in the arts and manufactures4 ,,contents ,,of ,,the ,,first ,,volume ,,page ,,translators' ,,preface v ,,author',s ,,preface ,,to ,,the ,,sixth ,,russian ,,edition vii ,,table ,,of ,,the ,,distribution ,,of ,,the ,,elements ,,in ,,groups ,,and ,,series xv ,,table ,,of ,,the ,,periodic ,,system ,,and ,,atomic ,,weights ,,of ,,the ,,elements xvi ,,introduction #a ,,chap4 ,i4 ,,on ,,water ,,and ,,its ,,compounds #dj ,,ii4 ,,the ,,composition ,,of ,,water4 ,,hydrogen #aac ,,iii4 ,,oxygen ,,and ,,the ,,chief ,,aspects ,,of ,,its ,,saline ,,combinations #aeb ,,iv4 ,,ozone ,,and ,,hydrogen ,,peroxide4 ,,dalton',s ,,law #aih ,v4 ,,nitrogen ,,and ,,air #bbc ,,vi4 ,,the ,,compounds ,,of ,,nitrogen ,,with ,,hydrogen ,,and ,,oxygen #bdf ,,vii4 ,,molecules ,,and ,,atoms4 ,,the ,,laws ,,of ,,gay-,,lussac ,,and ,,avogadro-,,gerhardt #bii ,,viii4 ,,carbon ,,and ,,the ,,hydrocarbons #cch ,,ix4 ,,compounds ,,of ,,carbon ,,with ,,oxygen ,,and ,,nitrogen #cgi ,x4 ,,sodium ,,chloride4 ,,berthollet',s ,,laws4 ,,hydrochloric ,,acid #dag ,,xi4 ,,the ,,halogens3 ,,chlorine1 ,,bromine1 ,,iodine ,,and ,,fluorine #dei ,,xii4 ,,sodium #eac ,,xiii4 ,,potassium1 ,,rubidium1 ,c,,sium ,,and ,,lithium4 ,,spectrum ,,analysis #edc ,,xiv4 ,,the ,,valency ,,and ,,specific ,,heat ,,of ,,the ,,metals4 ,,magnesium1 ,,calcium1 ,,strontium1 ,,barium1 ,,and ,,beryllium #eha ,,principles ,,of ,,chemistry ,,introduction ,the study of natural science1 whose rapid development dates from the days of ,galileo 7#afdb7 and ,newton 7#agbg71 and its closer application to the external universe#a led to the separation of ,chemistry as a particular branch of natural philosophy1 not only owing to the increasing store of observations and experiments relating to the mutual transformations of substances1 but also1 and more especially1 because in addition to gravity1 cohesion1 heat1 light and electricity it became necessary to recognise the existence of particular internal forces in the ultimate parts of all substances1 forces which make themselves manifest in the transformations of substances into one another1 but remain hidden 7latent7 under ordinary circumstances1 and whose existence cannot therefore be directly apprehended1 and so for a long time remained unrecognised4 ,the primary object of chemistry is the study of the homogeneous substances#b of which all the objects of the universe are made up1 with the transformations of these substances into each other1 and with the phenomena#c which accompany such transformations4 ,every chemical change or reaction1#d as it is called1 can only take place under a condition of most intimate and close contact of the re-acting substances1#e and is determined by the forces proper to the smallest invisible particles 7molecules7 of matter4 ,we must distinguish three chief classes of chemical transformations4 #a ,the investigation of a substance or a natural phenomenon consists 7a7 in determining the relation of the object under examination to that which is already known1 either from previous researches1 or from experiment1 or from the knowledge of the common surroundings of life--that is1 in determining and expressing the quality of the unknown by the aid of that which is known2 7b7 in measuring all that which can be subjected to measurement1 and thereby denoting the quantitative relation of that under investigation to that already known and its relation to the categories of time1 space1 temperature1 mass1 c42 7c7 in determining the position held by the object under investigation in the system of known objects guided by both qualitative and quantitative data2 7d7 in determining1 from the quantities which have been measured1 the empirical 7visible7 dependence 7function1 or 'law1' as it is sometimes termed7 of variable factors--for instance1 the dependence of the composition of the substance on its properties1 of temperature on time1 of time on locality1 c42 7e7 in framing hypotheses or propositions as to the actual cause and true nature of the relation between that studied 7measured or observed7 and that which is known or the categories of time1 space1 c42 7f7 in verifying the logical consequences of the hypotheses by experiment2 and 7g7 in advancing a theory which shall account for the nature of the properties of that studied in its relations with things already known and with those conditions or categories among which it exists4 ,it is certain that it is only possible to carry out these investigations when we have taken as a basis some incontestable fact which is self-evident to our understanding2 as1 for instance1 number1 time1 space1 motion1 or mass4 ,the determination of such primary or fundamental conceptions1 although not excluded from the possibility of investigation1 frequently does not subject itself to our present mode of scientific generalisation4 ,hence it follows that in the investigation of anything1 there always remains something which is accepted without investigation1 or admitted as a known factor4 ,the axioms of geometry may be taken as an example4 ,thus in the science of biology it is necessary to admit the faculty of organisms for multiplying themselves1 as a conception whose meaning is as yet unknown4 ,in the study of chemistry1 too1 the notion of elements must be accepted almost without any further analysis4 ,however1 by first investigating that which is visible and subject to direct observation by the organs of the senses1 we may hope that in the first place hypotheses will be arrived at1 and afterwards theories of that which has now to be placed at the basis of our investigations4 ,the minds of the ancients strove to seize at once the very fundamental categories of investigation1 whilst all the successes of recent knowledge are based on the above-cited method of investigation without the determination of 'the beginning of all beginnings4' ,by following this inductive method1 the exact sciences have already succeeded in becoming accurately acquainted with much of the invisible world1 which directly is imperceptible to the organs of sense 7for example1 the molecular motion of all bodies1 the composition of the heavenly luminaries1 the paths of their motion1 the necessity for the existence of substances which cannot be subjected to experiment1 c471 and have verified the knowledge thus obtained1 and employed it for increasing the interests of humanity4 ,it may therefore be safely said that the inductive method of investigation is a more perfect mode of acquiring knowledge than the deductive method alone 7starting from a little of the unknown accepted as incontestable to arrive at the much which is visible and observable7 by which the ancients strove to embrace the universe4 ,by investigating the universe by an inductive method 7endeavouring from the much which is observable to arrive at a little which may be verified and is indubitable7 the new science refuses to recognise dogma as truth1 but through reason1 by a slow and laborious method of investigation1 strives for and attains to true deductions4 #b ,a substance or material is that which occupies space and has weight2 that is1 which presents a mass attracted by the earth and by other masses of material1 and of which the objects of nature are composed1 and by means of which the motions and phenomena of nature are accomplished4 ,it is easy to discover by examining and investigating1 by various methods1 the objects met with in nature and in the arts1 that some of them are homogeneous1 whilst others are composed of a mixture of several homogeneous substances4 ,this is most clearly apparent in solid substances4 ,the metals used in the arts 7for example1 gold1 iron1 copper7 must be homogeneous1 otherwise they are brittle and unfit for many purposes4 ,homogeneous matter exhibits similar properties in all its parts4 ,by breaking up a homogeneous substance we obtain parts which1 although different in form1 resemble each other in their properties4 ,glass1 pure sugar1 marble1 c41 are examples of homogeneous substances4 ,examples of non-homogeneous substances are1 however1 much more frequent in nature and the arts4 ,thus the majority of the rocks are not homogeneous4 ,in porphyries bright pieces of a mineral called 'orthoclase' are often seen interspersed amongst the dark mass of the rock4 ,in ordinary red granite it is easy to distinguish large pieces of orthoclase mixed with dark semi-transparent quartz and flexible lamin of mica4 ,similarly1 plants and animals are non-homogeneous4 ,thus1 leaves are composed of a skin1 fibre1 pulp1 sap1 and a green colouring matter4 ,as an example of those non-homogeneous substances which are produced artificially1 gunpowder may be cited1 which is prepared by mixing together known proportions of sulphur1 nitre1 and charcoal4 ,many liquids1 also1 are not homogeneous1 as may be observed by the aid of the microscope1 when drops of blood are seen to consist of a colourless liquid in which red corpuscles1 invisible to the naked eye owing to their small size1 are floating about4 ,it is these corpuscles which give blood its peculiar colour4 ,milk is also a transparent liquid1 in which microscopical drops of fat are floating1 which rise to the top when milk is left at rest1 forming cream4 ,it is possible to extract from every non-homogeneous substance those homogeneous substances of which it is made up4 ,thus orthoclase may he separated from porphyry by breaking it off4 ,so also gold is extracted from auriferous sand by washing away the mixture of clay and sand4 ,chemistry deals only with the homogeneous substances met with in nature1 or extracted from natural or artificial non-homogeneous substances4 ,the various mixtures found in nature form the subjects of other natural sciences--as geognosy1 botany1 zoology1 anatomy1 c4 #c ,all those events which are accomplished by substances in time are termed 'phenomena4' ,phenomena in themselves form the fundamental subject of the study of physics4 ,motion is the primary and most generally understood form of phenomenon1 and therefore we endeavour to reason about other phenomena as clearly as when dealing with motion4 ,for this reason mechanics1 which treats of motion1 forms the fundamental science of natural philosophy1 and all other sciences endeavour to reduce the phenomena with which they are concerned to mechanical principles4 ,astronomy was the first to take to this path of reasoning1 and succeeded in many cases in reducing astronomical to purely mechanical phenomena4 ,chemistry and physics1 physiology and biology are proceeding in the same direction4 ,one of the most important questions of all natural science1 and one which has been handed down from the philosophers of classic times1 is1 whether the comprehension of all that is visible can be reduced to motion8 ,its participation in all1 from the 'fixed' stars to the most minute parts of the coldest bodies 7,dewar1 in #ahid showed that many substances cooled to -#ahj fluoresce more strongly than at the ordinary temperature2 i4e4 that there is a motion in them which produces light7 must now be recognised as undoubtable from direct experiment and observation1 but it does not follow from this that by motion alone can all be explained4 ,this follows1 however1 from the fact that we cannot apprehend motion otherwise than by recognising matter in a state of motion4 ,if light and electricity be understood as particular forms of motion1 then we must inevitably recognise the existence of a peculiar luminiferous 7universal7 ether as a material1 transmitting this form of motion4 ,and so1 under the present state of knowledge1 it is inevitably necessary to recognise the particular categories1 motion and matter1 and as chemistry is more closely concerned with the various forms of the latter1 it should1 together with mechanics or the study of motion1 lie at the basis of natural science4 #d ,the verb 'to react' means to act or change chemically4 #e ,if a phenomenon proceeds at visible or measurable distances 7as1 for instance1 magnetic attraction or gravity71 it cannot be described as chemical1 since these phenomena only take place at distances immeasurably small and undistinguishable to the eye or the microscope2 that is to say1 they are purely molecular4 #a4 ,combination is a reaction in which the union of two substances yields a new one1 or in general terms1 from a given number of substances1 a lesser number is obtained4 ,thus1 by heating a mixture of iron and sulphur#f a single new substance is produced1 iron sulphide1 in which the constituent substances cannot be distinguished even by the highest magnifying power4 ,before the reaction1 the iron could be separated from the mixture by a magnet1 and the sulphur by dissolving it in certain oily liquids2#g in general1 before combination they might be mechanically separated from each other1 but after combination both substances penetrate into each other1 and are then neither mechanically separable nor individually distinguishable4 ,as a rule1 reactions of direct combination are accompanied by an evolution of heat1 and the common case of combustion1 evolving heat1 consists in the combination of combustible substances with a portion 7oxygen7 of the atmosphere1 the gases and vapours contained in the smoke being the products of combination4 #b4 ,reactions of decomposition are cases the reverse of those of combination1 that is1 in which one substance gives two--or1 in general1 a given number of substances a greater number4 ,thus1 by heating wood 7and also coal and many animal or vegetable substances7 without access to air1 a combustible gas1 a watery liquid1 tar1 and carbon are obtained4 ,it is in this way that tar1 illuminating gas1 and charcoal are prepared on a large scale4#h ,all limestones1 for example1 flagstones1 chalk1 or marble1 are decomposed by heating to redness into lime and a peculiar gas called carbonic anhydride4 ,a similar decomposition1 taking place1 however1 at a much lower temperature1 proceeds with the green copper carbonate which is contained in natural malachite4 ,this example will be studied more in detail presently4 ,whilst heat is evolved in the ordinary reactions of combination1 it is1 on the contrary1 absorbed in the reactions of decomposition4 #c4 ,the third class of chemical reactions--where the number of re-acting substances is equal to the number of substances formed--may be considered as a simultaneous decomposition and combination4 ,if1 for instance1 two compounds ,a and ,b are taken and they react on each other to form the substances ,c and ,d1 then supposing that ,a is decomposed into ,d and ,e1 and that ,e combines with ,b to form ,c1 we have a reaction in which two substances ,a1 or ,d ,e1 and ,b were taken and two others ,c1 or ,e ,b1 and ,d were produced4 ,such reactions ought to be placed under the general term of reactions of 'rearrangement1' and the particular case where two substances give two fresh ones1 reactions of 'substitution4'#i ,thus1 if a piece of iron be immersed in a solution of blue vitriol 7copper sulphate71 copper is formed--or1 rather1 separated out1 and green vitriol 7iron sulphate1 which only differs from the blue vitriol in that the iron has replaced the copper7 is obtained in solution4 ,in this manner iron may be coated with copper1 so also copper with silver2 such reactions are frequently made use of in practice4 #f ,for this purpose a piece of iron may be made red hot in a forge1 and then placed in contact with a lump of sulphur1 when iron sulphide will be obtained as a molten liquid1 the combination being accompanied by a visible increase in the glow of the iron4 ,or else iron filings are mixed with powdered sulphur in the proportion of #e parts of iron to #c parts of sulphur1 and the mixture placed in a glass tube1 which is then heated in one place4 ,combination does not commence without the aid of external heat1 but when once started in any portion of the mixture it extends throughout the entire mass1 because the portion first heated evolves sufficient heat in forming iron sulphide to raise the adjacent parts of the mixture to the temperature required for starting the reaction4 ,the rise in temperature thus produced is so high as to soften the glass tube4 #g ,sulphur is slightly soluble in many thin oils2 it is very soluble in carbon bisulphide and in some other liquids4 ,iron is insoluble in carbon bisulphide1 and the sulphur therefore can be dissolved away from the iron4 #h ,decomposition of this kind is termed 'dry distillation1' because1 as in distillation1 the substance is heated and vapours are given off which1 on cooling1 condense into liquids4 ,in general1 decomposition1 in absorbing heat1 presents much in common to a physical change of state--such as1 for example1 that of a liquid into a gas4 ,deville likened complete decomposition to boiling1 and compared partial decomposition1 when a portion of a substance is not decomposed in the presence of its products of decomposition 7or dissociation71 to evaporation4 #i ,a reaction of rearrangement may in certain cases take place with one substance only2 that is to say1 a substance may by itself change into a new isomeric form4 ,thus1 for example1 if hard yellow sulphur be heated to a temperature of #bej and then poured into cold water it gives1 on cooling1 a soft1 brown variety4 ,ordinary phosphorus1 which is transparent1 poisonous1 and phosphorescent in the dark 7in the air71 gives1 after being heated at #bgj 7in an atmosphere incapable of supporting combustion1 such as steam71 an opaque1 red1 and non-poisonous isomeric variety1 which is not phosphorescent4 ,cases of isomerism point out the possibility of an internal rearrangement in a substance1 and are the result of an alteration in the grouping of the same elements1 just as a certain number of balls may be grouped in figures and forms of different shapes4 ,the majority of the chemical changes which occur in nature and are made use of technically are very complicated1 as they consist of an association of many separate and simultaneous combinations1 decompositions1 and replacements4 ,it is chiefly due to this natural complexity of chemical phenomena that for so many centuries chemistry did not exist as an exact science2 that is so say1 that although many chemical changes were known and made use of1#aj yet their real nature was unknown1 nor could they be predicted or directed at will4 ,another reason for the tardy progress of chemical knowledge is the participation of gaseous substances1 especially air1 in many reactions4 ,the true comprehension of air as a ponderable substance1 and of gases in general as peculiar elastic and dispersive states of matter1 was only arrived at in the sixteenth and seventeenth centuries1 and it was only after this that the transformations of substances could form a science4 ,up to that time1 without understanding the invisible and yet ponderable gaseous and vaporous states of substances1 it was impossible to obtain any fundamental chemical evidence1 because gases escaped from notice between the reacting and resultant substances4 ,it is easy from the impression conveyed to us by the phenomena we observe to form the opinion that matter is created and destroyed3 a whole mass of trees burn1 and there only remains a little charcoal and ash1 whilst from one small seed there grows little by little a majestic tree4 ,in one case matter seems to be destroyed1 and in the other to be created4 ,this conclusion is arrived at because the formation or consumption of gases1 being under the circumstances invisible to the eye1 is not observed4 ,when wood burns it undergoes a chemical change into gaseous products1 which escape as smoke4 ,a very simple experiment will prove this4 ,by collecting the smoke it may be observed that it contains gases which differ entirely from air1 being incapable of supporting combustion or respiration4 ,these gases may be weighed1 and it will then be seen that their weight exceeds that of the wood taken4 ,this increase in weight arises from the fact that1 in burning1 the component parts of the wood combine with a portion of the air2 in like manner iron increases in weight by rusting4 ,in burning gunpowder its substance is not destroyed1 but only converted into gases and smoke4 ,so also in the growth of a tree2 the seed does not increase in mass of itself and from itself1 but grows because it absorbs gases from the atmosphere and sucks water and substances dissolved therein from the earth through its roots4 ,the sap and solid substances which give plants their form are produced from these absorbed gases and liquids by complicated chemical processes4 ,the gases and liquids are converted into solid substances by the plants themselves4 ,plants not only do not increase in size1 but die1 in a gas which does not contain the constituents of air4 ,when moist substances dry they decrease in weight2 when water evaporates we know that it does not disappear1 but will return from the atmosphere as rain1 dew1 and snow4 ,when water is absorbed by the earth1 it does not disappear there for ever1 but accumulates somewhere underground1 from whence it afterwards flows forth as a spring4 ,thus matter does not disappear and is not created1 but only undergoes various physical and chemical transformations--that is to say1 changes its locality and form4 ,matter remains on the earth in the same quantity as before2 in a word it is1 so far as we are concerned1 everlasting4 ,it was difficult to submit this simple and primary truth of chemistry to investigation1 but when once made clear it rapidly spread1 and now seems as natural and simple as many truths which have been acknowledged for ages4 ,mariotte and other savants of the seventeenth century already suspected the existence of the law of the indestructibility of matter1 but they made no efforts to express it or to apply it to the requirements of science4 ,the experiments by means of which this simple law was arrived at were made during the latter half of the last century by the founder of modern chemistry1 ,,lavoisier1 the ,french ,academician and tax farmer4 ,the numerous experiments of this savant were conducted with the aid of the balance1 which is the only means of directly and accurately determining the quantity of matter4 #aj ,thus the ancients knew how to convert the juice of grapes containing the saccharine principle 7glucose7 into wine or vinegar1 how to extract metals from the ores which are found in the earth's crust1 and how to prepare glass from earthy substances4 ,lavoisier found1 by weighing all the substances1 and even the apparatus1 used in every experiment1 and then weighing the substances obtained after the chemical change1 that the sum of the weights of the substances formed was always equal to the sum of the weights of the substances taken2 or1 in other words3 ,,matter ,,is ,,not ,,created ,,and ,,does ,,not ,,disappear1 or that1 matter is everlasting4 ,this expression naturally includes a hypothesis1 but our only aim in using it is to concisely express the following lengthy period--,that in all experiments1 and in all the investigated phenomena of nature1 it has never been observed that the weight of the substances formed was less or greater 7as far as accuracy of weighing permits#aa7 than the weight of the substances originally taken1 and as weight is proportional to mass#aa bis or quantity of matter1 it follows that no one has ever succeeded in observing a disappearance of matter or its appearance in fresh quantities4 ,the law of the indestructibility of matter endows all chemical investigations with exactitude1 as1 on its basis1 an equation may be formed for every chemical reaction4 ,if in any reaction the weights of the substances taken be designated by the letters ,a1 ,b1 ,c1 c41 and the weights of the substances formed by the letters ,m1 ,n1 ,o1 c41 then ,a ,b ,c 444 444 444 ,m ,n ,o 444 444 444 ,therefore1 should the weight of one of the re-acting or resultant substances be unknown1 it may be determined by solving the equation4 ,the chemist1 in applying the law of the indestructibility of matter1 and in making use of the chemical balance1 must never lose sight of any one of the re-acting or resultant substances4 ,should such an over-sight be made1 it will at once be remarked that the sum of the weights of the substances taken is unequal to the sum of the weights of the substances formed4 ,all the progress made by chemistry during the end of the last1 and in the present1 century is entirely and immovably founded on the law of the indestructibility of matter4 ,it is absolutely necessary in beginning the study of chemistry to become familiar with the simple truth which is expressed by this law1 and for this purpose several examples elucidating its application will now be cited4 #aa ,the experiments conducted by ,staas 7described in detail in ,chap4 ,,xxiv4 on ,silver7 form some of the accurate researches1 proving that the weight of matter is not altered in chemical reactions1 because he accurately weighed 7introducing all the necessary corrections7 the reacting and resultant substances4 ,landolt 7#ahic7 carried on various reactions in inverted and sealed glass ,u-tubes1 and on weighing the tubes before reaction 7when the reacting solutions were separated in each of the branches of the tubes71 and after 7when the solutions had been well mixed together by shaking71 found that either the weight remained perfectly constant or that the variation was so small 7for instance1 #jb milligram in a total weight of about a million milligrams7 as to be ascribed to the inevitable errors of weighing4 #aa bis ,the idea of the mass of matter was first shaped into an exact form by ,galileo 7died #afdb71 and more especially by ,newton 7born #afdc1 died #agbg71 in the glorious epoch of the development of the principles of inductive reasoning enunciated by ,bacon and ,descartes in their philosophical treatises4 ,shortly after the death of ,newton1 ,lavoisier1 whose fame in natural philosophy should rank with that of ,galileo and ,newton1 was born on ,august #bf1 #agdc4 ,the death of ,lavoisier occurred during the ,reign of ,terror of the ,french ,revolution1 when he1 together with twenty-six other chief farmers of the revenue1 was guillotined on ,may #h1 #agid1 at ,paris2 but his works and ideas have made him immortal4 #a4 ,it is well known that iron rusts in damp air1#ab and that when heated to redness in air it becomes coated with scoria 7oxide71 having1 like rust1 the appearance of an earthy substance resembling some of the iron ores from which metallic iron is extracted4 ,if the iron is weighed before and after the formation of the scoria or rust1 it will be found that the metal has increased in weight during the operation4#ac ,it can easily be proved that this increase in weight is accomplished at the expense of the atmosphere1 and mainly1 as ,lavoisier proved1 at the expense of that portion which is called oxygen4 ,in fact1 in a vacuum1 or in gases which do not contain oxygen1 for instance1 in hydrogen or nitrogen1 the iron neither rusts nor becomes coated with scoria4 ,had the iron not been weighed1 the participation of the oxygen of the atmosphere in its transformation into an earthy substance might have easily passed unnoticed1 as was formerly the case1 when phenomena like the above were1 for this reason1 misunderstood4 ,it is evident from the law of the indestructibility of matter that as the iron increases in weight in its conversion into rust1 the latter must be a more complex substance than the iron itself1 and its formation is due to a reaction of combination4 ,we might form an entirely wrong opinion about it1 and might1 for instance1 consider rust to be a simpler substance than iron1 and explain the formation of rust as the removal of something from the iron4 ,such1 indeed1 was the general opinion prior to ,lavoisier1 when it was held that iron contained a certain unknown substance called 'phlogiston1' and that rust was iron deprived of this supposed substance4 #ab ,by covering iron with an enamel1 or varnish1 or with unrustable metals 7such as nickel71 or a coating of paraffin1 or other similar substances1 it is protected from the air and moisture1 and so kept from rusting4 #ac ,such an experiment may easily be made by taking the finest 7unrusted7 iron filings 7ordinary filings must be first washed in ether1 dried1 and passed through a very fine sieve74 ,the filings thus obtained are capable of burning directly in air 7by oxidising or forming rust71 especially when they hang 7are attracted7 on a magnet4 ,a compact piece of iron does not burn in air1 but spongy iron glows and smoulders like tinder4 ,in making the experiment1 a horse-shoe magnet is fixed1 with the poles downwards1 on one arm of a rather sensitive balance1 and the iron filings are applied to the magnet 7on a sheet of paper7 so as to form a beard about the poles4 ,the balance pan should be exactly under the filings on the magnet1 in order that any which might fall from it should not alter the weight4 ,the filings1 having been weighed1 are set light to by applying the flame of a candle2 they easily take fire1 and go on burning by themselves1 forming rust4 ,when the combustion is ended1 it will be clear that the iron has increased in weight2 from #e-a_/b parts by weight of iron filings taken1 there are obtained1 by complete combustion1 #g-a_/b parts by weight of rust4 ,illustration3 ,,fig4 #a4--;,apparatus for the decomposition of red mercury oxide4 #b4 ,copper carbonate 7in the form of a powder1 or as the well-known green mineral called 'malachite1' which is used for making ornaments1 or as an ore for the extraction of copper7 changes into a black substance called 'copper oxide' when heated to redness4#ad ,this black substance is also obtained by heating copper to redness in air--that is1 it is the scoria or oxidation product of copper4 ,the weight of the black oxide of copper left is less than that of the copper carbonate originally taken1 and therefore we consider the reaction which occurred to have been one of decomposition1 and that by it something was separated from the green copper carbonate1 and1 in fact1 by closing the orifice of the vessel in which the copper carbonate is heated with a well-fitting cork1 through which a gas delivery tube#ae passes whose end is immersed under water1 it will be observed that on heating1 a gas is formed which bubbles through the water4 ,this gas can be easily collected1 as will presently be described1 and it will be found to essentially differ from air in many respects2 for instance1 a burning taper is extinguished in it as if it had been plunged into water4 ,if weighing had not proved to us that some substance had been separated1 the formation of the gas might easily have escaped our notice1 for it is colourless and transparent like air1 and is therefore evolved without any striking feature4 ,the carbonic anhydride evolved may be weighed1#af and it will be seen that the sum of the weights of the black copper oxide and carbonic anhydride is equal to the weight of the copper carbonate#ag originally taken1 and thus by carefully following out the various stages of all chemical reactions we arrive at a confirmation of the law of the indestructibility of matter4 #ad ,for the purpose of experiment1 it is most convenient to take copper carbonate1 which may be prepared by the experimenter himself1 by adding a solution of sodium carbonate to a solution of copper sulphate4 ,the precipitate 7deposit7 so formed is collected on a filter1 washed1 and dried4 ,the decomposition of copper carbonate into copper oxide is effected by so moderate a heat that it may be performed in a glass vessel heated by a lamp4 ,for this purpose a thin glass tube1 closed at one end1 and called a 'test tube1' may be employed1 or else a vessel called a 'retort4' ,the experiment is carried on1 as described in example three on p4 #aa1 by collecting the carbonic anhydride over water1 as will be afterwards explained4 #ae ,gas delivery tubes are usually made of glass tubing of various diameters and thicknesses4 ,if of small diameter and thickness1 a glass tube is easily bent by heating in a gas jet or the flame of a spirit lamp1 and it may also be easily divided at a given point by making a deep scratch with a file and then breaking the tube at this point with a sharp jerk4 ,these properties1 together with their impermeability1 transparency1 hardness1 and regularity of bore1 render glass tubes most useful in experiments with gases4 ,naturally they might be replaced by straws1 india-rubber1 metallic1 or other tubes1 but these are more difficult to fix on to a vessel1 and are not entirely impervious to gases4 ,a glass gas delivery tube may be hermetically fixed into a vessel by fitting it into a perforated cork1 which should be soft and free from flaws1 and fixing the cork into the orifice of the vessel4 ,to protect the cork from the action of gases it is sometimes previously soaked in paraffin1 or it may be replaced by an india-rubber cork4 #af ,gases1 like all other substances1 may be weighed1 but1 owing to their extreme lightness and the difficulty of dealing with them in large masses1 they can only be weighed by very sensitive balances2 that is1 in such as1 with a considerable load1 indicate a very small difference in weight--for example1 a centigram or a milligram with a load of #a1jjj grams4 ,in order to weigh a gas1 a glass globe furnished with a tight-fitting stop-cock is first of all exhausted of air by an air-pump 7a ,sprengel pump is the best74 ,the stop-cock is then closed1 and the exhausted globe weighed4 ,if the gas to be weighed is then let into the globe1 its weight can be determined from the increase in the weight of the globe4 ,it is necessary1 however1 that the temperature and pressure of the air about the balance should remain constant for both weighings1 as the weight of the globe in air will 7according to the laws of hydrostatics7 vary with its density4 ,the volume of the air displaced1 and its weight1 must therefore be determined by observing the temperature1 density1 and moisture of the atmosphere during the time of experiment4 ,this will be partly explained later1 but may be studied more in detail by physics4 ,owing to the complexity of all these operations1 the mass of a gas is usually determined from its volume and density1 or from the weight of a known volume4 #ag ,the copper carbonate should be dried before weighing1 as otherwise--besides copper oxide and carbonic anhydride--water will be obtained in the decomposition4 ,water forms a part of the composition of malachite1 and has therefore to be taken into consideration4 ,the water produced in the decomposition may be all collected by absorbing it in sulphuric acid or calcium chloride1 as will be described further on4 ,in order to dry a salt it must be heated at about #ajj until its weight remains constant1 or be placed under an air pump over sulphuric acid1 as will also be presently described4 ,as water is met with almost everywhere1 and as it is absorbed by many substances1 the possibility of its presence should never be lost sight of4 #c4 ,red mercury oxide 7which is formed as mercury rust by heating mercury in air7 is decomposed like copper carbonate 7only by heating more slowly and at a somewhat higher temperature71 with the formation of the peculiar gas1 oxygen4 ,for this purpose the mercury oxide is placed in a glass tube or retort1#ah to which a gas delivery tube is attached by means of a cork4 ,this tube is bent downwards1 as shown in the drawing 7,fig4 #a74 ,the open end of the gas delivery tube is immersed in a vessel filled with water1 called a pneumatic trough4#ai ,when the gas begins to be evolved in the retort it is obliged1 having no other outlet1 to escape through the gas delivery tube into the water in the pneumatic trough1 and therefore its evolution will be rendered visible by the bubbles coming from this tube4 ,in heating the retort containing the mercury oxide1 the air contained in the apparatus is first partly expelled1 owing to its expansion by heat1 and then the peculiar gas called 'oxygen' is evolved1 and may be easily collected as it comes off4 ,for this purpose a vessel 7an ordinary cylinder1 as in the drawing7 is filled quite full with water and its mouth closed2 it is then inverted and placed in this position under the water in the trough2 the mouth is then opened4 ,the cylinder will remain full of water--that is1 the water will remain at a higher level in it than in the surrounding vessel1 owing to the atmospheric pressure4 ,the atmosphere presses on the surface of the water in the trough1 and prevents the water from flowing out of the cylinder4 ,the mouth of the cylinder is placed over the end of the gas delivery tube1#bj and the bubbles issuing from it will rise into the cylinder and displace the water contained in it4 ,gases are generally collected in this manner4 ,when a sufficient quantity of gas has accumulated in the cylinder it can be clearly shown that it is not air1 but another gas which is distinguished by its capacity for vigorously supporting combustion4 ,in order to show this1 the cylinder is closed1 under water1 and removed from the bath2 its mouth is then turned upwards1 and a smouldering taper plunged into it4 ,as is well known1 a smouldering taper will be extinguished in air1 but in the gas which is given off from red mercury oxide it burns clearly and vigorously1 showing the property possessed by this gas for supporting combustion more energetically than air1 and thus enabling it to be distinguished from the latter4 ,it may be observed in this experiment that1 besides the formation of oxygen1 metallic mercury is formed1 which1 volatilising at the high temperature required for the reaction1 condenses on the cooler parts of the retort as a mirror or in globules4 ,thus two substances1 mercury and oxygen1 are obtained by heating red mercury oxide4 ,in this reaction1 from one substance1 two new substances are produced--that is1 a decomposition has taken place4 ,the means of collecting and investigating gases were known before ,lavoisier's time1 but he first showed the real part they played in the processes of many chemical changes which before his era were either wrongly understood 7as will be afterwards explained7 or were not explained at all1 but only observed in their superficial aspects4 ,this experiment on red mercury oxide has a special significance in the history of chemistry contemporary with ,lavoisier1 because the oxygen gas which is here evolved is contained in the atmosphere1 and plays a most important part in nature1 especially in the respiration of animals1 in combustion in air1 and in the formation of rusts or scori 7earths1 as they were then called7 from metals--that is1 of earthy substances1 like the ores from which metals are extracted4 #ah ,as the decomposition of red oxide of mercury requires so high a temperature1 near redness1 as to soften ordinary glass1 it is necessary for this experiment to take a retort 7or test tube7 made of hard glass1 which is able to stand high temperatures without softening4 ,for the same reason1 the lamp used must give a strong heat and a large flame1 capable of embracing the whole bottom of the retort1 which should be as small as possible for the convenience of the experiment4 #ai ,illustration3 ,,fig4 #b4--;,apparatus for distilling under a diminished pressure liquids which decompose at their boiling points under the ordinary pressure4 ,the apparatus in which the liquid is distilled is connected with a large globe from which the air is pumped out2 the liquid is heated1 and the receiver cooled4 ,the pneumatic trough may naturally be made of any material 7china1 earthenware1 or metal1 c471 but usually a glass one1 as shown in the drawing1 is used1 as it allows the progress of the experiment to be better observed4 ,for this reason1 as well as the ease with which they are kept clean1 and from the fact also that glass is not acted on by many substances which affect other materials 7for instance1 metals71 glass vessels of all kinds--such as retorts1 test tubes1 cylinders1 beakers1 flasks1 globes1 c4--are preferred to any other for chemical experiments4 ,glass vessels may be heated without any danger if the following precautions be observed3 #ast1 they should be made of thin glass1 as otherwise they are liable to crack from the bad heat-conducting power of glass2 #bnd1 they should be surrounded by a liquid or with sand 7,fig4 #b71 or sand bath as it is called2 or else should stand in a current of hot gases without touching the fuel from which they proceed1 or in the flame of a smokeless lamp4 ,a common candle or lamp forms a deposit of soot on a cold object placed in their flames4 ,the soot interferes with the transmission of heat1 and so a glass vessel when covered with soot often cracks4 ,and for this reason spirit lamps1 which burn with a smokeless flame1 or gas burners of a peculiar construction1 are used4 ,in the ,bunsen burner the gas is mixed with air1 and burns with a non-luminous and smokeless flame4 ,on the other hand1 if an ordinary lamp 7petroleum or benzine7 does not smoke it may be used for heating a glass vessel without danger1 provided the glass is placed well above the flame in the current of hot gases4 ,in all cases1 the heating should be begun very carefully by raising the temperature by degrees4 #bj ,in order to avoid the necessity of holding the cylinder1 its open end is widened 7and also ground so that it may be closely covered with a ground-glass plate when necessary71 and placed on a stand below the level of the water in the bath4 ,this stand is called 'the bridge4' ,it has several circular openings cut through it1 and the gas delivery tube is placed under one of these1 and the cylinder for collecting the gas over it4 #d4 ,in order to illustrate by experiment one more example of chemical change and the application of the law of the indestructibility of matter1 we will consider the reaction between common table salt and lunar caustic1 which is well known from its use in cauterising wounds4 ,by taking a clear solution of each and mixing them together1 it will at once be observed that a solid white substance is formed1 which settles to the bottom of the vessel1 and is insoluble in water4 ,this substance may be separated from the solution by filtering2 it is then found to be an entirely different substance from either of those taken originally in the solutions4 ,this is at once evident from the fact that it does not dissolve in water4 ,on evaporating the liquid which passed through the filter1 it will be found to contain a new substance unlike either table salt or lunar caustic1 but1 like them1 soluble in water4 ,thus table salt and lunar caustic1 two substances soluble in water1 produced1 by their mutual chemical action1 two new substances1 one insoluble in water1 and the other remaining in solution4 ,here1 from two substances1 two others are obtained1 consequently there occurred a reaction of substitution4 ,the water served only to convert the re-acting substances into a liquid and mobile state4 ,if the lunar caustic and salt be dried#ba and weighed1 and if about #eh-a_/b grams#bb of salt and #agj grams of lunar caustic be taken1 then #adc-a_/b grams of insoluble silver chloride and #he grams of sodium nitrate will be obtained4 ,the sum of the weights of the re-acting and resultant substances are seen to be similar and equal to #bbh-a_/b grams1 which necessarily follows from the law of the indestructibility of matter4 #ba ,drying is necessary in order to remove any water which may be held in the salts 7see ,note #ag1 and ,chapter ,i41 ,notes #ac and #ad74 #bb ,the exact weights of the re-acting and resulting substances are determined with the greatest difficulty1 not only from the possible inexactitude of the balance 7every weighing is only correct within the limits of the sensitiveness of the balance7 and weights used in weighing1 not only from the difficulty in making corrections for the weight of air displaced by the vessels holding the substances weighed and by the weights themselves1 but also from the hygroscopic nature of many substances 7and vessels7 causing absorption of moisture from the atmosphere1 and from the difficulty in not losing any of the substance to be weighed in the several operations 7filtering1 evaporating1 and drying1 c47 which have to be performed before arriving at a final result4 ,all these circumstances have to be taken into consideration in exact researches1 and their elimination requires very many special precautions which are impracticable in preliminary experiments4 ,accepting the truth of the above law1 the question naturally arises as to whether there is any limit to the various chemical transformations1 or are they unrestricted in number--that is to say1 is it possible from a given substance to obtain an equivalent quantity of any other substance8 ,in other words1 does there exist a perpetual and infinite change of one kind of material into every other kind1 or is the cycle of these transformations limited8 ,this is the second essential problem of ,chemistry1 a question of quality of matter1 and one1 it is evident1 which is more complicated than the question of quantity4 ,it cannot be solved by a mere superficial glance at the subject4 ,indeed1 on seeing how all the varied forms and colours of plants are built up from air and the elements of the soil1 and how metallic iron can be transformed into colours such as inks and ,prussian blue1 we might be led to think that there is no end to the qualitative changes to which matter is susceptible4 ,but1 on the other hand1 the experiences of everyday life compel us to acknowledge that food cannot be made out of a stone1 or gold out of copper4 ,thus a definite answer can only be looked for in a close and diligent study of the subject1 and the problem has been resolved in different way at different times4 ,in ancient times the opinion most generally held was that everything visible was composed of four elements--,air1 ,water1 ,earth1 and ,fire4 ,the origin of this doctrine can be traced far back into the confines of ,asia1 whence it was handed down to the ,greeks1 and most fully expounded by ,empedocles1 who lived before #dfj ,b4,c4 ,this doctrine was not the result of exact research1 but apparently owes its origin to the clear division of bodies into gases 7like air71 liquids 7like water71 and solids 7like the earth74 ,the ,arabs appear to have been the first who attempted to solve the question by experimental methods1 and they introduced1 through ,spain1 the taste for the study of similar problems into ,europe1 where from that time there appear many adepts in chemistry1 which was considered as an unholy art1 and called 'alchemy4' ,as the alchemists were ignorant of any exact law which could guide them in their researches1 they obtained most anomalous results4 ,their chief service to chemistry was that they made a number of experiments1 and discovered many new chemical transformations2 but it is well known how they solved the fundamental problem of chemistry4 ,their view may be taken as a positive acknowledgment of the infinite transmutability of matter1 for they aimed at discovering the ,philosopher's ,stone1 capable of converting everything into gold and diamonds1 and of making the old young again4 ,this solution of the question was afterwards completely overthrown1 but it must not1 for this reason1 be thought that the hopes held by the alchemists were only the fruit of their imaginations4 ,the first chemical experiments might well lead them to their conclusions4 ,they took1 for instance1 the bright metallic mineral galena1 and extracted metallic lead from it4 ,thus they saw that from a metallic substance which is unfitted for use they could obtain another metallic substance which is ductile and valuable for many technical purposes4 ,furthermore1 they took this lead and obtained silver1 a still more valuable metal1 from it4 ,thus they might easily conclude that it was possible to ennoble metals by means of a whole series of transmutations--that is to say1 to obtain from them those which are more and more precious4 ,having got silver from lead1 they assumed that it would be possible to obtain gold from silver4 ,the mistake they made was that they never weighed or measured the substances used or produced in their experiments4 ,had they done so1 they would have learnt that the weight of the lead was much less than that of the galena from which it was obtained1 and the weight of the silver infinitesimal compared with that of the lead4 ,had they looked more closely into the process of the extraction of the silver from lead 7and silver at the present time is chiefly obtained from the lead ores7 they would have seen that the lead does not change into silver1 but that it only contains a certain small quantity of it1 and this amount having once been separated from the lead it cannot by any further operation give more4 ,the silver which the alchemists extracted from the lead was in the lead1 and was not obtained by a chemical change of the lead itself4 ,this is now well known from experiment1 but the first view of the nature of the process was very likely to be an erroneous one4#bc ,the methods of research adopted by the alchemists could give but little success1 for they did not set themselves clear and simple questions whose answers would aid them to make further progress4 ,thus though they did not arrive at any exact law1 they left nevertheless numerous and useful experimental data as an inheritance to chemistry2 they investigated1 in particular1 the transformations proper to metals1 and for this reason chemistry was for long afterwards entirely confined to the study of metallic substances4 #bc ,besides which1 in the majority of cases1 the first explanation of most subjects which do not repeat themselves in everyday experience under various aspects1 but always in one form1 or only at intervals and infrequently1 is usually wrong4 ,thus the daily evidence of the rising of the sun and stars evokes the erroneous idea that the heavens move and the earth stands still4 ,this apparent truth is far from being the real truth1 and1 as a matter of fact1 is contradictory to it4 ,similarly1 an ordinary mind and everyday experience concludes that iron is incombustible1 whereas it burns not only as filings1 but even as wire1 as we shall afterwards see4 ,with the progress of knowledge very many primitive prejudices have been obliged to give way to true ideas which have been verified by experiment4 ,in ordinary life we often reason at first sight with perfect truth1 only because we are taught a right judgment by our daily experience4 ,it is a necessary consequence of the nature of our minds to reach the attainment of truth through elementary and often erroneous reasoning and through experiment1 and it would be very wrong to expect a knowledge of truth from a simple mental effort4 ,naturally1 experiment itself cannot give truth1 but it gives the means of destroying erroneous representations whilst confirming those which are true in all their consequences4 ,in their researches1 the alchemists frequently made use of two chemical processes which are now termed 'reduction' and 'oxidation4' ,the rusting of metals1 and in general their conversion from a metallic into an earthy form1 is called 'oxidation1' whilst the extraction of a metal from an earthy substance is called 'reduction4' ,many metals--for instance1 iron1 lead1 and tin--are oxidised by heating in air alone1 and may be again reduced by heating with carbon4 ,such oxidised metals are found in the earth1 and form the majority of metallic ores4 ,the metals1 such as tin1 iron1 and copper1 may be extracted from these ores by heating them together with carbon4 ,all these processes were well studied by the alchemists4 ,it was afterwards shown that all earths and minerals are formed of similar metallic rusts or oxides1 or of their combinations4 ,thus the alchemists knew of two forms of chemical changes3 the oxidation of metals and the reduction of the oxides so formed into metals4 ,the explanation of the nature of these two classes of chemical phenomena was the means for the discovery of the most important chemical laws4 ,the first hypothesis on their nature is due to ,becker1 and more particularly to ,stahl1 a surgeon to the ,king of ,prussia4 ,stahl writes in his ',fundamenta ,chymi1' #agbc1 that all substances consist of an imponderable fiery substance called 'phlogiston' 7materia aut principium ignis non ipse ignis71 and of another element having particular properties for each substance4 ,the greater the capacity of a body for oxidation1 or the more combustible it is1 the richer it is in phlogiston4 ,carbon contains it in great abundance4 ,in oxidation or combustion phlogiston is emitted1 and in reduction it is consumed or enters into combination4 ,carbon reduces earthy substances because it is rich in phlogiston1 and gives up a portion of its phlogiston to the substance reduced4 ,thus ,stahl supposed metals to be compound substances consisting of phlogiston and an earthy substance or oxide4 ,this hypothesis is distinguished for its very great simplicity1 and for this and other reasons it acquired many supporters4#bd #bd ,it is true that ,stahl was acquainted with a fact which directly disproved his hypothesis4 ,it was already known 7from the experiments of ,geber1 and more especially of ,ray1 in #afcj7 that metals increase in weight by oxidation1 whilst1 according to ,stahl's hypothesis1 they should decrease in weight1 because phlogiston is separated by oxidation4 ,stahl speaks on this point as follows3--',i am well aware that metals1 in their transformation into earths1 increase in weight4 ,but not only does this fact not disprove my theory1 but1 on the contrary1 confirms it1 for phlogiston is lighter than air1 and1 in combining with substances1 strives to lift them1 and so decreases their weight2 consequently1 a substance which has lost phlogiston must be heavier4' ,this argument1 it will be seen1 is founded on a misconception of the properties of gases1 regarding them as having no weight and as not being attracted by the earth1 or else on a confused idea of phlogiston itself1 since it was first defined as imponderable4 ,the conception of imponderable phlogiston tallies well with the habit and methods of the last century1 when recourse was often had to imponderable fluids for explaining a large number of phenomena4 ,heat1 light1 magnetism1 and electricity were explained as being peculiar imponderable fluids4 ,in this sense the doctrine of ,stahl corresponds entirely with the spirit of his age4 ,if heat be now regarded as motion or energy1 then phlogiston also should be considered in this light4 ,in fact1 in combustion1 of coals for instance1 heat and energy are evolved1 and not combined in the coal1 although the oxygen and coal do combine4 ,consequently1 the doctrine of ,stahl contains the essence of a true representation of the evolution of energy1 but naturally this evolution is only a consequence of the combination occurring between the coal and oxygen4 ,as regards the history of chemistry prior to ,lavoisier1 besides ,stahl's work 7to which reference has been made above71 ,priestley's ,experiments and ,observations on ,different ,kinds of ,air1 ,london1 #agij1 and also ,scheele's ,opuscula ,chimica et ,physica1 ,lips41 #aghh-hi1 #b vols41 must be recommended as the two leading works of the ,english and ,scandinavian chemists showing the condition of chemical learning before the propagation of ,lavoisier's views1 and containing also many important observations which lie at the basis of the chemistry of our times4 ,a most interesting memoir on the history of phlogiston is that of ,rodwell1 in the ,philosophical ,magazine1 #ahfh1 in which it is shown that the idea of phlogiston dates very far back1 that ,basil ,valentine 7#acid-adae71 in the ,cursus ,triumphalis ,antimonii1 ,paracelsus 7#adic-aeda71 in his work1 ,de ,rerum ,natura1 ,glauber 7#afjd-affh71 and especially ,john ,joachim ,becher 7#afbe-afhb71 in his ,physica ,subterranea1 all referred to phlogiston1 but under different names4 ,illustration3 ,,fig4 #c4--,lavoisier's apparatus for determining the composition of air and the reason of metals increasing in weight when they are calcined in air4 ,lavoisier proved by means of the balance that every case of rusting of metals or oxidation1 or of combustion1 is accompanied by an increase in weight at the expense of the atmosphere4 ,he formed1 therefore1 the natural opinion that the heavier substance is more complex than the lighter one4#be ,lavoisier's celebrated experiment1 made in #aggd1 gave indubitable support to his opinion1 which in many respects was contradictory to ,stahl's doctrine4 ,lavoisier poured four ounces of pure mercury into a glass retort 7fig4 #c71 whose neck was bent as shown in the drawing and dipped into the vessel ,r ,s1 also full of mercury4 ,the projecting end of the neck was covered with a glass bell-jar ,p4 ,the weight of all the mercury taken1 and the volume of air remaining in the apparatus1 namely1 that in the upper portion of the retort1 and under the bell-jar1 were determined before beginning the experiment4 ,it was most important in this experiment to know the volume of air in order to learn what part it played in the oxidation of the mercury1 because1 according to ,stahl1 phlogiston is emitted into the air1 whilst1 according to ,lavoisier1 the mercury in oxidising absorbs a portion of the air2 and consequently it was absolutely necessary to determine whether the amount of air increased or decreased in the oxidation of the metal4 ,it was1 therefore1 most important to measure the volume of the air in the apparatus both before and after the experiment4 ,for this purpose it was necessary to know the total capacity of the retort1 the volume of the mercury poured into it1 the volume of the bell-jar above the level of the mercury1 and also the temperature and pressure of the air at the time of its measurement4 ,the volume of air contained in the apparatus and isolated from the surrounding atmosphere could be determined from these data4 ,having arranged his apparatus in this manner1 ,lavoisier heated the retort holding the mercury for a period of twelve days at a temperature near the boiling point of mercury4 ,the mercury became covered with a quantity of small red scales2 that is1 it was oxidised or converted into an earth4 ,this substance is the same mercury oxide which has already been mentioned 7example #c74 ,after the lapse of twelve days the apparatus was cooled1 and it was then seen that the volume of the air in the apparatus had diminished during the time of the experiment4 ,this result was in exact contradiction to ,stahl's hypothesis4 ,out of #ej cubic inches of air originally taken1 there only remained #db4 ,lavoisier's experiment led to other equally important results4 ,the weight of the air taken decreased by as much as the weight of the mercury increased in oxidising2 that is1 the portion of the air was not destroyed1 but only combined with mercury4 ,this portion of the air may be again separated from the mercury oxide and has1 as we saw 7example #c71 properties different from those of air4 ,it is called 'oxygen4' ,that portion of the air which remained in the apparatus and did not combine with the mercury does not oxidise metals1 and cannot support either combustion or respiration1 so that a lighted taper is immediately extinguished if it be dipped into the gas which remains in the bell-jar4 ',it is extinguished in the residual gas as if it had been plunged into water1' writes ,lavoisier in his memoirs4 ,this gas is called 'nitrogen4' ,thus air is not a simple substance1 but consists of two gases1 oxygen and nitrogen1 and therefore the opinion that air is an elementary substance is erroneous4 ,the oxygen of the air is absorbed in combustion and the oxidation of metals1 and the earths produced by the oxidation of metals are substances composed of oxygen and a metal4 ,by mixing the oxygen with the nitrogen the same air as was originally taken is re-formed4 ,it has also been shown by direct experiment that on reducing an oxide with carbon1 the oxygen contained in the oxide is transferred to the carbon1 and gives the same gas that is obtained by the combustion of carbon in air4 ,therefore this gas is a compound of carbon and oxygen1 just as the earthy oxides are composed of metals and oxygen4 #be ,an ,englishman1 named ,mayow1 who lived a whole century before ,lavoisier 7in #afff71 understood certain phenomena of oxidation in their true aspect1 but was not able to develop his views with clearness1 or support them by conclusive experiments2 he cannot therefore be considered1 like ,lavoisier1 as the founder of contemporary chemical learning4 ,science is a universal heritage1 and therefore it is only just to give the highest honour in science1 not to those who first enunciate a certain truth1 but to those who are first able to convince others of its authenticity and establish it for the general welfare4 ,but scientific discoveries are rarely made all at once2 as a rule1 the first teachers do not succeed in convincing others of the truth they have discovered2 with time1 however1 a true herald comes forward1 possessing every means for making the truth apparent to all1 but it must not be forgotten that such are entirely indebted to the labours and mass of data accumulated by many others4 ,such was ,lavoisier1 and such are all the great founders of science4 ,they are the enunciators of all past and present learning1 and their names will always be revered by posterity4 ,the many examples of the formation and decomposition of substances which are met with convince us that the majority of substances with which we have to deal are compounds made up of several other substances4 ,by heating chalk 7or else copper carbonate1 as in the second example7 we obtain lime and the same carbonic acid gas which is produced by the combustion of carbon4 ,on bringing lime into contact with this gas and water1 at the ordinary temperature1 we again obtain the compound1 carbonate of lime1 or chalk4 ,therefore chalk is a compound4 ,so also are those substances from which it may be built up4 ,carbonic anhydride is formed by the combination of carbon and oxygen2 and lime is produced by the oxidation of a certain metal called 'calcium4' ,by resolving substances in this manner into their component parts1 we arrive at last at such as are indivisible into two or more substances by any means whatever1 and which cannot be formed from other substances4 ,all we can do is to make such substances combine together to act on other substances4 ,substances which cannot be formed from or decomposed into others are termed simple substances 7elements74 ,thus all homogeneous substances may be classified into simple and compound substances4 ,this view was introduced and established as a scientific fact during the lifetime of ,lavoisier4 ,the number of these elements is very small in comparison with the number of compound substances which are formed by them4 ,at the present time1 only seventy elements are known with certainty to exist4 ,some of them are very rarely met with in nature1 or are found in very small quantities1 whilst the existence of others is still doubtful4 ,the number of elements with whose compounds we commonly deal in everyday life is very small4 ,elements cannot be transmuted into one another--at least up to the present not a single case of such a transformation has been met with2 it may therefore be said that1 as yet1 it is impossible to transmute one metal into another4 ,and as yet1 notwithstanding the number of attempts which have been made in this direction1 no fact has been discovered which could in any way support the idea of the complexity of such well-known elements#bf as oxygen1 iron1 sulphur1 c4 ,therefore1 from its very conception1 an element is not susceptible to reactions of decomposition4#bg #bf ,many of the ancient philosophers assumed the existence of one elementary form of matter4 ,this idea still appears in our times1 in the constant efforts which are made to reduce the number of the elements2 to prove1 for instance1 that bromine contains chlorine or that chlorine contains oxygen4 ,many methods1 founded both on experiment and theory1 have been tried to prove the compound nature of the elements4 ,all labour in this direction has as yet been in vain1 and the assurance that elementary matter is not so homogeneous 7single7 as the mind would desire in its first transport of rapid generalisation is strengthened from year to year4 ,all our knowledge shows that iron and other elements remain1 even at such a high temperature as there exists in the sun1 as different substances1 and are not converted into one common material4 ,admitting1 even mentally1 the possibility of one elementary form of matter1 a method must be imagined by which it could give rise to the various elements1 as also the modus operandi of their formation from one material4 ,if it be said that this diversitude only takes place at low temperatures1 as is observed with isomerides1 then there would be reason to expect1 if not the transition of the various elements into one particular and more stable form1 at least the mutual transformation of some into others4 ,but nothing of the kind has as yet been observed1 and the alchemist's hope to manufacture 7as ,berthollet puts it7 elements has no theoretical or practical foundation4 #bg ,the weakest point in the idea of elements is the negative character of the determinative signs given them by ,lavoisier1 and from that time ruling in chemistry4 ,they do not decompose1 they do not change into one another4 ,but it must be remarked that elements form the limiting horizon of our knowledge of matter1 and it is always difficult to determine a positive side on the borderland of what is known4 ,besides1 there is no doubt 7from the results of spectrum analysis7 that the elements are distributed as far as the most distant stars1 and that they support the highest attainable temperatures without decomposing4 ,the quantity1 therefore1 of each element remains constant in all chemical changes3 a fact which may be deduced as a consequence of the law of the indestructibility of matter1 and of the conception of elements themselves4 ,thus the equation expressing the law of the indestructibility of matter acquires a new and still more important signification4 ,if we know the quantities of the elements which occur in the re-acting substances1 and if from these substances there proceed1 by means of chemical changes1 a series of new compound substances1 then the latter will together contain the same quantity of each of the elements as there originally existed in the re-acting substances4 ,the essence of chemical change is embraced in the study of how1 and with what substances1 each element is combined before and after change4 ,in order to be able to express various chemical changes by equations1 it has been agreed to represent each element by the first or some two letters of its 7,latin7 name4 ,thus1 for example1 oxygen is represented by the letter ,o2 nitrogen by ,n2 mercury 7hydrargyrum7 by ,hg2 iron 7ferrum7 by ,fe2 and so on for all the elements1 as is seen in the tables on page #bd4 ,a compound substance is represented by placing the symbols representing the elements of which it is made up side by side4 ,for example1 red mercury oxide is represented by ,hg,o1 which shows that it is composed of oxygen and mercury4 ,besides this1 the symbol of every element corresponds with a certain relative quantity of it by weight1 called its 'combining' weight1 or the weight of an atom2 so that the chemical formula of a compound substance not only designates the nature of the elements of which it is composed1 but also their quantitative proportion4 ,every chemical process may be expressed by an equation composed of the formul corresponding with those substances which take part in it and are produced by it4 ,the amount by weight of the elements in every chemical equation must be equal on both sides of the equation1 since no element is either formed or destroyed in a chemical change4 ,on pages #bd1 #be1 and #bf a list of the elements1 with their symbols and combining or atomic weights1 is given1 and we shall see afterwards on what basis the atomic weights of elements are determined4 ,at present we will only point out that a compound containing the elements ,a and ,b is designated by the formula ,an ,bm1 where m and n are the coefficients or multiples in which the combining weights of the elements enter into the composition of the substance4 ,if we represent the combining weight of the substance ,a by a and that of the substance ,b by b1 then the composition of the substance ,an ,bm will be expressed thus3 it contains na parts by weight of the substance ,a and mb parts by weight of the substance ,b1 and consequently #ajj parts of our compound contain na #ajj_/na mb percentage parts by weight of the substance ,a and mb #ajj_/na mb of the substance ,b4 ,it is evident that as a formula shows the relative amounts of all the elements contained in a compound1 the actual weights of the elements contained in a given weight of a compound may be calculated from its formula4 ,for example1 the formula ,na,cl of table salt shows 7as ,na #bc and ,cl #cee7 that #ehe lbs4 of salt contain #bc lbs4 of sodium and #cee lbs4 of chlorine1 and that #ajj parts of it contain #cic per cent4 of sodium and #fjg per cent4 of chlorine4 ,what has been said above clearly limits the province of chemical changes1 because from substances of a given kind there can be obtained only such as contain the same elements4 ,even with this limitation1 however1 the number of possible combinations is infinitely great4 ,only a comparatively small number of compounds have yet been described or subjected to research1 and any one working in this direction may easily discover new compounds which had not before been obtained4 ,it often happens1 however1 that such newly-discovered compounds were foreseen by chemistry1 whose object is the apprehension of that uniformity which rules over the multitude of compound substances1 and whose aim is the comprehension of those laws which govern their formation and properties4 ,the conception of elements having been established1 the next objects of chemistry were3 the determination of the properties of compound substances on the basis of the determination of the quantity and kind of elements of which they are composed2 the investigation of the elements themselves2 the determination of what compound substances can be formed from each element and the properties which these compounds show2 and the apprehension of the nature of the connection between the elements in different compounds4 ,an element thus serves as the starting point1 and is taken as the primary conception on which all other substances are built up4 ,when we state that a certain element enters into the composition of a given compound 7when we say1 for instance1 that mercury oxide contains oxygen7 we do not mean that it contains oxygen as a gaseous substance1 but only desire to express those transformations which mercury oxide is capable of making2 that is1 we wish to say that it is possible to obtain oxygen from mercury oxide1 and that it can give up oxygen to various other substances2 in a word1 we desire only to express those transformations of which mercury oxide is capable4 ,or1 more concisely1 it may be said that the composition of a compound is the expression of those transformations of which it is capable4 ,it is useful in this sense to make a clear distinction between the conception of an element as a separate homogeneous substance1 and as a material but invisible part of a compound4 ,mercury oxide does not contain two simple bodies1 a gas and a metal1 but two elements1 mercury and oxygen1 which1 when free1 are a gas and a metal4 ,neither mercury as a metal nor oxygen as a gas is contained in mercury oxide2 it only contains the substance of these elements1 just as steam only contains the substance of ice1 but not ice itself1 or as corn contains the substance of the seed1 but not the seed itself4 ,the existence of an element may be recognised without knowing it in the uncombined state1 but only from an investigation of its combinations1 and from the knowledge that it gives1 under all possible conditions1 substances which are unlike other known combinations of substances4 ,fluorine is an example of this kind4 ,it was for a long time unknown in a free state1 and nevertheless was recognised as an element because its combinations with other elements were known1 and their difference from all other similar compound substances was determined4 ,in order to grasp the difference between the conception of the visible form of an element as we know it in the free state1 and of the intrinsic element 7or 'radicle1' as ,lavoisier called it7 contained in the visible form1 it should be remarked that compound substances also combine together forming yet more complex compounds1 and that they evolve heat in the process of combination4 ,the original compound may often be extracted from these new compounds by exactly the same methods as elements are extracted from their corresponding combinations4 ,besides1 many elements exist under various visible forms whilst the intrinsic element contained in these various forms is something which is not subject to change4 ,thus carbon appears as charcoal1 graphite1 and diamond1 but yet the element carbon alone1 contained in each1 is one and the same4 ,carbonic anhydride contains carbon1 and not charcoal1 or graphite1 or the diamond4 ,elements alone1 although not all of them1 have the peculiar lustre1 opacity1 malleability1 and the great heat and electrical conductivity which are proper to metals and their mutual combinations4 ,but elements are far from all being metals4 ,those which do not possess the physical properties of metals are called non-metals 7or metalloids74 ,it is1 however1 impossible to draw a strict line of demarcation between metals and non-metals1 there being many intermediary substances4 ,thus graphite1 from which pencils are manufactured1 is an element with the lustre and other properties of a metal2 but charcoal and the diamond1 which are composed of the same substance as graphite1 do not show any metallic properties4 ,both classes of elements are clearly distinguished in definite examples1 but in particular cases the distinction is not clear and cannot serve as a basis for the exact division of the elements into two groups4 ,the conception of elements forms the basis of chemical knowledge1 and in giving a list of them at the very beginning of our work1 we wish to tabulate our present knowledge on the subject4 ,altogether about seventy elements are now authentically known1 but many of them are so rarely met with in nature1 and have been obtained in such small quantities1 that we possess but a very insufficient knowledge of them4 ,the substances most widely distributed in nature contain a very small number of elements4 ,these elements have been more completely studied than the others1 because a greater number of investigators have been able to carry on experiments and observations on them4 ,the elements most widely distributed in nature are3-- ,hydrogen1 ,h #a4 ,in water1 and in animal and vegetable organisms4 ,carbon1 ,c #ab4 ,in organisms1 coal1 limestones4 ,nitrogen1 ,n #ad4 ,in air and in organisms4 ,oxygen1 ,o #af4 ,in air1 water1 earth4 ,it forms the greater part of the mass of the earth4 ,sodium1 ,na #bc4 ,in common salt and in many minerals4 ,magnesium1 ,mg #bd4 ,in sea-water and in many minerals4 ,aluminium1 ,al #bg4 ,in minerals and clay4 ,silicon1 ,si #bh4 ,in sand1 minerals1 and clay4 ,phosphorus1 ,p #ca4 ,in bones1 ashes of plants1 and soil4 ,sulphur1 ,s #cb4 ,in pyrites1 gypsum1 and in sea-water4 ,chlorine1 ,cl #cee4 ,in common salt1 and in the salts of sea-water4 ,potassium1 ,k #ci4 ,in minerals1 ashes of plants1 and in nitre4 ,calcium1 ,ca #dj4 ,in limestones1 gypsum1 and in organisms4 ,iron1 ,fe #ef4 ,in the earth1 iron ores1 and in organisms4 ,besides these1 the following elements1 although not very largely distributed in nature1 are all more or less well known from their applications to the requirements of everyday life or the arts1 either in a free state or in their compounds3-- ,lithium1 ,li #g4 ,in medicine 7,li#b;,,co#c71 and in photography 7,li,br74 ,boron1 ,b #aa4 ,as borax1 ,b#d,na#b,o#g1 and as boric anhydride1 ,b#b,o#c4 ,fluorine1 ,f #ai4 ,as fluor spar1 ,ca,f#b1 and as hydrofluoric acid1 ,,hf4 ,chromium1 ,cr #eb4 ,as chromic anhydride1 ,cr,o#c1 and potassium dichromate1 ,k#b;,cr#b,o#g4 ,manganese1 ,mn #ee4 ,as manganese peroxide1 ,mn,o#b1 and potassium permanganate1 ,mn,k,o#d4 ,cobalt1 ,co #eie ,in smalt and blue glass4 ,nickel1 ,ni #eie ,for electro-plating other metals4 ,copper1 ,cu #fc4 ,the well-known red metal4 ,zinc1 ,zn #fe4 ,used for the plates of batteries1 roofing1 c4 ,arsenic1 ,as #ge4 ,white arsenic 7poison71 ,as#b,o#c4 ,bromine1 ,br #hj4 ,a brown volatile liquid2 sodium bromide1 ,na,br4 ,strontium1 ,sr #hg4 ,in coloured fires 7,sr,n#b,o#f74 ,silver1 ,ag #aji4 ,the well-known white metal4 ,cadmium1 ,cd #aab4 ,in alloys4 ,yellow paint 7,cd,s74 ,tin1 ,sn #aai4 ,the well-known metal4 ,antimony1 ,sb #abj4 ,in alloys such as type metal4 ,iodine1 ,i #abg4 ,in medicine and photography2 free1 and as ,,ki4 ,barium1 ,ba #acg4 ,permanent white1 and as an adulterant in white lead1 and in heavy spar1 ,ba,s,o#d4 ,platinum1 ,pt #aif4 ,gold1 ,au #aig4 ,mercury1 ,hg #bjj4 ,well-known metals4 ,lead1 ,pb #bjg4 ,bismuth1 ,bi #bji4 ,in medicine and fusible alloys4 ,uranium1 ,u #bci4 ,in green fluorescent glass4 ,the compounds of the following metals and semi-metals have fewer applications1 but are well known1 and are somewhat frequently met with in nature1 although in small quantities3-- ,beryllium1 ,be #i4 ,palladium1 ,pd #ajg4 ,titanium1 ,ti #dh4 ,cerium1 ,ce #adj4 ,vanadium1 ,v #ea4 ,tungsten1 ,w #ahd4 ,selenium1 ,se #gi4 ,osmium1 ,os #aib4 ,zirconium1 ,zr #ia4 ,iridium1 ,ir #aic4 ,molybdenum1 ,mo #if4 ,thallium1 ,tl #bjd4 ,the following rare metals are still more seldom met with in nature1 but have been studied somewhat fully3-- ,scandium1 ,sc #dd4 ,germanium1 ,ge #gb4 ,gallium1 ,ga #gj4 ,rubidium1 ,rb #hf4 ,yttrium1 ,y #hi4 ,csium1 ,cs #acc4 ,niobium1 ,nb #id4 ,lanthanum1 ,la #ach4 ,ruthenium1 ,ru #ajb4 ,didymium1 ,di #adb4 ,rhodium1 ,rh #ajc4 ,ytterbium1 ,yb #agc4 ,indium1 ,in #aad4 ,tantalum1 ,ta #ahc4 ,tellurium1 ,te #abe4 ,thorium1 ,th #bcb4 ,besides these #ff elements there have been discovered3--,erbium1 ,terbium1 ,samarium1 ,thullium1 ,holmium1 ,mosandrium1 ,phillipium1 and several others4 ,but their properties and combinations1 owing to their extreme rarity1 are very little known1 and even their existence as independent substances#bh is doubtful4 #bh ,possibly some of their compounds are compounds of other already-known elements4 ,pure and incontestably independent compounds of these substances are unknown1 and some of them have not even been separated1 but are only supposed to exist from the results of spectroscopic researches4 ,there can be no mention of such contestable and doubtful elements in a short general handbook of chemistry4 ,it has been incontestably proved from observations on the spectra of the heavenly bodies that many of the commoner elements 7such as ,h1 ,na1 ,mg1 ,fe7 occur on the far distant stars4 ,this fact confirms the belief that those forms of matter which appear on the earth as elements are widely distributed over the entire universe4 ,but we do not yet know why1 in nature1 the mass of some elements should be greater than that of others4#bh bis #bh bis ,clark in ,america made an approximate calculation of the amount of the different elements contained in the earth's crust 7to a depth of #ae kilometres71 and found that the chief mass 7over #ej per cent47 is composed of oxygen2 then comes silicon1 c42 while the amount of hydrogen is less than #a per cent41 carbon scarcely #jbe per cent41 nitrogen even less than #jjc per cent4 ,the relative masses of such metals as ,cu1 ,ni1 ,au is minute4 ,judging from the density 7see ,chapter ,,viii47 of the earth1 a large proportion of its mass must be composed of iron4 ,the capacity of each element to combine with one or another element1 and to form compounds with them which are in a greater or less degree prone to give new and yet more complex substances1 forms the fundamental character of each element4 ,thus sulphur easily combines with the metals1 oxygen1 chlorine1 or carbon1 whilst gold and silver enter into combinations with difficulty1 and form unstable compounds1 which are easily decomposed by heat4 ,the cause or force which induces the elements to enter into chemical change must be considered1 as also the cause which holds different substances in combination--that is1 which endues the substances formed with their particular degree of stability4 ,this cause or force is called affinity 7affinitas1 affinit1 ,verwandtschaft71 or chemical affinity4#bi ,since this force must be regarded as exclusively an attractive force1 like gravity1 many writers 7for instance1 ,bergmann at the end of the last1 and ,berthollet at the beginning of this1 century7 supposed affinity to be essentially similar to the universal force of gravity1 from which it only differs in that the latter acts at observable distances whilst affinity only evinces itself at the smallest possible distances4 ,but chemical affinity cannot be entirely identified with the universal attraction of gravity1 which acts at appreciable distances and is dependent only on mass and distance1 and not on the quality of the material on which it acts1 whilst it is by the quality of matter that affinity is most forcibly influenced4 ,neither can it be entirely identified with cohesion1 which gives to homogeneous solid substances their crystalline form1 elasticity1 hardness1 ductility1 and other properties1 and to liquids their surface tension1 drop formation1 capillarity1 and other properties1 because affinity acts between the component parts of a substance and cohesion on a substance in its homogeneity1 although both act at imperceptible distances 7by contact7 and have much in common4 ,chemical force1 which makes one substance penetrate into another1 cannot be entirely identified with even those attracting forces which make different substances adhere to each other1 or hold together 7as when two plane-polished surfaces of solid substances are brought into close contact71 or which cause liquids to soak into solids1 or adhere to their surfaces1 or gases and vapours to condense on the surfaces of solids4 ,these forces must not be confounded with chemical forces1 which cause one substance to penetrate into the substance of another and to form a new substance1 which is never the case with cohesion4 ,but it is evident that the forces which determine cohesion form a connecting-link between mechanical and chemical forces1 because they only act by intimate contact4 ,for a long time1 and especially during the first half of this century1 chemical attraction and chemical forces were identified with electrical forces4 ,there is certainly an intimate relation between them1 for electricity is evolved in chemical reactions1 and has also a powerful influence on chemical processes--for instance1 compounds are decomposed by the action of an electrical current4 ,and the exactly similar relation which exists between chemical phenomena and the phenomena of heat 7heat being developed by chemical phenomena1 and heat being able to decompose compounds7 only proves the unity of the forces of nature1 the capability of one force to produce and to be transformed into others4 ,for this reason the identification of chemical force with electricity will not bear experimental proof4#cj ,as of all the 7molecular7 phenomena of nature which act on substances at immeasurably small distances1 the phenomena of heat are at present the best 7comparatively7 known1 having been reduced to the simplest fundamental principles of mechanics 7of energy1 equilibrium1 and movement71 which1 since ,newton1 have been subjected to strict mathematical analysis1 it is quite natural that an effort1 which has been particularly pronounced during recent years1 should have been made to bring chemical phenomena into strict correlation with the already investigated phenomena of heat1 without1 however1 aiming at any identification of chemical with heat phenomena4 ,the true nature of chemical force is still a secret to us1 just as is the nature of the universal force of gravity1 and yet without knowing what gravity really is1 by applying mechanical conceptions1 astronomical phenomena have been subjected not only to exact generalisation but to the detailed prediction of a number of particular facts2 and so1 also1 although the true nature of chemical affinity may be unknown1 there is reason to hope for considerable progress in chemical science by applying the laws of mechanics to chemical phenomena by means of the mechanical theory of heat4 ,as yet this portion of chemistry has been but little worked at1 and therefore1 while forming a current problem of the science1 it is treated more fully in that particular field which is termed either 'theoretical' or 'physical' chemistry1 or1 more correctly1 chemical mechanics4 ,as this province of chemistry requires a knowledge not only of the various homogeneous substances which have yet been obtained and of the chemical transformations which they undergo1 but also of the phenomena 7of heat and other kinds7 by which these transformations are accompanied1 it is only possible to enter on the study of chemical mechanics after an acquaintance with the fundamental chemical conceptions and substances which form the subject of this book4#ca #bi ,this word1 first introduced1 if ,i mistake not1 into chemistry by ,glauber1 is based on the idea of the ancient philosophers that combination can only take place when the substances combining have something in common--a medium4 ,as is generally the case1 another idea evolved itself in antiquity1 and has lived until now1 side by side with the first1 to which it is exactly contradictory2 this considers union as dependent on contrast1 on polar difference1 on an effort to fill up a want4 #cj ,especially conclusive are those cases of so-called metalepsis 7,dumas1 ,laurent74 ,chlorine1 in combining with hydrogen1 forms a very stable substance called 'hydrochloric acid1' which is split up by the action of an electrical current into chlorine and hydrogen1 the chlorine appearing at the positive and the hydrogen at the negative pole4 ,hence electro-chemists considered hydrogen to be an electro-positive and chlorine an electro-negative element1 and that they are held together in virtue of their opposite electrical charges4 ,it appears1 however1 from metalepsis1 that chlorine can replace hydrogen 7and1 inversely1 hydrogen can replace chlorine7 in its compounds without in any way changing the grouping of the other elements1 or altering their chief chemical properties4 ,for instance1 acetic acid in which hydrogen has been replaced by chlorine is still capable of forming salts4 ,it must be observed1 whilst considering this subject1 that the explanation suggesting electricity as the origin of chemical phenomena is unsound1 since it attempts to explain one class of phenomena whose nature is almost unknown by another class which is no better known4 ,it is most instructive to remark that together with the electrical theory of chemical attraction there arose and survives a view which explains the galvanic current as being a transference of chemical action through the circuit--i4e41 regards the origin of electricity as being a chemical one4 ,it is evident that the connection is very intimate1 although both phenomena are independent and represent different forms of molecular 7atomic7 motion1 whose real nature is not yet understood4 ,nevertheless1 the connection between the phenomena of both categories is not only in itself very instructive1 but it extends the applicability of the general idea of the unity of the forces of nature1 conviction of the truth of which has held so important a place in the science of the last ten years4 #ca ,i consider that in an elementary text-book of chemistry1 like the present1 it is only possible and advisable to mention1 in reference to chemical mechanics1 a few general ideas and some particular examples referring more especially to gases1 whose mechanical theory must be regarded as the most complete4 ,the molecular mechanics of liquids and solids is as yet in embryo1 and contains much that is disputable2 for this reason1 chemical mechanics has made less progress in relation to these substances4 ,it may not be superfluous here to remark1 with respect to the conception of chemical affinity1 that up to the present time gravity1 electricity1 and heat have all been applied to its elucidation4 ,efforts have also been made to introduce the luminiferous ether into theoretical chemistry1 and should that connection between the phenomena of light and electricity which was established by ,maxwell be worked out more in detail1 doubtless these efforts to elucidate all or a great deal by the aid of luminiferous ether will again appear in theoretical chemistry4 ,an independent chemical mechanics of the material particles of matter1 and of their internal 7atomic7 changes1 would1 in my opinion1 arise as the result of these efforts4 ,two hundred years ago ,newton laid the foundation of a truly scientific theoretical mechanics of external visible motion1 and on this foundation erected the edifice of celestial mechanics4 ,one hundred years ago ,lavoisier arrived at the first fundamental law of the internal mechanics of invisible particles of matter4 ,this subject is far from having been developed into a harmonious whole1 because it is much more difficult1 and1 although many details have been completely investigated1 it does not possess any starting points4 ,newton only came after ,copernicus and ,kepler1 who had discovered empirically the exterior simplicity of celestial phenomena4 ,lavoisier and ,dalton may1 in respect to the chemical mechanics of the molecular world1 be compared to ,copernicus and ,kepler4 ,but a ,newton has not yet appeared in the molecular world2 when he does1 ,i think that he will find the fundamental laws of the mechanics of the invisible motions of matter more easily and more quickly in the chemical structure of matter than in physical phenomena 7of electricity1 heat1 and light72 for these latter are accomplished by particles of matter already arranged1 whilst it is now clear that the problem of chemical mechanics mainly lies in the apprehension of those motions which are invisibly accomplished by the smallest atoms of matter4 ,as the chemical changes to which substances are liable proceed from internal forces proper to these substances1 as chemical phenomena certainly consist of motions of material parts 7from the laws of the indestructibility of matter and of elements71 and as the investigation of mechanical and physical phenomena proves the law of the indestructibility of forces1 or the conservation of energy--that is1 the possibility of the transformation of one kind of motion into another 7of visible or mechanical into invisible or physical7--we are inevitably obliged to acknowledge the presence in substances 7and especially in the elements of which all others are composed7 of a store of chemical energy or invisible motion inducing them to enter into combinations4 ,if heat be evolved in a reaction1 it means that a portion of chemical energy is transformed into heat2#cb if heat be absorbed in a reaction1#cc that it is partly transformed 7rendered latent7 into chemical energy4 ,the store of force or energy going to the formation of new compounds may1 after several combinations1 accomplished with an absorption of heat1 at last diminish to such a degree that indifferent compounds will be obtained1 although these sometimes1 by combining with energetic elements or compounds1 give more complex compounds1 which may be capable of entering into chemical combination4 ,among elements1 gold1 platinum1 and nitrogen have but little energy1 whilst potassium1 oxygen1 and chlorine have a very marked degree of energy4 ,when dissimilar substances enter into combination they often form substances of diminished energy4 ,thus sulphur and potassium when heated easily burn in air1 but when combined together their compound is neither inflammable nor burns in air like its component parts4 ,part of the energy of the potassium and of the sulphur was evolved in their combination in the form of heat4 ,just as in the passage of substances from one physical state into another a portion of their store of heat is absorbed or evolved1 so in combinations or decompositions and in every chemical process1 there occurs a change in the store of chemical energy1 and at the same time an evolution or absorption of heat4#cd #cb ,the theory of heat gave the idea of a store of internal motion or energy1 and therefore with it1 it became necessary to acknowledge chemical energy1 but there is no foundation whatever for identifying heat energy with chemical energy4 ,it may be supposed1 but not positively affirmed1 that heat motion is proper to molecules and chemical motion to atoms1 but that as molecules are made up of atoms1 the motion of the one passes to the other1 and that for this reason heat strongly influences reaction and appears or disappears 7is absorbed7 in reactions4 ,these relations1 which are apparent and hardly subject to doubt on general lines1 still present much that is doubtful in detail1 because all forms of molecular and atomic motion are able to pass into each other4 #cc ,the reactions which take place 7at the ordinary or at a high temperature7 directly between substances may be clearly divided into exothermal1 which are accompanied by an evolution of heat1 and endothermal1 which are accompanied by an absorption of heat4 ,it is evident that the latter require a source of heat4 ,they are determined either by the directly surrounding medium 7as in the formation of carbon bisulphide from charcoal and sulphur1 or in decompositions which take place at high temperatures71 or else by a secondary reaction proceeding simultaneously1 or by some other form of energy 7light1 electricity74 ,so1 for instance1 hydrogen sulphide is decomposed by iodine in the presence of water at the expense of the heat which is evolved by the solution in water of the hydrogen iodide produced4 ,this is the reason why this reaction1 as exothermal1 only takes place in the presence of water2 otherwise it would be accompanied by a cooling effect4 ,as in the combination of dissimilar substances1 the bonds existing between the molecules and atoms of the homogeneous substances have to be broken asunder1 whilst in reactions of rearrangement the formation of any one substance proceeds simultaneously with the formation of another1 and1 as in reactions1 a series of physical and mechanical changes take place1 it is impossible to separate the heat directly depending on a given reaction from the total sum of the observed heat effect4 ,for this reason1 thermochemical data are very complex1 and cannot by themselves give the key to many chemical problems1 as it was at first supposed they might4 ,they ought to form a part of chemical mechanics1 but alone they do not constitute it4 #cd ,as chemical reactions are effected by heating1 so the heat absorbed by substances before decomposition or change of state1 and called 'specific heat1' goes in many cases to the preparation1 if it may be so expressed1 of reaction1 even when the limit of the temperature of reaction is not attained4 ,the molecules of a substance ,a1 which is not able to react on a substance ,b below a temperature t1 by being heated from a somewhat lower temperature to t1 undergoes that change which had to be arrived at for the formation of ,a ,b4 ,for the comprehension of chemical phenomena as mechanical processes--i4e41 the study of the modus operandi of chemical phenomena--it is most important to consider3 7#a7 the facts gathered from stochiometry1 or that part of chemistry which treats of the quantitative relation1 by weight or volume1 of the reacting substances2 7#b7 the distinction between the different forms and classes of chemical reactions2 7#c7 the study of the changes in properties produced by alteration in composition2 7#d7 the study of the phenomena which accompany chemical transformation2 7#e7 a generalisation of the conditions under which reactions occur4 ,as regards stochiometry1 this branch of chemistry has been worked out most thoroughly1 and comprises laws 7of ,dalton1 ,avogadro-,gerhardt1 and others7 which bear so deeply on all parts of chemistry that at the present time the chief problem of chemical research consists in the application of general stochiometrical laws to concrete examples1 i4e41 the quantitative 7volumetric or gravimetric7 composition of substances4 ,all other branches of chemistry are clearly subordinate to this most important portion of chemical knowledge4 ,even the very signification of reactions of combination1 decomposition1 and rearrangement1 acquired1 as we shall see1 a particular and new character under the influence of the progress of exact ideas concerning the quantitative relations of substances entering into chemical changes4 ,furthermore1 in this sense there arose a new--and1 till then1 unknown--division of compound substances into definite and indefinite compounds4 ,even at the beginning of this century1 ,berthollet had not made this distinction4 ,but ,prout showed that a number of compounds contain the substances of which they are they break up1 in exact definite proportions by weight1 which are unalterable under any conditions4 ,thus1 for example1 red mercury oxide always contains sixteen parts by weight of oxygen for every #bjj parts by weight of mercury1 which is expressed by the formula ,hg,o4 ,but in an alloy of copper and silver one or the other metal may be added at will1 and in an aqueous solution of sugar1 the relative proportion of the sugar and water may be altered and nevertheless a homogeneous whole with the sum of the independent properties will be obtained--i4e41 in these cases there was indefinite chemical combination4 ,although in nature and chemical practice the formation of indefinite compounds 7such as alloys and solutions7 plays as essential a part as the formation of definite chemical compounds1 yet1 as the stochiometrical laws at present apply chiefly to the latter1 all facts concerning indefinite compounds suffer from inexactitude1 and it is only during recent years that the attention of chemists has been directed to this province of chemistry4 ,in chemical mechanics it is1 from a qualitative point of view1 very important to clearly distinguish at the very beginning between reversible and non-reversible reactions4 ,substances capable of reacting on each other at a certain temperature produce substances which at the same temperature either can or cannot give back the original substances4 ,for example1 salt dissolves in water at the ordinary temperature1 and the solution so obtained is capable of breaking up at the same temperature1 leaving salt and separating the water by evaporation4 ,carbon bisulphide is formed from sulphur and carbon at about the same temperature at which it can be resolved into sulphur and carbon4 ,iron1 at a certain temperature1 separates hydrogen from water1 forming iron oxide1 which1 in contact with hydrogen at the same temperature1 is able to produce iron and water4 ,it is evident that if two substances1 ,a and ,b1 give two others ,c and ,d1 and the reaction be reversible1 then ,c and ,d will form ,a and ,b1 and1 consequently1 by taking a definite mass of ,a and ,b1 or a corresponding mass of ,c and ,d1 we shall obtain1 in each case1 all four substances--that is to say1 there will be a state of chemical equilibrium between the reacting substances4 ,by increasing the mass of one of the substances we obtain a new condition of equilibrium1 so that reversible reactions present a means of studying the influence of mass on the modus operandi of chemical changes4 ,many of those reactions which occur with very complicated compounds or mixtures may serve as examples of non-reversible reactions4 ,thus many of the compound substances of animal and vegetable organisms are broken up by heat1 but cannot be re-formed from their products of decomposition at any temperature4 ,gunpowder1 as a mixture of sulphur1 nitre1 and carbon1 on being exploded1 forms gases from which the original substances cannot be re-formed at any temperature4 ,in order to obtain them1 recourse must be had to an indirect method of combination at the moment of separation4 ,if ,a does not under any circumstances combine directly with ,b1 it does not follow that it cannot give a compound ,a ,b4 ,for ,a can often combine with ,c and ,b with ,d1 and if ,c has a great affinity for ,d1 then the reaction of ,a ,c or ,b ,d produces not only ,c ,d1 but also ,a ,b4 ,as on the formation of ,c ,d1 the substances ,a and ,b 7previously in ,a ,c and ,b ,d7 are left in a peculiar state of separation1 it is supposed that their mutual combination occurs because they meet together in this nascent state at the moment of separation 7in statu nascendi74 ,thus chlorine does not directly combine with charcoal1 graphite1 or diamond2 there are1 nevertheless1 compounds of chlorine with carbon1 and many of them are distinguished by their stability4 ,they are obtained in the action of chlorine on hydrocarbons1 as the separation products from the direct action of chlorine on hydrogen4 ,chlorine takes up the hydrogen1 and the carbon liberated at the moment of its separation1 enters into combination with another portion of the chlorine1 so that in the end the chlorine is combined with both the hydrogen and the carbon4#ce #ce ,it is possible to imagine that the cause of a great many of such reactions is1 that substances taken in a separate state1 for instance1 charcoal1 present a complex molecule composed of separate atoms of carbon which are fastened together 7united1 as is usually said7 by a considerable affinity2 for atoms of the same kind1 just like atoms of different kinds1 possess a mutual affinity4 ,the affinity of chlorine for carbon1 although unable to break this bond asunder1 may be sufficient to form a stable compound with atoms of carbon1 which are already separate4 ,such a view of the subject presents a hypothesis which1 although dominant at the present time1 is without sufficiently firm foundation4 ,it is evident1 however1 that not only does chemical reaction itself consist of motions1 but that in the compound formed 7in the molecules7 the elements 7atoms7 forming it are in harmonious stable motion 7like the planets in the solar system71 and this motion will affect the stability and capacity for reaction1 and therefore the mechanical side of chemical action must be exceedingly complex4 ,just as there are solid1 physically constant non-volatile substances like rock1 gold1 charcoal1 c41 so are there stable and chemically constant bodies2 while corresponding to physically volatile substances there are bodies like camphor1 which are chemically unstable and variable4 ,as regards those phenomena which accompany chemical action1 the most important circumstance in reference to chemical mechanics is that not only do chemical processes produce a mechanical displacement 7a motion of particles71 heat1 light1 electrical potential and current2 but that all these agents are themselves capable of changing and governing chemical transformations4 ,this reciprocity or reversibility naturally depends on the fact that all the phenomena of nature are only different kinds and forms of visible and invisible 7molecular7 motions4 ,first sound1 and then light1 was shown to consist of vibratory motions1 as the laws of physics have proved and developed beyond a doubt4 ,the connection between heat and mechanical motion and work has ceased to be a supposition1 but has become an accepted fact1 and the mechanical equivalent of heat 7#dbe kilogrammetres of mechanical work correspond with one kilogram unit of heat or ,calorie7 gives a mechanical measure for thermal phenomena4 ,although the mechanical theory of electrical phenomena cannot be considered so fully developed as the theory of heat1 both statical and dynamical electricity are produced by mechanical means 7in common electrical machines or in ,gramme or other dynamos71 and conversely1 a current 7in electric motors7 can produce mechanical motion4 ,thus by connecting a current with the poles of a ,gramme dynamo it may be made to revolve1 and1 conversely1 by rotating it an electrical current is produced1 which demonstrates the reversibility of electricity into mechanical motion4 ,accordingly chemical mechanics must look for the fundamental lines of its advancement in the correlation of chemical with physical and mechanical phenomena4 ,but this subject1 owing to its complexity and comparative novelty1 has not yet been expressed by a harmonious theory1 or even by a satisfactory hypothesis1 and therefore we shall avoid lingering over it4 ,a chemical change in a certain direction is accomplished not only by reason of the difference of masses1 the composition of the substances concerned1 the distribution of their parts1 and their affinity or chemical energy1 but also by reason of the conditions under which the substances occur4 ,in order that a certain chemical reaction may take place between substances which are capable of reacting on each other1 it is often necessary to have recourse to conditions which are sometimes very different from those in which the substances usually occur in nature4 ,for example1 not only is the presence of air 7oxygen7 necessary for the combustion of charcoal1 but the latter must also be heated to redness4 ,the red-hot portion of the charcoal burns--i4e4 combines with the oxygen of the atmosphere--and in so doing evolves heat1 which raises the temperature of the adjacent parts of charcoal1 so that they burn4 ,just as the combustion of charcoal is dependent on its being heated to redness1 so also every chemical reaction only takes place under certain physical1 mechanical1 or other conditions4 ,the following are the chief conditions which exert an influence on the progress of chemical reactions4 7a7 ,temperature4--,chemical reactions of combination only take place within certain definite limits of temperature1 and cannot be accomplished outside these limits4 ,we may cite as examples not only that the combustion of charcoal begins at a red heat1 but also that chlorine and salt only combine with water at a temperature below #j4 ,these compounds cannot be formed at a higher temperature1 for they are then wholly or partially broken up into their component parts4 ,a certain rise in temperature is necessary to start combustion4 ,in certain cases the effect of this rise may be explained as causing one of the reacting bodies to change from a solid into a liquid or gaseous form4 ,the transference into a fluid form facilitates the progress of the reaction1 because it aids the intimate contact of the particles reacting on each other4 ,another reason1 and to this must be ascribed the chief influence of heat in exciting chemical action1 is that the physical cohesion1 or the internal chemical union1 of homogeneous particles is thereby weakened1 and in this way the separation of the particles of the original substances1 and their transference into new compounds1 is rendered easier4 ,when a reaction absorbs heat--as in decomposition--the reason why heat is necessary is self-evident4 ,at the present day it may be asserted upon the basis of existing data1 respecting the action of high temperature1 that all compound bodies are decomposed at a more or less high temperature4 ,we have already seen examples of this in describing the decomposition of mercury oxide into mercury and oxygen1 and the decomposition of wood under the influence of heat4 ,many substances are decomposed at a very moderate temperature2 for instance1 the fulminating salt which is employed in cartridges is decomposed at a little above #abj4 ,the majority of those compounds which make up the mass of animal and vegetable tissues are decomposed at #bjj4 ,on the other hand1 there is reason to think that at a very low temperature no reaction whatever can take place4 ,thus plants cease to carry on their chemical processes during the winter4 ,raoul ,pictet 7#ahib71 employing the very low temperatures 7as low as -#bjj;,c47 obtained by the evaporation of liquefied gases 7see ,chap4 ,,ii471 has recently again proved that at temperatures below -#abj1 even such reactions as those between sulphuric acid and caustic soda or metallic sodium do not take place1 and even the coloration of litmus by acids only commences at temperatures above -#hj4 ,if a given reaction does not take place at a certain low temperature1 it will at first only proceed slowly with a rise of temperature 7even if aided by an electric discharge71 and will only proceed rapidly1 with the evolution of heat1 when a certain definite temperature has been reached4 ,every chemical reaction requires certain limits of temperature for its accomplishment1 and1 doubtless1 many of the chemical changes observed by us cannot take place in the sun1 where the temperature is very high1 or on the moon1 where it is very low4 ,the influence of heat on reversible reactions is particularly instructive4 ,if1 for instance1 a compound which is capable of being reproduced from its products of decomposition be heated up to the temperature at which decomposition begins1 the decomposition of a mass of the substance contained in a definite volume is not immediately completed4 ,only a certain fraction of the substance is decomposed1 the other portion remaining unchanged1 and if the temperature be raised1 the quantity of the substance decomposed increases2 furthermore1 for a given volume1 the ratio between the part decomposed and the part unaltered corresponds with each definite rise in temperature until it reaches that at which the compound is entirely decomposed4 ,this partial decomposition under the influence of heat is called dissociation4 ,it is possible to distinguish between the temperatures at which dissociation begins and ends4 ,should dissociation proceed at a certain temperature1 yet should the product or products of decomposition not remain in contact with the still undecomposed portion of the compound1 then decomposition will go on to the end4 ,thus limestone is decomposed in a limekiln into lime and carbonic anhydride1 because the latter is carried off by the draught of the furnace4 ,but if a certain mass of limestone be enclosed in a definite volume--for instance1 in a gun barrel--which is then sealed up1 and heated to redness1 then1 as the carbonic anhydride cannot escape1 a certain proportion only of the limestone will be decomposed for every increment of heat 7rise in temperature7 higher than that at which dissociation begins4 ,decomposition will cease when the carbonic anhydride evolved presents a maximum dissociation pressure corresponding with each rise in temperature4 ,if the pressure be increased by increasing the quantity of gas1 then combination begins afresh2 if the pressure be diminished decomposition will recommence4 ,decomposition in this case is exactly similar to evaporation2 if the steam given off by evaporation cannot escape1 its pressure will reach a maximum corresponding with the given temperature1 and then evaporation will cease4 ,should steam be added it will be condensed in the liquid2 if its quantity be diminished--i4e4 if the pressure be lessened1 the temperature being constant--then evaporation will go on4 ,we shall afterwards discuss more fully these phenomena of dissociation1 which were first discovered by ,henri ,st4 ,claire ,deville4 ,we will only remark that the products of decomposition re-combine with greater facility the nearer their temperature is to that at which dissociation begins1 or1 in other words1 that the initial temperature of dissociation is near to the initial temperature of combination4 7b7 ,the influence of an electric current1 and of electricity in general1 on the progress of chemical transformations is very similar to the influence of heat4 ,the majority of compounds which conduct electricity are decomposed by the action of a galvanic current1 and as there is great similarity in the conditions under which decomposition and combination proceed1 combination often proceeds under the influence of electricity4 ,electricity1 like heat1 must be regarded as a peculiar form of molecular motion1 and all that refers to the influence of heat also refers to the phenomena produced by the action of an electrical current1 with this difference1 only that a substance can be separated into its component parts with much greater ease by electricity1 since the process goes on at the ordinary temperature4 ,the most stable compounds may be decomposed by this means1 and a most important fact is then observed--namely1 that the component parts appear at the different poles of electrodes by which the current passes through the substance4 ,those substances which appear at the positive pole 7anode7 are called 'electro-negative1' and those which appear at the negative pole 7cathode1 that in connection with the zinc of an ordinary galvanic battery7 are called 'electro-positive4' ,the majority of non-metallic elements1 such as chlorine1 oxygen1 c41 and also acids and substances analogous to them1 belong to the first group1 whilst the metals1 hydrogen1 and analogous products of decomposition appear at the negative pole4 ,chemistry is indebted to the decomposition of compounds by the electric current for many most important discoveries4 ,many elements have been discovered by this method1 the most important being potassium and sodium4 ,lavoisier and the chemists of his time were not able to decompose the oxygen compounds of these metals1 but ,davy showed that they might be decomposed by an electric current1 the metals sodium and potassium appearing at the negative pole4 ,now that the dynamo gives the possibility of producing an electric current by the combustion of fuel1 this method of ,sir ,h4 ,davy is advantageously employed for obtaining metals1 c4 on a large scale1 for instance1 sodium from fused caustic soda or chlorine from solutions of salt4 7c7 ,certain unstable compounds are also decomposed by the action of light4 ,photography is based on this property in certain substances 7for instance1 in the salts of silver74 ,the mechanical energy of those vibrations which determine the phenomena of light is very small1 and therefore only certain1 and these generally unstable1 compounds can be decomposed by light--at least under ordinary circumstances4 ,but there is one class of chemical phenomena dependent on the action of light which forms as yet an unsolved problem in chemistry--these are the processes accomplished in plants under the influence of light4 ,here there take place most unexpected decompositions and combinations1 which are often unattainable by artificial means4 ,for instance1 carbonic anhydride1 which is so stable under the influence of heat and electricity1 is decomposed and evolves oxygen in plants under the influence of light4 ,in other cases1 light decomposes unstable compounds1 such as are usually easily decomposed by heat and other agents4 ,chlorine combines with hydrogen under the influence of light1 which shows that combination1 as well as decomposition1 can be determined by its action1 as was likewise the case with heat and electricity4 7d7 ,mechanical causes exert1 like the foregoing agents1 an action both on the process of chemical combination and of decomposition4 ,many substances are decomposed by friction or by a blow--as1 for example1 the compound called iodide of nitrogen 7which is composed of iodine1 nitrogen1 and hydrogen71 and silver fulminate4 ,mechanical friction causes sulphur to burn at the expense of the oxygen contained in potassium chlorate4 ,pressure affects both the physical and chemical state of the reacting substances1 and1 together with the temperature1 determines the state of a substance4 ,this is particularly evident when the substance occurs in an elastic-gaseous form since the volume1 and hence also the number of points of encounter between the reacting substances is greatly altered by a change of pressure4 ,thus1 under equal conditions of temperature1 hydrogen when compressed acts more powerfully upon iodine and on the solutions of many salts4 7e7 ,besides the various conditions which have been enumerated above1 the progress of chemical reactions is accelerated or retarded by the condition of contact in which the reacting bodies occur4 ,other conditions remaining constant1 the rate of progress of a chemical reaction is accelerated by increasing the number of points of contact4 ,it will be enough to point out the fact that sulphuric acid does not absorb ethylene under ordinary conditions of contact1 but only after continued shaking1 by which means the number of points of contact is greatly increased4 ,to ensure complete action between solids1 it is necessary to reduce them to very fine powder and to mix them as thoroughly as possible4 ,m4 ,spring1 the ,belgian chemist1 has shown that finely powdered solids which do not react on each other at the ordinary temperature may do so under an increased pressure4 ,thus1 under a pressure of #f1jjj atmospheres1 sulphur combines with many metals at the ordinary temperature1 and mixtures of the powders of many metals form alloys4 ,it is evident that an increase in the number of points or surfaces must be regarded as the chief cause producing reaction1 which is doubtless accomplished in solids1 as in liquids and gases1 in virtue of an internal motion of the particles1 which motion1 although in different degrees and forms1 must exist in all the states of matter4 ,it is very important to direct attention to the fact that the internal motion or condition of the parts of the particles of matter must be different on the surface of a substance from what it is inside2 because in the interior of a substance similar particles are acting on all sides of every particle1 whilst at the surface they act on one side only4 ,therefore1 the condition of a substance at its surfaces of contact with other substances must be more or less modified by them--it may be in a manner similar to that caused by an elevation of temperature4 ,these considerations throw some light on the action in the large class of contact reactions2 that is1 such as appear to proceed from the mere presence 7contact7 of certain special substances4 ,porous or powdery substances are very prone to act in this way1 especially spongy platinum and charcoal4 ,for example1 sulphurous anhydride does not combine directly with oxygen1 but this reaction takes place in the presence of spongy platinum4#cf #cf ,contact phenomena are separately considered in detail in the work of ,professor ,konovaloff 7#ahhd74 ,in my opinion1 it must be held that the state of the internal motions of the atoms in molecules is modified at the points of contact of substances1 and this state determines chemical reactions1 and therefore1 that reactions of combination1 decomposition1 and rearrangement are accomplished by contact4 ,professor ,konovaloff showed that a number of substances1 under certain conditions of their surfaces1 act by contact2 for instance1 finely divided silica 7from the hydrate7 acts just like platinum1 decomposing certain compound ethers4 ,as reactions are only accomplished under close contact1 it is probable that those modifications in the distribution of the atoms in molecules which come about by contact phenomena prepare the way for them4 ,by this the rle of contact phenomena is considerably extended4 ,such phenomena should explain the fact why a mixture of hydrogen and oxygen yields water 7explodes7 at different temperatures1 according to the kind of heated substance which transmits this temperature4 ,in chemical mechanics1 phenomena of this kind have great importance1 but as yet they have been but little studied4 ,it must not be forgotten that contact is a necessary condition for every chemical reaction4 ,the above general and introductory chemical conceptions cannot be thoroughly grasped in their true sense without a knowledge of the particular facts of chemistry to which we shall now turn our attention4 ,it was1 however1 absolutely necessary to become acquainted on the very threshold with such fundamental principles as the laws of the indestructibility of matter and of the conservation of energy1 since it is only by their acceptance1 and under their direction and influence1 that the examination of particular facts can give practical and fruitful results4 ,,chapter ,i ,,on ,,water ,,and ,,its ,,compounds ,water is found almost everywhere in nature1 and in all three physical states4 ,as vapour1 water occurs in the atmosphere1 and in this form it is distributed over the entire surface of the earth4 ,the vapour of water in condensing1 by cooling1 forms snow1 rain1 hail1 dew1 and fog4 ,one cubic metre 7or #a1jjj1jjj cubic centimetres1 or #a1jjj litres1 or #cecaf cubic feet7 of air can contain at #j only #dh grams of water1 at #bj about #agj grams1 at #dj about #ejg grams2 but ordinary air only contains about #fj per cent4 of this maximum4 ,air containing less than #dj per cent4 is felt to be dry1 whilst air which contains more than #hj per cent4 of the same maximum is considered as distinctly damp4#a ,water in the liquid state1 in falling as rain and snow1 soaks into the soil and collects together into springs1 lakes1 rivers1 seas1 and oceans4 ,it is absorbed from the soil by the roots of plants1 which1 when fresh1 contain from #dj to #hj per cent4 of water by weight4 ,animals contain about the same amount of water4 ,in a solid state1 water appears as snow1 ice1 or in an intermediate form between these two1 which is seen on mountains covered with perpetual snow4 ,the water of rivers1#b springs1 oceans and seas1 lakes1 and wells contains various substances in solution mostly salt1--that is1 substances resembling common table salt in their physical properties and chief chemical transformations4 ,further1 the quantity and nature of these salts differ in different waters4#c ,everybody knows that there are salt1 fresh1 iron1 and other waters4 ,the presence of about #c-a_/b per cent4 of salts renders sea-water#d bitter to the taste and increases its specific gravity4 ,fresh water also contains salts1 but only in a comparatively small quantity4 ,their presence may be easily proved by simply evaporating water in a vessel4 ,on evaporation the water passes away as vapour1 whilst the salts are left behind4 ,this is why a crust 7incrustation71 consisting of salts1 previously in solution1 is deposited on the insides of kettles or boilers1 and other vessels in which water is boiled4 ,running water 7rivers1 c47 is charged with salts1 owing to its being formed from the collection of rain water percolating through the soil4 ,while percolating1 the water dissolves certain parts of the soil4 ,thus water which filters or passes through saline or calcareous soils becomes charged with salts or contains calcium carbonate 7chalk74 ,rain water and snow are much purer than river or spring water4 ,nevertheless1 in passing through the atmosphere1 rain and snow succeed in catching the dust held in it1 and dissolve air1 which is found in every water4 ,the dissolved gases of the atmosphere are partly disengaged1 as bubbles from water on heating1 and water after long boiling is quite freed from them4 #a ,in practice1 the chemist has to continually deal with gases1 and gases are often collected over water2 in which case a certain amount of water passes into vapour1 and this vapour mixes with the gases4 ,it is therefore most important that he should be able to calculate the amount of water or of moisture in air and other gases4 ,let us imagine a cylinder standing in a mercury bath1 and filled with a dry gas whose volume equals v1 temperature t1 and pressure or tension h mm4 7h millimetres of the column of mercury at #j74 ,we will introduce water into the cylinder in such a quantity that a small part remains in the liquid state1 and consequently that the gas will be saturated with aqueous vapour2 the volume of the gas will then increase 7if a larger quantity of water be taken some of the gas will he dissolved in it1 and the volume may therefore he diminished74 ,we will further suppose that1 after the addition of the water1 the temperature remains constant2 then since the volume increases1 the mercury in the cylinder falls1 and therefore the pressure as well as the volume is increased4 ,in order to investigate the phenomenon we will artificially increase the pressure1 and reduce the volume to the original volume v4 ,then the pressure or tension will be greater than h1 namely h f1 which means that by the introduction of aqueous vapour the pressure of the gas is increased4 ,the researches of ,dalton1 ,gay-,lussac1 and ,regnault showed that this increase is equal to the maximum pressure which is proper to the aqueous vapour at the temperature at which the observation is made4 ,the maximum pressure for all temperatures may be found in the tables made from observations on the pressure of aqueous vapour4 ,the quantity f will be equal to this maximum pressure of aqueous vapour4 ,this may be expressed thus3 the maximum tension of aqueous vapour 7and of all other vapours7 saturating a space in a vacuum or in any gas is the same4 ,this rule is known as ,dalton's law4 ,thus we have a volume of dry gas v1 under a pressure h1 and a volume of moist gas1 saturated with vapour1 under a pressure h f4 ,the volume v of the dry gas under a pressure h f occupies1 from ,boyle's law1 a volume vh_/h f2 consequently the volume occupied by the aqueous vapour under the pressure h f equals v-vh_/7h f71 or vf_/7h f74 ,thus the volumes of the dry gas and of the moisture which occurs in it1 at a pressure h f1 are in the ratio f 3 h4 ,and1 therefore1 if the aqueous vapour saturates a space at a pressure n1 the volumes of the dry air and of the moisture which is contained in it are in the ratio 7n-f7 3 f1 where f is the pressure of the vapour according to the tables of vapour tension4 ,thus1 if a volume ,n of a gas saturated with moisture be measured at a pressure ,h1 then the volume of the gas1 when dry1 will be equal to ,n7,h-f7_/,h4 ,in fact1 the entire volume ,n must be to the volume of dry gas x as ,h is to ,h-f2 therefore1 ,n 3 x ,h 3 ,h-f1 from which x ,n7,h-f7_/,h4 ,under any other pressure--for instance1 #gfj mm4--,the volume of dry gas will be x,h_/#gfj1 or 7,h-f7_/#gfj1 and we thus obtain the following practical rule3 ,if a volume of a gas saturated with aqueous vapour be measured at a pressure ,h mm41 then the volume of dry gas contained in it will be obtained by finding the volume corresponding to the pressure ,h1 less the pressure due to the aqueous vapour at the temperature observed4 ,for example1 #cge cubic centimetres of air saturated with aqueous vapour were measured at a temperature of #aec1 and under a pressure of #gdgc mm4 of mercury 7at #j74 ,what will be the volume of dry gas at #j and #gfj mm48 ,the pressure of aqueous vapour corresponding to #aec is equal to #abi mm41 and therefore the volume of dry gas at #aec and #gdgc mm4 is equal to #cge 7#gdgc-abi7_/gdgc2 at #gfj mm4 it will be equal to #cge 7#gcdd_/gfj72 and at #j the volume of dry gas will be #cge 7#gcdd_/gfj7 #bgc_/7bgc #aec7 #cdca c4c4 ,from this rule may also be calculated what fraction of a volume of gas is occupied by moisture under the ordinary pressure at different temperatures2 for instance1 at #cj ,c4 f #cae1 consequently #ajj volumes of a moist gas or air1 at #gfj mm41 contain a volume of aqueous vapour #ajj 7#cae_/gfj71 or #daaj2 it is also found that at #j there is contained #jfa p4c4 by volume1 at #aj #aba p4c41 at #bj #bbi p4c41 and at #ej up to #abaa p4c4 ,from this it may be judged how great an error might be made in the measurement of gases by volume if the moisture were not taken into consideration4 ,from this it is also evident how great are the variations in volume of the atmosphere when it loses or gains aqueous vapour1 which again explains a number of atmospheric phenomena 7winds1 variation of pressure1 rainfalls1 storms1 c47 ,if a gas is not saturated1 then it is indispensable that the degree of moisture should be known in order to determine the volume of dry gas from the volume of moist gas4 ,the preceding ratio gives the maximum quantity of water which can be held in a gas1 and the degree of moisture shows what fraction of this maximum quantity occurs in a given case1 when the vapour does not saturate the space occupied by the gas4 ,consequently1 if the degree of moisture equals #ej p4c4--that is1 half the maximum--then the volume of dry gas at #gfj mm4 is equal to the volume of dry gas at #gfj mm4 multiplied by 7h-#je;f7_/#gfj1 or1 in general1 by 7h-rf7_/#gfj where r is the degree of moisture4 ,thus1 if it is required to measure the volume of a moist gas1 it must either be thoroughly dried or quite saturated with moisture1 or else the degree of moisture determined4 ,the first and last methods are inconvenient1 and therefore recourse is usually had to the second4 ,for this purpose water is introduced into the cylinder holding the gas to be measured2 it is left for a certain time so that the gas may become saturated1 the precaution being taken that a portion of the water remains in a liquid state2 then the volume of the moist gas is determined1 from which that of the dry gas may be calculated4 ,in order to find the weight of the aqueous vapour in a gas it is necessary to know the weight of a cubic measure at #j and #gfj mm4 ,knowing that one cubic centimetre of air in these circumstances weighs #jjjabic gram1 and that the density of aqueous vapour is #jfb1 we find that one cubic centimetre of aqueous vapour at #j and #gfj mm4 weighs #jjjjh gram1 and at a temperature t and pressure h the weight of one cubic centimetre will be #jjjjh h_/#gfj #bgc_/7bgc t74 ,we already know that v volumes of a gas at a temperature t pressure h contain v f_/h volumes of aqueous vapour which saturate it1 therefore the weight of the aqueous vapour held in v volumes of a gas will be v x #jjjjh f_/#gfj #bgc_/7bgc t74 ,accordingly1 the weight of water which is contained in one volume of a gas depends only on the temperature and not on the pressure4 ,this also signifies that evaporation proceeds to the same extent in air as in a vacuum1 or1 in general terms 7this is ,dalton's law71 vapours and gases diffuse into each other as if into a vacuum4 ,in a given space1 at a given temperature1 a constant quantity of vapour enters1 whatever be the pressure of the gas filling that space4 ,from this it is clear that if the weight of the vapour contained in a given volume of a gas be known1 it is easy to determine the degree of moisture r p_/7v #jjjjh7 #gfj_/t 7#bgc t7_/#bgc4 ,on the is founded the very exact determination of the degree of moisture of air by the weight of water contained in a given volume4 ,it is easy to calculate from the preceding formula the number of grams of water contained at any pressure in one cubic metre or million cubic centimetres of air saturated with vapour at various temperatures2 for instance1 at #cj f #cae1 hence p #bihd grams4 ,the laws of ,mariotte1 ,dalton1 and ,gay-,lussac1 which are here applied to gases and vapours1 are not entirely exact1 but are approximately true4 ,if they were quite exact1 a mixture of several liquids1 having a certain vapour pressure1 would give vapours of a very high pressure1 which is not the case4 ,in fact the pressure of aqueous vapour is slightly less in a gas than in a vacuum1 and the weight of aqueous vapour held in a gas is slightly less than it should be according to ,dalton's law1 as was shown by the experiments of ,regnault and others4 ,this means that the tension of the vapour is less in air than in a vacuum4 ,the difference does not1 however1 exceed #e per cent4 of the total pressure of the vapours4 ,this decrement in vapour tension which occurs in the intermixture of vapours and gases1 although small1 indicates that there is then already1 so to speak1 a beginning of chemical change4 ,the essence of the matter is that in this case there occurs1 as on contact 7see preceding footnote71 an alteration in the motions of the atoms in the molecules1 and therefore also a change in the motion of the molecules themselves4 ,in the uniform intermixture of air and other gases with aqueous vapour1 and in the capacity of water to pass into vapour and form a uniform mixture with air1 we may perceive an instance of a physical phenomenon which is analogous to chemical phenomena1 forming indeed a transition from one class of phenomena to the other4 ,between water and dry air there exists a kind of affinity which obliges the water to saturate the air4 ,but such a homogeneous mixture is formed 7almost7 independently of the nature of the gas in which evaporation takes place2 even in a vacuum the phenomenon occurs in exactly the same way as in a gas1 and therefore it is not the property of the gas1 nor its relation to water1 but the property of the water itself1 which compels it to evaporate1 and therefore in this case chemical affinity is not yet operative--at least its action is not clearly pronounced4 ,that it does1 however1 play a certain part is seen from the deviation from ,dalton's law4 #b ,in falling through the atmosphere1 water dissolves the gases of the atmosphere1 nitric acid1 ammonia1 organic compounds1 salts of sodium1 magnesium1 and calcium1 and mechanically washes out a mixture of dust and microbes which are suspended in the atmosphere4 ,the amount of these and certain other constituents is very variable4 ,even in the beginning and end of the same rainfall a variation which is often very considerable may be remarked4 ,thus1 for example1 ,bunsen found that rain collected at the beginning of a shower contained #cg grams of ammonia per cubic metre1 whilst that collected at the end of the same shower contained only ,o#fd gram4 ,the water of the entire shower contained an average of #adg gram of ammonia per cubic metre4 ,in the course of a year rain supplies an acre of ground with as much as #e-a_/b kilos of nitrogen in a combined form4 ,marchand found in one cubic metre of snow water #aefc1 and in one cubic metre of rain water #ajjg1 grams of sodium sulphate4 ,angus ,smith showed that after a thirty hours' fall at ,manchester the rain still contained #cdc grams of salts per cubic metre4 ,a considerable amount of organic matter1 namely #be grams per cubic metre1 has been found in rain water4 ,the total amount of solid matter in rain water reaches #ej grams per cubic metre4 ,rain water generally contains very little carbonic acid1 whilst river water contains a considerable quantity of it4 ,in considering the nourishment of plants it is necessary to keep in view the substances which are carried into the soil by rain4 ,river water1 which is accumulated from springs and sources fed by atmospheric water1 contains from #ej to #a1fjj parts by weight of salts in #a1jjj1jjj parts4 ,the amount of solid matter1 per #a1jjj1jjj parts by weight1 contained in the chief rivers is as follows3--the ,don #abd1 the ,loire #ace1 the ,st4 ,lawrence #agj1 the ,rhone #ahb1 the ,dnieper #ahg1 the ,danube from #aag to #bcd1 the ,rhine from #aeh to #cag1 the ,seine from #aij to #dcb1 the ,thames at ,london from #djj to #dej1 in its upper parts #chg1 and in its lower parts up to #a1fag1 the ,nile #a1ehj1 the ,jordan #a1jeb4 ,the ,neva is characterised by the remarkably small amount of solid matter it contains4 ,from the investigations of ,prof4 ,g4 ,k4 ,trapp1 a cubic metre of ,neva water contains #cb grams of incombustible and #bc grams of organic matter1 or altogether about #ee grams4 ,this is one of the purest waters which is known in rivers4 ,the large amount of impurities in river water1 and especially of organic impurity produced by pollution with putrid matter1 makes the water of many rivers unfit for use4 ,the chief part of the soluble substances in river water consists of the calcium salts4 #ajj parts of the solid residues contain the following amounts of calcium carbonate--from the water of the ,loire #ec1 from the ,thames about #ej1 the ,elbe #ee1 the ,vistula #fe1 the ,danube #fe1 the ,rhine from #ee to #ge1 the ,seine #ge1 the ,rhone from #hb to #id4 ,the ,neva contains #dj parts of calcium carbonate per #ajj parts of saline matter4 ,the considerable amount of calcium carbonate which river water contains is very easily explained from the fact that water which contains carbonic acid in solution easily dissolves calcium carbonate1 which occurs all over the earth4 ,besides calcium carbonate and sulphate1 river water contains magnesium1 silica1 chlorine1 sodium1 potassium1 aluminium1 nitric acid1 iron and manganese4 ,the presence of salts of phosphoric acid has not yet been determined with exactitude for all rivers1 but the presence of nitrates has been proved with certainty in almost all kinds of well-investigated river water4 ,the quantity of calcium phosphate does not exceed #jd gram in the water of the ,dnieper1 and the ,don does not contain more than #e grams4 ,the water of the ,seine contains about #ae grams of nitrates1 and that of the ,rhone about #h grams4 ,the amount of ammonia is much less2 thus in the water of the ,rhine about #je gram in ,june1 and #jb gram in ,october2 the water of the ,seine contains the same amount4 ,this is less than in rain water4 ,notwithstanding this insignificant quantity1 the water of the ,rhine alone1 which is not so very large a river1 carries #af1bde kilograms of ammonia into the ocean every day4 ,the difference between the amount of ammonia in rain and river water depends on the fact that the soil through which the rain water passes is able to retain the ammonia4 7,soil can also absorb many other substances1 such as phosphoric acid1 potassium salts1 c47 ,the waters of springs1 rivers1 wells1 and in general of those localities from which it is taken for drinking purposes1 may be injurious to health if it contains much organic pollution1 the more so as in such water the lower organisms 7bacteria7 may rapidly develop1 and these organisms often serve as the carriers or causes of infectious diseases4 ,for instance1 certain pathogenic 7disease-producing7 bacteria are known to produce typhoid1 the ,siberian plague1 and cholera4 ,thanks to the work of ,pasteur1 ,metchnikoff1 ,koch1 and many others1 this province of research has made considerable progress4 ,it is possible to investigate the number and properties of the germs in water4 ,in bacteriological researches a gelatinous medium in which the germs can develop and multiply is prepared with gelatin and water1 which has previously been heated several times1 at intervals1 to #ajj 7it is thus rendered sterile--that is to say1 all the germs in it are killed74 ,the water to be investigated is added to this prepared medium in a definite and small quantity 7sometimes diluted with sterilised water to facilitate the calculation of the number of germs71 it is protected from dust 7which contains germs71 and is left at rest until whole families of lower organisms are developed from each germ4 ,these families 7colonies7 are visible to the naked eye 7as spots71 they may be counted1 and by examining them under the microscope and observing the number of organisms they produce1 their significance may be determined4 ,the majority of bacteria are harmless1 but there are decidedly pathogenic bacteria1 whose presence is one of the causes of malady and of the spread of certain diseases4 ,the number of bacteria in one cubic centimetre of water sometimes attains the immense figures of hundreds of thousands and millions4 ,certain well1 spring1 and river waters contain very few bacteria1 and are free from disease-producing bacteria under ordinary circumstances4 ,by boiling water1 the bacteria in it are killed1 but the organic matter necessary for their nourishment remains in the water4 ,the best kinds of water for drinking purposes do not contain more than #cjj bacteria in a cubic centimetre4 ,the amount of gases dissolved in river water is much more constant than that of its solid constituents4 ,one litre1 or #a1jjj c4c41 of water contains #dj to #ee c4c4 of gas measured at normal temperature and pressure4 ,in winter the amount of gas is greater than in summer or autumn4 ,assuming that a litre contains #ej c4c4 of gases1 it may be admitted that these consist1 on an average1 of #bj vols4 of nitrogen1 #bj vols of carbonic anhydride 7proceeding in all likelihood from the soil and not from the atmosphere71 and of #aj vols4 of oxygen4 ,if the total amount of gases be less1 the constituent gases are still in about the same proportion2 in many cases1 however1 carbonic anhydride predominates4 ,the water of many deep and rapid rivers contains less carbonic anhydride1 which shows their rapid formation from atmospheric water1 and that they have not succeeded1 during a long and slow course1 in absorbing a greater quantity of carbonic anhydride4 ,thus1 for instance1 the water of the ,rhine1 near ,strasburg1 according to ,deville1 contains #h c4c4 of carbonic anhydride1 #af c4c4 of nitrogen1 and #g c4c4 of oxygen per litre4 ,from the researches of ,prof4 ,m4 ,r4 ,kapoustin and his pupils1 it appears that in determining the quality of a water for drinking purposes1 it is most important to investigate the composition of the dissolved gases1 more especially oxygen4 #c ,spring water is formed from rain water percolating through the soil4 ,naturally a part of the rain water is evaporated directly from the surface of the earth and from the vegetation on it4 ,it has been shown that out of #ajj parts of water falling on the earth only #cf parts flow to the ocean2 the remaining #fd are evaporated1 or percolate far underground4 ,after flowing underground along some impervious strata1 water comes out at the surface in many places as springs1 whose temperature is determined by the depth from which the water has flowed4 ,springs penetrating to a great depth may become considerably heated1 and this is why hot mineral springs1 with a temperature of up to #cj and higher1 are often met with4 ,when a spring water contains substances which endow it with a peculiar taste1 and especially if these substances are such as are only found in minute quantities in river and other flowing waters1 then the spring water is termed a mineral water4 ,many such waters are employed for medicinal purposes4 ,mineral waters are classed according to their composition into--7a7 saline waters1 which often contain a large amount of common salt2 7b7 alkaline waters1 which contain sodium carbonate2 7c7 bitter waters1 which contain magnesia2 7d7 chalybeate waters1 which hold iron carbonate in solution2 7e7 arated waters1 which are rich in carbonic anhydride2 7f7 sulphuretted waters1 which contain hydrogen sulphide4 ,sulphuretted waters may be recognised by their smell of rotten eggs1 and by their giving a black precipitate with lead salts1 and also by their tarnishing silver objects4 ,arated waters1 which contain an excess of carbonic anhydride1 effervesce in the air1 have a sharp taste1 and redden litmus paper4 ,saline waters leave a large residue of soluble solid matter on evaporation1 and have a salt taste4 ,chalybeate waters have an inky taste1 and are coloured black by an infusion of galls2 on being exposed to the air they usually give a brown precipitate4 ,generally1 the character of mineral waters is mixed4 ,in the table below the analyses are given of certain mineral springs which are valued for their medicinal properties4 ,the quantity of the substances is expressed in millionths by weight4 ,column ,headings3 ,a3 ,calcium salts ,b3 ,sodium chloride ,c3 ,sodium sulphate ,d3 ,sodium carbonate ,e3 ,potassium iodide and bromide ---------------------------------------- ------- ,a ,b ,c ,d ,e ,f ---------------------------------------- ------- ,i4 #a1ibh -- #aeb -- -- #bd ,,ii4 #haf #chf #a1bci #bf -- #dc ,,iii4 #a1jhe #a1dcj #a1aje -- #d #ij ,,iv4 #cdc #c1ghc #af #c1dca -- #ad ,v4 #c1djf #ae1jdi -- -- #b -- ,,vi4 #ceb #c1ade -- #ie #ce #ej ,,vii4 #cjh #a1jcf #b1ehc #a1bfa #d #agh ,,viii4 #a1gbf #i1dhj -- -- #dj #abj ,,ix4 #eea #b1jdj #a1aej #iii -- #a ,x4 #bhe #eeh #bgi #c1hac -- -- ,,xi4 #cdj #iaj ,iron and aluminium #a1jbj sulphates3 #a1ffj ---------------------------------------- ---------- ,column ,headings3 ,g3 ,iron carbonate ,h3 ,magnesium salts ,i3 ,silica ,j3 ,carbonic anhydride ,k3 ,sulphuretted hydrogen ,l3 ,total solid contents ---------------------------------------- -------- ,g ,h ,i ,j ,k ,l ---------------------------------------- -------- ,i4 -- #ddh #aeb #a1cjj #hj #b1fji ,,ii4 #i #beg #df #a1dhe -- #b1hab ,,iii4 -- #ahg #fe #a1cbf #aa #c1iej ,,iv4 -- #bea #aab #b1hhc -- #g1iej ,v4 #ag #a1ehg #bbi -- #gf #bj1bij ,,vi4 #a #bfj #aa #bj -- #c1igj ,,vii4 #d #agh #ge -- -- #e1dea ,,viii4 #bf #bjh #dj -- -- #aa1gij ,,ix4 #cj #bji #ej #b1gdj -- #d1jgj ,x4 #g #de #de #b1bfh -- #e1jca ,sulphuric ,,xi4 #idj #aij #b1eej and hydrochloric #ccj acids ---------------------------------------- ---------- ,i4 ,sergieffsky1 a sulphur water1 ,gov4 of ,samara 7temp4 #h ,c471 analysis by ,clause4 ,,ii4 ,gelznovodskya water source ,no4 #aj1 near ,patigorsk1 ,caucasus 7temp4 #bbe71 analysis by ,fritzsche4 ,,iii4 ,aleksandroffsky1 alkaline-sulphur source1 ,patigorsk 7temp4 #dfe71 average of analyses by ,herman1 ,zinin and ,fritzsche4 ,,iv4 ,bougountouksky1 alkaline source1 ,no4 #ag1 ,essentoukah1 ,caucasus 7temp4 #baf71 analysis by ,fritzsche4 ,v4 ,saline water1 ,staro-,russi1 ,gov4 of ,novgorod1 analysis by ,nelubin4 ,,vi4 ,water from artesian well at the factory of state papers1 ,st4 ,petersburg1 analysis by ,struve4 ,,vii4 ,sprdel1 ,carlsbad 7temp4 #hcg71 analysis by ,berzelius4 ,,viii4 ,kreuznach spring 7,elisenquelle71 ,prussia 7temp4 #hh71 analysis by ,bauer4 ,,ix4 ,eau de ,seltz1 ,nassau1 analysis by ,henry4 ,x4 ,vichy water1 ,france1 analysis by ,berthier and ,puvy4 ,,xi4 ,paramo de ,ruiz1 ,new ,granada1 analysis by ,levy2 it is distinguished by the amount of free acids4 #d ,sea water contains more non-volatile saline constituents than the usual kinds of fresh water4 ,this is explained by the fact that the waters flowing into the sea supply it with salts1 and whilst a large quantity of vapour is given off from the surface of the sea1 the salts remain behind4 ,even the specific gravity of sea water differs considerably from that of pure water4 ,it is generally about #ajb1 but in this and also in respect of the amount of salts contained1 samples of sea water from different localities and from different depths offer rather remarkable variations4 ,it will be sufficient to point out that one cubic metre of water from the undermentioned localities contains the following quantity in grams of solid constituents3--,gulf of ,venice1 #ai1abb2 ,leghorn ,harbour #bd1cab2 ,mediterranean1 near ,cetta1 #cg1ffe2 the ,atlantic ,ocean from #cb1ehe to #ce1fie12 the ,pacific ,ocean from #ce1bcc to #cd1gjh4 ,in closed seas which do not communicate1 or are in very distant communication1 with the ocean1 the difference is often still greater4 ,thus the ,caspian ,sea contains #f1cjj grams2 the ,black ,sea and ,baltic #ag1gjj4 ,common salt forms the chief constituent of the saline matter of sea or ocean water2 thus in one cubic metre of sea water there are #be1jjj-ca1jjj grams of common salt1 #b1fjj-f1jjj grams of magnesium chloride1 #a1bjj-g1jjj grams of magnesium sulphate1 #a1ejj-f1jjj grams of calcium sulphate1 and #aj-gjj grams of potassium chloride4 ,the small amount of organic matter and of the salts of phosphoric acid in sea water is very remarkable4 ,sea water 7the composition of which is partially discussed in ,chapter ,x47 contains1 in addition to salts of common occurrence1 a certain and sometimes minute amount of the most varied elements1 even gold and silver1 and as the mass of water of the oceans is so enormous these 'traces' of rare substances amount to large quantities1 so that it may be hoped that in time methods will be found to extract even gold from sea water1 which by means of the rivers forms a vast reservoir for the numerous products of the changes taking place on the earth's surface4 ,the works of ,english1 ,american1 ,german1 ,russian1 ,swedish1 and other navigators and observers prove that a study of the composition of sea water not only explains much in the history of the earth's life1 but also gives the possibility 7especially since the researches of ,c4 ,o4 ,makaroff of the ,st4 ,petersburg ,academy7 of fixing one's position in the ocean in the absence of other means1 for instance1 in a fog1 or in the dark4 ,in general terms water is called pure when it is clear and free from insoluble particles held in suspension and visible to the naked eye1 from which it may be freed by filtration through charcoal1 sand1 or porous 7natural or artificial7 stones1 and when it possesses a clean fresh taste4 ,it depends on the absence of any taste1 decomposing organic matter1 on the quantity of air#e and atmospheric gases in solution1 and on the presence of mineral substances to the amount of about #cjj grams per ton 7or #ajjj kilograms per cubic metre1 or1 what is the same1 #cjj milligrams to a kilogram or a litre of water71 and of not more than #ajj grams of organic matter4#f ,such water is suitable for drinking and every practical application1 but evidently it is not pure in a chemical sense4 ,a chemically pure water is necessary not only for scientific purposes1 as an independent substance having constant and definite properties1 but also for many practical purposes--for instance1 in photography and in the preparation of medicines--because many properties of substances in solution are changed by the impurities of natural waters4 ,water is usually purified by distillation1 because the solid substances in solution are not transformed into vapours in this process4 ,such distilled water is prepared by chemists and in laboratories by boiling water in closed metallic boilers or stills1 and causing the steam produced to pass into a condenser--that is1 through tubes 7which should be made of tin1 or1 at all events1 tinned1 as water and its impurities do not act on tin7 surrounded by cold water1 and in which the steam1 being cooled1 condenses into water which is collected#g in a receiver4 ,by standing exposed to the atmosphere1 however1 the water in time absorbs air1 and dust carried in the air4 ,nevertheless1 in distillation1 water retains1 besides air1 a certain quantity of volatile impurities 7especially organic7 and the walls of the distillation apparatus are partly corroded by the water1 and a portion1 although small1 of their substance renders the water not entirely pure1 and a residue is left on evaporation4#h #e ,the taste of water is greatly dependent on the quantity of dissolved gases it contains4 ,these gases are given off on boiling1 and it is well known that1 even when cooled1 boiled water has1 until it has absorbed gaseous substances from the atmosphere1 quite a different taste from fresh water containing a considerable amount of gas4 ,the dissolved gases1 especially oxygen and carbonic anhydride1 have an important influence on the health4 ,the following instance is very instructive in this respect4 ,the ,grenelle artesian well at ,paris1 when first opened1 supplied a water which had an injurious effect on men and animals4 ,it appeared that this water did not contain oxygen1 and was in general very poor in gases4 ,as soon as it was made to fall in a cascade1 by which it absorbed air1 it proved quite fit for consumption4 ,in long sea voyages fresh water is sometimes not taken at all1 or only taken in a small quantity1 because it spoils by keeping1 and becomes putrid from the organic matter it contains undergoing decomposition4 ,fresh water may he obtained directly from sea-water by distillation4 ,the distilled water no longer contains sea salts1 and is therefore fit for consumption1 but it is very tasteless and has the properties of boiled water4 ,in order to render it palatable certain salts1 which are usually held in fresh water1 are added to it1 and it is made to flow in thin streams exposed to the air in order that it may become saturated with the component parts of the atmosphere--that is1 absorb gases4 #f ,hard water is such as contains much mineral matter1 and especially a large proportion of calcium salts4 ,such water1 owing to the amount of lime it contains1 does not form a lather with soap1 prevents vegetables boiled in it from softening properly1 and forms a large amount of incrustation on vessels in which it is boiled4 ,when of a high degree of hardness1 it is injurious for drinking purposes1 which is evident from the fact that in several large cities the death-rate has been found to decrease after introducing a soft water in the place of a hard water4 ,putrid water contains a considerable quantity of decomposing organic matter1 chiefly vegetable1 but in populated districts1 especially in towns1 chiefly animal remains4 ,such water acquires an unpleasant smell and taste1 by which stagnant bog water and the water of certain wells in inhabited districts are particularly characterised4 ,water of this kind is especially injurious at a period of epidemic4 ,it may be partially purified by being passed through charcoal1 which retains the putrid and certain organic substances1 and also certain mineral substances4 ,turbid water may be purified to a certain extent by the addition of alum1 which aids1 after standing some time1 the formation of a sediment4 ,condy's fluid 7potassium permanganate7 is another means of purifying putrid water4 ,a solution of this substance1 even if very dilute1 is of a red colour2 on adding it to a putrid water1 the permanganate oxidises and destroys the organic matter4 ,when added to water in such a quantity as to impart to it an almost imperceptible rose colour it destroys much of the organic substances it contains4 ,it is especially salutary to add a small quantity of ,condy's fluid to impure water in times of epidemic4 ,the presence in water of one gram per litre1 or #a1jjj grams per cubic metre1 of any substance whatsoever1 renders it unfit and even injurious for consumption by animals1 and this whether organic or mineral matter predominates4 ,the presence of #a p4c4 of chlorides makes water quite salt1 and produces thirst instead of assuaging it4 ,the presence of magnesium salts is most unpleasant2 they have a disagreeable bitter taste1 and1 in fact1 impart to sea water its peculiar taste4 ,a large amount of nitrates is only found in impure water1 and is usually injurious1 as they may indicate the presence of decomposing organic matter4 #g ,illustration3 ,,fig4 #d4--;,distillation by means of a metallic still4 ,the liquid in ,c is heated by the fire ,f4 ,the vapours rise through the head ,a and pass by the tube ,t to the worm ,s placed in a vessel ,r1 through which a current of cold water flows by means of the tubes ,d and ,p4 ,distilled water may be prepared1 or distillation in general carried on1 either in a metal still with worm condenser 7fig4 #d7 or on a small scale in the laboratory in a glass retort 7fig4 #e7 heated by a lamp4 ,fig4 #e illustrates the main parts of the usual glass laboratory apparatus used for distillation4 ,the steam issuing from the retort 7on the right-hand side7 passes through a glass tube surrounded by a larger tube1 through which a stream of cold water passes1 by which the steam is condensed1 and runs into a receiver 7on the left-hand side74 ,illustration3 ,,fig4 #e4--;,distillation from a glass retort4 ,the neck of the retort fits into the inner tube of the ,liebig's condenser4 ,the space between the inner and outer tube of the condenser is filled with cold water1 which enters by the tube g and flows out at f4 #h ,one of ,lavoisier's first memoirs 7#aggj7 referred to this question4 ,he investigated the formation of the earthy residue in the distillation of water in order to prove whether it was possible1 as was affirmed1 to convert water into earth1 and he found that the residue was produced by the action of water on the sides of the vessel containing it1 and not from the water itself4 ,he proved this to be the case by direct weighing4 ,for certain physical and chemical researches1 however1 it is necessary to have perfectly pure water4 ,to obtain it1 a solution of potassium permanganate is added to distilled water until the whole is a light rose colour4 ,by this means the organic matter in the water is destroyed 7converted into gases or non-volatile substances74 ,an excess of potassium permanganate does no harm1 because in the next distillation it is left behind in the distillation apparatus4 ,the second distillation should take place in a platinum retort with a platinum receiver4 ,platinum is a metal which is not acted on either by air or water1 and therefore nothing passes from it into the water4 ,the water obtained in the receiver still contains air4 ,it must then be boiled for a long time1 and afterwards cooled in a vacuum under the receiver of an air pump4 ,pure water does not leave any residue on evaporation2 does not in the least change1 however long it be kept2 does not decompose like water only once distilled or impure2 and it does not give bubbles of gas on heating1 nor does it change the colour of a solution of potassium permanganate4 ,water1 purified as above described1 has constant physical and chemical properties4 ,for instance1 it is of such water only that one cubic centimetre weighs one gram at #d ,c4--i4e4 it is only such pure water whose specific gravity equals #a at #d ,c4#i ,water in a solid state forms crystals of the hexagonal system#aj which are seen in snow1 which generally consists of star-like clusters of several crystals1 and also in the half-melted scattered ice floating on rivers in spring time4 ,at this time of the year the ice splits up into spars or prisms1 bounded by angles proper to substances crystallising in the hexagonal system4 #i ,taking the generally-accepted specific gravity of water at its greatest density--i4e4 at #d as one--it has been shown by experiment that the specific gravity of water at different temperatures is as follows3 ,at #j #jiiihg ,at #cj #jiiegd #aj #jiiigd #dj #jiibcc #ae #jiiiae #ej #jihhag #bj #jiihbg #ajj #jiehei ,a comparison of all the data at present known shows that the variation of the specific gravity ,st with the temperature t 7determined by the mercurial thermometer7 maybe expressed 7,mendeleff #ahia7 by the formula ,st #a - 7t-#d7b_/7ida t7 7#gjcea-t7 #ai ---------------------------------------- ---------------------- t ,c4 ,variation of sp4 gr4 with according ,sp4 gr4 ,st a rise of ,volume to the 7at #d --------------------------taking vol4 mercurial #a1jjj1jjj7 ,temp4 per #a;,c4,pressure per at #d #a thermometer or ds_/dt #a atmosphere or ds_/dp ---------------------------------------- --------------------- -#aj #iih1bha #bfd #ed #a1jja1gbb #j #iii1hgc #fe #ej #a1jjj1abg #aj #iii1gch -#he #dg #a1jjj1bfb #bj #iih1bgb -#bjc #de #a1jja1gca #cj #iie1gdc -#bii #dc #a1jjd1bgf #ej #ihh1agd -#dej #dj #a1jaa1ifg #gj #igg1idh -#efi #ci #a1jbb1edi #ij #ife1ecg -#fgj #da #a1jce1fib #ajj #ieh1eie -#gah #db #a1jdc1aid #abj #idc1had -#haj #dc #a1jfj1jic #afj #ijg1bfc -#iie #ee #a1ajb1baf #bjj #hfc1dgc -#a1bjj #gc #a1aeh1aad ---------------------------------------- --------------------- ,if the temperature be determined by the hydrogen thermometer1 whose indications between #j and #ajj are slightly lower than the mercurial 7for example1 about #ja ,c4 at #bj71 then a slightly smaller sp4 gr4 will be obtained for a given t4 ,thus ,chappuis 7#ahib7 obtained #jiihbcc for #bj4 ,water at #d is taken as the basis for reducing measures of length to measures of weight and volume4 ,the metric1 decimal1 system of measures of weights and volumes is generally employed in science4 ,the starting point of this system is the metre 7#cicg inches7 divided into decimetres 7 #ja metre71 centimetres 7 #jja metre71 millimetres 7 #jjja metre71 and micrometres 7 one millionth of a metre74 ,a cubic decimetre is called a litre1 and is used for the measurement of volumes4 ,the weight of a litre of water at #d in a vacuum is called a kilogram4 ,one thousandth part of a kilogram of water weighs one gram4 ,it is divided into decigrams1 centigrams1 and milligrams 7 #jjja gram74 ,an ,english pound equals #decei grams4 ,the great advantage of this system is that it is a decimal one1 and that it is universally adopted in science and in most international relations4 ,all the measures cited in this work are metrical4 ,the units most often used in science are3--,of length1 the centimetre2 of weight1 the gram2 of time1 the second2 of temperature1 the degree ,celsius or ,centigrade4 ,according to the most trustworthy determinations 7,kupfer in ,russia #ahda1 and ,chaney in ,england #ahib71 the weight of a c4 dcm4 of water at #d in vacuo is about #iiii grms4 ,for ordinary purposes the weight of a c4 dcg4 may be taken as equal to a kg4 ,hence the litre 7determined by the weight of water it holds7 is slightly greater than a cubic decimetre4 #aj ,as solid substances appear in independent1 regular1 crystalline forms which are dependent1 judging from their cleavage or lamination 7in virtue of which mica breaks1 up into laminae1 and ,iceland spar1 c41 into pieces bounded by faces inclined to each other at angles which are definite for each substance71 on an inequality of attraction 7cohesion1 hardness7 in different directions which intersect at definite angles the determination of crystalline form therefore affords one of the most important characteristics for identifying definite chemical compounds4 ,the elements of crystallography which comprise a special science should therefore he familiar to all who desire to work in scientific chemistry4 ,in this work we shall only have occasion to speak of a few crystalline forms1 some of which are shown in figs4 #f to #ab4 ,illustration3 ,,fig4 #f4--;,example of the form belonging to the regular system4 ,combination of an octahedron and a cube4 ,the former predominates4 ,alum1 fluor spar1 suboxide of copper1 and others4 ,illustration3 ,,fig4 #g4--,rhombic ,dodecahedron of the regular system4 ,garnet4 ,illustration3 ,,fig4 #h4--;,hexagonal prism terminated by hexagonal pyramids4 ,quartz1 c4 ,illustration3 ,,fig4 #i4--,rhombohedron4 ,calc spar1 c4 ,illustration3 ,,fig4 #aj4--,rhombic system4 ,desmine4 ,illustration3 ,,fig4 #aa4--,triclinic pyramid4 ,illustration3 ,,fig4 #ab4--,triclinic system4 ,albite1 c4 ,the temperatures at which water passes from one state to another are taken as fixed points on the thermometer scale2 namely1 the zero corresponds with the temperature of melting ice1 and the temperature of the steam disengaged from water boiling at the normal barometric pressure 7that is #gfj millimetres measured at #j1 at the latitude of #de1 at the sea level7 is taken as #ajj of the ,celsius scale4 ,thus1 the fact that water liquefies at #j and boils at #ajj is taken as one of its properties as a definite chemical compound4 ,the weight of a litre of water at #d is #a1jjj grams1 at #j it is #iiih grams4 ,the weight of a litre of ice at #j is less--namely1 #iag grams2 the weight of the same cubic measure of water vapour at #gfj mm4 pressure and #ajj is only #jfj gram2 the density of the vapour compared with air #jfb1 and compared with hydrogen #i4 ,these data briefly characterise the physical properties of water as a separate substance4 ,to this may be added that water is a mobile liquid1 colourless1 transparent1 without taste or smell1 c4 ,its latent heat of vaporisation is #ecd units1 of liquefaction #gi units of heat4#aa ,the large amount of heat stored up in water vapour and also in liquid water 7for its specific heat is greater than that of other liquids7 renders it available in both forms for heating purposes4 ,the chemical reactions which water undergoes1 and by means of which it is formed1 are so numerous1 and so closely allied to the reactions of many other substances1 that it is impossible to describe the majority of them at this early stage of chemical exposition4 ,we shall become acquainted with many of them afterwards1 but at present we shall only cite certain compounds formed by water4 ,in order to see clearly the nature of the various kinds of compounds formed by water we will begin with the most feeble1 which are determined by purely mechanical superficial properties of the reacting substances4#ab #aa ,of all known liquids1 water exhibits the greatest cohesion of particles4 ,indeed1 it ascends to a greater height in capillary tubes than other liquids2 for instance1 two and a half times as high as alcohol1 nearly three times as high as ether1 and to a much greater height than oil of vitriol1 c4 ,in a tube one mm4 in diameter1 water at #j ascends #aec mm41 measuring from the height of the liquid to two-thirds of the height of the meniscus1 and at #ajj it rises #abe mm4 ,the cohesion varies very uniformly with the temperature2 thus at #ej the height of the capillary column equals #aci mm4--that is1 the mean between the columns at #j and #ajj4 ,this uniformity is not destroyed even at temperatures near the freezing point1 and hence it may be assumed that at high temperatures cohesion will vary as uniformly as at ordinary temperatures2 that is1 the difference between the columns at #j and #ajj being #bh mm41 the height of the column at #ejj should be #aeb-7e #bh7 #ab mm42 or1 in other words1 at these high temperatures the cohesion between the particles of water would he almost nil4 ,only certain solutions 7sal ammoniac and lithium chloride71 and these only with a great excess of water1 rise higher than pure water in capillary tubes4 ,the great cohesion of water doubtless determines many of both its physical and chemical properties4 ,the quantity of heat required to raise the temperature of one part by weight of water from #j to #a1 i4e4 by #a ,c41 is called the unit of heat or calorie2 the specific heat of liquid water at #j is taken as equal to unity4 ,the variation of this specific heat with a rise in temperature is inconsiderable in comparison with the variation exhibited by the specific heats of other liquids4 ,according to ,ettinger1 the specific heat of water at #bj #ajaf1 at #ej #ajci1 and at #ajj #ajgc4 ,the specific heat of water is greater than that of any other known liquid2 for example1 the specific heat of alcohol at #j is #jee--;i4e4 the quantity of heat which raises #ee parts of water #a raises #ajj parts of alcohol #a4 ,the specific heat of oil of turpentine at #j is #jda1 of ether #jec1 of acetic acid #jebgd1 of mercury #jjcc4 ,hence water is the best condenser or absorber of heat4 ,this property of water has an important significance in practice and in nature4 ,water prevents rapid cooling or heating1 and thus tempers cold and heat4 ,the specific heats of ice and aqueous vapour are much less than that of water2 namely1 that of ice is #jejd1 and of steam #jdh4 ,with an increase in pressure equal to one atmosphere1 the compressibility of water 7see ,note #i7 is #jjjjjdg1 of mercury #jjjjjjceb1 of ether #jjjjab at #j1 of alcohol at #ac #jjjjjie4 ,the addition of various substances to water generally decreases both its compressibility and cohesion4 ,the compressibility of other liquids increases with a rise of temperature1 but for water it decreases up to #ec and then increases like other liquids4 ,the expansion of water by heat 7,note #i7 also exhibits many peculiarities which are not found in other liquids4 ,the expansion of water at low temperatures is very small compared with other liquids2 at #d it is almost zero1 and at #ajj it is equal to #jjjjh2 below #d it is negative--i4e4 water on cooling then expands1 and does not decrease in volume4 ,in passing into a solid state1 the specific gravity of water decreases2 at #j one c4c4 of water weighs #jiiihhg gram1 and one c4c4 of ice at the same temperature weighs only #jiage gram4 ,the ice formed1 however1 contracts on cooling like the majority of other substances4 ,thus #ajj volumes of ice are produced from #ib volumes of water--that is1 water expands considerably on freezing1 which fact determines a number of natural phenomena4 ,the freezing point of water falls with an increase in pressure 7#jjjg per atmosphere71 because in freezing water expands 7,thomson71 whilst with substances which contract in solidifying the melting point rises with an increase in pressure2 thus1 paraffin under one atmosphere melts at #df1 and under #ajj atmospheres at #di4 ,when liquid water passes into vapour1 the cohesion of its particles must be destroyed1 as the particles are removed to such a distance from each other that their mutual attraction no longer exhibits any influence4 ,as the cohesion of aqueous particles varies at different temperatures1 the quantity of heat which is expended in overcoming this cohesion--or the latent heat of evaporation--will for this reason alone be different at different temperatures4 ,the quantity of heat which is consumed in the transformation of one part by weight of water1 at different temperatures1 into vapour was determined by ,regnault with great accuracy4 ,his researches showed that one part by weight of water at #j1 in passing into vapour having a temperature t1 consumes #fjfe #jcjet units of heat1 at #ej #fbag1 at #ajj #fcgj1 at #aej #febb1 and at #bjj #ffge4 ,but this quantity includes also the quantity of heat required for heating the water from #j to t--i4e4 besides the latent heat of evaporation1 also that heat which is used in heating the water in a liquid state to a temperature t4 ,on deducting this amount of heat1 we obtain the latent heat of evaporation of water as #fjfe at #j1 #ega at #ej1 #ecd at #ajj1 #did at #aej1 and only #dec at #bjj1 which shows that the conversion of water at different temperatures into vapour at a constant temperature requires very different quantities of heat4 ,this is chiefly dependent on the difference of the cohesion of water at different temperatures2 the cohesion is greater at low than at high temperatures1 and therefore at low temperatures a greater quantity of heat is required to overcome the cohesion4 ,on comparing these quantities of heat1 it will be observed that they decrease rather uniformly1 namely their difference between #j and #ajj is #gb1 and between #ajj and #bjj is #ha units of heat4 ,from this we may conclude that this variation will be approximately the same for high temperatures also1 and therefore that no heat would be required for the conversion of water into vapour at a temperature of about #djj4 ,at this temperature1 water passes into vapour whatever be the pressure 7see ,chap4 ,,ii4 ,the absolute boiling point of water1 according to ,dewar1 is #cgj1 the critical pressure #aif atmospheres74 ,it must here be remarked that water1 in presenting a greater cohesion1 requires a larger quantity of heat for its conversion into vapour than other liquids4 ,thus alcohol consumes #bjh1 ether #ij1 turpentine #gj1 units of heat in their conversion into vapour4 ,the whole amount of heat which is consumed in the conversion of water into vapour is not used in overcoming the cohesion--that is1 in internal accomplished in the liquid4 ,a part of this heat is employed in moving the aqueous particles2 in fact1 aqueous vapour at #ajj occupies a volume #a1fei times greater than that of water 7at the ordinary pressure71 consequently a portion of the heat or work is employed in lifting the aqueous particles1 in overcoming pressure1 or in external work1 which may be usefully employed1 and which is so employed in steam engines4 ,in order to determine this work1 let us consider the variation of the maximum pressure or vapour tension of steam at different temperatures4 ,the observations of ,regnault in this respect1 as on those preceding1 deserve special attention from their comprehensiveness and accuracy4 ,the pressure or tension of aqueous vapour at various temperatures is given in the adjoining table1 and is expressed in millimetres of the barometric column reduced to #j4 ---------------------------------------- ---- ,temperature ,tension ,temperature ,tension ---------------------------------------- ---- -#bj #ji #gj #bccc -#aj #ba #ij #ebed #j #df #ajj #gfjj #aj #ia #aje #ijfd #ae #abg #aaj #ajged #bj #agd #aae #abfid #be #bce #abj #adiac #cj #cae #aej #cehaj #ej #ibj #bjj #aafhij ---------------------------------------- ---- ,the table shows the boiling points of water at different pressures4 ,thus on the summit of ,mont ,blanc1 where the average pressure is about #dbd mm41 water boils at #hdd4 ,in a rarefied atmosphere water boils even at the ordinary temperature1 but in evaporating it absorbs heat from the neighbouring parts1 and therefore it becomes cold and may even freeze if the pressure does not exceed #df mm41 and especially if the vapour be rapidly absorbed as it is formed4 ,oil of vitriol1 which absorbs the aqueous vapour1 is used for this purpose4 ,thus ice may be obtained artificially at the ordinary temperature with the aid of an air-pump4 ,this table of the tension of aqueous vapour also shows the temperature of water contained in a closed boiler if the pressure of the steam formed be known4 ,thus at a pressure of five atmospheres 7a pressure of five times the ordinary atmospheric pressure--i4e4 #e #gfj #c1hjj mm47 the temperature of the water would be #aeb4 ,the table also shows the pressure produced on a given surface by steam on issuing from a boiler4 ,thus steam having a temperature of #aeb exerts a pressure of #eag kilos on a piston whose surface equals #ajj sq4 cm41 for the pressure of one atmosphere on one sq4 cm4 equals #a1jcc kilos1 and steam at #aeb has a pressure of five atmospheres4 ,as a column of mercury #a mm4 high exerts a pressure of #aceiei grams on a surface of #a sq4 cm41 therefore the pressure of aqueous vapour at #j corresponds with a pressure of #fbe grams per square centimetre4 ,the pressures for all temperatures may be calculated in a similar way1 and it will be found that at #ajj it is equal to #a1jccbh grams4 ,this means that if a cylinder be taken whose sectional area equals #a sq4 cm41 and if water be poured into it and it be closed by a piston weighing #a1jcc grams1 then on heating it in a vacuum to #ajj no steam will be formed1 because the steam cannot overcome the pressure of the piston2 and if at #ajj #ecd units of heat be transmitted to each unit of weight of water1 then the whole of the water will be converted into vapour having the same temperature2 and so also for every other temperature4 ,the question now arises1 to what height does the piston rise under these circumstances8 that is1 in other words1 ,what is the volume occupied by the steam under a known pressure8 ,for this we must know the weight of a cubic centimetre of steam at various temperatures4 ,it has been shown by experiment that the density of steam1 which does not saturate a space1 varies very inconsiderably at all possible pressures1 and is nine times the density of hydrogen under similar conditions4 ,steam which saturates a space varies in density at different temperatures1 but this difference is very small1 and its average density with reference to air is #jfd4 ,we will employ this number in our calculation1 and will calculate what volume the steam occupies at #ajj4 ,one cubic centimetre of air at #j and #gfj mm4 weighs #jjjabic gram1 at #ajj and under the same pressure it will weigh #jjjabic_/acfh or about #jjjjidf gram1 and consequently one cubic centimetre of steam whose density is #jfd will weigh #jjjjfje gram at #ajj1 and therefore one gram of aqueous vapour will occupy a volume of about #afec c4c4 ,consequently1 the piston in the cylinder of #a sq4 cm4 sectional area1 and in which the water occupied a height of #a cm41 will be raised #a1fec cm4 on the conversion of this water into steam4 ,this piston1 as has been mentioned1 weighs #a1jcc grams1 therefore the external work of the steam--that is1 that work which the water does in its conversion into steam at #ajj--;is equal to lifting a piston weighing #a1jcc grams to a height of #a1fec cm41 or #agjg kilogram-metres of work--i4e4 is capable of lifting #ag kilograms #a metre1 or #a kilogram #ag metres4 ,one gram of water requires for its conversion into steam #ecd gram units of heat or #jecd kilogram unit of heat--i4e4 the quantity of heat absorbed in the evaporation of one gram of water is equal to the quantity of heat which is capable of heating #a kilogram of water #jecd4 ,each unit of heat1 as has been shown by accurate experiment1 is capable of doing #dbd kilogram-metres of work4 ,hence1 in evaporating1 one gram of water expends #dbd #jecd 7almost7 #bbg kilogram-metres of work4 ,the external work was found to be only #ag kilogram-metres1 therefore #baj kilogram-metres are expended in overcoming the internal cohesion of the aqueous particles1 and consequently about #ib p4c4 of the total heat or work is consumed in overcoming the internal cohesion4 ,the following figures are thus calculated approximately3-- ---------------------------------------- ------------------- ,total work of ,external work of ,temperature evaporation in vapour in ,internal kilogram-metres kilogram-metres work of vapour ---------------------------------------- ------------------- #j #bee #ac #bdb #ej #bdb #ae #bbg #ajj #bbf #ag #bji #aej #bji #ai #aij #bjj #aib #bj #agb ---------------------------------------- ------------------- ,the work necessary for overcoming the internal cohesion of water in its passage into vapour decreases with the rise in temperature--that is1 corresponds with the decrease of cohesion2 and1 in fact1 the variations which take place in this case are very similar to those which are observed in the heights to which water rises in capillary tubes at different temperatures4 ,it is evident1 therefore1 that the amount of external--or1 as it is termed1 useful--work which water can supply by its evaporation is very small compared with the amount which it expends in its conversion into vapour4 ,in considering certain physico-mechanical properties of water1 ,i had in view not only their importance for theory and practice1 but also their purely chemical significance2 for it is evident from the above considerations that even in a physical change of state the greatest part of the work done is employed in overcoming cohesion1 and that an enormous amount of internal energy must be expended in overcoming chemical cohesion or affinity4 #ab ,when it is necessary to heat a considerable mass of liquid in different vessels1 it would be very uneconomical to make use of metallic vessels and to construct a separate furnace for each2 such cases are continually met with in practice4 ,steam from a boiler is introduced into the liquid1 or1 in general1 into the vessel which it is required to heat4 ,the steam1 in condensing and passing into a liquid state1 parts with its latent heat1 and as this is very considerable a small quantity of steam will produce a considerable heating effect4 ,if it be required1 for instance1 to heat #a1jjj kilos of water from #bj to #ej1 which requires approximately #cj1jjj units of heat1 steam at #ajj is passed into the water from a boiler4 ,each kilogram of water at #ej contains about #ej units of heat1 and each kilogram of steam at #ajj contains #fcg units of heat2 therefore1 each kilogram of steam in cooling to #ej gives up #ehg units of heat1 and consequently #eb kilos of steam are capable of heating #a1jjj kilos of water from #bj to #ej4 ,water is very often applied for heating in chemical practice4 ,for this purpose metallic vessels or pans1 called 'water-baths1' are made use of4 ,they are closed by a cover formed of concentric rings lying on each other4 ,the vessels--such as beakers1 evaporating basins1 retorts1 c4--containing liquids1 are placed on these rings1 and the water in the bath is heated4 ,the steam given off heats the bottom of the vessels to be heated1 and thus effects the evaporation or distillation4 ,water is mechanically attracted by many substances2 it adheres to their surfaces just as dust adheres to objects1 or one piece of polished glass adheres to another4 ,such attraction is termed 'moistening1' 'soaking1' or 'absorption of water4' ,thus water moistens clean glass and adheres to its surface1 is absorbed by the soil1 sand1 and clay1 and does not flow away from them1 but lodges itself between their particles4 ,similarly1 water soaks into a sponge1 cloth1 hair1 or paper1 c41 but fat and greasy substances in general are not moistened4 ,attraction of this kind does not alter the physical or chemical properties of water4 ,for instance1 under these circumstances water1 as is known from everyday experience1 may be expelled from objects by drying4 ,water which is in any way held mechanically may be dislodged by mechanical means1 by friction1 pressure1 centrifugal force1 c4 ,thus water is squeezed from wet cloth by pressure or centrifugal machines4 ,but objects which in practice are called dry 7because they do not feel wet7 often still contain moisture1 as may be proved by heating the object in a glass tube closed at one end4 ,by placing a piece of paper1 dry earth1 or any similar object 7especially porous substances7 in such a glass tube1 and heating that part of the tube where the object is situated1 it will be remarked that water condenses on the cooler portions of the tube4 ,the presence of such absorbed1 or 'hygroscopic1' water is generally best detected in non-volatile substances by drying them at #ajj1 or under the receiver of an air-pump and over substances which attract water chemically4 ,by weighing a substance before and after drying1 it is easy to determine the amount of hygroscopic water from the loss in weight4#ac ,only in this case the amount of water must be judged with care1 because the loss in weight may sometimes proceed from the decomposition of the substance itself1 with disengagement of gases or vapour4 ,in making exact weighings the hygroscopic capacity of substances--that is1 their capacity to absorb moisture--must be continually kept in view1 as otherwise the weight will be untrue from the presence of moisture4 ,the quantity of moisture absorbed depends on the degree of moisture of the atmosphere 7that is1 on the tension of the aqueous vapour in it7 in which a substance is situated4 ,in an entirely dry atmosphere1 or in a vacuum1 the hygroscopic water is expelled1 being converted into vapour2 therefore1 substances containing hygroscopic water may be completely dried by placing them in a dry atmosphere or in a vacuum4 ,the process is aided by heat1 as it increases the tension of the aqueous vapour4 ,phosphoric anhydride 7a white powder71 liquid sulphuric acid1 solid and porous calcium chloride1 or the white powder of ignited copper sulphate1 are most generally employed in drying gases4 ,they absorb the moisture contained in air and all gases to a considerable1 but not unlimited1 extent4 ,phosphoric anhydride and calcium chloride deliquesce1 become damp1 sulphuric acid changes from an oily thick liquid into a more mobile liquid1 and ignited copper sulphate becomes blue2 after which changes these substances partly lose their capacity of holding water1 and can1 if it be in excess1 even give up their water to the atmosphere4 ,we may remark that the order in which these substances are placed above corresponds with the order in which they stand in respect to their capacity for absorbing moisture4 ,air dried by calcium chloride still contains a certain amount of moisture1 which it can give up to sulphuric acid4 ,the most complete desiccation takes place with phosphoric anhydride4 ,water is also removed from many substances by placing them in a dish over a vessel containing a substance absorbing water under a glass bell jar4#ad ,the bell jar1 like the receiver of an air pump1 should be hermetically closed4 ,in this case desiccation takes place2 because sulphuric acid1 for instance1 first dries the air in the bell jar by absorbing its moisture1 the substance to be dried then parts with its moisture to the dry air1 from which it is again absorbed by the sulphuric acid1 c4 ,desiccation proceeds still better under the receiver of an air pump1 for then the aqueous vapour is formed more quickly than in a bell jar full of air4 #ac ,illustration3 ,,fig4 #ac4--;,drying oven1 composed of brazed copper4 ,it is heated by a lamp4 ,the object to be dried is placed on the gauze inside the oven4 ,the thermometer indicates the temperature4 ,in order to dry any substance at about #ajj--that is1 at the boiling point of water 7hygroscopic water passes off at this temperature7--an apparatus called a 'drying-oven' is employed4 ,it consists of a double copper box2 water is poured into the space between the internal and external boxes1 and the oven is then heated over a stove or by any other means1 or else steam from a boiler is passed between the walls of the two boxes4 ,when the water boils1 the temperature inside the inner box will be approximately #ajj ,c4 ,the substance to be dried is placed inside the oven1 and the door is closed4 ,several holes are cut in the door to allow the free passage of air1 which carries off the aqueous vapour by the chimney on the top of the oven4 ,often1 however1 desiccation is carried on in copper ovens heated directly over a lamp 7fig4 #ac74 ,in this case any desired temperature may be obtained1 which is determined by a thermometer fixed in a special orifice4 ,there are substances which only part with their water at a much higher temperature than #ajj1 and then such air baths are very useful4 ,in order to determine directly the amount of water in a substance which does not part with anything except water at a red heat1 the substance is placed in a bulb tube4 ,by first weighing the tube empty and then with the substance to be dried in it1 the weight of the substance taken may be found4 ,the tube is then connected on one side with a gas-holder full of air1 which1 on opening a stop-cock1 passes first through a flask containing sulphuric acid1 and then into a vessel containing lumps of pumice stone moistened with sulphuric acid4 ,in passing through these vessels the air is thoroughly dried1 having given up all its moisture to the sulphuric acid4 ,thus dry air will pass into the bulb tube1 and as hygroscopic water is entirely given up from a substance in dry air even at the ordinary temperature1 and still more rapidly on heating1 the moisture given up by the substance in the tube will be carried off by the air passing through it4 ,this damp air then passes through a ,u-shaped tube full of pieces of pumice stone moistened with sulphuric acid1 which absorbs all the moisture given off from the substance in the bulb tube4 ,thus all the water expelled from the substance will collect in the ,u tube1 and so1 if this be weighed before and after1 the difference will show the quantity of water expelled from the substance4 ,if only water 7and not any gases7 come over1 the increase of the weight of the ,u tube will be equal to the decrease in the weight of the bulb tube4 #ad ,instead of under a glass bell jar1 drying over sulphuric acid is often carried on in a desiccator consisting of a shallow wide-mouthed glass vessel1 closed by a well-fitting ground-glass cover4 ,sulphuric acid is poured over the bottom of the desiccator1 and the substance to be dried is placed on a glass stand above the acid4 ,a lateral glass tube with a stop-cock is often fused into the desiccator in order to connect it with an air pump1 and so allow drying under a diminished pressure1 when the moisture evaporates more rapidly4 ,the fact that in the usual form of desiccator the desiccating substance 7sulphuric acid7 is placed beneath the substance to be dried has the disadvantage that the moist air being lighter than dry air distributes itself in the upper portion of the desiccator and not below4 ,hempel1 in his desiccator 7#ahia71 avoids this by placing the absorbent above the substance to be dried4 ,the process of desiccation can be further accelerated by cooling the upper portion of the desiccator1 and so inducing ascending and descending currents of air within the apparatus4 ,from what has been said above1 it is evident that the transference of moisture to gases and the absorption of hygroscopic moisture present great resemblance to1 but still are not1 chemical combinations with water4 ,water1 when combined as hygroscopic water1 does not lose its properties and does not form new substances4#ae #ae ,chappuis1 however1 determined that in wetting #a gram of charcoal with water #g units of heat are evolved1 and on pouring carbon bisulphide over #a gram of charcoal as much as #bd units of heat are evolved4 ,alumina 7#a gram71 when moistened with water1 evolves #b-a_/b calories4 ,this indicates that in respect to evolution of heat moistening already presents a transition towards exothermal combinations 7those evolving heat in their formation74 ,the attraction of water for substances which dissolve in it is of a different character4 ,in the solution of substances in water there proceeds a peculiar kind of indefinite combination2 a new homogeneous substance is formed from the two substances taken4 ,but here also the bond connecting the substances is very unstable4 ,water containing different substances in solution boils at a temperature near to its usual boiling point4 ,from the solution of substances which are lighter than water itself1 there are obtained solutions of a less density than water--as1 for example1 in the solution of alcohol in water2 whilst a heavier substance in dissolving in water gives it a higher specific gravity4 ,thus salt water is heavier than fresh4#af #af ,strong acetic acid 7,c#b;,h#d,o#b71 whose specific gravity at #ae is #ajee1 does not become lighter on the addition of water 7a lighter substance1 sp4 gr4 #jiii71 but heavier1 so that a solution of #hj parts of acetic acid and #bj parts of water has a specific gravity of #ajgd1 and even a solution of equal parts of acetic acid and water 7#ej p4c47 has a sp4 gr4 of #ajfe1 which is still greater than that of acetic acid itself4 ,this shows the high degree of contraction which takes place on solution4 ,in fact1 solutions--and1 in general1 liquids--on mixing with water1 decrease in volume4 ,we will consider aqueous solutions somewhat fully1 because1 among other reasons1 solutions are constantly being formed on the earth and in the waters of the earth1 in plants and in animals1 in chemical processes and in the arts1 and these solutions play an important part in the chemical transformations which are everywhere taking place1 not only because water is everywhere met with1 but chiefly because a substance in solution presents the most favourable conditions for the process of chemical changes1 which require a mobility of parts and a possible distension of parts4 ,in dissolving1 a solid substance acquires a mobility of parts1 and a gas loses its elasticity1 and therefore reactions often take place in solutions which do not proceed in the undissolved substances4 ,further1 a substance1 distributed in water1 evidently breaks up--that is1 becomes more like a gas and acquires a greater mobility of parts4 ,all these considerations require that in describing the properties of substances1 particular attention should be paid to their relation to water as a solvent4 ,illustration3 ,,fig4 #ad4--,method of transferring a gas into a cylinder filled with mercury and whose open end is immersed under the mercury in a bath having two glass sides4 ,the apparatus containing the gas is represented on the right4 ,its upper extremity is furnished with a tube extending under the cylinder4 ,the lower part of the vessel communicates with a vertical tube4 ,if mercury be poured into this tube1 the pressure of the gas in the apparatus is increased1 and it passes through the gas-conducting tube into the cylinder1 where it displaces the mercury1 and can be measured or subjected to the action of absorbing agents1 such as water4 ,it is well known that water dissolves many substances4 ,salt1 sugar1 alcohol1 and a number of other substances1 dissolve in water and form homogeneous liquids with it4 ,to demonstrate the solubility of gases in water1 a gas should be taken which has a high co-efficient of solubility--for instance1 ammonia4 ,this is introduced into a bell jar 7or cylinder1 as in fig4 #ad71 which is previously filled with mercury and stands in a mercury bath4 ,if water be then introduced into the cylinder1 the mercury will rise1 owing to the water dissolving the ammonia gas4 ,if the column of mercury be less than the barometric column1 and if there be sufficient water to dissolve the gas1 all the ammonia will be absorbed by the water4 ,the water is introduced into the cylinder by a glass pipette1 with a bent end4 ,the bent end is put into water1 and the air is sucked out from the upper end4 ,when full of water1 its upper end is closed with the finger1 and the bent end placed in the mercury bath under the orifice of the cylinder4 ,on blowing into the pipette the water will rise to the surface of the mercury in the cylinder owing to its lightness4 ,the solubility of a gas like ammonia may be demonstrated by taking a flask full of the gas1 and closed by a cork with a tube passing through it4 ,on placing the tube under water1 the water will rise into the flask 7this may be accelerated by previously warming the flask71 and begin to play like a fountain inside it4 ,both the rising of the mercury and the fountain clearly show the considerable affinity of water for ammonia gas1 and the force acting in this dissolution is rendered evident4 ,a certain period of time is required both for the homogeneous intermixture of gases 7diffusion7 and the process of solution1 which depends1 not only on the surface of the participating substances1 but also on their nature4 ,this is seen from experiment4 ,solutions of different substances heavier than water1 such as salt or sugar1 are poured into tall jars4 ,pure water is then very carefully poured into these jars 7through a funnel7 on to the top of the solutions1 so as not to disturb the lower stratum1 and the jars are then left undisturbed4 ,the line of demarcation between the solution and the pure water will be visible1 owing to their different co-efficients of refraction4 ,notwithstanding that the solutions taken are heavier than water1 after some time complete intermixture will ensue4 ,gay ,lussac convinced himself of this fact by this particular experiment1 which he conducted in the cellars under the ,paris ,astronomical ,observatory4 ,these cellars are well known as the locality where numerous interesting researches have been conducted1 because1 owing to their depth under ground1 they have a uniform temperature during the whole year2 the temperature does not change during the day1 and this was indispensable for the experiments on the diffusion of solutions1 in order that no doubt as to the results should arise from a daily change of temperature 7the experiment lasted several months71 which would set up currents in the liquids and intermix their strata4 ,notwithstanding the uniformity of the temperature1 the substance in solution in time ascended into the water and distributed itself uniformly through it1 proving that there exists between water and a substance dissolved in it a particular kind of attraction or striving for mutual interpenetration in opposition to the force of gravity4 ,further1 this effort1 or rate of diffusion1 is different for salt or sugar or for various other substances4#af bis ,it follows therefore that a peculiar force acts in solution1 as in actual chemical combinations1 and solution is determined by a particular kind of motion 7by the chemical energy of a substance7 which is proper to the substance dissolved and to the solvent4 #af bis ,graham1 in the jelly formed by gelatine1 and ,de ,vries in gelatinous silica 7,chapter ,,xviii47 most frequently employed coloured 7tinted7 substances1 for instance1 ,k#b;,cr#b,o#g1 which showed the rate of diffusion with very great clearness4 ,prof4 ,oumoff employed the method described in ,chapter ,x41 ,note #ag1 for this purpose4 ,graham made a series of experiments similar to those above described1 and showed that the rate of diffusion#ag in water is very variable--that is1 a uniform distribution of a substance in the water dissolving it is attained in different periods of time with different solutions4 ,graham compared diffusive capacity with volatility4 ,there are substances which diffuse easily1 and there are others which diffuse with difficulty1 just as there are more or less volatile substances4 ,seven hundred cubic centimetres of water were poured into a jar1 and by means of a syphon 7or a pipette7 #ajj cub4 centimetres of a solution containing #aj grams of a substance were cautiously poured in so as to occupy the lower portion of the jar4 ,after a lapse of several days1 successive layers of #ej cubic centimetres were taken from the top downwards1 and the quantity of substance dissolved in the different layers determined4 ,thus1 common table salt1 after fourteen days1 gave the following amounts 7in milligrams7 in the respective layers1 beginning from the top3 #ajd1 #abj1 #abf1 #aih1 #bfg1 #cdj1 #dbi1 #ece1 #fed1 #gff1 #hha1 #iia1 #a1jij1 #a1ahg1 and #b1bff in the remainder2 whilst albumin in the same time gave1 in the first seven layers1 a very small amount1 and beginning from the eighth layer1 #aj1 #ae1 #dg1 #aac1 #cdc1 #hee1 #a1hib1 and in the remainder #f1gbe milligrams4 ,thus1 the diffusive power of a solution depends on time and on the nature of the substance dissolved1 which fact may serve1 not only for explaining the process of solution1 but also for distinguishing one substance from another4 ,graham showed that substances which rapidly diffuse through liquids are able to rapidly pass through membranes and crystallise1 whilst substances which diffuse slowly and do not crystallise are colloids1 that is1 resemble glue1 and penetrate through a membrane slowly1 and form jellies2 that is1 occur in insoluble forms1#ah as will be explained in speaking of silica4 #ag ,the researches of ,graham1 ,fick1 ,nernst1 and others showed that the quantity of a dissolved substance which is transmitted 7rises7 from one stratum of liquid to another in a vertical cylindrical vessel is not only proportional to the time and to the sectional area of the cylinder1 but also to the amount and nature of the substance dissolved in a stratum of liquid1 so that each substance has its corresponding co-efficient of diffusion4 ,the cause of the diffusion of solutions must be considered as essentially the same as the cause of the diffusion of gases--that is1 as dependent on motions which are proper to their molecules2 but here most probably those purely chemical1 although feebly-developed1 forces1 which incline the substances dissolved to the formation of definite compounds1 also play their part4 #ah ,illustration3 ,,fig4 #ae4--;,dialyser4 ,apparatus for the separation of substances which pass through a membrane from those which do not4 ,description in text4 ,the rate of diffusion--like the rate of transmission--through membranes1 or dialysis 7which plays an important part in the vital processes of organisms and also in technical processes71 presents1 according to ,graham's researches1 a sharply defined change in passing from such crystallisable substances as the majority of salts and acids to substances which are capable of giving jellies 7gum1 gelatin1 c47 ,the former diffuse into solutions and pass through membranes much more rapidly than the latter1 and ,graham therefore distinguishes between crystalloids1 which diffuse rapidly1 and colloids1 which diffuse slowly4 ,on breaking solid colloids into pieces1 a total absence of cleavage is remarked4 ,the fracture of such substances is like that of glue or glass4 ,it is termed a 'conchoidal' fracture4 ,almost all the substances of which animal and vegetable bodies consist are colloids1 and this is1 at all events1 partly the reason why animals and plants have such varied forms1 which have no resemblance to the crystalline forms of the majority of mineral substances4 ,the colloid solid substances in organisms--that is1 in animals and plants--almost always contain water1 and take most peculiar forms1 of networks1 of granules1 of hairs1 of mucous1 shapeless masses1 c41 which are quite different from the forms taken by crystalline substances4 ,when colloids separate out from solutions1 or from a molten state1 they present a form which is similar to that of the liquid from which they were formed4 ,glass may he taken as the best example of this4 ,colloids are distinguishable from crystalloids1 not only by the absence of crystalline form1 but by many other properties which admit of clearly distinguishing both these classes of solids1 as ,graham showed4 ,nearly all colloids are capable of passing1 under certain circumstances1 from a soluble into an insoluble state4 ,the best example is shown by white of eggs 7albumin7 in the raw and soluble form1 and in the hard-boiled and insoluble form4 ,the majority of colloids1 on passing into an insoluble form in the presence of water1 give substances having a gelatinous appearance1 which is familiar to every one in starch1 solidified glue1 jelly1 c4 ,thus gelatin1 or common carpenter's glue1 when soaked in water1 swells up into an insoluble jelly4 ,if this jelly be heated1 it melts1 and is then soluble in water1 but on cooling it again forms a jelly which is insoluble in water4 ,one of the properties which distinguish colloids from crystalloids is that the former pass very slowly through a membrane1 whilst the latter penetrate very rapidly4 ,this may be shown by taking a cylinder1 open at both ends1 and by covering its lower end with a bladder or with vegetable parchment 7unsized paper immersed for two or three minutes in a mixture of sulphuric acid and half its volume of water1 and then washed71 or any other membranous substance 7all such substances are themselves colloids in an insoluble form74 ,the membrane must be firmly tied to the cylinder1 so as not to leave any opening4 ,such an apparatus is called a dialyser 7fig4 #ae71 and the process of separation of crystalloids from colloids by means of such a membrane is termed dialysis4 ,an aqueous solution of a crystalloid or colloid1 or a mixture of both1 is poured into the dialyser1 which is then placed in a vessel containing water1 so that the bottom of the membrane is covered with water4 ,then1 after a certain period of time1 the crystalloid passes through the membrane1 whilst the colloid1 if it does pass through at all1 does so at an incomparably slower rate4 ,the crystalloid naturally passes through into the water until the solution attains the same strength on both sides of the membrane4 ,by replacing the outside water with fresh water1 a fresh quantity of the crystalloid may be separated from the dialyser4 ,while a crystalloid is passing through the membrane1 a colloid remains almost entirely in the dialyser1 and therefore a mixed solution of these two kinds of substances may be separated from each other by a dialyser4 ,the study of the properties of colloids1 and of the phenomena of their passage through membranes1 should elucidate much respecting the phenomena which are accomplished in organisms4 ,hence1 if it be desired to increase the rate of solution1 recourse must be had to stirring1 shaking1 or some such mechanical motion4 ,but if once a uniform solution is formed1 it will remain uniform1 no matter how heavy the dissolved substance is1 or how long the solution be left at rest1 which fact again shows the presence of a force holding together the particles of the body dissolved and of the solvent4#ai #ai ,the formation of solutions may be considered in two aspects1 from a physical and from a chemical point of view1 and it is more evident in solutions than in any other department of chemistry how closely these provinces of natural science are allied together4 ,on the one hand solutions form a particular case of a physico-mechanical interpenetration of homogeneous substances1 and a juxtaposition of the molecules of the substance dissolved and of the solvent1 similar to the juxtaposition which is exhibited in homogeneous substances4 ,from this point of view this diffusion of solutions is exactly similar to the diffusion of gases1 with only this difference1 that the nature and store of energy are different in gases from what they are in liquids1 and that in liquids there is considerable friction1 whilst in gases there is comparatively little4 ,the penetration of a dissolved substance into water is likened to evaporation1 and solution to the formation of vapour4 ,this resemblance was clearly expressed even by ,graham4 ,in recent years the ,dutch chemist1 ,van't ,hoff1 has developed this view of solutions in great detail1 having shown 7in a memoir in the ,transactions of the ,swedish ,academy of ,science1 ,part #ba1 ,no4 #ag1 ',lois de l'quilibre chimique dans l'tat dilu1 gazeux ou dissous1' #ahhf71 that for dilute solutions the osmotic pressure follows the same laws of ,boyle1 ,mariotte1 ,gay-,lussac1 and ,avogadro-,gerhardt as for gases4 ,the osmotic pressure of a substance dissolved in water is determined by means of membranes which allow the passage of water1 but not of a substance dissolved in it1 through them4 ,this property is found in animal protoplasmic membranes and in porous substances covered with an amorphous precipitate1 such as is obtained by the action of copper sulphate on potassium ferrocyanide 7,pfeffer1 ,traube74 ,if1 for instance1 a one p4c4 solution of sugar he placed in such a vessel1 which is then closed and placed in water1 the water passes through the walls of the vessel and increases the pressure by #ej mm4 of the barometric column4 ,if the pressure be artificially increased inside the vessel1 then the water will be expelled through the walls4 ,de ,vries found a convenient means of determining isotonic solutions 7those presenting a similar osmotic pressure7 in the cells of plants4 ,for this purpose a portion of the soft part of the leaves of the ,tradescantis discolor1 for instance1 is cut away and moistened with the solution of a given salt and of a given strength4 ,if the osmotic pressure of the solution taken be less than that of the sap contained in the cells they will change their form or shrink2 if1 on the other hand1 the osmotic pressure be greater than that of the sap1 then the cells will expand1 as can easily be seen under the microscope4 ,by altering the amount of the different salts in solution it is possible to find for each salt the strength of solution at which the cells begin to swell1 and at which they will consequently have an equal osmotic pressure4 ,as it increases in proportion to the amount of a substance dissolved per #ajj parts of water1 it is possible1 knowing the osmotic pressure of a given substance--for instance1 sugar at various degrees of concentration of solution--and knowing the composition of isotonic solutions compared with sugar1 to determine the osmotic pressure of all the salts investigated4 ,the osmotic pressure of dilute solutions determined in this manner directly or indirectly 7from observations made by ,pfeffer and ,de ,vries7 was shown to follow the same laws as those of the pressure of gases2 for instance1 by doubling or increasing the quantity of a salt 7in a given volume7 n times1 the pressure is doubled or increases n times4 ,so1 for example1 in a solution containing one part of sugar per #ajj parts of water the osmotic pressure 7according to ,pfeffer7 #ehe cm4 of mercury1 if #b parts of sugar #ajaf1 if #d parts #bjhb and so on1 which proves that the ratio is true within the limits of experimental error4 7#b7 ,different substances for equal strengths of solutions1 show very different osmotic pressures1 just as gases for equal parts by weight in equal volumes show different tensions4 7#c7 ,if1 for a given dilute solution at #j1 the osmotic pressure equal p1 then at t it will be greater and equal to p7#a #jjjcfgt71 i4e4 it increases with the temperature in exactly the same manner as the tension of gases increases4 7#d7 ,if in dilute solutions of such substances as do not conduct an electric current 7for instance1 sugar1 acetone1 and many other organic bodies7 the substances be taken in the ratio of their molecular weights 7expressed by their formul1 see ,chapter ,,vii471 then not only will the osmotic pressure be equal1 but its magnitude will be determined by that tension which would be proper to the vapours of the given substances when they would be contained in the space occupied by the solution1 just as the tension of the vapours of molecular quantities of the given substances will be equal1 and determined by the laws of ,gay-,lussac1 ,mariotte1 and ,avogadro-,gerhardt4 ,those formul 7,chapter ,,vii41 ,notes #bc and #bd7 by which the gaseous state of matter is determined1 may also be applied in the present case4 ,so1 for example1 the osmotic pressure p1 in centimetres of mercury1 of a one per cent4 solution of sugar1 may be calculated according to the formula for gases3 ,mp #fbjjs7#bgc t71 where ,m is the molecular weight1 s the weight in grams of a cubic centimetre of vapour1 and t its temperature4 ,for sugar ,m #cdb 7because its molecular composition is ,c#ab;,h#bb,o#aa74 ,the specific gravity of the solution of sugar is #ajjc1 hence the weight of sugar s contained in a #a per cent4 solution #jjajjc gram4 ,the observation was made at t #ad4 ,hence1 according to the formula1 we find p #ebb centimetres4 ,and experiments carried on at #ad gave #ece centimetres1 which is very near to the above4 7#e7 ,for the solutions of salts1 acids1 and similar substances1 which conduct an electric current1 the calculated pressure is usually 7but not always in a definite or multiple number of times7 less than the observed by i times1 and this i for dilute solutions of ,mg,s,o#d is nearly #a1 for ,,co#b #a1 for ,k,cl1 ,na,cl1 ,,ki1 ,,kno#c greater than #a1 and approximates to #b1 for ,ba,cl#b1 ,mg,cl#b1 ,k#b;,,co#c1 and others between #b and #c1 for ,h,cl1 ,h#b,,so#d1 ,na,n,o#c1 ,ca,n#b,o#f1 and others nearly #b and so on4 ,it should be remarked that the above deductions are only applicable 7and with a certain degree of accuracy7 to dilute solutions1 and in this respect resemble the generalisations of ,michel and ,kraft 7see ,note #dd74 ,nevertheless1 the arithmetical relation found by ,van't ,hoff between the formation of vapours and the transition into dilute solutions forms an important scientific discovery1 which should facilitate the explanation of the nature of solutions1 while the osmotic pressure of solutions already forms a very important aspect of the study of solutions4 ,in this respect it is necessary to mention that ,prof4 ,konovaloff 7#ahia1 and subsequently others also7 discovered the dependence 7and it may be a sufficient explanation7 of the osmotic pressure upon the differences of the tensions of aqueous vapours and aqueous solutions2 this1 however1 already enters into a special province of physical chemistry 7certain data are given in ,note #di and following71 and to this physical side of the question also belongs one of the extreme consequences of the resemblance of osmotic pressure to gaseous pressure1 which is that the concentration of a uniform solution varies in parts which are heated or cooled4 ,soret 7#ahha7 indeed observed that a solution of copper sulphate containing #ag parts of the salt at #bj only contained #ad parts after heating the upper portion of the tube to #hj for a long period of time4 ,this aspect of solution1 which is now being very carefully and fully worked out1 may be called the physical side4 ,its other aspect is purely chemical1 for solution does not take place between any two substances1 but requires a special and particular attraction or affinity between them4 ,a vapour or gas permeates any other vapour or gas1 but a salt which dissolves in water may not be in the least soluble in alcohol1 and is quite insoluble in mercury4 ,in considering solutions as a manifestation of chemical force 7and of chemical energy71 it must be acknowledged that they are here developed to so feeble an extent that the definite compounds 7that is1 those formed according to the law of multiple proportions7 formed between water and a soluble substance dissociate even at the ordinary temperature1 forming a homogeneous system--that is1 one in which both the compound and the products into which it decomposes 7water and the aqueous compound7 occur in a liquid state4 ,the chief difficulty in the comprehension of solutions depends on the fact that the mechanical theory of the structure of liquids has not yet been so fully developed as the theory of gases1 and solutions are liquids4 ,the conception of solutions as liquid dissociated definite chemical compounds is based on the following considerations3 7#a7 that there exist certain undoubtedly definite chemical crystallised compounds 7such as ,h#b,,so#d1;,h#b,o2 or ,na,cl1#b;,h#b,o2 or ,ca,cl#b1f;,h#b,o2 c47 which melt on a certain rise of temperature1 and then form true solutions2 7#b7 that metallic alloys in a molten condition are real solutions1 but on cooling they often give entirely distinct and definite crystallised compounds1 which are recognised by the properties of alloys2 7#c7 that between the solvent and the substance dissolved there are formed1 in a number of cases1 many undoubtedly definite compounds1 such as compounds with water of crystallisation2 7#d7 that the physical properties of solutions1 and especially their specific gravities 7a property which can be very accurately determined71 vary with a change in composition1 and in such a manner as would be required by the formation of one or more definite but dissociating compounds4 ,thus1 for example1 on adding water to fuming sulphuric acid its density is observed to decrease until it attains the definite composition ,h#b,,so#d1 or ,,so#c ,h#b,o1 when the specific gravity increases1 although on further diluting with water it again falls4 ,moreover 7,mendeleff1 ,the ,investigation of ,aqueous ,solutions from their ,specific ,gravities1 #ahhg71 the increase in specific gravity 7ds71 varies in all well-known solutions with the proportion of the substance dissolved 7dp71 and this dependence can be expressed by a formula 7ds_/dp ,a ,bp7 between the limits of definite compounds whose existence in solutions must be admitted1 and this is in complete accordance with the dissociation hypothesis4 ,thus1 for instance1 from ,h#b,,so#d to ,h#b,,so#d ,h#b,o 7both these substances exist as definite compounds in a free state71 the fraction ds_/dp #jjgbi-jjjjgdip 7where p is the percentage amount of ,h#b,,so#d74 ,for alcohol ,c#b;,h#f,o1 whose aqueous solutions have been more accurately investigated than all others1 the definite compound ,c#b;,h#f,o #c;,h#b,o1 and others must be acknowledged in its solutions4 ,the two aspects of solution above mentioned1 and the hypotheses which have as yet been applied to the examination of solutions1 although they have somewhat different starting points1 will doubtless in time lead to a general theory of solutions1 because the same common laws govern both physical and chemical phenomena1 inasmuch as the properties and motions of molecules1 which determine physical properties1 depend on the motions and properties of atoms1 which determine chemical reactions4 ,for details of the questions dealing with theories of solution1 recourse must now be had to special memoirs and to works on theoretical 7physical7 chemistry2 for this subject forms one of special interest at the present epoch of the development of our science4 ,in working out chiefly the chemical side of solutions1 ,i consider it to be necessary to reconcile the two aspects of the question2 this seems to me to be all the more possible1 as the physical side is limited to dilute solutions only1 whilst the chemical side deals mainly with strong solutions4 ,in the consideration of the process of solution1 besides the conception of diffusion1 another fundamental conception is necessary--namely1 that of the saturation of solutions4 ,just as moist air may be diluted with any desired quantity of dry air1 so also an indefinitely large quantity of a liquid solvent may be taken1 and yet a uniform solution will be obtained4 ,but more than a definite quantity of aqueous vapour cannot be introduced into a certain volume of air at a certain temperature4 ,the excess above the point of saturation will remain in the liquid state4#bj ,the relation between water and substances dissolved in it is similar4 ,more than a definite quantity of a substance cannot1 at a certain temperature1 dissolve in a given quantity of water2 the excess does not unite with the water4 ,just as air or a gas becomes saturated with vapour1 so water becomes saturated with a substance dissolved in it4 ,if an excess of a substance be added to water which is already saturated with it1 it will remain in its original state1 and will not diffuse through the water4 ,the quantity of a substance 7either by volume with gases1 or by weight with solids and liquids7 which is capable of saturating #ajj parts of water is called the co-efficient of solubility or the solubility4 ,in #ajj grams of water at #ae1 there can be dissolved not more than #cehf grams of common salt4 ,consequently1 its solubility at #ae is equal to #cehf4ba ,it is most important to turn attention to the existence of the solid insoluble substances of nature1 because on them depends the shape of the substances of the earth's surface1 and of plants and animals4 ,there is so much water on the earth's surface1 that were the surface of substances formed of soluble matters it would constantly change1 and however substantial their forms might be1 mountains1 river banks and sea shores1 plants and animals1 or the habitations and coverings of men1 could not exist for any length of time4#bb #bj ,a system of 7chemically or physically7 re-acting substances in different states of aggregation--for instance1 some solid1 others liquid or gaseous--is termed a heterogeneous system4 ,up to now it is only systems of this kind which can be subjected to detailed examination in the sense of the mechanical theory of matter4 ,solutions 7i4e4 unsaturated ones7 form fluid homogeneous systems1 which at the present time can only be investigated with difficulty4 ,in the case of limited solution of liquids in liquids1 the difference between the solvent and the substance dissolved is clearly seen4 ,the former 7that is1 the solvent7 may be added in an unlimited quantity1 and yet the solution obtained will always be uniform1 whilst only a definite saturating proportion of the substance dissolved can be taken1 ,we will take water and common 7sulphuric7 ether4 ,on shaking the ether with the water1 it will be remarked that a portion of it dissolves in the water4 ,if the ether be taken in such a quantity that it saturates the water and a portion of it remains undissolved1 then this remaining portion will act as a solvent1 and water will diffuse through it and also form a saturated solution of water in the ether taken4 ,thus two saturated solutions will be obtained4 ,one solution will contain ether dissolved in water1 and the other solution will contain water dissolved in ether4 ,these two solutions will arrange themselves in two layers1 according to their density2 the ethereal solution of water will be on the top4 ,if the upper ethereal solution be poured off from the aqueous solution1 any quantity of ether may be added to it2 this shows that the dissolving substance is ether4 ,if water be added to it1 it is no longer dissolved in it2 this shows that water saturates the ether--here water is the substance dissolved4 ,if we act in the same manner with the lower layer1 we shall find that water is the solvent and ether the substance dissolved4 ,by taking different amounts of ether and water1 the degree of solubility of ether in water1 and of water in ether1 may be easily determined4 ,water approximately dissolves #a_/aj of its volume of ether1 and ether dissolves a very small quantity of water4 ,let us now imagine that the liquid poured in dissolves a considerable amount of water1 and that water dissolves a considerable amount of the liquid4 ,two layers could not be formed1 because the saturated solutions would resemble each other1 and therefore they would intermix in all proportions4 ,this is1 consequently1 a case of a phenomenon where two liquids present considerable co-efficients of solubility in each other1 but where it is impossible to say what these co-efficients are1 because it is impossible to obtain a saturated solution4 #ba ,the solubility1 or co-efficient of solubility1 of a substance is determined by various methods4 ,either a solution is expressly prepared with a clear excess of the soluble substance and saturated at a given temperature1 and the quantity of water and of the substance dissolved in it determined by evaporation1 desiccation1 or other means2 or else1 as is done with gases1 definite quantities of water and of the soluble substance are taken and the amount remaining undissolved is determined4 ,illustration3 ,,fig4 #af4--;,bunsen's absorptiometer4 ,apparatus for determining the solubility of gases in liquids4 ,the solubility of a gas in water is determined by means of an apparatus called an absorptiometer 7fig4 #af74 ,it consists of an iron stand f1 on which an india-rubber ring rests4 ,a wide glass tube is placed on this ring1 and is pressed down on it by the ring h and the screws i i4 ,the tube is thus firmly fixed on the stand4 ,a cock r1 communicating with a funnel r1 passes into the lower part of the stand4 ,mercury can be poured into the wide tube through this funnel1 which is therefore made of steel1 as copper would be affected by the mercury4 ,the upper ring h is furnished with a cover p1 which can be firmly pressed down on to the wide tube1 and hermetically closes it by means of an india-rubber ring4 ,the tube r r can be raised at will1 and so by pouring mercury into the funnel the height of the column of mercury1 which produces pressure inside the apparatus1 can be increased4 ,the pressure can also be diminished at will1 by letting mercury out through the cock r4 ,a graduated tube e1 containing the gas and liquid to be experimented on1 is placed inside the wide tube4 ,this tube is graduated in millimetres for determining the pressure1 and it is calibrated for volume1 so that the number of volumes occupied by the gas and liquid dissolving it can be easily calculated4 ,this tube can also be easily removed from the apparatus4 ,the lower portion of this tube when removed from the apparatus is shown to the right of the figure4 ,it will be observed that its lower end is furnished with a male screw b1 fitting in a nut a4 ,the lower surface of the nut a is covered with india-rubber1 so that on screwing up the tube its lower end presses upon the india-rubber1 and thus hermetically closes the whole tube1 for its upper end is fused up4 ,the nut a is furnished with arms c c1 and in the stand f there are corresponding spaces1 so that when the screwed-up internal tube is fixed into stand f1 the arms c c fix into these spaces cut in f4 ,this enables the internal tube to be fixed on to the stand f4 ,when the internal tube is fixed in the stand1 the wide tube is put into its right position1 and mercury and water are poured into the space between the two tubes1 and communication is opened between the inside of the tube e and the mercury between the interior and exterior tubes4 ,this is done by either revolving the interior tube e1 or by a key turning the nut about the bottom part of f4 ,the tube e is filled with gas and water as follows3 the tube is removed from the apparatus1 filled with mercury1 and the gas to be experimented on is passed into it4 ,the volume of the gas is measured1 the temperature and pressure determined1 and the volume it would occupy at #j and #gfj mm4 calculated4 ,a known volume of water is then introduced into the tube4 ,the water must be previously boiled1 so as to be quite freed from air in solution4 ,the tube is then closed by screwing it down on to the india-rubber on the nut4 ,it is then fixed on to the stand f1 mercury and water are poured into the intervening space between it and the exterior tube1 which is then screwed up and closed by the cover p1 and the whole apparatus is left at rest for some time1 so that the tube e1 and the gas in it1 may attain the same temperature as that of the surrounding water1 which is marked by a thermometer k tied to the tube e4 ,the interior tube is then again closed by turning it in the nut1 the cover p again shut1 and the whole apparatus is shaken in order that the gas in the tube e may entirely saturate the water4 ,after several shakings1 the tube e is again opened by turning it in the nut1 and the apparatus is left at rest for a certain time2 it is then closed and again shaken1 and so on until the volume of gas does not diminish after a fresh shaking--that is1 until saturation ensues4 ,observations are then made of the temperature1 the height of the mercury in the interior tube1 and the level of the water in it1 and also of the level of the mercury and water in the exterior tube4 ,all these data are necessary in order to calculate the pressure under which the solution of the gas takes place1 and what volume of gas remains undissolved1 and also the quantity of water which serves as the solvent4 ,by varying the temperature of the surrounding water1 the amount of gas dissolved at various temperatures may be determined4 ,bunsen1 ,carius1 and many others determined the solution of various gases in water1 alcohol1 and certain other liquids1 by means of this apparatus4 ,if in a determination of this kind it is found that n cubic centimetres of water at a pressure h dissolve m cubic centimetres of a given gas1 measured at #j and #gfj mm41 when the temperature under which solution took place was t1 then it follows that at the temperature t the co-efficient of solubility of the gas in #a volume of the liquid will be equal to m_/n #gfj_/;h4 ,this formula is very clearly understood from the fact that the co-efficient of solubility of gases is that quantity measured at #j and #gfj mm41 which is absorbed at a pressure of #gfj mm4 by one volume of a liquid4 ,if n cubic centimetres of water absorb m cubic centimetres of a gas1 then one cubic centimetre absorbs m_/n4 ,if m_/n c4c4 of a gas are absorbed under a pressure of h mm41 then1 according to the law of the variation of solubility of a gas with the pressure1 there would he dissolved1 under a pressure of #gfj mm41 a quantity varying in the same ratio to m_/n as #gfj 3 h4 ,in determining the residual volume of gas its moisture 7note #a7 must be taken into consideration4 ,below are given the number of grams of several substances saturating #ajj grams of water--that is1 their co-efficients of solubility by weight at three different temperatures3-- ---------------------------------------- ----------------------- ,at #j ,at #bj ,at #ajj ---------------------------------------- ----------------------- ,oxygen1 ,o#b #f_/ajjj #d_/ajjj -- ,gases ,carbonic anhydride1 ,,co#b #ce_/ajj #ah_/ajj -- ,ammonia1 ,,nh#c #ijj #eah #gc ,phenol1 ,c#f;,h#f,o #di #eb oo ,liquids ,amyl alcohol1 ,c#e;,h#ab,o #dd #bi -- ,sulphuric acid1 ,h#b,,so#d oo oo oo ,gypsum1 ,ca,s,o#d1b;,h#b,o #a_/e #a_/d #a_/e ,alum1 ,al,k,s#b,o#h1ab;,h#b,o #cc #aed #cege ,solids ,anhydrous sodium sulphate1 #de #bj #dc ,na#b,,so#d ,common ,salt1 ,na,cl #ceg #cfj #cig ,nitre1 ,,kno#c #acc #cag #bdfj ---------------------------------------- ----------------------- ,sometimes a substance is so slightly soluble that it may be considered as insoluble4 ,many such substances are met with both in solids and liquids1 and such a gas as oxygen1 although it does dissolve1 does so in so small a proportion by weight that it might be considered as zero did not the solubility of even so little oxygen play an important part in nature 7as in the respiration of fishes7 and were not an infinitesimal quantity of a gas by weight so easily measured by volume4 ,the sign oo1 which stands on a line with sulphuric acid in the above table1 indicates that it intermixes with water in all proportions4 ,there are many such cases among liquids1 and everybody knows1 for instance1 that spirit 7absolute alcohol7 can be mixed in any proportion with water4 #bb ,just as the existence must he admitted of substances which are completely undecomposable 7chemically7 at the ordinary temperature--and of substances which are entirely non-volatile at such a temperature 7as wood and gold71 although capable of decomposing 7wood7 or volatilising 7gold7 at a higher temperature--so also the existence must be admitted of substances which are totally insoluble in water without some degree of change in their state4 ,although mercury is partially volatile at the ordinary temperature1 there is no reason to think that it and other metals are soluble in water1 alcohol1 or other similar liquids4 ,however1 mercury forms solutions1 as it dissolves other metals4 ,on the other hand1 there are many substances found in nature which are so very slightly soluble in water1 that in ordinary practice they may be considered as insoluble 7for example1 barium sulphate74 ,for the comprehension of that general plan according to which a change of state of substances 7combined or dissolved1 solid1 liquid1 or gaseous7 takes place1 it is very important to make a distinction at this boundary line 7on approaching zero of decomposition1 volatility1 or solubility7 between an insignificant amount and zero1 but the present methods of research and the data at our disposal at the present time only just touch such questions 7by studying the electrical conductivity of dilute solutions and the development of micro-organisms in them74 ,it must be remarked1 besides1 that water in a number of cases does not dissolve a substance as such1 but acts on it chemically and forms a soluble substance4 ,thus glass and many rocks1 especially if taken as powder1 are chemically changed by water1 but are not directly soluble in it4 ,substances which are easily soluble in water bear a certain resemblance to it4 ,thus sugar and salt in many of their superficial features remind one of ice4 ,metals1 which are not soluble in water1 have no points in common with it1 whilst on the other hand they dissolve each other in a molten state1 forming alloys1 just as oily substances dissolve each other2 for example1 tallow is soluble in petroleum and in olive oil1 although they are all insoluble in water4 ,from this it is evident that the analogy of substances forming a solution plays an important part1 and as aqueous and all other solutions are liquids1 there is good reason to believe that in the process of solution solid and gaseous substances change in a physical sense1 passing into a liquid state4 ,these considerations elucidate many points of solution--as1 for instance1 the variation of the co-efficient of solubility with the temperature and the evolution or absorption of heat in the formation of solutions4 ,the solubility--that is1 the quantity of a substance necessary for saturation--varies with the temperature1 and1 further1 with an increase in temperature the solubility of solid substances generally increases1 and that of gases decreases2 this might be expected1 as solid substances by heating1 and gases by cooling1 approach to a liquid or dissolved state4#bc ,a graphic method is often employed to express the variation of solubility with temperature4 ,on the axis of absciss or on a horizontal line1 temperatures are marked out and perpendiculars are raised corresponding with each temperature1 whose length is determined by the solubility of the salt at that temperature--expressing1 for instance1 one part by weight of a salt in #ajj parts of water by one unit of length1 such as a millimetre4 ,by joining the summits of the perpendiculars1 a curve is obtained which expresses the degree of solubility at different temperatures4 ,for solids1 the curve is generally an ascending one--i4e4 recedes from the horizontal line with the rise in temperature4 ,these curves clearly show by their inclination the degree of rapidity of increase in solubility with the temperature4 ,having determined several points of a curve--that is1 having made a determination of the solubility for several temperatures--the solubility at intermediary temperatures may be determined from the form of the curve so obtained2 in this way the empirical law of solubility may be examined4#bd ,the results of research have shown that the solubility of certain salts--as1 for example1 common table salt--varies comparatively little with the temperature2 whilst for other substances the solubility increases by equal amounts for equal increments of temperature4 ,thus1 for example1 for the saturation of #ajj parts of water by potassium chloride there is required at #j1 #bib parts1 at #bj1 #cdg1 at #dj1 #djb1 at #fj1 #deg2 and so on1 for every #aj the solubility increases by #bge parts by weight of the salt4 ,therefore the solubility of the potassium chloride in water may be expressed by a direct equation3 a #bib #jbget1 where a represents the solubility at t4 ,for other salts1 more complicated equations are required4 ,for example1 for nitre3 a #acc #jegdt #jjagagt#b #jjjjjjcft#c1 which shows that when t #j a #acc1 when t #aj a #bjh1 and when t #ajj a #bdfj4 #bc ,beilby 7#ahhc7 experimented on paraffin1 and found that one litre of solid paraffin at #ba weighed #hgd grams1 and when liquid1 at its melting-point #ch1 #ghc grams1 at #di1 #gge grams1 and at #fj1 #gfg grams1 from which the weight of a litre of liquefied paraffin would be #gied grams at #ba if it could remain liquid at that temperature4 ,by dissolving solid paraffin in lubricating oil at #ba ,beilby found that #gief grams occupy one cubic decimetre1 from which he concluded that the solution contained liquefied paraffin4 #bd ,gay-,lussac was the first to have recourse to such a graphic method of expressing solubility1 and he considered1 in accordance with the general opinion1 that by joining up the summits of the ordinates in one harmonious curve it is possible to express the entire change of solubility with the temperature4 ,now1 there are many reasons for doubting the accuracy of such an admission1 for there are undoubtedly critical points in curves of solubility 7for example1 of sodium sulphate1 as shown further on71 and it may be that definite compounds of dissolved substances with water1 in decomposing within known limits of temperature1 give critical points more often than would be imagined2 it may even be1 indeed1 that instead of a continuous curve1 solubility should be expressed--if not always1 then not unfrequently--by straight or broken lines4 ,according to ,ditte1 the solubility of sodium nitrate1 ,na,n,o#c1 is expressed by the following figures per #ajj parts of water3-- #j #d #aj #ae #ba #bi #cf #ea #fh #ffg #gaj #gfc #hjf #heg #ibi #iid #aacf #abea ,in my opinion 7#ahha7 these data should be expressed with exactitude by a straight line1 #fge #jhgt1 which entirely agrees with the results of experiment4 ,according to this the figure expressing the solubility of salt at #j exactly coincides with the composition of a definite chemical compound--,na,n,o#c1g;,h#b,o4 ,the experiments made by ,ditte showed that all saturated solutions between #j and -#aeg have such a composition1 and that at the latter temperature the solution completely solidifies into one homogeneous whole4 ,between #j and -#aeg the solution ,na,n,o#c1g;,h#b,o does not deposit either salt or ice4 ,thus the solubility of sodium nitrate is expressed by a broken straight line4 ,in recent times 7#ahhh7 tard discovered a similar phenomenon in many of the sulphates4 ,brandes1 in #ahcj1 shows a diminution in solubility below #ajj for manganese sulphate4 ,the percentage by weight 7i4e4 per #ajj parts of the solution1 and not of water7 of saturation for ferrous sulphate1 ,fe,s,o#d1 from -#b to #fe #ace #jcghdt--that is1 the solubility of the salt increases4 ,the solubility remains constant from #fe to #ih 7according to ,brandes the solubility then increases2 this divergence of opinion requires proof71 and from #ih to #aej it falls as #ajdce - #jffhet4 ,hence1 at about #aef the solubility should #j1 and this has been confirmed by experiment4 ,i observe1 on my part1 that tard's formula gives #cha p4c4 of salt at #fe and #chh p4c4 at #ib1 and this maximum amount of salt in the solution very nearly corresponds with the composition ,fe,s,o#d1ad;,h#b,o1 which requires #cgf p4c4 ,from what has been said1 it is evident that the data concerning solubility require a new method of investigation1 which should have in view the entire scale of solubility--from the formation of completely solidified solutions 7cryohydrates1 which we shall speak of presently7 to the separation of salts from their solutions1 if this is accomplished at a higher temperature 7for manganese and cadmium sulphates there is an entire separation1 according to tard71 or to the formation of a constant solubility 7for potassium sulphate the solubility1 according to tard1 remains constant from #afc to #bbj and equals #bdi p4c47 7,see ,chapter ,,xiv41 note #ej1 solubility of ,ca,cl#b47 ,curves of solubility give the means of estimating the amount of salt separated by the cooling to a known extent of a solution saturated at a given temperature4 ,for instance1 if #bjj parts of a solution of potassium chloride in water saturated at a temperature of #fj be taken1 and it be asked how much of the salt will be separated by cooling the solution to #j1 if its solubility at #fj #deg and at #j #bib8 ,the answer is obtained in the following manner3 ,at #fj a saturated solution contains #deg parts of potassium chloride per #ajj parts by weight of water1 consequently #adeg parts by weight of the solution contain #deg parts1 or1 by proportion1 #bjj parts by weight of the solution contain #fbg parts of the salt4 ,the amount of salt remaining in solution at #j is calculated as follows2 ,in #bjj grams taken there will be #acgc grams of water2 consequently1 this amount of water is capable of holding only #dja grams of the salt1 and therefore in lowering the temperature from #fj to #j there should separate from the solution #fbg-dja #bbf grams of the dissolved salt4 ,the difference in the solubility of salts1 c41 with a rise or fall of temperature is often taken advantage of1 especially in technical work1 for the separation of salts1 in intermixture from each other4 ,thus a mixture of potassium and sodium chlorides 7this mixture is met with in nature at ,stassfurt7 is separated from a saturated solution by subjecting it alternately to boiling 7evaporation7 and cooling4 ,the sodium chloride separates out in proportion to the amount of water expelled from the solution by boiling1 and is removed1 whilst the potassium chloride separates out on cooling1 as the solubility of this salt rapidly decreases with a lowering in temperature4 ,nitre1 sugar1 and many other soluble substances are purified 7refined7 in a similar manner4 ,although in the majority of cases the solubility of solids increases with the temperature1 yet there are some solid substances whose solubilities decrease on heating4 ,glauber's salt1 or sodium sulphate1 forms a particularly instructive example of the case in question4 ,if this salt be taken in an ignited state 7deprived of its water of crystallisation71 then its solubility in #ajj parts of water varies with the temperature in the following manner3 at #j1 #e parts of the salt form a saturated solution2 at #bj1 #bj parts of the salt1 at #cc more than #ej parts4 ,the solubility1 as will be seen1 increases with the temperature1 as is the case with nearly all salts2 but starting from #cc it suddenly diminishes1 and at a temperature of #dj1 less than #ej parts of the salt dissolve1 at #fj only #de parts of the salt1 and at #ajj about #dc parts of the salt in #ajj parts of water4 ,this phenomenon may be traced to the following facts3 ,firstly1 that this salt forms various compounds with water1 as will be afterwards explained2 secondly1 that at #cc the compound ,na#b,,so#d #aj;,h#b,o formed from the solution at lower temperatures1 melts2 and thirdly1 that on evaporation at a temperature above #cc an anhydrous salt1 ,na#b,,so#d separates out4 ,it will be seen from this example how complicated such an apparently simple phenomenon as solution really is2 and all data concerning solutions lead to the same conclusion4 ,this complexity becomes evident in investigating the heat of solution4 ,if solution consisted of a physical change only1 then in the solution of gases there would be evolved--and in the solution of solids1 there would be absorbed--just that amount of heat corresponding to the change of state2 but in reality a large amount of heat is always evolved in solution1 depending on the fact that in the process of solution chemical combination takes place accompanied by an evolution of heat4 ,seventeen grams of ammonia 7this weight corresponds with its formula ,,nh#c71 in passing from a gaseous into a liquid state1 evolve #d1djj units of heat 7latent heat72 that is1 the quantity of heat necessary to raise the temperature of #d1djj grams of water #a4 ,the same quantity of ammonia1 in dissolving in an excess of water1 evolves twice as much heat--namely #h1hjj units--showing that the combination with water is accompanied by the evolution of #d1djj units of heat4 ,further1 the chief part of this heat is separated in dissolving in small quantities of water1 so that #ag grams of ammonia1 in dissolving in #ah grams of water 7this weight corresponds with its composition ,h#b,o71 evolve #g1ece units of heat1 and therefore the formation of the solution ,,nh#c ,h#b,o evolves #c1ace units of heat beyond that due to the change of state4 ,as in the solution of gases1 the heat of liquefaction 7of physical change of state7 and of chemical combination with water are both positive 771 therefore in the solution of gases in water a heat effect is always observed4 ,this phenomenon is different in the solution of solid substances1 because their passage from a solid to a liquid state is accompanied by an absorption of heat 7negative1-heat71 whilst their chemical combination with water is accompanied by an evolution of heat 7 heat72 consequently1 their sum may either be a cooling effect1 when the positive 7chemical7 portion of heat is less than the negative 7physical71 or it may be1 on the contrary1 a heating effect4 ,this is actually the case4 #abd grams of sodium thiosulphate 7employed in photography7 ,na#b,s#b,o#c1e;,h#b,o in melting 7at #dh7 absorbs #i1gjj units of heat1 but in dissolving in a large quantity of water at the ordinary temperature it absorbs #e1gjj units of heat1 which shows the evolution of heat 7about #d1jjj units71 notwithstanding the cooling effect observed in the process of solution1 in the act of the chemical combination of the salt with water4#be ,but in most cases solid substances in dissolving in water evolve heat1 notwithstanding the passage into a liquid state1 which indicates so considerable an evolution of 77 heat in the act of combination with water that it exceeds the absorption of 7-7 heat dependent on the passage into a liquid state1 ,thus1 for instance1 calcium chloride1 ,ca,cl#b1 magnesium sulphate1 ,mg,s,o#d1 and many other salts evolve heat in dissolving2 for example1 #fj grams of magnesium sulphate evolve about #aj1jjj units of heat4 ,therefore1 in the solution of solid bodies either a cooling#bf or a heating#bg effect is produced1 according to the difference of the reacting affinities4 ,when they are considerable--that is1 when water is with difficulty separated from the resultant solution1 and only with a rise of temperature 7such substances absorb water vapour7--then much heat is evolved in the process of solution1 just as in many reactions of direct combination1 and therefore a considerable heating of the solution is observed4 ,of such a kind1 for instance1 is the solution of sulphuric acid 7oil of vitriol ,h#b,,so#d71 and of caustic soda 7,na,h,o71 c41 in water4#bh #be ,the latent heat of fusion is determined at the temperature of fusion1 whilst solution takes place at the ordinary temperature1 and one must think that at this temperature the latent heat would be different1 just as the latent heat of evaporation varies with the temperature 7see ,note #aa74 ,besides which1 in dissolving1 disintegration of the particles of both the solvent and the substance dissolved takes place1 a process which in its mechanical aspect resembles evaporation1 and therefore must consume much heat4 ,the heat emitted in the solution of a solid must therefore be considered 7,personne7 as composed of three factors--7#a7 positive1 the effect of combination2 7#b7 negative1 the effect of transference into a liquid state2 and 7#c7 negative1 the effect of disintegration4 ,in the solution of a liquid by a liquid the second factor is removed2 and therefore1 if the heat evolved in combination is greater than that absorbed in disintegration a heating effect is observed1 and in the reverse case a cooling effect2 and1 indeed1 sulphuric acid1 alcohol1 and many liquids evolve heat in dissolving in each other4 ,but the solution of chloroform in carbon bisulphide 7,bussy and ,binget71 or of phenol 7or aniline7 in water 7,alexeff71 produces cold4 ,in the solution of a small quantity of water in acetic acid 7,abasheff71 or hydrocyanic acid 7,bussy and ,binget71 or amyl alcohol 7,alexeff71 cold is produced1 whilst in the solution of these substances in an excess of water heat is evolved4 ,the relation existing between the solubility of solid bodies and the heat and temperature of fusion and solution has been studied by many investigators1 and more recently 7#ahic7 by ,schrder1 who states that in the solution of a solid body in a solvent which does not act chemically upon it1 a very simple process takes place1 which differs but little from the intermixture of two gases which do not react chemically upon each other4 ,the following relation between the heat of solution ,q and the heat of fusion p may then be taken3 ,p_/,t#j ,q_/,t constant1 where ,t#j and ,t are the absolute 7from -#bgc7 temperatures of fusion and saturation4 ,thus1 for instance1 in the case of naphthalene the calculated and observed magnitudes of the heat of solution differ but slightly from each other4 ,the fullest information concerning the solution of liquids in liquids has been gathered by ,w4 ,t4 ,alexeff 7#ahhc-ahhe72 these data are1 however1 far from being sufficient to solve the mass of problems respecting this subject4 ,he showed that two liquids which dissolve in each other1 intermix together in all proportions at a certain temperature4 ,thus the solubility of phenol1 ,c#f;,h#f,o1 in water1 and the converse1 is limited up to #gj1 whilst above this temperature they intermix in all proportions4 ,this is seen from the following figures1 where p is the percentage amount of phenol and t the temperature at which the solution becomes turbid--that is1 that at which it is saturated3-- p #gab #ajbj #aeca #bfae #bhee #cfgj #dhhf #faae #gaig t #a #de #fj #fg #fg #fg #fe #ec #bj ,it is exactly the same with the solution of benzene1 aniline1 and other substances in molten sulphur4 ,alexeff discovered a similar complete intermixture for solutions of secondary butyl alcohol in water at about #ajg2 at lower temperatures the solubility is not only limited1 but between #ej and #gj it is at its minimum1 both for solutions of the alcohol in water and for water in the alcohol2 and at a temperature of #e both solutions exhibit a fresh change in their scale of solubility1 so that a solution of the alcohol in water which is saturated between #e and #dj will become turbid when heated to #fj4 ,in the solution of liquids in liquids1 ,alexeff observed a lowering in temperature 7an absorption of heat7 and an absence of change in specific heat 7calculated for the mixture7 much more frequently than had been done by previous observers4 ,as regards his hypothesis 7in the sense of a mechanical and not a chemical representation of solutions7 that substances in solution preserve their physical states 7as gases1 liquids1 or solids71 it is very doubtful1 for it would necessitate admitting the presence of ice in water or its vapour4 ,from what has been said above1 it will be clear that even in so very simple a case as solution1 it is impossible to calculate the heat emitted by chemical action alone1 and that the chemical process cannot be separated from the physical and mechanical4 #bf ,the cooling effect produced in the solution of solids 7and also in the expansion of gases and in evaporation7 is applied to the production of low temperatures4 ,ammonium nitrate is very often used for this purpose2 in dissolving in water it absorbs #gg units of heat per each part by weight4 ,on evaporating the solution thus formed1 the solid salt is re-obtained4 ,the application of the various freezing mixtures is based on the same principle4 ,snow or broken ice frequently enters into the composition of these mixtures1 advantage being taken of its latent heat of fusion in order to obtain the lowest possible temperature 7without altering the pressure or employing heat1 as in other methods of obtaining a low temperature74 ,for laboratory work recourse is most often had to a mixture of three parts of snow and one part of common salt1 which causes the temperature to fall from #j to -#ba ,c4 ,potassium thiocyanate1 ,,kcns1 mixed with water 7#c_/d by weight of the salt7 gives a still lower temperature4 ,by mixing ten parts of crystallised calcium chloride1 ,ca,cl#b1f;,h#b,o1 with seven parts of snow1 the temperature may even fall from #j to -#ee4 #bg ,the heat which is evolved in solution1 or even in the dilution of solutions1 is also sometimes made use of in practice4 ,thus caustic soda 7,na,h,o71 in dissolving or on the addition of water to a strong solution of it1 evolves so much heat that it can replace fuel4 ,in a steam boiler1 which has been previously heated to the boiling point1 another boiler is placed containing caustic soda1 and the exhaust steam is made to pass through the latter2 the formation of steam then goes on for a somewhat long period of time without any other heating4 ,norton makes use of this for smokeless street locomotives4 #bh ,illustration3 ,,fig4 #ag4--;,curves expressing the contraction1 quantity of heat1 and rises of temperature produced by mixing sulphuric acid with water4 ,percentage of ,h#b,,so#d is given along the axis of abscissae4 ,the temperatures obtained by mixing monohydrated sulphuric acid1 ,h#b,,so#d1 with different quantities of water1 are shown on the lowest curve in fig4 #ag1 the relative proportions of both substances being expressed in percentages by weight along the horizontal axis4 ,the greatest rise of temperature is #adi4 ,it corresponds with the greatest evolution of heat 7given on the middle curve7 corresponding with a definite volume 7#ajj c4c47 of the solution produced4 ,the top curve expresses the degree of contraction1 which also corresponds with #ajj volumes of the solution produced4 ,the greatest contraction1 as also the greatest rise of temperature1 corresponds with the formation of a trihydrate1 ,h#b,,so#d1b;,h#b,o 7 #gca p4c4 ,h#b,,so#d71 which very likely repeats itself in a similar form in other solutions1 although all the phenomena 7of contraction1 evolution of heat1 and rise of temperature7 are very complex and are dependent on many circumstances4 ,one would think1 however1 judging from the above examples1 that all other influences are feebler in their action than chemical attraction1 especially when it is so considerable as between sulphuric acid and water4 ,solution is a reversible reaction2 for1 if the water be expelled from a solution1 the substance originally taken is obtained again4 ,but it must be borne in mind that the expulsion of the water taken for solution is not always accomplished with equal facility1 because water has different degrees of chemical affinity for the substance dissolved4 ,thus1 if a solution of sulphuric acid1 which mixes with water in all proportions1 be heated1 it will be found that very different degrees of heat are required to expel the water4 ,when it is in a large excess1 water is given off at a temperature slightly above #ajj1 but if it be in but a small proportion there is such an affinity between it and the sulphuric acid that at #abj1 #aej1 #bjj1 and even at #cjj1 water is still retained by the sulphuric acid4 ,the bond between the remaining quantity of water and the sulphuric acid is evidently stronger than the bond between the sulphuric acid and the excess of water4 ,the force acting in solutions is consequently of different intensity1 starting from so feeble an attraction that the properties of water--as1 for instance1 its power of evaporation--are but very little changed1 and ending with cases of strong attraction between the water and the substance dissolved in or chemically combined with it4 ,in consideration of the very important significance of the phenomena1 and of the cases of the breaking up of solutions with separation of water or of the substance dissolved from them1 we shall further discuss them separately1 after having acquainted ourselves with certain peculiarities of the solution of gases and of solid bodies4 ,the solubility of gases1 which is usually measured by the volume of gas#bi 7at #j and #gfj mm4 pressure7 per #ajj volumes of water1 varies not only with the nature of the gas 7and also of the solvent71 and with the temperature1 but also with the pressure1 because gases themselves change their volume considerably with the pressure4 ,as might be expected1 7#a7 gases which are easily liquefied 7by pressure and cold7 are more soluble than those which are liquefied with difficulty4 ,thus1 in #ajj volumes of water only two volumes of hydrogen dissolve at #j and #gfj mm41 three volumes of carbonic oxide1 four volumes of oxygen1 c41 for these are gases which are liquefied with difficulty2 whilst there dissolve #ahj volumes of carbonic anhydride1 #acj of nitrous oxide1 and #dcg of sulphurous anhydride1 for these are gases which are rather easily liquefied4 7#b7 ,the solubility of a gas is diminished by heating1 which is easily intelligible from what has been said previously--the elasticity of a gas becomes greater1 it is removed further from a liquid state4 ,thus #ajj volumes of water at #j dissolve #be volumes of air1 and at #bj only #ag volume4 ,for this reason cold water1 when brought into a warm room1 parts with a portion of the gas dissolved in it4#cj 7#c7 ,the quantity of the gas dissolved varies directly with the pressure4 ,this rule is called the law of ,henry and ,dalton1 and is applicable to those gases which are little soluble in water4 ,therefore a gas is separated from its solution in water in a vacuum1 and water saturated with a gas under great pressure parts with it if the pressure be diminished4 ,thus many mineral springs are saturated underground with carbonic anhydride under the great pressure of the column of water above them4 ,on coming to the surface1 the water of these springs boils and foams on giving up the excess of dissolved gas4 ,sparkling wines and arated waters are saturated under pressure with the same gas4 ,they hold the gas so long as they are in a well-corked vessel4 ,when the cork is removed and the liquid comes in contact with air at a lower pressure1 part of the gas1 unable to remain in solution at a lower pressure1 is separated as froth with the hissing sound familiar to all4 ,it must be remarked that the law of ,henry and ,dalton belongs to the class of approximate laws1 like the laws of gases 7,gay-,lussac's and ,mariotte's7 and many others--that is1 it expresses only a portion of a complex phenomenon1 the limit towards which the phenomenon aims4 ,the matter is rendered complicated from the influence of the degree of solubility and of affinity of the dissolved gas for water4 ,gases which are little soluble--for instance1 hydrogen1 oxygen1 and nitrogen--follow the law of ,henry and ,dalton the most closely4 ,carbonic anhydride exhibits a decided deviation from the law1 as is seen from the determinations of ,wroblewski 7#ahhb74 ,he showed that at #j a cubic centimetre of water absorbs #ah cubic centimetre of the gas under a pressure of one atmosphere2 under #aj atmospheres1 #af cubic centimetres 7and not #ah1 as it should be according to the law72 under #bj atmospheres1 #bff cubic centimetres 7instead of #cf71 and under #cj atmospheres1 #ccg cubic centimetres4#ca ,however1 as the researches of ,sechenoff show1 the absorption of carbonic anhydride within certain limits of change of pressure1 and at the ordinary temperature1 by water--and even by solutions of salts which are not chemically changed by it1 or do not form compounds with it--very closely follows the law of ,henry and ,dalton1 so that the chemical bond between this gas and water is so feeble that the breaking up of the solution with separation of the gas is accomplished by a decrease of pressure alone4#cb ,the case is different if a considerable affinity exists between the dissolved gas and water4 ,then it might even be expected that the gas would not be entirely separated from water in a vacuum1 as should be the case with gases according to the law of ,henry and ,dalton4 ,such gases--and1 in general1 all those which are very soluble--exhibit a distinct deviation from the law of ,henry and ,dalton4 ,as examples1 ammonia and hydrochloric acid gas may be taken4 ,the former is separated by boiling and decrease of pressure1 while the latter is not1 but they both deviate distinctly from the law4 ---------------------------------------- ------------ ,pressure in mm4,ammonia dissolved ,hydrochloric acid of mercury in #ajj grams of gas dissolved in #ajj water at #j grams of water at #j ---------------------------------------- ------------ ,grams ,grams #ajj #bhj #feg #ejj #fib #ghb #a1jjj #aabf #hef #a1ejj #afef -- ---------------------------------------- ------------ #bi ,if a volume of gas v be measured under a pressure of h mm4 of mercury 7at #j7 and at a temperature t ,centigrade1 then1 according to the combined laws of ,boyle1 ,mariotte1 and of ,gay-,lussac1 its volume at #j and #gfj mm4 will equal the product of v into #gfj divided by the product of h into #a at1 where a is the co-efficient of expansion of gases1 which is equal to #jjjcfg4 ,the weight of the gas will be equal to its volume at #j and #gfj mm4 multiplied by its density referred to air and by the weight of one volume of air at #j and #gfj mm4 ,the weight of one litre of air under these conditions being #abic gram4 ,if the density of the gas be given in relation to hydrogen this must be divided by #add to bring it in relation to air4 ,if the gas be measured when saturated with aqueous vapour1 then it must be reduced to the volume and weight of the gas when dry1 according to the rules given in ,note #a4 ,if the pressure be determined by a column of mercury having a temperature t1 then by dividing the height of the column by #a #jjjjaht the corresponding height at #j is obtained4 ,if the gas be enclosed in a tube in which a liquid stands above the level of the mercury1 the height of the column of the liquid being ,h and its density ,d1 then the gas will be under a pressure which is equal to the barometric pressure less ,,hd_/#acei1 where #acei is the density of mercury4 ,by these methods the quantity of a gas is determined1 and its observed volume reduced to normal conditions or to parts by weight4 ,the physical data concerning vapours and gases must be continually kept in sight in dealing with and measuring gases4 ,the student must become perfectly familiar with the calculations relating to gases4 #cj ,according to ,bunsen1 ,winkler1 ,timofeeff1 and others1 #ajj vols4 of water under a pressure of one atmosphere absorb the following volumes of gas 7measured at #j and #gfj mm473-- #a #b #c #d #e #f #g #h #i #aj #aa #j #dhb #bce #bae #agig #ced #acje #dcga #fhhf #ed #ajdifj #gch #bj #caj #aed #ahc #ija #bcb #fgj #bije #cfbb #ce #fedjj #dga #a1 oxygen2 #b1 nitrogen2 #c1 hydrogen2 #d1 carbonic anhydride2 #e1 carbonic oxide2 #f1 nitrous oxide2 #g1 hydrogen sulphide2 #h1 sulphurous anhydride2 #i1 marsh gas2 #aj1 ammonia2 #aa1 nitric oxide4 ,the decrease of solubility with a rise of temperature varies for different gases2 it is greater1 the greater the molecular weight of the gas4 ,it is shown by calculation that this decrease varies 7,winkler7 as the cube root of the molecular weight of the gas4 ,this is seen from the following table3 ---------------------------------------- -- ,decrease of ,cube root of ,ratio between solubility molecular decrease and per #bj in weight4 cube root of per cent4 mol4 wt4 ---------------------------------------- -- ,h#b #aecb #abei #abag ,n#b #cdcc #cjcg #aacj ,,co #cddd #cjcg #aacd ,,no #cfbd #cajg #aaff ,o#b #cfee #cage #aaea ---------------------------------------- -- ,the decrease in the coefficient of absorption with the temperature must be connected with a change in the physical properties of the water4 ,winkler 7#ahia7 remarked a certain relation between the internal friction and the coefficient of absorption at various temperatures4 #ca ,these figures show that the co-efficient of solubility decreases with an increase of pressure1 notwithstanding that the carbonic anhydride approaches a liquid state4 ,as a matter of fact1 liquefied carbonic anhydride does not intermix with water1 and does not exhibit a rapid increase in solubility at its temperature of liquefaction4 ,this indicates1 in the first place1 that solution does not consist in liquefaction1 and in the second place that the solubility of a substance is determined by a peculiar attraction of water for the substance dissolving4 ,wroblewski even considered it possible to admit that a dissolved gas retains its properties as a gas4 ,this he deduced from experiments1 which showed that the rate of diffusion of gases in a solvent is1 for gases of different densities1 inversely proportional to the square roots of their densities1 just as the velocities of gaseous molecules 7see ,note #cd74 ,wroblewski showed the affinity of water1 ,h#b,o1 for carbonic anhydride1 ,,co#b1 from the fact that on expanding moist compressed carbonic anhydride 7compressed at #j under a pressure of #aj atmospheres7 he obtained 7a fall in temperature takes place from the expansion7 a very unstable definite crystalline compound1 ,,co#b #h;,h#b,o4 #cb ,as1 according to the researches of ,roscoe and his collaborators1 ammonia exhibits a considerable deviation at low temperatures from the law of ,henry and ,dalton1 whilst at #ajj the deviation is small1 it would appear that the dissociating influence of temperature affects all gaseous solutions2 that is1 at high temperatures1 the solutions of all gases will follow the law1 and at lower temperatures there will in all cases be a deviation from it4 ,it will be remarked1 for instance1 from this table that whilst the pressure increased #aj times1 the solubility of ammonia only increased #d-a_/b times4 ,a number of examples of such cases of the absorption of gases by liquids might be cited which do not in any way1 even approximately1 agree with the laws of solubility4 ,thus1 for instance1 carbonic anhydride is absorbed by a solution of caustic potash in water1 and if sufficient caustic potash be present it is not separated from the solution by a decrease of pressure4 ,this is a case of more intimate chemical combination4 ,a correlation less completely studied1 but similar and clearly chemical1 appears in certain cases of the solution of gases in water1 and we shall afterwards find an example of this in the solution of hydrogen iodide2 but we will first stop to consider a remarkable application of the law of ,henry and ,dalton#cc in the case of the solution of a mixture of two gases1 and this we must do all the more because the phenomena which there take place cannot be foreseen without a clear theoretical representation of the nature of gases4#cd #cc ,the ratio between the pressure and the amount of gas dissolved was discovered by ,henry in #ahje1 and ,dalton in #ahjg pointed out the adaptability of this law to cases of gaseous mixtures1 introducing the conception of partial pressures which is absolutely necessary for a right comprehension of ,dalton's law4 ,the conception of partial pressures essentially enters into that of the diffusion of vapours in gases 7footnote #a72 for the pressure of damp air is equal to the sum of the pressures of dry air and of the aqueous vapour in it1 and it is admitted as a corollary to ,dalton's law that evaporation in dry air takes place as in a vacuum4 ,it is1 however1 necessary to remark that the volume of a mixture of two gases 7or vapours7 is only approximately equal to the sum of the volumes of its constituents 7the same1 naturally1 also refers to their pressures7--that is to say1 in mixing gases a change of volume occurs1 which1 although small1 is quite apparent when carefully measured4 ,for instance1 in #ahhh ,brown showed that on mixing various volumes of sulphurous anhydride 7,,so#b7 with carbonic anhydride 7at equal pressures of #gfj mm4 and equal temperatures7 a decrease of pressure of #ci millimetres of mercury was observed4 ,the possibility of a chemical action in similar mixtures is evident from the fact that equal volumes of sulphurous and carbonic anhydrides at -#ai form1 according to ,pictet's researches in #ahhh1 a liquid which may be regarded as an unstable chemical compound1 or a solution similar to that given when sulphurous anhydride and water combine to an unstable chemical whole4 #cd ,the origin of the kinetic theory of gases now generally accepted1 according to which they are animated by a rapid progressive motion1 is very ancient 7,bernouilli and others in the last century had already developed a similar representation71 but it was only generally accepted after the mechanical theory of heat had been established1 and after the work of ,krnig 7#ahee71 and especially after its mathematical side had been worked out by ,clausius and ,maxwell4 ,the pressure1 elasticity1 diffusion1 and internal friction of gases1 the laws of ,boyle1 ,mariotte1 and of ,gay-,lussac and ,avogadro-,gerhardt are not only explained 7deduced7 by the kinetic theory of gases1 but also expressed with perfect exactitude2 thus1 for example1 the magnitude of the internal friction of different gases was foretold with exactitude by ,maxwell1 by applying the theory of probabilities to the impact of gaseous particles4 ,the kinetic theory of gases must therefore be considered as one of the most brilliant acquisitions of the latter half of the present century4 ,the velocity of the progressive motion of the particles of a gas1 one cubic centimetre of which weighs d grams1 is found1 according to the theory1 to be equal to the square root of the product of #cp,dq divided by d1 where p is the pressure under which d is determined expressed in centimetres of the mercury column1 ,d the weight of a cubic centimetre of mercury in grams 7,d #acei1 p #gf1 consequently the normal pressure #a1jcc grams on a sq4 cm471 and g the acceleration of gravity in centimetres 7g #ihje1 at the sea level and long4 #de #ihaib at ,st4 ,petersburg2 in general it varies with the longitude and altitude of the locality74 ,therefore1 at #j the velocity of hydrogen is #a1hdc1 and of oxygen #dfa1 metres per second4 ,this is the average velocity1 and 7according to ,maxwell and others7 it is probable that the velocities of individual particles are different2 that is1 they occur in1 as it were1 different conditions of temperature1 which it is very important to take into consideration in investigating many phenomena proper to matter4 ,it is evident from the above determination of the velocity of gases1 that different gases at the same temperature and pressure have average velocities1 which are inversely proportional to the square roots of their densities2 this is also shown by direct experiment on the flow of gases through a fine orifice1 or through a porous wall4 ,this dissimilar velocity of flow for different gases is frequently taken advantage of in chemical researches 7see ,chap4 ,,ii4 and also ,chap4 ,,vii47 in order to separate two gases having different densities and velocities4 ,the difference of the velocity of flow of gases also determines the phenomenon cited in the following footnote for demonstrating the existence of an internal motion in gases4 ,if for a certain mass of a gas which fully and exactly follows the laws of ,mariotte and ,gay-,lussac the temperature t and the pressure p be changed simultaneously1 then the entire change would be expressed by the equation pv ,c7#a at71 or1 what is the same1 pv ,,rt1 where ,t t #bgc and ,c and ,r are constants which vary not only with the units taken but with the nature of the gas and its mass4 ,but as there are discrepancies from both the fundamental laws of gases 7which will be discussed in the following chapter71 and as1 on the one hand1 a certain attraction between the gaseous molecules must be admitted1 while on the other hand the molecules of gases themselves must occupy a portion of a space1 hence for ordinary gases1 within any considerable variation of pressure and temperature1 recourse should be had to ,van der ,waal's formula-- 7p a_/v#b77v-p7 ,r7#a at7 where a is the true co-efficient of expansion of gases4 ,the formula of ,van der ,waals has an especially important significance in the case of the passage of a gas into a liquid state1 because the fundamental properties of both gases and liquids are equally well expressed by it1 although only in their general features4 ,the further development of the questions referring to the subjects here touched on1 which are of especial interest for the theory of solutions1 must be looked for in special memoirs and works on theoretical and physical chemistry4 ,a small part of this subject will be partially considered in the footnotes of the following chapter4 ,the law of partial pressures is as follows3--,the solubility of gases in intermixture with each other does not depend on the influence of the total pressure acting on the mixture1 but on the influence of that portion of the total pressure which is due to the volume of each given gas in the mixture4 ,thus1 for instance1 if oxygen and carbonic anhydride were mixed in equal volumes and exerted a pressure of #gfj millimetres1 then water would dissolve so much of each of these gases as would be dissolved if each separately exerted a pressure of half an atmosphere1 and in this case1 at #j one cubic centimetre of water would dissolve #jjb cubic centimetre of oxygen and #jij cubic centimetre of carbonic anhydride4 ,if the pressure of a gaseous mixture equals h1 and in n volumes of the mixture there be a volumes of a given gas1 then its solution will proceed as though this gas were dissolved under a pressure 7h a7_/n4 ,that portion of the pressure under influence of which the solution proceeds is termed the 'partial' pressure4 ,in order to clearly understand the cause of the law of partial pressures1 an explanation must be given of the fundamental properties of gases4 ,gases are elastic and disperse in all directions4 ,we are led from what we know of gases to the assumption that these fundamental properties of gases are due to a rapid progressive motion1 in all directions1 which is proper to their smallest particles 7molecules74#ce ,these molecules in impinging against an obstacle produce a pressure4 ,the greater the number of molecules impinging against an obstacle in a given time1 the greater the pressure4 ,the pressure of a separate gas or of a gaseous mixture depends on the sum of the pressures of all the molecules1 on the number of blows in a unit of time on a unit of surface1 and on the mass and velocity 7or the vis viva7 of the impinging molecules4 ,the nature of the different molecules is of no account2 the obstacle is acted on by a pressure due to the sum of their vis viva4 ,but1 in a chemical action such as the solution of gases1 the nature of the impinging molecules plays1 on the contrary1 the most important part4 ,in impinging against a liquid1 a portion of the gas enters into the liquid itself1 and is held by it so long as other gaseous molecules impinge against the liquid--exert a pressure on it4 ,as regards the solubility of a given gas1 for the number of blows it makes on the surface of a liquid1 it is immaterial whether other molecules of gases impinge side by side with it or not4 ,hence1 the solubility of a given gas will be proportional1 not to the total pressure of a gaseous mixture1 but to that portion of it which is due to the given gas separately4 ,moreover1 the saturation of a liquid by a gas depends on the fact that the molecules of gases that have entered into a liquid do not remain at rest in it1 although they enter in a harmonious kind of motion with the molecules of the liquid1 and therefore they throw themselves off from the surface of the liquid 7just like its vapour if the liquid be volatile74 ,if in a unit of time an equal number of molecules penetrate into 7leap into7 a liquid and leave 7or leap out of7 a liquid1 it is saturated4 ,it is a case of mobile equilibrium1 and not of rest4 ,therefore1 if the pressure be diminished1 the number of molecules departing from the liquid will exceed the number of molecules entering into the liquid1 and a fresh state of mobile equilibrium only takes place under a fresh equality of the number of molecules departing from and entering into the liquid4 ,in this manner the main features of the solution are explained1 and furthermore of that special 7chemical7 attraction 7penetration and harmonious motion7 of a gas for a liquid1 which determines both the measure of solubility and the degree of stability of the solution produced4 #ce ,although the actual motion of gaseous molecules1 which is accepted by the kinetic theory of gases1 cannot be seen1 yet its existence may be rendered evident by taking advantage of the difference in the velocities undoubtedly belonging to different gases which are of different densities under equal pressures4 ,the molecules of a light gas must move more rapidly than the molecules of a heavier gas in order to produce the same pressure4 ,let us take1 therefore1 two gases--hydrogen and air2 the former is #add times lighter than the latter1 and hence the molecules of hydrogen must move almost four times more quickly than air 7more exactly #ch1 according to the formula given in the preceding footnote74 ,consequently1 if a porous cylinder containing air is introduced into an atmosphere of hydrogen1 then in a given time the volume of hydrogen which succeeds in entering the cylinder will be greater than the volume of air leaving the cylinder1 and therefore the pressure inside the cylinder will rise until the gaseous mixture 7of air and hydrogen7 attains an equal density both inside and outside the cylinder4 ,if now the experiment be reversed and air surround the cylinder1 and hydrogen be inside the cylinder1 then more gas will leave the cylinder than enters it1 and hence the pressure inside the cylinder will be diminished4 ,in these considerations we have replaced the idea of the number of molecules by the idea of volumes4 ,we shall learn subsequently that equal volumes of different gases contain an equal number of molecules 7the law of ,avogadro-,gerhardt71 and therefore instead of speaking of the number of molecules we can speak of the number of volumes4 ,if the cylinder be partially immersed in water the rise and fall of the pressure can be observed directly1 and the experiment consequently rendered self-evident4 ,the consequences of the law of partial pressures are exceedingly numerous and important4 ,all liquids in nature are in contact with the atmosphere1 which1 as we shall afterwards see more fully1 consists of an intermixture of gases1 chiefly four in number--oxygen1 nitrogen1 carbonic anhydride1 and aqueous vapour4 #ajj volumes of air contain1 approximately1 #gh volumes of nitrogen1 and about #ba volumes of oxygen2 the quantity of carbonic anhydride1 by volume1 does not exceed #jje4 ,under ordinary circumstances1 the quantity of aqueous vapour is much greater than this1 but it varies of course with climatic conditions4 ,we conclude from these numbers that the solution of nitrogen in a liquid in contact with the atmosphere will proceed under a partial pressure of 7#gh_/ajj7 #gfj mm4 if the atmospheric pressure equal #gfj mm42 similarly1 under a pressure of #fjj mm4 of mercury1 the solution of oxygen will proceed under a partial pressure of about #afj mm41 and the solution of carbonic anhydride only under the very small pressure of #jd mm4 ,as1 however1 the solubility of oxygen in water is twice that of nitrogen1 the ratio of ,o to ,n dissolved in water will be greater than the ratio in air4 ,it is easy to calculate what quantity of each of the gases will be contained in water1 and taking the simplest case we will calculate what quantity of oxygen1 nitrogen1 and carbonic anhydride will be dissolved from air having the above composition at #j and #gfj mm4 pressure4 ,under a pressure of #gfj mm4 #a cubic centimetre of water dissolves #jjbjc cubic centimetre of nitrogen or under the partial pressure of #fjj mm4 it will dissolve #jjbjc #fjj_/gfj1 or #jjafj cubic centimetre2 of oxygen #jjdaa #afj_/gfj1 or #jjjhf cubic centimetre2 of carbonic anhydride #ah #jd_/gfj or #jjjjie cubic centimetre3 hence1 #ajj cubic centimetres of water will contain at #j altogether #bee cubic centimetres of atmospheric gases1 and #ajj volumes of air dissolved in water will contain about #fb p4c4 of nitrogen1 #cd p4c4 of oxygen1 and #d p4c4 of carbonic anhydride4 ,the water of rivers1 wells1 c4 usually contains more carbonic anhydride4 ,this proceeds from the oxidation of organic substances falling into the water4 ,the amount of oxygen1 however1 dissolved in water appears to be actually about #a_/c the dissolved gases1 whilst air contains only #a_/e of it by volume4 ,according to the law of partial pressures1 whatever gas be dissolved in water will be expelled from the solution in an atmosphere of another gas4 ,this depends on the fact that gases dissolved in water escape from it in a vacuum1 because the pressure is nil4 ,an atmosphere of another gas acts like a vacuum on a gas dissolved in water4 ,separation then proceeds1 because the molecules of the dissolved gas no longer impinge upon the liquid1 are not dissolved in it1 and those previously held in solution leave the liquid in virtue of their elasticity4#cf ,for the same reason a gas may be entirely expelled from a gaseous solution by boiling--at least1 in many cases when it does not form particularly stable compounds with water4 ,in fact the surface of the boiling liquid will be occupied by aqueous vapour1 and therefore all the pressure acting on the gas will be due to the aqueous vapour4 ,on this account1 the partial pressure of the dissolved gas will be very inconsiderable1 and this is the sole reason why a gas separates from a solution on boiling the liquid containing it4 ,at the boiling point of water the solubility of gases in water is still sufficiently great for a considerable quantity of a gas to remain in solution4 ,the gas dissolved in the liquid is carried away1 together with the aqueous vapour2 if boiling be continued for a long time1 all the gas will finally be separated4#cg #cf ,here two cases occur2 either the atmosphere surrounding the solution may be limited1 or it may be proportionally so vast as to be unlimited1 like the earth's atmosphere4 ,if a gaseous solution be brought into an atmosphere of another gas which is limited--for instance1 as in a closed vessel--then a portion of the gas held in solution will be expelled1 and thus pass over into the atmosphere surrounding the solution1 and will produce its partial pressure4 ,let us imagine that water saturated with carbonic anhydride at #j and under the ordinary pressure is brought into an atmosphere of a gas which is not absorbed by water2 for instance1 that #aj c4c4 of an aqueous solution of carbonic anhydride is introduced into a vessel holding #aj c4c4 of such a gas4 ,the solution will contain #ah c4c4 of carbonic anhydride4 ,the expulsion of this gas proceeds until a state of equilibrium is arrived at4 ,the liquid will then contain a certain amount of carbonic anhydride1 which is retained under the partial pressure of that gas which has been expelled4 ,now1 how much gas will remain in the liquid and how much will pass over into the surrounding atmosphere8 ,in order to solve this problem1 let us suppose that x cubic centimetres of carbonic anhydride are retained in the solution4 ,it is evident that the amount of carbonic anhydride which passed over into the surrounding atmosphere will be #ah-x1 and the total volume of gas will be #aj #ah-x or #bh-x cubic centimetres4 ,the partial pressure under which the carbonic anhydride is then dissolved will be 7supposing that the common pressure remains constant the whole time7 equal to 7#ah-x7_/7#bh-x71 hence there is not in solution #ah c4c4 of carbonic anhydride 7as would be the case were the partial pressure equal to the atmospheric pressure71 but only #ah7ah-x7_/7#bh-x71 which is equal to x1 and we therefore obtain the equation #ah7ah-x7_/7#bh-x7 x1 hence x #hfi4 ,again1 where the atmosphere into which the gaseous solution is introduced is not only that of another gas but also unlimited1 then the gas dissolved will1 on passing over from the solution1 diffuse into this atmosphere1 and produce an infinitely small pressure in the unlimited atmosphere4 ,consequently1 no gas can be retained in solution under this infinitely small pressure1 and it will be entirely expelled from the solution4 ,for this reason water saturated with a gas which is not contained in air1 will be entirely deprived of the dissolved gas if left exposed to the air4 ,water also passes off from a solution into the atmosphere1 and it is evident that there might be such a case as a constant proportion between the quantity of water vaporised and the quantity of a gas expelled from a solution1 so that not the gas alone1 but the entire gaseous solution1 would pass off4 ,a similar case is exhibited in solutions which are not decomposed by heat 7such as those of hydrogen chloride and iodide71 as will afterwards be considered4 #cg ,however1 in those cases when the variation of the co-efficient of solubility with the temperature is not sufficiently great1 and when a known quantity of aqueous vapour and of the gas passes off from a solution at the boiling point1 an atmosphere may be obtained having the same composition as the liquid itself4 ,in this case the amount of gas passing over into such an atmosphere will not be greater than that held by the liquid1 and therefore such a gaseous solution will distil over unchanged4 ,the solution will then represent1 like a solution of hydriodic acid in water1 a liquid which is not altered by distillation1 while the pressure under which this distillation takes place remains constant4 ,thus in all its aspects solution presents gradations from the most feeble affinities to examples of intimate chemical combination4 ,the amount of heat evolved in the solution of equal volumes of different gases is in distinct relation with these variations of stability and solubility of different gases4 #bbc litres of the following gases 7at #gfj mm4 pressure7 evolve the following number of 7gram7 units of heat in dissolving in a large mass of water2 carbonic anhydride #e1fjj1 sulphurous anhydride #g1gjj1 ammonia #h1hjj1 hydrochloric acid #ag1djj1 and hydriodic acid #ai1djj4 ,the two last-named gases1 which are not expelled from their solution by boiling1 evolve approximately twice as much heat as gases like ammonia1 which are separated from their solutions by boiling1 whilst gases which are only slightly soluble evolve very much less heat4 ,it is evident that the conception of the partial pressures of gases should be applied not only to the formations of solutions1 but also to all cases of chemical action of gases4 ,especially numerous are its applications to the physiology of respiration1 for in these cases it is only the oxygen of the atmosphere that acts4#ch #ch ,among the numerous researches concerning this subject1 certain results obtained by ,paul ,bert are cited in ,chapter ,,iii41 and we will here point out that ,prof4 ,sechenoff1 in his researches on the absorption of gases by liquids1 very fully investigated the phenomena of the solution of carbonic anhydride in solutions of various salts1 and arrived at many important results1 which showed that1 on the one hand1 in the solution of carbonic anhydride in solutions of salts on which it is capable of acting chemically 7for example1 sodium carbonate1 borax1 ordinary sodium phosphate71 there is not only an increase of solubility1 but also a distinct deviation from the law of ,henry and ,dalton2 whilst1 on the other hand1 that solutions of salts which are not acted on by carbonic anhydride 7for example1 the chlorides1 nitrates1 and sulphates7 absorb less of it1 owing to the 'competition' of the salt already dissolved1 and follow the law of ,henry and ,dalton1 but at the same time show undoubted signs of a chemical action between the salt1 water1 and carbonic anhydride4 ,sulphuric acid 7whose co-efficient of absorption is #ib vols4 per #ajj71 when diluted with water1 absorbs less and less carbonic anhydride1 until the hydrate ,h#b,,so#d1;,h#b,o 7co-eff4 of absorption then equals #ff vols47 is formed2 then on further addition of water the solubility again rises until a solution of #ajj p4c4 of water is obtained4 ,the solution of solids1 whilst depending only in a small measure on the pressure under which solution takes place 7because solids and liquids are almost incompressible71 is very clearly dependent on the temperature4 ,in the great majority of cases the solubility of solids in water increases with the temperature2 and further1 the rapidity of solution increases also4 ,the latter is determined by the rapidity of diffusion of the solution formed into the remainder of the water4 ,the solution of a solid in water1 although it is as with gases1 a physical passage into a liquid state1 is determined1 however1 by its chemical affinity for water2 this is clearly shown from the fact that in solution there occurs a diminution in volume1 a change in the boiling point of water1 a change in the tension of its vapour1 in the freezing point1 and in many similar properties4 ,if solution were a physical1 and not a chemical1 phenomenon1 it would naturally be accompanied by an increase and not by a diminution of volume1 because generally in melting a solid increases in volume 7its density diminishes74 ,contraction is the usual phenomenon accompanying solution and takes place even in the addition of solutions to water1#ci and in the solution of liquids in water1#dj just as happens in the combination of substances when evidently new substances are produced4#da ,the contraction which takes place in solution is1 however1 very small1 a fact which depends on the small compressibility of solids and liquids1 and on the insignificance of the compressing force acting in solution4#db ,the change of volume which takes place in the solution of solids and liquids1 or the alteration in specific gravity#dc corresponding with it1 depends on peculiarities of the dissolving substances1 and of water1 and1 in the majority of cases1 is not proportional to the quantity of the substance dissolved1#dd showing the existence of a chemical force between the solvent and the substance dissolved which is of the same nature as in all other forms of chemical reaction4#de #ci ,kremers made this observation in the following simple form3--,he took a narrow-necked flask1 with a mark on the narrow part 7like that on a litre flask which is used for accurately measuring liquids71 poured water into it1 and then inserted a funnel1 having a fine tube which reached to the bottom of the flask4 ,through this funnel he carefully poured a solution of any salt1 and 7having removed the funnel7 allowed the liquid to attain a definite temperature 7in a water bath72 he then filled the flask up to the mark with water4 ,in this manner two layers of liquid were obtained1 the heavy saline solution below and water above4 ,the flask was then shaken in order to accelerate diffusion1 and it was observed that the volume became less if the temperature remained constant4 ,this can be proved by calculation1 if the specific gravity of the solutions and water be known4 ,thus at #ae one c4c4 of a #bj p4c4 solution of common salt weighs #aaejj gram1 hence #ajj grams occupy a volume of #hfif c4c4 ,as the sp4 gr4 of water at #ae #jiiiaf1 therefore #ajj grams of water occupy a volume of #ajjjh c4c4 ,the sum of the volumes is #ahgjd c4c4 ,after mixing1 #bjj grams of a #aj p4c4 solution are obtained4 ,its specific gravity is #ajgbe 7at #ae and referred to water at its maximum density71 hence the #bjj grams will occupy a volume of #ahfdh c4c4 ,the contraction is consequently equal to #jef c4c4 #dj ,the contractions produced in the case of the solution of sulphuric acid in water are shown in the diagram ,fig4 #ag 7page #gg74 ,their maximum is #aja c4c4 per #ajj c4c4 of the solution formed4 ,a maximum contraction of #dae at #j1 #cgh at #ae1 and #cej at #cj1 takes place in the solution of #df parts by weight of anhydrous alcohol in #ed parts of water4 ,this signifies that if1 at #j1 #df parts by weight of alcohol be taken per #ed parts by weight of water1 then the sum of their separate volumes will he #ajdae1 and after mixing their total volume will be #ajj4 #da ,this subject will be considered later in this work1 and we shall then see that the contraction produced in reactions of combination 7of solids or liquids7 is very variable in its amount1 and that there are1 although rarely1 reactions of combination in which contraction does not take place1 or when an increase of volume is produced4 #db ,the compressibility of solutions of common salt is less1 according to ,grassi1 than that of water4 ,at #ah the compression of water per million volumes #dh vols4 for a pressure of one atmosphere2 for a #ae p4c4 solution of common salt it is #cb1 and for a #bd p4c4 solution #bf vols4 ,similar determinations were made by ,brown 7#ahhg7 for saturated solutions of sal ammoniac 7#ch vols471 alum 7#df vols471 common salt 7#bg vols471 and sodium sulphate at #a1 when the compressibility of water #dg per million volumes4 ,this investigator also showed that substances which dissolve with an evolution of heat and with an increase in volume 7as1 for instance1 sal ammoniac7 are partially separated from their saturated solutions by an increase of pressure 7this experiment was particularly conclusive in the case of sal ammoniac71 whilst the solubility of substances which dissolve with an absorption of heat or diminution in volume increases1 although very slightly1 with an increase of pressure4 ,sorby observed the same phenomenon with common salt 7#ahfc74 #dc ,the most trustworthy data relating to the variation of the specific gravity of solutions with a change of their composition and temperature1 are collected and discussed in my work cited in footnote #ai4 ,the practical 7for the amount of a substance in solution is determined by the aid of the specific gravities of solutions1 both in works and in laboratory practice7 and the theoretical 7for specific gravity can be more accurately observed than other properties1 and because a variation in specific gravity governs the variation of many other properties7 interest of this subject1 besides the strict rules and laws to which it is liable1 make one wish that this province of data concerning solutions may soon be enriched by further observations of as accurate a nature as possible4 ,their collection does not present any great difficulty1 although requiring much time and attention4 ,pickering in ,london and ,tourbaba in ,kharkoff must be ranked first among those who have pursued problems of this nature during recent years4 #dd ,inasmuch as the degree of change exhibited in many properties on the formation of solutions is not large1 so1 owing to the insufficient accuracy of observations1 a proportionality between this change and a change of composition may1 in a first rough approximation and especially within narrow limits of change of composition1 easily be imagined in cases where it does not even exist4 ,the conclusion of ,michel and ,kraft is particularly instructive in this respect2 in #ahed1 on the basis of their incomplete researches1 they supposed that the increment of the specific gravity of solutions was proportional to the increment of a salt in a given volume of a solution1 which is only true for determinations of specific gravity which are exact to the second decimal place--an accuracy insufficient even for technical determinations4 ,accurate measurements do not confirm a proportionality either in this case or in many others where a ratio has been generally accepted2 as1 for example1 for the rotatory power 7with respect to the plane of polarisation7 of solutions1 and for their capillarity1 c4 ,nevertheless1 such a method is not only still made use of1 but even has its advantages when applied to solutions within a limited scope--as1 for instance1 very weak solutions1 and for a first acquaintance with the phenomena accompanying solution1 and also as a means for facilitating the application of mathematical analysis to the investigation of the phenomenon of solution4 ,judging by the results obtained in my researches on the specific gravity of solutions1 ,i think that in many cases it would be nearer the truth to take the change of properties as proportional1 not to the amount of a substance dissolved1 but to the product of this quantity and the amount of water in which it is dissolved2 the more so since many chemical relations vary in proportion to the reacting masses1 and a similar ratio has been established for many phenomena of attraction studied by mechanics4 ,this product is easily arrived at when the quantity of water in the solutions to be compared is constant1 as is shown in investigating the fall of temperature in the formation of ice 7see footnote #di1 p4 #ia74 #de ,all the different forms of chemical reaction may be said to take place in the process of solution4 7#a7 ,combinations between the solvent and the substance dissolved1 which are more or less stable 7more or less dissociated74 ,this form of reaction is the most probable1 and is that most often observed4 7#b7 ,reactions of substitution or of double decomposition between the molecules4 ,thus it may be supposed that in the solution of sal ammoniac1 ,,nh#d;,cl1 the action of water produces ammonia1 ,,nh#d;,,ho1 and hydrochloric acid1 ,h,cl1 which are dissolved in the water and simultaneously attract each other4 ,as these solutions and many others do indeed exhibit signs1 which are sometimes indisputable1 of similar double decompositions 7thus solutions of sal-ammoniac yield a certain amount of ammonia71 it is probable that this form of reaction is more often met with than is generally thought4 7#c7 ,reactions of isomerism or replacement are also probably met with in solution1 all the more as here molecules of different kinds come into intimate contact1 and it is very likely that the configuration of the atoms in the molecules under these influences is somewhat different from what it was in its original and isolated state4 ,one is led to this supposition especially from observations made on solutions of substances which rotate the plane of polarisation 7and observations of this kind are very sensitive with respect to the atomic structure of molecules71 because they show1 for example 7according to ,schneider1 #ahha71 that strong solutions of malic acid rotate the plane of polarisation to the right1 whilst its ammonium salts in all degrees of concentration rotate the plane of polarisation to the left4 7#d7 ,reactions of decomposition under the influences of solution are not only rational in themselves1 but have in recent years been recognised by ,arrhenius1 ,ostwald1 and others1 particularly on the basis of electrolytic determinations4 ,if a portion of the molecules of a solution occur in a condition of decomposition1 the other portion may occur in a yet more complex state of combination1 just as the velocity of the motion of different gaseous molecules may be far from being the same 7see ,note #cd1 p4 #ha74 ,it is1 therefore1 very probable that the reactions taking place in solution vary both quantitatively and qualitatively with the mass of water in the solution1 and the great difficulty in arriving at a definite conclusion as to the nature of the chemical relations which take place in the process of solution will be understood1 and if besides this the existence of a physical process1 like the sliding between and interpenetration of two homogeneous liquids1 be also recognised in solution1 then the complexity of the problem as to the actual nature of solutions1 which is now to the fore1 appears in its true light4 ,however1 the efforts which are now being applied to the solution of this problem are so numerous and of such varied aspect that they will afford future investigators a vast mass of material towards the construction of a complete theory of solution4 ,for my part1 ,i am of opinion that the study of the physical properties of solutions 7and especially of weak ones7 which now obtains1 cannot give any fundamental and complete solution of the problem whatever 7although it should add much to both the provinces of physics and chemistry71 but that1 parallel with it1 should be undertaken the study of the influence of temperature1 and especially of low temperatures1 the application to solutions of the mechanical theory of heat1 and the comparative study of the chemical properties of solutions4 ,the beginning of all this is already established1 but it is impossible to consider in so short an exposition of chemistry the further efforts of this kind which have been made up to the present date4 ,the feeble development of the chemical affinities acting in solutions of solids becomes evident from those multifarious methods by which their solutions are decomposed1 whether they be saturated or not4 ,on heating 7absorption of heat71 on cooling1 and by internal forces alone1 aqueous solutions in many cases separate into their components or their definite compounds with water4 ,the water contained in solutions is removed from them as vapour1 or1 by freezing1 in the form of ice1#df but the tension of the vapour of water#dg held in solution is less than that of water in a free state1 and the temperature of the formation of ice from solutions is lower than #j4 ,further1 both the diminution of vapour tension and the lowering of the freezing point proceed1 in dilute solutions1 almost in proportion to the amount of a substance dissolved4#dh ,thus1 if per #ajj grams of water there be in solution #a1 #e1 #aj grams of common salt 7,na,cl71 then at #ajj the vapour tension of the solutions decreases by #d1 #ba1 #dc mm4 of the barometric column1 against #gfj mm41 or the vapour tension of water1 whilst the freezing points are -#jeh1 -#bia1 and -#faj respectively4 ,the above figures#di are almost proportional to the amounts of salt in solution 7#a1 #e1 and #aj per #ajj of water74 ,furthermore1 it has been shown by experiment that the ratio of the diminution of vapour tension to the vapour tension of water at different temperatures in a given solution is an almost constant quantity1#ej and that for every 7dilute7 solution the ratio between the diminution of vapour tension and of the freezing point is also a tolerably constant quantity4#ea #df ,if solutions are regarded as being in a state of dissociation 7see footnote #ai1 p4 #fd7 it would be expected that they would contain free molecules of water1 which form one of the products of the decomposition of those definite compounds whose formation is the cause of solution4 ,in separating as ice or vapour1 water makes1 with a solution1 a heterogeneous system 7made up of substances in different physical states7 similar1 for instance1 to the formation of a precipitate or volatile substance in reactions of double decomposition4 #dg ,if the substance dissolved is non-volatile 7like salt or sugar71 or only slightly volatile1 then the whole of the tension of the vapour given off is due to the water1 but if a solution of a volatile substance--for instance1 a gas or a volatile liquid--evaporates1 then only a portion of the pressure belongs to the water1 and the whole pressure observed consists of the sum of the pressures of the vapours of the water and of the substance dissolved4 ,the majority of researches bear on the first case1 which will be spoken of presently1 and the observations of ,d4 ,p4 ,konovaloff 7#ahha7 refer to the second case4 ,he showed that in the case of two volatile liquids1 mutually soluble in each other1 forming two layers of saturated solutions 7for example1 ether and water1 ,note #bj1 p4 #fg71 both solutions have an equal vapour tension 7in the case in point the tension of both is equal to #dca mm4 of mercury at #aih74 ,further1 he found that for solutions which are formed in all proportions1 the tension is either greater 7solutions of alcohol and water7 or less 7solutions of formic acid7 than that which answers to the rectilinear change 7proportional to the composition7 from the tension of water to the tension of the substance dissolved2 thus1 the tension1 for example1 of a #gj p4c4 solution of formic acid is less1 at all temperatures1 than the tension of water and of formic acid itself4 ,in this case the tension of a solution is never equal to the sum of the tensions of the dissolving liquids1 as ,regnault already showed when he distinguished this case from that in which a mixture of liquids1 which are insoluble in each other1 evaporates4 ,from this it is evident that a mutual action occurs in solution1 which diminishes the vapour tensions proper to the individual substances1 as would be expected on the supposition of the formation of compounds in solutions1 because the elasticity then always diminishes4 #dh ,this amount is usually expressed by the weight of the substance dissolved per #ajj parts by weight of water4 ,probably it would be better to express it by the quantity of the substance in a definite volume of the solution--for instance1 in a litre--or by the ratios of the number of molecules of water and of the substance dissolved4 #di ,the variation of the vapour tension of solutions has been investigated by many4 ,the best known researches are those of ,wllner in ,germany 7#aheh-ahfj7 and of ,tamman in ,russia 7#ahhg74 ,the researches on the temperature of the formation of ice from various solutions are also very numerous2 ,blagden 7#aghh71 ,rdorff 7#ahfa71 and ,de ,coppet 7#ahga7 established the beginning1 but this kind of investigation takes its chief interest from the work of ,raoult1 begun in #ahhb on aqueous solutions1 and afterwards continued for solutions in various other easily frozen liquids--for instance1 benzene1 ,c#f;,h#f 7melts at #dif71 acetic acid1 ,c#b;,h#d,o#b 7#afge71 and others4 ,an especially important interest is attached to these cryoscopic investigations of ,raoult in ,france on the depression of the freezing point1 because he took solutions of many well-known carbon-compounds and discovered a simple relation between the molecular weight of the substances and the temperature of crystallisation of the solvent1 which enabled this kind of research to be applied to the investigation of the nature of substances4 ,we shall meet with the application of this method later on 7see also ,chapter ,,vii471 and at present will only cite the deduction arrived at from these results4 ,the solution of one-hundredth part of that molecular gram weight which corresponds with the formula of a substance dissolved 7for example1 ,na,cl #ehe1 ,c#b;,h#f,o #df1 c47 in #ajj parts of a solvent lowers the freezing point of its solution in water #jahe1 in benzene #jdi1 and in acetic acid ,o#ci1 or twice as much as with water4 ,and as in weak solutions the depression or fall of freezing point is proportional to the amount of the substance dissolved1 it follows that the fall of freezing point for all other solutions may be calculated from this rule4 ,so1 for instance1 the weight which corresponds with the formula of acetone1 ,c#c;,h#f,o is #eh2 a solution containing #bdb1 #fbb1 and #abce grams of acetone per #ajj grams of water1 forms ice 7according to the determinations of ,beckmann7 at #jggj1 #aicj1 and #chbj1 and these figures show that with a solution containing #jeh gram of acetone per #ajj of water the fall of the temperature of the formation of ice will be #jahe1 #jahj1 and #jagi4 ,it must be remarked that the law of proportionality between the fall of temperature of the formation of ice1 and the composition of a solution1 is in general only approximate1 and is only applicable to weak solutions 7,pickering and others74 ,we will here remark that the theoretical interest of this subject was strengthened on the discovery of the connection existing between the fall of tension1 the fall of the temperature of the formation of ice1 of osmotic pressure 7,van't ,hoff1 ,note #ai71 and of the electrical conductivity of solutions1 and we will therefore supplement what we have already said on the subject by some short remarks on the method of cryoscopic investigations1 although the details of the subject form the subject of more special works on physical chemistry 7such as ,ostwald's ,lehrbuch der allgemeinen ,chemie1 #ahia-ahid1 #b vols47 ,in order to determine the temperature of the formation of ice 7or of crystallisation of other solvents71 a solution of known strength is prepared and poured into a cylindrical vessel surrounded by a second similar vessel1 leaving a layer of air between the two1 which1 being a bad conductor1 prevents any rapid change of temperature4 ,the bulb of a sensitive and corrected thermometer is immersed in the solution1 and also a bent platinum wire for stirring the solution2 the whole is then cooled 7by immersing the apparatus in a freezing mixture71 and the temperature at which ice begins to separate observed4 ,if the temperature at first falls slightly lower1 it nevertheless becomes constant when ice begins to form4 ,by then allowing the liquid to get just warm1 and again observing the temperature of the formation of ice1 an exact determination may be arrived at4 ,it is still better to take a large mass of solution1 and induce the formation of the first crystals by dropping a small lump of ice into the solution already partially over-cooled4 ,this only imperceptibly changes the composition of the solution4 ,the observation should be made at the point of formation of only a very small amount of crystals1 as otherwise the composition of the solution will become altered from their separation4 ,every precaution must be taken to prevent the access of moisture to the interior of the apparatus1 which might also alter the composition of the solution or properties of the solvent 7for instance1 when using acetic acid74 ,with respect to the depression of dilute solutions it is known--7#a7 ,that the depression increases in almost direct proportion to the amount of the substance in solution 7always per #ajj parts of water71 for example1 for ,k,cl when the solution contains #a part of salt 7per #ajj parts of water7 the depression #jde1 when the solution contains #b parts of salt #jij1 with #aj parts of salt #dd4 7#b7 ,the greater the molecular weight expressed by the formula 7see ,chapter ,,vii471 and designated by ,m1 the less1 under other similar conditions1 will be the depression d1 and therefore if the concentration of a solution 7the amount by weight of substance dissolved per #ajj parts of water7 be designated by p1 then the fraction ,md_/p or the molecular depression for a given class of substances will be a constant quantity2 for example1 in the case of methyl alcohol in water #agc1 for acetone about #ahj1 for sugar about #ahe4 7#c7 ,in general the molecular depression for substances whose solutions do not conduct an electric current is about #ahe1 while for acids1 salts1 and such like substances whose solutions do conduct electricity1 it is i times greater2 for instance1 for ,h,cl1 ,,ki1 ,,hno#c1 ,,kho1 c41 about #cf 7i is nearly #b71 for borax about #ff1 and so on where i varies in the same manner as it does in the case of the osmotic pressure of solutions 7,note #ai74 7#d7 ,different solvents 7water1 acetic acid1 benzene1 c47 have each their corresponding constants of molecular depression 7which have a certain remote connection with their molecular weight72 for example1 for acetic acid the molecular depression is about #ci and not #ai 7as it is for water71 for benzene #di1 for methyl alcohol about #ag1 c4 7#e7 ,if the molecular weight ,m of a substance be unknown1 then in the case of non-conductors of electricity or for a given group1 it may be found by determining the depression1 d1 for a given concentration1 p2 for example1 in the case of peroxide of hydrogen1 which is a non-conductor of electricity1 the molecular weight1 ,m1 was found to be nearly #cd1 i4e4 equal to ,h#b,o#b4 ,similar results have also been found for the fall in the vapour tension of solutions 7,note #ea71 and for the rise of their boiling points 7hence these data may also serve for determining the molecular weight of a substance in solution1 as is shortly described in ,chapter ,,vii41 ,note #bg bis74 ,and as these conclusions are also applicable in the case of osmotic pressure 7,note #ai71 and a variation in the magnitude of i1 in passing from solutions which do not conduct an electric current to those which do conduct electricity is everywhere remarked1 so it was natural to here seek that causal connection which ,arrhenius 7#ahhh71 ,ostwald1 and others expected to find in the supposition that a portion of the substance of the electrolyte is already decomposed in the very act of solution1 into its ions 7for example1 ,na,cl into ,na and ,cl71 or into the atoms of those individual substances which make their appearance in electrolysis1 and in this way to explain the fact that i is greater for those bodies which conduct an electric current4 ,we will not consider here this supposition1 known as the hypothesis of 'electrolytic dissociation1' not only because it wholly belongs to that special branch--physical chemistry1 and gives scarcely any help towards explaining the chemical relations of solutions 7particularly their passage into definite compounds1 their reactions1 and their very formation71 but also because--7#a7 all the above data 7for constant depression1 osmotic pressure1 c47 only refer to dilute solutions1 and are not applicable to strong solutions2 whilst the chemical interest in strong solutions is not less than in dilute solutions1 and the transition from the former into the latter is consecutive and inevitable2 7#b7 because in all homogeneous bodies 7although it may be insoluble and not an electrolyte7 a portion of the atoms may he supposed 7,clausius7 to be passing from one particle to another 7,chapter ,x41 ,note #bh71 and as it were dissociated1 but there are no reasons for believing that such a phenomenon is proper to the solutions of electrolytes only2 7#c7 because no essential mark of difference is observed between the solution of electrolytes and non-conductors1 although it might be expected there would be according to ,arrhenius' hypothesis2 7#d7 because it is most reasonable to suppose the formation of new1 more complex1 but unstable and easily dissociated compounds in the act of solution1 than a decomposition1 even partial1 of the substances taken2 7#e7 because if ,arrhenius' hypothesis be accepted it becomes necessary to admit the existence in solutions of free ions1 like the atoms ,cl or ,na1 without any apparent expenditure of the energy necessary for their disruption1 and if in this case it can be explained why i then #b1 it is not at all clear why solutions of ,mg,s,o#d give i #a1 although the solution does conduct an electric current2 7#f7 because in dilute solutions1 the approximative proportionality between the depression and concentration may be recognised1 while admitting the formation of hydrates1 with as much right as in admitting the solution of anhydrous substances1 and if the formation of hydrates be recognised it is easier to admit that a portion of these hydrates is decomposed than to accept the breaking-up into ions2 7#g7 because the best conductors of electricity are solutions like the sulphates in which it is necessary to recognise the formation of associated systems or hydrates2 7#h7 because the cause of electro-conductivity can be sooner looked for in this affinity and this combination of the substance dissolved with the solvent1 as is seen from the fact1 that 7,d4 ,p4 ,konovaloff7 neither aniline nor acetic acid alone conduct an electric current1 a solution of aniline in water conducts it badly 7and here the affinity is very small71 while a solution of aniline in acetic acid forms a good electrolyte1 in which1 without doubt1 chemical forces are acting1 bringing aniline1 like ammonia1 into combination with the acetic acid2 which is evident from the researches made by ,prof4 ,konovaloff upon mixtures 7solutions7 of aniline and other amines2 and1 lastly1 7#i7 because ,i1 together with many of the chemists of the present day1 cannot regard the hypothesis of electrolytic dissociation in the form given to it up to now by ,arrhenius and ,ostwald1 as answering to the sum total of the chemical data respecting solutions and dissociation in general4 ,thus1 although ,i consider it superfluous to discuss further the evolution of the above theory of solutions1 still ,i think that it would he most useful for students of chemistry to consider all the data referring to this subject1 which can be found in the ,zeitschrift fr physikalische ,chemie1 #ahhh-ahid4 #ej ,this fact1 which was established by ,gay-,lussac1 ,pierson1 and v4 ,babo1 is confirmed by the latest observations1 and enables us to express not only the fall of tension 7p-p7 itself1 but its ratio to the tension of water 7p-p7_/p4 ,it is to be remarked that in the absence of any chemical action1 the fall of pressure is either very small1 or does not exist at all 7note #cc71 and is not proportional to the quantity of the substance added4 ,as a rule1 the tension is then equal1 according to the law of ,dalton1 to the sum of the tensions of the substances taken4 ,hence liquids which are insoluble in each other 7for example1 water and chloride of carbon7 present a tension equal to the sum of their individual tensions1 and therefore such a mixture boils at a lower temperature than the more volatile liquid 7,magnus1 ,regnault74 #ea ,if1 in the example of common salt1 the fall of tension be divided by the tension of water1 a figure is obtained which is nearly #aje times less than the magnitude of the fall of temperature of formation of ice4 ,this correlation was theoretically deduced by ,goldberg1 on the basis of the application of the mechanical theory of heat1 and is repeated by many investigated solutions4 ,the diminution of the vapour tension of solutions explains the rise in boiling point due to the solution of solid non-volatile bodies in water4 ,the temperature of a vapour is the same as that of the solution from which it is generated1 and therefore it follows that the aqueous vapour given off from a solution will be superheated4 ,a saturated solution of common salt boils at #ajhd1 a solution of #cce parts of nitre in #ajj parts of water at #aaei1 and a solution of #cbe parts of potassium chloride in #ajj parts of water at #agi1 if the temperature of ebullition be determined by immersing the thermometer bulb in the liquid itself4 ,this is another proof of the bond which exists between water and the substance dissolved4 ,and this bond is seen still more clearly in those cases 7for example1 in the solution of nitric or formic acid in water7 where the solution boils at a higher temperature than either water or the volatile substance dissolved in it4 ,for this reason the solutions of certain gases--for instance1 hydriodic or hydrochloric acid--boil above #ajj4 ,the separation of ice from solutions#eb explains both the phenomenon1 well known to sailors1 that the ice formed from salt water gives fresh water1 and also the fact that by freezing1 just as by evaporation1 a solution is obtained which is richer in salts than before4 ,this is taken advantage of in cold countries for obtaining a liquor from sea water1 which is then evaporated for the extraction of salt4 #eb ,fritzsche showed that solutions of certain colouring matters yield colourless ice1 which clearly proves the passage of water only into a solid state1 without any intermixture of the substance dissolved1 although the possibility of the admixture in certain other cases cannot be denied4 ,on the removal of part of the water from a solution 7by evaporation or the separation of ice71 a saturated solution should be obtained1 and then the solid substance dissolved should separate out4 ,solutions saturated at a certain temperature should also separate out a corresponding portion of the substance dissolved if they be reduced1 by cooling1#ec to a temperature at which the water can no longer hold the former quantity of the substance in solution4 ,if this separation1 by cooling a saturated solution or by evaporation1 take place slowly1 crystals of the substance dissolved are in many cases formed2 and this is the method by which crystals of soluble salts are usually obtained4 ,certain solids very easily separate out from their solutions in perfectly formed crystals1 which may attain very large dimensions4 ,such are nickel sulphate1 alum1 sodium carbonate1 chrome-alum1 copper sulphate1 potassium ferricyanide1 and a whole series of other salts4 ,the most remarkable circumstance in this is that many solids in separating out from an aqueous solution retain a portion of water1 forming crystallised solid substances which contain water4 ,a portion of the water previously in the solution remains in the separated crystals4 ,the water which is thus retained is called the water of crystallisation4 ,alum1 copper sulphate1 ,glauber's salt1 and magnesium sulphate contain such water1 but neither sal-ammoniac1 table salt1 nitre1 potassium chlorate1 silver nitrate1 nor sugar1 contains any water of crystallisation4 ,one and the same substance may separate out from a solution with or without water of crystallisation1 according to the temperature at which the crystals are formed4 ,thus common salt in crystallising from its solution in water at the ordinary or at a higher temperature does not contain water of crystallisation4 ,but if its separation from the solution takes place at a low temperature1 namely below -#e1 then the crystals contain #ch parts of water in #ajj parts4 ,crystals of the same substance which separate out at different temperatures may contain different amounts of water of crystallisation4 ,this proves to us that a solid dissolved in water may form various compounds with it1 differing in their properties and composition1 and capable of appearing in a solid separate form like many ordinary definite compounds4 ,this is indicated by the numerous properties and phenomena connected with solutions1 and gives reason for thinking that there exist in solutions themselves such compounds of the substance dissolved1 and the solvent or compounds similar to them1 only in a liquid partly decomposed form4 ,even the colour of solutions may often confirm this opinion4 ,copper sulphate forms crystals having a blue colour and containing water of crystallisation4 ,if the water of crystallisation be removed by heating the crystals to redness1 a colourless anhydrous substance is obtained 7a white powder74 ,from this it may be seen that the blue colour belongs to the compound of the copper salt with water4 ,solutions of copper sulphate are all blue1 and consequently they contain a compound similar to the compound formed by the salt with its water of crystallisation4 ,crystals of cobalt chloride when dissolved in an anhydrous liquid--like alcohol1 for instance--give a blue solution1 but when they are dissolved in water a red solution is obtained4 ,crystals from the aqueous solution1 according to ,professor ,potilitzin1 contain six times as much water 7,co,cl#b1f;,h#b,o7 for a given weight of the salt1 as those violet crystals 7,co,cl#b1;,h#b,o7 which are formed by the evaporation of an alcoholic solution4 #ec ,as the solubility of certain substances 7for example1 coniine1 cerium sulphate1 and others7 decreases with a rise of temperature 7between certain limits--see1 for example1 note #bd71 so these substances do not separate from their saturated solutions on cooling but on heating4 ,thus a solution of manganese sulphate1 saturated at #gj1 becomes cloudy on further heating4 ,the point at which a substance separates from its solution with a change of temperature gives an easy means of determining the co-efficient of solubility1 and this was taken advantage of by ,prof4 ,alexeff for determining the solubility of many substances4 ,the phenomenon and method of observation are here essentially the same as in the determination of the temperature of formation of ice4 ,if a solution of a substance which separates out on heating be taken 7for example1 the sulphate of calcium or manganese71 then at a certain fall of temperature ice will separate out from it1 and at a certain rise of temperature the salt will separate out4 ,from this example1 and from general considerations1 it is clear that the separation of a substance dissolved from a solution should present a certain analogy to the separation of ice from a solution4 ,in both cases1 a heterogeneous system of a solid and a liquid is formed from a homogeneous 7liquid7 system4 ,that solutions contain particular compounds with water is further shown by the phenomena of supersaturated solutions1 of so-called cryohydrates1 of solutions of certain acids having constant boiling points1 and the properties of compounds containing water of crystallisation whose data it is indispensable to keep in view in the consideration of solutions4 ,supersaturated solutions exhibit the following phenomena3--,on the refrigeration of a saturated solution of certain salts1#ed if the liquid be brought under certain conditions1 the excess of the solid may sometimes remain in solution and not separate out4 ,a great number of substances1 and more especially sodium sulphate1 ,na#b,,so#d1 or ,glauber's salt1 easily form supersaturated solutions4 ,if boiling water be saturated with this salt1 and the solution be poured off from any remaining undissolved salt1 and1 the boiling being still continued1 the vessel holding the solution be well closed by cotton wool1 or by fusing up the vessel1 or by covering the solution with a layer of oil1 then it will he found that this saturated solution does not separate out any ,glauber's salt whatever on cooling down to the ordinary or even to a much lower temperature2 although without the above precautions a salt separates out on cooling1 in the form of crystals1 which contain ,na#b,,so#d1aj;,h#b,o--that is1 #ahj parts of water for #adb parts of anhydrous salt4 ,the supersaturated solution may be moved about or shaken inside the vessel holding it1 and no crystallisation will take place2 the salt remains in the solution in as large an amount as at a higher temperature4 ,if the vessel holding the supersaturated solution be opened and a crystal of ,glauber's salt be thrown in1 crystallisation suddenly takes place4#ee ,a considerable rise in temperature is noticed during this rapid separation of crystals1 which is due to the fact that the salt1 previously in a liquid state1 passes into a solid state4 ,this bears some resemblance to the fact that water maybe cooled below #j 7even to -#aj7 if it be left at rest1 under certain circumstances1 and evolves heat in suddenly crystallising4 ,although from this point of view there is a resemblance1 yet in reality the phenomenon of supersaturated solutions is much more complicated4 ,thus1 on cooling1 a saturated solution of ,glauber's salt deposits crystals containing ,na#b,,so#d1g;,h#bj1ef or #abf parts of water per #adb parts of anhydrous salt1 and not #ahj parts of water1 as in the above-mentioned salt4 ,the crystals containing #g;,h#b,o are distinguished for their instability2 if they stand in contact not only with crystals of ,na#b,,so#d1aj;,h#b,o1 but with many other substances1 they immediately become opaque1 forming a mixture of anhydrous and deca-hydrated salts4 ,it is evident that between water and a soluble substance there may be established different kinds of greater or less stable equilibrium1 of which solutions form a particular case4#eg #ed ,those salts which separate out with water of crystallisation and give several crystallohydrates form supersaturated solutions with the greatest facility1 and the phenomenon is much more common than was previously imagined4 ,the first data were given in the last century by ,loewitz1 in ,st4 ,petersburg4 ,numerous researches have proved that supersaturated solutions do not differ from ordinary solutions in any of their essential properties4 ,the variations in specific gravity1 vapour tension1 formation of ice1 c41 take place according to the ordinary laws4 #ee ,inasmuch as air1 as has been shown by direct experiment1 contains1 although in very small quantities1 minute crystals of salts1 and among them sodium sulphate1 air can bring about the crystallisation of a supersaturated solution of sodium sulphate in an open vessel1 but it has no effect on saturated solutions of certain other salts2 for example1 lead acetate4 ,according to the observations of ,de ,boisbaudran1 ,gernez1 and others1 isomorphous salts 7analogous in composition7 are capable of inducing crystallisation4 ,thus1 a supersaturated solution of nickel sulphate crystallises by contact with crystals of sulphates of other metals analogous to it1 such as those of magnesium1 cobalt1 copper1 and manganese4 ,the crystallisation of a supersaturated solution1 set up by the contact of a minute crystal1 starts from it in rays with a definite velocity1 and it is evident that the crystals as they form propagate the crystallisation in definite directions4 ,this phenomenon recalls the evolution of organisms from germs4 ,an attraction of similar molecules ensues1 and they dispose themselves in definite similar forms4 #ef ,at the present time a view is very generally accepted1 which regards supersaturated solutions as homogeneous systems1 which pass into heterogeneous systems 7composed of a liquid and a solid substance71 in all respects exactly resembling the passage of water cooled below its freezing point into ice and water1 or the passage of crystals of rhombic sulphur into monoclinic crystals1 and of the monoclinic crystals into rhombic4 ,although many phenomena of supersaturation are thus clearly understood1 yet the spontaneous formation of the unstable hepta-hydrated salt 7with #g;,h#b,o71 in the place of the more stable deca-hydrated salt 7with mol4 #aj;,h#b,o71 indicates a property of a saturated solution of sodium sulphate which obliges one to admit that it has a different structure from an ordinary solution4 ,stcherbacheff asserts1 on the basis of his researches1 that a solution of the deca-hydrated salt gives1 on evaporation1 without the aid of heat1 the deca-hydrated salt1 whilst after heating above #cc it forms a supersaturated solution and the hepta-hydrated salt4 ,but in order that this view should be accepted1 some facts must be discovered distinguishing solutions 7which are1 according to this view1 isomeric7 containing the hepta-hydrated salt from those containing the deca-hydrated salt1 and all efforts in this direction 7the study of the properties of the solutions7 have given negative results4 ,as some crystallohydrates of salts 7alums1 sugar of lead1 calcium chloride7 melt straightway 7without separating out anything71 whilst others 7like ,na#b,,so#d1aj;,h#b,o7 are broken up1 then it may be that the latter are only in a state of equilibrium at a higher temperature than their melting point4 ,it may here be observed that in melting crystals of the deca-hydrated salt1 there is formed1 besides the solid anhydrous salt1 a saturated solution giving the hepta-hydrated salt1 so that this passage from the deca-to the hepta-hydrated salt1 and the reverse1 takes place with the formation of the anhydrous 7or1 it may be1 monohydrated7 salt4 ,moreover1 supersaturation 7,potilitzin1 #ahhi7 only takes place with those substances which are capable of giving several modifications or several crystallohydrates1 i4e4 supersaturated solutions separate out1 besides the stable normal crystallohydrate1 hydrates containing less water and also the anhydrous salt4 ,this degree of saturation acts upon the substance dissolved in a like manner to heat4 ,sulphate of nickel in a solution at #ae to #bj separates out rhombic crystals with #g;,h#b,o1 at #cj to #dj cubical crystals1 with #f;,h#b,o1 at #ej to #gj monoclinic crystals1 also containing #f;,h#b,o4 ,crystals of the same composition separate out from supersaturated solutions at one temperature 7#ag to #ai71 but at different degrees of saturation1 as was shown by ,lecoq de ,boisbaudran4 ,the capacity to voluntarily separate out slightly hydrated or anhydrous salts by the introduction of a crystal into the solution is common to all supersaturated solutions4 ,if a salt forms a supersaturated solution1 then one would expect1 according to this view1 that it should exist in the form of several hydrates or in several modifications4 ,thus ,potilitzin concluded that chlorate of strontium1 which easily gives supersaturated solutions1 should be capable of forming several hydrates1 besides the anhydrous salt known2 and he succeeded in discovering the existence of two hydrates1 ,sr7,cl,o#c7b1c;,h#b,o and apparently ,sr7,cl,o#c7b1h;,h#b,o4 ,besides this1 three modifications of the common anhydrous salt were obtained1 differing from each other in their crystalline form4 ,one modification separated out in the form of rhombic octahedra1 another in oblique plates1 and a third in long brittle prisms or plates4 ,further researches showed that salts which are not capable of forming supersaturated solutions such as the bromates of calcium1 strontium1 and barium1 part with their water of hydration with difficulty 7they crystallise with #a;,h#b,o71 and decompose very slowly in a vacuum or in dry air4 ,in other words the tension of dissociation is very small in this class of hydrates4 ,as the hydrates characterised by a small dissociation tension are incapable of giving supersaturated solutions1 so conversely supersaturated solutions give hydrates whose tension of dissociation is great 7,potilitzin1 #ahic74 #eg ,emulsions1 like milk1 are composed of a solution of glutinous or similar substances1 or of oily liquids suspended in a liquid in the form of drops1 which are clearly visible under a microscope1 and form an example of a mechanical formation which resembles solution4 ,but the difference from solutions is here evident4 ,there are1 however1 solutions which approach very near to emulsions in the facility with which the substance dissolved separates from them4 ,it has long been known1 for example1 that a particular kind of ,prussian blue1 ,k,fe#b7;,,cn7#f1 dissolves in pure water1 but1 on the addition of the smallest quantity of either of a number of salts1 it coagulates and becomes quite insoluble4 ,if copper sulphide 7,cu,s71 cadmium sulphide 7,cd,s71 arsenic sulphide 7,as#b,s#e7 7the experiments with these substances proceed with great ease1 and the solution obtained is comparatively stable71 and many other metallic sulphides1 be obtained by a method of double decomposition 7by precipitating salts of these metals by hydrogen sulphide71 and be then carefully washed 7by allowing the precipitate to settle1 pouring off the liquid1 and again adding sulphuretted hydrogen water71 then1 as was shown by ,schulze1 ,spring1 ,prost1 and others1 the previously insoluble sulphides pass into transparent 7for mercury1 lead1 and silver1 reddish brown2 for copper and iron1 greenish brown2 for cadmium and indium1 yellow2 and for zinc1 colourless7 solutions1 which may be preserved 7the weaker they are the longer they keep7 and even boiled1 but which1 nevertheless1 in time coagulate--that is1 separate in an insoluble form1 and then sometimes become crystalline and quite incapable of re-dissolving4 ,graham and others observed the power shown by colloids 7see note #ah7 of forming similar hydrosols or solutions of gelatinous colloids1 and1 in describing alumina and silica1 we shall again have occasion to speak of such solutions4 ,in the existing state of our knowledge concerning solution1 such solutions may be looked on as a transition between emulsion and ordinary solutions1 but no fundamental judgment can be formed about them until a study has been made of their relations to ordinary solutions 7the solutions of even soluble colloids freeze immediately on cooling below #j1 and1 according to ,guthrie1 do not form cryohydrates71 and to supersaturated solutions1 with which they have certain points in common4 ,solutions of salts on refrigeration below #j deposit ice or crystals 7which then frequently contain water of crystallisation7 of the salt dissolved1 and on reaching a certain degree of concentration they solidify in their entire mass4 ,these solidified masses are termed cryohydrates4 ,my researches on solutions of common salt 7#ahfh7 showed that its solution solidifies when it reaches a composition ,na,cl #aj;,h#b,o 7#ahj parts of water per #ehe parts of salt71 which takes place at about -#bc4 ,the solidified solution melts at the same temperature1 and both the portion melted and the remainder preserve the above composition4 ,guthrie 7#ahgd-ahgf7 obtained the cryohydrates of many salts1 and he showed that certain of them are formed like the above at comparatively low temperatures1 whilst others 7for instance1 corrosive sublimate1 alums1 potassium chlorate1 and various colloids7 are formed on a slight cooling1 to -#b or even before4#eh ,in the case of common salt1 the cryohydrate with #aj molecules of water1 and in the case of sodium nitrate1 the cryohydrate#ei with #g molecules of water 7i4e4 #abf parts of water per #he of salt7 should be accepted as established substances1 capable of passing from a solid to a liquid state and conversely2 and therefore it may be thought that in cryohydrates we have solutions which are not only undecomposable by cold1 but also have a definite composition which would present a fresh case of definite equilibrium between the solvent and the substance dissolved4 #eh ,offer 7#ahhj7 concludes1 from his researches on cryohydrates1 that they are simple mixtures of ice and salts1 having a constant melting point1 just as there are alloys having a constant point of fusion1 and solutions of liquids with a constant boiling point 7see note #fj74 ,this does not1 however1 explain in what form a salt is contained1 for instance1 in the cryohydrate ,na,cl #aj;,h#b,o4 ,at temperatures above -#aj common salt separates out in anhydrous crystals1 and at temperatures near -#aj1 in combination with water of crystallisation1 ,na,cl #b;,h#b,o1 and1 therefore1 it is very improbable that at still lower temperatures it would separate without water4 ,if the possibility of the solidified cryohydrate containing ,na,cl #b;,h#b,o and ice be admitted1 then it is not clear why one of these substances does not melt before the other4 ,if alcohol does not extract water from the solid mass1 leaving the salt behind1 this does not prove the presence of ice1 because alcohol also takes up water from the crystals of many hydrated substances 7for instance1 from ,na,cl #b;,h#b,o7 at about their melting-points4 ,besides which1 a simple observation on the cryohydrate1 ,na,cl #aj;,h#b,o1 shows that with the most careful cooling it does not on the addition of ice deposit ice1 which would occur if ice were formed on solidification intermixed with the salt4 ,i may add with regard to cryohydrates that many of the solutions of acids solidify completely on prolonged cooling 7for example1 ,h#b,,so#d1;,h#b,o71 and then form perfectly definite compounds4 ,for the solutions of sulphuric acid 7see ,chapter ,,xx47 ,pickering obtained1 for instance1 a hydrate1 ,h#b,,so#d1d;,h#b,o at -#be4 ,hydrochloric1 nitric1 and other acids also give similar crystalline hydrates1 melting at low temperatures and presenting many similarities with the cryohydrates4 #ei ,see note #bd4 ,the formation of definite but unstable compounds in the process of solution becomes evident from the phenomena of a marked decrease of vapour tension1 or from the rise of the temperature of ebullition which occurs in the solution of certain volatile liquids and gases in water4 ,as an example1 we will take hydriodic acid1 ,,hi1 a gas which liquefies1 giving a liquid which boils at -#bj4 ,a solution of it containing #eg p4c4 of hydriodic acid is distinguished by the fact that if it be heated the hydriodic acid volatilises together with the water in the same proportions as they occur in the solution1 therefore such a solution may be distilled unchanged4 ,the solution boils at a higher temperature than water1 at #abg4 ,a portion of the physical properties of the gas and water have in this case already disappeared--a new substance is formed1 which has its definite boiling point4 ,to put it more correctly1 this is not the temperature of ebullition1 but the temperature at which the compound formed decomposes1 forming the vapours of the products of dissociation1 which1 on cooling1 re-combine4 ,should a less amount of hydriodic acid be dissolved in water than the above1 then1 on heating such a solution1 water only at first distils over1 until the solution attains the above-mentioned composition2 it will then distil over unaltered4 ,if more hydriodic acid be passed into such a solution a fresh quantity of the gas will dissolve1 but it passes off with great ease1 like air from water4 ,it must not1 however1 be thought that those forces which determine the formation of ordinary gaseous solutions play no part whatever in the formation of a solution having a definite boiling point2 that they do react is shown from the fact that such constant gaseous solutions vary in their composition under different pressures4#fj ,it is not1 therefore1 at every1 but only at the ordinary1 atmospheric pressure that a constant boiling solution of hydriodic acid will contain #eg p4c4 of the gas4 ,at another pressure the proportion of water and hydriodic acid will be different4 ,it varies1 however1 judging from observations made by ,roscoe1 very little for considerable variations of pressure4 ,this variation in composition directly indicates that pressure exerts an influence on the formation of unstable chemical compounds which are easily dissociated 7with formation of a gas71 just as it influences the solution of gases1 only the latter is influenced to a more considerable degree than the former4#fa ,hydrochloric1 nitric1 and other acids form solutions having definite boiling points1 like that of hydriodic acid4 ,they show further the common property1 if containing but a small proportion of water1 that they fume in air4 ,strong solutions of nitric1 hydrochloric1 hydriodic1 and other gases are even termed 'fuming acids4' ,the fuming liquids contain a definite compound whose temperature of ebullition 7decomposition7 is higher than #ajj1 and contain also an excess of the volatile substance dissolved1 which exhibits a capacity to combine with water and form a hydrate1 whose vapour tension is less than that of aqueous vapour4 ,on evaporating in air1 this dissolved substance meets the atmospheric moisture and forms a visible vapour 7fumes7 with it1 which consists of the above-mentioned compound4 ,the attraction or affinity which binds1 for instance1 hydriodic acid with water is evinced not only in the evolution of heat and the diminution of vapour tension 7rise of boiling point71 but also in many purely chemical relations4 ,thus hydriodic acid is produced from iodine and hydrogen sulphide in the presence of water1 but unless water is present this reaction does not take place4#fb #fj ,for this reason 7the want of entire constancy of the composition of constant boiling solutions with a change of pressure71 the existence of definite hydrates formed by volatile substances--for instance1 by hydrochloric acid and water--is frequently denied4 ,it is generally argued as follows3 ,if there did exist a constancy of composition1 then it would be unaltered by a change of pressure4 ,but the distillation of constant boiling hydrates is undoubtedly accompanied 7judging by the vapour densities determined by ,bineau71 like the distillation of sal ammoniac1 sulphuric acid1 c41 by a complete decomposition of the original compound--that is1 these substances do not exist in a state of vapour1 but their products of decomposition 7hydrochloric acid and water7 are gases at the temperature of volatilisation1 which dissolve in the volatilised and condensed liquids2 but the solubility of gases in liquids depends on the pressure1 and1 therefore1 the composition of constant boiling solutions may1 and even ought to1 vary with a change of pressure1 and1 further1 the smaller the pressure and the lower the temperature of volatilisation1 the more likely is a true compound to be obtained4 ,according to the researches of ,roscoe and ,dittmar 7#ahei71 the constant boiling solution of hydrochloric acid proved to contain #ah p4c4 of hydrochloric acid at a pressure of #c atmospheres1 #bj p4c4 at #a atmosphere1 and #bc p4c4 at #a_/aj of an atmosphere4 ,on passing air through the solution until its composition became constant 7i4e4 forcing the excess of aqueous vapour or of hydrochloric acid to pass away with the air71 then acid was obtained containing about #bj p4c4 at #ajj1 about #bc p4c4 at #ej1 and about #be p4c4 at #j4 ,from this it is seen that by decreasing the pressure and lowering the temperature of evaporation one arrives at the same limit1 where the composition should be taken as ,h,cl #f;,h#b,o1 which requires #bebf p4c4 of hydrochloric acid4 ,fuming hydrochloric acid contains more than this4 ,in the case already considered1 as in the case of formic acid in the researches of ,d4 ,p4 ,konovaloff 7note #dg71 the constant boiling solution corresponds with a minimum tension--that is1 with a boiling point higher than that of either of the component elements4 ,but there is another case of constant boiling solutions similar to the case of the solution of propyl alcohol1 ,c#c;,h#h,o1 when a solution1 undecomposed by distillation1 boils at a lower point than that of the more volatile liquid4 ,however1 in this case also1 if there be solution1 the possibility of the formation of a definite compound in the form ,c#c;,h#h,o ,h#b,o cannot be denied1 and the tension of the solution is not equal to the sum of tensions of the components4 ,there are possible cases of constant boiling mixtures even when there is no solution nor any loss of tension1 and consequently no chemical action1 since the amount of liquids that are volatilised is determined by the product of the vapour densities into their vapour tensions 7,wanklyn71 in consequence of which liquids whose boiling point is above #ajj--;for instance1 turpentine and ethereal oils in general--when distilled with aqueous vapour1 pass over at a temperature below #ajj4 ,consequently1 it is not in the constancy of composition and boiling point 7temperature of decomposition7 that evidence of a distinct chemical action is to be found in the above-described solutions of acids1 but in the great loss of tension1 which completely resembles the loss of tension observed1 for instance1 in the perfectly-definite combinations of substances with water of crystallisation 7see later1 note #fe74 ,sulphuric acid1 ,h#b,,so#d1 as we shall learn later1 is also decomposed by distillation1 like ,h,cl #f;,h#b,o1 and exhibits1 moreover1 all the signs of a definite chemical compound4 ,the study of the variation of the specific gravities of solutions as dependent on their composition 7see note #ai7 shows that phenomena of a similar kind1 although of different dimensions1 take place in the formation of both ,h#b,,so#d from ,h#b,o and ,,so#c1 and of ,h,cl #f;,h#b,o 7or of aqueous solutions analogous to it7 from ,h,cl and ,h#b,o4 #fa ,the essence of the matter may he thus represented4 ,a gaseous or easily volatile substance ,a forms with a certain quantity of water1 n,h#b,o1 a definite complex compound ,an,h#b,o1 which is stable up to a temperature t higher than #ajj4 ,at this temperature it is decomposed into two substances1 ,a ,h#b,o4 ,both boil below t at the ordinary pressure1 and therefore at t they distil over and re-combine in the receiver4 ,but if a part of the substance ,an,h#b,o is decomposed or volatilised1 a portion of the undecomposed liquid still remains in the vessel1 which can partially dissolve one of the products of decomposition1 and that in quantity varying with the pressure and temperature1 and therefore the solution at a constant boiling point will have a slightly different composition at different pressures4 #fb ,for solutions of hydrochloric acid in water there are still greater differences in reactions4 ,for instance1 strong solutions decompose antimony sulphide 7forming hydrogen sulphide1 ,h#b,s71 and precipitate common salt from its solutions1 whilst weak solutions do not act thus4 ,many compunds containing water of crystallisation are solid substances 7when melted they are already solutions--i4e4 liquids72 furthermore1 they are capable of being formed from solutions1 like ice or aqueous vapour4 ,they may be called crystallo-hydrates4 ,inasmuch as the direct presence of ice or aqueous vapour cannot be admitted in solutions 7for these are liquids71 although the presence of water may be1 so also there is no basis for acknowledging the presence in solutions of crystallo-hydrates1 although they are obtained from solutions as such4#fc ,it is evident that such substances present one of the many forms of equilibrium between water and a substance dissolved in it4 ,this form1 however1 reminds one1 in all respects1 of solutions--that is1 aqueous compounds which are more or less easily decomposed1 with separation of water and the formation of a less aqueous or an anhydrous compound4 ,in fact1 there are not a few crystals containing water which lose a part of their water at the ordinary temperature4 ,of such a kind1 for instance1 are the crystals of soda1 or sodium carbonate1 which1 when separated from an aqueous solution at the ordinary temperature1 are quite transparent2 but when left exposed to air1 lose a portion of their water1 becoming opaque1 and1 in the process1 lose their crystalline appearance1 although preserving their original form4 ,this process of the separation of water at the ordinary temperature is termed the efflorescence of crystals4 ,efflorescence takes place more rapidly under the receiver of an air pump1 and especially at a gentle heat4 ,this breaking up of a crystal is dissociation at the ordinary temperature4 ,solutions are decomposed in exactly the same manner4#fd ,the tension of the aqueous vapour which is given off from crystallo-hydrates is naturally1 as with solutions1 less than the vapour tension of water itself#fe at the same temperature1 and therefore many anhydrous salts which are capable of combining with water absorb aqueous vapour from moist air2 that is1 they act like a cold body on which water is deposited from steam4 ,it is on this that the desiccation of gases is based1 and it must further be remarked in this respect that certain substances--for instance1 potassium carbonate 7,k#b;,,co#c7 and calcium chloride 7,ca,cl#b7--not only absorb the water necessary for the formation of a solid crystalline compound1 but also give solutions1 or deliquesce1 as it is termed1 in moist air4 ,many crystals do not effloresce in the least at the ordinary temperature2 for example1 copper sulphate1 which may be preserved for an indefinite length of time without efflorescing1 but when placed under the receiver of an air pump1 if efflorescence be once started1 it goes on at the ordinary temperature4 ,the temperature at which the complete separation of water from crystals takes place varies considerably1 not only for different substances1 but also for different portions of the contained water4 ,very often the temperature at which dissociation begins is very much higher than the boiling point of water4 ,so1 for example1 copper sulphate1 which contains #cf p4c4 of water1 gives up #bhh p4c4 at #ajj1 and the remaining quantity1 namely #gb p4c41 only at #bdj4 ,alum1 out of the #dee p4c4 of water which it contains1 gives up #ahi p4c4 at #ajj1 #agg p4c4 at #abj1 #gg p4c4 at #ahj1 and #a p4c4 at #bhj2 it only loses the last quantity 7#a p4c47 at its temperature of decomposition4 ,these examples clearly show that the annexation of water of crystallisation is accompanied by a rather profound1 although1 in comparison with instances which we shall consider later1 still inconsiderable1 change of its properties4 ,in certain cases the water of crystallisation is only given off when the solid form of the substance is destroyed3 when the crystals melt on heating4 ,the crystals are then said to melt in their water of crystallisation4 ,further1 after the separation of the water1 a solid substance remains behind1 so that by further heating it acquires a solid form4 ,this is seen most clearly in crystals of sugar of lead or lead acetate1 which melt in their water of crystallisation at a temperature of #efbe1 and in so doing begin to lose water4 ,on reaching a temperature of #ajj the sugar of lead solidifies1 having lost all its water2 and then at a temperature of #bhj1 the anhydrous and solidified salt again melts4#fe bis #fc ,supersaturated solutions give an excellent proof in this respect4 ,thus a solution of copper sulphate generally crystallises in penta-hydrated crystals1 ,cu,s,o#d #e;,h#b,o1 and its saturated solution gives such crystals if it be brought into contact with the minutest possible crystal of the same kind4 ,but1 according to the observations of ,lecoq de ,boisbaudran1 if a crystal of ferrous sulphate 7an isomorphous salt1 see note #ee71 ,fe,s,o#d #g;,h#b,o1 be placed in a saturated solution of copper sulphate1 then crystals of hepta-hydrated salt1 ,cu,s,o#d #g;,h#b,o1 are obtained4 ,it is evident that neither the penta-nor the hepta-hydrated salt is contained as such in the solution4 ,the solution presents its own particular liquid form of equilibrium4 #fd ,efflorescence1 like every evaporation1 proceeds from the surface4 ,in the interior of crystals which have effloresced there is usually found a non-effloresced mass1 so that the majority of effloresced crystals of washing soda show1 in their fracture1 a transparent nucleus coated by an effloresced1 opaque1 powdery mass4 ,it is a remarkable circumstance in this respect that efflorescence proceeds in a completely regular and uniform manner1 so that the angles and planes of similar crystallographic character effloresce simultaneously1 and in this respect the crystalline form determines those parts of crystals where efflorescence starts1 and the order in which it continues4 ,in solutions evaporation also proceeds from the surface1 and the first crystals which appear on its reaching the required degree of saturation are also formed at the surface4 ,after falling to the bottom the crystals naturally continue to grow 7see ,chapter ,x474 #fe ,according to ,lescoeur 7#ahhc71 at #ajj a concentrated solution of barium hydroxide1 ,ba,h#b,o#b1 on first depositing crystals 7with ,h#b,o7 has a tension of about #fcj mm4 7instead of #gfj mm41 the tension of water71 which decreases 7because the solution evaporates7 to #de mm41 when all the water is expelled from the crystals1 ,ba,h#b,o#b ,h#b,o1 which are formed1 but they also lose water 7dissociate1 effloresce at #ajj71 leaving the hydroxide1 ,ba,h#b,o#b1 which is perfectly undecomposable at #ajj--that is1 does not part with water4 ,at #gc 7the tension of water is then #bfe mm47 a solution1 containing #cc;,h#b,o1 on crystallising has a tension of #bcj mm42 the crystals1 ,ba,h#b,o#b #h;,h#b,o1 which separate out1 have a tension of #afj mm42 on losing water they give ,ba,h#b,o#b ,h#b,o4 ,this substance does not decompose at #gc1 and therefore its tension #j4 ,in those crystallohydrates which effloresce at the ordinary temperature1 the tension of dissociation nearly approximates to that of the aqueous vapour1 as ,lescoeur 7#ahia7 showed4 ,to this category of compounds belong ,b#b,o#c7c x7,h#b,o1 ,c#b,o#d;,h#b7b x7,h#b,o1 ,ba,o7#i x7,h#b,o1 and ,sr,o7#i x7,h#b,o4 ,and a still greater tension is possessed by ,na#b,,so#daj;,h#b,o1 ,na#b;,,co#caj;,h#b,o1 and ,mg,s,o#d7g x7,h#b,o4 ,mller-,erzbach 7#ahhd7 determines the tension 7with reference to liquid water7 by placing tubes of the same length with water and the substances experimented with in a desiccator1 the rate of loss of water giving the relative tension4 ,thus1 at the ordinary temperature1 crystals of sodium phosphate1 ,na#b;,,hpo#d #ab;,h#b,o1 present a tension of #jg compared with water1 until they lose #e;,h#b,o1 then #jd until they lose #e;,h#b,o more1 and on losing the last equivalent of water the tension falls to #jjd compared with water4 ,it is clear that the different molecules of water are held by an unequal force4 ,out of the five molecules of water in copper sulphate the two first are comparatively easily separated even at the ordinary temperature 7but only after several days in a desiccator1 according to ,latchinoff72 the next two are more difficultly separated1 and the last equivalent is retained even at #ajj4 ,this is another indication of the capacity of ,cu,s,o#d to form three hydrates1 ,cu,s,o#de;,h#b,o1 ,cu,s,o#dc;,h#b,o1 and ,cu,s,o#d;,h#b,o4 ,the researches of ,andreae on the tension of dissociation of hydrated sulphate of copper showed 7#ahia7 the existence of three provinces1 characterised at a given temperature by a constant tension3 7#a7 between #c-e1 7#b7 between #a-c1 and lastly 7#c7 between #j-a molecule of water1 which again confirms the existence of three hydrates of the above composition for this salt4 #fe bis ,sodium acetate 7,c#b;,h#c,o#b,na1#c;,h#b,o7 melts at #eh1 but re-solidifies only on contact with a crystal1 otherwise it may remain liquid even at #j1 and may be used for obtaining a constant temperature4 ,according to ,jeannel1 the latent heat of fusion is about #bh calories1 and according to ,pickering the heat of solution #ce calories4 ,when melted this salt boils at #abc--that is1 the tension of the vapour given off at that temperature equals the atmospheric pressure4 ,it is most important to recognise in respect to the water of crystallisation that its ratio to the quantity of the substance with which it is combined is always a constant quantity4 ,however often we may prepare copper sulphate1 we shall always find #cfad p4c4 of water in its crystals1 and these crystals always lose four-fifths of their water at #ajj1 and one-fifth of the whole amount of the water contained remains in the crystals at #ajj1 and is only expelled from them at a temperature of about #bdj4 ,what has been said about crystals of copper sulphate refers also to crystals of every other substance1 which contain water of crystallisation4 ,it is impossible in any of these cases to increase either the relative proportion of the salt or of the water1 without changing the homogeneity of the substance4 ,if once a portion of the water be lost--for instance1 if once efflorescence takes place--a mixture is obtained1 and not a homogeneous substance1 namely a mixture of a substance deprived of water with a substance which has not yet lost water--i4e4 decomposition has already commenced4 ,this constant ratio is an example of the fact that in chemical compounds the quantity of the component parts is quite definite2 that is1 it is an example of the so-called definite chemical compounds4 ,they may be distinguished from solutions1 and from all other so-called indefinite chemical compounds1 in that at least one1 and sometimes both1 of the component parts may be added in a large quantity to an indefinite chemical compound1 without destroying its homogeneity1 as in solutions1 whilst it is impossible to add any one of the component parts to a definite chemical compound without destroying the homogeneity of the entire mass4 ,definite chemical compounds only decompose at a certain rise in temperature2 on a lowering in temperature they do not1 at least with very few exceptions1 yield their components like solutions which form ice or compounds with water of crystallisation4 ,this leads to the assumption that solutions contain water as water1#ff although it may sometimes be in a very small quantity4 ,therefore solutions which are capable of solidifying completely 7for instance1 crystallo-hydrates capable of melting7 such as the compound of #hd-a_/b parts of sulphuric acid1 ,h#b,,so#d1 with #ae-a_/b parts of water1 ,h#b,o1 or ,h#b,,so#d1;,h#b,o 7or ,h#d,,so#e71 appear as true definite chemical compounds4 ,if1 then1 we imagine such a definite compound in a liquid state1 and admit that it partially decomposes in this state1 separating water--not as ice or vapour 7for then the system would be heterogeneous1 including substances in different physical states71 but in a liquid form1 when the system will be homogeneous--we shall form an idea of a solution as an unstable1 dissociating fluid state of equilibrium between water and the substance dissolved4 ,moreover1 it should be remarked that1 judging by experiment1 many substances give with water not one but diverse compounds1#fg which is seen in the capacity of one substance to form with water many various crystallo-hydrates1 or compounds with water of crystallisation1 showing diverse and independent properties4 ,from these considerations1 solutions#fh may be regarded as fluid1 unstable1 definite chemical compounds in a state of dissociation4#fi #ff ,such a phenomenon frequently presents itself in purely chemical action4 ,for instance1 let a liquid substance ,a give1 with another liquid substance ,b1 under the conditions of an experiment1 a mere minute quantity of a solid or gaseous substance ,c4 ,this small quantity will separate out 7pass away from the sphere of action1 as ,berthollet expressed it71 and the remaining masses of ,a and ,b will again give ,c2 consequently1 under these conditions action will go on to the end4 ,such1 it seems to me1 is the action in solutions when they yield ice or vapour indicating the presence of water4 #fg ,certain substances are capable of forming together only one compound1 others several1 and these of the most varied degrees of stability4 ,the compounds of water are instances of this kind4 ,in solutions the existence of several different definite compounds must be acknowledged1 but many of these have not yet been obtained in a free state1 and it may be that they cannot be obtained in any other but a liquid form--that is1 dissolved2 just as there are many undoubted definite compounds which only exist in one physical state4 ,among the hydrates such instances occur4 ,the compound ,,co#b #h;,h#b,o 7see note #ca71 according to ,wroblewski1 only occurs in a solid form4 ,hydrates like ,h#b,s #ab;,h#b,o 7,de ,forcrand and ,villard71 ,h,br ,h#b,o 7,roozeboom71 can only be accepted on the basis of a decrease of tension1 but present themselves as very transient substances1 incapable of existing in a stable free state4 ,even sulphuric acid1 ,h#b,,so#d1 itself1 which undoubtedly is a definite compound1 fumes in a liquid form1 giving off the anhydride1 ,,so#c--that is1 it exhibits a very unstable equilibrium4 ,the crystallo-hydrates of chlorine1 ,cl#b #h;,h#b,o1 of hydrogen sulphide1 ,h#b,s #ab;,h#b,o 7it is formed at #j1 and is completely decomposed at #a1 as then #a vol4 of water only dissolves #d vols4 of hydrogen sulphide1 while at #ja it dissolves about #ajj vols471 and of many other gases1 are instances of hydrates which are very unstable4 #fh ,of such a kind are also other indefinite chemical compounds2 for example1 metallic alloys4 ,these are solid substances or solidified solutions of metals4 ,they also contain definite compounds1 and may contain an excess of one of the metals4 ,according to the experiments of ,laurie 7#ahhh71 the alloys of zinc with copper in respect to the electro-motive force in galvanic batteries behave just like zinc if the proportion of copper in the alloy does not exceed a certain percentage--that is1 until a definite compound is attained--for in that case particles of free zinc are present2 but if a copper surface be taken1 and it be covered by only one-thousandth part of its area of zinc1 then only the zinc will act in a galvanic battery4 #fi ,according to the above supposition1 the condition of solutions in the sense of the kinetic hypothesis of matter 7that is1 on the supposition of an internal motion of molecules and atoms7 may be represented in the following form3--,in a homogeneous liquid--for instance1 water--the molecules occur in a certain state of1 although mobile1 still stable1 equilibrium4 ,when a substance ,a dissolves in water1 its molecules form with several molecules of water1 systems ,an,h#b,o1 which are so unstable that when surrounded by molecules of water they decompose and re-form1 so that ,a passes from one mass of molecules of water to another1 and the molecules of water which were at this moment in harmonious motion with ,a in the form of the system ,an,h#b,o1 in the next instant may have already succeeded in getting free4 ,the addition of water or of molecules of ,a may either only alter the number of free molecules1 which in their turn enter into systems ,an,h#b,o1 or they may introduce conditions for the possibility of building up new systems ,am,h#b,o1 where m is either greater or less than n4 ,if in the solution the relation of the molecules be the same as in the system ,am,h#b,o1 then the addition of fresh molecules of water or of ,a would be followed by the formation of new molecules ,an,h#b,o4 ,the relative quantity1 stability1 and composition of these systems or definite compounds will vary in one or another solution4 ,i adopted this view of solutions 7#ahhg1 ,pickering subsequently put forward a similar view7 after a most intimate study of the variation of their specific gravities1 to which my book1 cited in note #ai1 is devoted4 ,definite compounds1 ,an#a;,h#b,o and ,am#a;,h#b,o1 existing in a free--for instance1 solid--form1 may in certain cases be held in solutions in a dissociated state 7although but partially72 they are similar in their structure to those definite substances which are formed in solutions1 but it is not necessary to assume that such systems as ,na#b,,so#d #aj;,h#b,o1 or ,na#b,,so#d #g;,h#b,o1 or ,na#b,,so#d1 are contained in solutions4 ,the comparatively more stable systems ,an#a;,h#b,o which exist in a free state and change their physical state must present1 although within certain limits of temperature1 an entirely harmonious kind of motion of ,a with n#a;,h#b,o2 the property also and state of systems ,an,h#b,o and ,am,h#b,o1 occurring in solutions1 is that they are in a liquid form1 although partially dissociated4 ,substances ,a#a1 which give solutions1 are distinguished by the fact that they can form such unstable systems ,an,h#b,o1 but besides them they can give other much more stable systems ,an#a;,h#b,o4 ,thus ethylene1 ,c#b;,h#d1 in dissolving in water1 probably forms a system ,c#b;,h#dn,h#b,o1 which easily splits up into ,c#b;,h#d and ,h#b,o1 but it also gives the system of alcohol1 ,c#b;,h#d1;,h#b,o or ,c#b;,h#f,o1 which is comparatively stable4 ,thus oxygen can dissolve in water1 and it can combine with it1 forming peroxide of hydrogen4 ,turpentine1 ,c#aj;,h#af1 does not dissolve in water1 but it combines with it as a comparatively stable hydrate4 ,in other words1 the chemical structure of hydrates1 or of the definite compounds which are contained in solutions1 is distinguished not only by its original peculiarities but also by a diversity of stability4 ,a similar structure to hydrates must be acknowledged in crystallo-hydrates4 ,on melting they give actual 7real7 solutions4 ,as substances which give crystallo-hydrates1 like salts1 are capable of forming a number of diverse hydrates1 and as the greater the number of molecules of water 7n7 they 7,an,h#b,o7 contain1 the lower is the temperature of their formation1 and as the more easily they decompose the more water they hold1 therefore1 in the first place1 the isolation of hydrates holding much water existing in aqueous solutions may be soonest looked for at low temperatures 7although1 perhaps1 in certain cases they cannot exist in the solid state72 and1 secondly1 the stability also of such higher hydrates will be at a minimum under the ordinary circumstances of the occurrence of liquid water4 ,hence a further more detailed investigation of cryohydrates may help to the elucidation of the nature of solutions4 ,but it may be foreseen that certain cryohydrates will1 like metallic alloys1 present solidified mixtures of ice with the salts themselves and their more stable hydrates1 and others will be definite compounds4 ,in regarding solutions from this point of view they come under the head of those definite compounds with which chemistry is mainly concerned4#gj #gj ,the above representation of solutions1 c41 considering them as a particular state of definite compounds1 excludes the independent existence of indefinite compounds2 by this means that unity of chemical conception is obtained which cannot be arrived at by admitting the physico-mechanical conception of indefinite compounds4 ,the gradual transition from typical solutions 7as of gases in water1 and of weak saline solutions7 to sulphuric acid1 and from it and its definite1 but yet unstable and liquid1 compounds1 to clearly defined compounds1 such as salts and their crystallo-hydrates1 is so imperceptible1 that in denying that solutions pertain to the number of definite but dissociating compounds1 we risk denying the definiteness of the atomic composition of such substances as sulphuric acid or of molten crystallo-hydrates4 ,i repeat1 however1 that for the present the theory of solutions cannot be considered as firmly established4 ,the above opinion about them is nothing more than a hypothesis which endeavours to satisfy those comparatively limited data which we have for the present about solutions1 and of those cases of their transition into definite compounds4 ,by submitting solutions to the ,daltonic conception of atomism1 ,i hope that we may not only attain to a general harmonious chemical doctrine1 but also that new motives for investigation and research will appear in the problem of solutions1 which must either confirm the proposed theory or replace it by another fuller and truer one2 and ,i for my part cannot consider this to be the case with any of the other present doctrines of solutions 7note #di74 ,we saw above that copper sulphate loses four-fifths of its water at #ajj and the remainder at #bdj4 ,this means that there are two definite compounds of water with the anhydrous salt4 ,washing soda or carbonate of sodium1 ,na#b;,,co#c separates out as crystals1 ,na#b;,,co#c1aj;,h#b,o1 containing #fbi p4c4 of water by weight1 from its solutions at the ordinary temperature4 ,when a solution of the same salt deposits crystals at a low temperature1 about -#bj1 then these crystals contain #gah parts of water per #bhb parts of anhydrous salt4 ,further1 the crystals are obtained together with ice1 and are left behind when it melts4 ,if ordinary soda1 with #fbi p4c4 of water1 be cautiously melted in its own water of crystallisation1 there remains a salt1 in a solid state1 containing only #ade p4c4 of water1 and a liquid is obtained which contains the solution of a salt which separates out crystals at #cd1 which contain #df p4c4 of water and do not effloresce in air4 ,lastly1 if a supersaturated solution of soda be prepared1 then at temperatures below #h it deposits crystals containing #edc p4c4 of water4 ,thus as many as five compounds of anhydrous soda with water are known2 and they are dissimilar in their properties and crystalline form1 and even in their solubility4 ,it is to be observed that the greatest amount of water in the crystals corresponds with a temperature of -#bj1 and the smallest to the highest temperature4 ,there is apparently no relation between the above quantities of water and the salts1 but this is only because in each case the amount of water and anhydrous salt was given in percentages2 but if it be calculated for one and the same quantity of anhydrous salt1 or of water1 a great regularity will be observed in the amounts of the component parts in all these compounds4 ,it appears that for #ajf parts of anhydrous salt in the crystals separated out at -#bj there are #bgj parts of water2 in the crystals obtained at #ae there are #ahj parts of water2 in the crystals obtained from a supersaturated solution #abf parts1 in the crystals which separate out at #cd1 #ij parts1 and the crystals with the smallest amount of water1 #ah parts4 ,on comparing these quantities of water it may easily be seen that they are in simple proportion to each other1 for they are all divisible by #ah1 and are in the ratio #ae 3 #aj 3 #g 3 #e 3 #a4 ,naturally1 direct experiment1 however carefully it be conducted1 is hampered with errors1 but taking these unavoidable experimental errors into consideration1 it will be seen that for a given quantity of an anhydrous substance there occur1 in several of its compounds with water1 quantities of water which are in very simple multiple proportion4 ,this is observed in1 and is common to1 all definite chemical compounds4 ,this rule is called the law of multiple proportions4 ,it was discovered by ,dalton1 and will be evolved in further detail subsequently in this work4 ,for the present we will only state that the law of definite composition enables the composition of substances to be expressed by formul1 and the law of multiple proportions permits the application of whole numbers as coefficients of the symbols of the elements in these formul4 ,thus the formula ,na#b;,,co#c1aj;,h#b,o shows directly that in this crystallo-hydrate there are #ahj parts of water to #ajf parts by weight of the anhydrous salt1 because the formula of soda1 ,na#b;,,co#c1 directly answers to a weight of #ajf1 and the formula of water to #ah parts1 by weight1 which are here taken #aj times4 ,in the above examples of the combinations of water1 we saw the gradually increasing intensity of the bond between water and a substance with which it forms a homogeneous compound4 ,there is a series of such compounds with water1 in which the water is held with very great force1 and is only given up at a very high temperature1 and sometimes cannot be separated by any degree of heat without the entire decomposition of the substance4 ,in these compounds there is generally no outward sign whatever of their containing water4 ,a perfectly new substance is formed from an anhydrous substance and water1 in which sometimes the properties of neither one nor the other substance are observable4 ,in the majority of cases1 a considerable amount of heat is evolved in the formation of such compounds with water4 ,sometimes the heat evolved is so intense that a red heat is produced and light is emitted4 ,it is hardly to be wondered at1 after this1 that stable compounds are formed by such a combination4 ,their decomposition requires great heat2 a large amount of work is necessary to separate them into their component parts4 ,all such compounds are definite1 and1 generally1 completely and clearly definite4 ,the number of such definite compounds with water or hydrates1 in the narrow sense of the word1 is generally inconsiderable for each anhydrous substance2 in the greater number of cases1 there is formed only one such combination of a substance with water1 one hydrate1 having so great a stability4 ,the water contained in these compounds is often called water of constitution--i4e4 water which enters into the structure or composition of the given substance4 ,by this it is desired to express1 that in other cases the molecules of water are1 as it were1 separate from the molecules of that substance with which it is combined4 ,it is supposed that in the formation of hydrates this water1 even in the smallest particles1 forms one complete whole with the anhydrous substance4 ,many examples of the formation of such hydrates might be cited4 ,the most familiar example in practice is the hydrate of lime1 or so-called 'slaked' lime4 ,lime is prepared by burning limestone1 by which the carbonic anhydride is expelled from it1 and there remains a white stony mass1 which is dense1 compact1 and rather tenacious4 ,lime is usually sold in this form1 and bears the name of 'quick' or 'unslaked' lime4 ,if water be poured over such lime1 a great rise in temperature is remarked either directly1 or after a certain time4 ,the whole mass becomes hot1 part of the water is evaporated1 the stony mass in absorbing water crumbles into powder1 and if the water be taken in sufficient quantity and the lime be pure and well burnt1 not a particle of the original stony mass is left--it all crumbles into powder4 ,if the water be in excess1 then naturally a portion of it remains and forms a solution4 ,this process is called 'slaking' lime4 ,slaked lime is used in practice in intermixture with sand as mortar4 ,slaked lime is a definite hydrate of lime4 ,if it is dried at #ajj it retains #bdc p4c4 of water4 ,this water can only be expelled at a temperature above #djj1 and then quicklime is re-obtained4 ,the heat evolved in the combination of lime with water is so intense that it can set fire to wood1 sulphur1 gunpowder1 c4 ,even on mixing lime with ice the temperature rises to #ajj4 ,if lime be moistened with a small quantity of water in the dark1 a luminous effect is observed4 ,but1 nevertheless1 water may still be separated from this hydrate4#ga ,if phosphorus be burnt in dry air1 a white substance called 'phosphoric anhydride' is obtained4 ,it combines with water with such energy1 that the experiment must be conducted with great caution4 ,a red heat is produced in the formation of the compound1 and it is impossible to separate the water from the resultant hydrate at any temperature4 ,the hydrate formed by phosphoric anhydride is a substance which is totally undecomposable into its original component parts by the action of heat4 ,almost as energetic a combination occurs when sulphuric anhydride1 ,,so#c1 combines with water1 forming its hydrate1 sulphuric acid1 ,h#b,,so#d4 ,in both cases definite compounds are produced1 but the latter substance1 as a liquid1 and capable of decomposition by heat1 forms an evident link with solutions4 ,if #hj parts of sulphuric anhydride retain #ah parts of water1 this water cannot be separated from the anhydride1 even at a temperature of #cjj4 ,it is only by the addition of phosphoric anhydride1 or by a series of chemical transformations1 that this water can be separated from its compound with sulphuric anhydride4 ,oil of vitriol1 or sulphuric acid1 is such a compound4 ,if a larger proportion of water be taken1 it will combine with the ,h#b,,so#d2 for instance1 if #cf parts of water per #hj parts of sulphuric anhydride be taken1 a compound is formed which crystallises in the cold1 and melts at #h1 whilst oil of vitriol does not solidify even at -#cj4 ,if still more water be taken1 the oil of vitriol will dissolve in the remaining quantity of water4 ,an evolution of heat takes place1 not only on the addition of the water of constitution1 but in a less degree on further additions of water4#gb ,and therefore there is no distinct boundary1 but only a gradual transition1 between those chemical phenomena which are expressed in the formation of solutions and those which take place in the formation of the most stable hydrates4#gc #ga ,in combining with water one part by weight of lime evolves #bde units of heat4 ,a high temperature is obtained1 because the specific heat of the resulting product is small4 ,sodium oxide1 ,na#b,o1 in reacting on water1 ,h#b,o1 and forming caustic soda 7sodium hydroxide71 ,na,h,o1 evolves #eeb units of heat for each part by weight of sodium oxide4 #gb ,the diagram given in note #bh shows the evolution of heat on the mixture of sulphuric acid1 or monohydrate 7,h#b,,so#d1 i4e4 ,,so#c ,h#b,o71 with different quantities of water per #ajj vols4 of the resultant solution4 ,every #ih grams of sulphuric acid 7,h#b,,so#d7 evolve1 on the addition of #ah grams of water1 #f1cgi units of heat2 with twice or three times the quantity of water #i1dah and #aa1acg units of heat1 and with an infinitely large quantity of water #ag1hfj units of heat1 according to the determinations of ,thomsen4 ,he also showed that when ,h#b,,so#d is formed from ,,so#c 7 #hj7 and ,h#b,o 7 #ah71 #ba1cjh units of heat are evolved per #ih parts by weight of the resultant sulphuric acid4 #gc ,thus1 for different hydrates the stability with which they hold water is very dissimilar4 ,certain hydrates hold water very loosely1 and in combining with it evolve little heat4 ,from other hydrates the water cannot be separated by any degree of heat1 even if they are formed from anhydrides 7i4e4 anhydrous substances7 and water with little evolution of heat2 for instance1 acetic anhydride in combining with water evolves an inconsiderable amount of heat1 but the water cannot then be expelled from it4 ,if the hydrate 7acetic acid7 formed by this combination be strongly heated it either volatilises without change1 or decomposes into new substances1 but it does not again yield the original substances--i4e41 the anhydride and water1 at least in a liquid form4 ,here is an instance which gives the reason for calling the water entering into the composition of the hydrate1 water of constitution4 ,such1 for example1 is the water entering into the so-called caustic soda or sodium hydroxide 7see note #ga74 ,but there are hydrates which easily part with their water2 yet this water cannot be considered as water of crystallisation1 not only because sometimes such hydrates have no crystalline form1 but also because1 in perfectly analogous cases1 very stable hydrates are formed1 which are capable of particular kinds of chemical reactions1 as we shall subsequently learn4 ,such1 for example1 is the unstable hydrated oxide of copper1 which is not formed from water and oxide of copper1 but which is obtained just like far more stable hydrates1 for example1 the hydrated oxide of barium ,ba,h#b,o#b equal to ,ba,o ,h#b,o1 by the double decomposition of the solution of salts with alkalies4 ,in a word1 there is no distinct boundary either between the water of hydrates and of crystallisation1 or between solution and hydration4 ,it must be observed that in separating from an aqueous solution1 many substances1 without having a crystalline form1 hold water in the same unstable state as in crystals2 only this water cannot be termed 'water of crystallisation' if the substance which separates out has no crystalline form4 ,the hydrates of alumina and silica are examples of such unstable hydrates4 ,if these substances are separated from an aqueous solution by a chemical process1 then they always contain water4 ,the formation of a new chemical compound containing water is here particularly evident1 for alumina and silica in an anhydrous state have chemical properties differing from those they show when combined with water1 and do not combine directly with it4 ,the entire series of colloids on separating from water form similar compounds with it1 which have the aspect of solid gelatinous substances4 ,water is held in a considerable quantity in solidified glue or boiled albumin4 ,it cannot be expelled from them by pressure2 hence1 in this case there has ensued some kind of combination of the substance with water4 ,this water1 however1 is easily separated on drying2 but not the whole of it1 a portion being retained1 and this portion is considered to belong to the hydrate1 although in this case it is very difficult1 if not impossible1 to obtain definite compounds4 ,the absence of any distinct boundary lines between solutions1 crystallo-hydrates1 and ordinary hydrates above referred to1 is very clearly seen in such examples4 ,we have thus considered many aspects and degrees of combination of various substances with water1 or instances of the compounds of water1 when it and other substances form new homogeneous substances1 which in this case will evidently be complex--i4e4 made up of different substances--and although they are homogeneous1 yet it must be admitted that in them there exist those component parts which entered into their composition1 inasmuch as these parts may be re-obtained from them4 ,it must not be imagined that water really exists in hydrate of lime1 any more than that ice or steam exists in water4 ,when we say that water occurs in the composition of a certain hydrate1 we only wish to point out that there are chemical transformations in which it is possible to obtain that hydrate by means of water1 and other transformations in which this water may be separated out from the hydrate4 ,this is all simply expressed by the words1 that water enters into the composition of this hydrate4 ,if a hydrate be formed by feeble bonds1 and be decomposed even at the ordinary temperature1 and be a liquid1 then the water appears as one of the products of dissociation1 and this gives an idea of what solutions are1 and forms the fundamental distinction between them and other hydrates in which the water is combined with greater stability4 ,,chapter ,,ii ,,the ,,composition ,,of ,,water1 ,,hydrogen ,the question now arises1 ,is not water itself a compound substance8 ,cannot it be formed by the mutual combination of some component parts8 ,cannot it be broken up into its component parts8 ,there cannot be the least doubt that if it does split up1 and if it is a compound1 then it is a definite one characterised by the stability of the union between those component parts from which it is formed4 ,from the fact alone that water passes into all physical states as a homogeneous whole1 without in the least varying chemically in its properties and without splitting up into its component parts 7neither solutions nor many hydrates can be distilled--they are split up71 we must conclude1 from this fact alone1 that if water is a compound then it is a stable and definite chemical compound capable of entering into many other combinations4 ,like many other great discoveries in the province of chemistry1 it is to the end of the last century that we are indebted for the important discovery that water is not a simple substance1 that it is composed of two substances like a number of other compound substances4 ,this was proved by two of the methods by which the compound nature of bodies may be directly determined2 by analysis and by synthesis--that is1 by a method of the decomposition of water into1 and of the formation of water from1 its component parts4 ,in #agha ,cavendish first obtained water by burning hydrogen in oxygen1 both of which gases were already known to him4 ,he concluded from this that water was composed of two substances4 ,but he did not make more accurate experiments1 which would have shown the relative quantities of the component parts in water1 and which would have determined its complex nature with certainty4 ,although his experiments were the first1 and although the conclusion he drew from them was true1 yet such novel ideas as the complex nature of water are not easily recognised so long as there is no series of researches which entirely and indubitably proves the truth of such a conclusion4 ,the fundamental experiments which proved the complexity of water by the method of synthesis1 and of its formation from other substances1 were made in #aghi by ,monge1 ,lavoisier1 ,fourcroy1 and ,vauquelin4 ,they obtained four ounces of water by burning hydrogen1 and found that water consists of #ae parts of hydrogen and #he parts of oxygen4 ,it was also proved that the weight of water formed was equal to the sum of the weights of the component parts entering into its composition2 consequently1 water contains all the matter entering into oxygen and hydrogen4 ,the complexity of water was proved in this manner by a method of synthesis4 ,but we will turn to its analysis--i4e4 to its decomposition into its component parts4 ,the analysis may be more or less complete4 ,either both component parts may be obtained in a separate state1 or else only one is separated and the other is converted into a new compound in which its amount may be determined by weighing4 ,this will be a reaction of substitution1 such as is often taken advantage of for analysis4 ,the first analysis of water was thus conducted in #aghd by ,lavoisier and ,meusnier4 ,the apparatus they arranged consisted of a glass retort containing water previously purified1 and of which the weight had been determined4 ,the neck of the retort was inserted into a porcelain tube1 placed inside an oven1 and heated to a red heat by charcoal4 ,iron filings1 which decompose water at a red heat1 were placed inside this tube4 ,the end of the tube was connected with a worm1 for condensing any water which might pass through the tube undecomposed4 ,this condensed water was collected in a separate flask4 ,the gas formed by the decomposition was collected over water in a bell jar4 ,the aqueous vapour in passing over the red-hot iron was decomposed1 and a gas was formed from it whose weight could be determined from its volume1 its density being known4 ,besides the water which passed through the tube unaltered1 a certain quantity of water disappeared in the experiment1 and this quantity1 in the experiments of ,lavoisier and ,meusnier1 was equal to the weight of gas which was collected in the bell jar plus the increase in weight of the iron filings4 ,hence the water was decomposed into a gas1 which was collected in the bell jar1 and a substance1 which combined with the iron2 consequently1 it is composed of these two component parts4 ,this was the first analysis of water ever made2 but here only one 7and not both7 of the gaseous component parts of water was collected separately4 ,both the component parts of water can1 however1 be simultaneously obtained in a free state4 ,for this purpose the decomposition is brought about by a galvanic current or by heat1 as we shall learn directly4#a #a ,the first experiments of the synthesis and decomposition of water did not afford1 however1 an entirely convincing proof that water was composed of hydrogen and oxygen only4 ,davy1 who investigated the decomposition of water by the galvanic current1 thought for a long time that1 besides the gases1 an acid and alkali were also obtained4 ,he was only convinced of the fact that water contains nothing but hydrogen and oxygen by a long series of researches1 which showed him that the appearance of an acid and alkali in the decomposition of water proceeds from the presence of impurities 7especially from the presence of ammonium nitrate7 in water4 ,a final comprehension of the composition of water is obtained from the accurate determination of the quantities of the component parts which enter into its composition4 ,it will be seen from this how many data are necessary for proving the composition of water--that is1 of the transformations of which it is capable4 ,what has been said of water refers to all other compounds2 the investigation of each one1 the entire proof of its composition1 can only be obtained by the accumulation of a large mass of data referring to it4 ,water is a bad conductor of electricity--that is1 pure water does not transmit a feeble current2 but if any salt or acid be dissolved in it1 then its conductivity increases1 and on the passage of a current through acidified water it is decomposed into its component parts4 ,some sulphuric acid is generally added to the water4 ,by immersing platinum plates 7electrodes7 in this water 7platinum is chosen because it is not acted on by acids1 whilst many other metals are chemically acted on by acids71 and connecting them with a galvanic battery1 it will be observed that bubbles of gas appear on these plates4 ,the gas which separates is called detonating gas1#b because1 on ignition1 it very easily explodes4#c ,what takes place is as follows3--,first1 the water1 by the action of the current1 is decomposed into two gases4 ,the mixture of these gases forms detonating gas4 ,when detonating gas is brought into contact with an incandescent substance--for instance1 a lighted taper--the gases re-combine1 forming water1 the combination being accompanied by a great evolution of heat1 and therefore the vapour of the water formed expands considerably1 which it does very rapidly1 and as a consequence1 an explosion takes place--that is1 sound and increase of pressure1 and atmospheric disturbance1 as in the explosion of gunpowder4 #b ,this gas is collected in a voltameter4 #c ,in order to observe this explosion without the slightest danger1 it is best to proceed in the following manner4 ,some soapy water is prepared1 so that it easily forms soap bubbles1 and it is poured into an iron trough4 ,in this water1 the end of a gas-conducting tube is immersed4 ,this tube is connected with any suitable apparatus1 in which detonating gas is evolved4 ,soap bubbles1 full of this gas1 are then formed4 ,if the apparatus in which the gas is produced be then removed 7otherwise the explosion might travel into the interior of the apparatus71 and a lighted taper be brought to the soap bubbles1 a very sharp explosion takes place4 ,the bubbles should be small to avoid any danger2 ten1 each about the size of a pea1 suffice to give a sharp report1 like a pistol shot4 ,in order to discover what gases are obtained by the decomposition of water1 the gases which separate at each electrode must be collected separately4 ,for this purpose a ,v-shaped tube is taken2 one of its ends is open and the other fused up4 ,a platinum wire1 terminating inside the tube in a plate1 is fused into the closed end2 the closed end is entirely filled with water#d acidified with sulphuric acid1 and another platinum wire1 terminating in a plate1 is immersed in the open end4 ,if a current from a galvanic battery be now passed through the wires an evolution of gases will be observed1 and the gas which is obtained in the open branch passes into the air1 while that in the closed branch accumulates above the water4 ,as this gas accumulates it displaces the water1 which continues to descend in the closed and ascend into the open branch of the tubes4 ,when the water1 in this way1 reaches the top of the open end1 the passage of the current is stopped1 and the gas which was evolved from one of the electrodes only is obtained in the apparatus4 ,by this means it is easy to prove that a particular gas appears at each electrode4 ,if the closed end be connected with the negative pole--i4e4 with that joined to the zinc--then the gas collected in the apparatus is capable of burning4 ,this may be demonstrated by the following experiment3--,the bent tube is taken off the stand1 and its open end stopped up with the thumb and inclined in such a manner that the gas passes from the closed to the open end4 ,it will then be found1 on applying a lighted lamp or taper1 that the gas burns4 ,this combustible gas is hydrogen4 ,if the same experiment be carried on with a current passing in the opposite direction--that is1 if the closed end be joined up with the positive pole 7i4e4 with the carbon1 copper1 or platinum71 then the gas which is evolved from it does not itself burn1 but it supports combustion very vigorously1 so that a smouldering taper in it immediately bursts into flame4 ,this gas1 which is collected at the anode or positive pole1 is oxygen1 which is obtained1 as we saw before 7in the ,introduction71 from mercury oxide and is contained in air4 #d ,in order to fill the tube with water1 it is turned up1 so that the closed end points downwards and the open end upwards1 and water acidified with sulphuric acid is poured into it4 ,thus in the decomposition of water oxygen appears at the positive pole and hydrogen at the negative pole1#d bis so that detonating gas will be a mixture of both4 ,hydrogen burns in air from the fact that in doing so it re-forms water1 with the oxygen of the air4 ,detonating gas explodes from the fact that the hydrogen burns in the oxygen mixed with it4 ,it is very easy to measure the relative quantities of one and the other gas which are evolved in the decomposition of water4 ,for this purpose a funnel is taken1 whose orifice is closed by a cork through which two platinum wires pass4 ,these wires are connected with a battery4 ,acidified water is poured into the funnel1 and a glass cylinder full of water is placed over the end of each wire 7fig4 #ah74 ,on passing a current1 hydrogen and oxygen collect in these cylinders1 and it will easily be seen that two volumes of hydrogen are evolved for every one volume of oxygen4 ,this signifies that1 in decomposing1 water gives two volumes of hydrogen and one volume of oxygen4 #d bis ,owing to the gradual but steady progress made during the last twenty-five years in the production of an electric current from the dynamo and its transmission over considerable distances1 the electrolytic decomposition of many compound bodies has acquired great importance1 and the use of the electric current is making its way into many chemical manufactures4 ,hence1 ,prof4 ,d4 ,a4 ,lachinoff's proposal to obtain hydrogen and oxygen 7both of which have many applications7 by means of electrolysis 7either of a #aj to #ae per cent4 solution of caustic soda or a #ae per cent4 solution of sulphuric acid7 may find a practical application1 at all events in the future4 ,in general1 owing to their simplicity1 electrolytic methods have a great future1 but as yet1 so long as the production of an electric current remains so costly1 their application is limited4 ,and for this reason1 although certain of these methods are mentioned in this work1 they are not specially considered1 the more so since a profitable and proper use of the electric current for chemical purposes requires special electro-technical knowledge which beginners cannot he assumed to have1 and therefore1 an exposition of the principles of electrotechnology as applied1 to the production of chemical transformations1 although referred to in places1 does not come within the scope of the present work4 ,illustration3 ,,fig4 #ah4--;,decomposition of water by the galvanic current1 for determining the relation between the volumes of hydrogen and oxygen4 ,water is also decomposed into its component parts by the action of heat4 ,at the melting point of silver 7#ifj71 and in its presence1 water is decomposed and the oxygen absorbed by the molten silver1 which dissolves it so long as it is liquid4 ,but directly the silver solidifies the oxygen is expelled from it4 ,however1 this experiment is not entirely convincing2 it might be thought that in this case the decomposition of the water did not proceed from the action of heat1 but from the action of the silver on water--that silver decomposes water1 taking up the oxygen4 ,if steam be passed through a red-hot tube1 whose internal temperature attains #a1jjj1 then a portion#e of the water decomposes into its component parts1 forming detonating gas4 ,but on passing into the cooler portions of the apparatus this detonating gas again reunites and forms water4 ,the hydrogen and oxygen obtained combine together at a lower temperature4#f ,apparently the problem--to show the decomposability of water at high temperatures--is unattainable4 ,it was considered as such before ,henri ,sainte-,claire ,deville 7in the fifties7 introduced the conception of dissociation into chemistry1 as of a change of chemical state resembling evaporation1 if decomposition be likened to boiling1 and before he had demonstrated the decomposability of water by the action of heat in an experiment which will presently be described4 ,in order to demonstrate clearly the dissociation of water1 or its decomposability by heat1 at a temperature approaching that at which it is formed1 it was necessary to separate the hydrogen from the oxygen at a high temperature1 without allowing the mixture to cool4 ,deville took advantage of the difference between the densities of hydrogen and oxygen4 #e ,as water is formed by the combination of oxygen and hydrogen1 with a considerable evolution of heat1 and as it can also be decomposed1 this reaction is a reversible one 7see ,introduction71 and consequently at a high temperature the decomposition of water cannot be complete--it is limited by the opposite reaction4 ,strictly speaking1 it is not known how much water is decomposed at a given temperature1 although many efforts 7,bunsen1 and others7 have been made in various directions to solve this question4 ,not knowing the coefficient of expansion1 and the specific heat of gases at such high temperatures1 renders all calculations 7from observations of the pressure on explosion7 doubtful4 #f ,grove1 in #ahdg1 observed that a platinum wire fused in the oxyhydrogen flame--that is1 having acquired the temperature of the formation of water--and having formed a molten drop at its end which fell into water1 evolved detonating gas--that is1 decomposed water4 ,it therefore follows that water already decomposes at the temperature of its formation4 ,at that time1 this formed a scientific paradox2 this we shall unravel only with the development of the conceptions of dissociation1 introduced into science by ,henri ,sainte-,claire ,deville1 in #aheg4 ,these conceptions form an important epoch in science1 and their development is one of the problems of modern chemistry4 ,the essence of the matter is that1 at high temperatures1 water exists but also decomposes1 just as a volatile liquid1 at a certain temperature1 exists both as a liquid and as a vapour4 ,similarly as a volatile liquid saturates a space1 attaining its maximum tension1 so also the products of dissociation have their maximum tension1 and once that is attained decomposition ceases1 just as evaporation ceases4 ,under like conditions1 if the vapour be allowed to escape 7and therefore its partial pressure be diminished71 evaporation recommences1 so also if the products of decomposition be removed1 decomposition again continues4 ,these simple conceptions of dissociation introduce infinitely varied consequences into the mechanism of chemical reactions1 and therefore we shall have occasion to return to them very often4 ,we may add that ,grove also concluded that water was decomposed at a white heat1 from the fact that he obtained detonating gas by passing steam through a tube with a wire heated strongly by an electric current1 and also by passing steam over molten oxide of lead1 he obtained1 on the one hand1 litharge 7 oxide of lead and oxygen71 and on the other1 metallic lead formed by the action of hydrogen4 ,illustration3 ,,fig4 #ai4--;,decomposition of water by the action of heat1 and the separation of the hydrogen formed by its permeating through a porous tube4 ,a wide porcelain tube ,p 7fig4 #ai7 is placed in a furnace1 which can be raised to a high temperature 7it should be heated with small pieces of good coke74 ,in this tube there is inserted a second tube ,t1 of smaller diameter1 made of unglazed earthenware and therefore porous4 ,the ends of the tube are luted to the wide tube1 and two tubes1 ,c and ,c'1 are inserted into the ends1 as shown in the drawing4 ,with this arrangement it is possible for a gas to pass into the annular space between the walls of the two tubes1 from whence it can be collected4 ,steam from a retort or flask is passed through the tube ,d1 into the inner porous tube ,t4 ,this steam on entering the red-hot space is decomposed into hydrogen and oxygen4 ,the densities of these gases are very different1 hydrogen being sixteen times lighter than oxygen4 ,light gases1 as we saw above1 penetrate through porous surfaces very much more rapidly than denser gases1 and therefore the hydrogen passes through the pores of the tube into the annular space very much more rapidly than the oxygen4 ,the hydrogen which separates out into the annular space can only be collected when this space does not contain any oxygen4 ,if any air remains in this space1 then the hydrogen which separates out will combine with its oxygen and form water4 ,for this reason a gas incapable of supporting combustion--for instance1 nitrogen or carbonic anhydride--is previously passed into the annular space4 ,thus the carbonic anhydride is passed through the tube ,c1 and the hydrogen1 separated from the steam1 is collected through the tube ,c'1 and will be partly mixed with carbonic anhydride4 ,a certain portion of the carbonic anhydride will penetrate through the pores of the unglazed tube into the interior of the tube ,t4 ,the oxygen will remain in this tube1 and the volume of the remaining oxygen will be half that of the volume of hydrogen which separates out from the annular space4#f bis #f bis ,part of the oxygen will also penetrate through the pores of the tube2 but1 as was said before1 a much smaller quantity than the hydrogen1 and as the density of oxygen is sixteen times greater than that of hydrogen1 the volume of oxygen which passes through the porous walls will be four times less than the volume of hydrogen 7the quantities of gases passing through porous walls are inversely proportional to the square roots of their densities74 ,the oxygen which separates out into the annular space will combine1 at a certain fall of temperature1 with the hydrogen2 but as each volume of oxygen only requires two volumes of hydrogen1 whilst at least four volumes of hydrogen will pass through the porous walls for every volume of oxygen that passes1 therefore1 part of the hydrogen will remain free1 and can be collected from the annular space4 ,a corresponding quantity of oxygen remaining from the decomposition of the water can be collected from the internal tube4 ,the decomposition of water is effected much more easily by a method of substitution1 taking advantage of the affinity of substances for the oxygen or the hydrogen of water4 ,if a substance be added to water1 which takes up the oxygen and replaces the hydrogen--then we shall obtain the latter gas from the water4 ,thus with sodium1 water gives hydrogen1 and with chlorine1 which takes up the hydrogen1 oxygen is obtained4 ,hydrogen is evolved from water by many metals1 which are capable of forming oxides in air--that is1 which are capable of burning or combining with oxygen4 ,the capacity of metals for combining with oxygen1 and therefore for decomposing water1 or for the evolution of hydrogen1 is very dissimilar4#g ,among metals1 potassium and sodium exhibit considerable energy in this respect4 ,the first occurs in potash1 the second in soda4 ,they are both lighter than water1 soft1 and easily change in air4 ,by bringing one or the other of them in contact with water at the ordinary temperature1#h a quantity of hydrogen1 corresponding with the amount of the metal taken1 may be directly obtained4 ,one gram of hydrogen1 occupying a volume of #aaaf litres at #j and #gfj mm41 is evolved from every #ci grams of potassium1 or #bc grams of sodium4 ,the phenomenon may be observed in the following way3 a solution of sodium in mercury--or 'sodium amalgam1' as it is generally called--is poured into a vessel containing water1 and owing to its weight sinks to the bottom2 the sodium held in the mercury then acts on the water like pure sodium1 liberating hydrogen4 ,the mercury does not act here1 and the same amount of it as was taken for dissolving the sodium is obtained in the residue4 ,the hydrogen is evolved gradually in the form of bubbles1 which pass through the liquid4 #g ,in order to demonstrate the difference of the affinity of oxygen for different elements1 it is enough to compare the amounts of heats which are evolved in their combination with #af parts by weight of oxygen2 in the case of sodium 7when ,na#b,o is formed1 or #df parts of ,na combine with #af parts of oxygen1 according to ,beketoff7 #ajj1jjj calories 7or units of heat71 are evolved1 for hydrogen 7when water1 ,h#b,o1 is formed7 #fi1jjj calories1 for iron 7when the oxide ,fe,o is formed7 #fi1jjj1 and if the oxide ,fe#b,o#c is formed1 #fd1jjj calories1 for zinc 7,zn,o is formed7 #hf1jjj calories1 for lead 7when ,pb,o is formed7 #ea1jjj calories1 for copper 7when ,cu,o is formed7 #ch1jjj calories1 and for mercury 7,hg,o is formed7 #ca1jjj calories4 ,these figures cannot correspond directly with the magnitude of the affinities1 for the physical and mechanical side of the matter is very different in the different cases4 ,hydrogen is a gas1 and1 in combining with oxygen1 gives a liquid2 consequently it changes its physical state1 and1 in doing so1 evolves heat4 ,but zinc and copper are solids1 and1 in combining with oxygen1 give solid oxides4 ,the oxygen1 previously a gas1 now passes into a solid or liquid state1 and1 therefore1 also must have given up its store of heat in forming oxides4 ,as we shall afterwards see1 the degree of contraction 7and consequently of mechanical work7 was different in the different cases1 and therefore the figures expressing the heat of combination cannot directly depend on the affinities1 on the loss of internal energy previously in the elements4 ,nevertheless1 the figures above cited correspond1 in a certain degree1 with the order in which the elements stand in respect to their affinity for oxygen1 as may be seen from the fact that the mercury oxide1 which evolves the least heat 7among the above examples71 is the least stable is easily decomposed1 giving up its oxygen2 whilst sodium1 the formation of whose oxide is accompanied by the greatest evolution of heat1 is able to decompose all the other oxides1 taking up their oxygen4 ,in order to generalise the connection between affinity and the evolution and the absorption of heat1 which is evident in its general features1 and was firmly established by the researches of ,favre and ,silbermann 7about #ahdj71 and then of ,thomsen 7in ,denmark7 and ,berthelot 7in ,france71 many investigators1 especially the one last mentioned1 established the law of maximum work4 ,this states that only those chemical reactions take place of their own accord in which the greatest amount of chemical 7latent1 potential7 energy is transformed into heat4 ,but1 in the first place1 we are not able1 judging from what has been said above1 to distinguish that heat which corresponds with purely chemical action from the sum total of the heat observed in a reaction 7in the calorimeter72 in the second place1 there are evidently endothermal reactions which proceed under the same circumstances as exothermal 7carbon burns in the vapour of sulphur with absorption of heat1 whilst in oxygen it evolves heat72 and1 in the third place1 there are reversible reactions1 which when taking place in one direction evolve heat1 and when taking place in the opposite direction absorb it2 and1 therefore1 the principle of maximum work in its elementary form is not supported by science4 ,but the subject continues to be developed1 and will probably lead to a general law1 such as thermal chemistry does not at present possess4 #h ,if a piece of metallic sodium be thrown into water1 it floats on it 7owing to its lightness71 keeps in a state of continual motion 7owing to the evolution of hydrogen on all sides71 and immediately decomposes the water1 evolving hydrogen1 which can be lighted4 ,this experiment may1 however1 lead to an explosion should the sodium stick to the walls of the vessel1 and begin to act on the limited mass of water immediately adjacent to it 7probably in this case ,na,h,o forms with ,na1 ,na#b,o1 which acts on the water1 evolving much heat and rapidly forming steam71 and the experiment should therefore be carried on with caution4 ,the decomposition of water by sodium may he better demonstrated1 and with greater safety1 in the following manner4 ,into a glass cylinder filled with mercury1 and immersed in a mercury bath1 water is first introduced1 which will1 owing to its lightness1 rise to the top1 and then a piece of sodium wrapped in paper is introduced with forceps into the cylinder4 ,the metal rises through the mercury to the surface of the water1 on which it remains1 and evolves hydrogen1 which collects in the cylinder1 and may be tested after the experiment has been completed4 ,the safest method of making this experiment is1 however1 as follows4 ,the sodium 7cleaned from the naphtha in which it is kept7 is either wrapped in fine copper gauze and held by forceps1 or else held in forceps at the end of which a small copper cage is attached1 and is then held under water4 ,the evolution of hydrogen goes on quietly1 and it may he collected in a bell jar and then lighted4 ,beyond the hydrogen evolved and a solid substance1 which remains in solution 7it may be obtained by evaporating the resultant solution7 no other products are here obtained4 ,consequently1 from the two substances 7water and sodium7 taken1 the same number of new substances 7hydrogen and the substance dissolved in water7 have been obtained1 from which we may conclude that the reaction which here takes place is a reaction of double decomposition or of substitution4 ,the resultant solid is nothing else but the so-called caustic soda 7sodium hydroxide71 which is made up of sodium1 oxygen1 and half of the hydrogen contained in the water4 ,therefore1 the substitution took place between the hydrogen and the sodium1 namely half of the hydrogen in the water was replaced by the sodium1 and was evolved in a free state4 ,hence the reaction which takes place here may be expressed by the equation ,h#b,o ,na ,na,h,o ,h2 the meaning of this is clear from what has already been said4#i #i ,this reaction is vigorously exothermal1 i4e4 it is accompanied by the evolution of heat4 ,if a sufficient quantity of water be taken the whole of the sodium hydroxide1 ,na,h,o1 formed is dissolved1 and about #db1ejj units of heat are evolved per #bc grams of sodium taken4 ,as #dj grams of sodium hydroxide are produced1 and they in dissolving1 judging from direct experiment1 evolve about #aj1jjj calories2 therefore1 without an excess of water1 and without the formation of a solution1 the reaction would evolve about #cb1ejj calories4 ,we shall afterwards learn that hydrogen contains in its smallest isolable particles ,h#b and not ,h1 and therefore it follows that the reaction should be written thus--#b,na #b;,h#b,o ,h#b #b,na,o,h1 and it then corresponds with an evolution of heat of #fe1jjj calories4 ,and as ,n4 ,n4 ,beketoff showed that ,na#b,o1 or anhydrous oxide of sodium1 forms the hydrate1 or sodium hydroxide 7caustic soda71 #b,na,h,o1 with water1 evolving about #ce1ejj calories1 therefore the reaction #b,na ,h#b,o ,h#b ,na#b,o corresponds to #bi1ejj calories4 ,this quantity of heat is less than that which is evolved in combining with water1 in the formation of caustic soda1 and therefore it is not to be wondered at that the hydrate1 ,na,h,o1 is always formed and not the anhydrous substance ,na#b,o4 ,that such a conclusion1 which agrees with facts1 is inevitable is also seen from the fact that1 according to ,beketoff1 the anhydrous sodium oxide1 ,na#b,o1 acts directly on hydrogen1 with separation of sodium1 ,na#b,o ,h ,na,h,o ,na4 ,this reaction is accompanied by an evolution of heat equal to about #c1jjj calories1 because ,na#b,o ,h#b,o gives1 as we saw1 #ce1ejj calories and ,na ,h#b,o evolves #cb1ejj calories4 ,however1 an opposite reaction also takes place--,na,h,o ,na ,na#b,o ,h 7both with the aid of heat7--consequently1 in this case heat is absorbed4 ,in this we see an example of calorimetric calculations and the limited application of the law of maximum work for the general phenomena of reversible reactions1 to which the case just considered belongs4 ,but it must be remarked that all reversible reactions evolve or absorb but little heat1 and the reason of the law of maximum work1 not being universal must first of all be looked for in the fact that we have no means of separating the heat which corresponds with the purely chemical process from the sum total of the heat observed1 and as the structure of a number of substances is altered by heat and also by contact1 we can scarcely hope that the time approaches when such a distinction will be possible4 ,a heated substance1 in point of fact1 has no longer the original energy of its atoms--that is1 the act of heating not only alters the store of motion of the molecules but also of the atoms forming the molecules1 in other words1 it makes the beginning of or preparation for chemical change4 ,from this it must be concluded that thermochemistry1 or the study of the heat accompanying chemical transformations1 cannot he identified with chemical mechanics4 ,thermo-chemical data form a part of it1 but they alone cannot give it4 ,sodium and potassium act on water at the ordinary temperature4 ,other heavier metals only act on it with a rise of temperature1 and then not so rapidly or vigorously4 ,thus magnesium and calcium only liberate hydrogen from water at its boiling point1 and zinc and iron only a red heat1 whilst a whole series of heavy metals1 such as copper1 lead1 mercury1 silver1 gold1 and platinum1 do not in the least decompose water at any temperature1 and do not replace its hydrogen4 ,from this it is clear that hydrogen may be obtained by the decomposition of steam by the action of iron 7or zinc7 with a rise of temperature4 ,the experiment is conducted in the following manner3 pieces of iron 7filings1 nails1 c471 are placed in a porcelain tube1 which is then subjected to a strong heat and steam passed through it4 ,the steam1 coming into contact with the iron1 gives up its oxygen to it1 and thus the hydrogen is set free and passes out at the other end of the tube together with undecomposed steam4 ,this method1 which is historically very significant1#aj is practically inconvenient1 as it requires a rather high temperature4 ,further1 this reaction1 as a reversible one 7a red-hot mass of iron decomposes a current of steam1 forming oxide and hydrogen2 and a mass of oxide of iron1 heated to redness in a stream of hydrogen1 forms iron and steam71 does not proceed in virtue of the comparatively small difference between the affinity of oxygen for iron 7or zinc7 and for hydrogen1 but only because the hydrogen escapes1 as it is formed1 in virtue of its elasticity4#aa ,if the oxygen compounds--that is1 the oxides--which are obtained from the iron or zinc1 be able to pass into solution1 then the affinity acting in solution is added1 and the reaction may become non-reversible1 and proceed with comparatively much greater facility4#ab ,as the oxides of iron and zinc1 by themselves insoluble in water1 are capable of combining with 7have an affinity for7 acid oxides 7as we shall afterwards fully consider71 and form saline and soluble substances1 with acids1 or hydrates having acid properties1 hence by the action of such hydrates1 or of their aqueous solutions1#ac iron and zinc are able to liberate hydrogen with great ease at the ordinary temperature--that is1 they act on solutions of acids just as sodium acts on water4#ad ,sulphuric acid1 ,h#b,,so#d1 is usually chosen for this purpose2 the hydrogen is displaced from it by many metals with much greater facility than directly from water1 and such a displacement is accompanied by the evolution of a large amount of heat4#ae ,when the hydrogen in sulphuric acid is replaced by a metal1 a substance is obtained which is called a salt of sulphuric acid or a sulphate4 ,thus1 by the action of zinc on sulphuric acid1 hydrogen and zinc sulphate ,zn,s,o#d1ae bis are obtained4 ,the latter is a solid substance1 soluble in water4 ,in order that the action of the metal on the acid should go on regularly1 and to the end1 it is necessary that the acid should be diluted with water1 which dissolves the salt as it is formed2 otherwise the salt covers the metal1 and hinders the acid from attacking it4 ,usually the acid is diluted with from three to five times its volume of water1 and the metal is covered with this solution4 ,in order that the metal should act rapidly on the acid1 it should present a large surface1 so that a maximum amount of the reacting substances may come into contact in a given time4 ,for this purpose the zinc is used as strips of sheet zinc1 or in the granulated form 7that is1 zinc which has been poured from a certain height1 in a molten state1 into water74 ,the iron should be in the form of wire1 nails1 filings1 or cuttings4 ,illustration3 ,,fig4 #bj4--;,apparatus for the preparation of hydrogen from zinc and sulphuric acid4 #aj ,the composition of water1 as we saw above1 was determined by passing steam over red-hot iron2 the same method has been used for making hydrogen for filling balloons4 ,an oxide having the composition ,fe#c,o#d is formed in the reaction1 so that it is expressed by the equation #c;,fe #d;,h#b,o ,fe#c,o#d #h;,h4 #aa ,the reaction between iron and water 7note #aj7 is reversible4 ,by heating the oxide in a current of hydrogen1 water and iron are obtained4 ,from this it follows1 from the principle of chemical equilibria1 that if iron and hydrogen be taken1 and also oxygen1 but in such a quantity that it is insufficient for combination with both substances1 then it will divide itself between the two2 part of it will combine with the iron and the other part with the hydrogen1 but a portion of both will remain in an uncombined state4 ,therefore1 if iron and water be placed in a closed space1 decomposition of the water will proceed on heating to the temperature at which the reaction #c;,fe #d;,h#b,o ,fe#c,o#d #h;,h commences2 but it ceases1 does not go on to the end1 because the conditions for a reverse reaction are attained1 and a state of equilibrium will ensue after the decomposition of a certain quantity of water4 ,here again 7see note #i7 the reversibility is connected with the small heat effect1 and again both reactions 7direct and reverse7 proceed at a red heat4 ,but if1 in the above-described reaction1 the hydrogen escapes as it is evolved1 then its partial pressure does not increase with its formation1 and therefore all the iron can he oxidised by the water4 ,in this we see the elements of that influence of mass to which we shall have occasion to return later4 ,with copper and lead there will be no decomposition1 either at the ordinary or at a high temperature1 because the affinity of these metals for oxygen is much less than that of hydrogen4 #ab ,in general1 if reversible as well as non-reversible reactions can take place between substances acting on each other1 then1 judging by our present knowledge1 the non-reversible reactions take place in the majority of cases1 which obliges one to acknowledge the action1 in this case1 of comparatively strong affinities4 ,the reaction1 ,zn ,h#b,,so#d ,h#b ,zn,s,o#d1 which takes place in solutions at the ordinary temperature1 is scarcely reversible under these conditions1 but at a certain high temperature it becomes reversible1 because at this temperature zinc sulphate and sulphuric acid split up1 and the action must take place between the water and zinc4 ,from the preceding proposition results proceed which are in some cases verified by experiment4 ,if the action of zinc or iron on a solution of sulphuric acid presents a non-reversible reaction1 then we may by this means obtain hydrogen in a very compressed state1 and compressed hydrogen will not act on solutions of sulphates of the above-named metals4 ,this is verified in reality as far as was possible in the experiments to keep up the compression or pressure of the hydrogen4 ,those metals which do not evolve hydrogen with acids1 on the contrary1 should1 at least at an increase of pressure1 be displaced by hydrogen4 ,and in fact ,brunner showed that gaseous hydrogen displaces platinum and palladium from the aqueous solutions of their chlorine compounds1 but not gold1 and ,beketoff succeeded in showing that silver and mercury1 under a considerable pressure1 are separated from the solutions of certain of their compounds by means of hydrogen4 ,reaction already commences under a pressure of six atmospheres1 if a weak solution of silver sulphate be taken2 with a stronger solution a much greater pressure is required1 however1 for the separation of the silver4 #ac ,for the same reason1 many metals in acting on solutions of the alkalis displace hydrogen4 ,aluminium acts particularly clearly in this respect1 because its oxide gives a soluble compound with alkalis4 ,for the same reason tin1 in acting on hydrochloric acid1 evolves hydrogen1 and silicon does the same with hydrofluoric acid4 ,it is evident that in such cases the sum of all the affinities plays a part2 for instance1 taking the action of zinc on sulphuric acid1 we have the affinity of zinc for oxygen 7forming zinc oxide1 ,zn,o71 the affinity of its oxide for sulphuric anhydride1 ,,so#c 7forming zinc sulphate1 ,zn,s,o#d71 and the affinity of the resultant salt1 ,zn,s,o#d1 for water4 ,it is only the first-named affinity that acts in the reaction between water and the metal1 if no account is taken of those forces 7of a physico-mechanical character7 which act between the molecules 7for instance1 the cohesion between the molecules of the oxide7 and those forces 7of a chemical character7 which act between the atoms forming the molecule1 for instance1 between the atoms of hydrogen giving the molecule ,h#b containing two atoms4 ,i consider it necessary to remark1 that the hypothesis of the affinity or endeavour of heterogeneous atoms to enter into a common system and in harmonious motion 7i4e4 to form a compound molecule7 must inevitably be in accordance with the hypothesis of forces including homogeneous atoms to form complex molecules 7for instance1 ,h#b71 and to build up the latter into solid or liquid substances1 in which the existence of an attraction between the homogeneous particles must certainly be admitted4 ,therefore1 those forces which bring about solution must also be taken into consideration4 ,these are all forces of one and the same series1 and in this may be seen the great difficulties surrounding the study of molecular mechanics and its province--chemical mechanics4 #ad ,it is acknowledged that zinc itself acts on water1 even at the ordinary temperature1 but that the action is confined to small masses and only proceeds at the surface4 ,in reality1 zinc1 in the form of a very fine powder1 or so-called 'zinc dust1' is capable of decomposing water with the formation of oxide 7hydrated7 and hydrogen4 ,the oxide formed acts on sulphuric acid1 water then dissolves the salt produced1 and the action continues because one of the products of the action of water on zinc1 zinc oxide1 is removed from the surface4 ,one might naturally imagine that the reaction does not proceed directly between the metal and water1 but between the metal and the acid1 but such a simple representation1 which we shall cite afterwards1 hides the mechanism of the reaction1 and does not permit of its actual complexity being seen4 #ae ,according to ,thomsen the reaction between zinc and a very weak solution of sulphuric acid evolves about #ch1jjj calories 7zinc sulphate being formed7 per #fe parts by weight of zinc2 and #ef parts by weight of iron--which combine1 like #fe parts by weight of zinc1 with #af parts by weight of oxygen--evolve about #be1jjj calories 7forming ferrous sulphate1 ,fe,s,o#d74 ,paracelsus observed the action of metals on acids in the seventeenth century2 but it was not until the eighteenth century that ,lmery determined that the gas which is evolved in this action is a particular one which differs from air and is capable of burning4 ,even ,boyle confused it with air4 ,cavendish determined the chief properties of the gas discovered by ,paracelsus4 ,at first it was called 'inflammable air'2 later1 when it was recognised that in burning it gives water1 it was called hydrogen1 from the ,greek words for water and generator4 #ae bis ,if1 when the sulphuric acid is poured over the zinc1 the evolution of the hydrogen proceed too slowly1 it may be greatly accelerated by adding a small quantity of a solution of ,cu,s,o#d or ,pt,cl#d to the acid4 ,the reason of this is explained in ,chap4 ,,xvi41 note #aj bis4 ,the usual method of obtaining hydrogen is as follows3--,a certain quantity of granulated zinc is put into a double-necked1 or ,woulfe's1 bottle4 ,into one neck a funnel is placed1 reaching to the bottom of the bottle1 so that the liquid poured in may prevent the hydrogen from escaping through it4 ,the gas escapes through a special gas conducting tube1 which is firmly fixed1 by a cork1 into the other neck1 and ends in a water bath 7fig4 #bj71 under the orifice of a glass cylinder full of water4#af ,if sulphuric acid be now poured into the ,woulfe's bottle it will soon be seen that bubbles of a gas are evolved1 which is hydrogen4 ,the first part of the gas evolved should not be collected1 as it is mixed with the air originally in the apparatus4 ,this precaution should be taken in the preparation of all gases4 ,time must be allowed for the gas evolved to displace all the air from the apparatus1 otherwise in testing the combustibility of the hydrogen an explosion may occur from the formation of detonating gas 7the mixture of the oxygen of the air with the hydrogen74#ag #af ,as laboratory experiments with gases require a certain preliminary knowledge1 we will describe certain practical methods for the collection and preparation of gases4 ,when in laboratory practice an intermittent supply of hydrogen 7or other gas which is evolved without the aid of heat7 is required the apparatus represented in fig4 #ba is the most convenient4 ,it consists of two bottles1 having orifices at the bottom1 in which corks with tubes are placed1 and these tubes are connected by an india-rubber tube 7sometimes furnished with a spring clamp74 ,zinc is placed in one bottle1 and dilute sulphuric acid in the other4 ,the neck of the former is closed by a cork1 which is fitted with a gas-conducting tube with a stopcock4 ,if the two bottles are connected with each other and the stopcock be opened1 the acid will flow to the zinc and evolve hydrogen4 ,if the stopcock be closed1 the hydrogen will force out the acid from the bottle containing the zinc1 and the action will cease4 ,or the vessel containing the acid may be placed at a lower level than that containing the zinc1 when all the liquid will flow into it1 and in order to start the action the acid vessel may be placed on a higher level than the other1 and the acid will flow to the zinc4 ,it can also be employed for collecting gases 7as an aspirator or gasometer74 ,illustration3 ,,fig4 #ba4--;,a very convenient apparatus for the preparation of gases obtained without heat4 ,it may also replace an aspirator or gasometer4 ,in laboratory practice1 however1 other forms of apparatus are generally employed for exhausting1 collecting1 and holding gases4 ,we will here cite the most usual forms4 ,an aspirator usually consists of a vessel furnished with a stopcock at the bottom4 ,a stout cork1 through which a glass tube passes1 is fixed into the neck of this vessel4 ,if the vessel be filled up with water to the cork and the bottom stopcock is opened1 then the water will run out and draw gas in4 ,for this purpose the glass tube is connected with the apparatus from which it is desired to pump out or exhaust the gas4 ,illustration3 ,,fig4 #bb4--;,continuous aspirator4 ,the tube d should be more than #cb feet long4 ,the aspirator represented in fig4 #bb may be recommended for its continuous action4 ,it consists of a tube d which widens out at the top1 the lower part being long and narrow4 ,in the expanded upper portion c1 two tubes are sealed2 one1 e1 for drawing in the gas1 whilst the other1 b1 is connected to the water supply w4 ,the amount of water supplied through the tube b must be less than the amount which can be carried off by the tube d4 ,owing to this the water in the tube d will flow through it in cylinders alternating with cylinders of gas1 which will be thus carried away4 ,the gas which is drawn through may be collected from the end of the tube d1 but this form of pump is usually employed where the air or gas aspirated is not to be collected4 ,if the tube d is of considerable length1 say #dj ft4 or more1 a very fair vacuum will be produced1 the amount of which is shown by the gauge g2 it is often used for filtering under reduced pressure1 as shown in the figure4 ,if water be replaced by mercury1 and the length of the tube d be greater than #gfj mm41 the aspirator may be employed as an air-pump1 and all the air may be exhausted from a limited space2 for instance1 by connecting g with a hollow sphere4 ,illustration3 ,,fig4 #bc4--;,gasholder4 ,gasholders are often used for collecting and holding gases4 ,they are made of glass1 copper1 or tin plate4 ,the usual form is shown in fig4 #bc4 ,the lower vessel ,b is made hermetically tight--i4e41 impervious to gases--and is filled with water4 ,a funnel is attached to this vessel 7on several supports74 ,the vessel ,b communicates with the bottom of the funnel by a stopcock b and a tube a1 reaching to the bottom of the vessel ,b4 ,if water be poured into the funnel and the stopcocks a and b opened1 the water will run through a1 and the air escape from the vessel ,b by b4 ,a glass tube f runs up the side of the vessel ,b1 with which it communicates at the top and bottom1 and shows the amount of water and gas the gasholder contains4 ,in order to fill the gasholder with a gas1 it is first filled with water1 the cocks a1 b and e are closed1 the nut d unscrewed1 and the end of the tube conducting the gas from the apparatus in which it is generated is passed into d4 ,as the gas fills the gasholder1 the water runs out at d4 ,if the pressure of a gas be not greater than the atmospheric pressure and it be required to collect it in the gasholder1 then the stopcock e is put into communication with the space containing the gas4 ,then1 having opened the orifice d1 the gasholder acts like an aspirator2 the gas will pass through e1 and the water run out at d4 ,if the cocks be closed1 the gas collected in the gasholder may be easily preserved and transported4 ,if it be desired to transfer this gas into another vessel1 then a gas-conducting tube is attached to e1 the cock a opened1 b and d closed1 and the gas will then pass out at e1 owing to its pressure in the apparatus being greater than the atmospheric pressure1 due to the pressure of the water poured into the funnel4 ,if it be required to fill a cylinder or flask with the gas1 it is filled with water and inverted in the funnel1 and the stopcocks b and a opened4 ,then water will run through a1 and the gas will escape from the gasholder into the cylinder through b4 #ag ,when it is required to prepare hydrogen in large quantities for filling balloons1 copper vessels or wooden casks lined with lead are employed2 they are filled with scrap iron1 over which dilute sulphuric acid is poured4 ,the hydrogen generated from a number of casks is carried through lead pipes into special casks containing water 7in order to cool the gas7 and lime 7in order to remove acid fumes74 ,to avoid loss of gas all the joints are made hermetically tight with cement or tar4 ,in order to fill his gigantic balloon 7of #be1jjj cubic metres capacity71 ,giffard1 in #ahgh1 constructed a complicated apparatus for giving a continuous supply of hydrogen1 in which a mixture of sulphuric acid and water was continually run into vessels containing iron1 and from which the solution of iron sulphate formed was continually drawn off4 ,when coal gas1 extracted from coal1 is employed for filling balloons1 it should be as light1 or as rich in hydrogen1 as possible4 ,for this reason1 only the last portions of the gas coming from the retorts are collected1 and1 besides this1 it is then sometimes passed through red-hot vessels1 in order to decompose the hydrocarbons as much as possible2 charcoal is deposited in the red-hot vessels1 and hydrogen remains as gas4 ,coal gas may be yet further enriched in hydrogen1 and consequently rendered lighter1 by passing it over an ignited mixture of charcoal and lime4 ,l4 ,mond 7,london7 proposes to manufacture hydrogen on a large scale from water gas 7see infra1 and ,chapters ,,viii4 and ,,ix471 which contains a mixture of oxide of carbon 7,,co7 and hydrogen1 and is produced by the action of steam upon incandescent coke 7,c ,h#b,o ,,co ,h#b74 ,he destroys the oxide of carbon by converting it into carbon and carbonic anhydride 7#b;,,co ,c ,,co#b71 which is easily done by means of incandescent1 finely-divided metallic nickel2 the carbon then remains with the nickel1 from which it may be removed by burning it in air1 and the nickel can then be used over again 7see ,chapter ,,ix41 ,note #bd bis74 ,the ,,co#b formed is removed from the hydrogen by passing it through milk of lime4 ,this process should apparently give hydrogen on a large scale more economically than any of the methods hitherto proposed4 ,hydrogen1 besides being contained in water1 is also contained in many other substances1#ah and may be obtained from them4 ,as examples of this1 it may be mentioned 7#a7 that a mixture of formate of sodium1 ,c,h,na,o#b1 and caustic soda1 ,na,h,o1 when heated to redness1 forms sodium carbonate1 ,na#b;,,co#c1 and hydrogen1 ,h#b2ai 7#b7 that a number of organic substances are decomposed at a red heat1 forming hydrogen1 among other gases1 and thus it is that hydrogen is contained in ordinary coal gas4 #ah ,of the metals1 only a very few combine with hydrogen 7for example1 sodium71 and give substances which are easily decomposed4 ,of the non-metals1 the halogens 7fluorine1 chlorine1 bromine1 and iodine7 most easily form hydrogen compounds2 of these the hydrogen compound of chlorine1 and still more that of fluorine1 is stable1 whilst those of bromine and iodine are easily decomposed1 especially the latter4 ,the other non-metals--for instance1 sulphur1 carbon1 and phosphorus--give hydrogen compounds of different composition and properties1 but they are all less stable than water4 ,the number of the carbon compounds of hydrogen is enormous1 but there are very few among them which are not decomposed1 with separation of the carbon and hydrogen1 at a red heat4 #ai ,the reaction expressed by the equation ,c,na,h,o#b ,na,h,o ,c,na#b,o#c ,h#b may be effected in a glass vessel1 like the decomposition of copper carbonate or mercury oxide 7see ,introduction72 it is non-reversible1 and takes place without the presence of water1 and therefore ,pictet 7see later7 made use of it to obtain hydrogen under great pressure4 ,charcoal itself liberates hydrogen from steam at a high temperature2#bj but the reaction which here takes place is distinguished by a certain complexity1 and will therefore be considered later4 #bj ,the reaction between charcoal and superheated steam is a double one--that is1 there may be formed either carbonic oxide1 ,,co 7according to the equation ,h#b,o ,c ,h#b ,,co71 or carbonic anhydride ,,co#b 7according to the equation #b;,h#b,o ,c #b;,h#b ,,co#b71 and the resulting mixture is called water-gas2 we shall speak of it in ,chapter ,,ix4 ,the properties of hydrogen4--,hydrogen presents us with an example of a gas which at first sight does not differ from air4 ,it is not surprising1 therefore1 that ,paracelsus1 having discovered that an ariform substance is obtained by the action of metals on sulphuric acid1 did not determine exactly its difference from air4 ,in fact1 hydrogen1 like air1 is colourless1 and has no smell2#ba but a more intimate acquaintance with its properties proves it to be entirely different from air4 ,the first sign which distinguishes hydrogen from air is its combustibility4 ,this property is so easily observed that it is the one to which recourse is usually had in order to recognise hydrogen1 if it is evolved in a reaction1 although there are many other combustible gases4 ,but before speaking of the combustibility and other chemical properties of hydrogen1 we will first describe the physical properties of this gas1 as we did in the case of water4 ,it is easy to show that it is one of the lightest gases4#bb ,if passed into the bottom of a flask full of air1 hydrogen will not remain in it1 but1 owing to its lightness1 rapidly escapes and mixes with the atmosphere4 ,if1 however1 a cylinder whose orifice is turned downwards be filled with hydrogen1 it will not escape1 or1 more correctly1 it will only slowly mix with the atmosphere4 ,this may be demonstrated by the fact that a lighted taper sets fire to the hydrogen at the orifice of the cylinder1 and is itself extinguished inside the cylinder4 ,hence1 hydrogen1 being itself combustible1 does not support combustion4 ,the great lightness of hydrogen is taken advantage of for balloons4 ,ordinary coal gas1 which is often also used for the same purpose1 is only about twice as light as air1 whilst hydrogen is #ad-a_/b times lighter than air4 ,a very simple experiment with soap bubbles very well illustrates the application of hydrogen for filling balloons4 ,charles1 of ,paris1 showed the lightness of hydrogen in this way1 and constructed a balloon filled with hydrogen almost simultaneously with ,montgolfier4 ,one litre of pure and dry hydrogen#bc at #j and #gfj mm4 pressure weighs #jjhihf gram2 that is1 hydrogen is almost #ad-a_/b 7more exactly1 #adci7 times lighter than air4 ,it is the lightest of all gases4 ,the small density of hydrogen determines many remarkable properties which it shows2 thus1 hydrogen passes exceedingly rapidly through fine orifices1 its molecules 7,chapter ,i47 being endued with the greatest velocity4#bd ,at pressures somewhat higher than the atmospheric pressure1 all other gases exhibit a greater compressibility and co-efficient of expansion than they should according to the laws of ,mariotte and ,gay-,lussac2 whilst hydrogen1 on the contrary1 is compressed to a less degree than it should be from the law of ,mariotte1#be and with a rise of pressure it expands slightly less than at the atmospheric pressure4#bf ,however1 hydrogen1 like air and many other gases which are permanent at the ordinary temperature1 does not pass into a liquid state under a very considerable pressure1#bg but is compressed into a lesser volume than would follow from ,mariotte's law4#bh ,from this it may be concluded that the absolute boiling point of hydrogen1 and of gases resembling it1#bi lies very much below the ordinary temperature2 that is1 that the liquefaction of this gas is only possible at low temperatures1 and under great pressures4#cj ,this conclusion was verified 7#ahgg7 by the experiments of ,pictet and ,cailletet4#ca ,they compressed gases at a very low temperature1 and then allowed them to expand1 either by directly decreasing the pressure or by allowing them to escape into the air1 by which means the temperature fell still lower1 and then1 just as steam when rapidly rarefied#cb deposits liquid water in the form of a fog1 hydrogen in expanding forms a fog1 thus indicating its passage into a liquid state4 ,but as yet it has been impossible to preserve this liquid1 even for a short time1 to determine its properties1 notwithstanding the employment of a temperature of -#bjj and a pressure of #bjj atmospheres1#cc although by these means the gases of the atmosphere may be kept in a liquid state for a long time4 ,this is due to the fact that the absolute boiling point of hydrogen lies lower than that of all other known gases1 which also depends on the extreme lightness of hydrogen4#cd #ba ,hydrogen obtained by the action of zinc or iron on sulphuric acid generally smells of hydrogen sulphide 7like rotten eggs71 which it contains in admixture4 ,as a rule such hydrogen is not so pure as that obtained by the action of an electric current or of sodium on water4 ,the impurity of the hydrogen depends on the impurities contained in the zinc1 or iron1 and sulphuric acid1 and on secondary reactions which take place simultaneously with the main reaction4 ,impure hydrogen may be easily freed from the impurities it contains3 some of them--namely1 those having acid properties--are absorbed by caustic soda1 and therefore may be removed by passing the hydrogen through a solution of this substance2 another series of impurities is absorbed by a solution of mercuric chloride2 and1 lastly1 a third series is absorbed by a solution of potassium permanganate4 ,if absolutely pure hydrogen be required1 it is sometimes obtained by the decomposition of water 7previously boiled to expel all air1 and mixed with pure sulphuric acid7 by the galvanic current4 ,only the gas evolved at the negative electrode is collected4 ,or else1 an apparatus like that which gives detonating gas is used1 the positive electrode1 however1 being immersed under mercury containing zinc in solution4 ,the oxygen which is evolved at this electrode then immediately1 at the moment of its evolution1 combines with the zinc1 and this compound dissolves in the sulphuric acid and forms zinc sulphate1 which remains in solution1 and therefore the hydrogen generated will be quite free from oxygen4 #bb ,an inverted beaker is attached to one arm of the beam of a tolerably sensitive balance1 and its weight counterpoised by weights in the pan attached to the other arm1 ,if the beaker be then filled with hydrogen it rises1 owing to the air being replaced by hydrogen4 ,thus1 at the ordinary temperature of a room1 a litre of air weighs about #ab gram1 and on replacing the air by hydrogen a decrease in weight of about #a gram per litre is obtained4 ,moist hydrogen is heavier than dry--for aqueous vapour is nine times heavier than hydrogen4 ,in filling balloons it is usually calculated that 7it being impossible to have perfectly dry hydrogen or to obtain it quite free from air7 the lifting force due to the difference between the weights of equal volumes of hydrogen and air is equal to #a kilogram 7 #a1jjj grams7 per cubic metre 7 #a1jjj litres74 #bc ,the density of hydrogen in relation to the air has been repeatedly determined by accurate experiments4 ,the first determination1 made by ,lavoisier1 was not very exact2 taking the density of air as unity1 he obtained #jjgfi for that of hydrogen--that is1 hydrogen as thirteen times lighter than air4 ,more accurate determinations are due to ,thomsen1 who obtained the figure #jjfic2 ,berzelius and ,dulong1 who obtained #jjfhh2 and ,dumas and ,boussingault1 who obtained #jjfide4 ,regnault1 and more recently ,le ,duc 7#ahib71 took two spheres of considerable capacity1 which contained equal volumes of air 7thus avoiding the necessity of any correction for weighing them in air74 ,both spheres were attached to the scale pans of a balance4 ,one was sealed up1 and the other first weighed empty and then full of hydrogen4 ,thus1 knowing the weight of the hydrogen filling the sphere1 and the capacity of the sphere1 it was easy to find the weight of a litre of hydrogen2 and1 knowing the weight of a litre of air at the same temperature and pressure1 it was easy to calculate the density of hydrogen4 ,regnault1 by these experiments1 found the average density of hydrogen to be #jjfibf in relation to air2 ,le ,duc1 #jjfidh 7with a possible error of #jjjjja71 and this latter figure must now be looked upon as near to the truth4 ,in this work ,i shall always refer the densities of all gases to hydrogen1 and not to air2 ,i will therefore give1 for the sake of clearness1 the weight of a litre of dry pure hydrogen in grams at a temperature t and under a pressure ,h 7measured in millimetres of mercury at #j1 in lat4 #de74 ,the weight of a litre of hydrogen #jjhihf 7,h_/#gfj7 #a_/7a #jjjcfgt7 gram4 ,for aronauts it is very useful to know1 besides this1 the weight of the air at different heights1 and ,i therefore insert the adjoining table1 constructed on the basis of ,glaisher's data1 for the temperature and moisture of the atmospheric strata in clear weather4 ,all the figures are given in the metrical system--#a1jjj millimetres #cicg inches1 #a1jjj kilograms #bbjdccge lbs41 #a1jjj cubic metres #ce1caff cubic feet4 ,the starting temperature at the earth's surface is taken as #ae ,c41 its moisture #fj p4c41 pressure #gfj millimetres4 ,the pressures are taken as indicated by an aneroid barometer1 assumed to be corrected at the sea level and at lat4 #de ,c4 ,if the height above the level of the sea equal z kilometres1 then the weight of #a cubic metre of air may be approximately taken as #abbb- #jabz #jjjcggz#b kilogram4 ---------------------------------------- --------------- ,pressure,temperature,moisture ,height ,weight of the air 7metres77#a1jjj cubic metres7 ---------------------------------------- --------------- #gfj mm4 #ae ,c4 #fj p4c4 #j #abbb kilos4 #gjj #aaj #fd #fij #aada #fej #gf #fd #acjj #ajgc #fjj #dc #fc #aifj #ajjc #eej -#aj #fb #bffj #ica #ejj -#bd #eh #cdbj #heg #dej -#eh #eb #dbej #gha #djj -#ia #dd #eagj #gjc #cej -#abe #cf #faij #fbd #cjj -#aei #bg #gcfj #edb #bej -#aib #ah #hgbj #deg ---------------------------------------- --------------- ,although the figures in this table are calculated with every possible care from average data1 yet they can only be taken approximately1 for in every separate case the conditions1 both at the earth's surface and in the atmosphere1 will differ from those here taken4 ,in calculating the height to which a balloon can ascend1 it is evident that the density of gas in relation to air must be known4 ,this density for ordinary coal gas is from #jf to #jce1 and for hydrogen with its ordinary contents of moisture and air from #ja to #jae4 ,hence1 for instance1 it may be calculated that a balloon of #a1jjj cubic metres capacity filled with pure hydrogen1 and weighing 7the envelope1 tackle1 people1 and ballast7 #gbg kilograms1 will only ascend to a height of about #d1bej metres4 #bd ,if a cracked flask be filled with hydrogen and its neck immersed under water or mercury1 then the liquid will rise up into the flask1 owing to the hydrogen passing through the cracks about #ch times quicker than the air is able to pass through these cracks into the flask4 ,the same phenomenon may be better observed if1 instead of a flask1 a tube be employed1 whose end is closed by a porous substance1 such as graphite1 unglazed earthenware1 or a gypsum plate4 #be ,according to ,boyle and ,mariotte's law1 for a given gas at a constant temperature the volume decreases by as many times as the pressure increases2 that is1 this law requires that the product of the volume v and the pressure p for a given gas should be a constant quantity3 pv ,c1 a constant quantity which does not vary with a change of pressure4 ,this equation does very nearly and exactly express the observed relation between the volume and pressure1 but only within comparatively small variations of pressure1 density1 and volume4 ,if these variations be in any degree considerable1 the quantity pv proves to be dependent on the pressure1 and it either increases or diminishes with an increase of pressure4 ,in the former case the compressibility is less than it should he according to ,mariotte's law1 in the latter case it is greater4 ,we will call the first case a positive discrepancy 7because then d7pv7_/d7p7 is greater than zero71 and the second case a negative discrepancy 7because then d7pv7_/d7p7 is less than zero74 ,determinations made by myself 7in the seventies71 ,m4 ,l4 ,kirpicheff1 and ,v4 ,a4 ,hemilian showed that all known gases at low pressures--i4e4 when considerably rarefied--present positive discrepancies4 ,on the other hand1 it appears from the researches of ,cailletet1 ,natterer1 and ,amagat that all gases under great pressures 7when the volume obtained is #ejj-a1jjj times less than under the atmospheric pressure7 also present positive discrepancies4 ,thus under a pressure of #b1gjj atmospheres air is compressed1 not #b1gjj times1 but only #hjj1 and hydrogen #a1jjj times4 ,hence the positive kind of discrepancy is1 so to say1 normal to gases4 ,and this is easily intelligible4 ,if a gas followed ,mariotte's law1 or if it were compressed to a greater extent than is shown by this law1 then under great pressures it would attain a density greater than that of solid and liquid substances1 which is in itself improbable and even impossible by reason of the fact that solid and liquid substances are themselves but little compressible4 ,for instance1 a cubic centimetre of oxygen at #j and under the atmospheric pressure weighs about #jjjad gram1 and at a pressure of #c1jjj atmospheres 7this pressure is attained in guns7 it would1 if it followed ,mariotte's law1 weigh #db grams--that is1 would be about four times heavier than water--and at a pressure of #aj1jjj atmospheres it would be heavier than mercury4 ,besides this1 positive discrepancies are probable because the molecules of a gas themselves must occupy a certain volume4 ,considering that ,mariotte's law1 strictly speaking1 applies only to the intermolecular space1 we can understand the necessity of positive discrepancies4 ,if we designate the volume of the molecules of a gas by b 7like van der ,waals1 see ,chap4 ,i41 ,note #cd71 then it must be expected that p7v-b7 ,c4 ,hence pv ,c bp1 which expresses a positive discrepancy4 ,supposing that for hydrogen pv #a1jjj1 at a pressure of one metre of mercury1 according to the results of ,regnault's1 ,amagat's1 and ,natterer's experiments1 we obtain b as approximately #jg to #ji4 ,thus the increase of pv with the increase of pressure must be considered as the normal law of the compressibility of gases4 ,hydrogen presents such a positive compressibility at all pressures1 for it presents positive discrepancies from ,mariotte's law1 according to ,regnault1 at all pressures above the atmospheric pressure4 ,hence hydrogen is1 so to say1 a perfect gas4 ,no other gas behaves so simply with a change of pressure4 ,all other gases at pressures from #a to #cj atmospheres present negative discrepancies--that is1 they are then compressed to a greater degree than should follow from ,mariotte's law1 as was shown by the determinations of ,regnault1 which were verified when repeated by myself and ,boguzsky4 ,thus1 for example1 on changing the pressure from #d to #bj metres of mercury--that is1 on increasing the pressure five times--the volume only decreased #dic times when hydrogen was taken1 and #ejf when air was taken4 ,the positive discrepancies from the law at low pressures are of particular interest1 and1 according to the above-mentioned determinations made by myself1 ,kirpicheff1 and ,hemilian1 and verified 7by two methods7 by ,k4 ,d4 ,kraevitch and ,prof4 ,ramsay 7,london1 #ahid71 they are proper to all gases 7even to those which are easily compressed into a liquid state1 such as carbonic and sulphurous anhydrides74 ,these discrepancies approach the case of a very high rarefaction of gases1 where a gas is near to a condition of maximum dispersion of its molecules1 and perhaps presents a passage towards the substance termed 'luminiferous ether' which fills up interplanetary and interstellar space4 ,if we suppose that gases are rarefiable to a definite limit only1 having attained which they 7like solids7 do not alter in volume with a decrease of pressure1 then on the one hand the passage of the atmosphere at its upper limits into a homogeneous ethereal medium becomes comprehensible1 and on the other hand it would be expected that gases would1 in a state of high rarefaction 7i4e4 when small masses of gases occupy large volumes1 or when furthest removed from a liquid state71 present positive discrepancies from ,boyle and ,mariotte's law4 ,our present acquaintance with this province of highly rarefied gases is very limited 7because direct measurements are exceedingly difficult to make1 and are hampered by possible errors of experiment1 which may be considerable71 and its further development promises to elucidate much in respect to natural phenomena4 ,to the three states of matter 7solid1 liquid1 and gaseous7 it is evident a fourth must yet be added1 the ethereal or ultra-gaseous 7as ,crookes proposed71 understanding by this1 matter in its highest possible state of rarefaction4 #bf ,the law of ,gay-,lussac states that all gases in all conditions present one coefficient of expansion #jjjcfg2 that is1 when heated from #j to #ajj they expand like air2 namely1 a thousand volumes of a gas measured at #j will occupy #acfg volumes at #ajj4 ,regnault1 about #ahej1 showed that ,gay-,lussac's law is not entirely correct1 and that different gases1 and also one and the same gas at different pressures1 have not quite the same coefficients of expansion4 ,thus the expansion of air between #j and #ajj is #jcfg under the ordinary pressure of one atmosphere1 and at three atmospheres it is #jcga1 the expansion of hydrogen is #jcff1 and of carbonic anhydride #jcg4 ,regnault1 however1 did not directly determine the change of volume between #j and #ajj1 but measured the variation of tension with the change of temperature2 but since gases do not entirely follow ,mariotte's law1 the change of volume cannot be directly judged by the variation of tension4 ,the investigations carried on by myself and ,kayander1 about #ahgj1 showed the variation of volume on heating from #j to #ajj under a constant pressure4 ,these investigations confirmed ,regnault's conclusion that ,gay-,lussac's law is not entirely correct1 and further showed 7#a7 that the expansion per volume from #j to #ajj under a pressure of one atmosphere1 for air #jcfh1 for hydrogen #jcfg1 for carbonic anhydride #jcgc1 for hydrogen bromide #jchf1 c42 7#b7 that for gases which are more compressible than should follow from ,mariotte's law the expansion by heat increases with the pressure--for example1 for air at a pressure of three and a half atmospheres1 it equals #jcga1 for carbonic anhydride at one atmosphere it equals #jcgc1 at three atmospheres #jchi1 and at eight atmospheres #jdac2 7#c7 that for gases which are less compressible than should follow from ,mariotte's law1 the expansion by heat decreases with an increase of pressure--for example1 for hydrogen at one atmosphere #jcfg1 at eight atmospheres #jcfi1 for air at a quarter of an atmosphere #jcgj1 at one atmosphere #jcfh2 and hydrogen like air 7and all gases7 is less compressed at low pressures than should follow from ,mariotte's law 7see ,note #be74 ,hence1 hydrogen1 starting from zero to the highest pressures1 exhibits a gradually1 although only slightly1 varying coefficient of expansion1 whilst for air and other gases at the atmospheric and higher pressures1 the coefficient of expansion increases with the increase of pressure1 so long as their compressibility is greater than should follow from ,mariotte's law4 ,but when at considerable pressures1 this kind of discrepancy passes into the normal 7see ,note #be71 then the coefficient of expansion of all gases decreases with an increase of pressure1 as is seen from the researches of ,amagat4 ,the difference between the two coefficients of expansion1 for a constant pressure and for a constant volume1 is explained by these relations4 ,thus1 for example1 for air at a pressure of one atmosphere the true coefficient of expansion 7the volume varying at constant pressure7 #jjjcfh 7according to ,mendeleff and ,kayander7 and the variation of tension 7at a constant volume1 according to ,regnault7 #jjjcfg4 #bg ,permanent gases are those which cannot be liquefied by an increase of pressure alone4 ,with a rise of temperature1 all gases and vapours become permanent gases4 ,as we shall afterwards learn1 carbonic anhydride becomes a permanent gas at temperatures above #ca1 and at lower temperatures it has a maximum tension1 and may be liquefied by pressure alone4 ,the liquefaction of gases1 accomplished by ,faraday 7see ,ammonia1 ,chapter ,,vi47 and others1 in the first half of this century1 showed that a number of substances are capable1 like water1 of taking all three physical states1 and that there is no essential difference between vapours and gases1 the only distinction being that the boiling points 7or the temperature at which the tension #gfj mm47 of liquids lie above the ordinary temperature1 and those of liquefied gases below1 and consequently a gas is a superheated vapour1 or vapour heated above the boiling point1 or removed from saturation1 rarefied1 having a lower tension than that maximum which is proper to a given temperature and substance4 ,we will here cite the maximum tensions of certain liquids and gases at various temperatures1 because they may be taken advantage of for obtaining constant temperatures by changing the pressure at which boiling or the formation of saturated vapours takes place4 7,i may remark that the dependence between the tension of the saturated vapours of various substances and the temperature is very complex1 and usually requires three or four independent constants1 which vary with the nature of the substance1 and are found from the dependence of the tension p on the temperature t given by experiment2 but in #ahib ,k4 ,d4 ,kraevitch showed that this dependence is determined by the properties of a substance1 such as its density1 specific heat1 and latent heat of evaporation47 ,the temperatures 7according to the air thermometer7 are placed on the left1 and the tension in millimetres of mercury 7at #j7 on the right-hand side of the equations4 ,carbon bisulphide1 ,,cs#b1 #j #abgi2 #aj #aihe2 #bj #biha2 #cj #dcaf2 #dj #fage2 #ej #hega4 ,chlorobenzene1 ,c#f;,h#e;,cl1 #gj #igi2 #hj #adah2 #ij #bjhd2 #ajj #bibh2 #aaj #djbf2 #abj #edbh2 #acj #gaij4 ,aniline1 ,c#f;,h#g,n1 #aej #bhcg2 #afj #chgj2 #agj #eaef2 #ahj #fggb2 #ahe #ggae4 ,methyl salicylate1 ,c#h;,h#h,o#c1 #ahj #bidd2 #aij #ccji2 #bjj #dcbd2 #baj #eege2 #bbj #gajb2 #bbd #ggii4 ,mercury1 ,hg1 #cjj #bdfh2 #caj #cjdi2 #cbj #cgcg2 #ccj #dedd2 #cdj #edhf2 #cej #fehj2 #cei #ggji4 ,sulphur1 ,s1 #cie #cjj2 #dbc #ejj2 #ddc #gjj2 #deb #hjj2 #dei #ijj4 ,these figures 7,ramsay and ,young7 show the possibility of obtaining constant temperatures in the vapours of boiling liquids by altering the pressure4 ,we may add the following boiling points under a pressure of #gfj mm4 7according to the air thermometer by ,collendar and ,griffiths1 #ahia73 aniline1 #ahd #ac2 naphthalene1 #bag #id2 benzophenone1 #cje #hb2 mercury1 #cef #gf2 triphenyl-methane1 #cef #dd2 sulphur1 #ddd #ec4 ,and melting points3 tin1 #bca #fh2 bismuth1 #bfi #bb2 lead1 #cbg #fi2 and zinc1 #dag #eg4 ,these data may be used for obtaining a constant temperature and for verifying thermometers4 ,the same object may be attained by the melting points of certain salts1 determined according to the air thermometer by ,v4 ,meyer and ,riddle 7#ahic73 ,na,cl1 #hea2 ,na,br1 #gbg2 ,na,i1 #fej2 ,k,cl1 #gfj2 ,k,br1 #gae2 ,,ki1 #fbc2 ,k#b;,,co#c1 #ajde2 ,na#b;,,co#c1 #ajih2 ,na#b;,b#d,o#g1 #hgc2 ,na#b,,so#d1 #hdc2 ,k#b,,so#d1 #ajgc4 ,the tension of liquefied gases is expressed in atmospheres4 ,sulphurous anhydride1 ,,so#b1-cj #jd2-bj #jf2-aj #a2 #j #ae2 #aj #bc2 #bj #cb2 #cj #ec4 ,ammonia1 ,,nh#c1-dj #jg2-cj #aa2-bj #ah2 -#aj #bh2 #j #db2 #aj #fj2 #bj #hd4 ,carbonic anhydride1 ,,co#b1-aae #jjcc2 -#hj #a2-gj #ba2-fj #ci2-ej #fh2-dj #aj2-bj #bc2 #j #ce2 #aj #df2 #bj #eh4 ,nitrous oxide1 ,n#b,o1-#abe #jjcc2-ib #a2-hj #ai2-ej #gf2 -#bj #bca2 #j #cfa2 #bj #eec4 ,ethylene1 ,c#b;,h#d1-adj #jjcc2-acj #ja2 -#ajc #a2-dj #ac2-a #db4 ,air1-#aia #a2-aeh #ad2-adj #ci4 ,nitrogen1 ,n#b1-bjc #jjhe2-aic #a2-afj #ad2-adf #cb4 ,the methods of liquefying gases 7by pressure and cold7 will be described under ammonia1 nitrous oxide1 sulphurous anhydride1 and in later footnotes4 ,we will now turn our attention to the fact that the evaporation of volatile liquids1 under various1 and especially under low1 pressures1 gives an easy means for obtaining low temperatures4 ,thus liquefied carbonic anhydride1 under the ordinary pressure1 reduces the temperature to -#hj1 and when it evaporates in a rarefied atmosphere 7under an air-pump7 to #be mm4 7 #jjcc atmosphere7 the temperature1 judging by the above-cited figures1 falls to -#aae 7,dewar74 ,even the evaporation of liquids of common occurrence1 under low pressures easily attainable with an air-pump1 may produce low temperatures1 which may be again taken advantage of for obtaining still lower temperatures4 ,water boiling in a vacuum becomes cold1 and under a pressure of less than #de mm4 it freezes1 because its tension at #j is #de mm4 ,a sufficiently low temperature may be obtained by forcing fine streams of air through common ether1 or liquid carbon bisulphide1 ,,cs#b1 or methyl chloride1 ,,ch#c;,cl1 and other similar volatile liquids4 ,in the adjoining table are given1 for certain gases1 7#a7 the number of atmospheres necessary for their liquefaction at #ae1 and 7#b7 the boiling points of the resultant liquids under a pressure of #gfj mm4 ,c#b;,h#d ,n#b,o ,,co#b1 ,h#b,s 7#a7 #db #ca #eb #aj 7#b7 -#ajc -#ib -#hj -#gd ,as,h#c1 ,,nh#c ,h,cl ,,ch#c;,cl ,c#b,n#b ,,so#b 7#a7 #h #g #be #d #d #c 7#b7 -#eh -#ch -#ce -#bd -#ba -#aj #bh ,natterer's determinations 7#ahea-ahed71 together with ,amagat's results 7#ahhj-ahhh71 show that the compressibility of hydrogen1 under high pressures1 may be expressed by the following figures3-- p #a #ajj #ajjj #bejj v #a #jjajg #jjjai #jjjac pv #a #ajg #ai #cbe s #jaa #ajc #eh #he where p the pressure in metres of mercury1 v the volume1 if the volume taken under a pressure of #a metre #a1 and s the weight of a litre of hydrogen at #bj in grams4 ,if hydrogen followed ,mariotte's law1 then under a pressure of #b1ejj metres1 one litre would contain not #he1 but #bfe grams4 ,it is evident from the above figures that the weight of a litre of the gas approaches a limit as the pressure increases1 which is doubtless the density of the gas when liquefied1 and therefore the weight of a litre of liquid hydrogen will probably be near #ajj grams 7density about #ja1 being less than that of all other liquids74 #bi ,cagniard de ,latour1 on heating ether in a closed tube to about #aij1 observed that at this temperature the liquid is transformed into vapour occupying the original volume--that is1 having the same density as the liquid4 ,the further investigations made by ,drion and myself showed that every liquid has such an absolute boiling point1 above which it cannot exist as a liquid and is transformed into a dense gas4 ,in order to grasp the true signification of this absolute boiling temperature1 it must be remembered that the liquid state is characterised by a cohesion of its particles which does not exist in vapours and gases4 ,the cohesion of liquids is expressed in their capillary phenomena 7the breaks in a column of liquid1 drop formation1 and rise in capillary tubes1 c471 and the product of the density of a liquid into the height to which it rises in a capillary tube 7of a definite diameter7 may serve as the measure of the magnitude of cohesion4 ,thus1 in a tube of #a mm4 diameter1 water at #ae rises 7the height being corrected for the meniscus7 #adh mm41 and ether at t to a height #ece-jjbht mm4 ,the cohesion of a liquid is lessened by heating1 and therefore the capillary heights are also diminished4 ,it has been shown by experiment that this decrement is proportional to the temperature1 and hence by the aid of capillary observations we are able to form an idea that at a certain rise of temperature the cohesion may become #j4 ,for ether1 according to the above formula1 this would occur at #aia4 ,if the cohesion disappear from a liquid it becomes a gas1 for cohesion is the only point of difference between these two states4 ,a liquid in evaporating and overcoming the force of cohesion absorbs heat4 ,therefore1 the absolute boiling point was defined by me 7#ahfa7 as that temperature at which 7a7 a liquid cannot exist as a liquid1 but forms a gas which cannot pass into a liquid state under any pressure whatever2 7b7 cohesion #j2 and 7c7 the latent heat of evaporation #j4 ,this definition was but little known until ,andrews 7#ahfi7 explained the matter from another aspect4 ,starting from gases1 he discovered that carbonic anhydride cannot be liquefied by any degree of compression at temperatures above #ca1 whilst at lower temperatures it can be liquefied4 ,he called this temperature the critical temperature4 ,it is evident that it is the same as the absolute boiling point4 ,we shall afterwards designate it by tc4 ,at low temperatures a gas which is subjected to a pressure greater than its maximum tension 7,note #bg7 is transformed into a liquid1 which1 in evaporating1 gives a saturated vapour possessing this maximum tension2 whilst at temperatures above tc the pressure to which the gas is subjected may increase indefinitely4 ,however1 under these conditions the volume of the gas does not change indefinitely but approaches a definite limit 7see ,note #bh7--that is1 it resembles in this respect a liquid or a solid which is altered but little in volume by pressure4 ,the volume which a liquid or gas occupies at tc is termed the critical volume1 and corresponds with the critical pressure1 which we will designate by pc and express in atmospheres4 ,it is evident from what has been said that the discrepancies from ,mariotte and ,boyle's law1 the absolute boiling point1 the density in liquid and compressed gaseous states1 and the properties of liquids1 must all he intimately connected together4 ,we will consider these relations in one of the following notes4 ,at present we will supplement the above observations by the values of tc and pc for certain liquids and gases which have been investigated in this respect-- ---------------------------------------- --------------- tc pc tc pc ---------------------------------------- --------------- ,n#b -#adf #cc ,h#b,s #ajh #ib ,,co -#adj #ci ,c#b,n#b #abd #fb ,o#b -#aai #ej ,,nh#c #aca #aad ,,ch#d -#ajj #ej ,,ch#c;,cl #ada #gc ,,no -#ic #ga ,,so#b #aee #gi ,c#b;,h#d #aj #ea ,c#e;,h#aj #aib #cd ,,co#b #cb #gg ,c#d;,h#aj,o #aic #dj ,n#b,o #ec #ge ,c,h,cl#c #bfh #ee ,c#b;,h#b #cg #fh ,,cs#b #bgh #gh ,h,cl #eb #hf ,c#f;,h#f #bib #fj ,h#b,o #cfe #bjj ,c#f;,h#e;,f #bhg #de ,,ch#c,,oh #bdj #gi ,c#f;,h#e;,cl #cfj #de ,c#b;,h#e,,oh #bdc #fc ,c#f;,h#e;,br #cig #de ,,ch#c;,,cooh #cbb #eg ,c#f;,h#e;,i #ddh #de ---------------------------------------- --------------- ,young and ,guy 7#ahia7 showed that tc and pc clearly depend upon the composition and molecular weight4 #cj ,i came to this conclusion in #ahgj 7,ann4 ,phys4 ,chem4 #ada1 #fbc74 #ca ,pictet1 in his researches1 effected the direct liquefaction of many gases which up to that time had not been liquefied4 ,he employed the apparatus used for the manufacture of ice on a large scale1 employing the vaporisation of liquid sulphurous anhydride1 which may be liquefied by pressure alone4 ,this anhydride is a gas which is transformed into a liquid at the ordinary temperature under a pressure of several atmospheres 7see ,note #bg71 and boils at -#aj at the ordinary atmospheric pressure4 ,this liquid1 like all others1 boils at a lower temperature under a diminished pressure1 and by continually pumping out the gas which comes off by means of a powerful air-pump its boiling point falls as low as -#ge4 ,consequently1 if on the one hand we force liquid sulphurous anhydride into a vessel1 and on the other hand pump out the gas from the same vessel by powerful air-pumps1 then the liquefied gas will boil in the vessel1 and cause the temperature in it to fall to -#ge4 ,if a second vessel is placed inside this vessel1 then another gas may be easily liquefied in it at the low temperature produced by the boiling liquid sulphurous anhydride4 ,pictet in this manner easily liquefied carbonic anhydride1 ,,co#b 7at -#fj under a pressure of from four to six atmospheres74 ,this gas is more refractory to liquefaction than sulphurous anhydride1 but for this reason it gives on evaporating a still lower temperature than can be attained by the evaporation of sulphurous anhydride4 ,a temperature of -#hj may be obtained by the evaporation of liquid carbonic anhydride at a pressure of #gfj mm41 and in an atmosphere rarefied by a powerful pump the temperature falls to -#adj4 ,by employing such low temperatures1 it was possible1 with the aid of pressure1 to liquefy the majority of the other gases4 ,it is evident that special pumps which are capable of rarefying gases are necessary to reduce the pressure in the chambers in which the sulphurous and carbonic anhydride boil2 and that1 in order to re-condense the resultant gases into liquids1 special force pumps are required for pumping the liquid anhydrides into the refrigerating chamber4 ,thus1 in ,pictet's apparatus 7fig4 #bd71 the carbonic anhydride was liquefied by the aid of the pumps ,e ,f1 which compressed the gas 7at a pressure of #d-f atmospheres7 and forced it into the tube ,k1 vigorously cooled by being surrounded by boiling liquid sulphurous anhydride1 which was condensed in the tube ,c by the pump ,b1 and rarefied by the pump ,a4 ,the liquefied carbonic anhydride flowed down the tube ,k into the tube ,h1 in which it was subjected to a low pressure by the pump ,e1 and thus gave a very low temperature of about -#adj4 ,the pump ,e carried off the vapour of the carbonic anhydride1 and conducted it to the pump ,f1 by which it was again liquefied4 ,the carbonic anhydride thus made an entire circuit--that is1 it passed from a rarefied vapour of small tension and low temperature into a compressed and cooled gas1 which was transformed into a liquid1 which again vaporised and produced a low temperature4 ,illustration3 ,,fig4 #bd4--;,general arrangement of the apparatus employed by ,pictet for liquefying gases4 ,inside the wide inclined tube ,h1 where the carbonic acid evaporated1 was placed a second and narrow tube ,m containing hydrogen1 which was generated in the vessel ,l from a mixture of sodium formate and caustic soda 7,,cho#b,na ,na,h,o ,na#b;,,co#c ,h#b74 ,this mixture gives hydrogen on heating the vessel ,l4 ,this vessel and the tube ,m were made of thick copper1 and could withstand great pressures4 ,they were1 moreover1 hermetically connected together and closed up4 ,thus the hydrogen which was evolved had no outlet1 accumulated in a limited space1 and its pressure increased in proportion to the amount of it evolved4 ,this pressure was recorded on a metallic manometer ,r attached to the end of the tube ,m4 ,as the hydrogen in this tube was submitted to a very low temperature and a powerful pressure1 all the necessary conditions were present for its liquefaction4 ,when the pressure in the tube ,h became steady--i4e4 when the temperature had fallen to -#adj and the manometer ,r indicated a pressure of #fej atmospheres in the tube ,m--then this pressure did not rise with a further evolution of hydrogen in the vessel ,l4 ,this served as an indication that the tension of the vapour of the hydrogen had attained a maximum corresponding with -#adj1 and that consequently all the excess of the gas was condensed to a liquid4 ,pictet convinced himself of this by opening the cock ,n1 when the liquid hydrogen rushed out from the orifice4 ,but1 on leaving a space where the pressure was equal to #fej atmospheres1 and coming into contact with air under the ordinary pressure1 the liquid or powerfully compressed hydrogen expanded1 began to boil1 absorbed still more heat1 and became still colder4 ,in doing so a portion of the liquid hydrogen1 according to ,pictet1 passed into a solid state1 and did not fall in drops into a vessel placed under the outlet ,n1 but as pieces of solid matter1 which struck against the sides of the vessel like shot and immediately vaporised4 ,thus1 although it was impossible to see and keep the liquefied hydrogen1 still it was clear that it passed not only into a liquid1 but also into a solid state4 ,pictet in his experiments obtained other gases which had not previously been liquefied1 especially oxygen and nitrogen1 in a liquid and solid state4 ,pictet supposed that liquid and solid hydrogen has the properties of a metal1 like iron4 #cb ,at the same time 7#ahgi7 as ,pictet was working on the liquefaction of gases in ,switzerland1 ,cailletet1 in ,paris1 was occupied on the same subject1 and his results1 although not so convincing as ,pictet's1 still showed that the majority of gases1 previously unliquefied1 were capable of passing into a liquid state4 ,cailletet subjected gases to a pressure of several hundred atmospheres in narrow thick-walled glass tubes 7fig4 #be72 he then cooled the compressed gas as far as possible by surrounding it with a freezing mixture2 a cock was then rapidly opened for the outlet of mercury from the tube containing the gas1 which consequently rapidly and vigorously expanded4 ,this rapid expansion of the gas would produce great cold1 just as the rapid compression of a gas evolves heat and causes a rise in temperature4 ,this cold was produced at the expense of the gas itself1 for in rapidly expanding its particles were not able to absorb heat from the walls of the tube1 and in cooling a portion of the expanding gas was transformed into liquid4 ,this was seen from the formation of cloud-like drops like a fog which rendered the gas opaque4 ,thus ,cailletet proved the possibility of the liquefaction of gases1 but he did not isolate the liquids4 ,the method of ,cailletet allows the passage of gases into liquids being observed with greater facility and simplicity than ,pictet's method1 which requires a very complicated and expensive apparatus4 ,illustration3 ,,fig4 #be4--;,cailletet's apparatus for liquefying gases4 ,the methods of ,pictet and ,cailletet were afterwards improved by ,olszewski1 ,wroblewski1 ,dewar1 and others4 ,in order to obtain a still lower temperature they employed1 instead of carbonic acid gas1 liquid ethylene or nitrogen and oxygen1 whose evaporation at low pressures produces a much lower temperature 7to -#bjj74 ,they also improved on the methods of determining such low temperatures1 but the methods were not essentially altered2 they obtained nitrogen and oxygen in a liquid1 and nitrogen even in a solid1 state1 but no one has yet succeeded in seeing hydrogen in a liquid form4 ,the most illustrative and instructive results 7because they gave the possibility of maintaining a very low temperature and the liquefied gas1 even air1 for a length of time7 were obtained in recent years by ,prof4 ,dewar in the ,royal ,institution of ,london1 which is glorified by the names of ,davy1 ,faraday1 and ,tyndall4 ,dewar1 with the aid of powerful pumps1 obtained many kilograms of oxygen and air 7the boiling point under the atmospheric pressure -#aij7 in a liquid state and kept them in this state for a length of time by means of open glass vessels with double walls1 having a vacuum between them1 which prevented the rapid transference of heat1 and so gave the possibility of maintaining very low temperatures inside the vessel for a long period of time4 ,the liquefied oxygen or air can be poured from one vessel into another and used for any investigations4 ,thus in ,june #ahid1 ,prof4 ,dewar showed that at the low temperature produced by liquid oxygen many substances become phosphorescent 7become self-luminous2 for instance1 oxygen on passing into a vacuum7 and fluoresce 7emit light after being illuminated2 for instance1 paraffin1 glue1 c47 much more powerfully than at the ordinary temperature2 also that solids then greatly alter in their mechanical properties1 c4 ,i had the opportunity 7#ahid7 at ,prof4 ,dewar's of seeing many such experiments in which open vessels containing pounds of liquid oxygen were employed1 and in following the progress made in researches conducted at low temperatures1 it is my firm impression that the study of many phenomena at low temperatures should widen the horizon of natural science as much as the investigation of phenomena made at the highest temperatures attained in the voltaic arc4 #cc ,the investigations of ,s4 ,wroblewski in ,cracow give reason to believe that ,pictet could not have obtained liquid hydrogen in the interior of his apparatus1 and that if he did obtain it1 it could only have been at the moment of its outrush due to the fall in temperature following its sudden expansion4 ,pictet calculated that he obtained a temperature of -#adj1 but in reality it hardly fell below -#abj1 judging from the latest data for the vaporisation of carbonic anhydride under low pressure4 ,the difference lies in the method of determining low temperatures4 ,judging from other properties of hydrogen 7see ,note #cd71 one would think that its absolute boiling point lies far below -#abj1 and even -#adj 7according to the calculation of ,sarrau1 on the basis of its compressibility1 at -#agd74 ,but even at -#bjj 7if the methods of determining such low temperatures be correct7 hydrogen does not give a liquid even under a pressure of several hundred atmospheres4 ,however1 on expansion a fog is formed and a liquid state attained1 but the liquid does not separate4 #cd ,after the idea of the absolute temperature of ebullition 7tc1 ,note #bi7 had been worked out 7about #ahgj71 and its connection with the deviations from ,mariotte's law had become evident1 and especially after the liquefaction of permanent gases1 general attention was turned to the development of the fundamental conceptions of the gaseous and liquid states of matter4 ,some investigators directed their energies to the further study of vapours 7for instance1 ,ramsay and ,young71 gases 7,amagat71 and liquids 7,zaencheffsky1 ,nadeschdin1 and others71 especially to liquids near tc and pc2 others 7,konovaloff and ,de ,heen7 endeavoured to discover the relation between liquids under ordinary conditions 7removed from tc and pc7 and gases1 whilst a third class of investigators 7van der ,waals1 ,clausius1 and others71 starting from the generally-accepted principles of the mechanical theory of heat and the kinetic theory of gases1 and assuming in gases the existence of those forces which certainly act in liquids1 deduced the connection between the properties of one and the other4 ,it would be out of place in an elementary handbook like the present to enunciate the whole mass of conclusions arrived at by this method1 but it is well to give an idea of the results of van der ,waals' considerations1 for they explain the gradual uninterrupted passage from a liquid into a gaseous state in the simplest manner1 and1 although the deduction cannot be considered as complete and decisive 7see ,note #be71 nevertheless it penetrates so deeply into the essence of the matter that its signification is not only reflected in a great number of physical investigations1 but also in the province of chemistry1 where instances of the passage of substances from a gaseous to a liquid state are so common1 and where the very processes of dissociation1 decomposition1 and combination must be identified with a change of physical state of the participating substances1 which has been elaborated by ,gibbs1 ,lavenig1 and others4 ,for a given quantity 7weight1 mass7 of a definite substance1 its state is expressed by three variables--volume v1 pressure 7elasticity1 tension7 p1 and temperature t4 ,although the compressibility--i4e41 d7v7_/d7p7--of liquids is small1 still it is clearly expressed1 and varies not only with the nature of liquids but also with their pressure and temperature 7at tc the compressibility of liquids is very considerable74 ,although gases1 according to ,mariotte's law1 with small variations of pressure1 are uniformly compressed1 nevertheless the dependence of their volume v on t and p is very complex4 ,this also applies to the coefficient of expansion d7v7_/d7t71 or d7p7_/d7t71 which also varies with t and p1 both for gases 7see ,note #bf71 and for liquids 7at tc it is very considerable1 and often exceeds that of gases1 #jjjcfg74 ,hence1 the equation of condition must include three variables1 v1 p1 and t4 ,for a so-called perfect 7ideal7 gas1 or for inconsiderable variations of density1 the elementary expression pv ,r,greek3 a7#a ,greek3 at71 or pv ,r7#bgc t7 should be accepted1 where ,r is a constant varying with the mass and nature of a gas1 as expressing this dependence1 because it includes in itself the laws of ,gay-,lussac and ,mariotte1 for at a constant pressure the volume varies proportionally to #a ,greek3 at1 and when t is constant the product of tv is constant4 ,in its simplest form the equation may be expressed thus3 pv ,,rt2 where ,t denotes what is termed the absolute temperature1 or the ordinary temperature #bgc--that is1 ,t t #bgc4 ,starting from the supposition of the existence of an attraction or internal pressure 7expressed by a7 proportional to the square of the density 7or inversely proportional to the square of the volume71 and of the existence of a real volume or diminished length of path 7expressed by b7 for each gaseous molecule1 van der ,waals gives for gases the following more complex equation of condition3-- 7p a_/v#b77v-b7 #a #jjjcfgt2 if at #j under a pressure p #a 7for example1 under the atmospheric pressure71 the volume 7for instance1 a litre7 of a gas or vapour he taken as #a1 and therefore v and b be expressed by the same units as p and a4 ,the deviations from both the laws of ,mariotte and ,gay-,lussac are expressed by the above equation4 ,thus1 for hydrogen a must be taken as infinitely small1 and b #jjjji1 judging by the data for #a1jjj and #b1ejj metres pressure 7,note #bh74 ,for other permanent gases1 for which 7,note #bh7 ,i showed 7about #ahgj7 from ,regnault's and ,natterer's data1 a decrement of pv1 followed by an increment1 which was confirmed 7about #ahhj7 by fresh determinations made by ,amagat1 this phenomena may be expressed in definite magnitudes of a and b 7although van der ,waals' formula is not applicable in the case of very small pressures7 with sufficient accuracy for contemporary requirements4 ,it is evident that van der ,waals' formula can also express the difference of the coefficients of expansion of gases with a change of pressure1 and according to the methods of determination 7,note #bf74 ,besides this1 van der ,waals' formula shows that at temperatures above #bgc7h;a_/#bg;b-#a7 only one actual volume 7gaseous7 is possible1 whilst at lower temperatures1 by varying the pressure1 three different volumes--liquid1 gaseous1 and partly liquid1 partly saturated-vaporous--are possible4 ,it is evident that the above temperature is the absolute boiling point--that is 7tc7 #bgc7h;a_/#bg;b-#a74 ,it is found under the condition that all three possible volumes 7the three roots of van der ,waals' cubic equation7 are then similar and equal 7vc #c;b74 ,the pressure in this case 7pc7 a_/7#bg;b#b74 ,these ratios between the constants a and b and the conditions of critical state--i4e4 7tc7 and 7pc7--give the possibility of determining the one magnitude from the other4 ,thus for ether 7,note #bi71 7tc7 #aic1 7tp7 #dj1 hence a #jjcjg1 b #jjjecc1 and 7vc7 #jjaf4 ,that mass of ether which at a pressure of one atmosphere at #j occupies one volume--for instance1 a litre--occupies1 according to the above-mentioned condition1 this critical volume4 ,and as the density of the vapour of ether compared with hydrogen #cg1 and a litre of hydrogen at #j and under the atmospheric pressure weighs #jjhif gram1 then a litre of ether vapour weighs #ccb grams2 therefore1 in a critical state 7at #aic and #dj atmospheres7 #ccb grams occupy #jjaf litre1 or #af c4c42 therefore #a gram occupies a volume of about #e c4c41 and the weight of #a c4c4 of ether will then be #jba4 ,according to the investigations of ,ramsay and ,young 7#ahhg71 the critical volume of ether was approximately such at about the absolute boiling point1 but the compressibility of the liquid is so great that the slightest change of pressure or temperature has a considerable effect on the volume4 ,but the investigations of the above savants gave another indirect demonstration of the truth of van der ,waals' equation4 ,they also found for ether that the isochords1 or the lines of equal volumes 7if both t and p vary71 are generally straight lines4 ,thus the volume of #aj c4c4 for #a gram of ether corresponds with pressures 7expressed in metres of mercury7 equal to #jacet-#cc 7for example1 at #ahj the pressure #ba metres1 and at #bhj it #cde metres74 ,the rectilinear form of the isochord 7when v a constant quantity7 is a direct result of van der ,waals' formula4 ,when1 in #ahhc1 ,i demonstrated that the specific gravity of liquids decreases in proportion to the rise of temperature ,st ,s#j-,kt or ,st ,s#j7a-,kt71 or that the volumes increase in inverse proportion to the binomial #a-,kt1 that is1 ,vt ,v#j7a-,kt7-#a1 where ,k is the modulus of expansion1 which varies with the nature of the liquid1 then1 in general1 not only does a connection arise between gases and liquids with respect to a change of volume1 but also it would appear possible1 by applying van der ,waals' formula1 to judge1 from the phenomena of the expansion of liquids1 as to their transition into vapour1 and to connect together all the principal properties of liquids1 which up to this time had not been considered to be in direct dependence4 ,thus ,thorpe and ,rcker found that #b7tc7 #bgc #a_/,k1 where ,k is the modulus of expansion in the above-mentioned formula4 ,for example1 the expansion of ether is expressed with sufficient accuracy from #j to #ajj by the equation ,st #jgcf_/7a-jjjaedt71 or ,vt #a_/7a-jjjaedt71 where #jjjaed is the modulus of expansion1 and therefore 7tc7 #ahh1 or by direct observation #aic4 ,for silicon tetrachloride1 ,si,cl#d1 the modulus equals #jjjacf1 from whence 7tc7 #bca1 and by experiment #bcj4 ,on the other hand1 ,d4 ,p4 ,konovaloff1 admitting that the external pressure p in liquids is insignificant when compared with the internal 7a in van der ,waals' formula71 and that the work in the expansion of liquids is proportional to their temperature 7as in gases71 directly deduced1 from van der ,waals' formula1 the above-mentioned formula for the expansion of liquids1 ,vt #a_/7a-,kt71 and also the magnitude of the latent heat of evaporation1 cohesion1 and compressibility under pressure4 ,in this way van der ,waals' formula embraces the gaseous1 critical1 and liquid states of substances1 and shows the connection between them4 ,on this account1 although van der ,waals' formula cannot be considered as perfectly general and accurate1 yet it is not only very much more exact than pv ,,rt1 but it is also more comprehensive1 because it applies both to gases and liquids4 ,further research will naturally give a closer proximity to truth1 and will show the connection between composition and the constants 7a and b72 but a great scientific progress is seen in this form of the equation of state4 ,clausius 7in #ahhj71 taking into consideration the variability of a1 in van der ,waals' formula1 with the temperature1 gave the following equation of condition3-- 7p a_/7,t7v c7#b77 7v-b7 ,,rt4 ,sarrau applied this formula to ,amagat's data for hydrogen1 and found a #jjeea1 c -#jjjjdc1 b #jjjjhi1 and therefore calculated its absolute boiling point as -#agd1 and 7pc7 #ii atmospheres4 ,but as similar calculations for oxygen 7-#aje71 nitrogen 7-#abd71 and marsh gas 7-#gf7 gave tc higher than it really is1 the absolute boiling point of hydrogen must lie below -#agd4 ,although a substance which passes with great difficulty into a liquid state by the action of physico-mechanical forces1 hydrogen loses its gaseous state 7that is1 its elasticity1 or the physical energy of its molecules1 or their rapid progressive motion7 with comparative ease under the influence of chemical attraction1#ce which is not only shown from the fact that hydrogen and oxygen 7two permanent gases7 form liquid water1 but also from many phenomena of the absorption of hydrogen4 #ce ,this and a number of similar cases clearly show how great are the internal chemical forces compared with physical and mechanical forces4 ,hydrogen is vigorously condensed by certain solids2 for example1 by charcoal and by spongy platinum4 ,if a piece of freshly ignited charcoal be introduced into a cylinder full of hydrogen standing in a mercury bath1 then the charcoal absorbs as much as twice its volume of hydrogen4 ,spongy platinum condenses still more hydrogen4 ,but palladium1 a grey metal which occurs with platinum1 absorbs more hydrogen than any other metal4 ,graham showed that when heated to a red heat and cooled in an atmosphere of hydrogen1 palladium retains as much as #fjj volumes of hydrogen4 ,when once absorbed it retains the hydrogen at the ordinary temperature1 and only parts with it when heated to a red heat4#cf ,this capacity of certain dense metals for the absorption of hydrogen explains the property of hydrogen of passing through metallic tubes4#cg ,it is termed occlusion1 and presents a similar phenomenon to solution2 it is based on the capacity of metals of forming unstable easily dissociating compounds#ch with hydrogen1 similar to those which salts form with water4 #cf ,the property of palladium of absorbing hydrogen1 and of increasing in volume in so doing1 may be easily demonstrated by taking a sheet of palladium varnished on one side1 and using it as a cathode4 ,the hydrogen which is evolved by the action of the current is retained by the unvarnished surface1 as a consequence of which the sheet curls up4 ,by attaching a pointer 7for instance1 a quill7 to the end of the sheet this bending effect is rendered strikingly evident1 and on reversing the current 7when oxygen will be evolved and combine with the absorbed hydrogen1 forming water7 it may be shown that on losing the hydrogen the palladium regains its original form4 #cg ,deville discovered that iron and platinum become pervious to hydrogen at a red heat4 ,he speaks of this in the following terms3--',the permeability of such homogeneous substances as platinum and iron is quite different from the passage of gases through such non-compact substances as clay and graphite4 ,the permeability of metals depends on their expansion1 brought about by heat1 and proves that metals and alloys have a certain porosity4' ,however1 ,graham proved that it is only hydrogen which is capable of passing through the above-named metals in this manner4 ,oxygen1 nitrogen1 ammonia1 and many other gases1 only pass through in extremely minute quantities4 ,graham showed that at a red heat about #ejj c4c4 of hydrogen pass per minute through a surface of one square metre of platinum #aa mm4 thick1 but that with other gases the amount transmitted is hardly perceptible4 ,indiarubber has the same capacity for allowing the transference of hydrogen through its substance 7see ,chapter ,,iii471 but at the ordinary temperature one square metre1 #jjad mm4 thick1 transmits only #abg c4c4 of hydrogen per minute4 ,in the experiment on the decomposition of water by heat in porous tubes1 the clay tube may be exchanged for a platinum one with advantage4 ,graham showed that by placing a platinum tube containing hydrogen under these conditions1 and surrounding it by a tube containing air1 the transference of the hydrogen may be observed by the decrease of pressure in the platinum tube4 ,in one hour almost all the hydrogen 7#ig p4c47 had passed from the tube1 without being replaced by air4 ,it is evident that the occlusion and passage of hydrogen through metals capable of occluding it are not only intimately connected together1 but are dependent on the capacity of metals to form compounds of various degrees of stability with hydrogen--like salts with water4 #ch ,it appeared on further investigation that palladium gives a definite compound1 ,pd#b;,h 7see further7 with hydrogen2 but what was most instructive was the investigation of sodium hydride1 ,na#b;,h1 which clearly showed that the origin and properties of such compounds are in entire accordance with the conceptions of dissociation4 ,since hydrogen is a gas which is difficult to condense1 it is little soluble in water and other liquids4 ,at #j a hundred volumes of water dissolve #ai volume of hydrogen1 and alcohol #fi volumes measured at #j and #gfj mm4 ,molten iron absorbs hydrogen1 but in solidifying1 it expels it4 ,the solution of hydrogen by metals is to a certain degree based on its affinity for metals1 and must be likened to the solution of metals in mercury and to the formation of alloys4 ,in its chemical properties hydrogen1 as we shall see later1 has much of a metallic character4 ,pictet 7see ,note #ca7 even affirms that liquid hydrogen has metallic properties4 ,the metallic properties of hydrogen are also evinced in the fact that it is a good conductor of heat1 which is not the case with other gases 7,magnus74 ,at the ordinary temperature hydrogen very feebly and rarely enters into chemical reaction4 ,the capacity of gaseous hydrogen for reaction becomes evident only under a change of circumstances--by compression1 heating1 or the action of light1 or at the moment of its evolution4 ,however1 under these circumstances it combines directly with only a very few of the elements4 ,hydrogen combines directly with oxygen1 sulphur1 carbon1 potassium1 and certain other elements1 but it does not combine directly with either the majority of the metals or with nitrogen1 phosphorus1 c4 ,compounds of hydrogen with certain elements on which it does not act directly are1 however1 known2 they are not obtained by a direct method1 but by reactions of decomposition1 or of double decomposition1 of other hydrogen compounds4 ,the property of hydrogen of combining with oxygen at a red heat determines its combustibility4 ,we have already seen that hydrogen easily takes fire1 and that it then burns with a pale--that is1 non-luminous--flame4#ci ,hydrogen does not combine with the oxygen of the atmosphere at the ordinary temperature2 but this combination takes place at a red heat1#dj and is accompanied by the evolution of much heat4 ,the product of this combination is water--that is1 a compound of oxygen and hydrogen4 ,this is the synthesis of water1 and we have already noticed its analysis or decomposition into its component parts4 ,the synthesis of water may be very easily observed if a cold glass bell jar be placed over a burning hydrogen flame1 and1 better still1 if the hydrogen flame be lighted in the tube of a condenser4 ,the water will condense in drops as it is formed on the walls of the condenser and trickle down4#da #ci ,if it be desired to obtain a perfectly colourless hydrogen flame1 it must issue from a platinum nozzle1 as the glass end of a gas-conducting tube imparts a yellow tint to the flame1 owing to the presence of sodium in the glass4 #dj ,let us imagine that a stream of hydrogen passes along a tube1 and let us mentally divide this stream into several parts1 consecutively passing out from the orifice of the tube4 ,the first part is lighted--that is1 brought to a state of incandescence1 in which state it combines with the oxygen of the atmosphere4 ,a considerable amount of heat is evolved in the combination4 ,the heat evolved then1 so to say1 ignites the second part of hydrogen coming from the tube1 and1 therefore1 when once ignited1 the hydrogen continues to burn1 if there be a continual supply of it1 and if the atmosphere in which it burns be unlimited and contains oxygen4 #da ,the combustibility of hydrogen may be shown by the direct decomposition of water by sodium4 ,if a pellet of sodium be thrown into a vessel containing water1 it floats on the water and evolves hydrogen1 which may be lighted4 ,the presence of sodium imparts a yellow tint to the flame4 ,if potassium be taken1 the hydrogen bursts into flame spontaneously1 because sufficient heat is evolved in the reaction to ignite the hydrogen4 ,the flame is coloured violet by the potassium4 ,if sodium be thrown not on to water1 but on to an acid1 it will evolve more heat1 and the hydrogen will then also burst into flame4 ,these experiments must be carried on with caution1 as1 sometimes towards the end1 a mass of sodium oxide 7,note #h7 is produced1 and flies about2 it is therefore best to cover the vessel in which the experiment is carried on4 ,light does not aid the combination of hydrogen and oxygen1 so that a mixture of these two gases does not change when exposed to the action of light2 but an electric spark acts just like a flame1 and this is taken advantage of for inflaming a mixture of oxygen and hydrogen1 or detonating gas1 inside a vessel1 as will be explained in the following chapters4 ,as hydrogen 7and oxygen also7 is condensed by spongy platinum1 by which a rise of temperature ensues1 and as platinum acts by contact 7,introduction71 therefore hydrogen also combines with oxygen1 under the influence of platinum1 as ,dbereiner showed4 ,if spongy platinum be thrown into a mixture of hydrogen and oxygen1 an explosion takes place4 ,if a mixture of the gases be passed over spongy platinum1 combination also ensues1 and the platinum becomes red-hot4#db #db ,this property of spongy platinum is made use of in the so-called hydrogen cigar-lighter4 ,it consists of a glass cylinder or beaker1 inside which there is a small lead stand 7which is not acted on by sulphuric acid71 on which a piece of zinc is laid4 ,this zinc is covered by a bell1 which is open at the bottom and furnished with a cock at the top4 ,sulphuric acid is poured into the space between the bell and the sides of the outer glass cylinder1 and will thus compress the gas in the bell4 ,if the cock of the cylinder be opened the gas will escape by it1 and will be replaced by the acid1 which1 coming into contact with the zinc1 evolves hydrogen1 and it will escape through the cock4 ,if the cock be closed1 then the hydrogen evolved will increase the pressure of the gas in the bell1 and thus again force the acid into the space between the bell and the walls of the outer cylinder4 ,thus the action of the acid on the zinc may be stopped or started at will by opening or shutting the cock1 and consequently a stream of hydrogen may be always turned on4 ,now1 if a piece of spongy platinum be placed in this stream1 the hydrogen will take light1 because the spongy platinum becomes hot in condensing the hydrogen and inflames it4 ,the considerable rise in temperature of the platinum depends1 among other things1 on the fact that the hydrogen condensed in its pores comes into contact with previously absorbed and condensed atmospheric oxygen1 with which hydrogen combines with great facility in this form4 ,in this manner the hydrogen cigar-lighter gives a stream of burning hydrogen when the cock is open4 ,in order that it should work regularly it is necessary that the spongy platinum should be quite clean1 and it is best enveloped in a thin sheet of platinum foil1 which protects it from dust4 ,in any case1 after some time it will be necessary to clean the platinum1 which may be easily done by boiling it in nitric acid1 which does not dissolve the platinum1 but clears it of all dirt4 ,this imperfection has given rise to several other forms1 in which an electric spark is made to pass before the orifice from which the hydrogen escapes4 ,this is arranged in such a manner that the zinc of a galvanic element is immersed when the cock is turned1 or a small coil giving a spark is put into circuit on turning the hydrogen on4 ,although gaseous hydrogen does not act directly#dc on many substances1 yet in a nascent state reaction often takes place4 ,thus1 for instance1 water on which sodium amalgam is acting contains hydrogen in a nascent state4 ,the hydrogen is here evolved from a liquid1 and at the first moment of its formation must be in a condensed state4#dd ,in this condition it is capable of reacting on substances on which it does not act in a gaseous state4#dd bis ,reactions of substitution or displacement of metals by hydrogen at the moment of its formation are particularly numerous4#de #dc ,under conditions similar to those in which hydrogen combines with oxygen it is also capable of combining with chlorine4 ,a mixture of hydrogen and chlorine explodes on the passage of an electric spark through it1 or on contact with an incandescent substance1 and also in the presence of spongy platinum2 but1 besides this1 the action of light alone is enough to bring about the combination of hydrogen and chlorine4 ,if a mixture of equal volumes of hydrogen and chlorine be exposed to the action of sunlight1 complete combination rapidly ensues1 accompanied by a report4 ,hydrogen does not combine directly with carbon1 either at the ordinary temperature or by the action of heat and pressure4 ,but if an electric current be passed through carbon electrodes at a short distance from each other 7as in the electric light or voltaic arc71 so as to form an electric arc in which the particles of carbon are carried from one pole to the other1 then1 in the intense heat to which the carbon is subjected in this case1 it is capable of combining with hydrogen4 ,a gas of peculiar smell called acetylene1 ,c#b;,h#b1 is thus formed from carbon and hydrogen4 #dd ,there is another explanation of the facility with which hydrogen reacts in a nascent state4 ,we shall afterwards learn that the molecule of hydrogen contains two atoms1 ,h#b1 but there are elements the molecules of which only contain one atom--for instance1 mercury4 ,therefore1 every reaction of gaseous hydrogen must be accompanied by the disruption of that bond which exists between the atoms forming a molecule4 ,at the moment of evolution1 however1 it is supposed that free atoms exist1 and in this condition1 according to the hypothesis1 act energetically4 ,this hypothesis is not based upon facts1 and the idea that hydrogen is condensed at the moment of its evolution is more natural1 and is in accordance with the fact 7,note #ab7 that compressed hydrogen displaces palladium and silver 7,brunner1 ,beketoff7--that is1 acts as at the moment of its liberation4 #dd bis ,there is a very intimate and evident relation between the phenomena which take place in the action of spongy platinum and the phenomena of the action in a nascent state4 ,the combination of hydrogen with aldehyde may be taken as an example4 ,aldehyde is a volatile liquid with an aromatic smell1 boiling at #ba1 soluble in water1 and absorbing oxygen from the atmosphere1 and in this absorption forming acetic acid--the substance which is found in ordinary vinegar4 ,if sodium amalgam be thrown into an aqueous solution of aldehyde1 the greater part of the hydrogen evolved combines with the aldehyde1 forming alcohol--a substance also soluble in water1 which forms the principle of all spirituous liquors1 boils at #gh1 and contains the same amount of oxygen and carbon as aldehyde1 but more hydrogen4 ,the composition of aldehyde is ,c#b;,h#d,o1 that of alcohol ,c#b;,h#f,o4 #de ,when1 for instance1 an acid and zinc are added to a salt of silver1 the silver is reduced2 but this may be explained as a reaction of the zinc1 and not of the hydrogen at the moment of its formation4 ,there are1 however1 examples to which this explanation is entirely inapplicable2 thus1 for instance1 hydrogen1 at the moment of its liberation easily takes up oxygen from its compounds with nitrogen if they be in solution1 and converts the nitrogen into its hydrogen-compound4 ,here the nitrogen and hydrogen1 so to speak1 meet at the moment of their liberation1 and in this state combine together4 ,it is evident from this that the elastic gaseous state of hydrogen fixes the limit of its energy3 prevents it from entering into those combinations of which it is capable4 ,in the nascent state we have hydrogen which is not in a gaseous state1 and its action is then much more energetic4 ,at the moment of evolution that heat1 which would be latent in the gaseous hydrogen1 is transmitted to its molecules1 and consequently they are in a state of strain1 and can hence act on many substances4 ,metals1 as we shall afterwards see1 are in many cases able to replace each other2 they also1 and in some cases still more easily1 replace and are replaced by hydrogen4 ,we have already seen examples of this in the formation of hydrogen from water1 sulphuric acid1 c4 ,in all these cases the metals sodium1 iron1 or zinc displace the hydrogen which occurs in these compounds4 ,hydrogen may be displaced from many of its compounds by metals in exactly the same manner as it is displaced from water2 so1 for example1 hydrochloric acid1 which is formed directly by the combination of hydrogen with chlorine1 gives hydrogen by the action of a great many metals1 just as sulphuric acid does4 ,potassium and sodium also displace hydrogen from its compounds with nitrogen2 it is only from its compounds with carbon that hydrogen is not displaced by metals4 ,hydrogen1 in its turn1 is able to replace metals2 this is accomplished most easily on heating1 and with those metals which do not themselves displace hydrogen4 ,if hydrogen be passed over the compounds of many metals with oxygen at a red heat1 it takes up the oxygen from the metals and displaces them just as it is itself displaced by metals4 ,if hydrogen be passed over the compound of oxygen with copper at a red heat1 then metallic copper and water are obtained--,cu,o ,h#b ,h#b,o ,cu4 ,this kind of double decomposition is called reduction with respect to the metal1 which is thus reduced to a metallic state from its combination with oxygen4 ,but it must be recollected that all metals do not displace hydrogen from its compound with oxygen1 and1 conversely1 hydrogen is not able to displace all metals from their compounds with oxygen2 thus it does not displace potassium1 calcium1 or aluminium from its compounds with oxygen4 ,if the metals be arranged in the following series3 ,k1 ,na1 ,ca1 ,al 444 ,fe1 ,zn1 ,hg 444 ,cu1 ,pb1 ,ag1 ,au1 then the first are able to take up oxygen from water--that is1 displace hydrogen--whilst the last do not act thus1 but are1 on the contrary1 reduced by hydrogen--that is1 have1 as is said1 a less affinity for oxygen than hydrogen1 whilst potassium1 sodium1 and calcium have more4 ,this is also expressed by the amount of heat evolved in the act of combination with oxygen 7see ,note #g71 and is shown by the fact that potassium and sodium and other similar metals evolve heat in decomposing water2 but copper1 silver1 and the like do not do this1 because in combining with oxygen they evolve less heat than hydrogen does1 and therefore it happens that when hydrogen reduces these metals heat is evolved4 ,thus1 for example1 if #af grams of oxygen combine with copper1 #ch1jjj units of heat are evolved2 and when #af grams of oxygen combine with hydrogen1 forming water1 #fi1jjj units of heat are evolved2 whilst #bc grams of sodium1 in combining with #af grams of oxygen1 evolve #ajj1jjj units of heat4 ,this example clearly shows that chemical reactions which proceed directly and unaided evolve heat4 ,sodium decomposes water and hydrogen reduces copper1 because they are exothermal reactions1 or those which evolve heat2 copper does not decompose water1 because such a reaction would be accompanied by an absorption 7or secretion7 of heat1 or belongs to the class of endothermal reactions in which heat is absorbed2 and such reactions do not generally proceed directly1 although they may take place with the aid of energy 7electrical1 thermal1 c47 borrowed from some foreign source4#df #df ,several numerical data and reflections bearing on this matter are enumerated in ,notes #g1 #i1 and #aa4 ,it must be observed that the action of iron or zinc on water is reversible4 ,but the reaction ,cu,o ,h#b ,cu ,h#b,o is not reversible2 the difference between the degrees of affinity is very great in this case1 and1 therefore1 so far as is at present known1 no hydrogen is liberated even in the presence of a large excess of water4 ,it is to be further remarked1 that under the conditions of the dissociation of water1 copper is not oxidised by water1 because the oxide of copper is reduced by free hydrogen4 ,if a definite amount of a metal and acid be taken and their reaction be carried on in a closed space1 then the evolution of hydrogen will cease1 when its tension equals that at which compressed hydrogen displaces the metal4 ,the result depends upon the nature of the metal and the strength of the solution of acid4 ,tammann and ,nernst 7#ahib7 found that the metals stand in the following order in respect to this limiting tension of hydrogen3--,na1 ,mg1 ,zn1 ,al1 ,cd1 ,fe1 ,ni4 ,illustration3 ,,fig4 #bf4--;,apparatus employed by ,dumas for determining the composition of water4 ,described in text4 ,the reduction of metals by hydrogen is taken advantage of for determining the exact composition of water by weight4 ,copper oxide is usually chosen for this purpose4 ,it is heated to redness in hydrogen1 and the quantity of water thus formed is determined1 when the quantity of oxygen which occurs in it is found from the loss of weight of the copper oxide4 ,the copper oxide must be weighed immediately before and after the experiment4 ,the difference shows the weight of the oxygen which entered into the composition of the water formed4 ,in this manner only solids have to be weighed1 which is a very great gain in the accuracy of the results obtained4#dg ,dulong and ,berzelius 7#ahai7 were the first to determine the composition of water by this method1 and they found that water contains #hhia of oxygen and #aaji of hydrogen in #ajj parts by weight1 or #hjjh parts of oxygen per one part of hydrogen4 ,dumas 7#ahdb7 improved on this method1#dh and found that water contains #abege parts of hydrogen per #ajj parts of oxygen--that is1 #giij parts of oxygen per #a part of hydrogen--and therefore it is usually accepted that water contains eight parts by weight of oxygen to one part by weight of hydrogen4 ,by whatever method water be obtained1 it will always present the same composition4 ,whether it be taken from nature and purified1 or whether it be obtained from hydrogen by oxidation1 or whether it be separated from any of its compounds1 or obtained by some double decomposition--it will in every case contain one part by weight of hydrogen and eight parts of oxygen4 ,this is because water is a definite chemical compound4 ,detonating gas1 from which it may be formed1 is a simple mixture of oxygen and hydrogen1 although a mixture of the same composition as water4 ,all the properties of both constituent gases are preserved in detonating gas4 ,either one or the other gas may be added to it without destroying its homogeneity4 ,the fundamental properties of oxygen and hydrogen are not found in water1 and neither of the gases can be directly combined with it4 ,but they may be evolved from it4 ,in the formation of water there is an evolution of heat2 for the decomposition of water heat is required4 ,all this is expressed by the words1 ,water is a definite chemical compound of hydrogen with oxygen4 ,taking the symbol of hydrogen1 ,h1 as expressing a unit quantity by weight of this substance1 and expressing #af parts by weight of oxygen by ,o1 we can formulate all the above statements by the chemical symbol of water1 ,h#b,o4 ,as only definite chemical compounds are denoted by formul1 having denoted the formula of a compound substance we express by it the entire series of properties which go to make up our conception of a definite compound1 and at the same time the quantitative composition of the substance by weight4 ,further1 as we shall afterwards see1 formul express the volume of the gases contained in a substance4 ,thus the formula of water shows that it contains two volumes of hydrogen and one volume of oxygen4 ,besides which1 we shall learn that the formula expresses the density of the vapour of a compound1 and on this many properties of substances depend1 and1 as we shall learn1 determine the quantities of the bodies entering into reactions4 ,this vapour density we shall find also determines the quantity of a substance entering into a reaction4 ,thus the letters ,h#b,o tell the chemist the entire history of the substance4 ,this is an international language1 which endows chemistry with a simplicity1 clearness1 stability1 and trustworthiness founded on the investigation of the laws of nature4 #dg ,this determination may be carried on in an apparatus like that mentioned in ,note #ac of ,chapter ,i4 #dh ,we will proceed to describe ,dumas' method and results4 ,for this determination pure and dry copper oxide is necessary4 ,dumas took a sufficient quantity of copper oxide for the formation of #ej grams of water in each determination4 ,as the oxide of copper was weighed before and after the experiment1 and as the amount of oxygen contained in water was determined by the difference between these weights1 it was essential that no other substance besides the oxygen forming the water should be evolved from the oxide of copper during its ignition in hydrogen4 ,it was necessary1 also1 that the hydrogen should be perfectly pure1 and free not only from traces of moisture1 but from any other impurities which might dissolve in the water or combine with the copper and form some other compound with it4 ,the bulb containing the oxide of copper 7fig4 #bf71 which was heated to redness1 should be quite free from air1 as otherwise the oxygen in the air might1 in combining with the hydrogen passing through the vessel1 form water in addition to that formed by the oxygen of the oxide of copper4 ,the water formed should be entirely absorbed in order to accurately determine its quantity4 ,the hydrogen was evolved in the three-necked bottle4 ,the sulphuric acid1 for acting on the zinc1 is poured through funnels into the middle neck4 ,the hydrogen evolved in the ,woulfe's bottle passes through ,u tubes1 in which it is purified1 to the bulb1 where it comes into contact with the copper oxide1 forms water1 and reduces the oxide to metallic copper2 the water formed is condensed in the second bulb1 and any passing off is absorbed in the second set of ,u tubes4 ,this is the general arrangement of the apparatus4 ,the bulb with the copper oxide is weighed before and after the experiment4 ,the loss in weight shows the quantity of oxygen which entered into the composition of the water formed1 the weight of the latter being shown by the gain in weight of the absorbing apparatus4 ,knowing the amount of oxygen in the water formed1 we also know the quantity of hydrogen contained in it1 and consequently we determine the composition of water by weight4 ,this is the essence of the determination4 ,we will now turn to certain particulars4 ,in one neck of the three-necked bottle a tube is placed dipping under mercury4 ,this serves as a safety-valve to prevent the pressure inside the apparatus becoming too great from the rapid evolution of hydrogen4 ,if the pressure rose to any considerable extent1 the current of gases and vapours would be very rapid1 and1 as a consequence1 the hydrogen would not be perfectly purified1 or the water entirely absorbed in the tubes placed for this purpose4 ,in the third neck of the ,woulfe's bottle is a tube conducting the hydrogen to the purifying apparatus1 consisting of eight ,u tubes1 destined for the purification and testing of the hydrogen4 ,the hydrogen1 evolved by zinc and sulphuric acid1 is purified by passing it first through a tube full of pieces of glass moistened with a solution of lead nitrate next through silver sulphate2 the lead nitrate retains sulphurette hydrogen1 and arseniuretted hydrogen is retained by the tube with silver sulphate4 ,caustic potash in the next ,u tube retains any acid which might come over4 ,the two following tubes are filled with lumps of dry caustic potash in order to absorb any carbonic anhydride and moisture which the hydrogen might contain4 ,the next two tubes1 to remove the last traces of moisture1 are filled with phosphoric anhydride1 mixed with lumps of pumice-stone4 ,they are immersed in a freezing mixture4 ,the small ,u tube contains hygroscopic substances1 and is weighed before the experiment3 this is in order to know whether the hydrogen passing through still retains any moisture4 ,if it does not1 then the weight of this tube will not vary during the whole experiment1 but if the hydrogen evolved still retains moisture1 the tube will increase in weight4 ,the copper oxide is placed in the bulb1 which1 previous to the experiment1 is dried with the copper oxide for a long period of time4 ,the air is then exhausted from it1 in order to weigh the oxide of copper in a vacuum and to avoid the need of a correction for weighing in air4 ,the bulb is made of infusible glass1 that it may be able to withstand a lengthy 7#bj hours7 exposure to a red heat without changing in form4 ,the weighed bulb is only connected with the purifying apparatus after the hydrogen has passed through for a long time1 and after experiment has shown that the hydrogen passing from the purifying apparatus is pure and does not contain any air4 ,on passing from the condensing bulb the gas and vapour enter into an apparatus for absorbing the last traces of moisture4 ,the first ,u tube contains pieces of ignited potash1 the second and third tubes phosphoric anhydride or pumice-stone moistened with sulphuric acid4 ,the last of the two is employed for determining whether all the moisture is absorbed1 and is therefore weighed separately4 ,the final tube only serves as a safety-tube for the whole apparatus1 in order that the external moisture should not penetrate into it4 ,the glass cylinder contains sulphuric acid1 through which the excess of hydrogen passes2 it enables the rate at which the hydrogen is evolved to be judged1 and whether its amount should be decreased or increased4 ,when the apparatus is fitted up it must be seen that all its parts are hermetically tight before commencing the experiment4 ,when the previously weighed parts are connected together and the whole apparatus put into communication1 then the bulb containing the copper oxide is heated with a spirit lamp 7reduction does not take place without the aid of heat71 and the reduction of the copper oxide then takes place1 and water is formed4 ,when nearly all the copper oxide is reduced the lamp is removed and the apparatus allowed to cool1 the current of hydrogen being kept up all the time4 ,when cool1 the drawn-out end of the bulb is fused up1 and the hydrogen remaining in it is exhausted1 in order that the copper may be again weighed in a vacuum4 ,the absorbing apparatus remains full of hydrogen1 and would therefore present a less weight than if it were full of air1 as it was before the experiment1 and for this reason1 having disconnected the copper oxide bulb1 a current of dry air is passed through it until the gas passing from the glass cylinder is quite free from hydrogen4 ,the condensing bulb and the two tubes next to it are then weighed1 in order to determine the quantity of water formed4 ,dumas repeated this experiment many times4 ,the average result was that water contains #abecc parts of hydrogen per #aj1jjj parts of oxygen4 ,making a correction for the amount of air contained in the sulphuric acid employed for producing the hydrogen1 ,dumas obtained the average figure #abeae1 between the extremes #abdgb and #abefb4 ,this proves that per #a part of hydrogen water contains #giijd parts of oxygen1 with a possible error of not more than #a_/bej1 or #jjc1 in the amount of oxygen per #a part of hydrogen4 ,erdmann and ,marchand1 in eight determinations1 found that per #aj1jjj parts of oxygen water contains an average of #a1beb parts of hydrogen1 with a difference of from #a1behe to #a1bdhg2 hence per #a part of hydrogen there would be #giieb of oxygen1 with an error of at least #jje4 ,keiser 7#ahhh71 in ,america by employing palladium hydride1 and by introducing various fresh precautions for obtaining accurate results1 found the composition of water to be #aeie parts of oxygen per #b of hydrogen4 ,certain of the latest determinations of the composition of water1 as also those made by ,dumas1 always give less than #h1 and on the average #gih1 of oxygen per #a part of hydrogen4 ,however1 not one of these figures is to be entirely depended on1 and for ordinary accuracy it may be considered that ,o #af when ,h #a4 ,,chapter ,,iii ,,oxygen ,,and ,,the ,,chief ,,aspects ,,of ,,its ,,saline ,,combinations ,on the earth's surface there is no other element which is so widely distributed as oxygen in its various compounds4#a ,it makes up eight-ninths of the weight of water1 which occupies the greater part of the earth's surface4 ,nearly all earthy substances and rocks consist of compounds of oxygen with metals and other elements4 ,thus1 the greater part of sand is formed of silica1 ,si,o#b1 which contains #ec p4c4 of oxygen2 clay contains water1 alumina 7formed of aluminium and oxygen71 and silica4 ,it may be considered that earthy substances and rocks contain up to one-third of their weight of oxygen2 animal and vegetable substances are also very rich in oxygen4 ,without counting the water present in them1 plants contain up to #dj1 and animals up to #bj p4c4 by weight of oxygen4 ,thus1 oxygen compounds predominate on the earth's surface4 ,besides this1 a portion exists in a free state1 and is contained in admixture with nitrogen in the atmosphere1 forming about one-fourth of its mass1 or one-fifth of its volume4 #a ,as regards the interior of the earth1 it probably contains far less oxygen compounds than the surface1 judging by the accumulated evidences of the earth's origin1 of meteorites1 of the earth's density1 c4 7see ,chapter ,,viii41 ,note #eh1 and ,chapter ,,xxii41 ,note #b74 ,being so widely distributed in nature1 oxygen plays a very important part in it1 for a number of the phenomena which take place before us are mainly dependent on it4 ,animals breathe air in order to obtain only oxygen from it1 the oxygen entering into their respiratory organs 7the lungs of human beings and animals1 the gills of fishes1 and the trach of insects72 they1 so to say1 drink in air in order to absorb the oxygen4 ,the oxygen of the air 7or dissolved in water7 passes through the membranes of the respiratory organs into the blood1 is retained in it by the blood corpuscles1 is transmitted by their means to all parts of the body1 aids their transformations1 bringing about chemical processes in them1 and chiefly extracting carbon from them in the form of carbonic anhydride1 the greater part of which passes into the blood1 is dissolved by it1 and is thrown off by the lungs during the absorption of the oxygen4 ,thus1 in the process of respiration carbonic anhydride 7and water7 is given off1 and the oxygen of the air absorbed1 by which means the blood is changed from a red venous to a dark-red arterial blood4 ,the cessation of this process causes death1 because then all those chemical processes1 and the consequent heat and work which the oxygen introduced into the system brought about1 cease4 ,for this reason suffocation and death ensue in a vacuum1 or in a gas which does not contain free oxygen1 i4e4 which does not support combustion4 ,if an animal be placed in an atmosphere of free oxygen1 at first its movements are very active and a general invigoration is remarked1 but a reaction soon sets in1 and death may ensue4 ,the oxygen of the air when it enters the lungs is diluted with four volumes of nitrogen1 which is not absorbed into the system1 so that the blood absorbs but a small quantity of oxygen from the air1 whilst in an atmosphere of pure oxygen a large quantity of oxygen would be absorbed1 and would produce a very rapid change of all parts of the organism1 and destroy it4 ,from what has been said1 it will be understood that oxygen may be employed in respiration1 at any rate for a limited time1 when the respiratory organs suffer under certain forms of suffocation and impediment to breathing4#b #b ,it is evident that the partial pressure 7see ,chapter ,i47 acts in respiration4 ,the researches of ,paul ,bert showed this with particular clearness4 ,under a pressure of one-fifth of an atmosphere consisting of oxygen only1 animals and human beings remain under the ordinary conditions of the partial pressure of oxygen1 but organisms cannot support air rarefied to one-fifth1 for then the partial pressure of the oxygen falls to one-twenty-fifth of an atmosphere4 ,even under a pressure of one-third of an atmosphere the regular life of human beings is impossible1 by reason of the impossibility of respiration 7because of the decrease of solubility of oxygen in the blood71 owing to the small partial pressure of the oxygen1 and not from any mechanical effect of the decrease of pressure4 ,paul ,bert illustrated all this by many experiments1 some of which he conducted on himself4 ,this explains1 among other things1 the discomfort felt in the ascent of high mountains or in balloons when the height reached exceeds eight kilometres1 and at pressures below #bej mm4 7,chapter ,,ii41 ,note #bc74 ,it is evident that an artificial atmosphere has to be employed in the ascent to great heights1 just as in submarine work4 ,the cure by compressed and rarefied air which is practised in certain illnesses is based partly on the mechanical action of the change of pressure1 and partly on the alteration in the partial pressure of the respired oxygen4 ,the combustion of organic substances--that is1 substances which make up the composition of plants and animals--proceeds in the same manner as the combustion of many inorganic substances1 such as sulphur1 phosphorus1 iron1 c41 from the combination of these substances with oxygen1 as was described in the ,introduction4 ,the decomposition1 rotting1 and similar transformations of substances1 which proceed around us1 are also very often dependent on the action of the oxygen of the air1 and also reduce it from a free to a combined state4 ,the majority of the compounds of oxygen are1 like water1 very stable1 and do not give up their oxygen under the ordinary conditions of nature4 ,as these processes are taking place everywhere1 it might be expected that the amount of free oxygen in the atmosphere should decrease1 and this decrease should proceed somewhat rapidly4 ,this is1 in fact1 observed where combustion or respiration proceeds in a closed space4 ,animals suffocate in a closed space because in consuming the oxygen the air remains unfit for respiration4 ,in the same manner combustion1 after a time1 ceases in a closed space1 which may be proved by a very simple experiment4 ,an ignited substance--for instance1 a piece of burning sulphur--has only to be placed in a glass flask1 which is then closed with a stout cork to prevent the access of the external air2 combustion will proceed for a certain time1 so long as the flask contains any free oxygen1 but it will cease when the oxygen of the enclosed air has combined with the sulphur4 ,from what has been said1 it is evident that regularity of combustion or respiration requires a constant renewal of air--that is1 that the burning substance or respiring animal should have access to a fresh supply of oxygen4 ,this is attained in dwellings by having many windows1 outlets1 and ventilators1 and by the current of air produced by fires and stoves4 ,as regards the air over the entire earth's surface its amount of oxygen hardly decreases1 because in nature there is a process going on which renews the supply of free oxygen4 ,plants1 or rather their leaves1 during daytime1#c under the influence of light1 absorb carbonic anhydride ,,co#b1 and evolve free oxygen4 ,thus the loss of oxygen which occurs in consequence of the respiration of animals and of combustion is made good by plants4 ,if a leaf be placed in a bell jar containing water1 and carbonic anhydride 7because this gas is absorbed and oxygen evolved from it by plants7 be passed into the bell1 and the whole apparatus placed in sunlight1 then oxygen will accumulate in the bell jar4 ,this experiment was first made by ,priestley at the end of the last century4 ,thus the life of plants on the earth not only serves for the formation of food for animals1 but also for keeping up a constant percentage of oxygen in the atmosphere4 ,in the long period of the life of the earth an equilibrium has been attained between the processes absorbing and evolving oxygen1 by which a definite quantity of free oxygen is preserved in the entire mass of the atmosphere4#d #c ,at night1 without the action of light1 without the absorption of that energy which is required for the decomposition of carbonic anhydride into free oxygen and carbon 7which is retained by the plants7 they breathe like animals1 absorbing oxygen and evolving carbonic anhydride4 ,this process also goes on side by side with the reverse process in the daytime1 but it is then far feebler than that which gives oxygen4 #d ,the earth's surface is equal to about #eaj million square kilometres1 and the mass of the air 7at a pressure of #gfj mm47 on each kilometre of surface is about #aj-a_/c thousand millions of kilograms1 or about #aj-a_/c million tons2 therefore the whole weight of the atmosphere is about #e1ajj million million 7 #ea #ajad7 tons4 ,consequently there are about #b #ajae tons of free oxygen in the earth's atmosphere4 ,the innumerable series of processes which absorb a portion of this oxygen are compensated for by the plant processes4 ,assuming that #ajj million tons of vegetable matter1 containing #dj p4c4 of carbon1 formed from carbonic acid1 are produced 7and the same process proceeds in water7 per year on the #ajj million square kilometres of dry land 7ten tons of roots1 leaves1 stems1 c41 per hectare1 or #a_/ajj of a square kilometre71 we find that the plant life of the dry land gives about #ajj1jjj tons of oxygen1 which is an insignificant fraction of the entire mass of the oxygen of the air4 ,oxygen was obtained as an independent gas in #aggd by ,priestley in ,england and in the same year by ,scheele in ,sweden1 but its nature and great importance were only perfectly elucidated by ,lavoisier4 ,free oxygen may be obtained by one or other method from all the substances in which it occurs4 ,thus1 for instance1 the oxygen of many substances may be transferred into water1 from which1 as we have already seen1 oxygen may be obtained4#e ,we will first consider the methods of extracting oxygen from air as being a substance everywhere distributed4 ,the separation of oxygen from it is1 however1 hampered by many difficulties4 #e ,the extraction of oxygen from water may be effected by two processes3 either by the decomposition of water into its constituent parts by the action of a galvanic current 7,chapter ,,ii471 or by means of the removal of the hydrogen from water4 ,but1 as we have seen and already know1 hydrogen enters into direct combination with very few substances1 and then only under special circumstances2 whilst oxygen1 as we shall soon learn1 combines with nearly all substances4 ,only gaseous chlorine 7and1 especially1 fluorine7 is capable of decomposing water1 taking up the hydrogen from it1 without combining with the oxygen4 ,chlorine is soluble in water1 and if an aqueous solution of chlorine1 so-called chlorine water1 be poured into a flask1 and this flask be inverted in a basin containing the same chlorine water1 then we shall have an apparatus by means of which oxygen may be extracted from water4 ,at the ordinary temperature1 and in the dark1 chlorine does not act on water1 or only acts very feebly2 but under the action of direct sunlight chlorine decomposes water1 with the evolution of oxygen4 ,the chlorine then combines with the hydrogen1 and gives hydrochloric acid1 which dissolves in the water1 and therefore free oxygen only will be separated from the liquid1 and it will only contain a small quantity of chlorine in admixture1 which can be easily removed by passing the gas through a solution of caustic potash4 ,from air1 which contains a mixture of oxygen and nitrogen1 the nitrogen alone cannot be removed1 because it has no inclination to combine directly or readily with any substance2 and although it does combine with certain substances 7boron1 titanium71 these substances combine simultaneously with the oxygen of the atmosphere4#f ,however1 oxygen may be separated from air by causing it to combine with substances which may be easily decomposed by the action of heat1 and1 in so doing1 give up the oxygen absorbed--that is1 by making use of reversible reactions4 ,thus1 for instance1 the oxygen of the atmosphere may be made to oxidise sulphurous anhydride1 ,,so#b 7by passing directly over ignited spongy platinum71 and to form sulphuric anhydride1 or sulphur trioxide1 ,,so#c2 and this substance 7which is a solid and volatile1 and therefore easily separated from the nitrogen and sulphurous anhydride71 on further heating1 gives oxygen and sulphurous anhydride4 ,caustic soda or lime extracts 7absorbs7 the sulphurous anhydride from this mixture1 whilst the oxygen is not absorbed1 and thus it is isolated from the air4 ,on a large scale in works1 as we shall afterwards see1 sulphurous anhydride is transformed into hydrate of sulphuric trioxide1 or sulphuric acid1 ,h#b,,so#d2 if this is allowed to drop on to red-hot flagstones1 water1 sulphurous anhydride1 and oxygen are obtained4 ,the oxygen is easily isolated from this mixture by passing the gases over lime4 ,the extraction of oxygen from oxide of mercury 7,priestley1 ,lavoisier71 which is obtained from mercury and the oxygen of the atmosphere1 is also a reversible reaction by which oxygen may be obtained from the atmosphere4 ,so also1 by passing dry air through a red-hot tube containing barium oxide1 it is made to combine with the oxygen of the air4 ,in this reaction the so-called barium peroxide1 ,ba,o#b1 is formed from the barium oxide1 ,ba,o1 and at a higher temperature the former evolves the absorbed oxygen1 and leaves the barium oxide originally taken4#g #f ,a difference in the physical properties of both gases cannot be here taken advantage of1 because they are very similar in this respect4 ,thus the density of oxygen is #af times and of nitrogen #ad times greater than the density of hydrogen1 and therefore porous vessels cannot be here employed--the difference between the times of their passage through a porous surface would be too insignificant4 ,illustration3 ,,fig4 #bg4--;,graham's apparatus for the decomposition of air by pumping it through india-rubber4 ,graham1 however1 succeeded in enriching air in oxygen by passing it through india-rubber4 ,this may be done in the following way3--,a common india-rubber cushion1 ,e 7,fig4 #bg71 is taken1 and its orifice hermetically connected with an air-pump1 or1 better still1 a mercury aspirator 7the ,sprengel pump is designated by the letters ,a1 ,c1 ,b74 ,when the aspirator 7,chapter ,,ii41 ,note #af7 has pumped out the air1 which will be seen by the mercury running out in an almost uninterrupted stream1 and from its standing approximately at the barometric height1 then it may be clearly observed that gas passes through the india-rubber4 ,this is also seen from the fact that bubbles of gas continually pass along with the mercury4 ,a minus pressure may be constantly maintained in the cushion by pouring mercury into the funnel ,a1 and screwing up the pinchcock ,c1 so that the stream flowing from it is small1 and then a portion of the air passing through the india-rubber will be carried along with the mercury4 ,this air may be collected in the cylinder1 ,r4 ,its composition proves to be about #db volumes of oxygen with #eg volumes of nitrogen1 and one volume of carbonic anhydride1 whilst ordinary air contains only #ba volumes of oxygen in #ajj volumes4 ,a square metre of india-rubber surface 7of the usual thickness7 passes about #de c4c4 of such air per hour4 ,this experiment clearly shows that india-rubber is permeable to gases4 ,this may1 by the way1 be observed in common toy balloons filled with coal-gas4 ,they fall after a day or two1 not because there are holes in them1 but because air penetrates into1 and the gas from1 their interior1 through the surface of the india-rubber of which they are made4 ,the rate of the passage of gases through india-rubber does not1 as ,mitchell and ,graham showed1 depend on their densities1 and consequently its permeability is not determined by orifices4 ,it more resembles dialysis--that is1 the penetration of liquids through colloid surfaces4 ,equal volumes of gases penetrate through india-rubber in periods of time which are related to each other as follows3--carbonic anhydride1 #ajj2 hydrogen1 #bdg2 oxygen1 #ecb2 marsh gas1 #fcc2 carbonic oxide1 #a1bbj2 nitrogen1 #a1ceh4 ,hence nitrogen penetrates more slowly than oxygen1 and carbonic anhydride more quickly than other gases4 #beef volumes of oxygen and #acehe volumes of carbonic anhydride penetrate in the same time as one volume of nitrogen4 ,by multiplying these ratios by the amounts of these gases in air1 we obtain figures which are in almost the same proportion as the volumes of the gases penetrating from air through india-rubber4 ,if the process of dialysis be repeated on the air which has already passed through india-rubber1 then a mixture containing #fe p4c4 by volume of oxygen is obtained4 ,it may be thought that the cause of this phenomenon is the absorption or occlusion 7see ,chap4 ,,ii41 ,note #cg7 of gases by india-rubber and the evolution of the gas dissolved in a vacuum2 and1 indeed1 india-rubber does absorb gases1 especially carbonic anhydride4 ,graham called the above method of the decomposition of air atmolysis4 #g ,the preparation of oxygen by this method1 which is due to ,boussingault1 is conducted in a porcelain tube1 which is placed in a stove heated by charcoal1 so that its ends project beyond the stove4 ,barium oxide 7which may be obtained by igniting barium nitrate1 previously dried7 is placed in the tube1 one end of which is connected with a pair of bellows1 or a gas-holder1 for keeping up a current of air through it4 ,the air is previously passed through a solution of caustic potash1 to remove all traces of carbonic anhydride1 and it is very carefully dried 7for the hydrate ,ba,h#b,o#b does not give the peroxide74 ,at a dark-red heat 7#ejj-fjj7 the oxide of barium absorbs oxygen from the air1 so that the gas leaving the tube consists almost entirely of nitrogen4 ,when the absorption ceases1 the air will pass through the tube unchanged1 which may be recognised from the fact that it supports combustion4 ,the barium oxide is converted into peroxide under these circumstances1 and eleven parts of barium oxide absorb about one part of oxygen by weight4 ,when the absorption ceases1 one end of the tube is closed1 a cork with a gas-conducting tube is fixed into the other end1 and the heat of the stove is increased to a bright-red heat 7#hjj74 ,at this temperature the barium peroxide gives up all that oxygen which it acquired at a dark-red heat--i4e4 about one part by weight of oxygen is evolved from twelve parts of barium peroxide4 ,after the evolution of the oxygen there remains the barium oxide which was originally taken1 so that air may be again passed over it1 and thus the preparation of oxygen from one and the same quantity of barium oxide may be repeated many times4 ,oxygen has been produced one hundred times from one mass of oxide by this method2 all the necessary precautions being taken1 as regards the temperature of the mass and the removal of moisture and carbonic acid from the air4 ,unless these precautions be taken1 the mass of oxide soon spoils4 ,as oxygen may become of considerable technical use1 from its capacity for giving high temperatures and intense light in the combustion of substances1 its preparation directly from air by practical methods forms a problem whose solution many investigators continue to work at up to the present day4 ,the most practical methods are those of ,tessi du ,motay and ,kassner4 ,the first is based on the fact that a mixture of equal weights of manganese peroxide and caustic soda at an incipient red heat 7about #cej7 absorbs oxygen from air1 with the separation of water1 according to the equation ,mn,o#b #b,na,h,o ,o ,na#b,mn,o#d ,h#b,o4 ,if superheated steam1 at a temperature of about #dej1 be then passed through the mixture1 the manganese peroxide and caustic soda originally taken are regenerated1 and the oxygen held by them is evolved1 according to the reverse equation ,na#b,mn,o#d ,h#b,o ,mn,o#b #b,na,h,o ,o4 ,this mode of preparing oxygen may be repeated for an infinite number of times4 ,the oxygen in combining liberates water1 and steam1 acting on the resultant substance1 evolves oxygen4 ,hence all that is required for the preparation of oxygen by this method is fuel and the alternate cutting off the supply of air and steam4 ,in ,kassner's process 7#ahia7 a mixture of oxide of lead and lime 7,pb,o #b;,ca,o7 is heated to redness in the presence of air1 oxygen is then absorbed and calcium plumbate1 ,ca#b,pb,o#d1 formed4 ,the latter is of a chocolate colour1 and on further heating evolves oxygen and gives the original mixture ,pb,o #b;,ca,o--that is1 the phenomenon is essentially the same as in ,boussingault's process 7with ,ba,o71 but according to ,le ,chatelier 7#ahic7 the dissociation tension of the oxygen evolved from ,ca#b,pb,o#d is less than with ,ba,o#b at equal temperatures2 for instance1 at #idj1 #aab mm4 of mercury for the first1 and for the latter #baj mm4 at #gbj1 and #fgj mm4 at #gij1 while for ,ca#b,pb,o#d this tension is only reached at #a1jhj4 ,however1 in ,kassner's process the oxygen is absorbed more rapidly1 and the influence of the presence of moisture and ,,co#b in the air is not so marked1 so that this process1 like that of ,tessi du ,motay1 deserves consideration4 ,oxygen is evolved with particular ease by a whole series of unstable oxygen compounds1 of which we shall proceed to take a general survey1 remarking that many of these reactions1 although not all1 belong to the number of reversible reactions2#h so that in order to obtain many of these substances 7for instance1 potassium chlorate7 rich in oxygen1 recourse must be had to indirect methods 7see ,introduction7 with which we shall become acquainted in the course of this book4 #h ,even the decomposition of manganese peroxide is reversible1 and it may be re-obtained from that suboxide 7or its salts71 which is formed in the evolution of oxygen 7,chap4 ,,xi41 ,note #f74 ,the compounds of chromic acid containing the trioxide ,cr,o#c in evolving oxygen give chromium oxide1 ,cr#b,o#c1 but they re-form the salt of chromic acid when heated to redness in air with an alkali4 #a4 ,the compounds of oxygen with certain metals1 and especially with the so-called noble metals--that is1 mercury1 silver1 gold1 and platinum--having once been obtained1 retain their oxygen at the ordinary temperature1 but part with it at a red heat4 ,the compounds are solids1 generally amorphous and infusible1 and are easily decomposed by heat into the metal and oxygen4 ,we have seen an example of this in speaking of the decomposition of mercury oxide4 ,priestley1 in #aggd1 obtained pure oxygen for the first time by heating mercury oxide by means of a burning-glass1 and clearly showed its difference from air4 ,he showed its characteristic property of supporting combustion 'with remarkable vigour1' and named it dephlogisticated air4 #b4 ,the substances called peroxides#i evolve oxygen at a greater or less heat 7and also by the action of many acids74 ,they usually contain metals combined with a large quantity of oxygen4 ,peroxides are the highest oxides of certain metals2 those metals which form them generally give several compounds with oxygen4 ,those of the lowest degrees of oxidation1 containing the least amount of oxygen1 are generally substances which are capable of easily reacting with acids--for instance1 with sulphuric acid4 ,such low oxides are called bases4 ,peroxides contain more oxygen than the bases formed by the same metals4 ,for example1 lead oxide contains #ga parts of oxygen in #ajj parts1 and is basic1 but lead peroxide contains #acc parts of oxygen in #ajj parts4 ,manganese peroxide is a similar substance1 and is a solid of a dark colour1 which occurs in nature4 ,it is employed for technical purposes under the name of black oxide of manganese 7in ,german1 ',braunstein1' the pyrolusite of the mineralogist74 ,peroxides are able to evolve oxygen at a more or less elevated temperature4 ,they do not then part with all their oxygen1 but with only a portion of it1 and are converted into a lower oxide or base4 ,thus1 for example1 lead peroxide1 on heating1 gives oxygen and lead oxide4 ,the decomposition of this peroxide proceeds tolerably easily on heating1 even in a glass vessel1 but manganese peroxide only evolves oxygen at a strong red heat1 and therefore oxygen can only be obtained from it in iron1 or other metallic1 or clay vessels4 ,this was formerly the method for obtaining oxygen4 ,manganese peroxide only parts with one-third of its oxygen 7according to the equation #c,mn,o#b ,mn#c,o#d ,o#b71 whilst two-thirds remain in the solid substance which forms the residue after heating4 ,metallic peroxides are also capable of evolving oxygen on heating with sulphuric acid4 ,they then evolve just that amount of oxygen which is in excess of that necessary for the formation of the base1 the latter reacting on the sulphuric acid forming a compound 7salt7 with it4 ,thus barium peroxide1 when heated with sulphuric acid1 forms oxygen and barium oxide1 which gives a compound with sulphuric acid termed barium sulphate 7,ba,o#b ,h#b,,so#d ,ba,s,o#d ,h#b,o ,o74#i bis ,this reaction usually proceeds with greater ease than the decomposition of peroxides by heat alone4 ,for the purposes of experiment powdered manganese peroxide is usually taken and mixed with strong sulphuric acid in a flask1 and the apparatus set up as shown in ,fig4 #bh4 ,the gas which is evolved is passed through a ,woulfe's bottle containing a solution of caustic potash1 to purify it from carbonic anhydride and chlorine1 which accompany the evolution of oxygen from commercial manganese peroxide1 and the gas is not collected until a thin smouldering taper placed in front of the escape orifice bursts into flame1 which shows that the gas coming off is oxygen4 ,by this method of decomposition of the manganese peroxide by sulphuric acid there is evolved1 not1 as in heating1 one-third1 but one-half of the oxygen contained in the peroxide 7,mn,o#b ,h#b,,so#d ,mn,s,o#d ,h#b,o ,o7--that is1 from #ej grams of peroxide about #g-a_/e grams1 or about #e-a_/b litres1 of oxygen1#aj whilst by heating only about #c-a_/b litres are obtained4 ,the chemists of ,lavoisier's time generally obtained oxygen by heating manganese peroxide4 ,at the present time more convenient methods are known4 ,illustration3 ,,fig4 #bh4--,preparation of oxygen from manganese peroxide and sulphuric acid4 ,the gas evolved is passed through a ,woulfe's bottle containing caustic potash4 #i ,we shall afterwards see that it is only substances like barium peroxide 7which give hydrogen peroxide7 which should be counted as true peroxides1 and that ,mn,o#b1 ,pb,o#b1 c41 should be distinguished from them 7they do not give hydrogen peroxide with acids71 and therefore it is best to call them dioxides4 #i bis ,peroxide of barium also gives oxygen at the ordinary temperature in the presence of the solutions of many substances in a higher degree of oxidation4 ,in this respect we may mention that ,kassner 7#ahij7 proposes to obtain oxygen for laboratory purposes by mixing ,ba,o#b with ,fe,k#c7;,,cn7#f 7red prussiate of potash1 ,chapter ,,xxii473 the reaction proceeds with the evolution of oxygen even on the addition of a very small quantity of water4 ,in order to ensure a gradual evolution of gas the author proposes to introduce both substances into the reaction1 little by little1 instead of all at once1 which may be done with the following arrangement 7,gavaloffsky73 finely powdered peroxide of barium is placed in an ordinary flask and sufficient water is added to fill the flask one-third full4 ,the cork closing the flask has three holes2 7#a7 for the gas-conducting tube2 7#b7 for a rod to stir the ,ba,o#b2 and 7#c7 for a glass rod terminating in a perforated glass vessel containing crystals of ,fe,k#c7;,,cn7#f4 ,when it is desired to start the evolution of the oxygen1 the vessel is lowered until it is immersed in the liquid in the flask1 and the ,ba,o#b is stirred with the other rod4 ,the reaction proceeds according to the equation1 ,ba,o#b #b;,fe,k#c7;,,cn7#f ,fe,k#d7;,,cn7#f ,fe,k#b;,ba7,,cn7#f ,o#b4 ,the double salt1 ,fe,ba#b7;,,cn7#f1 crystallises out from the mother liquor4 ,to understand the course of the reaction1 it must be remembered ,ba,o#b is of a higher degree of oxidation1 and that it parts with oxygen and gives the base ,ba,o which enters into the complex salt ,fe,k#b;,ba7,,cn7#f ,fe7,,cn7#b #b,,kcn ,ba7,,cn7#b1 and this latter ,ba,o #b;,,hcn-,h#b,o4 ,moreover1 ,fe,k#c7;,,cn7#f contains the salt ,fe#b7;,,cn7#f which also corresponds to the higher degree of oxidation of iron1 ,fe#b,o#c1 whilst after the reaction a salt is obtained which contains ,fe7,,cn7#b1 and corresponds to the lower degree of oxidation1 ,fe,o1 so that 7in the presence of water7 oxygen is also set free on this side also1 i4e4 the reaction gives lower degrees of oxidation and oxygen4 #aj ,scheele1 in #aghe1 discovered the method of obtaining oxygen by treating manganese peroxide with sulphuric acid4 #c4 ,a third source to which recourse may be had for obtaining oxygen is represented in acids and salts containing much oxygen1 which are capable1 by parting with a portion or all of their oxygen1 of being converted into other compounds 7lower products of oxidation7 which are more difficultly decomposed4 ,these acids and salts 7like peroxides7 evolve oxygen either on heating alone1 or only when in the presence of some other substance4 ,sulphuric acid may be taken as an example of an acid which is decomposed by the action of heat alone1#aa for it breaks up at a red heat into water1 sulphurous anhydride1 and oxygen1 as was mentioned before4 ,priestley1 in #aggb1 and ,scheele1 somewhat later1 obtained oxygen by heating nitre to a red heat4 ,the best examples of the formation of oxygen by the heating of salts is given in potassium chlorate1 or ,berthollet's salt1 so called after the ,french chemist who discovered it4 ,potassium chlorate is a salt composed of the elements potassium1 chlorine1 and oxygen1 ,k,cl,o#c4 ,it occurs as transparent colourless plates1 is soluble in water1 especially in hot water1 and resembles common table salt in some of its reactions and physical properties2 it melts on heating1 and in melting begins to decompose1 evolving oxygen gas4 ,this decomposition ends in all the oxygen being evolved from the potassium chlorate1 potassium chloride being left as a residue1 according to the equation ,k,cl,o#c ,k,cl ,o#c4ab ,this decomposition proceeds at a temperature which allows of its being conducted in a glass vessel4 ,however1 in decomposing1 the molten potassium chlorate swells up and boils1 and gradually solidifies1 so the evolution of the oxygen is not regular1 and the glass vessel may crack4 ,in order to overcome this inconvenience1 the potassium chlorate is crushed and mixed with a powder of a substance which is infusible1 incapable of combining with the oxygen evolved1 and is a good conductor of heat4 ,usually it is mixed with manganese peroxide4#ac ,the decomposition of the potassium chlorate is then considerably facilitated1 and proceeds at a lower temperature 7because the entire mass is then better heated1 both externally and internally71 without swelling up1 and this method is therefore more convenient than the decomposition of the salt alone4 ,this method for the preparation of oxygen is very convenient2 it is generally employed when a small quantity of oxygen is required4 ,further1 potassium chlorate is easily obtained pure1 and it evolves much oxygen4 #ajj grams of the salt give as much as #ci grams1 or #cj litres1 of oxygen4 ,this method is so simple and easy1#ad that a course of practical chemistry is often commenced by the preparation of oxygen by this method1 and of hydrogen by the aid of zinc and sulphuric acid1 since by means of these gases many interesting and striking experiments may be performed4#ae #aa ,all acids rich in oxygen1 and especially those whose elements form lower oxides1 evolve oxygen either directly at the ordinary temperature 7for instance1 ferric acid71 or on heating 7nitric1 manganic1 chromic1 chloric1 and others71 or if basic lower oxides are formed from them1 by heating with sulphuric acid4 ,thus the salts of chromic acid 7for example1 potassium dichromate1 ,k#b;,cr#b,o#g7 give oxygen with sulphuric acid2 first potassium sulphate1 ,k#b,,so#d1 is formed1 and then the chromic acid set free gives a sulphuric acid salt of the lower oxide1 ,cr#b,o#c4 #ab ,this reaction is not reversible1 and is exothermal--that is1 it does not absorb heat1 but1 on the contrary1 evolves #i1gac calories per molecular weight ,k,cl,o#c1 equal to #abb parts of salt 7according to the determination of ,thomsen1 who burnt hydrogen in a calorimeter either alone or with a definite quantity of potassium chlorate mixed with oxide of iron74 ,it does not proceed at once1 but first forms perchlorate1 ,k,cl,o#d 7see ,chlorine and ,potassium74 ,it is to be remarked that potassium chloride melts at #gff1 potassium chlorate at #cei1 and potassium perchlorate at #faj4 7,concerning the decomposition of ,k,cl,o#c1 see ,chapter ,,ii41 ,note #dg47 #ac ,the peroxide does not evolve oxygen in this case4 ,it may be replaced by many oxides--for instance1 by oxide of iron4 ,it is necessary to take the precaution that no combustible substances 7such as bits of paper1 splinters1 sulphur1 c47 fall into the mixture1 as they might cause an explosion4 #ad ,the decomposition of a mixture of fused and well-crushed potassium chlorate with powdered manganese peroxide proceeds at so low a temperature 7the salt does not melt7 that it may be effected in an ordinary glass flask4 ,the apparatus is arranged in the same manner as in the decomposition of mercury oxide 7,introduction71 or as shown in the last drawing4 ,as the reaction is exothermal1 the decomposition of potassium chlorate with the formation of oxygen may probably be accomplished1 under certain conditions 7for example1 under contact action71 at very low temperatures4 ,substances mixed with the potassium chlorate probably act partially in this manner4 #ae ,many other salts evolve oxygen by heat1 like potassium chlorate1 but they only part with it either at a very high temperature 7for instance1 common nitre7 or else are unsuited for use on account of their cost 7potassium manganate71 or evolve impure oxygen at a high temperature 7zinc sulphate at a red heat gives a mixture of sulphurous anhydride and oxygen71 and are not therefore used in practice4 ,a solution of bleaching powder1 which contains calcium hypochlorite1 ,ca,cl#b,o#b1 evolves oxygen on gently heating when a small quantity of certain oxides is added--for instance1 cobalt oxide1 which in this case acts by contact 7see ,introduction74 ,when heated by itself1 a solution of bleaching powder does not evolve oxygen1 but it oxidises the cobalt oxide to a higher degree of oxidation2 this higher oxide of cobalt in contact with the bleaching powder decomposes into oxygen and lower oxidation products1 and the resultant lower oxide of cobalt with bleaching powder again gives the higher oxide1 which again gives up its oxygen1 and so on4#af ,the calcium hypochlorite is here decomposed according to the equation ,ca,cl#b,o#b ,ca,cl#b ,o#b4 ,in this manner a small quantity of cobalt oxide#ag is sufficient for the decomposition of an indefinitely large quantity of bleaching powder4 #af ,such is1 at present1 the only possible method of explaining the phenomenon of contact action4 ,in many cases1 such as the present one1 it is supported by observations based on facts4 ,thus1 for instance1 it is known1 as regards oxygen1 that often two substances rich in oxygen retain it so long as they are separate1 but directly they come into contact free oxygen is evolved from both of them4 ,thus1 an aqueous solution of hydrogen peroxide 7containing twice as much oxygen as water7 acts in this manner on silver oxide 7containing silver and oxygen74 ,this reaction takes place at the ordinary temperature1 and the oxygen is evolved from both compounds4 ,to this class of phenomena may be also referred the fact that a mixture of barium peroxide and potassium manganate with water and sulphuric acid evolves oxygen at the ordinary temperature 7,note #i bis74 ,it would seem that the essence of phenomena of this kind is entirely and purely a property of contact2 the distribution of the atoms is changed by contact1 and if the equilibrium be unstable it is destroyed4 ,this is more especially evident in the case of those substances which change exothermally--that is1 for those reactions which are accompanied by an evolution of heat4 ,the decomposition ,ca,cl#b,o#b ,ca,cl#b ,o#b belongs to this class 7like the decomposition of potassium chlorate74 #ag ,generally a solution of bleaching powder is alkaline 7contains free lime71 and therefore1 a solution of cobalt chloride is added directly to it1 by which means the oxide of cobalt required for the reaction is formed4 ,the properties of oxygen4#ah--;,it is a permanent gas--that is1 it cannot be liquefied by pressure at the ordinary temperature1 and further1 is only liquefied with difficulty 7although more easily than hydrogen7 at temperatures below -#abj1 because this is its absolute boiling point4 ,as its critical pressure#ai is about #ej atmospheres1 it can be easily liquefied under pressures greater than #ej atmospheres at temperatures below -#abj4 ,according to ,dewar1 the density of oxygen in a critical state is #jfe 7water #a71 but1 like all other substances in this state1#bj it varies considerably in density with a change of pressure and temperature1 and therefore many investigators who made their observations under high pressures give a greater density1 as much as #aa4 ,liquefied oxygen is an exceedingly mobile transparent liquid1 with a faint blue tint and boiling 7tension #a atmosphere7 about -#ahj4 ,oxygen1 like all gases1 is transparent1 and like the majority of gases1 colourless4 ,it has no smell or taste1 which is evident from the fact of its being a component of air4 ,the weight of one litre of oxygen gas at #j and #gfj mm4 pressure is #adbih gram2 it is therefore slightly denser than air4 ,its density in respect to air #aajef and in respect to hydrogen #af4ba #ah ,it must be remarked that in all the reactions above mentioned the formation of oxygen may be prevented by the admixture of substances capable of combining with it--for example1 charcoal1 many carbon 7organic7 compounds1 sulphur1 phosphorus1 and various lower oxidation products1 c4 ,these substances absorb the oxygen evolved1 combine with it1 and a compound containing oxygen is formed4 ,thus1 if a mixture of potassium chlorate and charcoal be heated1 no oxygen is obtained1 but an explosion takes place from the rapid formation of gases resulting from the combination of the oxygen of the potassium chlorate with the charcoal and the evolution of gaseous ,,co#b4 ,the oxygen obtained by any of the above-described methods is rarely pure4 ,it generally contains aqueous vapour1 carbonic anhydride1 and very often small traces of chlorine4 ,the oxygen may be freed from these impurities by passing it through a solution of caustic potash1 and by drying it4 ,if the potassium chlorate be dry and pure1 it gives almost pure oxygen4 ,however1 if the oxygen be required for respiration in cases of sickness1 it should be washed by passing it through a solution of caustic alkali and through water4 ,the best way to obtain pure oxygen directly is to take potassium perchlorate 7,k,cl,o#d71 which can be well purified and then evolves pure oxygen on heating4 #ai ,with regard to the absolute boiling point1 critical pressure1 and the critical state in general1 see ,chapter ,,ii41 ,notes #bi and #cd4 #bj ,judging from what has been said in ,note #cd of the last chapter1 and also from the results of direct observation1 it is evident that all substances in a critical state have a large coefficient of expansion1 and are very compressible4 #ba ,as water consists of #a volume of oxygen and #b volumes of hydrogen1 and contains #af parts by weight of oxygen per #b parts by weight of hydrogen1 it therefore follows directly that oxygen is #af times denser than hydrogen4 ,conversely1 the composition of water by weight may be deduced from the densities of hydrogen and oxygen1 and the volumetric composition of water4 ,this method of mutual and reciprocal correction strengthens the practical data of the exact sciences1 whose conclusions require the greatest possible exactitude and variety of corrections4 ,it must he observed that the specific heat of oxygen at constant pressure is #jbage1 consequently it is to the specific heat of hydrogen 7#cdji7 as #a is to #aef4 ,hence1 the specific heats are inversely proportional to the weights of equal volumes4 ,this signifies that equal volumes of both gases have 7nearly7 equal specific heats--that is1 they require an equal quantity of heat for raising their temperature by #a4 ,we shall afterwards consider the specific heat of different substances more fully in ,chap4 ,,xiv4 ,oxygen1 like the majority of difficultly-liquefiable gases1 is but slightly soluble in water and other liquids4 ,the solubility is given in ,note #cj1 ,chap4 ,i4 ,from this it is evident that water standing in air must absorb--i4e4 dissolve--oxygen4 ,this oxygen serves for the respiration of fishes4 ,fishes cannot exist in boiled water1 because it does not contain the oxygen necessary for their respiration 7see ,chap4 ,i47 ,illustration3 ,,fig4 #bi4--,mode of burning sulphur1 phosphorus1 sodium1 c41 in oxygen4 ,in its chemical properties oxygen is remarkable from the fact that it very easily--and1 in a chemical sense1 vigorously--reacts on a number of substances1 forming oxygen compounds4 ,however1 only a few substances and mixtures of substances 7for example1 phosphorus1 copper with ammonia1 decomposing organic matter1 aldehyde1 pyrogallol with an alkali1 c47 combine directly with oxygen at the ordinary temperature1 whilst many substances easily combine with oxygen at a red heat1 and often this combination presents a rapid chemical reaction accompanied by the evolution of a large quantity of heat4 ,every reaction which takes place rapidly1 if it be accompanied by so great an evolution of heat as to produce incandescence1 is termed combustion4 ,thus combustion ensues when many metals are plunged into chlorine1 or oxide of sodium or barium into carbonic anhydride1 or when a spark falls on gunpowder4 ,a great many substances are combustible in oxygen1 and1 owing to its presence1 in air also4 ,in order to start combustion it is generally necessary#bb that the combustible substance should be brought to a state of incandescence4 ,the continuation of the process does not require the aid of fresh external heat1 because sufficient heat#bc is evolved to raise the temperature of the remaining parts of the combustible substance to the required degree4 ,examples of this are familiar to all from every-day experience4 ,combustion proceeds in oxygen with greater rapidity1 and is accompanied by a more powerful incandescence1 than in ordinary air4 ,this may be demonstrated by a number of very convincing experiments4 ,if a piece of charcoal1 attached to a wire and previously brought to red-heat1 be plunged into a flask full of oxygen1 it burns rapidly at a white heat--i4e4 it combines with the oxygen1 forming a gaseous product of combustion called carbonic anhydride1 or carbonic acid gas1 ,,co#b4 ,this is the same gas that is evolved in the act of respiration1 for charcoal is one of the substances which is obtained by the decomposition of all organic substances which contain it1 and in the process of respiration part of the constituents of the body1 so to speak1 slowly burn4 ,if a piece of burning sulphur be placed in a small cup attached to a wire and introduced into a flask full of oxygen1 then the sulphur1 which burns in air with a very feeble flame1 burns in the oxygen with a violet flame1 which1 although pale1 is much larger than in air4 ,if the sulphur be exchanged for a piece of phosphorus1#bd then1 unless the phosphorus be heated1 it combines very slowly with the oxygen2 but1 if heated1 although on only one spot1 it burns with an exceedingly brilliant white flame4 ,in order to heat the phosphorus inside the flask1 the simplest way is to bring a red-hot wire into contact with it4 ,before the charcoal can burn1 it must be brought to a state of incandescence4 ,sulphur also will not burn under #ajj1 whilst phosphorus inflames at #dj4 ,phosphorus which has been already lighted in air cannot so well be introduced into the flask1 because it burns very rapidly and with a large flame in air4 ,if a small lump of metallic sodium be put in a small cup made of lime1#be melted1 and ignited1#bf it burns very feebly in air4 ,but if burning sodium be introduced into oxygen1 the combustion is invigorated and is accompanied by a brighter yellow flame4 ,metallic magnesium1 which burns brightly in air1 continues to burn with still greater vigour in oxygen1 forming a white powder1 which is a compound of magnesium with oxygen 7magnesium oxide2 magnesia74 ,a strip of iron or steel does not burn in air1 but an iron wire or steel spring may be easily burnt in oxygen4#bg ,the combustion of steel or iron in oxygen is not accompanied by a flame1 but sparks of oxide fly in all directions from the burning portions of the iron4#bh #bb ,certain substances 7with which we shall afterwards become acquainted71 however1 ignite spontaneously in air2 for example1 impure phosphuretted hydrogen1 silicon hydride1 zinc ethyl1 and pyrophorus 7very finely divided iron1 c47 #bc ,if so little heat is evolved that the adjacent parts are not heated to the temperature of combustion1 then combustion will cease4 #bd ,the phosphorus must be dry2 it is usually kept in water1 as it oxidises in air4 ,it should be cut under water1 as otherwise the freshly-cut surface oxidises4 ,it must be dried carefully and quickly by wrapping it in blotting-paper4 ,if damp1 it splutters on burning4 ,a small piece should be taken1 as otherwise the iron spoon will melt4 ,in this and the other experiments on combustion1 water should be poured over the bottom of the vessel containing the oxygen1 to prevent it from cracking4 ,the cork closing the vessel should not fit tightly1 in order to allow for the expansion of the gas due to the heat of the combustion4 #be ,an iron cup will melt with sodium in oxygen4 #bf ,in order to rapidly heat the lime crucible containing the sodium1 it is heated in the flame of a blowpipe described in ,chap4 ,,viii4 #bg ,in order to burn a watch spring1 a piece of tinder 7or paper soaked in a solution of nitre1 and dried7 is attached to one end4 ,the tinder is lighted1 and the spring is then plunged into the oxygen4 ,the burning tinder heats the end of the spring1 the heated part burns1 and in so doing heats the further portions of the spring1 which then burns completely if sufficient oxygen be present4 #bh ,the sparks of rust are produced1 owing to the fact that the volume of the oxide of iron is nearly twice that of the volume of the iron1 and as the heat evolved is not sufficient to entirely melt the oxide or the iron1 the particles must be torn off and fly about4 ,similar sparks are formed in the combustion of iron1 in other cases also4 ,we saw the combustion of iron filings in the ,introduction4 ,in the welding of iron small iron splinters fly off in all directions and burn in the air1 as is seen from the fact that whilst flying through the air they remain red hot1 and also because1 on cooling1 they are seen to be no longer iron1 but a compound of it with oxygen4 ,the same thing takes place when the hammer of a gun strikes against the flint4 ,small scales of steel are heated by the friction1 and glow and burn in the air4 ,the combustion of iron is still better seen by taking it as a very fine powder1 such as is obtained by the decomposition of certain of its compounds--for instance1 by heating ,prussian blue1 or by the reduction of its compounds with oxygen by hydrogen2 when this fine powder is strewn in air1 it burns by itself1 even without being previously heated 7it forms a pyrophorus74 ,this obviously depends on the fact that the powder of iron presents a larger surface of contact with air than an equal weight in a compact form4 ,illustration3 ,,fig4 #cj4--,mode of burning a steel spring in oxygen4 ,in order to demonstrate by experiment the combustion of hydrogen in oxygen1 a gas-conducting tube1 bent so as to form a convenient jet1 is led from the vessel evolving hydrogen4 ,the hydrogen is first set light to in air1 and then the gas-conducting tube is let down into a flask containing oxygen4 ,the combustion in oxygen will be similar to that in air2 the flame remains pale1 notwithstanding the fact that its temperature rises considerably4 ,it is instructive to remark that oxygen may burn in hydrogen1 just as hydrogen in oxygen4 ,in order to show the combustion of oxygen in hydrogen1 a tube bent vertically upwards and ending in a fine orifice is attached to the stopcock of a gas-holder full of oxygen4 ,two wires1 placed at such a distance from each other as to allow the passage of a constant series of sparks from a ,ruhmkorff's coil1 are fixed in front of the orifice of the tube4 ,this is in order to ignite the oxygen1 which may also be done by attaching tinder round the orifice1 and burning it4 ,when the wires are arranged at the orifice of the tube1 and a series of sparks passes between them1 then an inverted 7because of the lightness of the hydrogen7 jar full of hydrogen is placed over the gas-conducting tube4 ,when the jar covers the orifice of the gas-conducting tube 7and not before1 as otherwise an explosion might take place7 the cock of the gasometer is opened1 and the oxygen flows into the hydrogen and is set light to by the sparks4 ,the flame obtained is similar to that formed by the combustion of hydrogen in oxygen4#bi ,from this it is evident that the flame is the locality where the oxygen combines with the hydrogen1 therefore a flame of burning oxygen can be obtained as well as a flame of burning hydrogen4 #bi ,the experiment may be conducted without the wires1 if the hydrogen be lighted in the orifice of an inverted cylinder1 and at the same time the cylinder be brought over the end of a gas-conducting tube connected with a gas-holder containing oxygen4 ,thomsen's method may be adopted for a lecture experiment4 ,two glass tubes1 with platinum ends1 are passed through orifices1 about #a-a-a_/b centimetre apart1 in a cork4 ,one tube is connected with a gas-holder containing oxygen1 and the other with a gas-holder full of hydrogen4 ,having turned on the gases1 the hydrogen is lighted1 and a common lamp glass1 tapering towards the top1 is placed over the cork4 ,the hydrogen continues to burn inside the lamp glass1 at the expense of the oxygen4 ,if the current of oxygen be then decreased little by little1 a point is reached when1 owing to the insufficient supply of oxygen1 the flame of the hydrogen increases in size1 disappears for several moments1 and then reappears at the tube supplying the oxygen4 ,if the flow of oxygen be again increased1 the flame reappears at the hydrogen tube4 ,thus the flame may be made to appear at one or the other tube at will1 only the increase or decrease of the current of gas must take place by degrees and not suddenly4 ,further1 air may be taken instead of oxygen1 and ordinary coal-gas instead of hydrogen1 and it will then be shown how air burns in an atmosphere of coal-gas1 and it can easily be proved that the lamp glass is full of a gas combustible in air1 because it may be lighted at the top4 ,if1 instead of hydrogen1 any other combustible gas be taken--for example1 ordinary coal gas--then the phenomenon of combustion will be exactly the same1 only a bright flame will be obtained1 and the products of combustion will be different4 ,however1 as coal gas contains a considerable amount of free and combined hydrogen1 it will also form a considerable quantity of water in its combustion4 ,if hydrogen be mixed with oxygen in the proportion in which they form water--i4e4 if two volumes of hydrogen be taken for each volume of oxygen--then the mixture will be the same as that obtained by the decomposition of water by a galvanic current--detonating gas4 ,illustration3 ,,fig4 #ca4--;,cavendish's apparatus for exploding detonating gas4 ,the bell jar standing in the bath is filled with a mixture of two volumes of hydrogen and one volume of oxygen1 and the thick glass vessel ,a is then screwed on to it4 ,the air is first pumped out of this vessel1 so that when the stopcock ,c is opened1 it becomes filled with detonating gas4 ,the stopcock is then re-closed1 and the explosion produced by means of a spark from a ,leyden jar4 ,after the explosion has taken place the stopcock is again opened1 and the water rises into the vessel ,a4 ,we have already mentioned in the last chapter that the combination of these gases1 or their explosion1 may be brought about by the action of an electric spark1 because the spark heats the space through which it passes1 and acts consequently in a manner similar to ignition by means of contact with an incandescent or burning substance4#bi bis ,cavendish made this experiment on the ignition of detonating gas1 at the end of the last century1 in the apparatus shown in fig4 #ca4 ,ignition by the aid of the electric spark is convenient1 for the reason that it may then be brought about in a closed vessel1 and hence chemists still employ this method when it is required to ignite a mixture of oxygen with a combustible gas in a closed vessel4 ,for this purpose1 especially since ,bunsen's time1#cj an eudiometer is employed4 ,it consists of a thick glass tube graduated along its length in millimetres 7for indicating the height of the mercury column71 and calibrated for a definite volume 7weight of mercury74 ,two platinum wires are fused into the upper closed end of the tube1 as shown in fig4 #cb4ca ,by the aid of the eudiometer we may not only determine the volumetric composition of water1#cb and the quantitative contents of oxygen in air1#cc but also make a number of experiments explaining the phenomenon of combustion4 ,illustration3 ,,fig4 #cb4--;,eudiometer #bi bis ,in fact1 instead of a spark a fine wire may be taken1 and an electric current passed through it to bring it to a state of incandescence2 in this case there will be no sparks1 but the gases will inflame if the wire be fine enough to become red hot by the passage of the current4 #cj ,now1 a great many other different forms of apparatus1 sometimes designed for special purposes1 are employed in the laboratory for the investigation of gases4 ,detailed descriptions of the methods of gas analysis1 and of the apparatus employed1 must be looked for in works on analytical and applied chemistry4 #ca ,they must be sealed into the tube in such a manner as to leave no aperture between them and the glass4 ,in order to test this1 the eudiometer is filled with mercury1 and its open end inverted into mercury4 ,if there be the smallest orifice at the wires1 the external air will enter into the cylinder and the mercury will fall1 although not rapidly if the orifice be very fine4 #cb ,the eudiometer is used for determining the composition of combustible gases4 ,a detailed account of gas analysis would be out of place in this work 7see ,note #cj71 but1 as an example1 we will give a short description of the determination of the composition of water by the eudiometer4 ,pure and dry oxygen is first introduced into the eudiometer4 ,when the eudiometer and the gas in it acquire the temperature of the surrounding atmosphere--which is recognised by the fact of the meniscus of the mercury not altering its position during a long period of time--then the heights at which the mercury stands in the eudiometer and in the bath are observed4 ,the difference 7in millimetres7 gives the height of the column of mercury in the eudiometer4 ,it must be reduced to the height at which the mercury would stand at #j and deducted from the atmospheric pressure1 in order to find the pressure under which the oxygen is measured 7see ,chap4 ,i4 ,note #bi74 ,the height of the mercury also shows the volume of the oxygen4 ,the temperature of the surrounding atmosphere and the height of the barometric column must also be observed1 in order to know the temperature of the oxygen and the atmospheric pressure4 ,when the volume of the oxygen has been measured1 pure and dry hydrogen is introduced into the eudiometer1 and the volume of the gases in the eudiometer again measured4 ,they are then exploded4 ,this is done by a ,leyden jar1 whose outer coating is connected by a chain with one wire1 so that a spark passes when the other wire1 fused into the eudiometer1 is touched by the terminal of the jar4 ,or else an electrophorus is used1 or1 better still1 a ,ruhmkorff's coil1 which has the advantage of working equally well in damp or dry air1 whilst a ,leyden jar or electrical machine does not act in damp weather4 ,further1 it is necessary to close the lower orifice of the eudiometer before the explosion 7for this purpose the eudiometer1 which is fixed in a stand1 is firmly pressed down from above on to a piece of india-rubber placed at the bottom of the bath71 as otherwise the mercury and gas would be thrown out of the apparatus by the explosion4 ,it must also be remarked that to ensure complete combustion the proportion between the volumes of oxygen and hydrogen must not exceed twelve of hydrogen to one volume of oxygen1 or fifteen volumes of oxygen to one volume of hydrogen1 because no explosion will take place if one of the gases be in great excess4 ,it is best to take a mixture of one volume of hydrogen with several volumes of oxygen4 ,the combustion will then be complete4 ,it is evident that water is formed1 and that the volume 7or tension7 is diminished1 so that on opening the end of the eudiometer the mercury will rise in it4 ,but the tension of the aqueous vapour is now added to the tension of the gas remaining after the explosion4 ,this must be taken into account 7,chap4 ,i4 ,note #a74 ,if but little gas remain1 the water which is formed will be sufficient for its saturation with aqueous vapour4 ,this may be learnt from the fact that drops of water are visible on the sides of the eudiometer after the mercury has risen in it4 ,if there be none1 a certain quantity of water must be introduced into the eudiometer4 ,then the number of millimetres expressing the pressure of the vapour corresponding with the temperature of the experiment must be subtracted from the atmospheric pressure at which the remaining gas is measured1 otherwise the result will be inaccurate 7,chap4 ,i4 ,note #a74 ,this is essentially the method of the determination of the composition of water which was made for the first time by ,gay-,lussac and ,humboldt with sufficient accuracy4 ,their determinations led them to the conclusion that water consists of two volumes of hydrogen 7more exactly #bjjc1 ,le ,duc #ahib71 and one volume of oxygen4 ,every time they took a greater quantity of oxygen1 the gas remaining after the explosion was oxygen4 ,when they took an excess of hydrogen1 the remaining gas was hydrogen2 and when the oxygen and hydrogen were taken in exactly the above proportion1 neither one nor the other remained4 ,the composition of water was thus definitely confirmed4 #cc ,concerning this application of the eudiometer1 see the chapter on ,nitrogen4 ,it may be mentioned as illustrating the various uses of the eudiometer that ,prof4 ,timeraseeff employed microscopically small eudiometers to analyse the bubbles of gas given off from the leaves of plants4 ,thus1 for example1 it may be demonstrated1 by the aid of the eudiometer1 that for the ignition of detonating gas1 a definite temperature is required4 ,if the temperature be below that required1 combination will not take place1 but if at any spot within the tube it rises to the temperature of inflammation1 then combination will ensue at that spot1 and evolve enough heat for the ignition of the adjacent portions of the detonating mixture4 ,if to #a volume of detonating gas there be added #aj volumes of oxygen1 or #d volumes of hydrogen1 or #c volumes of carbonic anhydride1 then we shall not obtain an explosion by passing a spark through the diluted mixture4 ,this depends on the fact that the temperature falls with the dilution of the detonating gas by another gas1 because the heat evolved by the combination of the small quantity of hydrogen and oxygen brought to incandescence by the spark is not only transmitted to the water proceeding from the combination1 but also to the foreign substance mixed with the detonating gas4#cd ,the necessity of a definite temperature for the ignition of detonating gas is also seen from the fact that pure detonating gas explodes in the presence of a red-hot iron wire1 or of charcoal heated to #bge1 but with a lower degree of incandescence there is not any explosion4 ,it may also be brought about by rapid compression1 when1 as is known1 heat is evolved4#ce ,experiments made in the eudiometer showed that the ignition of detonating gas takes place at a temperature between #dej and #efj4cf #cd ,thus #a_/d volume of carbonic oxide1 an equal volume of marsh gas1 two volumes of hydrogen chloride or of ammonia1 and six volumes of nitrogen or twelve volumes of air added to one volume of detonating gas1 prevent its explosion4 #ce ,if the compression be brought about slowly1 so that the heat evolved succeeds in passing to the surrounding space1 then the combination of the oxygen and hydrogen does not take place1 even when the mixture is compressed by #aej times2 for the gases are not heated4 ,if paper soaked with a solution of platinum 7in aqua regia7 and sal ammoniac be burnt1 then the ash obtained contains very finely-divided platinum1 and in this form it is best fitted for igniting hydrogen and detonating gas4 ,platinum wire requires to be heated1 but platinum in so finely divided a state as it occurs in this ash inflames hydrogen1 even at -#bj4 ,many other metals1 such as palladium 7#age71 iridium1 and gold1 act with a slight rise of temperature1 like platinum2 but mercury1 at its boiling point1 does not inflame detonating gas1 although the slow formation of water then begins at #cje4 ,all data of this kind show that the explosion of detonating gas presents one of the many cases of contact phenomena4 ,this conclusion is further confirmed by the researches of ,v4 ,meyer 7#ahib74 ,he showed that only a very slow formation of steam begins at #ddh1 and that it only proceeds more rapidly at #eah4 ,the temperature of the explosion of detonating gas1 according to the same author1 varies according as to whether the explosion is produced in open vessels or in closed tubes4 ,in the first case the temperature of explosion lies between #ecj-fjf1 and in the second between #fcj-gcj4 ,in general it may be remarked that the temperature of explosion of gaseous mixtures is always lower in closed vessels than when the detonating mixture flows freely through tubes4 ,according to ,freyer and ,v4 ,meyer1 the following gases when mixed with the requisite amount of oxygen explode at the following temperatures3 ---------------------------------------- ---------------- -- ,when flowing freely ,in closed vessels ---------------------------------------- ---------------- ,h#b #fcj-gcj #ecj-fjf ,,ch#d #fej-gcj #fjf-fej ,c#b;,h#f #fjf-fej #ecj-fjf ,c#b;,h#d #fjf-fej #ecj-fjf ,,co #fej-gcj #fej-gcj ,h#b,s #cae-cbj #bej-bgj ,h#b ,cl#b #dcj-ddj #bdj-bgj ---------------------------------------- ---------------- ,the velocity of the transmission of explosion in gaseous mixtures is as characteristic a quantity for gaseous systems as the velocity of the transmission of sound4 ,berthelot showed that this velocity depends neither upon the pressure nor upon the size of the tubes in which the gaseous mixture is contained1 nor upon the material out of which the tube is made4 ,dixon 7#ahia7 determined the magnitude of these velocities for various mixtures1 and his results proved very near to those previously given by ,berthelot4 ,for comparison we give the velocities expressed in metres per second3 ----------------------------------- -- ,dixon ,berthelot ----------------------------------- ,h#b ,o #b1hba #b1haj ,h#b ,n#b,o #b1cje #b1bhd ,,ch#d #d,o #b1cbb #b1bhg ,c#b;,h#b #f,o #b1cfd #b1baj ,c#b;,h#b #e,o #b1cia #b1dhb ,c#b,n#b #d,o #b1cba #b1aie ----------------------------------- ,the addition of oxygen to detonating gas lowers the velocity of the transmission of explosion almost as much as the introduction of nitrogen4 ,an excess of hydrogen on the contrary raises the velocity of transmission4 ,it is remarked that the explosion of mixtures of oxygen with marsh gas1 ethylene and cyanogen is transmitted more quickly if the oxygen be taken in such a proportion that the carbon should burn to oxide of carbon1 i4e4 the velocity of the explosion is less if the oxygen be taken in sufficient quantity to form carbonic anhydride4 ,observations upon liquid and solid explosives 7,berthelot7 show that in this case the velocity of transmission of explosion is dependent upon the material of the tube4 ,thus the explosion of liquid nitro-methyl ether in glass tubes travels at the rate 7in dependence upon the diam41 from #a mm4-#de mm47 of from #a1hij to #b1dhb metres1 and in tubes of ,britannia metal 7#c mm4 in diam7 at the rate of #a1bcj metres4 ,the harder the tube the greater the velocity of transmission of explosion4 ,the following are the velocities for certain bodies3 metres ,nitro-glycerine #a1cjj ,dynamite #b1ejj ,nitro-mannite #g1gjj ,picric acid #f1ejj ,in conclusion we may add that ,mallard and ,le ,chatelier 7#ahhb7 observed that in the explosion of a mixture of #a volume of detonating gas with n volumes of an inert gas1 the pressure is approximately equal to #ib-jin atmospheres4 #cf ,from the very commencement of the promulgation of the idea of dissociation1 it might have been imagined that reversible reactions of combination 7the formation of ,h#b and ,o belongs to this number7 commence at the same temperature as that at which dissociation begins4 ,and in many cases this is so1 but not always1 as may be seen from the facts 7#a7 that at #dej-efj1 when detonating gas explodes1 the density of aqueous vapour not only does not vary 7and it hardly varies at higher temperatures1 probably because the amount of the products of dissociation is small71 but there are not1 as far as is yet known1 any traces of dissociation2 7#b7 that under the influence of contact the temperature at which combination takes place falls even to the ordinary temperature1 when water and similar compounds naturally are not dissociated and1 judging from the data communicated by ,d4 ,p4 ,konovaloff 7,introduction1 ,note #ci7 and others1 it is impossible to escape the phenomena of contact2 all vessels1 whether of metal or glass1 show the same influence as spongy platinum1 although to a much less degree4 ,the phenomena of contact1 judging from a review of the data referring to it1 must be especially sensitive in reactions which are powerfully exothermal1 and the explosion of detonating gas is of this kind4 ,the combination of hydrogen with oxygen is accompanied by the evolution of a very considerable amount of heat2 according to the determinations of ,favre and ,silbermann1#cg #a part by weight of hydrogen in forming water evolves #cd1dfb units of heat4 ,many of the most recent determinations are very close to this figure1 so that it may be taken that in the formation of #ah parts of water 7,h#b,o7 there are evolved #fi major calories1 or #fi1jjj units of heat4#ch ,if the specific heat of aqueous vapour 7#jdh7 remained constant from the ordinary temperature to that at which the combustion of detonating gas takes place 7but there is now no doubt that it increases71 were the combustion concentrated at one point#ci 7but it occurs in the whole region of a flame71 were there no loss from radiation and heat conduction1 and did dissociation not take place--that is1 did not a state of equilibrium between the hydrogen1 oxygen1 and water come about--then it would be possible to calculate the temperature of the flame of detonating gas4 ,it would then be #h1jjj4dj ,in reality it is very much lower1 but it is nevertheless higher than the temperature attained in furnaces and flames1 and is as high as #b1jjj4 ,the explosion of detonating gas is explained by this high temperature1 because the aqueous vapour formed must occupy a volume at least #e times greater than that occupied by the detonating gas at the ordinary temperature4 ,detonating gas emits a sound1 not only as a consequence of the commotion which occurs from the rapid expansion of the heated vapour1 but also because it is immediately followed by a cooling effect1 the conversion of the vapour into water1 and a rapid contraction4#da #cg ,illustration3 ,,fig4 #cc4--;,favre and ,silbermann's calorimeter for determining the heat evolved in combustion4 ,the amount of heat evolved in the combustion of a known weight 7for instance1 #a gram7 of a given substance is determined by the rise in temperature of water1 to which the whole of the heat evolved in the combustion is transmitted4 ,a calorimeter1 for example that shown in fig4 #cc1 is employed for this purpose4 ,it consists of a thin 7in order that it may absorb less heat71 polished 7that it should transmit a minimum of heat7 metallic vessel1 surrounded by down 7c71 or some other bad conductor of heat1 and an outer metallic vessel4 ,this is necessary in order that the least possible amount of heat should be lost from the vessels2 nevertheless1 there is always a certain loss1 whose magnitude is determined by preliminary experiment 7by taking warm water1 and determining its fall in temperature after a definite period of time7 as a correction for the results of observations4 ,the water to which the heat of the burning substance is transmitted is poured into the vessel4 ,the stirrer g allows of all the layers of water being brought to the same temperature1 and the thermometer serves for the determination of the temperature of the water4 ,the heat evolved passes1 naturally1 not to the water only1 but to all the parts of the apparatus4 ,the quantity of water corresponding to the whole amount of those objects 7the vessels1 tubes1 c47 to which the heat is transmitted is previously determined1 and in this manner another most important correction is made in the calorimetric determinations4 ,the combustion itself is carried on in the vessel a4 ,the ignited substance is introduced through the tube at the top1 which closes tightly4 ,in fig4 #cc the apparatus is arranged for the combustion of a gas1 introduced by a tube4 ,the oxygen required for the combustion is led into a by the tube e1 and the products of combustion either remain in the vessel a 7if liquid or solid71 or escape by the tube f into an apparatus in which their quantity and properties can easily be determined4 ,thus the heat evolved in combustion passes to the walls of the vessel a1 and to the gases which are formed in it1 and these transmit it to the water of the calorimeter4 #ch ,this quantity of heat corresponds with the formation of liquid water at the ordinary temperature from detonating gas at the same temperature4 ,if the water be as vapour the heat evolved #eh major calories2 if as ice #gjd major calories4 ,a portion of this heat is due to the fact that #b vols4 of hydrogen and #a vol4 of oxygen give #b vols4 of aqueous vapour--that is to say1 contraction ensues--and this evolves heat4 ,this quantity of heat may be calculated1 but it cannot be said how much is expended in the separation of the atoms of oxygen from each other1 and1 therefore1 strictly speaking1 we do not know the quantity of heat which is evolved in the reaction alone1 although the number of units of heat evolved in the combustion of detonating gas is accurately known4 ,the construction of the calorimeter and even the method of determination vary considerably in different cases4 ,since the beginning of the nineties1 a large number of determinations of the heat of combustion have been conducted in closed bombs containing compressed oxygen4 ,the greatest number of calorimetric determinations were made by ,berthelot and ,thomsen4 ,they are given in their works ,essai de mcanique chimique fonde sur la thermochimie1 by ,m4 ,berthelot1 #ahgi 7#b vols471 and thermochemische ,untersuchungen1 by ,j4 ,thomsen1 #ahhf 7#d vols47 ,the most important methods of recent thermochemistry1 and all the trustworthy results of experiment1 are given in ,prof4 ,p4 ,f4 ,louginin's ,description of the ,different ,modes of ,determining the ,heat of ,combustion of ,organic ,compounds1 ,moscow1 #ahid4 ,the student must refer to works on theoretical and physical chemistry for a description of the elements and methods of thermochemistry1 into the details of which it is impossible to enter in this work4 ,one of the originators of thermochemistry1 ,hess1 was a member of the ,st4 ,petersburg ,academy of ,sciences4 ,since #ahgj a large amount of research has been carried out in this province of chemistry1 especially in ,france and ,germany1 after the investigations of the ,french ,academician1 ,berthelot1 and ,professor ,thomsen1 of ,copenhagen4 ,among ,russians1 ,beketoff1 ,louginin1 ,cheltzoff1 ,chroustchoff1 and others are known by their thermochemical researches4 ,the present epoch of thermochemistry must be considered rather as a collective one1 wherein the material of facts is amassed1 and the first consequences arising from them are noticed4 ,in my opinion two essential circumstances prevent the possibility of deducing any exact consequences1 of importance to chemical mechanics1 from the immense store of thermochemical data already collected3 7#a7 ,the majority of the determinations are conducted in weak aqueous solutions1 and1 the heat of solution being known1 are referred to the substances in solution2 yet there is much 7,chapter ,i47 which leads to the conclusion that in solution water does not play the simple part of a diluting medium1 but of itself acts independently in a chemical sense on the substance dissolved4 7#b7 ,physical and mechanical changes 7decrease of volume1 diffusion1 and others7 invariably proceed side by side with chemical changes1 and for the present it is impossible1 in a number of cases1 to distinguish the thermal effect of the one and the other kind of change4 ,it is evident that the one kind of change 7chemical7 is essentially inseparable and incomprehensible without the other 7mechanical and physical72 and therefore it seems to me that thermochemical data will only acquire their true meaning when the connection between the phenomena of both kinds 7on the one hand chemical and atomic1 and on the other hand mechanical and molecular or between entire masses7 is explained more clearly and fully than is at present the case4 ,as there is no doubt that the simple mechanical contact1 or the action of heat alone1 on substances sometimes causes an evident and always a latent 7incipient7 chemical change--that is1 a different distribution or motion of the atoms in the molecules--it follows that purely chemical phenomena are inseparable from physical and mechanical phenomena4 ,a mechanical change may be imagined without a physical change1 and a physical without a chemical change1 but it is impossible to imagine a chemical change without a physical and mechanical one1 for without the latter we should not be able to recognise the former1 and it is by their means that we are enabled to do so4 #ci ,the flame1 or locality where the combustion of gases and vapours takes place1 is a complex phenomenon1 'an entire factory1' as ,faraday says1 and therefore we will consider flame in some detail in one of the following notes4 #dj ,if #cd1ejj units of heat are evolved in the combustion of #a part of hydrogen1 and this heat is transmitted to the resulting #i parts by weight of aqueous vapour1 then we find that1 taking the specific heat of the latter as #jdge1 each unit of heat raises the temperature of #a part by weight of aqueous vapour #ba and #i parts by weight 7#ba #i7 ,o#bc2 hence the #cd1ejj units of heat raise its temperature #g1ice4 ,if detonating gas is converted into water in a closed space1 then the aqueous vapour formed cannot expand1 and therefore1 in calculating the temperature of combustion1 the specific heat at a constant volume must be taken into consideration2 for aqueous vapour it is #jcf4 ,this figure gives a still higher temperature for the flame4 ,in reality it is much lower1 but the results given by different observers are very contradictory 7from #a1gjj to #b1djj71 the discrepancies depending on the fact that flames of different sizes are cooled by radiation to a different degree1 but mainly on the fact that the methods and apparatus 7pyrometers7 for the determination of high temperatures1 although they enable relative changes of temperature to be judged1 are of little use for determining their absolute magnitude4 ,by taking the temperature of the flame of detonating gas as #b1jjj1 ,i give1 ,i think1 the average of the most trustworthy determinations and calculations based upon the determination of the variation of the specific heat of aqueous vapour and other gases 7see ,chapter ,,xli47 #da ,it is evident that not only hydrogen1 but every other combustible gas1 will give an explosive mixture with oxygen4 ,for this reason coal-gas mixed with air explodes when the mixture is ignited4 ,the pressure obtained in the explosions serves as the motive power of gas engines4 ,in this case advantage is taken1 not only of the pressure produced by the explosion1 but also of that contraction which takes place after the explosion4 ,on this is based the construction of several motors1 of which ,lenoir's was formerly1 and ,otto's is now1 the best known4 ,the explosion is usually produced by coal-gas and air1 but of late the vapours of combustible liquids 7kerosene1 benzene7 are also being employed in place of gas 7,chapter ,,ix47 ,in ,lenoir's engine a mixture of coal-gas and air is ignited by means of sparks from a ,ruhmkorff's coil1 but in the most recent machines the gases are ignited by the direct action of a gas jet1 or by contact with the hot walls of a side tube4 ,illustration3 ,,fig4 #cd4--,safety burner for detonating gas1 described in text4 ,mixtures of hydrogen and of various other gases with oxygen are taken advantage of for obtaining high temperatures4 ,by the aid of such high temperatures metals like platinum may be melted on a large scale1 which cannot be performed in furnaces heated with charcoal and fed by a current of air4 ,the burner1 shown in fig4 #cd1 is constructed for the application of detonating gas to the purpose4 ,it consists of two brass tubes1 one fixed inside the other1 as shown in the drawing4 ,the internal central tube ,c ,c conducts oxygen1 and the outside1 enveloping1 tube ,e' ,e' conducts hydrogen4 ,previous to their egress the gases do not mix together1 so that there can be no explosion inside the apparatus4 ,when this burner is in use ,c is connected with a gas-holder containing oxygen1 and ,e with a gas-holder containing hydrogen 7or sometimes coal-gas74 ,the flow of the gases can be easily regulated by the stopcocks ,o ,h4 ,the flame is shortest and evolves the greatest heat when the gases burning are in the proportion of #a volume of oxygen to #b volumes of hydrogen4 ,the degree of heat may be easily judged from the fact that a thin platinum wire placed in the flame of a properly proportioned mixture easily melts4 ,by placing the burner in the orifice of a hollow piece of lime1 a crucible ,a ,b is obtained in which the platinum may be easily melted1 even in large quantities if the current of oxygen and hydrogen be sufficiently great 7,deville74 ,the flame of detonating gas may also be used for illuminating purposes4 ,it is by itself very pale1 but owing to its high temperature it may serve for rendering infusible objects incandescent1 and at the very high temperature produced by the detonating gas the incandescent substance gives a most intense light4 ,for this purpose lime1 magnesia1 or oxide of zirconium are used1 as they are not fusible at the very high temperature evolved by the detonating gas4 ,a small cylinder of lime placed in the flame of detonating gas1 if regulated to the required point1 gives a very brilliant white light1 which was at one time proposed for illuminating lighthouses4 ,at present in the majority of cases the electric light1 owing to its constancy and other advantages1 has replaced it for this purpose4 ,the light produced by the incandescence of lime in detonating gas is called the ,drummond light or limelight4 ,the above cases form examples of the combustion of elements in oxygen1 but exactly similar phenomena are observed in the combustion of compounds4 ,so1 for instance1 the solid1 colourless1 shiny substance1 naphthalene1 ,c#aj;,h#h1 burns in the air with a smoky flame1 whilst in oxygen it continues to burn with a very brilliant flame4 ,alcohol1 oil1 and other substances burn brilliantly in oxygen on conducting the oxygen by a tube to the flame of lamps burning these substances4 ,a high temperature is thus evolved1 which is sometimes taken advantage of in chemical practice4 ,in order to understand why combustion in oxygen proceeds more rapidly1 and is accompanied by a more intense heat effect1 than combustion in air1 it must be recollected that air is oxygen diluted with nitrogen1 which does not support combustion1 and therefore fewer particles of oxygen flow to the surface of a substance burning in air than when burning in pure oxygen1 besides which the reason of the intensity of combustion in oxygen is the high temperature acquired by the substance burning in it4#da bis #da bis ,let us consider as an example the combustion of sulphur in air and in oxygen4 ,if #a gram of sulphur burns in air or oxygen it evolves in either case #bbej units of heat--i4e4 evolves sufficient heat for heating #b1bej grams of water #a ,c4 ,this heat is first of all transmitted to the sulphurous anhydride1 ,,so#b1 formed by the combination of sulphur with oxygen4 ,in its combustion #a gram of sulphur forms #b grams of sulphurous anhydride--i4e4 the sulphur combines with #a gram of oxygen4 ,in order that #a gram of sulphur should have access to #a gram of oxygen in air1 it is necessary that #cd grams of nitrogen should simultaneously reach the sulphur1 because air contains seventy-seven parts of nitrogen 7by weight7 per twenty-three parts of oxygen4 ,thus in the combustion of #a gram of sulphur1 the #b1bej units of heat are transmitted to #b grams of sulphurous oxide and to at least #cd grams of nitrogen4 ,as #jaee unit of heat is required to raise #a gram of sulphurous anhydride #a ,c41 therefore #b grams require #jca unit4 ,so also #cd grams of nitrogen require #cd #jbdd or #jhc unit of heat1 and therefore in order to raise both gases #a ,c4 #jca #jhc or #aad unit of heat is required2 but as the combustion of the sulphur evolves #b1bej units of heat1 therefore the gases might be heated 7if their specific heats remained constant7 to #bbej_/aad or #a1igd ,c4 ,that is1 the maximum possible temperature of the flame of the sulphur burning in air will be #a1igd ,c4 ,in the combustion of the sulphur in oxygen the heat evolved 7#b1bej units7 can only pass to the #b grams of sulphurous anhydride1 and therefore the highest possible temperature of the flame of the sulphur in oxygen will be #bbej_/jca or #gbeh4 ,in the same manner it may be calculated that the temperature of charcoal burning in air cannot exceed #b1gjj1 while in oxygen it may attain #aj1ajj ,c4 ,for this reason the temperature in oxygen will always be higher than in air1 although 7judging from what has been said respecting detonating gas7 neither one temperature nor the other will ever approximate to the theoretical amount4 ,illustration3 ,,fig4 #ce4--;,faraday's experiment for investigating the different parts of a candle flame4 ,among the phenomena accompanying the combustion of certain substances1 the phenomenon of flame attracts attention4 ,sulphur1 phosphorus1 sodium1 magnesium1 naphthalene1 c41 burn like hydrogen with a flame1 whilst in the combustion of other substances no flame is observed1 as1 for instance1 in the combustion of iron and of charcoal4 ,the appearance of flame depends on the capacity of the combustible substance to yield gases or vapours at the temperature of combustion4 ,at the temperature of combustion1 sulphur1 phosphorus1 sodium1 and naphthalene pass into vapour1 whilst wood1 alcohol1 oil1 c41 are decomposed into gaseous and vaporous substances4 ,the combustion of gases and vapours forms flames1 and therefore a flame is composed of the hot and incandescent gases and vapours produced by combustion4 ,it may easily be proved that the flames of such non-volatile substances as wood contain volatile and combustible substances formed from them1 by placing a tube in the flame connected with an aspirator4 ,besides the products of combustion1 combustible gases and liquids1 previously in the flame as vapours1 collect in the aspirator4 ,for this experiment to succeed--i4e4 in order to really extract combustible gases and vapours from the flame it is necessary that the suction tube should be placed inside the flame4 ,the combustible gases and vapours can only remain unburnt inside the flame1 for at the surface of the flame they come into contact with the oxygen of the air and burn4#db ,flames are of different degrees of brilliancy1 according to whether solid incandescent particles occur in the combustible gas or vapour1 or not4 ,incandescent gases and vapours emit but little light by themselves1 and therefore give a paler flame4#dc ,if a flame does not contain solid particles it is transparent1 pale1 and emits but little light4#dd ,the flames of burning alcohol1 sulphur1 and hydrogen are of this kind4 ,a pale flame may be rendered luminous by placing fine particles of solid matter in it4 ,thus1 if a very fine platinum wire be placed in the pale flame of burning alcohol--or1 better still1 of hydrogen--the flame emits a bright light4 ,this is still better seen by sifting the powder of an incombustible substance1 such as fine sand1 into the flame1 or by placing a bunch of asbestos threads in it4 ,every brilliant flame always contains some kind of solid particles1 or at least some very dense vapour4 ,the flame of sodium burning in oxygen has a brilliant yellow colour1 from the presence of particles of solid sodium oxide4 ,the flame of magnesium is brilliant from the fact that in burning it forms solid magnesia1 which becomes white hot1 and similarly the brilliancy of the ,drummond light is due to the heat of the flame raising the solid non-volatile lime to a state of incandescence4 ,the flames of a candle1 wood1 and similar substances are brilliant1 because they contain particles of charcoal or soot4 ,it is not the flame itself which is luminous1 but the incandescent soot it contains4 ,these particles of charcoal which occur in flames may be easily observed by introducing a cold object1 like a knife1 into the flame4#de ,the particles of charcoal burn at the outer surface of the flame if the supply of air be sufficient1 but if the supply of air--that is1 of oxygen--be insufficient for their combustion the flame smokes1 because the unconsumed particles of charcoal are carried off by the current of air4#df #db ,faraday proved this by a very convincing experiment on a candle flame4 ,if one arm of a bent glass tube be placed in a candle flame above the wick in the dark portion of the flame1 then the products of the partial combustion of the stearin will pass up the tube1 condense in the other arm1 and collect in a flask placed under it 7fig4 #ce7 as heavy white fumes which burn when lighted4 ,if the tube be raised into the upper luminous portion of the flame1 then a dense black smoke which will not inflame accumulates in the flask4 ,lastly1 if the tube be let down until it touches the wick1 then little but stearic acid condenses in the flask4 #dc ,all transparent substances which transmit light with great ease 7that is1 which absorb but little light7 are but little luminous when heated2 so also substances which absorb but few heat rays1 when heated transmit few rays of heat4 #dd ,there is1 however1 no doubt but that very heavy dense vapours or gases under pressure 7according to the experiments of ,frankland7 are luminous when heated1 because1 as they become denser they approach a liquid or solid state4 ,thus detonating gas when exploded under pressure gives a brilliant light4 #de ,if hydrogen gas be passed through a volatile liquid hydrocarbon--for instance1 through benzene 7the benzene may be poured directly into the vessel in which hydrogen is generated7--then its vapour burns with the hydrogen and gives a very bright flame1 because the resultant particles of carbon 7soot7 become incandescent4 ,benzene1 or platinum gauze1 introduced into a hydrogen flame may be employed for illuminating purposes4 #df ,in flames the separate parts may be distinguished with more or less distinctness4 ,that portion of the flame whither the combustible vapours or gases flow1 is not luminous because its temperature is still too low for the process of combustion to take place in it4 ,this is the space which in a candle surrounds the wick1 or in a gas jet is immediately above the orifice from which the gas escapes4 ,in a candle the combustible vapours and gases which are formed by the action of heat on the melted tallow or stearin rise in the wick1 and are heated by the high temperature of the flame4 ,by the action of the heat1 the solid or liquid substance is here1 as in other cases1 decomposed1 forming products of dry distillation4 ,these products occur in the central portion of the flame of a candle4 ,the air travels to it from the outside1 and is not able to intermix at once with the vapours and gases in all parts of the flame equally2 consequently1 in the outer portion of the flame the amount of oxygen will be greater than in the interior portions4 ,but1 owing to diffusion1 the oxygen1 of course mixed with nitrogen1 flowing towards the combustible substance1 does finally penetrate to the interior of the flame 7when the combustion takes place in ordinary air74 ,the combustible vapours and gases combine with this oxygen1 evolve a considerable amount of heat1 and bring about that state of incandescence which is so necessary both for keeping up the combustion and also for the uses to which the flame is applied4 ,passing from the colder envelope of air through the interior of the flame1 to the source of the combustible vapours 7for instance1 the wick71 we evidently first traverse layers of higher and higher temperature1 and then portions which are less and less hot1 in which the combustion is less complete1 owing to the limited supply of oxygen4 ,illustrationtion3 ,,fig4 #cf4--;,in the candle flame the portion ,c contains the vapours and products of decomposition2 in the bright zone ,a the combustion has commenced1 and particles of carbon are emitted2 and in the pale zone ,b the combustion is completed4 ,thus unburnt products of the decomposition of organic substances occur in the interior of the flame4 ,but there is always free hydrogen in the interior of the flame1 even when oxygen is introduced there1 or when a mixture of hydrogen and oxygen burns1 because the temperature evolved in the combustion of hydrogen or the carbon of organic matter is so high that the products of combustion are themselves partially decomposed--that is1 dissociated--at this temperature4 ,hence1 in a flame a portion of the hydrogen and of the oxygen which might combine with the combustible substances must always be present in a free state4 ,if a hydrocarbon burns1 and we imagine that a portion of the hydrogen is in a free state1 then a portion of the carbon must also occur in the same form in the flame1 because1 other conditions being unchanged1 carbon burns after hydrogen1 and this is actually observed in the combustion of various hydrocarbons4 ,charcoal1 or the soot of a common flame1 arises from the dissociation of organic substances contained in the flame4 ,the majority of hydrocarbons1 especially those containing much carbon--for instance1 naphthalene--burn1 even in oxygen1 with separation of soot4 ,in that portion of the flame where the hydrogen burns the carbon remains unburnt1 or at least partly so4 ,it is this free carbon which causes the brilliancy of the flame4 ,that the interior of the flame contains a mixture which is still capable of combustion may be proved by the following experiment3 ,a portion of the gases may be withdrawn by an aspirator from the central portion of the flame of carbonic oxide1 which is combustible in air4 ,for this purpose ,deville passed water through a metallic tube having a fine lateral orifice1 which is placed in the flame4 ,as the water flows along the tube portions of the gases of the flame enter1 and1 passing along the tube alternately with cylinders of water1 are carried away into an apparatus where they can be investigated4 ,it appears that all portions of the flame obtained by the combustion of a mixture of carbonic oxide and oxygen contain a portion of this mixture still unburnt4 ,the researches of ,deville and ,bunsen showed that in the explosion of a mixture of hydrogen and of carbonic oxide with oxygen in a closed space1 complete combustion does not ever take place immediately4 ,if two volumes of hydrogen and one volume of oxygen be confined in a closed space1 then on explosion the pressure does not attain that magnitude which it would were there immediate and complete combustion4 ,it may be calculated that the pressure should attain twenty-six atmospheres4 ,in reality1 it does not exceed nine and a half atmospheres4 ,hence the admixture of the products of combustion with an explosive mixture prevents the combustion of the remaining mass1 although capable of burning4 ,the admixture of carbonic anhydride prevents carbonic oxide from burning4 ,the presence of any other foreign gas interferes in the same manner4 ,this shows that every portion of a flame must contain combustible1 burning1 and already burnt substances--i4e4 oxygen1 carbon1 carbonic oxide1 hydrogen1 hydrocarbons1 carbonic anhydride1 and water4 ,consequently1 it is impossible to attain instantaneous complete combustion1 and this is one of the reasons of the phenomenon of flame4 ,a certain space is required1 and the temperature must be unequal in different parts of it4 ,in this space different quantities of the component parts are successively subjected to combustion1 or are cooled under the influence of adjacent objects1 and combustion only ends where the flame ends4 ,if the combustion could be concentrated at one spot1 then the temperature would be incomparably higher than it is under the actual circumstances4 ,the various regions of the flame have formed the frequent subject of experimental research1 and the experiments conducted by ,smithells and ,ingle 7#ahib7 are particularly instructive2 they show that the reducing 7interior7 and oxidising 7exterior7 portions of the flame of a burning gas may be divided by taking a ,bunsen burner and surrounding the flame of the gas burnt in it1 by another wider tube 7without the access of air to the annular space or allowing only a small current of air to pass71 when a gaseous mixture1 containing oxide of carbon and capable of further combustion1 will issue from this enveloping tube1 so that a second flame1 corresponding to the exterior 7oxidising7 portion of an ordinary flame1 may be obtained above the enveloping tube4 ,this division of the flame into two portions is particularly clear when cyanogen ,c#b,n#b is burnt1 because the interior portion 7where ,,co is chiefly formed according to the equation ,c#b,n#b ,o#b #b;,,co ,n#b1 but a portion of the nitrogen is oxidised7 is of a rose colour1 while the exterior portion 7where the ,,co burns into ,,co#b at the expense of a fresh quantity of oxygen and of the oxides of nitrogen proceeding from the interior portions7 is of a bluish-grey colour4 ,the combination of various substances with oxygen may not present any signs of combustion--that is1 the temperature may rise but inconsiderably4 ,this may either proceed from the fact that the reaction of the substance 7for example1 tin1 mercury1 lead at a high temperature1 or a mixture of pyrogallol with caustic potash at the ordinary temperature7 evolves but little heat1 or that the heat evolved is transmitted to good conductors of heat1 like metals1 or that the combination with oxygen takes place so slowly that the heat evolved succeeds in passing to the surrounding objects4 ,combustion is only a particular1 intense1 and evident case of combination with oxygen4 ,respiration is also an act of combination with oxygen2 it also serves1 like combustion1 for the development of heat by those chemical processes which accompany it 7the transformation of oxygen into carbonic anhydride74 ,lavoisier enunciated this in the lucid expression1 'respiration is slow combustion4' ,reactions involving slow combination of substances with oxygen are termed oxidations4 ,combination of this kind 7and also combustion7 often results in the formation of acid substances1 and hence the name oxygen 7,sauerstoff74 ,combustion is only rapid oxidation4 ,phosphorus1 iron1 and wine may be taken as examples of substances which slowly oxidise in air at the ordinary temperature4 ,if such a substance be left in contact with a definite volume of air or oxygen1 it absorbs the oxygen little by little1 as may be seen by the decrease in volume of the gas4 ,this slow oxidation is not often accompanied by a sensible evolution of heat2 an evolution of heat really does occur1 only it is not apparent to our senses owing to the small rise in temperature which takes place2 this is owing to the slow rate of the reaction and to the transmission of the heat formed as radiant heat1 c4 ,thus1 in the oxidation of wine and its transformation into vinegar by the usual method of preparation of the latter1 the heat evolved cannot be observed because it extends over several weeks1 but in the so-called rapid process of the manufacture of vinegar1 when a large quantity of wine is comparatively rapidly oxidised1 the evolution of heat is quite apparent4 ,such slow processes of oxidation are always taking place in nature by the action of the atmosphere4 ,dead organisms and the substances obtained from them--such as bodies of animals1 wood1 wool1 grass1 c4--are especially subject to this action4 ,they rot and decompose--that is1 their solid matter is transformed into gases1 under the influence of moisture and atmospheric oxygen1 and generally under the influence of other organisms1 such as moulds1 worms1 micro-organisms 7bacteria71 and the like4 ,these are processes of slow combustion1 of slow combination with oxygen4 ,it is well known that manure rots and develops heat1 that stacks of damp hay1 damp flour1 straw1 c41 become heated and are changed in the process4#dg ,in all these transformations the same chief products of combustion are formed as those which are contained in smoke2 the carbon gives carbonic anhydride1 and the hydrogen water4 ,hence these processes require oxygen just like combustion4 ,this is the reason why the entire prevention of access of air hinders these transformations1#dh and an increased supply of air accelerates them4 ,the mechanical treatment of arable lands by the plough1 harrow1 and other similar means has not only the object of facilitating the spread of roots in the ground1 and of making the soil more permeable to water1 but it also serves to facilitate the access of the air to the component parts of the soil2 as a consequence of which the organic remains of soil rot--so to speak1 breathe air and evolve carbonic anhydride4 ,one acre of good garden land in the course of a summer evolves more than sixteen tons of carbonic anhydride4 #dg ,cotton waste 7used in factories for cleaning machines from lubricating oil7 soaked in oil and lying in heaps is self-combustible1 being oxidised by the air4 #dh ,when it is desired to preserve a supply of vegetable and animal food1 the access of the oxygen of the atmosphere 7and also of the germs of organisms present in the air7 is often prevented4 ,with this object articles of food are often kept in hermetically closed vessels1 from which the air has been withdrawn2 vegetables are dried and soldered up while hot in tin boxes2 sardines are immersed in oil1 c4 ,the removal of water from substances is also sometimes resorted to with the same object 7the drying of hay1 corn1 fruits71 as also is saturation with substances which absorb oxygen 7such as sulphurous anhydride71 or which hinder the growth of organisms forming the first cause of putrefaction1 as in processes of smoking1 embalming1 and in the keeping of fishes and other animal specimens in spirit1 c4 ,it is not only vegetable and animal substances which are subject to slow oxidation in the presence of water4 ,some metals even rust under these conditions4 ,copper very easily absorbs oxygen in the presence of acids4 ,many metallic sulphides 7for example1 pyrites7 are very easily oxidised with access of air and moisture4 ,thus processes of slow oxidation proceed throughout nature4 ,however1 there are many elements which do not under any circumstances combine directly with gaseous oxygen2 nevertheless their compounds with oxygen may be obtained4 ,platinum1 gold1 iridium1 chlorine1 and iodine are examples of such elements4 ,in this case recourse is had to a so-called indirect method--i4e4 the given substance is combined with another element1 and by a method of double decomposition this element is replaced by oxygen4 ,substances which do not directly combine with oxygen1 but form compounds with it by an indirect method1 often readily lose the oxygen which they had absorbed by double decomposition or at the moment of its evolution4 ,such1 for example1 are the compounds of oxygen with chlorine1 nitrogen1 and platinum1 which evolve oxygen on heating--that is1 they may be used as oxidising agents4 ,in this respect oxidising agents1 or those compounds of oxygen which are employed in chemical and technical practice for transferring oxygen to other substances1 are especially remarkable4 ,the most important among these is nitric acid or aqua fortis--a substance rich in oxygen1 and capable of evolving it when heated1 which easily oxidises a great number of substances4 ,thus nearly all metals and organic substances containing carbon and hydrogen are more or less oxidised when heated with nitric acid4 ,if strong nitric acid be taken1 and a piece of burning charcoal be immersed in the acid1 it continues to burn4 ,chromic acid acts like nitric acid2 alcohol burns when mixed with it4 ,although the action is not so marked1 even water may oxidise with its oxygen4 ,sodium is not oxidised in perfectly dry oxygen at the ordinary temperature1 but it burns very easily in water and aqueous vapour4 ,charcoal can burn in carbonic anhydride--a product of combustion--forming carbonic oxide4 ,magnesium burns in the same gas1 separating carbon from it4 ,speaking generally1 combined oxygen can pass from one compound to another4 ,the products of combustion or oxidation--and in general the definite compounds of oxygen--are termed oxides4 ,some oxides are not capable of combining with other oxides--or combine with only a few1 and then with the evolution of very little heat2 others1 on the contrary1 enter into combination with very many other oxides1 and in general have remarkable chemical energy4 ,the oxides incapable of combining with others1 or only showing this quality in a small degree1 are termed indifferent oxides4 ,such are the peroxides1 of which mention has before been made4 ,the class of oxides capable of entering into mutual combination we will term saline oxides4 ,they fall into two chief groups--at least1 as regards the most extreme members4 ,the members of one group combine with the members of the other group with particular ease4 ,as representative of one group may be taken the oxides of the metals1 magnesium1 sodium1 calcium1 c4 ,representatives of the other group are the oxides formed by the non-metals1 sulphur1 phosphorus1 carbon4 ,thus1 if we take the oxide of calcium1 or lime1 and bring it into contact with oxides of the second group1 combination very readily ensues4 ,for instance1 if we mix calcium oxide with oxide of phosphorus they combine with great facility and with the evolution of much heat4 ,if we pass the vapour of sulphuric anhydride1 obtained by the combination of sulphurous oxide with oxygen1 over pieces of lime heated to redness1 the sulphuric anhydride is absorbed by the lime with the formation of a substance called calcium sulphate4 ,the oxides of the first kind1 which contain metals1 are termed basic oxides or bases4 ,lime is a familiar example of this class4 ,the oxides of the second group1 which are capable of combining with the bases1 are termed anhydrides of the acids or acid oxides4 ,sulphuric anhydride1 ,,so#c1 may be taken as a type of the latter group4 ,it is a compound of sulphur with oxygen formed not directly but by the addition of a fresh quantity of oxygen to sulphurous anhydride1 ,,so#b1 by passing it together with oxygen over incandescent spongy platinum4 ,carbonic anhydride 7often termed 'carbonic acid'71 ,,co#b1 phosphoric anhydride1 sulphurous anhydride1 are all acid oxides1 for they can combine with such oxides as lime or calcium oxide1 magnesia or magnesium oxide1 ,mg,o1 soda or sodium oxide1 ,na#b,o1 c4 ,if a given element form but one basic oxide1 it is termed the oxide2 for example1 calcium oxide1 magnesium oxide1 potassium oxide4 ,some indifferent oxides are also called 'oxides' if they have not the properties of peroxides1 and at the same time do not show the properties of acid anhydrides--for example1 carbonic oxide1 of which mention has already been made4 ,if an element forms two basic oxides 7or two indifferent oxides not having the characteristics of a peroxide7 then that of the lower degree of oxidation is called a suboxide--that is1 suboxides contain less oxygen than oxides4 ,thus1 when copper is heated to redness in a furnace it increases in weight and absorbs oxygen1 until for #fc parts of copper there is absorbed not more than #h parts of oxygen by weight1 forming a red mass1 which is suboxide of copper2 but if the roasting be prolonged1 and the draught of air increased1 #fc parts of copper absorb #af parts of oxygen1 and form black oxide of copper4 ,sometimes to distinguish between the degrees of oxidation a change of suffix is made in the oxidised element1 -ic oxide denoting the higher degree of oxidation1 and -ous oxide the lower degree4 ,thus ferrous oxide and ferric oxide are the same as suboxide of iron and oxide of iron4 ,if an element forms one anhydride only1 then it is named by an adjective formed from the name of the element made to end in -ic and the word anhydride4 ,when an element forms two anhydrides1 then the suffixes -ous and -ic are used to distinguish them3 -ous signifying less oxygen than -ic2 for example1 sulphurous and sulphuric anhydrides4#di ,when several oxides are formed from the same element1 the prefixes mon1 di1 tri1 tetra are used1 thus3 chlorine monoxide1 chlorine dioxide1 chlorine trioxide1 and chlorine tetroxide or chloric anhydride4 #di ,it must be remarked that certain elements form oxides of all three kinds--i4e4 indifferent1 basic1 and acid2 for example1 manganese forms manganous oxide1 manganic oxide1 peroxide of manganese1 red oxide of manganese1 and manganic anhydride1 although some of them are not known in a free state but only in combination4 ,the basic oxides contain less oxygen than the peroxides1 and the peroxides less than the acid anhydrides4 ,thus they must be placed in the following general normal order with respect to the amount of oxygen entering into their composition--7#a7 basic oxides1 suboxides1 and oxides2 7#b7 peroxides2 7#c7 acid anhydrides4 ,the majority of elements1 however1 do not give all three kinds of oxides1 some giving only one degree of oxidation4 ,it must further be remarked that there are oxides formed by the combination of acid anhydrides with basic oxides1 or1 in general1 of oxides with oxides4 ,for every oxide having a higher and a lower degree of oxidation1 it might be said that the intermediate oxide was formed by the combination of the higher with the lower oxide4 ,but this is not true in all cases--for instance1 when the oxide under consideration forms a whole series of independent compounds--for oxides which are really formed by the combination of two other oxides do not give such independent compounds1 but in many cases decompose into the higher and lower oxides4 ,the oxides themselves rarely undergo chemical transformations1 and in the few cases where they are subject to such changes a particularly important part is played by their combinations with water4 ,the majority of1 if not all1 basic and acid oxides combine with water1 either by a direct or an indirect method forming hydrates--that is1 compounds which split up into water and an oxide of the same kind only4 ,it is well known that many substances are capable of combining with water4 ,oxides possess this property in the highest degree4 ,we have already seen examples of this 7,chapter ,i47 in the combination of lime1 and of sulphuric and phosphoric anhydrides1 with water4 ,the resulting combinations are basic and acid hydrates4 ,acid hydrates are called acids because they have an acid taste when dissolved in water 7or saliva71 for then only can they act on the palate4 ,vinegar1 for example1 has an acid taste because it contains acetic acid dissolved in water4 ,sulphuric acid1 to which we have frequently referred1 because it is the acid of the greatest importance both in practical chemistry and for its technical applications1 is really a hydrate formed by the combination of sulphuric anhydride with water4 ,besides their acid taste1 dissolved acids or acid hydrates have the property of changing the blue colour of certain vegetable dyes to red4 ,of these dyes litmus is particularly remarkable and much used4 ,it is the blue substance extracted from certain lichens1 and is used for dyeing tissues blue2 it gives a blue infusion with water4 ,this infusion1 on the addition of an acid1 changes from blue to red4#ej #ej ,blotting or unsized paper1 soaked in a solution of litmus1 is usually employed for detecting the presence of acids4 ,this paper is cut into strips1 and is called test paper2 when dipped into acid it immediately turns red4 ,this is a most sensitive reaction1 and may be employed for testing for the smallest traces of acids4 ,if #aj1jjj parts by weight of water be mixed with #a part of sulphuric acid1 the coloration is distinct1 and it is even perceptible on the addition of ten times more water4 ,certain precautions must1 however1 be taken in the preparation of such very sensitive litmus paper4 ,litmus is sold in lumps4 ,take1 say1 #ajj grams of it2 powder it1 and add it to cold pure water in a flask2 shake and decant the water4 ,repeat this three times4 ,this is done to wash away easily-soluble impurities1 especially alkalis4 ,transfer the washed litmus 7it is washed with absolute alcohol to remove the non-sensitive reddish colouring matter7 to a flask1 and pour in #fjj c4c4 of water1 heat1 and allow the hot infusion to remain for some hours in a warm place4 ,then filter1 and divide the filtrate into two parts4 ,add a few drops of nitric acid to one portion1 so that a faint red tinge is obtained1 and then mix the two portions4 ,add spirit to the mixture1 and keep it in a stoppered bottle 7it soon spoils if left open to the air74 ,this infusion may be employed directly2 it reddens in the presence of acids1 and turns blue in the presence of alkalis4 ,if evaporated1 a solid mass is obtained which is soluble in water1 and may be kept unchanged for any length of time4 ,the test paper may be prepared as follows3--,take a strong infusion of litmus1 and soak blotting-paper with it2 dry it1 and cut it into strips1 and use it as test-paper for acids4 ,for the detection of alkalis1 the paper must be soaked in a solution of litmus just reddened by a few drops of acid2 if too much acid be taken1 the paper will not be sensitive4 ,such acids as sulphuric acid colour litmus1 and especially its infusion1 a brick-red colour1 whilst more feeble acids1 such as carbonic1 give a faint red-wine tinge4 ,test-paper of a yellow colour is also employed2 it is dyed by an infusion of turmeric roots in spirit4 ,in alkalis it turns brown1 but regains its original hue in acids4 ,many blue and other vegetable colouring matters may be used for the detection of acids and alkalis2 for example1 infusions of cochineal1 violets1 log-wood1 c4 ,certain artificially prepared substances and dyes may also be employed4 ,thus rosolic acid1 ,c#bj;,h#af,o#c and phenolphthalen1 ,c#bj;,h#ad,o#d 7it is used in an alcoholic solution1 and is not suitable for the detection of ammonia71 are colourless in an acid1 and red in an alkaline1 solution4 ,cyanine is also colourless in the presence of acids1 and gives a blue coloration with alkalis4 ,methyl-orange 7yellow in an aqueous solution7 is not altered by alkalis but becomes pink with acids 7weak acids have no action71 c4 ,these are very sensitive tests4 ,their behaviour in respect to various acids1 alkalis1 and salts sometimes give the means of distinguishing substances from each other4 ,basic oxides1 in combining with water1 form hydrates1 of which1 however1 very few are soluble in water4 ,those which are soluble in water have an alkaline taste like that of soap or of water in which wood ashes have been boiled1 and are called alkalis4 ,further1 alkalis have the property of restoring the blue colour to litmus which has been reddened by the action of acids4 ,the hydrates of the oxides of sodium and potassium1 ,na,h,o and ,,kho1 are examples of basic hydrates easily soluble in water4 ,they are true alkalis1 and are termed caustic1 because they act very powerfully on the skin of animals and plants4 ,thus ,na,h,o is called 'caustic' soda4 ,the saline oxides are capable of combining together and with water4 ,water itself is an oxide1 and not an indifferent one1 for it can1 as we have seen1 combine with basic and acid oxides2 it is a representative of a whole series of saline oxides1 intermediate oxides1 capable of combining with both basic and acid oxides4 ,there are many such oxides1 which1 like water1 combine with basic and acid anhydrides--for instance1 the oxides of aluminium and tin1 c4 ,from this it may be concluded that all oxides might be placed1 in respect to their capacity for combining with one another1 in one uninterrupted series1 at one extremity of which would stand those oxides which do not combine with the bases--that is1 the alkalis--while at the other end would be the acid oxides1 and in the interval those oxides which combine with one another and with both the acid and basic oxides4 ,the further apart the members of this series are1 the more stable are the compounds they form together1 the more energetically do they act on each other1 the greater the quantity of heat evolved in their reaction1 and the more marked is their saline chemical character4 ,we said above that basic and acid oxides combine together1 but rarely react on each other2 this depends on the fact that the majority of them are solids or gases--that is1 they occur in the state least prone to chemical reaction4 ,the gaseo-elastic state is with difficulty destroyed1 because it necessitates overcoming the elasticity proper to the gaseous particles4 ,the solid state is characterised by the immobility of its particles2 whilst chemical action requires contact1 and hence a displacement and mobility4 ,if solid oxides be heated1 and especially if they be melted1 then reaction proceeds with great ease4 ,but such a change of state rarely occurs in nature or in practice4 ,only in a few furnace processes is this the case4 ,for example2 in the manufacture of glass1 the oxides contained in it combine together in a molten state4 ,but when oxides combine with water1 and especially when they form hydrates soluble in water1 then the mobility of their particles increases to a considerable extent1 and their reaction is greatly facilitated4 ,reaction then takes place at the ordinary temperature--easily and rapidly2 so that this kind of reaction belongs to the class of those which take place with unusual facility1 and are1 therefore1 very often taken advantage of in practice1 and also have been and are going on in nature at every step4 ,we will now consider the reactions of oxides in the state of hydrates1 not losing sight of the fact that water is itself an oxide with definite properties1 and has1 therefore1 no little influence on the course of those changes in which it takes part4 ,if we take a definite quantity of an acid1 and add an infusion of litmus to it1 it turns red2 the addition of an alkaline solution does not immediately alter the red colour of the litmus1 but on adding more and more of the alkaline solution a point is reached when the red colour changes to violet1 and then the further addition of a fresh quantity of the alkaline solution changes the colour to blue4 ,this change of the colour of the litmus is a consequence of the formation of a new compound4 ,this reaction is termed the saturation or neutralisation of the acid by the base1 or vice vers4 ,the solution in which the acid properties of the acid are saturated by the alkaline properties of the base is termed a neutral solution4 ,such a solution1 although derived from the mixture of a base with an acid1 does not exhibit either the acid or basic reaction on litmus1 yet it preserves many other signs of the acid and alkali4 ,it is observed that in such a definite admixture of an acid with an alkali1 besides the changes in the colour of litmus there is a heating effect--i4e4 an evolution of heat--which is alone sufficient to prove that there was chemical action4 ,and1 indeed1 if the resultant violet solution be evaporated1 there separates out1 not the acid or the alkali originally taken1 but a substance which has neither acid nor alkaline properties1 but is usually solid and crystalline1 having a saline appearance2 this is a salt in the chemical sense of the word4 ,hence a salt is derived from the reaction of an acid on an alkali1 in a certain definite proportion4 ,the water here taken for solution plays no other part than merely facilitating the progress of the reaction4 ,this is seen from the fact that the anhydrides of the acids are able to combine with basic oxides1 and give the same salts as do the acids with the alkalis or hydrates4 ,hence1 a salt is a compound of definite quantities of an acid with an alkali4 ,in the latter reaction1 water is separated out if the substance formed be the same as is produced by the combination of anhydrous oxides together4#ea ,examples of the formation of salts from acids and bases are easily observed1 and are very often applied in practice4 ,if we take1 for instance1 insoluble magnesium oxide 7magnesia7 it is easily dissolved in sulphuric acid1 and on evaporation gives a saline substance1 bitter1 like all the salts of magnesium1 and familiar to all under the name of ,epsom salts1 used as a purgative4 ,if a solution of caustic soda--which is obtained1 as we saw1 by the action of water on sodium oxide--be poured into a flask in which charcoal has been burnt2 or if carbonic anhydride1 which is produced under so many circumstances1 be passed through a solution of caustic soda1 then sodium carbonate or soda1 ,na#b;,,co#c1 is obtained1 of which we have spoken several times1 and which is prepared on a large scale and often used in manufactures4 ,this reaction is expressed by the equation1 #b,na,h,o ,,co#b ,na#b;,,co#c ,h#b,o4 ,thus1 the various bases and acids form an innumerable number of different salts4#eb ,salts constitute an example of definite chemical compounds1 and both in the history and practice of science are most often cited as confirming the conception of definite chemical compounds4 ,indeed1 all the indications of a definite chemical combination are clearly seen in the formation and properties of salts4 ,thus1 they are produced with a definite proportion of oxides1 heat is evolved in their formation1#ec and the chemical character of the oxides and many of the physical properties become hidden in their salts4 ,for example1 when gaseous carbonic anhydride combines with a base to form a solid salt1 the elasticity of the gas quite disappears in its passage into the salt4#ed #ea ,that water really is separated in the reaction of acid on alkaline hydrates1 may be shown by taking some other intermediate hydrate--for example1 alumina--instead of water4 ,thus1 if a solution of alumina in sulphuric acid be taken1 it will have1 like the acid1 an acid reaction1 and will therefore colour litmus red4 ,if1 on the other hand1 a solution of alumina in an alkali--say1 potash--be taken1 it will have an alkaline reaction1 and will turn red litmus blue4 ,on adding the alkaline to the acid solution until neither an alkaline nor an acid reaction is produced1 a salt is formed1 consisting of sulphuric anhydride and potassium oxide4 ,in this1 as in the reaction of hydrates1 an intermediate oxide is separated out--namely1 alumina4 ,its separation will be very evident in this case1 as alumina is insoluble in water4 #eb ,the mutual interaction of hydrates1 and their capacity of forming salts1 may be taken advantage of for determining the character of those hydrates which are insoluble in water4 ,let us imagine that a given hydrate1 whose chemical character is unknown1 is insoluble in water4 ,it is therefore impossible to test its reaction on litmus4 ,it is then mixed with water1 and an acid--for instance1 sulphuric acid--is added to the mixture4 ,if the hydrate taken be basic1 reaction will take place1 either directly or by the aid of heat1 with the formation of a salt4 ,in certain cases1 the resultant salt is soluble in water1 and this will at once show that combination has taken place between the insoluble basic hydrate and the acid1 with the formation of a soluble saline substance4 ,in those cases where the resultant salt is insoluble1 still the water loses its acid reaction1 and therefore it may he ascertained1 by the addition of an acid1 whether a given hydrate has a basic character1 like the hydrates of oxide of copper1 lead1 c4 ,if the acid does not act on the given insoluble hydrate 7at any temperature71 then it has not a basic character1 and it should be tested as to whether it has an acid character4 ,this is done by taking an alkali1 instead of the acid1 and by observing whether the unknown hydrate then dissolves1 or whether the alkaline reaction disappears4 ,thus it may he proved that hydrate of silica is acid1 because it dissolves in alkalis and not in acids4 ,if it be a case of an insoluble intermediate hydrate1 then it will be observed to react on both the acid and alkali4 ,hydrate of alumina is an instance in question1 which is soluble both in caustic potash and in sulphuric acid4 ,the degree of affinity or chemical energy proper to oxides and their hydrates is very dissimilar2 some extreme members of the series possess it to a great extent4 ,when acting on each other they evolve a large quantity of heat1 and when acting on intermediate hydrates they also evolve heat to a considerable degree1 as we saw in the combination of lime and sulphuric anhydride with water4 ,when extreme oxides combine they form stable salts1 which are decomposed with difficulty1 and often show characteristic properties4 ,the compounds of the intermediate oxides with each other1 or even with basic and acid oxides1 present a very different case4 ,however much alumina we may dissolve in sulphuric acid1 we cannot saturate the acid properties of the sulphuric acid1 the resulting solution will always have an acid reaction4 ,so also1 whatever quantity of alumina is dissolved in an alkali1 the resulting solution will always present an alkaline reaction4 #ec ,in order to give an idea of the quantity of heat evolved in the formation of salts ,i append a table of data for very dilute aqueous solutions of acids and alkalis1 according to the determinations of ,berthelot and ,thomsen4 ,the figures are given in major calories--that is1 in thousands of units of heat4 ,for example1 #di grams of sulphuric acid1 ,h#b,,so#d1 taken in a dilute aqueous solution1 when mixed with such an amount of a weak solution of caustic soda1 ,na,h,o1 that a neutral salt is formed 7when all the hydrogen of the acid is replaced by the sodium71 evolves #ae1hjj units of heat4 #di parts of #fc parts of ,h#b,,so#d ,,hno#c ,na,h,o #aeh #acg ,,kho #aeg #ach ,,nh#c #ade #abe ,ca,o #aef #aci ,ba,o #ahd #aci ,mg,o #aef #ach ,fe,o #abe #ajg 787 ,zn,o #aag #ih ,fe#b,o#c #eg #ei ,these figures cannot be considered as the heat of neutralisation1 because the water here plays an important part4 ,thus1 for instance1 sulphuric acid and caustic soda in dissolving in water evolve very much heat1 and the resultant sodium sulphate very little2 consequently1 the amount of heat evolved in an anhydrous combination will be different from that evolved in a hydrated combination4 ,those acids which are not energetic in combining with the same quantity of alkalis required for the formation of normal salts of sulphuric or nitric acids always1 however1 give less heat4 ,for instance1 with caustic soda3 carbonic acid gives #ajb1 hydrocyanic1 #bi1 hydrogen sulphide1 #ci major calories4 ,and as feeble bases 7for example1 ,fe#b,o#c7 also evolve less heat than those which are more powerful1 so a certain general correlation between thermochemical data and the degree of affinity shows itself here1 as in other cases 7see ,chapter ,,ii41 ,note #g72 this does not1 however1 give any reason for measuring the affinity which binds the elements of salts by the heat of their formation in dilute solutions4 ,this is very clearly demonstrated by the fact that water is able to decompose many salts1 and is separated in their formation4 #ed ,carbonic anhydride evolves heat in dissolving in water4 ,the solution easily dissociates and evolves carbonic anhydride1 according to the law of ,henry and ,dalton 7see ,chapter ,i47 ,in dissolving in caustic soda1 it either gives a normal salt1 ,na#b;,,co#c1 which does not evolve carbonic anhydride1 or an acid salt1 ,na,h,c,o#c which easily evolves carbonic anhydride when heated4 ,the same gas1 when dissolved in solutions of salts1 acts in one or the other manner 7see ,chapter ,,ii41 ,note #ch74 ,here it is seen what a successive series of relations exists between compounds of a different order1 between substances of different degrees of stability4 ,by making a distinction between the phenomena of solutions and chemical compounds1 we overlook those natural transitions which in reality exist4 ,judging from the above1 a salt is a compound of basic and acid oxides1 or the result of the action of hydrates of these classes on each other with separation of water4 ,but salts may be obtained by other methods4 ,it must not be forgotten that basic oxides are formed by metals1 and acid oxides usually by non-metals4 ,but metals and non-metals are capable of combining together1 and a salt is frequently formed by the oxidation of such a compound4 ,for example1 iron very easily combines with sulphur1 forming iron sulphide ,fe,s 7as we saw in the ,introduction72 this in air1 and especially moist air1 absorbs oxygen1 with the formation of the same salt ,fe,s,o#d1 that may be obtained by the combination of the oxides of iron and sulphur1 or of the hydrates of these oxides4 ,hence1 it cannot be said or supposed that a salt has the properties of the oxides1 or must necessarily contain two kinds of oxides in itself4 ,the derivation of salts from oxides is merely one of the methods of their preparation4 ,we saw1 for instance1 that in sulphuric acid it was possible to replace the hydrogen by zinc1 and that by this means zinc sulphate was formed2 so likewise the hydrogen in many other acids may be replaced by zinc1 iron1 potassium1 sodium1 and a whole series of similar metals1 corresponding salts being obtained4 ,the hydrogen of the acid1 in all these cases1 is exchanged for a metal1 and a salt is obtained from the hydrate4 ,regarding a salt from this point of view1 it may be said that a salt is an acid in which hydrogen is replaced by a metal4 ,this definition shows that a salt and an acid are essentially compounds of the same series1 with the difference that the latter contains hydrogen and the former a metal4 ,such a definition is more exact than the first definition of salts1 inasmuch as it likewise includes those acids which do not contain oxygen1 and1 as we shall afterwards learn1 there is a series of such acids4 ,such elements as chlorine and bromine form compounds with hydrogen in which the hydrogen may be replaced by a metal1 forming substances which1 in their reactions and external characters1 resemble the salts formed from oxides4 ,table salt1 ,na,cl1 is an example of this4 ,it may be obtained by the replacement of hydrogen in hydrochloric acid1 ,h,cl1 by the metal sodium1 just as sulphate of sodium1 ,na#b,,so#d1 may be obtained by the replacement of hydrogen in sulphuric acid1 ,h#b,,so#d1 by sodium4 ,the exterior appearance of the resulting products1 their neutral reaction1 and even their saline taste1 show their resemblance to one another4 ,to the fundamental properties of salts yet another must be added--namely1 that they are more or less decomposed by the action of a galvanic current4 ,the results of this decomposition are very different according to whether the salt be taken in a fused or dissolved state4 ,but the decomposition may generally be so represented1 that the metal appears at the electro-negative pole or cathode 7like hydrogen in the decomposition of water1 or its mixture with sulphuric acid71 and the remaining parts of the salt appear at the electro-positive pole or anode 7where the oxygen of water appears74 ,if1 for instance1 an electric current acts on an aqueous solution of sodium sulphate1 then the sodium appears at the negative pole1 and oxygen and the anhydride of sulphuric acid at the positive pole4 ,but in the solution itself the result is different1 for sodium1 as we know1 decomposes water with evolution of hydrogen1 forming caustic soda2 consequently hydrogen will be evolved1 and caustic soda appear at the negative pole3 while at the positive pole the sulphuric anhydride immediately combines with water and forms sulphuric acid1 and therefore oxygen will be evolved and sulphuric acid formed round this pole4#ee ,in other cases1 when the metal separated is not able to decompose water1 it will be deposited in a free state4 ,thus1 for example1 in the decomposition of copper sulphate1 copper separates out at the cathode1 and oxygen and sulphuric acid appear at the anode1 and if a copper plate be attached to the positive pole1 then the oxygen evolved will oxidise the copper1 and the oxide of copper will dissolve and be deposited at the negative pole--that is1 a transfer of copper from the positive to the negative pole ensues4 ,the galvanoplastic art 7electro-typing7 is based on this principle4#ef ,therefore the most radical and general properties of salts 7including also such salts as table salt1 which contain no oxygen7 may be expressed by representing the salt as composed of a metal ,m and a haloid ,x--that is1 by expressing the salt by ,,mx4 ,in common table salt the metal is sodium1 and the haloid an elementary body1 chlorine4 ,in sodium sulphate1 ,na#b,,so#d1 sodium is again the metal1 but the complex group1 ,,so#d1 is the haloid4 ,in sulphate of copper1 ,cu,s,o#d1 the metal is copper and the haloid the same as in the preceding salt4 ,such a representation of salts expresses with great simplicity the capacity of every salt to enter into saline double decompositions with other salts2 consisting in the mutual replacement of the metals in the salts4 ,this exchange of their metals is the fundamental property of salts4 ,in the case of two salts with different metals and haloids1 which are in solution or fusion1 or in any other manner brought into contact1 the metals of these salts will always partially or wholly exchange places4 ,if we designate one salt by ,,mx1 and the other by ,,ny1 then we either partially or wholly obtain from them new salts1 ,,my and ,,nx4 ,thus we saw in the ,introduction1 that on mixing solutions of table salt1 ,na,cl1 and silver nitrate1 ,ag,n,o#c1 a white insoluble precipitate of silver chloride1 ,ag,cl1 is formed and a new salt1 sodium nitrate1 ,na,n,o#c1 is obtained in solution4 ,if the metals of salts exchange places in reactions of double decomposition1 it is clear that metals themselves1 taken in a separate state1 are able to act on salts1 as zinc evolves hydrogen from acids1 and as iron separates copper from copper sulphate4 ,when1 to what extent1 and which metals displace each other1 and how the metals are distributed between the haloids1 will be discussed in ,chapter ,x41 where we shall be guided by those reflections and deductions which ,berthollet introduced into the science at the beginning of this century4 #ee ,this kind of decomposition may be easily observed by pouring a solution of sodium sulphate into a ,u-shaped tube and inserting electrodes in the two branches4 ,if the solution be coloured with an infusion of litmus1 it will easily be seen that it turns blue at the cathode1 owing to the formation of sodium hydroxide1 and red at the electro-positive pole1 from the formation of sulphuric acid4 #ef ,in other cases the decomposition of salts by the electric current may be accompanied by much more complex results4 ,thus1 when the metal of the salt is capable of a higher degree of oxidation1 such a higher oxide may be formed at the positive pole by the oxygen which is evolved there4 ,this takes place1 for instance1 in the decomposition of salts of silver and manganese by the galvanic current1 peroxides of these metals being formed4 ,thus in the electrolysis of a solution of ,k,cl1 ,k,cl,o#c is formed1 and of sulphuric acid 7corresponding to ,,so#c7 persulphuric acid1 corresponding to ,s#b,o#g4 ,but all the phenomena as yet known may be expressed by the above law--that the current decomposes salts into metals1 which appear at the negative pole1 and into the remaining component parts1 which appear at the positive pole4 ,according to the above observations1 an acid is nothing more than a salt of hydrogen4 ,water itself may be looked on as a salt in which the hydrogen is combined with either oxygen or the aqueous radicle1 ,,oh2 water will then be ,,hoh1 and alkalis or basic hydrates1 ,,moh4 ,the group ,,oh1 or the aqueous radicle1 otherwise called hydroxyl1 may be looked on as a haloid like the chlorine in table salt1 not only because the element ,cl and the group ,,oh very often change places1 and combine with one and the same element1 but also because free chlorine is very similar in many properties and reactions to peroxide of hydrogen1 which is the same in composition as the aqueous radicle1 as we shall afterwards see in ,chapter ,,iv4 ,alkalis and basic hydrates are also salts consisting of a metal and hydroxyl--for instance1 caustic soda1 ,na,o,h2 this is therefore termed sodium hydroxide4 ,according to this view1 acid salts are those in which a portion only of the hydrogen is replaced by a metal1 and a portion of the hydrogen of the acid remains4 ,thus sulphuric acid 7,h#b,,so#d7 not only gives the normal salt ,na#b,,so#d1 with sodium1 but also an acid salt1 ,na,h,s,o#d4 ,a basic salt is one in which the metal is combined not only with the haloids of acids1 but also with the aqueous radicale of basic hydrates--for example1 bismuth gives not only a normal salt of nitric acid1 ,bi7,,no#c7c1 but also basic salts like ,bi7,,oh7#b7,,no#c74 ,as basic and acid salts of the oxygen acids contain hydrogen and oxygen1 they are able to part with these as water and to give anhydro-salts1 which it is evident will be compounds of normal salts with anhydrides of the acids or with bases4 ,thus the above-mentioned acid sodium sulphate corresponds with the anhydro-salt1 ,na#b,s#b,o#g1 equal to #b,na,h,s,o#d1 less ,h#b,o4 ,the loss of water is here1 and frequently in other cases1 brought about by heat alone1 and therefore such salts are frequently termed pyro-salts--for instance1 the preceding is sodium pyrosulphate 7,na#b,s#b,o#g71 or it may be regarded as the normal salt ,na#b,,so#d sulphuric anhydride1 ,,so#c4 ,double salts are those which contain either two metals1 ,k,al7,,so#d7b1 or two haloids4#eg #eg ,the above-enunciated generalisation of the conception of salts as compounds of the metals 7simple1 or compound like ammonium1 ,,nh#d71 with the haloids 7simple1 like chlorine1 or compound1 like cyanogen1 ,,cn1 or the radical of sulphuric acid1 ,,so#d71 capable of entering into double saline decomposition1 which is in accordance with the general data respecting salts1 was only formed little by little after a succession of most varied propositions as to the chemical structure of salts4 ,salts belong to the class of substances which have been known since very early times1 and have long been investigated in many directions4 ,at first1 however1 no distinction was made between salts1 acids1 and bases4 ,glauber prepared many artificial salts during the latter half of the seventeenth century4 ,up to that time the majority of salts were obtained from natural sources1 and that salt which we have referred to several times--namely1 sodium sulphate--was named ,glauber's salt after this chemist4 ,rouelle distinguished normal1 acid1 and basic salts1 and showed their action on vegetable dyes1 still he confounded many salts with acids 7even now every acid salt ought to be regarded as an acid1 because it contains hydrogen1 which may be replaced by metals--that is1 it is the hydrogen of an acid74 ,baum disputed ,rouelle's opinion concerning the subdivision of salts1 contending that normal salts only are true salts1 and that basic salts are simple mixtures of normal salts with bases and acid salts with acids1 considering that washing alone could remove the base or acid from them4 ,rouelle1 in the middle of the last century1 however1 rendered a great service to the study of salts and the diffusion of knowledge respecting this class of compounds in his attractive lectures4 ,he1 like the majority of the chemists of that period1 did not employ the balance in his researches1 but satisfied himself with purely qualitative data4 ,the first quantitative researches on salts were carried on about this time by ,wenzel1 who was the director of the ,freiburg mines1 in ,saxony4 ,wenzel studied the double decomposition of salts1 and observed that in the double decomposition of neutral salts a neutral salt was always obtained4 ,he proved1 by a method of weighing1 that this is due to the fact that the saturation of a given quantity of a base requires such relative quantities of different acids as are capable of saturating every other base4 ,having taken two neutral salts--for example1 sodium sulphate and calcium nitrate--let us mix their solutions together4 ,double decomposition takes place1 because calcium sulphate is formed1 which is almost insoluble4 ,however much we might add of each of the salts1 the neutral reaction will still be preserved1 consequently the neutral character of the salts is not destroyed by the interchange of metals2 that is to say1 that quantity of sulphuric acid which saturated the sodium is sufficient for the saturation of the calcium1 and that amount of nitric acid which saturated the calcium is enough to saturate the sodium contained in combination with sulphuric acid in sodium sulphate4 ,wenzel was even convinced that matter does not disappear in nature1 and on this principle he corrects1 in his ,doctrine of ,affinity1 the results of his experiments when he found that he obtained less than he had originally taken4 ,although ,wenzel deduced the law of the double decomposition of salts quite correctly1 he did not determine those quantities in which acids and bases act on each other4 ,this was carried out at the end of the last century by ,richter4 ,he determined the quantities by weight of the bases which saturate acids and of the acids which saturate bases1 and obtained comparatively correct results1 although his conclusions were not correct1 for he states that the quantity of a base saturating a given acid varies in arithmetical progression1 and the quantity of an acid saturating a given base in geometrical progression4 ,richter studied the deposition of metals from their salts by other metals1 and observed that the neutral reaction of the solution is not destroyed by this exchange4 ,he also determined the quantities by weight of the metals replacing one another in salts4 ,he showed that copper displaces silver from its salt1 and that zinc displaces copper and a whole series of other metals4 ,those quantities of metals which were capable of replacing one another were termed equivalents4 ,richter's teaching found no followers1 because1 although he fully believed in the discoveries of ,lavoisier1 yet he still held to the phlogistic reasonings which rendered his expositions very obscure4 ,the works of the ,swedish savant ,berzelius freed the facts discovered by ,wenzel and ,richter from the obscurity of former conceptions1 and led to their being explained in accordance with ,lavoisier's views1 and in the sense of the law of multiple proportions which had already been discovered by ,dalton4 ,on applying to salts those conclusions which ,berzelius arrived at by a whole series of researches of remarkable accuracy1 we arrive at the following law of equivalents--one part by weight of hydrogen in an acid is replaced by the corresponding equivalent weight of any metal2 and1 therefore1 when metals replace each other their weights are in the same ratio as their equivalents4 ,thus1 for instance1 one part by weight of hydrogen is replaced by #bc parts of sodium1 #ci parts of potassium1 #ab parts of magnesium1 #bj parts of calcium1 #bh parts of iron1 #ajh parts of silver1 #cc parts of zinc1 c42 and1 therefore1 if zinc replaces silver1 then #cc parts of zinc will take the place of #ajh parts of silver1 or #cc parts of zinc will he substituted by #bc parts of sodium1 c4 ,the doctrine of equivalents would be precise and simple did every metal only give one oxide or one salt4 ,it is rendered complicated from the fact that many metals form several oxides1 and consequently offer different equivalents in their different degrees of oxidation4 ,for example1 there are oxides containing iron in which its equivalent is #bh--this is in the salts formed by the suboxide2 and there is another series of salts in which the equivalent of iron equals #ah-b_/c--which contain less iron1 and consequently more oxygen1 and correspond with a higher degree of oxidation--ferric oxide4 ,it is true that the former salts are easily formed by the direct action of metallic iron on acids1 and the latter only by a further oxidation of the compound formed already2 but this is not always so4 ,in the case of copper1 mercury1 and tin1 under different circumstances1 salts are formed which correspond with different degrees of oxidation of these metals1 and many metals have two equivalents in their different salts--that is1 in salts corresponding with the different degrees of oxidation4 ,thus it is impossible to endow every metal with one definite equivalent weight4 ,hence the conception of equivalents1 while playing an important part from an historical point of view1 appears1 with a fuller study of chemistry1 to be but subordinate to a higher conception1 with which we shall afterwards become acquainted4 ,the fate of the theoretical views of chemistry was for a long time bound up with the history of salts4 ,the clearest representation of this subject dates back to ,lavoisier1 and was systematically developed by ,berzelius4 ,this representation is called the binary theory4 ,all compounds1 and especially salts1 are represented as consisting of two parts4 ,salts are represented as compounds of a basic oxide 7a base7 and an acid 7that is1 an anhydride of an acid1 then termed an acid71 whilst hydrates are represented as compounds of anhydrous oxides with water4 ,such an expression was employed not only to denote the most usual method of formation of these substances 7where it would be quite true71 but also to express that internal distribution of the elements by which it was proposed to explain all the properties of these substances4 ,copper sulphate was supposed to contain two most intimate component parts--copper oxide and sulphuric anhydride4 ,this is an hypothesis4 ,it arose from the so-called electro-chemical hypothesis1 which supposed the two component parts to be held in mutual union1 because one component 7the anhydride of the acid7 has electro-negative properties1 and the other 7the base in salts7 electro-positive4 ,the two parts are attracted together1 like substances having opposite electrical charges4 ,but as the decomposition of salts in a state of fusion by an electric current always gives a metal1 that representation of the constitution and decomposition of salts called the hydrogen theory of acids is nearer the truth than that which considers salts as made up of a base and an anhydride of an acid4 ,but the hydrogen theory of acids is also a binary hypothesis1 and does not contradict the electro-chemical hypothesis1 but is rather a modification of it4 ,the binary theory dates from ,rouelle and ,lavoisier1 the electro-chemical aspect was zealously developed by ,berzelius1 and the hydrogen theory of acids is due to ,davy and ,liebig4 ,these hypothetical views simplified and generalised the study of a complicated subject1 and served to support further arguments1 but when salts were in question it was equally convenient to follow one or the other of these hypotheses4 ,but these theories were brought to bear on all other substances1 on all compound substances4 ,those holding the binary and electro-chemical hypotheses searched for two anti-polar component parts1 and endeavoured to express the process of chemical reactions by electro-chemical and similar differences4 ,if zinc replaces hydrogen1 they concluded that it is more electro-positive than hydrogen1 whilst they forgot that hydrogen may1 under different circumstances1 displace zinc--for instance1 at a red heat4 ,chlorine and oxygen were considered as being of opposite polarity to hydrogen because they easily combine with it1 nevertheless both are capable of replacing hydrogen1 and1 what is very characteristic1 in the replacement of hydrogen by chlorine in carbon compounds not only does the chemical character often remain unaltered1 but even the external form may remain unchanged1 as ,laurent and ,dumas demonstrated4 ,these considerations undermine the binary1 and more especially the electro-chemical theory4 ,an explanation of known reactions then began to be sought for not in the difference of the polarity of the different substances1 but in the joint influences of all the elements on the properties of the compound formed4 ,this is the reverse of the preceding hypothesis4 ,this reversal was not1 however1 limited to the destruction of the tottering foundations of the preceding theory2 it proposed a new doctrine1 and laid the foundation for the modern course of our science4 ,this doctrine may be termed the unitary theory--that is1 it strictly acknowledges the joint influences of the elements in a compound substance1 denies the existence of separate and contrary components in them1 regards copper sulphate1 for instance1 as a strictly definite compound of copper1 sulphur1 and oxygen2 then seeks for compounds which are analogous in their properties1 and1 placing them side by side1 endeavours to express the influence of each element in determining the united properties of its compound4 ,in the majority of cases it arrives at conclusions similar to those which are obtained by the above-mentioned hypotheses1 but in certain special cases the conclusions of the unitary theory are in entire opposition to those of the binary theory and its corollaries4 ,cases of this kind are most often met with in the consideration of compounds of a more complex nature than salts1 especially organic compounds containing hydrogen4 ,but it is not in this change from an artificial to a natural system1 important as it is1 that the chief service and strength of the unitary doctrine lies4 ,by a simple review of the vast store of data regarding the reactions of typical substances1 it succeeded from its first appearance in establishing a new and important law1 it introduced a new conception into science--namely1 the conception of molecules1 with which we shall soon become acquainted4 ,the deduction of the law and of the conception of molecules has been verified by facts in a number of cases1 and was the cause of the majority of chemists of our times deserting the binary theory and accepting the unitary theory1 which forms the basis of the present work4 ,laurent and ,gerhardt must be considered as the founders of this doctrine4 ,inasmuch as oxygen compounds predominate in nature1 it should be expected from what has been said above1 that salts1 rather than acids or bases1 would occur most frequently in nature1 for these latter would always tend to combine forming salts1 especially through the medium of the all-pervading water4 ,and1 as a matter of fact1 salts are found everywhere in nature4 ,they occur in animals and plants1 although in but small quantity1 because1 as forming the last stage of chemical reaction1 they are capable of only a few chemical transformations4 ,and organisms are bodies in which a series of uninterrupted1 varied1 and active chemical transformations proceed1 whilst salts1 which only enter into double decompositions between each other1 are little prone to such changes4 ,but organisms always contain salts4 ,thus1 for instance1 bones contain calcium phosphate1 the juice of grapes potassium tartrate 7cream of tartar71 certain lichens calcium oxalate1 and the shells of mollusca calcium carbonate1 c4 ,as regards water and soil1 portions of the earth in which the chemical processes are less active1 they are full of salts4 ,thus the waters of the oceans1 and all others 7,chap4 ,i471 abound in salts1 and in the soil1 in the rocks of the earth's crust1 in the upheaved lavas1 and in the falling meteorites the salts of silicic acid1 and especially its double salts1 predominate4 ,saline substances also make up the composition of those limestones which often form mountain chains and whole thicknesses of the earth's strata1 these consisting of calcium carbonate1 ,ca,c,o#c4 ,thus we have seen oxygen in a free state and in various compounds of different degrees of stability1 from the unstable salts1 like ,berthollet's salt and nitre1 to the most stable silicon compounds1 such as exist in granite4 ,we saw an entirely similar gradation of stability in the compounds of water and of hydrogen4 ,in all its aspects oxygen1 as an element1 or single substance1 remains the same however varied its chemical states1 just as a substance may appear in many different physical states of aggregation4 ,but our notion of the immense variety of the chemical states in which oxygen can occur would not be completely understood if we did not make ourselves acquainted with it in the form in which it occurs in ozone and peroxide of hydrogen4 ,in these it is most active1 its energy seems to have increased4 ,they illustrate fresh aspects of chemical correlations1 and the variety of the forms in which matter can appear stand out clearly4 ,we will therefore consider these two substances somewhat in detail4 ,,chapter ,,iv ,,ozone ,,and ,,hydrogen ,,peroxide--,,dalton',s ,,law ,,van ,,marum1 during the last century1 observed that oxygen in a glass tube1 when subjected to the action of a series of electric sparks1 acquired a peculiar smell1 and the property of combining with mercury at the ordinary temperature4 ,this was afterwards confirmed by a number of fresh experiments4 ,even in the simple revolution of an electrical machine1 when electricity diffuses into the air or passes through it1 the peculiar and characteristic smell of ozone1 proceeding from the action of the electricity on the oxygen of the atmosphere1 is recognised4 ,in #ahdj ,prof4 ,schnbein1 of ,basle1 turned his attention to this odoriferous substance1 and showed that it is also formed1 with the oxygen evolved at the positive pole1 in the decomposition of water by the action of a galvanic current2 in the oxidation of phosphorus in damp air1 and also in the oxidation of a number of substances1 although it is distinguished for its instability and capacity for oxidising other substances4 ,the characteristic smell of this substance gave it its name1 from the ,greek ,greek3 oz1 ',i emit an odour4' ,schnbein pointed out that ozone is capable of oxidising many substances on which oxygen does not act at the ordinary temperature4 ,it will be sufficient to point out for instance that it oxidises silver1 mercury1 charcoal1 and iron with great energy at the ordinary temperature4 ,it might be thought that ozone was some new compound substance1 as it was at first supposed to be2 but careful observations made in this direction have long led to the conclusion that ozone is nothing but oxygen altered in its properties4 ,this is most strikingly proved by the complete transformation of oxygen containing ozone into ordinary oxygen when it is passed through a tube heated to #bej4 ,further1 at a low temperature pure oxygen gives ozone when electric sparks are passed through it 7,marignac and ,de la ,rive74 ,hence it is proved both by synthesis and analysis that ozone is that same oxygen with which we are already acquainted1 only endowed with particular properties and in a particular state4 ,however1 by whatever method it be obtained1 the amount of it contained in the oxygen is inconsiderable1 generally only a few fractions per cent41 rarely #b per cent41 and only under very propitious circumstances as much as #bj per cent4 ,the reason of this must be looked for first in the fact that ozone in its formation from oxygen absorbs heat4 ,if any substance be burnt in a calorimeter at the expense of ozonised oxygen1 then more heat is evolved than when it is burnt in ordinary oxygen1 and ,berthelot showed that this difference is very large--namely1 #bi1fjj heat units correspond with every forty-eight parts by weight of ozone4 ,this signifies that the transformation of forty-eight parts of oxygen into ozone is accompanied by the absorption of this quantity of heat1 and that the reverse process evolves this quantity of heat4 ,therefore the passage of ozone into oxygen should take place easily and fully 7as an exothermal reaction71 like combustion2 and this is proved by the fact that at #bej ozone entirely disappears1 forming oxygen4 ,any rise of temperature may thus bring about the breaking up of ozone1 and as a rise of temperature takes place in the action of an electrical discharge1 there are in an electric discharge the conditions both for the preparation of ozone and for its destruction4 ,hence it is clear that the transformation of oxygen into ozone as a reversible reaction has a limit when a state of equilibrium is arrived at between the products of the two opposite reactions1 that the phenomena of this transformation accord with the phenomena of dissociation1 and that a fall of temperature should aid the formation of a large quantity of ozone4#a ,further1 it is evident1 from what has been said1 that the best way of preparing ozone is not by electric sparks1#b which raise the temperature1 but by the employment of a continual discharge or flow of electricity--that is1 by the action of a silent discharge4#c ,for this reason all ozonisers 7which are of most varied construction71 or forms of apparatus for the preparation of ozone from oxygen 7or air7 by the action of electricity1 now usually consist of sheets of metal--for instance1 tinfoil--a solution of sulphuric acid mixed with chromic acid1 c4 separated by thin glass surfaces placed at short distances from each other1 and between which the oxygen or air to be ozonised is introduced and subjected to the action of a silent discharge4#d ,thus in ,siemens' apparatus 7fig4 #cg7 the exterior of the tube a and the interior of the tube b c are coated with tinfoil and connected with the poles of a source of electricity 7with the terminals of a ,ruhmkorff's coil74 ,a silent discharge passes through the thin walls of the glass cylinders a and b c over all their surfaces1 and consequently1 if oxygen be passed through the apparatus by the tube d1 fused into the side of a1 it will be ozonised in the annular space between a and b c4 ,the ozonised oxygen escapes by the tube e1 and may be introduced into any other apparatus4#e ,illustration3 ,,fig4 #cg4--,siemens' apparatus for preparing ozone by means of a silent discharge4 #a ,this conclusion1 deduced by me as far back as #ahgh 7,moniteur ,scientifique7 by conceiving the molecules of ozone 7see later7 as more complex than those of oxygen1 and ozone as containing a greater quantity of heat than oxygen1 has been proved experimentally by the researches of ,mailfert 7#ahhj71 who showed that the passage of a silent discharge through a litre of oxygen at #j may form up to #ad milligrams of ozone1 and at -#cj up to #fj milligrams2 but best of all in the determinations of ,chappuis and ,hautefeuille 7#ahhj71 who found that at a temperature of -#be a silent discharge converted #bj p4c4 of oxygen into ozone1 whilst at #bj it was impossible to obtain more than #ab p4c41 and at #ajj less than #b p4c4 of ozone was obtained4 #b ,a series of electric sparks may be obtained by an ordinary electrical machine1 the electrophorus machines of ,holtz and ,teploff1 c41 ,leyden jars1 ,ruhmkorff coils1 or similar means1 when the opposite electricities are able to accumulate at the terminals of conductors1 and a discharge of sufficient electrical intensity passes through the non-conductors air or oxygen4 #c ,a silent discharge is such a combination of opposite statical 7potential7 electricities as takes place 7generally between large surfaces7 regularly1 without sparks1 slowly1 and quietly 7as in the dispersion of electricity74 ,the discharge is only luminous in the dark2 there is no observable rise of temperature1 and therefore a larger amount of ozone is formed4 ,but1 nevertheless1 on continuing the passage of a silent discharge through ozone it is destroyed4 ,for the action to be observable a large surface is necessary1 and consequently a source of electricity at a high potential4 ,for this reason the silent discharge is best produced by a ,ruhmkorff coil1 as the most convenient means of obtaining a considerable potential of statical electricity with the employment of the comparatively feeble current of a galvanic battery4 #d v4 ,babo's apparatus was one of the first constructed for ozonising oxygen by means of a silent discharge 7and it is still one of the best74 ,it is composed of a number 7twenty and more7 of long1 thin capillary glass tubes closed at one end4 ,a platinum wire1 extending along their whole length1 is introduced into the other end of each tube1 and this end is then fused up round the wire1 the end of which protrudes outside the tube4 ,the protruding ends of the wires are arranged alternately in two sides in such a manner that on one side there are ten closed ends and ten wires4 ,a bunch of such tubes 7forty should make a bunch of not more than #a c4m4 diameter7 is placed in a glass tube1 and the ends of the wires are connected with two conductors1 and are fused to the ends of the surrounding tube4 ,the discharge of a ,ruhmkorff coil is passed through these ends of the wires1 and the dry air or oxygen to be ozonised is passed through the tube4 ,if oxygen be passed through1 ozone is obtained in large quantities1 and free from oxides of nitrogen1 which are partially formed when air is acted on4 ,at low temperatures ozone is formed in large quantities4 ,as ozone acts on corks and india-rubber1 the apparatus should be made entirely of glass4 ,with a powerful ,ruhmkorff coil and forty tubes the ozonation is so powerful that the gas when passed through a solution of iodide of potassium not only sets the iodine free1 but even oxidises it to potassium iodate1 so that in five minutes the gas-conducting tube is choked up with crystals of the insoluble iodate4 #e ,in order to connect the ozoniser with any other apparatus it is impossible to make use of india-rubber1 mercury1 or cements1 c41 because they are themselves acted on by1 and act on1 ozone4 ,all connections must1 as was first proposed by ,brodie1 be hermetically closed by sulphuric acid1 which is not acted on by ozone4 ,thus1 a cork is passed over the vertical end of a tube1 over which a wide tube passes so that the end of the first tube protrudes above the cork2 mercury is first poured over the cork 7to prevent its being acted on by the sulphuric acid71 and then sulphuric acid is poured over the mercury4 ,the protruding end of the first tube is covered by the lower end of a third tube immersed in the sulphuric acid4 ,the properties of ozone obtained by such a method#f distinguish it in many respects from oxygen4 ,ozone very rapidly decolorises indigo1 litmus1 and many other dyes by oxidising them4 ,silver is oxidised by it at the ordinary temperature1 whilst oxygen is not able to oxidise silver even at high temperatures2 a bright silver plate rapidly turns black 7from oxidation7 in ozonised oxygen4 ,it is rapidly absorbed by mercury1 forming oxide2 it transforms the lower oxides into higher--for instance1 sulphurous anhydride into sulphuric1 nitrous oxide into nitric1 arsenious anhydride 7,as#b,o#c7 into arsenic anhydride 7,as#b,o#e7 c4#g ,but what is especially characteristic in ozone is the decomposing action it exerts on potassium iodide4 ,oxygen does not act on it1 but ozone passed into a solution of potassium iodide liberates iodine1 whilst the potassium is obtained as caustic potash1 which remains in solution1 #b,,ki ,h#b,o ,o #b,,kho ,i#b4 ,as the presence of minute traces of free iodine may be discovered by means of starch paste1 with which it forms a very dark blue-coloured substance1 a mixture of potassium iodide with starch paste will detect the presence of very small traces of ozone4#h ,ozone is destroyed or converted into ordinary oxygen not only by heat1 but also by long keeping1 especially in the presence of alkalis1 peroxide of manganese1 chlorine1 c4 #f ,the method above described is the only one which has been well investigated4 ,the admixture of nitrogen1 or even of hydrogen1 and especially of silicon fluoride1 appears to aid the formation and preservation of ozone4 ,amongst other methods for preparing ozone we may mention the following3 #a4 ,in the action of oxygen on phosphorus at the ordinary temperature a portion of the oxygen is converted into ozone4 ,at the ordinary temperature a stick of phosphorus1 partially immersed in water and partially in air in a large glass vessel1 causes the air to acquire the odour of ozone4 ,it must further be remarked that if the air be left for long in contact with the phosphorus1 or without the presence of water1 the ozone formed is destroyed by the phosphorus4 #b4 ,by the action of sulphuric acid on peroxide of barium4 ,if the latter be covered with strong sulphuric acid 7the acid1 if diluted with only one-tenth of water1 does not give ozone71 then at a low temperature the oxygen evolved contains ozone1 and in much greater quantities than in that ozone is obtained by the action of electric sparks or phosphorus4 #c4 ,ozone may also be obtained by decomposing strong sulphuric acid by potassium manganate especially with the addition of barium peroxide4 #g ,ozone takes up the hydrogen from hydrochloric acid2 chlorine is liberated1 and can dissolve gold4 ,iodine is directly oxidised by ozone1 but not by oxygen4 ,ammonia1 ,,nh#c1 is oxidised by ozone into ammonium nitrite 7and nitrate71 #b,,nh#c ,o#c ,,nh#d,,no#b ,h#b,o1 and therefore a drop of ammonia1 on falling into the gas1 gives a thick cloud of the salts formed4 ,ozone converts lead oxide into peroxide1 and suboxide of thallium 7which is colourless7 into oxide 7which is brown71 so that this reaction is made use of for detecting the presence of ozone4 ,lead sulphide1 ,pb,s 7black71 is converted into sulphate1 ,pb,s,o#d 7colourless71 by ozone4 ,a neutral solution of manganese sulphate gives a precipitate of manganese peroxide1 and an acid solution may be oxidised into permanganic acid1 ,h,mn,o#d4 ,with respect to the oxidising action of ozone on organic substances1 it may be mentioned that with ether1 ,c#d;,h#aj,o1 ozone gives ethyl peroxide1 which is capable of decomposing with explosion 7according to ,berthelot71 and is decomposed by water into alcohol1 #b;,c#b;,h#f,o1 and hydrogen peroxide1 ,h#b,o#b4 #h ,this reaction is the one usually made use of for detecting the presence of ozone4 ,in the majority of cases paper is soaked in solutions of potassium iodide and starch4 ,such ozonometrical or iodised starch-paper when damp turns blue in the presence of ozone1 and the tint obtained varies considerably1 according to the length of time it is exposed and to the amount of ozone present4 ,the amount of ozone in a given gas may even to a certain degree he judged by the shade of colour acquired by the paper1 if preliminary tests be made4 ,test-paper for ozone is prepared in the following manner3--,one gram of neutral potassium iodide is dissolved in #ajj grams of distilled water2 #aj grams of starch are then shaken up in the solution1 and the mixture is boiled until the starch is converted into a jelly4 ,this jelly is then smeared over blotting-paper and left to dry4 ,it must always he remembered1 however1 that the colour of iodised starch-paper is changed not only by the action of ozone1 but of many other oxidisers2 for example1 by the oxides of nitrogen 7especially ,n#b,o#c7 and hydrogen peroxide4 ,houzeau proposed soaking common litmus-paper with a solution of potassium iodide1 which in the presence of iodine would turn blue1 owing to the formation of ,,kho4 ,in order to determine if the blue colour is not produced by an alkali 7ammonia7 in the gas1 a portion of the paper is not soaked in the potassium iodide1 but moistened with water2 this portion will then also turn blue if ammonia be present4 ,a reagent for distinguishing ozone from hydrogen peroxide with certainty is not known1 and therefore these substances in very small quantities 7for instance1 in the atmosphere7 may easily he confounded4 ,until recent years the mistake has frequently been made of ascribing the alteration of iodised starch-paper in the air to the presence of ozone2 at the present time there is reason to believe that it is most often due to the presence of nitrous acid 7,ilosva1 #ahhi74 ,hence ozone1 although it has the same composition as oxygen1 differs from it in stability1 and by the fact that it oxidises a number of substances very energetically at the ordinary temperature4 ,in this respect ozone resembles the oxygen of certain unstable compounds1 or oxygen at the moment of its liberation4#h bis #h bis ,fluorine 7,chap4 ,,xi471 acting upon water at the ordinary temperature1 takes up the hydrogen1 and evolves the oxygen in the form of ozone 7,moissan1 #ahhi71 and therefore the reaction must be expressed thus3--#c;,h#b,o #c;,f#b #f;,,hf ,o#c4 ,in ordinary oxygen and ozone we see an example of one and the same substance1 in this case an element1 appearing in two states4 ,this indicates that the properties of a substance1 and even of an element1 may vary without its composition varying4 ,very many such cases are known4 ,such cases of a chemical transformation which determine a difference in the properties of one and the same element are termed cases of isomerism4 ,the cause of isomerism evidently lies deep within the essential conditions of a substance1 and its investigation has already led to a number of results of unexpected importance and of immense scientific significance4 ,it is easy to understand the difference between substances containing different elements or the same elements in different proportions4 ,that a difference should exist in these cases necessarily follows1 if1 as our knowledge compels us1 we admit that there is a radical difference in the simple bodies or elements4 ,but when the composition--i4e4 the quality and quantity of the elements in two substances is the same and yet their properties are different1 then it becomes clear that the conceptions of diverse elements and of the varying composition of compounds1 alone1 are insufficient for the expression of all the diversity of properties of matter in nature4 ,something else1 still more profound and internal than the composition of substances1 must1 judging from isomerism1 determine the properties and transformation of substances4 ,on what are the isomerism of ozone and oxygen1 and the peculiarities of ozone1 dependent8 ,in what1 besides the extra store of energy1 which is one of the peculiarities of ozone1 resides the cause of its difference from oxygen8 ,these questions for long occupied the minds of investigators1 and were the motive for the most varied1 exact1 and accurate researches1 which were chiefly directed to the study of the volumetric relations exhibited by ozone4 ,in order to acquaint the reader with the previous researches of this kind1 ,i cite the following from a memoir by ,soret1 in the ',transactions of the ,french ,academy of ,sciences' for #ahff3 ',our present knowledge of the volumetric relations of ozone may be expressed in the following manner3 '#a4 ,ordinary oxygen in changing into ozone under the action of electricity shows a diminution in volume4 ,this was discovered by ,andrews and ,tait4 '#b4 ,in acting on ozonised oxygen with potassium iodide and other substances capable of being oxidised1 we destroy the ozone1 but the volume of the gas remains unchanged4 ,for the researches of ,andrews1 ,soret1 v4 ,babo1 and others showed that the proportion of ozonised oxygen absorbed by the potassium iodide is equal to the original contraction of volume of the oxygen--that is1 in the absorption of the ozone the volume of the gas remains unchanged4 ,from this it might be imagined that ozone1 so to say1 does not occupy any space--is indefinitely dense4 '#c4 ,by the action of heat ozonised oxygen increases in volume1 and is transformed into ordinary oxygen4 ,this increase in volume corresponds with the quantity of ozonised oxygen which is given up to the potassium iodide in its decomposition 7the same observers74 '#d4 ,these unquestionable experimental results lead to the conclusion that ozone is denser than oxygen1 and that in its oxidising action it gives off that portion of its substance to which is due its extra density distinguishing it from ordinary oxygen4' ,if we imagine 7says ,weltzien7 that n volumes of ozone consist of n volumes of oxygen combined with m volumes of the same substance1 and that ozone in oxidising gives up m volumes of oxygen and leaves n volumes of ordinary oxygen gas1 then all the above facts can be explained2 otherwise it must be supposed that ozone is infinitely dense4 ',in order to determine the density of ozone' 7we again cite ,soret7 'recourse cannot be had to the direct determination of the weight of a given volume of the gas1 because ozone cannot be obtained in a pure state4 ,it is always mixed with a very large quantity of oxygen4 ,it was necessary1 therefore1 to have recourse to such substances as would absorb ozone without absorbing oxygen and without destroying the ozone4 ,then the density might be deduced from the decrease of volume produced in the gas by the action of this solvent in comparison with the quantity of oxygen given up to potassium iodide4 ,advantage must also be taken of the determination of the increase of volume produced by the action of heat on ozone1 if the volume occupied by the ozone before heating be known4' ,soret found two such substances1 turpentine and oil of cinnamon4 ',ozone disappears in the presence of turpentine4 ,this is accompanied by the appearance of a dense vapour1 which fills a vessel of small capacity 7#jad litre7 to such an extent that it is impenetrable to direct solar-rays4 ,on leaving the vessel at rest1 it is observed that the cloud of vapour settles2 the clearing is first remarked at the upper portion of the vessel1 and the brilliant colours of the rainbow are seen on the edge of a cloud of vapour4' ,oil of cinnamon--that is1 the volatile or essential oil of the well-known spice1 cinnamon--gives under similar circumstances the same kind of vapours1 but they are much less voluminous4 ,on measuring the gaseous volume before and after the action of both volatile oils1 a considerable decrease is remarked4 ,on applying all the necessary corrections 7for the solubility of oxygen in the oily liquids named above1 for the tension of their vapour1 for the change of pressure1 c47 and making a series of comparative determinations1 ,soret obtained the following result3 two volumes of ozone capable of being dissolved1 when changed to ordinary 7by heating a wire to a red-heat by a galvanic current7 increase by one volume4 ,hence it is evident that in the formation of ozone three volumes of oxygen give two volumes of ozone--that is1 its density 7referred to hydrogen7 #bd4 ,the observations and determinations of ,soret showed that ozone is heavier than oxygen1 and even than carbonic anhydride 7because ozonised oxygen passes through fine orifices more slowly than oxygen and than its mixtures with carbonic anhydride71 although lighter than chlorine 7it flows more rapidly through such orifices than chlorine71 and they indicated that ozone is one and a half times denser than oxygen1 which may be expressed by designating a molecule of oxygen by ,o#b and of ozone by ,o#c1 and hence ozone ,,oo#b is comparable with compound substances#i formed by oxygen1 as for instance ,,co#b1 ,,so#b1 ,,no#b1 c4 ,this explains the chief differences between ozone and oxygen and the cause of the isomerism1 and at the same time leads one to expect#aj that ozone1 being a gas which is denser than oxygen1 would be liquefied much more easily4 ,this was actually shown to be the case in #ahhj1 by ,chappuis and ,hautefeuille in their researches on the physical properties of ozone4 ,its boiling point under a pressure of #gfj mm4 is about -#ajf1 and consequently compressed and refrigerated ozone when rapidly expanded forms drops1 i4e4 is liquefied4 ,liquid and compressed#aa ozone is blue4 ,in dissolving in water ozone partly passes into oxygen4 ,it explodes violently when suddenly compressed and heated1 changing into ordinary oxygen and evolving1 like all explosive substances1#ab that extra heat which distinguishes it from oxygen4 #i ,ozone is1 so to say1 an oxide of oxygen1 just as water is an oxide of hydrogen4 ,just as aqueous vapour is composed of two volumes of hydrogen and one volume of oxygen1 which on combining condense into two volumes of aqueous vapour1 so also two volumes of oxygen are combined in ozone with one volume of oxygen to give two volumes of ozone4 ,in the action of ozone on different substances it is only that additional portion of its molecule by which it differs from ordinary oxygen that combines with other bodies1 and that is why1 under these circumstances1 the volume of the ozonised oxygen does not change4 ,starting with two volumes of ozone1 one-third of its weight is parted with1 and two volumes of oxygen remain4 ,the above observations of ,soret on the capacity of turpentine for dissolving ozone1 together with ,schnbein's researches on the formation of ozone in the oxidation of turpentine and of similar volatile vegetable oils 7entering into the composition of perfumes71 also explain the action of this ethereal oil on a great many substances4 ,it is known that turpentine oil1 when mixed with many substances1 promotes their oxidation4 ,in this case it probably not only itself promotes the formation of ozone1 but also dissolves ozone from the atmosphere1 and thus acquires the property of oxidising many substances4 ,it bleaches linen and cork1 decolorises indigo1 promotes the oxidation and hardening of boiled linseed oil1 c4 ,these properties of turpentine oil are made use of in practice4 ,dirty linen and many stained materials are easily cleaned by turpentine1 not only because it dissolves the grease1 but also because it oxidises it4 ,the admixture of turpentine with drying 7boiled7 oil1 oil-colours1 and lacs aids their rapid drying because it attracts ozone4 ,various oils occurring in plants1 and entering into the composition of perfumes and certain scent extracts1 also act as oxidisers4 ,they act in the same manner as oil of turpentine and oil of cinnamon4 ,this perhaps explains the refreshing influence they have in scents and other similar preparations1 and also the salubrity of the air of pine forests4 ,water upon which a layer of turpentine oil has been poured acquires1 when left standing in the light1 the disinfecting and oxidising properties in general of ozonised turpentine 7is this due to the formation of ,h#b,o#b874 #aj ,the densest1 most complex1 and heaviest particles of matter should1 under equal conditions1 evidently be less capable of passing into a state of gaseous motion1 should sooner attain a liquid state1 and have a greater cohesive force4 #aa ,the blue colour proper to ozone may be seen through a tube one metre long1 filled with oxygen1 containing #aj p4c4 of ozone4 ,the density of liquid ozone has not1 so far as ,i am aware1 been determined4 #ab ,all explosive bodies and mixtures 7gunpowder1 detonating gas1 c47 evolve heat in exploding--that is1 the reactions which accompany explosions are exothermal4 ,in this manner ozone in decomposing evolves latent heat1 although generally heat is absorbed in decomposition4 ,this shows the meaning and cause of explosion4 ,thus1 judging by what has been said above1 ozone should he formed in nature not only in the many processes of oxidation which go on1 but also by the condensation of atmospheric oxygen4 ,the significance of ozone in nature has often arrested the attention of observers4 ,there is a series of ozonometrical observations which show the different amounts of ozone in the air at different localities1 at different times of the year1 and under different circumstances4 ,but the observations made in this direction cannot be considered as sufficiently exact1 because the methods in use for determining ozone were not quite accurate4 ,it is however indisputable#ac that the amount of ozone in the atmosphere is subject to variation2 that the air of dwellings contains no ozone 7it disappears in oxidising organic matter72 that the air of fields and forests always contains ozone1 or substances 7peroxide of hydrogen7 which act like it 7on iodised starch paper c47#ac bis2 that the amount of ozone increases after storms2 and that miasms1 c41 are destroyed by ozonising the atmosphere4 ,it easily oxidises organic substances1 and miasms are produced by organic substances and the germs of organisms1 all of which are easily changed and oxidised4 ,indeed1 many miasms--for instance1 the volatile substance of decomposing organisms--are clearly destroyed or changed not only by ozone1 but also by many other powerfully oxidising substances1 such as chlorine water1 potassium permanganate1 and the like4#ad ,all that is now known respecting the presence of ozone in the air may be summed up in the following words3 ,a small quantity of an oxidising substance1 resembling ozone in its reactions1 has undoubtedly been observed and determined in the atmosphere1 especially in fresh air1 for instance after a storm1 and it is very likely that this substance contains a mixture of such oxidising substances as ozone1 peroxide of hydrogen1 and the lower oxides of nitrogen 7especially nitrous acid and its ammonia salt7 produced from the elements of the atmosphere by oxidation and by the action of electrical discharges4 #ac ,in ,paris it has been found that the further from the centre of the town the greater the amount of ozone in the air4 ,the reason of this is evident3 in a city there are many conditions for the destruction of ozone4 ,this is why we distinguish country air as being fresh4 ,in spring the air contains more ozone than in autumn2 the air of fields more than the air of towns4 #ac bis ,the question of the presence of ozone in the air has not yet been fully elucidated1 as those reactions by which ozone is generally detected are also common to nitrous acid 7and its ammonia salt74 ,ilosvay de ,ilosva 7#ahhi71 in order to exclude the influence of such bodies1 passed air through a #dj per cent4 solution of caustic soda1 and then through a #bj per cent4 solution of sulphuric acid 7these solutions do not destroy ozone71 and tested the air thus purified for the presence of ozone4 ,as no ozone was then detected the author concludes that all the effects which were formerly ascribed to ozone should be referred to nitrous acid4 ,but this conclusion requires more careful verification1 since the researches of ,prof4 ,schnbein on the presence of peroxide of hydrogen in the atmosphere4 #ad ,the oxidising action of ozone may be taken advantage of for technical purposes2 for instance1 for destroying colouring matters4 ,it has even been employed for bleaching tissues and for the rapid preparation of vinegar1 although these methods have not yet received wide application4 ,thus in ozone we see 7#a7 the capacity of elements 7and it must be all the more marked in compounds7 of changing in properties without altering in composition2 this is termed isomerism2#ae 7#b7 the capacity of certain elements for condensing themselves into molecules of different densities2 this forms a special case of isomerism called polymerism2 7#c7 the capacity of oxygen for appearing in a still more active and energetic chemical state than that in which it occurs in ordinary gaseous oxygen2 and 7#d7 the formation of unstable equilibria1 or chemical states1 which are illustrated both by the ease with which ozone acts as an oxidiser and by its capacity for decomposing with explosion4#af #ae ,isomerism in elements is termed allotropism4 #af ,a number of substances resemble ozone in one or other of these respects4 ,thus cyanogen1 ,c#b,n#b1 nitrogen chloride1 c41 decompose with an explosion and evolution of heat4 ,nitrous anhydride1 ,n#b,o#c1 forms a blue liquid like ozone1 and in a number of cases oxidises like ozone4 ,hydrogen peroxide4--,many of those properties which we have seen in ozone belong also to a peculiar substance containing oxygen and hydrogen and called hydrogen peroxide or oxygenated water4 ,this substance was discovered in #ahah by ,thnard4 ,when heated it is decomposed into water and oxygen1 evolving as much oxygen as is contained in the water remaining after the decomposition4 ,that portion of oxygen by which hydrogen peroxide differs from water behaves in a number of cases just like the active oxygen in ozone1 which distinguishes it from ordinary oxygen4 ,in ,h#b,o#b1 and in ,o#c1 one atom of oxygen acts as a powerful oxidiser1 and on separating out it leaves ,h#b,o or ,o#b1 which do not act so energetically1 although they still contain oxygen4#ag ,both ,h#b,o#b and ,o#c contain the oxygen in a compressed state1 so to speak1 and when freed from pressure by the forces 7internal7 of the elements in another substance1 this oxygen is easily evolved1 and therefore acts as oxygen does at the moment of its liberation4 ,both substances in decomposing1 with the separation of a portion of their oxygen1 evolve heat1 whilst decomposition is usually accompanied by an absorption of heat4 #ag ,it is evident that there is a want of words here for distinguishing oxygen1 ,o1 as an ultimate element1 from oxygen1 ,o#b1 as a free element4 ,the latter should be termed oxygen gas1 did not custom and the length of the expression render it inconvenient4 ,hydrogen peroxide is formed under many circumstances by combustion and oxidation1 but in very limited quantities2 thus1 for instance1 it is sufficient to shake up zinc with sulphuric acid1 or even with water1 to observe the formation of a certain quantity of hydrogen peroxide in the water4#ah ,from this cause1 probably1 a series of diverse oxidation processes are accomplished in nature1 and according to ,prof4 ,schne of ,moscow1 hydrogen peroxide occurs in the atmosphere1 although in variable and small quantities1 and probably its formation is connected with ozone1 with which it has much in common4 ,the usual mode of the formation of hydrogen peroxide1 and the method by which it may be indirectly obtained1#ai is by the double decomposition of an acid and the peroxides of certain metals1 especially those of potassium1 calcium1 and barium4#bj ,we saw when speaking of ,oxygen 7,chap4 ,,iii47 that it is only necessary to heat the anhydrous oxide of barium to a red heat in a current of air or oxygen 7or1 better still1 to heat it with potassium chlorate1 and then to wash away the potassium chloride formed7 to obtain peroxide of barium4#ba ,barium peroxide gives hydrogen peroxide by the action of acids in the cold4#bb ,the process of decomposition is very clear in this case2 the hydrogen of the acid replaces the barium of the peroxide1 a barium salt of the acid being formed1 while the hydrogen peroxide formed in the reaction remains in solution4#bc #ah ,schnbein states that the formation of hydrogen peroxide is to be remarked in every oxidation in water or in the presence of aqueous vapour4 ,according to ,struve1 hydrogen peroxide is contained in snow and in rain-water1 and its formation1 together with ozone and ammonium nitrate1 is even probable in the processes of respiration and combustion4 ,a solution of tin in mercury1 or liquid tin amalgam1 when shaken up in water containing sulphuric acid1 produces hydrogen peroxide1 whilst iron under the same circumstances does not give rise to its formation4 ,the presence of small quantities of hydrogen peroxide in these and similar cases is recognised by many reactions4 ,amongst them1 its action on chromic acid in the presence of ether is very characteristic4 ,hydrogen peroxide converts the chromic acid into a higher oxide1 ,cr#b,o#g1 which is of a dark-blue colour and dissolves in ether4 ,this ethereal solution is to a certain degree stable1 and therefore the presence of hydrogen peroxide may be recognised by mixing the liquid to be tested with ether and adding several drops of a solution of chromic acid4 ,on shaking the mixture the ether dissolves the higher oxide of chromium which is formed1 and acquires a blue colour4 ,the formation of hydrogen peroxide in the combustion and oxidation of substances containing or evolving hydrogen must be understood in the light of the conception1 to be considered later1 of molecules occupying equal volumes in a gaseous state4 ,at the moment of its evolution a molecule ,h#b combines with a molecule ,o#b1 and gives ,h#b,o#b4 ,as this substance is unstable1 a large proportion of it is decomposed1 a small amount only remaining unchanged4 ,if it is obtained1 water is easily formed from it2 this reaction evolves heat1 and the reverse action is not very probable4 ,direct determinations show that the reaction ,h#b,o#b ,h#b,o ,o evolves #bb1jjj heat units4 ,from this it will be understood how easy is the decomposition of hydrogen peroxide1 as well as the fact that a number of substances which are not directly oxidised by oxygen are oxidised by hydrogen peroxide and by ozone1 which also evolves heat on decomposition4 ,such a representation of the origin of hydrogen peroxide has been developed by me since #ahgj4 ,recently 7#ahij7 ,traube has pronounced a similar opinion1 stating that ,zn under the action of water and air gives1 besides ,zn,h#b,o#b1 also ,h#b,o#b4 #ai ,the formation of hydrogen peroxide from barium peroxide by a method of double decomposition is an instance of a number of indirect methods of preparation4 ,a substance ,a does not combine with ,b1 but ,a ,b is obtained from ,a ,c in its action on ,b ,d 7see ,introduction7 when ,c ,d is formed4 ,water does not combine with oxygen1 but as a hydrate of acids it acts on the compound of oxygen with barium oxide1 because this oxide gives a salt with an acid anhydride2 or1 what is the same thing1 hydrogen with oxygen does not directly form hydrogen peroxide1 but when combined with a haloid 7for example1 chlorine71 under the action of barium peroxide1 ,ba,o#b1 it leads to the formation of a salt of barium and ,h#b,o#b4 ,it is to be remarked that the passage of barium oxide1 ,ba,o1 into the peroxide1 ,ba,o#b1 is accompanied by the evolution of #ab1ajj heat units per #af parts of oxygen by weight combined1 and the passage of ,h#b,o into the peroxide ,h#b,o#b does not proceed directly1 because it would be accompanied by the absorption of #bb1jjj units of heat by #af parts by weight of oxygen combined4 ,barium peroxide1 in acting on an acid1 evidently evolves less heat than the oxide1 and it is this difference of heat that is absorbed in the hydrogen peroxide4 ,its energy is obtained from that evolved in the formation of the salt of barium4 #bj ,peroxides of lead and manganese1 and other analogous peroxides 7see ,chap4 ,,iii41 ,note #i71 do not give hydrogen peroxide under these conditions1 but yield chlorine with hydrochloric acid4 #ba ,the impure barium peroxide obtained in this manner may be easily purified4 ,for this purpose it is dissolved in a dilute solution of nitric acid4 ,a certain quantity of an insoluble residue always remains1 from which the solution is separated by filtration4 ,the solution will contain not only the compound of the barium peroxide1 but also a compound of the barium oxide itself1 a certain quantity of which always remains uncombined with oxygen4 ,the acid compounds of the peroxide and oxide of barium are easily distinguishable by their stability4 ,the peroxide gives an unstable compound1 and the oxide a stable salt4 ,by adding an aqueous solution of barium oxide to the resultant solution1 the whole of the peroxide contained in the solution may be precipitated as a pure aqueous compound 7,kouriloff1 #ahhi1 obtained the same result by adding an excess of ,ba,o#b74 ,the first portions of the precipitate will consist of impurities--for instance1 oxide of iron4 ,the barium peroxide then separates out1 and is collected on a filter and washed2 it forms a substance having a definite composition1 ,ba,o#b1h;,h#b,o1 and is very pure4 ,pure hydrogen peroxide should always be prepared from such purified barium peroxide4 #bb ,in the cold1 strong sulphuric acid with barium peroxide gives ozone2 when diluted with a certain amount of water it gives oxygen 7see ,note #f71 and hydrogen peroxide is only obtained by the action of very weak sulphuric acid4 ,hydrochloric1 hydrofluoric1 carbonic1 and hydrosilicofluoric acids1 and others1 when diluted with water also give hydrogen peroxide with barium peroxide4 ,professor ,schne1 who very carefully investigated hydrogen peroxide1 showed that it is formed by the action of many of the above-mentioned acids on barium peroxide4 ,in preparing peroxide of hydrogen by means of sulphuric acid1 the solution must be kept cold4 ,a solution of maximum concentration may be obtained by successive treatments with sulphuric acid of increasing strength4 ,in this manner a solution containing #b to #c grams of pure peroxide in #ajj c4c4 of water may be obtained 7,v4 ,kouriloff74 #bc ,with the majority of acids1 that salt of barium which is formed remains in solution2 thus1 for instance1 by employing hydrochloric acid1 hydrogen peroxide and barium chloride remain in solution4 ,complicated processes would be required to obtain pure hydrogen peroxide from such a solution4 ,it is much more convenient to take advantage of the action of carbonic anhydride on the pure hydrate of barium peroxide4 ,for this purpose the hydrate is stirred up in water1 and a rapid stream of carbonic anhydride is passed through the water4 ,barium carbonate1 insoluble in water1 is formed1 and the hydrogen peroxide remains in solution1 so that it may be separated from the carbonate by filtering only4 ,on a large scale hydrofluosilicic acid is employed1 its barium salt being also insoluble in water4 ,the reaction is expressed by the equation ,ba,o#b ,h#b,,so#d ,h#b,o#b ,ba,s,o#d4 ,it is best to take a weak cold solution of sulphuric acid and to almost saturate it with barium peroxide1 so that a small excess of acid remains2 insoluble barium sulphate is formed4 ,a more or less dilute aqueous solution of hydrogen peroxide is obtained4 ,this solution may be concentrated in a vacuum over sulphuric acid4 ,in this way the water may even be entirely evaporated from the solution of the hydrogen peroxide2 only in this case it is necessary to work at a low temperature1 and not to keep the peroxide for long in the rarefied atmosphere1 as otherwise it decomposes4#bc bis ,a solution of peroxide of hydrogen 7mixed with the solution of a salt of sodium ,na,x7 is used for bleaching 7especially silk and wool7 on a large scale1 and is now usually prepared from peroxide of sodium ,na#b,o#b by the action of acids4 ,na#b,o#b #b;,,hx #b,na,x ,h#b,o#bbd4 #bc bis ,hydrogen peroxide may be extracted from very dilute solutions by means of ether1 which dissolves it1 and when mixed with it the hydrogen peroxide may even be distilled4 ,a solution of hydrogen peroxide in water may be strengthened by cooling it to a low temperature1 when the water crystallises out--that is1 is converted into ice--whilst the hydrogen peroxide remains in solution1 as it only freezes at very low temperatures4 ,it must be observed that hydrogen peroxide1 in a strong solution in a pure state1 is exceedingly unstable even at the ordinary temperature1 and therefore it must be preserved in vessels always kept cold1 as otherwise it evolves oxygen and forms water4 #bd ,peroxide of sodium 7,chap4 ,,xii41 ,note #di7 is prepared by burning sodium in dry air4 ,when pure1 hydrogen peroxide is a colourless liquid1 without smell1 and having a very unpleasant taste--such as belongs to the salts of many metals--the so-called 'metallic' taste4 ,water stored in zinc vessels has this taste1 which is probably due to its containing hydrogen peroxide4 ,the tension of the vapour of hydrogen peroxide is less than that of aqueous vapour2 this enables its solutions to be concentrated in a vacuum4 ,the specific gravity of anhydrous hydrogen peroxide is #adee4 ,hydrogen peroxide decomposes1 with the evolution of oxygen1 when heated even to #bj4 ,but the more dilute its aqueous solution the more stable it is4 ,very weak solutions may be distilled without decomposing the hydrogen peroxide4 ,it decolorises solutions of litmus and turmeric1 and acts in a similar manner on many colouring matters of organic origin 7for which reason it is employed for bleaching tissues74#bd bis #bd bis ,peroxide of hydrogen should apparently find an industrial application in the arts1 for instance1 7#a7 as a bleaching agent1 it having the important advantage over chloride of lime1 ,,so#b1 c41 of not acting upon the material under treatment4 ,it may be used for bleaching feathers1 hair1 silk1 wool1 wood1 c41 it also removes stains of all kinds1 such as wine1 ink1 and fruit stains2 7#b7 it destroys bacteria like ozone without having any injurious effect upon the human body4 ,it can also be used for washing all kinds of wounds1 for purifying the air in the sick room1 c41 and 7#c7 as a preserving agent for potted meats1 c4 ,many substances decompose hydrogen peroxide1 forming water and oxygen1 without apparently suffering any change4 ,in this case substances in a state of fine division show a much quicker action than compact masses1 from which it is evident that the action is here based on contact 7see ,introduction74 ,it is sufficient to bring hydrogen peroxide into contact with charcoal1 gold1 the peroxide of manganese or lead1 the alkalis1 metallic silver1 and platinum1 to bring about the above decomposition4#be ,besides which1 hydrogen peroxide forms water and parts with its oxygen with great ease to a number of substances which are capable of being oxidised or of combining with oxygen1 and in this respect is very like ozone and other powerful oxidisers4#bf ,to the class of contact phenomena1 which are so characteristic of hydrogen peroxide as a substance which is unstable and easily decomposable with the evolution of heat1 must be referred the following--that in the presence of many substances containing oxygen it evolves1 not only its own oxygen1 but also that of the substances which are brought into contact with it--that is1 it acts in a reducing manner4 ,it behaves thus with ozone1 the oxides of silver1 mercury1 gold and platinum1 and lead peroxide4 ,the oxygen in these substances is not stable1 and therefore the feeble influence of contact is enough to destroy its position4 ,hydrogen peroxide1 especially in a concentrated form1 in contact with these substances1 evolves an immense quantity of oxygen1 so that an explosion takes place and an exceedingly powerful evolution of heat is observed if hydrogen peroxide in a concentrated form be made to drop upon these substances in dry powder4 ,slow decomposition also proceeds in dilute solutions4#bg #be ,as the result of careful research1 certain of the catalytic or contact phenomena have been subjected to exact explanation1 which shows the participation of a substance present in the process of a reaction1 whilst1 however1 it does not alter the series of changes proceeding from mechanical actions only4 ,professor ,schne1 of the ,petroffsky ,academy1 has already explained a number of reactions of hydrogen peroxide which previously were not understood4 ,thus1 for instance1 he showed that with hydrogen peroxide1 alkalis give peroxides of the alkaline metals1 which combine with the remaining hydrogen peroxide1 forming unstable compounds which are easily decomposed1 and therefore alkalis evince a decomposing 7catalytic7 influence on solutions of hydrogen peroxide4 ,only acid solutions of hydrogen peroxide1 and then only dilute ones1 can be preserved well4 #bf ,hydrogen peroxide1 as a substance containing much oxygen 7namely1 #af parts to one part by weight of hydrogen71 exhibits many oxidising reactions4 ,thus1 it oxidises arsenic1 converts lime into calcium peroxide1 the oxides of zinc and copper into peroxides2 it parts with its oxygen to many sulphides1 converting them into sulphates1 c4 ,so1 for example1 it converts black lead sulphide1 ,pb,s1 into white lead sulphate1 ,pb,s,o#d1 copper sulphide into copper sulphate1 and so on4 ,the restoration of old oil paintings by hydrogen peroxide is based on this action4 ,oil colours are usually admixed with white lead1 and in many cases the colour of oil-paints becomes darker in process of time4 ,this is partly due to the sulphuretted hydrogen contained in the air1 which acts on white lead1 forming lead sulphide1 which is black4 ,the intermixture of the black colour darkens the rest4 ,in cleaning a picture with a solution of hydrogen peroxide1 the black lead sulphide is converted into white sulphate1 and the colours brighten owing to the disappearance of the black substance which previously darkened them4 ,hydrogen peroxide oxidises with particular energy substances containing hydrogen and capable of easily parting with it to oxidising substances4 ,thus it decomposes hydriodic acid1 setting the iodine free and converting the hydrogen it contains into water2 it also decomposes sulphuretted hydrogen in exactly the same manner1 setting the sulphur free4 ,starch paste with potassium iodide is not1 however1 directly coloured by peroxide of hydrogen in the entire absence of free acids2 but the addition of a small quantity of iron sulphate 7green vitriol7 or of lead acetate to the mixture is enough to entirely blacken the paste4 ,this is a very sensitive reagent 7test7 for peroxide of hydrogen1 like the test with chromic acid and ether 7see ,note #h74 #bg ,to explain the phenomenon1 an hypothesis has been put forward by ,brodie1 ,clausius1 and ,schnbein which supposes ordinary oxygen to be an electrically neutral substance1 composed1 so to speak1 of two electrically opposite kinds of oxygen--positive and negative4 ,it is supposed that hydrogen peroxide contains one kind of such polar oxygen1 whilst in the oxides of the above-named metals the oxygen is of opposite polarity4 ,it is supposed that in the oxides of the metals the oxygen is electro-negative1 and in hydrogen peroxide electro-positive1 and that on the mutual contact of these substances ordinary neutral oxygen is evolved as a consequence of the mutual attraction of the oxygens of opposite polarity4 ,brodie admits the polarity of oxygen in combination1 but not in an uncombined state1 whilst ,schnbein supposes uncombined oxygen to be polar also1 considering ozone as electro-negative oxygen4 ,the supposition that the oxygen of ozone is different from that of hydrogen peroxide is contradicted by the fact that in acting on barium peroxide strong sulphuric acid forms ozone1 and dilute acid forms hydrogen peroxide4 ,just as a whole series of metallic compounds1 and especially the oxides and their hydrates1 correspond with water1 so also there are many substances analogous to hydrogen peroxide4 ,thus1 for instance1 calcium peroxide is related to hydrogen peroxide in exactly the same way as calcium oxide or lime is related to water4 ,in both cases the hydrogen is replaced by a metal--namely1 by calcium4#bg bis ,but it is most important to remark that the nearest approach to the properties of hydrogen peroxide is afforded by a non-metallic element1 chlorine2 its action on colouring matters1 its capacity for oxidising1 and for evolving oxygen from many oxides1 is analogous to that exhibited by hydrogen peroxide4 ,even the very formation of chlorine is closely analogous to the formation of peroxide of hydrogen2 chlorine is obtained from manganese peroxide1 ,mn,o#b1 and hydrochloric acid1 ,h,cl1 and hydrogen peroxide from barium peroxide1 ,ba,o#b1 and the same acid4 ,the result in one case is essentially water1 chlorine1 and manganese chloride2 and in the other case barium chloride and hydrogen peroxide are produced4 ,hence water chlorine corresponds with hydrogen peroxide1 and the action of chlorine in the presence of water is analogous to the action of hydrogen peroxide4 ,this analogy between chlorine and hydrogen peroxide is expressed in the conception of an aqueous radicle1 which 7,chapter ,,iii47 has been already mentioned4 ,this aqueous radicle 7or hydroxyl7 is that which is left from water if it be imagined as deprived of half of its hydrogen4 ,according to this method of expression1 caustic soda will be a compound of sodium with the aqueous radicle1 because it is formed from water with the evolution of half the hydrogen4 ,this is expressed by the following formul3 water1 ,h#b,o1 caustic soda1 ,na,h,o1 just as hydrochloric acid is ,h,cl and sodium chloride ,na,cl4 ,hence the aqueous radicle ,,ho is a compound radicle1 just as chlorine1 ,cl1 is a simple radicle4 ,they both give hydrogen compounds1 ,,hho1 water1 and ,h,cl1 hydrochloric acid2 sodium compounds1 ,na,h,o and ,na,cl1 and a whole series of analogous compounds4 ,free chlorine in this sense will be ,cl,cl1 and hydrogen peroxide ,,hoho1 which indeed expresses its composition1 because it contains twice as much oxygen as water does4#bh #bg bis ,it should be mentioned that ,schiloff 7#ahic7 on taking a #c per cent4 solution of ,h#b,o#b1 adding soda to it1 and then extracting the peroxide of hydrogen from the mixture by shaking it with ether1 obtained a #ej per cent4 solution of ,h#b,o#b1 which1 although perfectly free from other acids1 gave a distinctly acid reaction with litmus4 ,and here attention should first of all be turned to the fact that the peroxides of the metals correspond to ,h#b,o#b1 like salts to an acid1 for instance1 ,na#b,o#b and ,ba,o#b1 c4 ,furthermore1 it must be remembered that ,o is an analogue of ,s 7,chapters ,,xv4 and ,,xx471 and sulphur gives ,h#b,s1 ,h#b,,so#c1 and ,h#b,,so#d4 ,and sulphurous acid1 ,h#b,,so#c1 is unstable as a hydrate1 and gives water and the anhydride ,,so#b4 ,if the sulphur be replaced by oxygen1 then instead of ,h#b,,so#c and ,,so#b1 we have ,h#b,,oo#c and ,,oo#b4 ,the latter is ozone1 while the salt ,k#b,o#d 7peroxide of potassium7 corresponds to the hydrate ,h#b,o#d as to an acid4 ,and between ,h#b,o and ,h#b,o#d there may exist intermediate acid compounds1 the first of which would be ,h#b,o#b1 in which1 from analogy to the sulphur compounds1 one would expect acid properties4 ,besides which we may mention that for sulphur1 besides ,h#b,s 7which is a feeble acid71 ,h#b,s#b1 ,h#b,s#c1 ,h#b,s#e are known4 ,thus in many respects ,h#b,o#b offers points of resemblance to acid compounds1 and as regards its qualitative 7reactive7 analogies1 it not only resembles ,na#b,o#b1 ,ba,o#b1 c41 but also persulphuric acid ,,hso#d 7to which the anhydride ,s#b,o#g corresponds7 and ,cu#b,o#g1 c41 which will be subsequently described4 #bh ,tamman and ,carrara 7#ahib7 showed by determining the depression 7fall of the temperature of the formation of ice1 ,chapters ,i4 and ,,vii47 that the molecule of peroxide of hydrogen contains ,h#b,o#b1 and not ,,ho or ,h#c,o#c4 ,thus in ozone and hydrogen peroxide we see examples of very unstable1 easily decomposable 7by time1 spontaneously1 and on contact7 substances1 full of the energy necessary for change1#bh bis capable of being easily reconstituted 7in this case decomposing with the evolution of heat72 they are therefore examples of unstable chemical equilibria4 ,if a substance exists1 it signifies that it already presents a certain form of equilibrium between those elements of which it is built up4 ,but chemical1 like mechanical1 equilibria exhibit different degrees of stability or solidity4#bi #bh bis ,the lower oxides of nitrogen and chlorine and the higher oxides of manganese are also formed with the absorption of heat1 and therefore1 like hydrogen peroxide1 act in a powerfully oxidising manner1 and are not formed by the same methods as the majority of other oxides4 ,it is evident that1 being endowed with a richer store of energy 7acquired in combination or by absorption of heat71 such substances1 compared with others poorer in energy1 will exhibit a greater diversity of cases of chemical action with other substances4 #bi ,if the point of support of a body lies in a vertical line below the centre of gravity1 it is in unstable equilibrium4 ,if the centre of gravity lies below the point of support2 the state of equilibrium is very stable1 and a vibration may take place about this position of stable equilibrium1 as in a pendulum or balance1 when finally the body assumes a position of stable equilibrium4 ,but if1 keeping to the same mechanical example1 the body be supported not on a point1 in the geometrical sense of the word1 but on a small plane1 then the state of unstable equilibrium may be preserved1 unless destroyed by external influences4 ,thus a man stands upright supported on the plane1 or several points of the surfaces of his feet1 having the centre of gravity above the points of support4 ,vibration is then possible1 but it is limited1 otherwise on passing outside the limit of possible equilibrium another more stable position is attained about which vibration becomes more possible4 ,a prism immersed in water may have several more or less stable positions of equilibrium4 ,the same is also true with the atoms in molecules4 ,some molecules present a state of more stable equilibrium than others4 ,hence from this simple comparison it will be at once evident that the stability of molecules may vary considerably1 that one and the same elements1 taken in the same number1 may give isomerides of different stability1 and1 lastly1 that there may exist states of equilibria which are so unstable1 so ephemeral1 that they will only arise under particularly special conditions--such1 for example1 as certain hydrates mentioned in the first chapter 7see ,notes #eg1 #fg1 and others74 ,and if in one case the instability of a given state of equilibrium is expressed by its instability with a change of temperature or physical state1 then in other cases it is expressed by the facility with which it decomposes under the influence of contact or of the chemical influence of other substances4 ,besides this1 hydrogen peroxide presents another side of the subject which is not less important1 and is much clearer and more general4 ,hydrogen unites with oxygen in two degrees of oxidation3 water or hydrogen oxide1 and oxygenated water or hydrogen peroxide2 for a given quantity of hydrogen1 the peroxide contains twice as much oxygen as does water4 ,this is a fresh example confirming the correctness of the law of multiple proportions1 to which we have already referred in speaking of the water of crystallisation of salts4 ,we can now formulate this law--the law of multiple proportions4 ,if two substances ,a and ,b 7either simple or compound71 unite together to form several compounds1 ,an,bm1 ,aq,br 4441 then having expressed the compositions of all these compounds in such a way that the quantity 7by weight or volume7 of one of the component parts will be a constant quantity ,a1 it will be observed that in all the compounds ,,aba1 ,,abb 444 the quantities of the other component part1 ,b1 will always be in commensurable relation3 generally in simple multiple proportion--that is1 that a 3 b 4441 or m_/n is to r_/q as whole numbers1 for instance as #b 3 #c or #c 3 #d4444 ,the analysis of water shows that in #ajj parts by weight it contains #aaaab parts by weight of hydrogen and #hhhhh of oxygen1 and the analysis of peroxide of hydrogen shows that it contains #idaab parts of oxygen to #ehhh parts of hydrogen4 ,in this the analysis is expressed1 as analyses generally are1 in percentages2 that is1 it gives the amounts of the elements in a hundred parts by weight of the substance4 ,the direct comparison of the percentage compositions of water and hydrogen peroxide does not give any simple relation4 ,but such a relation is immediately apparent if we calculate the composition of water and of hydrogen peroxide1 having taken either the quantity of oxygen or the quantity of hydrogen as a constant quantity--for instance1 as unity4 ,the most simple proportions show that in water there are contained eight parts of oxygen to one part of hydrogen1 and in hydrogen peroxide sixteen parts of oxygen to one part of hydrogen2 or one-eighth part of hydrogen in water and one-sixteenth part of hydrogen in hydrogen peroxide to one part of oxygen4 ,naturally1 the analysis does not give these figures with absolute exactness--it gives them within a certain degree of error--but they approximate1 as the error diminishes1 to that limit which is here given4 ,the comparison of the quantities of hydrogen and oxygen in the two substances above named1 taking one of the components as a constant quantity1 gives an example of the application of the law of multiple proportions1 because water contains eight parts and hydrogen peroxide sixteen parts of oxygen to one part of hydrogen1 and these figures are commensurable and are in the simple proportion of #a 3 #b4 ,an exactly similar multiple proportion is observed in the composition of all other well-investigated definite chemical compounds1#cj and therefore the law of multiple proportions is accepted in chemistry as the starting point from which other considerations proceed4 #cj ,when1 for example1 any element forms several oxides1 they are subject to the law of multiple proportions4 ,for a given quantity of the non-metal or metal the quantities of oxygen in the different degrees of oxidation will stand as #a 3 #b1 or as #a 3 #c1 or as #b 3 #c1 or as #b 3 #g1 and so on4 ,thus1 for instance1 copper combines with oxygen in at least two proportions1 forming the oxides found in nature1 and called the suboxide and the oxide of copper1 ,cu#b,o and ,cu,o2 the oxide contains twice as much oxygen as the suboxide4 ,lead also presents two degrees of oxidation1 the oxide and peroxide1 and in the latter there is twice as much oxygen as in the former1 ,pb,o and ,pb,o#b4 ,when a base and an acid are capable of forming several kinds of salts1 normal1 acid1 basic1 and anhydro-1 it is found that they also clearly exemplify the law of multiple proportions4 ,this was demonstrated by ,wollaston soon after the discovery of the law in question4 ,we saw in the first chapter that salts show different degrees of combination with water of crystallisation1 and that they obey the law of multiple proportions4 ,and1 more than this1 the indefinite chemical compounds existing as solutions may1 as we saw in the same chapter1 be brought under the law of multiple proportions by the hypothesis that solutions are unstable hydrates formed according to the law of multiple proportions1 but occurring in a state of dissociation4 ,by means of this hypothesis the law of multiple proportions becomes still more general1 and all the aspects of chemical compounds are subject to it4 ,the direction of the whole contemporary state of chemistry was determined by the discoveries of ,lavoisier and ,dalton4 ,by endeavouring to prove that in solutions we have nothing else than the liquid products of the dissociation of definite hydrates1 it is my aim to bring also this category of indefinite compounds under the general principle enunciated by ,dalton2 just as astronomers have discovered a proof and not a negation of the laws of ,newton in perturbations4 ,the law of multiple proportions was discovered at the beginning of this century by ,john ,dalton1 of ,manchester1 in investigating the compounds of carbon with hydrogen4 ,it appeared that two gaseous compounds of these substances--marsh gas1 ,,ch#d1 and olefiant gas1 ,c#b;,h#d1 contain for one and the same quantity of hydrogen1 quantities of carbon which stand in multiple proportion2 namely1 marsh gas contains relatively half as much carbon as olefiant gas4 ,although the analysis of that time was not exact1 still the accuracy of this law1 recognised by ,dalton1 was further confirmed by more accurate investigations4 ,on establishing the law of multiple proportions1 ,dalton gave a hypothetical explanation for it4 ,this explanation is based on the atomic theory of matter4 ,in fact1 the law of multiple proportions may be very easily understood by admitting the atomic structure of matter4 ,the essence of the atomic theory is that matter is supposed to consist of an agglomeration of small and indivisible parts--atoms--which do not fill up the whole space occupied by a substance1 but stand apart from each other1 as the sun1 planets1 and stars do not fill up the whole space of the universe1 but are at a distance from each other4 ,the form and properties of substances are determined by the position of their atoms in space and by their state of motion1 whilst the reactions accomplished by substances are understood as redistributions of the relative positions of atoms and changes in their motion4 ,the atomic representation of matter arose in very ancient times1#ca and up to recent times was at variance with the dynamical hypothesis1 which considers matter as only a manifestation of forces4 ,at the present time1 however1 the majority of scientific men uphold the atomic hypothesis1 although the present conception of an atom is quite different from that of the ancient philosophers4 ,an atom at the present day is regarded rather as an individual or unit which is indivisible by physical#cb and chemical forces1 whilst the atom of the ancients was actually mechanically and geometrically indivisible4 ,when ,dalton 7#ahjd7 discovered the law of multiple proportions1 he pronounced himself in favour of the atomic doctrine1 because it enables this law to be very easily understood4 ,if the divisibility of every element has a limit1 namely the atom1 then the atoms of elements are the extreme limits of all divisibility1 and if they differ from each other in their nature1 the formation of a compound from elementary matter must consist in the aggregation of several different atoms into one whole or system of atoms1 now termed particles or molecules4 ,as atoms can only combine in their entire masses1 it is evident that not only the law of definite composition1 but also that of multiple proportions1 must apply to the combination of atoms with one another2 for one atom of a substance can combine with one1 two1 or three atoms of another substance1 or in general one1 two1 three atoms of one substance are able to combine with one1 two1 or three atoms of another2 this being the essence of the law of multiple proportions4 ,chemical and physical data are very well explained by the aid of the atomic theory4 ,the displacement of one element by another follows the law of equivalency4 ,in this case one or several atoms of a given element take the place of one or several atoms of another element in its compounds4 ,the atoms of different substances can be mixed together in the same sense as sand can be mixed with clay4 ,they do not unite into one whole--i4e4 there is not a perfect blending in the one or other case1 but only a juxtaposition1 a homogeneous whole being formed from individual parts4 ,this is the first and most simple method of applying the atomic theory to the explanation of chemical phenomena4#cc #ca ,leucippus1 ,democritus1 and especially ,lucretius1 in the classical ages1 represented matter as made up of atoms--that is1 of parts incapable of further division4 ,the geometrical impossibility of such an admission1 as well as the conclusions which were deduced by the ancient atomists from their fundamental propositions1 prevented other philosophers from following them1 and the atomic doctrine1 like very many others1 lived1 without being ratified by fact1 in the imaginations of its followers4 ,between the present atomic theory and the doctrine of the above-named ancient philosophers there is naturally a remote historical connection1 as between the doctrine of ,pythagoras and ,copernicus1 but they are essentially different4 ,for us the atom is indivisible1 not in the geometrical abstract sense1 but only in a physical and chemical sense4 ,it would be better to call the atoms indivisible individuals4 ,the ,greek atom the ,latin individual1 both according to the etymology and original sense of the words1 but in course of time these two words have acquired a different meaning4 ,the individual is mechanically and geometrically divisible1 and only indivisible in a special sense4 ,the earth1 the sun1 a man or a fly are individuals1 although geometrically divisible4 ,thus the 'atoms' of contemporary science1 indivisible in a chemical sense1 form those units with which we are concerned in the investigation of the natural phenomena of matter1 just as a man is an indivisible unit in the investigation of social relations1 or as the stars1 planets1 and luminaries serve as units in astronomy4 ,the formation of the vortex hypothesis1 in which1 as we shall afterwards see1 atoms are entire whirls mechanically complex1 although physico-chemically indivisible1 clearly shows that the scientific men of our time in holding to the atomic theory have only borrowed the word and form of expression from the ancient philosophers1 and not the essence of their atomic doctrine4 ,it is erroneous to imagine that the contemporary conceptions of the atomists are nothing but the repetition of the metaphysical reasonings of the ancients4 ,to show the true meaning of the atomism of the ancient philosophers1 and the profound difference between their points of argument and those of contemporary men of science1 ,i cite the following fundamental propositions of ,democritus 7,b4,c4 #dgj-chj7 as the best expounder of the atomic doctrine of the ancients3--7#a7 ,nothing can proceed from nothing1 nothing that exists can disappear or be destroyed 7and hence matter71 and every change only consists of a combination or separation4 7#b7 ,nothing is accidental1 there is a reason and necessity for everything4 7#c7 ,all except atoms and vacua is reason and not existence4 7#d7 ,the atoms1 which are infinite in number and form1 constitute the visible universe by their motion1 impact1 and consequent revolving motion4 7#e7 ,the variety of objects depends only upon a difference in the number1 form1 and order of the atoms of which they are formed1 and not upon a qualitative difference of their atoms1 which only act upon each other by pressure and impact4 7#f7 ,the spirit1 like fire1 consists of minute1 spherical1 smooth1 and very mobile and all-penetrating atoms1 whose motion forms the phenomenon of life4 ,these ,democritian1 chiefly metaphysical1 principles of atomism are so essentially different from the principles of the present atomic doctrine1 which is exclusively applied to explaining the phenomena of the external world1 that it may be useful to mention the essence of the atomic propositions of ,boscovitch1 a ,slav who lived in the middle of the eighteenth century1 and who is regarded as the founder of the modern atomic doctrines which1 however1 did not take hold upon the minds of scientific men1 and were rarely applied prior to ,dalton--i4e4 until the beginning of the nineteenth century4 ,the doctrine of ,boscovitch was enunciated by him in #ageh-agfd in his ',philosophi naturalis theoria reducta ad unicam legem virium in natura existentium4' ,boscovitch considers matter to be composed of atoms1 and the atoms to be the points or centres of forces 7just as the stars and planets may be considered as points of space71 acting between bodies and their parts4 ,these forces vary with the distance1 so that beyond a certain very small distance all atoms1 and hence also their aggregates1 are attracted according to ,newton's law1 but at less distances1 there alternate wave-like spheres of gradually decreasing attraction and increasing 7as the distance decreases7 repulsion1 until at last at a minimum distance only the repellent action remains4 ,atoms1 therefore1 cannot merge into each other4 ,consequently1 the atoms are held at a certain distance from each other1 and therefore occupy space4 ,boscovitch compares the sphere of repulsion surrounding the atoms to the spheres of action of firing of a detachment of soldiers4 ,according to his doctrine1 atoms are indestructible1 do not merge into each other1 have mass1 are everlasting and mobile under the action of the forces proper to them4 ,maxwell rightly calls this hypothesis the 'extreme' among those existing to explain matter1 but many aspects of ,boscovitch's doctrine repeat themselves in the views of our day1 with this essential difference1 that instead of a mathematical point furnished with the properties of mass1 the atoms are endowed with a corporality1 just as the stars and planets are corporal1 although in certain aspects of their interaction they may be regarded as mathematical points4 ,in my opinion1 the atomism of our day must first of all be regarded merely as a convenient method for the investigation of ponderable matter4 ,as a geometrician in reasoning about curves represents them as formed of a succession of right lines1 because such a method enables him to analyse the subject under investigation1 so the scientific man applies the atomic theory as a method of analysing the phenomena of nature4 ,naturally there are people now1 as in ancient times1 and as there always will be1 who apply reality to imagination1 and therefore there are to be found atomists of extreme views2 but it is not in their spirit that we should acknowledge the great services rendered by the atomic doctrine to all science1 which1 while it has been essentially independently developed1 is1 if it be desired to reduce all ideas to the doctrines of the ancients1 a union of the ancient dynamical and atomic doctrines4 #cb ,dalton and many of his successors distinguished the atoms of elements and compounds1 in which they clearly symbolised the difference of their opinion from the representations of the ancients4 ,now only the individuals of the elements1 indivisible by physical and chemical forces1 are termed atoms1 and the individuals of compounds indivisible under physical changes are termed molecules2 these are divisible into atoms by chemical forces4 #cc ,in the present condition of science1 either the atomic or the dynamical hypothesis is inevitably obliged to admit the existence of an invisible and imperceptible motion in matter1 without which it is impossible to understand either light or heat1 or gaseous pressure1 or any of the mechanical1 physical1 or chemical phenomena4 ,the ancients saw vital motion in animals only1 but to us the smallest particle of matter1 endued with vis viva1 or energy in some degree or other1 is incomprehensible without self-existent motion4 ,thus motion has become a conception inseparably knit with the conception of matter1 and this has prepared the ground for the revival of the dynamical hypothesis of the constitution of matter4 ,in the atomic theory there has arisen that generalising idea by which the world of atoms is constructed1 like the universe of heavenly bodies1 with its suns1 planets1 and meteors1 endued with everlasting force of motion1 forming molecules as the heavenly bodies form systems1 like the solar system1 which molecules are only relatively indivisible in the same way as the planets of the solar system are inseparable1 and stable and lasting as the solar system is lasting4 ,such a representation1 without necessitating the absolute indivisibility of atoms1 expresses all that science can require for an hypothetical representation of the constitution of matter4 ,in closer proximity to the dynamical hypothesis of the constitution of matter is the oft-times revived vortex hypothesis4 ,descartes first endeavoured to raise it2 ,helmholtz and ,thomson 7,lord ,kelvin7 gave it a fuller and more modern form2 many scientific men applied it to physics and chemistry4 ,the idea of vortex rings serves as the starting point of this hypothesis2 these are familiar to all as the rings of tobacco smoke1 and may be artificially obtained by giving a sharp blow to the sides of a cardboard box having a circular orifice and filled with smoke4 ,phosphuretted hydrogen1 as we shall see later on1 when bubbling from water always gives very perfect vortex rings in a still atmosphere4 ,in such rings it is easy to observe a constant circular motion about their axes1 and to notice the stability the rings possess in their motion of translation4 ,this unchangeable mass1 endued with a rapid internal motion1 is likened to the atom4 ,in a medium deprived of friction1 such a ring1 as is shown by theoretical considerations of the subject from a mechanical point of view1 would be perpetual and unchangeable4 ,the rings are capable of grouping together1 and in combining1 without being absolutely indivisible1 remain indivisible4 ,the vortex hypothesis has been established in our times1 but it has not been fully developed2 its application to chemical phenomena is not clear1 although not impossible2 it does not satisfy a doubt in respect to the nature of the space existing between the rings 7just as it is not clear what exists between atoms1 and between the planets71 neither does it tell us what is the nature of the moving substance of the ring1 and therefore for the present it only presents the germ of an hypothetical conception of the constitution of matter2 consequently1 ,i consider that it would be superfluous to speak of it in greater detail4 ,however1 the thoughts of investigators are now 7and naturally will be in the future71 as they were in the time of ,dalton1 often turned to the question of the limitation of the mechanical division of matter1 and the atomists have searched for an answer in the most diverse spheres of nature4 ,i select one of the methods attempted1 which does not in any way refer to chemistry1 in order to show how closely all the provinces of natural science are bound together4 ,wollaston proposed the investigation of the atmosphere of the heavenly bodies as a means for confirming the existence of atoms4 ,if the divisibility of matter be infinite1 then air must extend throughout the entire space of the heavens as it extends all over the earth by its elasticity and diffusion4 ,if the infinite divisibility of matter be admitted1 it is impossible that any portion of the whole space of the universe can be entirely void of the component parts of our atmosphere4 ,but if matter be divisible up to a certain limit only--namely1 up to the atom--then there can exist a heavenly body void of an atmosphere2 and if such a body be discovered1 it would serve as an important factor for the acceptation of the validity of the atomic doctrine4 ,the moon has long been considered as such a luminary and this circumstance1 especially from its proximity to the earth1 has been cited as the best proof of the validity of the atomic doctrine4 ,this proof is apparently 7,poisson7 deprived of some of its force from the possibility of the transformation of the component parts of our atmosphere into a solid or liquid state at immense heights above the earth's surface1 where the temperature is exceedingly low2 but a series of researches 7,pouillet7 has shown that the temperature of the heavenly space is comparatively not so very low1 and is attainable by experimental means1 so that at the low existing pressure the liquefaction of the gases of the atmosphere cannot he expected even on the moon4 ,therefore the absence of an atmosphere about the moon1 if it were not subject to doubt1 would be counted as a forcible proof of the atomic theory4 ,as a proof of the absence of a lunar atmosphere1 it is cited that the moon1 in its independent motion between the stars1 when eclipsing a star--that is1 when passing between the eye and the star--does not show any signs of refraction at its edge2 the image of the star does not alter its position in the heavens on approaching the moon's surface1 consequently there is no atmosphere on the moon's surface capable of refracting the rays of light4 ,such is the conclusion by which the absence of a lunar atmosphere is acknowledged4 ,but this conclusion is most feeble1 and there are even facts in exact contradiction to it1 by which the existence of a lunar atmosphere may be proved4 ,the entire surface of the moon is covered with a number of mountains1 having in the majority of cases the conical form natural to volcanoes4 ,the volcanic character of the lunar mountains was confirmed in ,october #ahff1 when a change was observed in the form of one of them 7the crater ,linnea74 ,these mountains must be on the edge of the lunar disc4 ,seen in profile1 they screen one another and interfere with observations on the surface of the moon1 so that when looking at the edge of the lunar disc we are obliged to make our observations not on the moon's surface1 but at the summits of the lunar mountains4 ,these mountains are higher than those on our earth1 and consequently at their summits the lunar atmosphere must he exceedingly rarefied even if it possess an observable density at the surface4 ,knowing the mass of the moon to be eighty-two times less than the mass of the earth1 we are able to determine approximately that our atmosphere at the moon's surface would be about twenty-eight times lighter than it is on the earth1 and consequently at the very surface of the moon the refraction of light by the lunar atmosphere must he very slight1 and at the heights of the lunar mountains it must be imperceptible1 and would be lost within the limits of experimental error4 ,therefore the absence of refraction of light at the edge of the moon's disc cannot yet be urged in favour of the absence of a lunar atmosphere4 ,there is even a series of observations obliging us to admit the existence of this atmosphere4 ,these researches are due to ,sir ,john ,herschel4 ,this is what he writes3 ',it has often been remarked that during the eclipse of a star by the moon there occurs a peculiar optical illusion2 it seems as if the star before disappearing passed over the edge of the moon and is seen through the lunar disc1 sometimes for a rather long period of time4 ,i myself have observed this phenomenon1 and it has been witnessed by perfectly trustworthy observers4 ,i ascribe it to optical illusion1 but it must be admitted that the star might have been seen on the lunar disc through some deep ravine on the moon4' ,geniller1 in ,belgium 7#ahef71 following the opinion of ,cassini1 ,eiler1 and others1 gave an explanation of this phenomenon3 he considers it due to the refraction of light in the valleys of the lunar mountains which occur on the edge of the lunar disc4 ,in fact1 although these valleys do not probably present the form of straight ravines1 yet it may sometimes happen that the light of a star is so refracted that its image might he seen1 notwithstanding the absence of a direct path for the light-rays4 ,he then goes on to remark that the density of the lunar atmosphere must be variable in different parts1 owing to the very long nights on the moon4 ,on the dark1 or non-illuminated portion1 owing to these long nights1 which last thirteen of our days and nights1 there must be excessive cold1 and hence a denser atmosphere1 while1 on the contrary1 on the illuminated portion the atmosphere must be much more rarefied4 ,this variation in the temperature of the different parts of the moon's surface explains also the absence of clouds1 notwithstanding the possible presence of air and aqueous vapour1 on the visible portion of the moon4 ,the presence of an atmosphere round the sun and planets1 judging from astronomical observations1 may be considered as fully proved4 ,on ,jupiter and ,mars even bands of clouds may be distinguished4 ,thus the atomic doctrine1 admitting a finite mechanical divisibility only1 must he1 as yet at least1 only accepted as a means1 similar to that means which a mathematician employs when he breaks up a continuous curvilinear line into a number of straight lines4 ,there is a simplicity of representation in atoms1 but there is no absolute necessity to have recourse to them4 ,the conception of the individuality of the parts of matter exhibited in chemical elements only is necessary and trustworthy4 ,a certain number of atoms n of an element ,a in combining with several atoms m of another element ,b give a compound ,an ,bm1 each molecule of which will contain the atoms of the elements ,a and ,b in this ratio1 and therefore the compound will present a definite composition1 expressed by the formula ,an,bm1 where ,a and ,b are the weights of the atoms and n and m their relative number4 ,if the same elements ,a and ,b1 in addition to ,an,bm1 also yield another compound ,ar,bq1 then by expressing the composition of the first compound by ,anr,bmr 7and this is the same composition as ,an,bm71 and of the second compound by ,arn,bqn1 we have the law of multiple proportions1 because for a given quantity of the first element1 ,arn1 there occur quantities of the second element bearing the same ratio to each other as mr is to qn2 and as m1 r1 q1 and n are whole numbers1 their products are also whole numbers1 and this is expressed by the law of multiple proportion4 ,consequently the atomic theory is in accordance with and evokes the first laws of definite chemical compounds3 the law of definite composition and the law of multiple proportions4 ,so1 also1 is the relation of the atomic theory to the third law of definite chemical compounds1 the law of reciprocal combining weights1 which is as follows3--,if a certain weight of a substance ,c combine with a weight a of a substance ,a1 and with a weight b of a substance ,b1 then1 also1 the substances ,a and ,b will combine together in quantities a and b 7or in multiples of them74 ,this should be the case from the conception of atoms4 ,let ,a1 ,b1 and ,c be the weights of the atoms of the three substances1 and for simplicity of reasoning suppose that combination takes place between single atoms4 ,it is evident that if the substance gives ,,ac and ,,bc1 then the substances ,a and ,b will give a compound ,,ab1 or their multiple1 ,an,bm4 ,and so it is in reality in nature4 ,sulphur combines with hydrogen and with oxygen4 ,sulphuretted hydrogen contains thirty-two parts by weight of sulphur to two parts by weight of hydrogen2 this is expressed by the formula ,h#b,s4 ,sulphur dioxide1 ,,so#b1 contains thirty-two parts of sulphur and thirty-two parts of oxygen1 and therefore we conclude1 from the law of combining weights1 that oxygen and hydrogen will combine in the proportion of two parts of hydrogen and thirty-two parts of oxygen1 or multiple numbers of them4 ,and we have seen this to be the case4 ,hydrogen peroxide contains thirty-two parts of oxygen1 and water sixteen parts1 to two parts of hydrogen2 and so it is in all other cases4 ,this consequence of the atomic theory is in accordance with nature1 with the results of analysis1 and is one of the most important laws of chemistry4 ,it is a law1 because it indicates the relation between the weights of substances entering into chemical combination4 ,further1 it is an eminently exact law1 and not an approximate one4 ,the law of combining weights is a law of nature1 and by no means an hypothesis1 for even if the entire theory of atoms be refuted1 still the laws of multiple proportions and of combining weights will remain1 inasmuch as they deal with facts4 ,they may be guessed at from the sense of the atomic theory1 and historically the law of combining weights is intimately connected with this theory2 but they are not identical1 but only connected1 with it4 ,the law of combining weights is formulated with great ease1 and is an immediate consequence of the atomic theory2 without it1 it is even difficult to understand4 ,data for its evolution existed previously1 but it was not formulated until those data were interpreted by the atomic theory1 an hypothesis which up to the present time has contradicted neither experiment nor fact1 and is useful and of general application4 ,such is the nature of hypotheses4 ,they are indispensable to science2 they bestow an order and simplicity which are difficultly attainable without their aid4 ,the whole history of science is a proof of this4 ,and therefore it may be truly said that it is better to hold to an hypothesis which may afterwards prove untrue than to have none at all4 ,hypotheses facilitate scientific work and render it consistent4 ,in the search for truth1 like the plough of the husbandman1 they help forward the work of the labourer4 ,,chapter ,v ,,nitrogen ,,and ,,air ,gaseous nitrogen forms about four-fifths 7by volume7 of the atmosphere2 consequently the air contains an exceedingly large mass of it4 ,whilst entering in so considerable a quantity into the composition of air1 nitrogen does not seem to play any active part in the atmosphere1 the chemical action of which is mainly dependent on the oxygen it contains4 ,but this is not an entirely correct idea1 because animal life cannot exist in pure oxygen1 in which animals pass into an abnormal state and die2 and the nitrogen of the air1 although slowly1 forms diverse compounds1 many of which play a most important part in nature1 especially in the life of organisms4 ,however1 neither plants#a nor animals directly absorb the nitrogen of the air1 but take it up from already prepared nitrogenous compounds2 further1 plants are nourished by the nitrogenous substances contained in the soil and water1 and animals by the nitrogenous substances contained in plants and in other animals4 ,atmospheric electricity is capable of aiding the passage of gaseous nitrogen into nitrogenous compounds1 as we shall afterwards see1 and the resultant substances are carried to the soil by rain1 where they serve for the nourishment of plants4 ,plentiful harvests1 fine crops of hay1 vigorous growth of trees--other conditions being equal--are only obtained when the soil contains ready prepared nitrogenous compounds1 consisting either of those which occur in air and water1 or of the residues of the decomposition of other plants or animals 7as in manure74 ,the nitrogenous substances contained in animals have their origin in those substances which are formed in plants4 ,thus the nitrogen of the atmosphere is the origin of all the nitrogenous substances occurring in animals and plants1 although not directly so1 but after first combining with the other elements of air4 #a ,see ,note #ae bis4 ,the nitrogenous compounds which enter into the composition of plants and animals are of primary importance2 no vegetable or animal cell--that is1 the elementary form of organism--exists without containing a nitrogenous substance1 and moreover organic life manifests itself primarily in these nitrogenous substances4 ,the germs1 seeds1 and those parts by which cells multiply themselves abound in nitrogenous substances2 the sum total of the phenomena which are proper to organisms depend primarily on the chemical properties of the nitrogenous substances which enter into their composition4 ,it will be sufficient1 for instance1 to point out the fact that vegetable and animal organisms1 clearly distinguishable as such1 are characterised by a different degree of energy in their nature1 and at the same time by a difference in the amount of nitrogenous substances they contain4 ,in plants1 which compared with animals possess but little activity1 being incapable of independent movement1 c41 the amount of nitrogen is very much less than in animals1 whose tissues are almost exclusively formed of nitrogenous substances4 ,it is remarkable that the nitrogenous parts of plants1 chiefly of the lower orders1 sometimes present both forms and properties which approach to those of animal organisms2 for example1 the zoospores of sea-weeds1 or those parts by means of which the latter multiply themselves4 ,these zoospores on leaving the sea-weed in many respects resemble the lower orders of animal life1 having1 like the latter1 the property of moving4 ,they also approach the animal kingdom in their composition1 their outer coating containing nitrogenous matter4 ,directly the zoospore becomes covered with that non-nitrogenous or cellular coating which is proper to all the ordinary cells of plants1 it loses all resemblance to an animal organism and becomes a small plant4 ,it may be thought from this that the cause of the difference in the vital processes of animals and plants is the different amount of nitrogenous substances they contain4 ,the nitrogenous substances which occur in plants and animals appertain to a series of exceedingly complex and very changeable chemical compounds2 their elementary composition alone shows this2 besides nitrogen1 they contain carbon1 hydrogen1 oxygen1 and sulphur4 ,being distinguished by a very great instability under many conditions in which other compounds remain unchanged1 these substances are fitted for those perpetual changes which form the first condition of vital activity4 ,these complex and changeable nitrogenous substances of the organism are called proted substances4 ,the white of eggs is a familiar example of such a substance4 ,they are also contained in the flesh of animals1 the curdy elements of milk1 the glutinous matter of wheaten flour1 or so-called gluten1 which forms the chief component of macaroni1 c4 ,nitrogen occurs in the earth's crust1 in compounds either forming the remains of plants and animals1 or derived from the nitrogen of the atmosphere as a consequence of its combination with the other component parts of the air4 ,it is not found in other forms in the earth's crust2 so that nitrogen must be considered1 in contradistinction to oxygen1 as an element which is purely superficial1 and does not extend to the depths of the earth4#a bis #a bis ,the reason why there are no other nitrogenous substances within the earth's mass beyond those which have come there with the remains of organisms1 and from the air with rain-water1 must be looked for in two circumstances4 ,in the first place1 in the instability of many nitrogenous compounds1 which are liable to break up with the formation of gaseous nitrogen2 and in the second place in the fact that the salts of nitric acid1 forming the product of the action of air on many nitrogenous and especially organic compounds1 are very soluble in water1 and on penetrating into the depths of the earth 7with water7 give up their oxygen4 ,the result of the changes of the nitrogenous organic substances which fall into the earth is without doubt frequently1 if not invariably1 the formation of gaseous nitrogen4 ,thus the gas evolved from coal always contains much nitrogen 7together with marsh gas1 carbonic anhydride1 and other gases74 ,nitrogen is liberated in a free state in the decomposition of the nitrogenous organic substances entering into the composition of organisms--for instance1 on their combustion4 ,all organic substances burn when heated to redness with oxygen 7or substances readily yielding it1 such as oxide of copper72 the oxygen combines with the carbon1 sulphur1 and hydrogen1 and the nitrogen is evolved in a free state1 because at a high temperature it does not form any stable compound1 but remains uncombined4 ,carbonic anhydride and water are formed from the carbon and hydrogen respectively1 and therefore to obtain pure nitrogen it is necessary to remove the carbonic anhydride from the gaseous products obtained4 ,this may be done very easily by the action of alkalis--for instance1 caustic soda4 ,the amount of nitrogen in organic substances is determined by a method founded on this4 ,it is also very easy to obtain nitrogen from air1 because oxygen combines with many substances4 ,either phosphorus or metallic copper is usually employed for removing the oxygen from air1 but1 naturally1 a number of other substances may also be used4 ,if a small saucer on which a piece of phosphorus is laid be placed on a cork floating on water1 and the phosphorus be lighted1 and the whole covered with a glass bell jar1 then the air under the jar will be deprived of its oxygen1 and nitrogen only will remain1 owing to which1 on cooling1 the water will rise to a certain extent in the bell jar4 ,the same object 7procuring nitrogen from air7 is attained much more conveniently and perfectly by passing air through a red-hot tube containing copper filings4 ,at a red heat1 metallic copper combines with oxygen and gives a black powder of copper oxide4 ,if the layer of copper be sufficiently long and the current of air slow1 all the oxygen will be absorbed1 and nitrogen alone will pass from the tube4#b #b ,copper 7best as turnings1 which present a large surface7 absorbs oxygen1 forming ,cu,o1 at the ordinary temperature in the presence of solutions of acids1 or1 better still1 in the presence of a solution of ammonia1 when it forms a bluish-violet solution of oxide of copper in ammonia4 ,nitrogen is very easily procured by this method4 ,a flask filled with copper turnings is closed with a cork furnished with a funnel and stopcock4 ,a solution of ammonia is poured into the funnel1 and caused to drop slowly upon the copper4 ,if at the same time a current of air be slowly passed through the flask 7from a gasholder71 then all the oxygen will be absorbed from it and the nitrogen will pass from the flask4 ,it should be washed with water to retain any ammonia that may be carried off with it4 ,nitrogen may also be procured from many of its compounds with oxygen#c and hydrogen1#d but the best fitted for this purpose is a saline mixture containing1 on the one hand1 a compound of nitrogen with oxygen1 termed nitrous anhydride1 ,n#b,o#c1 and on the other hand1 ammonia1 ,,nh#c--that is1 a compound of nitrogen with hydrogen4 ,by heating such a mixture1 the oxygen of the nitrous anhydride combines with the hydrogen of the ammonia1 forming water1 and gaseous nitrogen is evolved1 #b,,nh#c ,n#b,o#c #c;,h#b,o ,n#d4 ,nitrogen is procured by this method in the following manner3--,a solution of caustic potash is saturated with nitrous anhydride1 by which means potassium nitrite is formed4 ,on the other hand1 a solution of hydrochloric acid saturated with ammonia is prepared2 a saline substance called sal-ammoniac1 ,,nh#d;,cl1 is thus formed in the solution4 ,the two solutions thus prepared are mixed together and heated4 ,reaction takes place according to the equation ,,kno#b ,,nh#d;,cl ,k,cl #b;,h#b,o ,n#b4 ,this reaction proceeds in virtue of the fact that potassium nitrite and ammonium chloride are salts which1 on interchanging their metals1 give potassium chloride and ammonium nitrite1 ,,nh#d,,no#b1 which breaks up into water and nitrogen4 ,this reaction does not take place without the aid of heat1 but it proceeds very easily at a moderate temperature4 ,of the resultant substances1 the nitrogen only is gaseous4 ,pure nitrogen may be obtained by drying the resultant gas and passing it through a solution of sulphuric acid 7to absorb a certain quantity of ammonia which is evolved in the reaction74#d bis #c ,the oxygen compounds of nitrogen 7for example1 ,n#b,o1 ,,no1 ,,no#b7 are decomposed at a red heat by themselves1 and under the action of red-hot copper1 iron1 sodium1 c41 they give up their oxygen to the metals1 leaving the nitrogen free4 ,according to ,meyer and ,langer 7#ahhe71 nitrous oxide1 ,n#b,o1 decomposes below #ijj1 although not completely4 #d ,chlorine and bromine 7in excess71 as well as bleaching powder 7hypochlorites71 take up the hydrogen from ammonia1 ,,nh#c1 leaving nitrogen4 ,nitrogen is best procured from ammonia by the action of a solution of sodium hypobromite on solid sal-ammoniac4 #d bis ,lord ,rayleigh in #ahid1 when determining the weight of a volume of carefully purified nitrogen by weighing it in one and the same globe1 found that the gas obtained from air1 by the action of incandescent copper 7or iron or by removing the oxygen by ferrous oxide7 was always #a_/bjj heavier than the nitrogen obtained from its compounds1 for instance1 from the oxide or suboxide of nitrogen1 decomposed by incandescent pulverulent iron or from the ammonia salt of nitrous acid4 ,for the nitrogen procured from air1 he obtained1 at #j and #gfjd mm4 pressure1 a weight #bcaj grms41 while for the nitrogen obtained from its compounds1 #bbii grms4 ,this difference of about #a_/bjj could not be explained by the nitrogen not having been well purified1 or by inaccuracy of experiment1 and was the means for the remarkable discovery of the presence of a heavy gas in air1 which will be mentioned in ,note #af bis4 ,nitrogen is a gaseous substance which does not differ much in physical properties from air2 its density1 referred to hydrogen1 is approximately equal to #ad--that is1 it is slightly lighter than air1 its density referred to air being #jigb2 one litre of nitrogen weighs #abeg gram4 ,nitrogen mixed with oxygen1 which is slightly heavier than air1 forms air4 ,it is a gas which1 like oxygen and hydrogen1 is liquefied with difficulty1 and is but little soluble in water and other liquids4 ,its absolute boiling point#e is about -#adj2 above this temperature it is not liquefiable by pressure1 and at lower temperatures it remains a gas at a pressure of #ej atmospheres4 ,liquid nitrogen boils at -#aic1 so that it may be employed as a source of great cold4 ,at about -#bjc1 in vaporising under a decrease of pressure1 nitrogen solidifies into a colourless snow-like mass4 ,nitrogen does not burn1#e bis does not support combustion1 is not absorbed by any of the reagents used in gas analysis1 at least at the ordinary temperature--in a word1 it presents a whole series of negative chemical properties2 this is expressed by saying that this element has no energy for combination4 ,although it is capable of forming compounds both with oxygen and hydrogen as well as with carbon1 yet these compounds are only formed under particular circumstances1 to which we will directly turn our attention4 ,at a red heat nitrogen combines with boron1 titanium1 and silicon1 barium1 magnesium1 c41 forming very stable nitrogenous compounds1#f whose properties are entirely different from those of nitrogen with hydrogen1 oxygen and carbon4 ,however1 the combination of nitrogen with carbon1 although it does not take place directly between the elements at a red heat1 yet proceeds with comparative ease by heating a mixture of charcoal with an alkaline carbonate1 especially potassium carbonate or barium carbonate1 to redness1 carbo-nitrides or cyanides of the metals being formed2 for instance1 ,k#b;,,co#c #d;,c ,n#b #b,,kcn #c;,,co4#g #e ,see ,chapter ,,ii4 ,note #bi4 #e bis ,see ,note #aa bis4 #f ,the combination of boron with nitrogen is accompanied by the evolution of sufficient heat to raise the mass to redness2 titanium combines so easily with nitrogen that it is difficult to obtain it free from that element2 magnesium easily absorbs nitrogen at a red heat4 ,it is a remarkable and instructive fact that these compounds of nitrogen are very stable and non-volatile4 ,carbon 7,c #ab7 with nitrogen gives cyanogen1 ,c#b,n#b1 which is gaseous and very unstable1 and whose molecule is not large1 whilst boron 7,b #aa7 forms a nitrogenous compound which is solid1 non-volatile1 and very stable4 ,its composition1 ,,bn1 is similar to that of cyanogen1 but its molecular weight1 ,bn,nn1 is probably greater4 ,its composition1 like that of ,n#b,mg#c1 ,n,na#c1 ,n#b;,hg#c and of many of the metallic nitrides1 corresponds to ammonia with the substitution of all its hydrogen by a metal4 ,in my opinion1 a detailed study of the transformations of the nitrides now known1 should lead to the discovery of many facts in the history of nitrogen4 #g ,this reaction1 so far as is known1 does not proceed beyond a certain limit1 probably because cyanogen1 ,,cn1 itself breaks up into carbon and nitrogen4 ,nitrogen is found with oxygen in the air1 but they do not readily combine4 ,cavendish1 however1 in the last century1 showed that nitrogen combines with oxygen under the influence of a series of electric sparks4 ,electric sparks in passing through a moist#h mixture of nitrogen and oxygen cause these elements to combine1 forming reddish-brown fumes of oxides of nitrogen1#i which form nitric acid1#aj ,,nho#c4 ,the presence of the latter is easily recognised1 not only from its reddening litmus paper1 but also from its acting as a powerful oxidiser even of mercury4 ,conditions similar to these occur in nature1 during a thunderstorm or in other electrical discharges which take place in the atmosphere2 whence it may be taken for granted that air and rain-water always contain traces of nitric and nitrous acids4#aa ,besides which ,crookes 7#ahib7 showed that under certain circumstances and when electricity of high potential#aa bis passes through the air1 the combination of nitrogen with oxygen is accompanied by the formation of a true flame4 ,this was also observed previously 7#ahhj7 during the passage of electrical discharges through the air4 #h ,frmy and ,becquerel took dry air1 and observed the formation of brown vapours of oxides of nitrogen on the passage of sparks4 #i ,if a mixture of one volume of nitrogen and fourteen volumes of hydrogen be burnt1 then water and a considerable quantity of nitric acid are formed4 ,it may be partly due to this that a certain quantity of nitric acid is produced in the slow oxidation of nitrogenous substances in an excess of air4 ,this is especially facilitated by the presence of an alkali with which the nitric acid formed can combine4 ,if a galvanic current be passed through water containing the nitrogen and oxygen of the air in solution1 then the hydrogen and oxygen set free combine with the nitrogen1 forming ammonia and nitric acid4 ,when copper is oxidised at the expense of the air at the ordinary temperature in the presence of ammonia1 oxygen is absorbed1 not only for combination with the copper1 but also for the formation of nitric acid4 ,the combination of nitrogen with oxygen1 even1 for example1 by the action of electric sparks1 is not accompanied by an explosion or rapid combination1 as in the action of a spark on a mixture of oxygen and hydrogen4 ,this is explained by the fact that heat is not evolved in the combination of nitrogen with oxygen1 but is absorbed--an expenditure of energy is required1 there is no evolution of energy4 ,in fact1 there will not be the transmission of heat from particle to particle which occurs in the explosion of detonating gas4 ,each spark will aid the formation of a certain quantity of the compound of oxygen and nitrogen1 but will not excite the same in the neighbouring particles4 ,in other words1 the combination of hydrogen with oxygen is an exothermal reaction1 and the combination of nitrogen with oxygen an endothermal reaction4 ,a condition particularly favourable for the oxidation of nitrogen is the explosion of detonating gas and air if the former be in excess4 ,if a mixture of two volumes of detonating gas and one volume of air be exploded1 then one-tenth of the air is converted into nitric acid1 and consequently after the explosion has taken place there remain only nine-tenths of the volume of air originally taken4 ,if a large proportion of air be taken--for instance1 four volumes of air to two volumes of detonating gas--then the temperature of the explosion is lowered1 the volume of air taken remains unchanged1 and no nitric acid is formed4 ,this gives a rule to be observed in making use of the eudiometer--namely that to weaken the force of the explosion not less than an equal volume of air should be added to the explosive mixture4 ,on the other hand a large excess must not be taken as no explosion would then ensue 7see ,chapter ,,iii4 ,note #cd74 ,probably in the future means will be found for obtaining compounds of nitrogen on a large industrial scale by the aid of electric discharges1 and by making use of the inexhaustible mass of nitrogen in the atmosphere4 #aj ,in reality nitric oxide1 ,,no1 is first formed1 but with oxygen and water it gives 7brown fumes7 nitrous anhydride1 which1 as we shall afterwards learn1 in the presence of water and oxygen gives nitric acid4 #aa ,the nitric acid contained in the soil1 river water 7,chapter ,i41 ,note #b71 wells1 c41 proceeds 7like carbonic anhydride7 from the oxidation of organic compounds which have fallen into water1 soil1 c4 #aa bis ,crookes employed a current of #ae ampres and #fe volts1 and passed it through an induction coil with #ccj vibrations per second1 and obtained a flame between the poles placed at a distance of #df mm4 which after the appearance of the arc and flame could be increased to #bjj mm4 ,a platinum wire fused in the flame4 ,further observations showed that under the influence of electrical discharges1#ab silent as well as with sparks1 nitrogen is able to enter into many reactions with hydrogen and with many hydrocarbons2 although these reactions cannot be effected by exposure to a red heat4 ,thus1 for instance1 a series of electric sparks passed through a mixture of nitrogen and hydrogen causes them to combine and form ammonia#ac or nitrogen hydride1 ,,nh#c1 composed of one volume of nitrogen and three volumes of hydrogen4 ,this combination is limited to the formation of #f per cent4 of ammonia1 because ammonia is decomposed1 although not entirely 7#id_/ajj7 by electric sparks4 ,this signifies that under the influence of an electrical discharge the reaction ,,nh#c ,n #c;,h is reversible1 consequently it is a dissociation1 and in it a state of equilibrium is arrived at4 ,the equilibrium may be destroyed by the addition of gaseous hydrochloric acid1 ,h,cl1 because with ammonia it forms a solid saline compound1 sal-ammoniac1 ,,nh#d;,cl1 which 7being formed from a gaseous mixture of #c;,h1 ,n1 and ,h,cl7 fixes the ammonia4 ,the remaining mass of nitrogen and hydrogen1 under the action of the sparks1 again forms ammonia1 and in this manner solid sal-ammoniac is obtained to the end by the action of a series of electric sparks on a mixture of gaseous ,n1 ,h#c1 and ,h,cl4#ad ,berthelot 7#ahgf7 showed that under the action of a silent discharge many non-nitrogenous organic substances 7benzene1 ,c#f;,h#f1 cellulose in the form of paper1 resin1 glucose1 ,c#f;,h#aj,o#e1 and others7 absorb nitrogen and form complex nitrogenous compounds1 which are capable1 like albuminous substances1 of evolving their nitrogen as ammonia when heated with alkalis4#ae #ab ,this property of nitrogen1 which under normal conditions is inactive1 leads to the idea that under the influence of an electric discharge gaseous nitrogen changes in its properties2 if not permanently like oxygen 7electrolysed oxygen or ozone does not react on nitrogen1 according to ,berthelot71 it may be temporarily at the moment of the action of the discharge1 just as some substances under the action of heat are permanently affected 7that is1 when once changed remain so--for instance1 white phosphorus passes into red1 c471 whilst others are only temporarily altered 7the dissociation of ,s#f into ,s#b or of sal-ammoniac into ammonia and hydrochloric acid74 ,such a proposition is favoured by the fact that nitrogen gives two kinds of spectra1 with which we shall afterwards become acquainted4 ,it may be that the molecules ,n#b then give less complex molecules1 ,n containing one atom1 or form a complex molecule ,n#c1 like oxygen in passing into ozone4 ,probably under a silent discharge the molecules of oxygen1 ,o#b1 are partly decomposed and the individual atoms ,o combine with ,o#b1 forming ozone1 ,o#c4 #ac ,this reaction1 discovered by ,chabri and investigated by ,thnard1 was only rightly understood when ,deville applied the principles of dissociation to it4 #ad ,the action of nitrogen on acetylene 7,berthelot7 resembles this reaction4 ,a mixture of these gases under the influence of a silent discharge gives hydrocyanic acid1 ,c#b;,h#b ,n#b #b;,,cnh4 ,this reaction cannot proceed beyond a certain limit because it is reversible4 #ae ,berthelot successfully employed electricity of even feeble potential in these experiments1 which fact led him to think that in nature1 where the action of electricity takes place very frequently1 a part of the complex nitrogenous substances may proceed from the gaseous nitrogen of the air by this method4 ,as the nitrogenous substances of organisms play a very important part in them 7organic life cannot exist without them71 and as the nitrogenous substances introduced into the soil are capable of invigorating its crops 7of course in the presence of the other nourishing principles required by plants71 the question of the means of converting the atmospheric nitrogen into the nitrogenous compounds of the soil1 or into assimilable nitrogen capable of being absorbed by plants and of forming complex 7albuminous7 substances in them1 is one of great theoretical and practical interest4 ,the artificial 7technical7 conversion of the atmospheric nitrogen into nitrogenous compounds1 notwithstanding repeated attempts1 cannot yet be considered as fulfilled in a practical remunerative manner although its possibility is already evident4 ,electricity will probably aid in solving this very important practical problem4 ,when the theoretical side of the question is further advanced1 then without doubt an advantageous means will be found for the manufacture of nitrogenous substances from the nitrogen of the air2 and this is needed1 before all1 for the agriculturist1 to whom nitrogenous fertilisers form an expensive item1 and are more important than all other manures4 ,one thousand tons of farmyard manure do not generally contain more than four tons of nitrogen in the form of complex nitrogenous substances1 and this amount of nitrogen is contained in twenty tons of ammonium sulphate1 therefore the effect of a mass of farmyard manure in respect to the introduction of nitrogen may be produced by small quantities of artificial nitrogenous fertilisers 7see ,note #ae bis74 ,by such indirect methods does the gaseous nitrogen of the atmosphere yield its primary compounds1 in which form it enters into plants1 and is elaborated in them into complex albuminous substances4#ae bis ,but1 starting from a given compound of nitrogen with hydrogen or oxygen1 we may1 without the aid of organisms1 obtain1 as will afterwards be partially indicated1 most diverse and complex nitrogenous substances1 which cannot by any means be formed directly from gaseous nitrogen4 ,in this we see an example not only of the difference between an element in the free state and an intrinsic element1 but also of those circuitous or indirect methods by which substances are formed in nature4 ,the discovery1 prognostication1 and1 in general1 the study of such indirect methods of the preparation and formation of substances forms one of the existing problems of chemistry4 ,from the fact that ,a does not act at all on ,b1 it must not be concluded that a compound ,,ab is not to be formed4 ,the substances ,a and ,b contain atoms which occur in ,,ab1 but their state or the nature of their motion may not be at all that which is required for the formation of ,,ab1 and in this substance the chemical state of the elements may be as different as the state of the atoms of oxygen in ozone and in water4 ,thus free nitrogen is inactive2 but in its compounds it very easily enters into changes and is distinguished by great activity4 ,an acquaintance with the compounds of nitrogen confirms this4 ,but1 before entering on this subject1 let us consider air as a mass containing free nitrogen4 #ae bis ,although the numerous1 and as far as possible accurate and varied researches made in the physiology of plants have proved that the higher forms of plants are not capable of directly absorbing the nitrogen of the atmosphere and converting it into complex albuminous substances1 still it has been long and repeatedly observed that the amount of nitrogenous substances in the soil is increased by the cultivation of plants of the bean 7leguminous7 family such as pea1 acacia1 c4 ,a closer study of these plants has shown that this is connected with the formation of peculiar nodular swellings in their roots caused by the growth of peculiar micro-organisms 7bacteria7 which cohabit the soil with the roots1 and are capable of absorbing nitrogen from the air1 i4e4 of converting it into assimilated nitrogen4 ,this branch of plant physiology1 which forms another proof of the important part played by micro-organisms in nature1 cannot be discussed in this work1 but it should be mentioned1 since it is of great theoretical and practical interest1 and1 moreover1 phenomena of this kind1 which have recently been discovered1 promise to explain1 to some extent at least1 certain of the complex problems concerning the development of life on the earth4 ,judging from what has been already stated1 it will be evident that atmospheric air#af contains a mixture of several gases and vapours4 ,some of them are met with in it in nearly constant proportions1 whilst others1 on the contrary1 are very variable in their amount4 ,the chief component parts of air1 placed in the order of their relative amounts1 are the following3 nitrogen1#af bis oxygen1 aqueous vapour1 carbonic anhydride1 nitric acid1 salts of ammonia1 oxides of nitrogen1 and also ozone1 hydrogen peroxide1 and complex organic nitrogenous substances4 ,besides these1 air generally contains water1 as spray1 drops1 and snow1 and particles of solids1 perhaps of cosmic origin in certain instances1 but in the majority of cases proceeding from the mechanical translation of solid particles from one locality to another by the wind4 ,these small solid and liquid particles 7having a large surface in proportion to their weight7 are suspended in air as solid matter is suspended in turbid water2 they often settle on the surface of the earth1 but the air is never entirely free from them because they are never in a state of complete rest4 ,then1 air not unfrequently contains incidental traces of various substances as everyone knows by experience4 ,these incidental substances sometimes belong to the order of those which act injuriously1 the germs of lower organisms--for instance of moulds--and the class of carriers of infectious diseases4 #af ,under the name of atmospheric air the chemist and physicist understand ordinary air containing nitrogen and oxygen only1 notwithstanding that the other component parts of air have a very important influence on the living matter of the earth's surface4 ,that air is so represented in science is based on the fact that only the two components above-named are met with in air in a constant quantity1 whilst the others are variable4 ,the solid impurities may be separated from air required for chemical or physical research by simple filtration through a long layer of cotton-wool placed in a tube4 ,organic impurities are removed by passing the air through a solution of potassium permanganate4 ,the carbonic anhydride contained in air is absorbed by alkalis--best of all1 soda-lime1 which in a dry state in porous lumps absorbs it with exceeding rapidity and completeness4 ,aqueous vapour is removed by passing the air over calcium chloride1 strong sulphuric acid1 or phosphoric anhydride4 ,air thus purified is accepted as containing only nitrogen and oxygen1 although in reality it still contains a certain quantity of hydrogen and hydrocarbons1 from which it may be purified by passing over copper oxide heated to redness4 ,the copper oxide then oxidises the hydrogen and hydrocarbons--it burns them1 forming water and carbonic anhydride1 which may be removed as above described4 ,when it is said that in the determination of the density of gases the weight of air is taken as unity1 it is understood to be such air1 containing only nitrogen and oxygen4 #af bis ,thanks to the remarkable discovery made in the summer of #ahid by ,lord ,rayleigh and ,prof4 ,ramsay1 the well-known component elements of air must now he supplemented by #a p4c4 7by volume7 of a heavy gas 7density about #ai1 ,h #a71 inactive like nitrogen1 which was discovered in the researches made by ,lord ,rayleigh on the density of nitrogen as mentioned in note #d bis4 ,up to the present time this gas has been always determined together with nitrogen1 because it combines with neither the hydrogen in the eudiometer nor with the copper in the gravimetric method of determining the composition of air1 and therefore has always remained with the nitrogen4 ,it has been possible to separate it from nitrogen since magnesium absorbs nitrogen at a red heat1 while this gas remains unabsorbed1 and was found to have a density nearly one and a half time greater than that of nitrogen 7is it not a polymer of nitrogen1 ,n#c874 ,it is now known also that this gas gives a luminous spectrum1 which contains the bright blue line observed in the spectrum of nitrogen4 ,owing to the fact that it is an exceedingly inert substance1 even more so than nitrogen1 it has been termed ,argon4 ,further reference will be made to it in the ,appendix4 ,in the air of the various countries of the earth1 at different longitudes and at different altitudes above its surface1 on the ocean or on the dry land--in a word1 in the air of most diverse localities of the earth--the oxygen and nitrogen are found everywhere to be in a constant ratio4 ,this is1 moreover1 self-evident from the fact that the air constantly diffuses 7intermixes by virtue of the internal motion of the gaseous particles7 and is also put into motion and intermixed by the wind1 by which processes it is equalised in its composition over the entire surface of the earth4 ,in those localities where the air is subject to change1 and is in a more or less enclosed space1 or1 at any rate1 in an unventilated space1 it may alter very considerably in its composition4 ,for this reason the air in dwellings1 cellars1 and wells1 in which there are substances absorbing oxygen1 contains less of this gas1 whilst the air on the surface of standing water1 which abounds in the lower orders of plant life evolving oxygen1 contains an excess of this gas4#ag ,the constant composition of air over the whole surface of the earth has been proved by a number of most careful researches4#ah #ag ,as a further proof of the fact that certain circumstances may change the composition of air1 it will be enough to point out that the air contained in the cavities of glaciers contains only up to #aj p4c4 of oxygen4 ,this depends on the fact that at low temperatures oxygen is much more soluble in snow-water and snow than nitrogen4 ,when shaken up with water the composition of air should change1 because the water dissolves unequal quantities of oxygen and nitrogen4 ,we have already seen 7,chapter ,i47 that the air boiled off from water saturated at about #j contains about thirty-five volumes of oxygen and sixty-five volumes of nitrogen1 and we have considered the reason of this4 #ah ,the analysis of air by weight conducted by ,dumas and ,boussingault in ,paris1 which they repeated many times between ,april #bg and ,september #bb1 #ahda1 under various conditions of weather1 showed that the amount by weight of oxygen only varies between #bbhi p4c4 and #bcjh p4c41 the average amount being #bcjg p4c4 ,brunner1 at ,bern in ,switzerland1 and ,bravais1 at ,faulhorn in the ,bernese ,alps1 at a height of two kilometres above the level of the sea1 ,marignac at ,geneva1 ,lewy at ,copenhagen1 and ,stas at ,brussels1 have analysed the air by the same methods1 and found that its composition does not exceed the limits determined for ,paris4 ,the most recent determinations 7with an accuracy of #jje p4c47 confirm the conclusion that the composition of the atmosphere is constant4 ,as there are some grounds 7which will be mentioned shortly7 for considering that the composition of the air at great altitudes is slightly different from that at attainable heights--namely1 that it is richer in the lighter nitrogen--several fragmentary observations made at ,munich 7,jolly1 #ahhj7 gave reason for thinking that in the upward currents 7that is in the region of minimum barometric pressure or at the centres of meteorological cyclones7 the air is richer in oxygen than in the descending currents of air 7in the regions of anticyclones or of barometric maxima72 but more carefully conducted observations showed this supposition to be incorrect4 ,improved methods for the analysis of air have shown that certain slight variations in its composition do actually occur1 but in the first place they depend on incidental local influences 7on the passage of the air over mountains and large surfaces of water1 regions of forest and herbage1 and the like71 and in the second place are limited to quantities which are scarcely distinguishable from possible errors in the analyses4 ,the researches made by ,kreisler in ,germany 7#ahhe7 are particularly convincing4 ,the considerations which lead to the supposition that the atmosphere at great altitudes contains less oxygen than at the surface of the earth are based on the law of partial pressures 7,chapter ,i47 ,according to this law1 the equilibrium of the oxygen in the strata of the atmosphere is not dependent on the equilibrium of the nitrogen1 and the variation in the densities of both gases with the height is determined by the pressure of each gas separately4 ,details of the calculations and considerations here involved are contained in my work ,on ,barometric ,levellings1 #ahgf1 p4 #dh4 ,on the basis of the law of partial pressure and of hypsometrical formul1 expressing the laws of the variation of pressures at different altitudes1 the conclusion may be deduced that at the upper strata of the atmosphere the proportion of the nitrogen with respect to the oxygen increases1 but the increase will not exceed a fraction per cent41 even at altitudes of four and a half to six miles1 the greatest height within the reach of men either by climbing mountains or by means of balloons4 ,this conclusion is confirmed by the analyses of air collected by ,welch in ,england during his aronautic ascents4 ,the analysis of air is effected by converting the oxygen into a non-gaseous compound1 so as to separate it from the air4 ,the original volume of the air is first measured1 and then the volume of the remaining nitrogen4 ,the quantity of oxygen is calculated either from the difference between these volumes or by the weight of the oxygen compound formed4 ,all the volumetric measurements have to be corrected for pressure1 temperature1 and moisture 7,chapters ,i4 and ,,ii47 ,the medium employed for converting the oxygen into a non-gaseous substance should enable it to be taken up from the nitrogen to the very end without evolving any gaseous substance4 ,so1 for instance1#ai a mixture of pyrogallol1 ,c#f;,h#f,o#c1 with a solution of a caustic alkali absorbs oxygen with great ease at the ordinary temperature 7the solution turns black71 but it is unsuited for accurate analysis because it requires an aqueous solution of an alkali1 and it alters the composition of the air by acting on it as a solvent4#bj ,however1 for approximate determinations this simple method gives results which are entirely satisfactory4 #ai ,the complete absorption of the oxygen may be attained by introducing moist phosphorus into a definite volume of air2 the occurrence of this is recognised by the fact of the phosphorus becoming non-luminous in the dark4 ,the amount of oxygen may be determined by measuring the volume of nitrogen remaining4 ,this method however cannot give accurate results1 owing to a portion of the air being dissolved in the water1 to the combination of some of the nitrogen with oxygen and to the necessity of introducing and withdrawing the phosphorus1 which cannot be accomplished without introducing bubbles of air4 #bj ,for rapid and approximate analyses 7technical and hygienic71 such a mixture is very suitable for determining the amount of oxygen in mixtures of gases from which the substances absorbed by alkalis have first been removed4 ,according to certain observers1 this mixture evolves a certain 7small7 quantity of carbonic oxide after absorbing oxygen4 ,the determinations in a eudiometer 7,chapter ,,iii47 give more exact results1 if all the necessary corrections for changes of pressure1 temperature1 and moisture be taken into account4 ,this determination is carried out essentially as follows3--,a certain amount of air is introduced into the eudiometer1 and its volume is determined4 ,about an equal volume of dry hydrogen is then passed into the eudiometer1 and the volume again determined4 ,the mixture is then exploded1 in the way described for the determination of the composition of water4 ,the remaining volume of the gaseous mixture is again measured2 it will be less than the second of the previously measured volumes4 ,out of three volumes which have disappeared1 one belonged to the oxygen and two to the hydrogen1 consequently one-third of the loss of volume indicates the amount of oxygen contained in the air4#ba #ba ,details of eudiometrical analysis must1 as was pointed out in ,chap4 ,,iii41 ,note #cb1 be looked for in works on analytical chemistry4 ,the same remark applies to the other analytical methods mentioned in this work4 ,they are only described for the purpose of showing the diversity of the methods of chemical research4 ,the most complete method for the analysis of air1 and one which is accompanied by the least amount of error1 consists in the direct weighing1 as far as is possible1 of the oxygen1 nitrogen1 water1 and carbonic anhydride contained in it4 ,for this purpose the air is first passed through an apparatus for retaining the moisture and carbonic anhydride 7which will be considered presently71 and is then led through a tube which contains shavings of metallic copper and has been previously weighed4 ,a long layer of such copper heated to redness absorbs all the oxygen from the air1 and leaves pure nitrogen1 whose weight must be determined4 ,this is done by collecting it in a weighed and exhausted globe1 while the amount by weight of oxygen is shown by the increase in weight of the tube with the copper after the experiment4 ,illustration3 ,,fig4 #ch4--;,dumas and ,boussingault's apparatus for the analysis of air by weight4 ,the globe ,b contains #aj-ae litres4 ,the air is first pumped out of it1 and it is weighed empty4 ,the tube ,t connected with it is filled with copper1 and is weighed empty of air4 ,it is heated in a charcoal furnace4 ,when the copper has become red-hot1 the stopcock r 7near ,r7 is slightly opened1 and the air passes through the vessels ,l1 containing a solution of potash1 f1 containing solutions and pieces of caustic potash1 which remove the carbonic anhydride from the air1 and then through o and t1 containing sulphuric acid 7which has been previously boiled to expel dissolved air7 and pumice-stone1 which removes the moisture from the air4 ,the pure air then gives up its oxygen to the copper in ,t4 ,when the air passes into ,t the stopcock ,r of the globe ,b is opened1 and it becomes filled with nitrogen4 ,when the air ceases to flow in1 the stopcocks are closed1 and the globe ,b and tube ,t weighed4 ,the nitrogen is then pumped out of the tube and it is weighed again4 ,the increase in weight of the tube shows the amount of oxygen1 and the difference of the second and third weighings of the tube1 with the increase in weight of the globe1 gives the weight of the nitrogen4 ,air free from moisture and carbonic anhydride#bb contains #bjie to #bjhhbc parts by volume of oxygen2 the mean amount of oxygen will therefore be #bjib #jje per cent4 ,taking the density of air #a and of oxygen #aaje and nitrogen #jigb the composition of air by weight will be #bcab per cent4 of oxygen and #gfhh per cent4 of nitrogen4#bd #bb ,air free from carbonic anhydride indicates after explosion the presence of a small quantity of carbonic anhydride1 as ,de ,saussure remarked1 and air free from moisture1 after being passed over red-hot copper oxide1 appears invariably to contain a small quantity of water1 as ,boussingault has observed4 ,these observations lead to the assumption that air always contains a certain quantity of gaseous hydrocarbons1 like marsh gas1 which1 as we shall afterwards learn1 is evolved from the earth1 marshes1 c4 ,its amount1 however1 does not exceed a few hundredths per cent4 #bc ,the analyses of air are accompanied by errors1 and there are variations of composition attaining hundredths per cent42 the average normal composition of air is therefore only correct to the first decimal place4 #bd ,these figures express the mean composition of air from an average of the most accurate determinations2 they are accurate within #jje p4c4 ,illustration3 ,,fig4 #ci4--;,apparatus for the absorption and washing of gases1 known as ,liebig's bulbs4 ,the gas enters m1 presses on the absorptive liquid1 and passes from m into b1 c1 d1 and e consecutively1 and escapes through f4 ,illustration3 ,,fig4 #dj4--;,geisler's potash bulbs4 ,the gas enters at a1 and passes through a solution of potash in the lower bulbs1 where the carbonic anhydride is absorbed1 and the gas escapes from b4 ,the lower bulbs are arranged in a triangle1 so that the apparatus can stand without support4 ,the possibility of the composition of air being altered by the mere action of a solvent very clearly shows that the component parts of air are in a state of mixture1 in which any gases may occur2 they do not in this case form a definite compound1 although the composition of the atmosphere does appear constant under ordinary conditions4 ,the fact that its composition varies under different conditions confirms the truth of this conclusion1 and therefore the constancy of the composition of air must not be considered as in any way dependent on the nature of the gases entering into its composition1 but only as proceeding from cosmic phenomena co-operating towards this constancy4 ,it must be admitted1 therefore1 that the processes evolving oxygen1 and chiefly the processes of the respiration of plants1 are of equal force with those processes which absorb oxygen over the entire surface of the earth4#be #be ,in ,chapter ,,iii41 ,note #d1 an approximate calculation is made for the determination of the balance of oxygen in the entire atmosphere2 it may therefore he supposed that the composition of air will vary from time to time1 the relation between vegetation and the oxygen absorbing processes changes2 but as the atmosphere of the earth can hardly have a definite limit and we have already seen 7,chapter ,,iv41 ,note #cc7 that there are observations confirming this1 it follows that our atmosphere should vary in its component parts with the entire heavenly space1 and therefore it must he supposed that any variation in the composition by weight of the air can only take place exceedingly slowly1 and in a manner imperceptible by experiment4 ,illustration3 ,,fig4 #da4--,tube for the absorption of carbonic acid4 ,a plug of cotton wool is placed in the bulb to prevent the powder of soda-lime being carried off by the gas4 ,the tube contains soda-lime and chloride of calcium4 ,air always contains more or less moisture#bf and carbonic anhydride produced by the respiration of animals and the combustion of carbon and carboniferous compounds4 ,the latter shows the properties of an acid anhydride4 ,in order to determine the amount of carbonic anhydride in air1 substances are employed which absorb it--namely1 alkalis either in solution or solid4 ,a solution of caustic potash1 ,,kho1 is poured into light glass vessels1 through which the air is passed1 and the amount of carbonic anhydride is determined by the increase in weight of the vessel4 ,but it is best to take a solid porous alkaline mass such as soda-lime4#bg ,with a slow current of air a layer of soda-lime #bj cm4 in length is sufficient to completely deprive #a cubic metre of air of the carbonic anhydride it contains4 ,a series of tubes containing calcium chloride for absorbing the moisture#bh is placed before the apparatus for the absorption of the carbonic anhydride1 and a measured mass of air is passed through the whole apparatus by means of an aspirator4 ,in this manner the determination of the moisture is combined with the absorption of the carbonic anhydride4 ,the arrangement shown in fig4 #ch is such a combination4 #bf ,the amount of moisture contained in the air is considered in greater detail in the study of physics and meteorology and the subject has been mentioned above1 in ,chapter ,i41 ,note #a1 where the methods of absorbing moisture from gases were pointed out4 #bg ,soda-lime is prepared in the following manner3--,unslaked lime is finely powdered and mixed with a slightly warmed and very strong solution of caustic soda4 ,the mixing should be done in an iron dish1 and the materials should be well stirred together until the lime begins to slake4 ,when the mass becomes hot1 it boils1 swells up1 and solidifies1 forming a porous mass very rich in alkali and capable of rapidly absorbing carbonic anhydride4 ,a lump of caustic soda or potash presents a much smaller surface for absorption and therefore acts much less rapidly4 ,it is necessary to place an apparatus for absorbing water after the apparatus for absorbing the carbonic anhydride1 because the alkali in absorbing the latter gives off water4 #bh ,it is evident that the calcium chloride employed for absorbing the water should be free from lime or other alkalis in order that it may not retain carbonic anhydride4 ,such calcium chloride may be prepared in the following manner3 ,a perfectly neutral solution of calcium chloride is prepared from lime and hydrochloric acid2 it is then carefully evaporated first on a water-bath and then on a sand-bath4 ,when the solution attains a certain strength a scum is formed1 which solidifies at the surface4 ,this scum is collected1 and will be found to be free from caustic alkalis4 ,it is necessary in any case to test it before use1 as otherwise a large error may be introduced into the results1 owing to the presence of free alkali 7lime74 ,it is best to pass carbonic anhydride through the tube containing the calcium chloride for some time before the experiment1 in order to saturate any free alkali that may remain from the decomposition of a portion of the calcium chloride by water1 ,ca,cl#b #b;,h#b,o ,ca,o,h#b,o #b;,h,cl4 ,the amount of carbonic anhydride#bi in free air is incomparably more constant than the amount of moisture4 ,the average amount in #ajj volumes of dry air is approximately #jjc volume--that is1 #aj1jjj volumes of air contain about three volumes of carbonic anhydride1 most frequently about #bie volumes4 ,as the specific gravity of carbonic anhydride referred to air #aeb1 it follows that #ajj parts by weight of air contain #jjde part by weight of carbonic anhydride4 ,this quantity varies according to the time of year 7more in winter71 the altitude above the level of the sea 7less at high altitudes71 the proximity to forests and fields 7less7 or cities 7greater71 c4 ,but the variation is small and rarely exceeds the limits of #b-a_/b to #d ten-thousandths by volume4#cj ,as there are many natural local influences which either increase the amount of carbonic anhydride in the air 7respiration1 combustion1 decomposition1 volcanic eruptions1 c471 or diminish it 7absorption by plants and water71 the reason of the great constancy in the amount of this gas in the air must be looked for1 in the first place1 in the fact that the wind mixes the air of various localities together1 and1 in the second place1 in the fact that the waters of the ocean1 holding carbonic acid in solution1#ca form an immense reservoir for regulating the amount of this gas in the atmosphere4 ,immediately the partial pressure of the carbonic anhydride in the air decreases1 the water evolves it1 and when the partial pressure increases1 it absorbs it1 and thus nature supplies the conditions for a natural state of moving equilibrium in this as in so many other instances4#cb #bi ,recourse is had to special methods when the determination only takes note of the carbonic anhydride of the air4 ,for instance1 it is absorbed by an alkali which does not contain carbonates 7by a solution of baryta or caustic soda mixed with baryta71 and then the carbonic anhydride is expelled by an excess of an acid1 and its amount determined by the volume given off4 ,a rapid method of determining ,,co#b 7for hygienic purposes7 is given by the fall of tension produced by the introduction of an alkali 7the air having been either brought to dryness or saturated with moisture74 ,dr4 ,schidloffsky's apparatus is based upon this principle4 ,the question as to the amount of carbonic anhydride present in the air has been submitted to many voluminous and exact researches1 especially those of ,reiset1 ,schloesing1 ,mntz1 and ,aubin1 who showed that the amount is not subject to such variations as at first announced on the basis of incomplete and insufficiently accurate determinations4 #cj ,it is a different case in enclosed spaces1 in dwellings1 cellars1 wells1 caves1 and mines1 where the renewal of air is impeded4 ,under these circumstances large quantities of carbonic anhydride may accumulate4 ,in cities1 where there are many conditions for the evolution of carbonic anhydride 7respiration1 decomposition1 combustion71 its amount is greater than in free air1 yet even in still weather the difference does not often exceed one ten-thousandth 7that is1 rarely attains #d instead of #bi vols4 in #ajjjj vols4 of air74 #ca ,in the sea as well as in fresh water1 carbonic acid occurs in two forms1 directly dissolved in the water1 and combined with lime as calcium bicarbonate 7hard waters sometimes contain very much carbonic acid in this form74 ,the tension of the carbonic anhydride in the first form varies with the temperature1 and its amount with the partial pressure1 and that in the form of acid salts is under the same conditions1 for direct experiments have shown a similar dependence in this case1 although the quantitative relations are different in the two cases4 #cb ,in studying the phenomena of nature the conclusion is arrived at that the universally reigning state of mobile equilibrium forms the chief reason for that harmonious order which impresses all observers4 ,it not unfrequently happens that we do not see the causes regulating the order and harmony2 in the particular instance of carbonic anhydride1 it is a striking circumstance that in the first instance a search was made for an harmonious and strict uniformity1 and in incidental 7insufficiently accurate and fragmentary7 observations conditions were even found for concluding it to be absent4 ,when1 later1 the rule of this uniformity was confirmed1 then the causes regulating such order were also discovered4 ,the researches of ,schloesing were of this character4 ,deville's idea of the dissociation of the acid carbonates of sea-water is suggested in them4 ,in many other cases also1 a correct interpretation can only follow from a detailed investigation4 ,besides nitrogen1 oxygen1 moisture1 and carbonic acid1 all the other substances occurring in air are found in infinitesimally small quantities by weight1 and therefore the weight of a cubic measure of air depends1 to a sensible degree1 on the above-named components alone4 ,we have already mentioned that at #j and #gfj mm4 pressure the weight of a cubic litre of air is #abic gram4 ,this weight varies with the acceleration of gravity1 g1 so that if g be expressed in metres the weight of a litre of air1 e g #jacahdd gram4 ,for ,st4 ,petersburg g is about #ihahh1 and therefore e is about #abidf1cc the air being understood to be dry and free from carbonic anhydride4 ,taking the amount of the latter as #jjc per #ajj volumes1 we obtain a greater weight2 for example1 for ,st4 ,petersburg e #abidh instead of #abidf gram4 ,the weight of one litre of moist air in which the tension#cd of the aqueous vapour 7partial pressure7 f mm41 at a pressure 7total7 of air of ,h millimetres1 at a temperature t1 will be 7i4e41 if at #j and #gfj mm4 the weight of dry air e7 equal to e_/7#a #jjjcfgt7 7,h - #jch;f7_/#gfj4 ,for instance1 if ,h #gcj mm41 t #bj1 and f #aj mm4 7the moisture is then slightly below #fj p4c471 the weight of a litre of air at ,st4 ,petersburg #aaebg gram4#ce #cc ,the difference of the weight of a litre of dry air 7free from carbonic anhydride7 at #j and #gfj mm41 at different longitudes and altitudes1 depends on the fact that the force of gravity varies under these conditions1 and with it the pressure of the barometrical column also varies4 ,this is treated in detail in my works ,on the ,elasticity of ,gases and ,on ,barometric ,levellings1 and ',the ,publications of the ,weights and ,measures ,department' 7,journal of the ,russian ,physico-,chemical ,society1 #ahid74 ,in reality the weight is not measured in absolute units of weight 7in pressure--refer to works on mechanics and physics71 but in relative units 7grams1 scale weights7 whose mass is invariable1 and therefore the variation of the weight of the weights itself with the change of gravity must not be here taken into account1 for we are here dealing with weights proportional to masses1 since with a change of locality the weight of the weights varies as the weight of a given volume of air does4 ,in other words3 the mass of a substance always remains constant1 but the pressure produced by it varies with the acceleration of gravity3 the gram1 pound1 and other units of weight are really units of mass4 #cd ,the tension of the aqueous vapour in the air is determined by hygrometers and other similar methods4 ,it may also be determined by analysis 7see ,chapter ,i41 ,note #a74 #ce ,for rapid calculation the weight of a litre of air 7in a room7 in ,st4 ,petersburg1 may under these conditions 7,h1 t1 and f7 be obtained by the formula e #abjfga #jjjaf ,h#a-gee #bf7ah-t7 where ,h#a ,h-#jch;f4 ,in determining the weight of small and heavy objects 7crucibles1 c4 in analysis1 and in determining the specific gravities of liquids1 c47 a correction may be introduced for the loss of weight in the air of the room1 by taking the weight of a litre of air displaced as #ab gram1 and consequently #jjjab gram for every cubic centimetre4 ,but if gases or1 in general1 large vessels are weighed1 and the weighings require to be accurate1 it is necessary to take into account all the data for the determination of the density of the air 7t1 ,h1 and f71 because sensitive balances can determine the possible variations of the weight of air1 as in the case of a litre the weight of air varies in centigrams1 even at a constant temperature1 with variations of ,h and f4 ,some time ago 7#ahei7 ,i proposed the following method and applied it for this purpose4 ,a large light and closed vessel is taken1 and its volume and weight in a vacuum are accurately determined1 and verified from time to time4 ,on weighing it we obtain the weight in air of a given density1 and by subtracting this weight from its absolute weight and dividing by its volume we obtain the density of the air4 ,the presence of ammonia1 a compound of nitrogen and hydrogen1 in the air1 is indicated by the fact that all acids exposed to the air absorb ammonia from it after a time4 ,de ,saussure observed that aluminium sulphate is converted by air into a double sulphate of ammonium and aluminium1 or the so-called ammonia alum4 ,quantitative determinations have shown that the amount of ammonia#cf contained in air varies at different periods4 ,however1 it may be accepted that #ajj cubic metres of air do not contain less than #a or more than #e milligrams of ammonia4 ,it is remarkable that mountain air contains more ammonia than the air of valleys4 ,the air in those places where animal substances undergoing change are accumulated1 and especially that of stables1 generally contains a much greater quantity of this gas4 ,this is the reason of the peculiar pungent smell noticed in such places4 ,moreover ammonia1 as we shall learn in the following chapter1 combines with acids1 and should therefore be found in air in the form of such combinations1 since air contains carbonic and nitric acids4 #cf ,schloesing studied the equilibrium of the ammonia of the atmosphere and of the rivers1 seas1 c41 and showed that the amount of the gas is interchangeable between them4 ,the ratio between the amount of ammonia in a cubic metre of air and a litre of water at #j #jjjd1 at #aj #jjaj1 at #be #jjdj to #a1 and therefore in nature there is a state of equilibrium in the amount of ammonia in the atmosphere and waters4 ,the presence of nitric acid in air is proved without doubt by the fact that rain-water contains an appreciable amount of it4 ,further 7as already mentioned in ,chapter ,,iv471 air contains ozone and hydrogen peroxide and nitrous acid 7and its ammonia salt71 i4e4 substances having a direct oxidising action 7for instance1 upon iodized starch-paper71 but they are present in very small quantities4#cg #cg ,whilst formed in the air these oxidising substances 7,n#b,o#c1 ozone and hydrogen peroxide7 at the same time rapidly disappear from it by oxidising those substances which are capable of being oxidised4 ,owing to this instability their amounts vary considerably1 and1 as would be expected1 they are met with to an appreciable amount in pure air1 whilst their amount decreases to zero in the air of cities1 and especially in dwellings where there is a maximum of substances capable of oxidisation and a minimum of conditions for the formation of such bodies4 ,there is a causal connection between the amount of these substances present in the air and its purity--that is1 the amount of foreign residues of organic origin liable to oxidation present in the air4 ,where there is much of such residues their amount must be small4 ,when they are present the amount of organic substances must be small1 as otherwise they would be destroyed4 ,for this reason efforts have been made to apply ozone for purifying the air by evolving it by artificial means in the atmosphere2 for instance1 by passing a series of electrical sparks through the ventilating pipes conveying air into a building4 ,air thus ozonised destroys by oxidation--that is1 brings about the combustion of--the organic residues present in the air1 and thus will serve for purifying it4 ,for these reasons the air of cities contains less ozone and such like oxidising agents than country air4 ,this forms the distinguishing feature of country air4 ,however1 animal life cannot exist in air containing a comparatively large amount of ozone4 ,besides substances in a gaseous or vaporous state1#ch there is always found a more or less considerable quantity of substances which are not known in a state of vapour4 ,these substances are present in the air as dust4 ,if a linen surface1 moistened with an acid1 be placed in perfectly pure air1 then the washings are found to contain sodium1 calcium1 iron1 and potassium4#ci ,linen moistened with an alkali absorbs carbonic1 sulphuric1 phosphoric1 and hydrochloric acids4 ,further1 the presence of organic substances in air has been proved by a similar experiment4 ,if a glass globe be filled with ice and placed in a room where are a number of people1 then the presence of organic substances1 like albuminous substances1 may be proved in the water which condenses on the surface of the globe4 ,it may be that the miasmas causing infection in marshy localities1 hospitals1 and in certain epidemic illnesses proceed from the presence of such substances in the air 7and especially in water1 which contains many micro-organisms71 as well as from the presence of germs of lower organisms in the air as a minute dust4 ,pasteur proved the existence of such germs in the air by the following experiment3--,he placed gun-cotton 7pyroxylin71 which has the appearance of ordinary cotton1 in a glass tube4 ,gun-cotton is soluble in a mixture of ether and alcohol1 forming the so-called collodion4 ,a current of air was passed through the tube for a long period of time1 and the gun-cotton was then dissolved in a mixture of ether and alcohol4 ,an insoluble residue was thus obtained which actually contained the germs of organisms1 as was shown by microscopical observations1 and by their capacity to develop into organisms 7mould1 c47 under favourable conditions4 ,the presence of these germs determines the property of air of bringing about the processes of putrefaction and fermentation--that is the fundamental alteration of organic substances1 which is accompanied by an entire change in their properties4 ,the appearance of lower organisms1 both vegetable and animal1 is always to be remarked in these processes4 ,thus1 for instance1 in the process of fermentation1 when1 for example1 wine is procured from the sweet juice of grapes1 a sediment separates out which is known under the name of lees1 and contains peculiar yeast organisms4 ,germs are required before these organisms can appear4#dj ,they are floating in the air1 and fall into the sweet fermentable liquid from it4 ,finding themselves under favourable conditions1 the germs develop into organisms2 they are nourished at the expense of the organic substance1 and during growth change and destroy it1 and bring about fermentation and putrefaction4 ,this is why1 for instance1 the juice of the grape when contained in the skin of the fruit1 which allows access of the air but is impenetrable to the germs1 does not ferment1 does not alter so long as the skin remains intact4 ,this is also the reason why animal substances when kept from the access of air may be preserved for a great length of time4 ,preserved foods for long sea voyages are kept in this way4#da ,hence it is evident that however infinitesimal the quantity of germs carried in the atmosphere may be1 still they have an immense significance in nature4#db #ch ,amongst them we may mention iodine and alcohol1 ,c#b;,h#f,o1 which ,mntz found to be always present in air1 the soil1 and water1 although in minute traces only4 #ci ,a portion of the atmospheric dust is of cosmic origin2 this is undoubtedly proved by the fact of its containing metallic iron as do meteorites4 ,nordenskild found iron in the dust covering snow1 and ,tissandier in every kind of air1 although naturally in very small quantities4 #dj ,the idea of the spontaneous growth of organisms in a suitable medium1 although still upheld by many1 has since the work of ,pasteur and his followers 7and to a certain extent of his predecessors7 been discarded1 because it has been proved how1 when1 and whence 7from the air1 water1 c47 the germs appear2 that fermentation as well as infectious diseases cannot take place without them2 and chiefly because it has been shown that any change accompanied by the development of the organisms introduced may be brought about at will by the introduction of the germs into a suitable medium4 #da ,in further confirmation of the fact that putrefaction and fermentation depend on germs carried in the air1 we may cite the circumstance that poisonous substances destroying the life of organisms stop or hinder the appearance of the above processes4 ,air which has been heated to redness or passed through sulphuric acid no longer contains the germs of organisms1 and loses the faculty of producing fermentation and putrefaction4 #db ,their presence in the air is naturally due to the diffusion of germs into the atmosphere1 and owing to their microscopical dimensions1 they1 as it were1 hang in the air in virtue of their large surfaces compared to their weight4 ,in ,paris the amount of dust suspended in the air equals from #f 7after rain7 to #bc grams per #a1jjj c4m4 of air4 ,thus we see that air contains a great variety of substances4 ,the nitrogen1 which is found in it in the largest quantity1 has the least influence on those processes which are accomplished by the action of air4 ,the oxygen1 which is met with in a lesser quantity than the nitrogen1 on the contrary takes a very important part in a number of reactions2 it supports combustion and respiration1 it brings about decomposition and every process of slow oxidation4 ,the part played by the moisture of air is well known4 ,the carbonic anhydride1 which is met with in still smaller quantities1 has an immense significance in nature1 inasmuch as it serves for the nourishment of plants4 ,the importance of the ammonia and nitric acid is very great1 because they are the sources of the nitrogenous substances comprising an indispensable element in all living organisms4 ,and1 lastly1 the infinitesimal quantity of germs also have a great significance in a number of processes4 ,thus it is not the quantitative but the qualitative relations of the component parts of the atmosphere which determine its importance in nature4#dc #dc ,we see similar cases everywhere4 ,for example1 the predominating mass of sand and clay in the soil takes hardly any chemical part in the economy of the soil in respect to the nourishment of plants4 ,the plants by their roots search for substances which are diffused in comparatively small quantities in the soil4 ,if a large quantity of these nourishing substances are removed1 then the plants will not develop in the soil1 just as animals die in oxygen4 ,air1 being a mixture of various substances1 may suffer considerable changes in consequence of incidental circumstances4 ,it is particularly necessary to remark those changes in the composition of air which take place in dwellings and in various localities where human beings have to remain during a lengthy period of time4 ,the respiration of human beings and animals alters the air4#dd ,a similar deterioration of air is produced by the influence of decomposing organic substances1 and especially of substances burning in it4#de ,hence it is necessary to have regard to the purification of the air of dwellings4 ,the renewal of air1 the replacing of respired by fresh air1 is termed 'ventilation1'#df and the removal of foreign and injurious admixtures from the air is called 'disinfection4'#dg ,the accumulation of all kinds of impurities in the air of dwellings and cities is the reason why the air of mountains1 forests1 seas1 and non-marshy localities1 covered with vegetation or snow1 is distinguished for its freshness1 and1 in all respects1 beneficial action4 #dd ,a man in breathing burns about #aj grams of carbon per hour--that is1 he produces about #hhj grams1 or 7as #a cub4m4 of carbonic anhydride weighs about #b1jjj grams7 about #e_/ab c4m4 of carbonic anhydride4 ,the air coming from the lungs contains #d p4c4 of carbonic anhydride by volume4 ,the exhaled air acts as a direct poison1 owing to this gas and to other impurities4 #de ,for this reason candles1 lamps1 and gas change the composition of air almost in the same way as respiration4 ,in the burning of #a kilogram of stearin candles1 #ej cubic metres of air are changed as by respiration--that is1 #d p4c4 of carbonic anhydride will be formed in this volume of air4 ,the respiration of animals and exhalations from their skins1 and especially from the intestines and the excrements and the transformations taking place in them1 contaminate the air to a still greater extent1 because they introduce other volatile substances besides carbonic anhydride into the air4 ,at the same time that carbonic anhydride is formed the amount of oxygen in the air decreases1 and there is noticed the appearance of miasmata which occur in but small quantity1 but which are noticeable in passing from fresh air into a confined space full of such adulterated air4 ,the researches of ,schmidt and ,leblanc and others show that even with #bjf p4c4 of oxygen 7instead of #bji p4c471 when the diminution is due to respiration1 air becomes noticeably less fit for respiration1 and that the heavy feeling experienced in such air increases with a lesser percentage of oxygen4 ,it is difficult to remain for a few minutes in air containing #agb p4c4 of oxygen4 ,these observations were chiefly obtained by observations on the air of different mines1 at different depths below the surface4 ,the air of theatres and buildings full of people also proves to contain less oxygen2 it was found on one occasion that at the end of a theatrical representation the air in the stalls contained #bjge p4c4 of oxygen1 whilst the air at the upper part of the theatre contained only #bjcf p4c4 ,the amount of carbonic anhydride in the air may be taken as a measure of its purity 7,pettenkofer74 ,when it reaches #a p4c4 it is very difficult for human beings to remain long in such air1 and it is necessary to set up a vigorous ventilation for the removal of the adulterated air4 ,in order to keep the air in dwellings in a uniformly good state1 it is necessary to introduce at least #aj cubic metres of fresh air per hour per person4 ,we saw that a man exhales about five-twelfths of a cubic metre of carbonic anhydride per day4 ,accurate observations have shown that air containing one-tenth p4c4 of exhaled carbonic anhydride 7and consequently also a corresponding amount of the other substances evolved together with it7 is not felt to be oppressive2 and therefore the five-twelfth cubic metres of carbonic anhydride should be diluted with #dbj cubic metres of fresh air if it be desired to keep not more than one-tenth p4c4 7by volume7 of carbonic anhydride in the air4 ,hence a man requires #dbj cubic metres of air per day1 or #ah cubic metres per hour4 ,with the introduction of only #aj cubic metres of fresh air per person1 the amount of carbonic anhydride may reach one-fifth p4c41 and the air will not then be of the requisite freshness4 #df ,the ventilation of inhabited buildings is most necessary1 and is even indispensable in hospitals1 schools1 and similar buildings4 ,in winter it is carried on by the so-called calorifiers or stoves heating the air before it enters4 ,the best kind of calorifiers in this respect are those in which the fresh cold air is led through a series of pipes heated by the hot gases coming from a stove4 ,in ventilation1 particularly during winter1 care is taken that the incoming air shall be moist1 because in winter the amount of moisture in the air is very small4 ,ventilation1 besides introducing fresh air into a dwelling-place1 must also withdraw the air already spoilt by respiration and other causes--that is1 it is necessary to construct channels for the escape of the bad air1 besides those for the introduction of fresh air4 ,in ordinary dwelling-places1 where not many people are congregated1 the ventilation is conducted by natural means1 in the heating by fires1 through crevices1 windows1 and various orifices in walls1 doors1 and windows4 ,in mines1 factories1 and workrooms ventilation is of the greatest importance4 ,animal vitality may still continue for a period of several minutes in air containing up to #cj p4c4 of carbonic anhydride1 if the remaining #gj p4c4 consist of ordinary air2 but respiration ceases after a certain time1 and death may even ensue4 ,the flame of a candle is very easily extinguished in an atmosphere containing from #e to #f p4c4 of carbonic anhydride1 but animal vitality can be sustained in it for a somewhat long time1 although the effect of such air is exceedingly painful even to the lower animals4 ,there are mines in which a lighted candle easily goes out from the excess of carbonic anhydride1 but in which the miners have to remain for a long time4 ,the presence of #a p4c4 of carbonic oxide is deadly even to cold-blooded animals4 ,the air in the galleries of a mine where blasting has taken place1 is known to produce a state of insensibility resembling that produced by charcoal fumes4 ,deep wells and vaults not unfrequently contain similar substances1 and their atmosphere often causes suffocation4 ,the atmospheres of such places cannot be tested by lowering a lighted candle into it1 as these poisonous gases would not extinguish the flame4 ,this method only suffices to indicate the amount of carbonic anhydride4 ,if a candle keeps alight1 it signifies that there is less than #f p4c4 of this gas4 ,in doubtful cases it is best to lower a dog or other animal into the air to be tested4 ,if ,,co#b be very carefully added to air1 the flame of a candle is not extinguished 7although it becomes very much smaller7 even when the gas amounts to #ab p4c4 of air4 ,researches made by ,f4 ,clowes 7#ahid7 show that the flames 7in every case #c_/d in4 long7 of different combustible substances are extinguished by the gradual addition of different percentages of nitrogen and carbonic acid to the air2 the percentage sufficient to extinguish the flame being as follows 7the percentage of oxygen is given in parenthesis73 p4c4 ,,co#b p4c4 ,n4 ,absolute alcohol #ad 7#aha7 #ba 7#aff7 ,candle #ad 7#aha7 #bb 7#afd7 ,hydrogen #eh 7 #hh7 #gj 7 #fc7 ,coal gas #cc 7#ada7 #df 7#aac7 ,carbonic oxide #bd 7#afj7 #bh 7#aea7 ,methane #aj 7#ahi7 #ag 7#agd7 ,the flames of all solid and liquid substances is extinguished by almost the same percentage of ,,co#b or ,n#b1 but the flames of different gases vary in this respect1 and hydrogen continues to burn in mixtures which are far poorer in oxygen than those in which the flames of other combustible gases are extinguished2 the flame of methane ,,ch#d is the most easily extinguished4 ,the percentage of nitrogen may be greater than that of ,,co#b4 ,this1 together with the fact that1 under the above circumstances1 the flame of a gas before going out becomes fainter and increases in size1 seems to indicate that the chief reason for the extinction of the flame is the fall in its temperature4 #dg ,different so-called disinfectants purify the air1 and prevent the injurious action of certain of its components by changing or destroying them4 ,disinfection is especially necessary in those places where a considerable amount of volatile substances are evolved into the air1 and where organic substances are decomposed2 for instance1 in hospitals1 closets1 c4 ,the numerous disinfectants are of the most varied nature4 ,they may be divided into oxidising1 antiseptic1 and absorbent substances4 ,to the oxidising substances used for disinfection belong chlorine1 and various substances evolving it1 because chlorine in the presence of water oxidises the majority of organic substances1 and this is why chlorine is used as a disinfectant for ,siberian plagues4 ,further1 to this class belong the permanganates of the alkalis and peroxide of hydrogen1 as substances easily oxidising matters dissolved in water2 these salts are not volatile like chlorine1 and therefore act much more slowly1 and in a much more limited sphere4 ,antiseptic substances are those which convert organic substances into such as are little prone to change1 and prevent putrefaction and fermentation4 ,they most probably kill the germs of organisms occurring in miasmata4 ,the most important of these substances are creosote and phenol 7carbolic acid71 which occur in tar1 and act in preserving smoked meat4 ,phenol is a substance little soluble in water1 volatile1 oily1 and having the characteristic smell of smoked objects4 ,its action on animals in considerable quantities is injurious1 but in small quantities1 used in the form of a weak solution1 it prevents the change of animal matter4 ,the smell of privies1 which depends on the change of excremental matter1 may be easily removed by means of chlorine or phenol4 ,salicylic acid1 thymol1 common tar1 and especially its solution in alkalis as proposed by ,nensky1 c41 are also substances having the same property4 ,absorbent substances are of no less importance1 especially as preventatives1 than the preceding two classes of disinfectants1 inasmuch as they are innocuous4 ,they are those substances which absorb the odoriferous gases and vapours emitted during putrefaction1 which are chiefly ammonia1 sulphuretted hydrogen1 and other volatile compounds4 ,to this class belong charcoal1 certain salts of iron1 gypsum1 salts of magnesia1 and similar substances1 as well as peat1 mould1 and clay4 ,questions of disinfection and ventilation appertain to the most serious problems of common life and hygiene4 ,these questions are so vast that we are here able only to give a short outline of their nature4 ,,chapter ,,vi ,,the ,,compounds ,,of ,,nitrogen ,,with ,,hydrogen ,,and ,,oxygen ,illustration3 ,,fig4 #db4--,the dry distillation of bones on a large scale4 ,the bones are heated in the vertical cylinders ,c 7about #a-a_/b metre high and #cj centimetres in diameter74 ,the products of distillation pass through the tubes ,t1 into the condenser ,b1 and receiver ,f4 ,when the distillation is completed the trap ,h is opened1 and the burnt bones are loaded into trucks ,v4 ,the roof ,m is then opened1 and the cylinders are charged with a fresh quantity of bones4 ,the ammonia water is preserved1 and goes to the preparation of ammoniacal salts1 as described in the following drawing4 ,in the last chapter we saw that nitrogen does not directly combine with hydrogen1 but that a mixture of these gases in the presence of hydrochloric acid gas1 ,h,cl1 forms ammonium chloride1 ,,nh#d;,cl1 on the passage of a series of electric sparks4#a ,in ammonium chloride1 ,h,cl is combined with ,,nh#c1 consequently ,n with ,h#c forms ammonia4#b ,almost all the nitrogenous substances of plants and animals evolve ammonia when heated with an alkali4 ,but even without the presence of an alkali the majority of nitrogenous substances1 when decomposed or heated with a limited supply of air1 evolve their nitrogen1 if not entirely1 at all events partially1 in the form of ammonia4 ,when animal substances such as skins1 bones1 flesh1 hair1 horns1 c41 are heated without access of air in iron retorts--they undergo what is termed dry distillation4 ,a portion of the resultant substances remains in the retort and forms a carbonaceous residue1 whilst the other portion1 in virtue of its volatility1 escapes through the tube leading from the retort4 ,the vapours given off1 on cooling1 form a liquid which separates into two layers2 the one1 which is oily1 is composed of the so-called animal oils 7oleum animale73 the other1 an aqueous layer1 contains a solution of ammonia salts4 ,if this solution be mixed with lime and heated1 the lime takes up the elements of carbonic acid from the ammonia salts1 and ammonia is evolved as a gas4#c ,in ancient times ammonia compounds were imported into ,europe from ,egypt1 where they were prepared from the soot obtained in the employment of camels' dung as fuel in the locality of the temple of ,jupiter ,ammon 7in ,lybia71 and therefore the salt obtained was called 'sal-ammoniacale1' from which the name of ammonia is derived4 ,at the present time ammonia is obtained exclusively1 on a large scale1 either from the products of the dry distillation of animal or vegetable refuse1 from urine1 or from the ammoniacal liquors collected in the destructive distillation of coal for the preparation of coal gas4 ,this ammoniacal liquor is placed in a retort with lime and heated2 the ammonia is then evolved together with steam4#d ,in the arts1 only a small amount of ammonia is used in a free state--that is1 in an aqueous solution2 the greater portion of it is converted into different salts having technical uses1 especially sal-ammoniac1 ,,nh#d;,cl1 and ammonium sulphate1 7,,nh#d7b,,so#d4 ,they are saline substances which are formed because ammonia1 ,,nh#c1 combines with all acids1 ,,hx1 forming ammonia salts1 ,,nh#d,x4 ,sal-ammoniac1 ,,nh#d;,cl1 is a compound of ammonia with hydrochloric acid4 ,it is prepared by passing the vapours of ammonia and water1 evolved1 as above described1 from ammoniacal liquor1 into an aqueous solution of hydrochloric acid1 and on evaporating the solution sal-ammoniac is obtained in the form of soluble crystals#e resembling common salt in appearance and properties4 ,ammonia may be very easily prepared from this sal-ammoniac1 ,,nh#d;,cl1 as from any other ammoniacal salt1 by heating it with lime4 ,calcium hydroxide1 ,ca,h#b,o#b1 as an alkali takes up the acid and sets free the ammonia1 forming calcium chloride1 according to the equation #b,,nh#d;,cl ,ca,h#b,o#b #b;,h#b,o ,ca,cl#b #b,,nh#c4 ,in this reaction the ammonia is evolved as a gas4#f ,illustration3 ,,fig4 #dc4--,method of abstracting ammonia1 on a large scale1 from ammonia water obtained at gas works by the dry distillation of coal1 or by the fermentation of urine1 c4 ,this water is mixed with lime and poured into the boiler ,c1 and from thence into ,c and ,c consecutively4 ,the last boiler is heated directly over a furnace1 and hence no ammonia remains in solution after the liquid has been boiled in it4 ,the liquid is therefore then thrown away4 ,the ammonia vapour and steam pass from the boiler ,c1 through the tube ,t1 into the boiler ,c1 and then into ,c1 so that the solution in ,c becomes stronger than that in ,c1 and still stronger in ,c4 ,the boilers are furnished with stirrers ,a1 ,a1 and ,a to prevent the lime settling4 ,from ,c the ammonia and steam pass through the tube ,t into worm condensers surrounded with cold water1 thence into the ,woulfe's bottle ,p1 where the solution of ammonia is collected1 and finally the still uncondensed ammonia vapour is led into the flat vessel ,r1 containing acid which absorbs the last traces of ammonia4 #a ,the ammonia in the air1 water1 and soil proceeds from the decomposition of the nitrogenous substances of plants and animals1 and also probably from the reduction of nitrates4 ,ammonia is always formed in the rusting of iron4 ,its formation in this case depends in all probability on the decomposition of water1 and on the action of the hydrogen at the moment of its evolution on the nitric acid contained in the air 7,cloez71 or on the formation of ammonium nitrite1 which takes place under many circumstances4 ,the evolution of vapours of ammonia compounds is sometimes observed in the vicinity of volcanoes4 ,at a red heat nitrogen combines directly with ,b ,ca ,mg1 and with many other metals1 and these compounds1 when heated with a caustic alkali1 or in the presence of water1 give ammonia 7see ,chapter ,,xiv41 ,note #ad1 and ,chapter ,,xvii41 ,note #ab74 ,these are examples of the indirect combination of nitrogen with hydrogen4 #b ,if a silent discharge or a series of electric sparks be passed through ammonia gas1 it is decomposed into nitrogen and hydrogen4 ,this is a phenomenon of dissociation2 therefore1 a series of sparks do not totally decompose the ammonia1 but leave a certain portion undecomposed4 ,one volume of nitrogen and three volumes of hydrogen are obtained from two volumes of ammonia decomposed4 ,ramsay and ,young 7#ahhd7 investigated the decomposition of ,,nh#c under the action of heat1 and showed that at #ejj1 #a-a_/b p4c4 is decomposed1 at #fjj about #ah p4c41 at #hjj #fe p4c41 but these results were hardly free from the influence of 'contact4' ,the presence of free ammonia--that is1 ammonia not combined with acids--in a gas or aqueous solution may be recognised by its characteristic smell4 ,but many ammonia salts do not possess this smell4 ,however1 on the addition of an alkali 7for instance1 caustic lime1 potash1 or soda71 they evolve ammonia gas1 especially when heated4 ,the presence of ammonia may be made visible by introducing a substance moistened with strong hydrochloric acid into its neighbourhood4 ,a white cloud1 or visible white vapour1 then makes its appearance4 ,this depends on the fact that both ammonia and hydrochloric acid are volatile1 and on coming into contact with each other produce solid sal-ammoniac1 ,,nh#d;,cl1 which forms a cloud4 ,this test is usually made by dipping a glass rod into hydrochloric acid1 and holding it over the vessel from which the ammonia is evolved4 ,with small amounts of ammonia this test is1 however1 untrustworthy1 as the white vapour is scarcely observable4 ,in this case it is best to take paper moistened with mercurous nitrate1 ,hg,n,o#c4 ,this paper turns black in the presence of ammonia1 owing to the formation of a black compound of ammonia with mercurous oxide4 ,the smallest traces of ammonia 7for instance1 in river water7 may be detected by means of the so-called ,nessler's reagent1 containing a solution of mercuric chloride and potassium iodide1 which forms a brown coloration or precipitate with the smallest quantities of ammonia4 ,it will be useful here to give the thermochemical data 7in thousands of units of heat1 according to ,thomsen71 or the quantities of heat evolved in the formation of ammonia and its compounds in quantities expressed by their formul4 ,thus1 for instance1 7,n ,h#c7 #bfg indicates that #ad grams of nitrogen in combining with #c grams of hydrogen develop sufficient heat to raise the temperature of #bfg kilograms of water #a4 7,,nh#c n,h#b,o7 #hd 7heat of solution72 7,,nh#c1n,h#b,o ,h,cl1n,h#b,o7 #abc2 7,n ,h#d ,cl7 #ijf2 7,,nh#c ,h,cl7 #dai4 #c ,the same ammonia water is obtained1 although in smaller quantities1 in the dry distillation of plants and of coal1 which consists of the remains of fossil plants4 ,in all these cases the ammonia proceeds from the destruction of the complex nitrogenous substances occurring in plants and animals4 ,the ammonia salts employed in the arts are prepared by this method4 #d ,the technical methods for the preparation of ammonia water1 and for the extraction of ammonia from it1 are to a certain extent explained in the figures accompanying the text4 #e ,usually these crystals are sublimed by heating them in crucibles or pots1 when the vapours of sal-ammoniac condense on the cold covers as a crust1 in which form the salt comes into the market4 #f ,on a small scale ammonia may be prepared in a glass flask by mixing equal parts by weight of slaked lime and finely-powdered sal-ammoniac1 the neck of the flask being connected with an arrangement for drying the gas obtained4 ,in this instance neither calcium chloride nor sulphuric acid can be used for drying the gas1 since both these substances absorb ammonia1 and therefore solid caustic potash1 which is capable of retaining the water1 is employed4 ,the gas-conducting tube leading from the desiccating apparatus is introduced into a mercury bath1 if dry gaseous ammonia be required1 because water cannot be employed in collecting ammonia gas4 ,ammonia was first obtained in this dry state by ,priestley1 and its composition was investigated by ,berthollet at the end of the last century4 ,oxide of lead mixed with sal-ammoniac 7,isambert7 evolves ammonia with still greater ease than lime4 ,the cause and process of the decomposition are almost the same1 #b,pb,o #b,,nh#d;,cl ,pb#b,o,cl#b ,h#b,o #b,,nh#c4 ,lead oxychloride is 7probably7 formed4 ,it must be observed that all the complex nitrogenous substances of plants1 animals1 and soils are decomposed when heated with an excess of sulphuric acid1 the whole of their nitrogen being converted into ammonium sulphate1 from which it may be liberated by treatment with an excess of alkali4 ,this reaction is so complete that it forms the basis of ,kjeldahl's method for estimating the amount of nitrogen in its compounds4 ,ammonia is a colourless gas1 resembling those with which we are already acquainted in its outward appearance1 but clearly distinguishable from any other gas by its very characteristic and pungent smell4 ,it irritates the eyes1 and it is positively impossible to inhale it4 ,animals die in it4 ,its density1 referred to hydrogen1 is #he2 hence it is lighter than air4 ,it belongs to the class of gases which are easily liquefied4#g ,faraday employed the following method for liquefying ammonia4 ,ammonia when passed over dry silver chloride1 ,ag,cl1 is absorbed by it to a considerable extent1 especially at low temperatures4#h ,the solid compound ,ag,cl1#c,,nh#c thus obtained is introduced into a bent tube 7fig4 #de71 whose open end c is then fused up4 ,the compound is then slightly heated at a1 and the ammonia comes off1 owing to the easy dissociation of the compound4 ,the other end of the tube is immersed in a freezing mixture4 ,the pressure of the gas coming off1 combined with the low temperature at one end of the tube1 causes the ammonia evolved to condense into a liquid1 in which form it collects at the cold end of the tube4 ,if the heating be stopped1 the silver chloride again absorbs the ammonia4 ,in this manner one tube may serve for repeated experiments4 ,ammonia may also be liquefied by the ordinary methods--that is1 by means of pumping dry ammonia gas into a refrigerated space4 ,liquefied ammonia is a colourless and very mobile liquid1#i whose specific gravity at #j is #jfc 7,e4 ,andreff74 ,at the temperature 7about -#gj7 given by a mixture of liquid carbonic anhydride and ether1 liquid ammonia crystallises1 and in this form its odour is feeble1 because at so low a temperature its vapour tension is very inconsiderable4 ,the boiling point 7at a pressure of #gfj mm47 of liquid ammonia is about -#cb4 ,hence this temperature may be obtained at the ordinary pressure by the evaporation of liquefied ammonia4 ,illustration3 ,,fig4 #de4--,the liquefaction of ammonia in a thick bent glass tube4 ,a compound of chloride of silver and ammonia is placed in the end a1 and the end c is then sealed up4 #g ,illustration3 ,,fig4 #dd4--;,carr's apparatus4 ,described in text4 ,this is evident from the fact that its absolute boiling point lies at about #acj 7,chapter ,,ii41 ,note #bi74 ,it may therefore be liquefied by pressure alone at the ordinary1 and even at much higher temperatures4 ,the latent heat of evaporation of #ag parts by weight of ammonia equals #d1djj units of heat1 and hence liquid ammonia may be employed for the production of cold4 ,strong aqueous solutions of ammonia1 which in parting with their ammonia act in a similar manner1 are not unfrequently employed for this purpose4 ,suppose a saturated solution of ammonia to be contained in a closed vessel furnished with a receiver4 ,if the ammoniacal solution be heated1 the ammonia1 with a small quantity of water1 will pass off from the solution1 and in accumulating in the apparatus will produce a considerable pressure1 and will therefore liquefy in the cooler portions of the receiver4 ,hence liquid ammonia will be obtained in the receiver4 ,the heating of the vessel containing the aqueous solution of ammonia is then stopped4 ,after having been heated it contains only water1 or a solution poor in ammonia4 ,when once it begins to cool the ammonia vapours commence dissolving in it1 the space becomes rarefied1 and a rapid vaporisation of the liquefied ammonia left in the receiver takes place4 ,in evaporating in the receiver it will cause the temperature in it to fall considerably1 and will itself pass into the aqueous solution4 ,in the end1 the same ammoniacal solution as originally taken is re-obtained4 ,thus1 in this case1 on heating the vessel the pressure increases by itself1 and on cooling it diminishes1 so that here heat directly replaces mechanical work4 ,this is the principle of the simplest forms of ,carr's ice-making machines1 shown in fig4 #dd4 ,c is a vessel made of boiler plates into which the saturated solution of ammonia is poured2 m is a tube conducting the ammonia vapour to the receiver ,a4 ,all parts of the apparatus should be hermetically joined together1 and should be able to withstand a pressure reaching ten atmospheres4 ,the apparatus should be freed from air1 which would otherwise hinder the liquefaction of the ammonia4 ,the process is carried on as follows3--,the apparatus is first so inclined that any liquid remaining in ,a may flow into ,c4 ,the vessel ,c is then placed upon a stove ,f1 and heated until the thermometer t indicates a temperature of #acj ,c4 ,during this time the ammonia has been expelled from ,c1 and has liquefied in ,a4 ,in order to facilitate the liquefaction1 the receiver ,a should be immersed in a tank of water ,r 7see the left-hand drawing in fig4 #dd74 ,after about half an hour1 when it may be supposed that the ammonia has been expelled1 the fire is removed from under ,c1 and this is now immersed in the tank of water ,r4 ,the apparatus is represented in this position in the right-hand drawing of fig4 #dd4 ,the liquefied ammonia then evaporates1 and passes over into the water in ,c4 ,this causes the temperature of ,a to fall considerably4 ,the substance to be refrigerated is placed in a vessel ,g1 in the cylindrical space inside the receiver ,a4 ,the refrigeration is also kept on for about half an hour1 and with an apparatus of ordinary dimensions 7containing about two litres of ammonia solution71 five kilograms of ice are produced by the consumption of one kilogram of coal4 ,in industrial works more complicated types of ,carr's machines are employed4 #h ,below #ae 7according to ,isambert71 the compound ,ag,cl1#c,,nh#c is formed1 and above #bj the compound #b;,ag,cl1#c,,nh#c4 ,the tension of the ammonia evolved from the latter substance is equal to the atmospheric pressure at #fh1 whilst for ,ag,cl1#c,,nh#c the pressures are equal at about #bj2 consequently1 at higher temperatures it is greater than the atmospheric pressure1 whilst at lower temperatures the ammonia is absorbed and forms this compound4 ,consequently1 all the phenomena of dissociation are here clearly to be observed4 ,joannis and ,croisier 7#ahid7 investigated similar compounds with ,ag,br1 ,ag,i1 ,ag,c,n and ,ag,n,o#c1 and found that they all give definite compounds with ,,nh#c1 for instance ,ag,br1#c,,nh#c1 #b;,ag,br1#c,,nh#c and ,ag,br1#b,,nh#c2 they are all colourless1 solid substances which decompose under the atmospheric pressure at #ce1 #cd and #ea4 #i ,the liquefaction of ammonia may be accomplished without an increase of pressure1 by means of refrigeration alone1 in a carefully prepared mixture of ice and calcium chloride 7because the absolute boiling point of ,,nh#c is high1 about #acj74 ,it may even take place in the severe frosts of a ,russian winter4 ,the application of liquid ammonia as a motive power for engines forms a problem which has to a certain extent been solved by the ,french engineer ,tellier4 ,ammonia1 containing1 as it does1 much hydrogen1 is capable of combustion2 it does not1 however1 burn steadily1 and sometimes not at all1 in ordinary atmospheric air4 ,in pure oxygen it burns with a greenish-yellow flame1#aj forming water1 whilst the nitrogen set free gives its oxygen compounds--that is1 oxides of nitrogen4 ,the decomposition of ammonia into hydrogen and nitrogen not only takes place at a red heat and under the action of electric sparks1 but also by means of many oxidising substances2 for instance1 by passing ammonia through a tube containing red-hot copper oxide4 ,the water thus formed may be collected by substances absorbing it1 and the quantity of nitrogen may be measured in a gaseous form1 and thus the composition of ammonia determined4 ,in this manner it is very easy to prove that ammonia contains #c parts by weight of hydrogen to #ad parts by weight of nitrogen2 and1 by volume1 #c vols4 of hydrogen and #a vol4 of nitrogen form #b vols4 of ammonia4#aa #aj ,the combustion of ammonia in oxygen may be effected by the aid of platinum4 ,a small quantity of an aqueous solution of ammonia1 containing about #bj p4c4 of the gas1 is poured into a wide-necked beaker of about one litre capacity4 ,a gas-conducting tube about #aj mm4 in diameter1 and supplying oxygen1 is immersed in the aqueous solution of ammonia4 ,but before introducing the gas an incandescent platinum spiral is placed in the beaker2 the ammonia in the presence of the platinum is oxidised and burns1 whilst the platinum wire becomes still more incandescent4 ,the solution of ammonia is heated1 and oxygen passed through the solution4 ,the oxygen1 as it bubbles off from the ammonia solution1 carries with it a part of the ammonia1 and this mixture explodes on coming into contact with the incandescent platinum4 ,this is followed by a certain cooling effect1 owing to the combustion ceasing1 but after a short interval this is renewed1 so that one feeble explosion follows after another4 ,during the period of oxidation without explosion1 white vapours of ammonium nitrite and red-brown vapours of oxides of nitrogen make their appearance1 while during the explosion there is complete combustion and consequently water and nitrogen are formed4 #aa ,this may be verified by their densities4 ,nitrogen is #ad times denser than hydrogen1 and ammonia is #h-a_/b times4 ,if #c volumes of hydrogen with #a volume of nitrogen gave #d volumes of ammonia1 then these #d volumes would weigh #ag times as much as #a volume of hydrogen2 consequently #a volume of ammonia would be #d-a_/d times heavier than the same volume of hydrogen4 ,but if these #d volumes only give #b volumes of ammonia1 the latter will be #h-a_/b times as dense as hydrogen1 which is found to be actually the case4 ,ammonia is capable of combining with a number of substances1 forming1 like water1 substances of various degrees of stability4 ,it is more soluble than any of the gases yet described1 both in water and in many aqueous solutions4 ,we have already seen1 in the first chapter1 that one volume of water1 at the ordinary temperature1 dissolves about #gjj vols4 of ammonia gas4 ,the great solubility of ammonia enables it to be always kept ready for use in the form of an aqueous solution1#ab which is commercially known as spirits of hartshorn4 ,ammonia water is continually evolving ammoniacal vapour1 and so has the characteristic smell of ammonia itself4 ,it is a very characteristic and important fact that ammonia has an alkaline reaction1 and colours litmus paper blue1 just like caustic potash or lime2 it is therefore sometimes called caustic ammonia 7volatile alkali74 ,acids may be saturated by ammonia water or gas in exactly the same way as by any other alkali4 ,in this process ammonia combines directly with acids1 and this forms the most essential chemical reaction of this substance4 ,if sulphuric1 nitric1 acetic1 or any other acid be brought into contact with ammonia it absorbs it1 and in so doing evolves a large amount of heat and forms a compound having all the properties of a salt4 ,thus1 for example1 sulphuric acid1 ,h#b,,so#d1 in absorbing ammonia1 forms 7on evaporating the solution7 two salts1 according to the relative quantities of ammonia and acid4 ,one salt is formed from ,,nh#c ,h#b,,so#d1 and consequently has the composition ,,nh#e,,so#d1 and the other is formed from #b,,nh#c ,h#b,,so#d1 and its composition is therefore ,n#b;,h#h,,so#d4 ,the former has an acid reaction and the latter a neutral reaction1 and they are called respectively acid ammonium sulphate 7ammonium hydrogen sulphate71 and normal ammonium sulphate1 or simply ammonium sulphate4 ,the same takes place in the action of all other acids2 but certain of them are able to form normal ammonium salts only1 whilst others give both acid and normal ammonium salts4 ,this depends on the nature of the acid and not on the ammonia1 as we shall afterwards see4 ,ammonium salts are very similar in appearance and in many of their properties to metallic salts2 for instance1 sodium chloride1 or table salt1 resembles sal-ammoniac1 or ammonium chloride1 not only in its outward appearance but even in crystalline form1 in its property of giving precipitates with silver salts1 in its solubility in water1 and in its evolving hydrochloric acid when heated with sulphuric acid--in a word1 a most perfect analogy is to be remarked in an entire series of reactions4 ,an analogy in composition is seen if sal-ammoniac1 ,,nh#d;,cl1 be compared with table salt1 ,na,cl2 and the ammonium hydrogen sulphate1 ,,nh#d;,,hso#d1 with the sodium hydrogen sulphate1 ,na,h,s,o#d2 or ammonium nitrate1 ,,nh#d,,no#c1 with sodium nitrate1 ,na,n,o#c4ac ,it is seen1 on comparing the above compounds1 that the part which sodium takes in the sodium salts is played in ammonium salts by a group ,,nh#d1 which is called ammonium4 ,if table salt be called 'sodium chloride1' then sal-ammoniac should be and is called 'ammonium chloride4' #ab ,aqueous solutions of ammonia are lighter than water1 and at #ae1 taking water at #d #aj1jjj1 their specific gravity1 as dependent on p1 or the percentage amount 7by weight7 of ammonia1 is given by the expression s #i1iib-dbep #jbap#b2 for instance1 with #aj p4c4 s #i1ehg4 ,if t represents the temperature between the limits of #aj and #bj1 then the expression 7#ae-t77#ae #jadp7 must be added to the formula for the specific gravity4 ,solutions containing more than #bd p4c4 have not been sufficiently investigated in respect to the variation of their specific gravity4 ,it is1 however1 easy to obtain more concentrated solutions1 and at #j solutions approaching ,,nh#c1;,h#b,o 7#dhf p4c4 ,,nh#c7 in their composition1 and of sp4 gr4 #jhe1 may be prepared4 ,but such solutions give up the bulk of their ammonia at the ordinary temperature1 so that more than #bd p4c4 ,,nh#c is rarely contained in solution4 ,ammoniacal solutions containing a considerable amount of ammonia give ice-like crystals which seem to contain ammonia at temperatures far below #j 7for instance1 an #h p4c4 solution at -#ad1 the strongest solutions at -#dh74 ,the whole of the ammonia may be expelled from a solution by heating1 even at a comparatively low temperature2 hence on heating aqueous solutions containing ammonia a very strong solution of ammonia is obtained in the distillate4 ,alcohol1 ether1 and many other liquids are also capable of dissolving ammonia4 ,solutions of ammonia1 when exposed to the atmosphere1 give off a part of their ammonia in accordance with the laws of the solution of gases in liquids1 which we have already considered4 ,but the ammoniacal solutions at the same time absorb carbonic anhydride from the air1 and ammonium carbonate remains in the solution4 ,illustration3 ,,fig4 #df4--;,apparatus for preparing solutions of ammonia4 ,solutions of ammonia are required both for laboratory and factory operations1 and have therefore to be frequently prepared4 ,for this purpose the arrangement shown in fig4 #df is employed in the laboratory4 ,in works the same arrangement is used1 only on a larger scale 7with earthenware or metallic vessels74 ,the gas is prepared in the retort1 from whence it is led into the two-necked globe ,a1 and then through a series of ,woulfe's bottles1 ,b1 ,c1 ,d1 ,e4 ,the impurities spurting over collect in ,a1 and the gas is dissolved in ,b1 but the solution soon becomes saturated1 and a purer 7washed7 ammonia passes over into the following vessels1 in which only a pure solution is obtained4 ,the bent funnel tube in the retort preserves the apparatus from the possibility both of the pressure of the gas evolved in it becoming too great 7when the gas escapes through it into the air71 and also from the pressure incidentally falling too low 7for instance1 owing to a cooling effect1 or from the reaction stopping74 ,if this takes place1 the air passes into the retort1 otherwise the liquid from ,b would be drawn into ,a4 ,the safety tubes in each ,woulfe's bottle1 open at both ends1 and immersed in the liquid1 serve for the same purpose4 ,without them1 in case of an accidental stoppage in the evolution of so soluble a gas as ammonia1 the solution would be sucked from one vessel to another--for instance1 from ,e into ,d1 c4 ,in order to clearly see the necessity for safety tubes in a gas apparatus1 it must be remembered that the gaseous pressure in the interior of the arrangement must exceed the atmospheric pressure by the height of the sum of the columns of liquid through which the gas has to pass4 #ac ,the analogy between the ammonium and sodium salts might seem to be destroyed by the fact that the latter are formed from the alkali or oxide and an acid1 with the separation of water1 whilst the ammonium salts are directly formed from ammonia and an acid1 without the separation of water2 but the analogy is restored if we compare soda to ammonia water1 and liken caustic soda to a compound of ammonia with water4 ,then the very preparation of ammonium salts from such a hydrate of ammonia will completely resemble the preparation of sodium salts from soda4 ,we may cite as an example the action of hydrochloric acid on both substances4 ,na,h,o ,h,cl ,h#b,o ,na,cl ,sodium hydroxide ,hydrochloric acid ,water ,table salt ,,nh#d;,,ho ,h,cl ,h#bj ,,nh#d;,cl ,ammonium hydroxide ,hydrochloric acid ,water ,sal-ammoniac ,just as in soda the hydroxyl or aqueous radicle ,,oh is replaced by chlorine1 so it is in ammonia hydrate4 ,the hypothesis that ammoniacal salts correspond with a complex metal ammonium bears the name of the ammonium theory4 ,it was enunciated by the famous ,swedish chemist ,berzelius after the proposition made by ,ampre4 ,the analogy admitted between ammonium and metals is probable1 owing to the fact that mercury is able to form an amalgam with ammonium similar to that which it forms with sodium or many other metals4 ,the only difference between ammonium amalgam and sodium amalgam consists in the instability of the ammonium1 which easily decomposes into ammonia and hydrogen4#ad ,ammonium amalgam may be prepared from sodium amalgam4 ,if the latter be shaken up with a strong solution of sal-ammoniac1 the mercury swells up violently and loses its mobility whilst preserving its metallic appearance4 ,in so doing1 the mercury dissolves ammonium--that is1 the sodium in the mercury is replaced by the ammonium1 and replaces it in the sal-ammoniac1 forming sodium chloride1 ,,nh#d;,cl ,hg,na ,na,cl ,hg,n,h#d4 ,naturally1 the formation of ammonium amalgam does not entirely prove the existence of ammonium itself in a separate state2 but it shows the possibility of this substance existing1 and its analogy with the metals1 because only metals dissolve in mercury4#ae ,ammonium amalgam crystallises in cubes1 three times heavier than water2 it is only stable in the cold1 and particularly at very low temperatures4 ,it begins to decompose at the ordinary temperature1 evolving ammonia and hydrogen in the proportion of two volumes of ammonia and one volume of hydrogen1 ,,nh#d ,,nh#c ,h4 ,by the action of water1 ammonium amalgam gives hydrogen and ammonia water1 just as sodium amalgam gives hydrogen and sodium hydroxide2 and therefore1 in accordance with the ammonium theory1 ammonia water must be looked on as containing ammonium hydroxide1 ,,nh#d,,oh1#af just as an aqueous solution of sodium hydroxide1 contains ,na,o,h4 ,the ammonium hydroxide1 like ammonium itself1 is an unstable substance1 which easily dissociates1 and can only exist in a free state at low temperatures4#ag ,ordinary solutions of ammonia must be looked on as the products of the dissociation of this hydroxide1 inasmuch as ,,nh#d,,oh ,,nh#c ,h#b,o4 #ad ,weyl 7#ahfd7 by subjecting sodium to the action of ammonia at the ordinary temperature and under considerable pressures1 obtained a liquid1 which was subsequently investigated by ,joannis 7#ahhi71 who confirmed the results obtained by ,weyl4 ,at #j and the atmospheric pressure the composition of this substance is ,na #ec,,nh#c4 ,the removal 7at #j7 of ammonia from the liquid gives a solid copper-red body having the composition ,,nh#c,na4 ,the determination of the molecular weight of this substance by the fall of the tension of liquid ammonia gave ,n#b;,h#f,na#b4 ,it is1 therefore1 free ammonium in which one ,h is replaced by ,na4 ,the compound with potassium1 obtained under the same conditions1 proved to have an analogous composition4 ,by the decomposition of ,,nh#c,na at the ordinary temperature1 ,joannis 7#ahia7 obtained hydrogen and sodium-amide ,,nh#b,na in small colourless crystals which were soluble in water4 ,the addition of liquid ammonia to metallic sodium and a saturated solution of sodium chloride1 gives ,,nh#b,na#b;,cl1 and this substance is sal-ammoniac1 in which ,h#b is replaced by ,na#b4 ,if pure oxygen be passed through a solution of these compounds in ammonia at a temperature of about -#ej1 it is seen that the gas is rapidly absorbed4 ,the liquid gradually loses its dark red colour and becomes lighter1 and when it has become quite colourless a gelatinous precipitate is thrown down4 ,after the removal of the ammonia1 this precipitate dissolves easily in water with a considerable evolution of heat1 but without giving off any gaseous products4 ,the composition of the sodium compound thus obtained is ,,nh#b,na#b;,,ho1 which shows that it is a hydrate of bisodium-ammonium4 ,thus1 although free ammonium has not been obtained1 still a sodium substitution product of it is known which corresponds to it as a salt to a hydrate4 ,ammonium amalgam was originally obtained in exactly the same way as sodium amalgam 7,davy72 namely1 a piece of sal-ammoniac was taken1 and moistened with water 7in order to render it a conductor of electricity74 ,a cavity was made in it1 into which mercury was poured1 and it was laid on a sheet of platinum connected with the positive pole of a galvanic battery1 while the negative pole was put into connection with the mercury4 ,on passing a current the mercury increased considerably in volume1 and became plastic1 whilst preserving its metallic appearance1 just as would be the case were the sal-ammoniac replaced by a lump of a sodium salt or of many other metals4 ,in the analogous decomposition of common metallic salts1 the metal contained in a given salt separates out at the negative pole1 immersed in mercury1 by which the metal is dissolved4 ,a similar phenomenon is observed in the case of sal-ammoniac2 the elements of ammonium1 ,,nh#d1 in this case are also collected in the mercury1 and are retained by it for a certain time4 #ae ,we may mention1 however1 that under particular conditions hydrogen is also capable of forming an amalgam resembling the amalgam of ammonium4 ,if an amalgam of zinc be shaken up with an aqueous solution of platinum chloride1 without access of air1 then a spongy mass is formed which easily decomposes1 with the evolution of hydrogen4 #af ,we saw above that the solubility of ammonia in water at low temperatures attains to the molecular ratio ,,nh#c ,h#b,o1 in which these substances are contained in caustic ammonia1 and perhaps it may be possible at exceedingly low temperatures to obtain ammonium hydroxide1 ,,nh#d;,,ho1 in a solid form4 ,regarding solutions as dissociated definite compounds1 we should see a confirmation of this view in the property shown by ammonia of being extremely soluble in water1 and in so doing of approaching to the limit ,,nh#d;,,ho4 #ag ,in confirmation of the truth of this conclusion we may cite the remarkable fact that there exist1 in a free state and as comparatively stable compounds1 a series of alkaline hydroxides1 ,,nr#d;,,ho1 which are perfectly analogous to ammonium hydroxide1 and present a striking resemblance to it and to sodium hydroxide1 with the only difference that the hydrogen in ,,nh#d;,,ho is replaced by complex groups1 ,r ,,ch#c1 ,c#b;,h#e1 c41 for instance ,n7,,ch#c7d;,,ho4 ,details will be found in organic chemistry4 ,all ammoniacal salts decompose at a red heat into ammonia and an acid1 which1 on cooling in contact with each other1 re-combine together4 ,if the acid be non-volatile1 the ammoniacal salt1 when heated1 evolves the ammonia1 leaving the non-volatile acid behind2 if the acid be volatile1 then1 on heating1 both the acid and ammonia volatilise together1 and on cooling re-combine into the salt which originally served for the formation of their vapours4#ah #ah ,the fact that ammoniacal salts are decomposed when ignited1 and not simply sublimed1 may be proved by a direct experiment with sal-ammoniac1 ,,nh#d;,cl1 which in a state of vapour is decomposed into ammonia1 ,,nh#c1 and hydrochloric acid1 ,h,cl1 as will be explained in the following chapter4 ,the readiness with which ammonium salts decompose is seen from the fact that a solution of ammonium oxalate is decomposed with the evolution of ammonia even at -#a4 ,dilute solutions of ammonium salts1 when boiled give aqueous vapour having an alkaline reaction1 owing to the presence of free ammonia given off from the salt4 ,ammonia is not only capable of combining with acids1 but also with many salts1 as was seen from its forming definite compounds1 ,ag,cl1#c,,nh#c and #b;,ag,cl1#c,,nh#c1 with silver chloride4 ,just as ammonia is absorbed by various oxygen salts of the metals1 so also is it absorbed by the chlorine1 iodine1 and bromine compounds of many metals1 and in so doing evolves heat4 ,certain of these compounds part with their ammonia even when left exposed to the air1 but others only do so at a red heat2 many give up their ammonia when dissolved1 whilst others dissolve without decomposition1 and when evaporated separate from their solutions unchanged4 ,all these facts only indicate that ammoniacal1 like aqueous1 compounds dissociate with greater or lesser facility4#ai ,certain metallic oxides also absorb ammonia and are dissolved in ammonia water4 ,such are1 for instance1 the oxides of zinc1 nickel1 copper1 and many others2 the majority of such compounds are unstable4 ,the property of ammonia of combining with certain oxides explains its action on certain metals4#bj ,by reason of such action1 copper vessels are not suitable for holding liquids containing ammonia4 ,iron is not acted on by such liquids4 #ai ,isambert studied the dissociation of ammoniacal compounds1 as we have seen in ,note #h1 and showed that at low temperatures many salts are able to combine with a still greater amount of ammonia1 which proves an entire analogy with hydrates2 and as in this case it is easy to isolate the definite compounds1 and as the least possible tension of ammonia is greater than that of water1 therefore the ammoniacal compounds present a great and peculiar interest1 as a means for explaining the nature of aqueous solutions and as a confirmation of the hypothesis of the formation of definite compounds in them2 for these reasons we shall frequently refer to these compounds in the further exposition of this work4 #bj ,chapter ,v41 ,note #b4 ,the similarity between the relation of ammonia and water to salts and other substances is more especially marked in those cases in which the salt is capable of combining with both ammonia and water4 ,take1 for example1 copper sulphate1 ,cu,s,o#d4 ,as we saw in ,chapter ,i41 it gives with water blue crystals1 ,cu,s,o#d1e;,h#b,o2 but it also absorbs ammonia in the same molecular proportion1 forming a blue substance1 ,cu,s,o#d1e,,nh#c1 and therefore the ammonia combining with salts may be termed ammonia of crystallisation4 ,such are the reactions of combination proper to ammonia4 ,let us now turn our attention to the reactions of substitution proper to this substance4 ,if ammonia be passed through a heated tube containing metallic sodium1 hydrogen is evolved1 and a compound is obtained containing ammonia in which one atom of hydrogen is replaced by an atom of sodium1 ,,nh#b,na 7according to the equation ,,nh#c ,na ,,nh#b,na ,h74 ,this body is termed sodium amide4 ,we shall afterwards see that iodine and chlorine are also capable of directly displacing hydrogen from ammonia1 and of replacing it4 ,in fact1 the hydrogen of ammonia may be replaced in many ways by different elements4 ,if in this replacement ,,nh#b remains1 the resultant substances ,,nh#b,r are called amides1 whilst the substitution products1 ,,nhr#b1 in which only ,,nh remains1 are called imides1#bj bis and those in which none of the ammoniacal hydrogen remains1 ,,nr#c1 are known as nitrides4 ,free amidogen1 ,n#b;,h#d1 is now known in a state of hydration under the name of hydrazine2#ba it combines with acids and resembles ammonia in this respect4 ,in the action of different substances on ammonia it is the hydrogen that is substituted1 whilst the nitrogen remains in the resultant compound1 so to say1 untouched4 ,the same phenomenon is to be observed in the action of various substances on water4 ,in the majority of cases the reactions of water consist in the hydrogen being evolved1 and in its being replaced by different elements4 ,this also takes place1 as we have seen1 in acids in which the hydrogen is easily displaced by metals4 ,this chemical mobility of hydrogen is perhaps connected with the great lightness of the atoms of this element4 #bj bis ,imide1 ,,nh1 has not been obtained in a free state1 but its hydrochloric acid salt1 ,n,h,h,cl1 has apparently been obtained 7#ahij7 by ,maumen by igniting the double bichloride of platinum and ammonium chloride1 ,pt,cl#b,,nh#d;,cl ,pt #b;,h,cl ,n,h,h,cl4 ,it is soluble in water1 and crystallises from its solution in hexagonal rhombic prisms4 ,it gives a double salt with ,fe,cl#c of the composition ,fe,cl#cc,n,h,h,cl4 ,the salt ,n,h,h,cl is similar 7isomeric7 with the first possible product of the metalepsis of ammonia1 ,,nh#b;,cl1 although it does not resemble it in any of its properties4 #ba ,free amidogen or hydrazine1 ,n#b;,h#d1 or #b,,nh#b1 was prepared by ,curtius 7#ahhg7 by means of ethyl diazoacetate1 or triazoacetic acid4 ,curtius and ,jay 7#ahhi7 showed that triazoacetic acid1 ,,chn#b4;,,cooh 7the formula should be tripled71 when heated with water or a mineral acid1 gives 7quantitatively7 oxalic acid and amidogen 7hydrazine71 ,,chn#b4;,,cooh #b;,h#b,o ,c#b,o#b7,,oh7#b ,n#b;,h#d--;i4e4 7empirically71 the oxygen of the water replaces the nitrogen of the azoacetic acid4 ,the amidogen is thus obtained in the form of a salt4 ,with acids1 amidogen forms very stable salts of the two types ,n#b;,h#d;,,hx and ,n#b;,h#d;,h#b,x#b1 as1 for example1 with ,h,cl1 ,h#b,,so#d1 c4 ,these salts are easily crystallised2 in acid solutions they act as powerful reducing agents1 evolving nitrogen2 when ignited they are decomposed into ammoniacal salts1 nitrogen1 and hydrogen2 with nitrites they evolve nitrogen4 ,the sulphate ,n#b;,h#d1;,h#b,,so#d is sparingly soluble in cold water 7#c parts in #ajj of water71 but is very soluble in hot water2 its specific gravity is #acgh1 it fuses at #bed with decomposition4 ,the hydrochloride ,n#b;,h#d1b;,h,cl crystallises in octahedra1 is very soluble in water1 but not in alcohol2 it fuses at #aih1 evolving hydrogen chloride and forming the salt ,n#b;,h#d;,h,cl2 when rapidly heated it decomposes with an explosion2 with platinic chloride it immediately evolves nitrogen1 forming platinous chloride4 ,by the action of alkalis the salts ,n#b;,h#d1b;,,hx give hydrate of amidogen1 ,n#b;,h#d1;,h#b,o1 which is a fuming liquid 7specific gravity #ajc71 boiling at #aai1 almost without odour1 and whose aqueous solution corrodes glass and india-rubber1 has an alkaline taste and poisonous properties4 ,the reducing capacities of the hydrate are clearly seen from the fact that it reduces the metals platinum and silver from their solutions4 ,with mercuric oxide it explodes4 ,it reacts directly with the aldehydes ,,ro1 forming ,n#b,r#b and water2 for example1 with benzaldehydes it gives the very stable insoluble benzalazine 7,c#f;,h#e;,,chn7#b of a yellow colour4 ,we may add that hydrazine often forms double salts2 for example1 ,mg,s,o#d,n#b;,h#d;,h#b,,so#d or ,k,cl,n#b;,h#d;,h,cl1 and that it is also formed by the action of nitrous acid upon aldehyde-ammonia4 ,the products of the substitution of the hydrogen in hydrazine by hydrocarbon groups ,r 7,r ,,ch#c1 ,c#b;,h#e1 ,c#f;,h#e1 c47 were obtained before hydrazine itself2 for example1 ,,nhrnh#b1 ,,nr#b,,nh#b1 and 7,,nrh7#b4 ,the heat of solution of the sulphuric acid salt 7#a part in #bjj and #cjj parts of water at #ajh7 is equal to -#hg ,c4 ,according to ,berthelot and ,matigon 7#ahib71 the heat of neutralisation of hydrazine by sulphuric acid is #ee ,c and by hydrochloric acid #eb ,c4 ,thus hydrazine is a very feeble base1 for its heat of saturation is not only lower than that of ammonia 7#abd ,c4 for ,h,cl71 but even below that of hydroxylamine 7#ic ,c47 ,the heat of formation from the elements of hydrated hydrazine -#ie ,c was deduced from the heat of combustion1 determined by burning ,n#b;,h#d;,h#b,,so#d in a calorimetric bomb1 #abgg ,c4 ,thus hydrazine is an endothermal compound2 its passage into ammonia by the combination of hydrogen is accompanied by the evolution of #eae ,c4 ,in the presence of an acid these figures were greater by #add ,c4 ,hence the direct converse passage from ammonia into hydrazine is impossible4 ,as regards the passage of hydroxylamine into hydrazine1 it would be accompanied by the evolution of heat 7#bae ,c47 in an aqueous solution4 ,amidogen must be regarded as a compound which stands to ammonia in the same relation as hydrogen peroxide stands to water4 ,water1 ,h7,,oh71 gives1 according to the law of substitution1 as was clearly to be expected1 7,,oh77,,oh7--that is1 peroxide of hydrogen is the free radicle of water 7hydroxyl74 ,so also ammonia1 ,h7,,nh#b71 forms hydrazine1 7,,nh#b77,,nh#b7--that is1 the free radicle of ammonia1 ,,nh#b1 or amidogen4 ,in the case of phosphorus a similar substance1 as we shall afterwards see1 has long been known under the name of liquid phosphuretted hydrogen1 ,p#b;,h#d4 ,in practical chemistry#ba bis ammonia is often employed1 not only for saturating acids1 but also for effecting reactions of double decomposition with salts1 and especially for separating insoluble basic hydroxides from soluble salts4 ,let ,,mho stand for an insoluble basic hydroxide and ,,hx for an acid4 ,the salt formed by them will have a composition ,,mho ,,xh-,h#b,o ,,mx4 ,if aqueous ammonia1 ,,nh#d,,oh1 be added to a solution of this salt1 the ammonia will change places with the metal ,m1 and thus form the insoluble basic hydroxide1 or1 as it is said1 give a precipitate4 ,,mx ,,nh#d7,,oh7 ,,nh#d,x ,,mho ,salt of the metal4 ,aqueous ammonia4 ,ammonium salt4 ,basic hydrate4 ,in solution ,in solution ,in solution ,as precipitate #ba bis ,in practice1 the applications of ammonia are very varied4 ,the use of ammonia as a stimulant1 in the forms of the so-called 'smelling salts' or of spirits of hartshorn1 in cases of faintness1 c41 is known to everyone4 ,the volatile carbonate of ammonium1 or a mixture of an ammonium salt with an alkali1 is also employed for this purpose4 ,ammonia also produces a well-known stimulating effect when rubbed on the skin1 for which reason it is sometimes employed for external applications4 ,thus1 for instance1 the well-known volatile salve is prepared from any liquid oil shaken up with a solution of ammonia4 ,a portion of the oil is thus transformed into a soapy substance4 ,the solubility of greasy substances in ammonia1 which proceeds from the formation both of emulsions and soaps1 explains its use in extracting grease spots4 ,it is also employed as an external application for stings from insects1 and for bites from poisonous snakes1 and in general in medicine4 ,it is also remarkable that in cases of drunkenness a few drops of ammonia in water taken internally rapidly renders a person sober4 ,a large quantity of ammonia is used in dyeing1 either for the solution of certain dyes--for example1 carmine--or for changing the tints of others1 or else for neutralising the action of acids4 ,it is also employed in the manufacture of artificial pearls4 ,for this purpose the small scales of a peculiar small fish are mixed with ammonia1 and the liquid so obtained is blown into small hollow glass beads shaped like pearls4 ,in nature and the arts1 however1 ammonium salts1 and not free ammonia1 are most frequently employed4 ,in this form a portion of that nitrogen which is necessary for the formation of albuminous substances is supplied to plants4 ,owing to this1 a large quantity of ammonium sulphate is now employed as a fertilising substance4 ,but the same effect may be produced by nitre1 or by animal refuse1 which in decomposing gives ammonia4 ,for this reason1 an ammoniacal 7hydrogen7 compound may be introduced into the soil in the spring which will be converted into a nitrate 7oxygen salt7 in the summer4 ,thus1 for instance1 if aqueous ammonia is added to a solution of a salt of aluminium1 then alumina hydrate is separated out as a colourless gelatinous precipitate4#bb #bb ,as certain basic hydrates form peculiar compounds with ammonia1 in some cases it happens that the first portions of ammonia added to a solution of a salt produce a precipitate1 whilst the addition of a fresh quantity of ammonia dissolves this precipitate if the ammoniacal compound of the base be soluble in water4 ,this1 for example1 takes place with the copper salts4 ,but alumina does not dissolve under these circumstances4 ,in order to grasp the relation between ammonia and the oxygen compounds of nitrogen it is necessary to recognise the general law of substitution1 applicable to all cases of substitution between elements1#bc and therefore showing what may be the cases of substitution between oxygen and hydrogen as component parts of water4 ,the law of substitution may be deduced from mechanical principles if the molecule be conceived as a system of elementary atoms occurring in a certain chemical and mechanical equilibrium4 ,by likening the molecule to a system of bodies in a state of motion--for instance1 to the sum total of the sun1 planets1 and satellites1 existing in conditions of mobile equilibrium--then we should expect the action of one part1 in this system1 to be equal and opposite to the other1 according to ,newton's third law of mechanics4 ,hence1 given a molecule of a compound1 for instance1 ,h#b,o1 ,,nh#c1 ,na,cl1 ,h,cl1 c41 its every two parts must in a chemical sense represent two things somewhat alike in force and properties1 and therefore every two parts into which a molecule of a compound may be divided are capable of replacing each other4 ,in order that the application of the law should become clear it is evident that among compounds the most stable should be chosen4 ,we will therefore take hydrochloric acid and water as the most stable compounds of hydrogen4#bd ,according to the above law of substitution1 if the elements ,h and ,cl are able to form a molecule1 ,h,cl1 and a stable one1 they are able to replace each other4 ,and1 indeed1 we shall afterwards see 7,chapter ,,xi47 that in a number of instances a substitution between hydrogen and chlorine can take place4 ,given ,,rh1 then ,r,cl is possible1 because ,h,cl exists and is stable4 ,the molecule of water1 ,h#b,o1 may be divided in two ways1 because it contains #c atoms3 into ,h and 7,,ho7 on the one hand1 and into ,h#b and ,o on the other4 ,consequently1 being given ,,rh1 its substitution products will be ,r7,,ho7 according to the first form1 and ,r#b,o according to the second2 being given ,,rh#b1 its corresponding substitution products will be ,,rh7,,oh71 ,r7,,oh7#b1 ,,ro1 7,,rh7#b,o1 c4 ,the group 7,,oh7 is the same hydroxyl or aqueous radicle which we have already mentioned in the third chapter as a component part of hydroxides and alkalis--for instance1 ,na7,,oh71 ,ca7,,oh7#b1 c4 ,it is evident1 judging from ,h7,,ho7 and ,h,cl1 that 7,,oh7 can be substituted by ,cl1 because both are replaceable by ,h2 and this is of common occurrence in chemistry1 because metallic chlorides--for example1 ,na,cl and ,,nh#d;,cl--correspond with hydroxides of the alkalis ,na7,,oh7 or ,,nh#d7,,oh74 ,in hydrocarbons--for instance1 ,c#b;,h#f--the hydrogen is replaceable by chlorine and by hydroxyl4 ,thus ordinary alcohol is ,c#b;,h#f1 in which one atom of ,h is replaced by 7,,oh72 that is1 ,c#b;,h#e7,,oh74 ,it is evident that the replacement of hydrogen by hydroxyl essentially forms the phenomenon of oxidation1 because ,,rh gives ,r7,,oh71 or ,,rho4 ,hydrogen peroxide may in this sense be regarded as water in which the hydrogen is replaced by hydroxyl2 ,h7,,oh7 gives 7,,oh7#b or ,h#b,o#b4 ,the other form of substitution--namely1 that of ,o in the place of ,h#b--;is also a common chemical phenomenon4 ,thus alcohol1 ,c#b;,h#f,o1 or ,c#b;,h#e7,,oh71 when oxidising in the air1 gives acetic acid1 ,c#b;,h#d,o#b1 or ,c#b;,h#c,o7,,oh71 in which ,h#b is replaced by ,o4 #bc ,when the element chlorine1 as we shall afterwards more fully learn1 replaces the element hydrogen1 the reaction by which such an exchange is accomplished proceeds as a substitution1 ,,ah ,cl#b ,a,cl ,h,cl1 so that two substances1 ,,ah and chlorine1 react on each other1 and two substances1 ,a,cl and ,h,cl1 are formed2 and further1 two molecules react on each other1 and two others are formed4 ,the reaction proceeds very easily1 but the substitution of one element1 ,a1 by another1 ,x1 does not always proceed with such ease1 clearness1 or simplicity4 ,the substitution between oxygen and hydrogen is very rarely accomplished by the reaction of the free elements1 but the substitution between these elements1 one for another1 forms the most common case of oxidation and reduction4 ,in speaking of the law of substitution1 ,i have in view the substitution of the elements one by another1 and not the direct reaction of substitution4 ,the law of substitution determines the cycle of the combinations of a given element1 if a few of its compounds 7for instance1 the hydrogen compounds7 be known4 ,a development of the conceptions of the law of substitution may be found in my lecture given at the ,royal ,institution in ,london1 #ahhi4 #bd ,if hydrogen peroxide be taken as a starting point1 then still higher forms of oxidation than those corresponding with water should be looked for4 ,they should possess the properties of hydrogen peroxide1 especially that of parting with their oxygen with extreme ease 7even by contact74 ,such compounds are known4 ,pernitric1 persulphuric1 and similar acids present these properties1 as we shall see in describing them4 ,in the further course of this work we shall have occasion to refer to the law of substitution for explaining many chemical phenomena and relations4 ,we will now apply these conceptions to ammonia in order to see its relation to the oxygen compounds of nitrogen4 ,it is evident that many substances should be obtainable from ammonia1 ,,nh#c1 or aqueous ammonia1 ,,nh#d7,,oh71 by substituting their hydrogen by hydroxyl1 or ,h#b by oxygen4 ,and such is the case4 ,the two extreme cases of such substitution will be as follows3 7#a7 ,one atom of ,h in ,,nh#c is substituted by 7,,oh71 and ,,nh#b7,,oh7 is produced4 ,such a substance1 still containing much hydrogen1 should have many of the properties of ammonia4 ,it is known under the name of hydroxylamine1#be and1 in fact1 is capable1 like ammonia1 of giving salts with acids2 for example1 with hydrochloric acid1 ,,nh#c7,,oh7,cl--which is a substance corresponding to sal-ammoniac1 in which one atom of hydrogen is replaced by hydroxyl4#be bis 7#b7 ,the other extreme case of substitution is that given by ammonium hydroxide1 ,,nh#d7,,oh71 when the whole of the hydrogen of the ammonium is replaced by oxygen2 and1 as ammonium contains #d atoms of hydrogen1 the highest oxygen compound should be ,,no#b7,,oh71 or ,,nho#c1 as we find to be really the case1 for ,,nho#c is nitric acid1 exhibiting the highest degree of oxidation of nitrogen4#bf ,if instead of the two extreme aspects of substitution we take an intermediate one1 we obtain the intermediate oxygen compounds of nitrogen4 ,for instance1 ,n7,,oh7#c is orthonitrous acid1#bg to which corresponds nitrous acid1 ,,no7,,oh71 or ,,nho#b1 equal to ,n7,,oh7#c-;,h#b,o1 and nitrous anhydride1 ,n#b,o#c #b,n7,,oh7#c-c;,h#b,o4 ,thus nitrogen gives a series of oxygen compounds1 which we will proceed to describe4 ,we will1 however1 first show by two examples that in the first place the passage of ammonia into the oxygen compounds of nitrogen up to nitric acid1 as well as the converse preparation of ammonia 7and consequently of the intermediate compounds also7 from nitric acid1 are reactions which proceed directly and easily under many circumstances1 and in the second place that the above general principle of substitution gives the possibility of understanding many1 at first sight unexpected and complex1 relations and transformations1 such as the preparation of hydronitrous acid1 ,,hn#c4 ,in nature the matter is complicated by a number of influences and circumstances1 but in the law the relations are presented in their simplest aspect4 #be ,the compound of hydroxylamine with hydrochloric acid has the composition ,,nh#b7,,oh7,h,cl ,,nh#d;,cl,o--that is1 it is as it were oxidised sal-ammoniac4 ,it was prepared by ,lossen in #ahfe by the action of tin and hydrochloric acid in the presence of water on a substance called ethyl nitrate1 in which case the hydrogen liberated from the hydrochloric acid by the tin acts upon the elements of nitric acid-- ,c#b;,h#e,,no#c #f;,h ,h,cl ,,nh#d,o,cl ,h#b,o ,c#b;,h#e,,oh ,ethyl nitrate ,hydrogen ,hydroxylamine ,water ,alcohol from ,h,cl ,h,cl and ,sn ,thus in this case the nitric acid is deoxidised1 not directly into nitrogen1 but into hydroxylamine4 ,hydroxylamine is also formed by passing nitric oxide1 ,,no1 into a mixture of tin and hydrochloric acid--that is1 by the action of the hydrogen evolved on the nitric oxide1 ,,no #c;,h ,h,cl ,,nh#d,o,cl--and in many other cases4 ,according to ,lossen's method1 a mixture of #cj parts of ethyl nitrate1 #abj parts of tin1 and #dj parts of a solution of hydrochloric acid of sp4 gr4 #ajf are taken4 ,after a certain time the reaction commences spontaneously4 ,when the reaction has ceased the tin is separated by means of hydrogen sulphide1 the solution is evaporated1 and a large amount of sal-ammoniac is thus obtained 7owing to the further action of hydrogen on the hydroxylamine compound1 the hydrogen taking up oxygen from it and forming water72 a solution ultimately remains containing the hydroxylamine salt2 this salt is dissolved in anhydrous alcohol and purified by the addition of platinum chloride1 which precipitates any ammonium salt still remaining in the solution4 ,after concentrating the alcoholic solution the hydroxylamine hydrochloride separates in crystals4 ,this substance melts at about #aej1 and in so doing decomposes into nitrogen1 hydrogen chloride1 water1 and sal-ammoniac4 ,a sulphuric acid compound of hydroxylamine may be obtained by mixing a solution of the above salt with sulphuric acid4 ,the sulphate is also soluble in water like the hydrochloride2 this shows that hydroxylamine1 like ammonia itself1 forms a series of salts in which one acid may be substituted for another4 ,it might he expected that by mixing a strong solution of a hydroxylamine salt with a solution of a caustic alkali hydroxylamine itself would be liberated1 just as an ammonia salt under these circumstances evolves ammonia2 but the liberated hydroxylamine is immediately decomposed with the formation of nitrogen and ammonia 7and probably nitrous oxide71 #c,,nh#c,o ,,nh#c #c;,h#b,o ,n#b4 ,dilute solutions give the same reaction1 although very slowly1 but by decomposing a solution of the sulphate with barium hydroxide a certain amount of hydroxylamine is obtained in solution 7it is partly decomposed74 ,hydroxylamine in aqueous solution1 like ammonia1 precipitates basic hydrates1 and it deoxidises the oxides of copper1 silver1 and other metals4 ,free hydroxylamine was obtained by ,lobry de ,bruyn 7#ahia74 ,it is a solid1 colourless1 crystalline substance1 without odour1 which does not melt below #bg4 ,it has the property of dissolving metallic salts2 for instance1 sodium chloride4 ,hydroxylamine1 when rapidly heated with platinum1 decomposes with a flash and the formation of a yellow flame4 ,it is almost insoluble in ordinary solvents like chloroform1 benzine1 acetic ether1 and carbon bisulphide4 ,its aqueous solutions are tolerably stable1 contain up to #fj per cent4 7sp4 gr4 #aae at #bj71 and may be kept for many weeks without undergoing any change4 ,lobry de ,bruyn used the hydrochloric salt to prepare pure hydroxylamine4 ,the salt was first treated with sodium methylate 7,,ch#c,na,o71 and then methyl alcohol was added to the mixture4 ,the precipitated sodium chloride was separated from the solution by filtration4 7,the methyl alcohol is added to prevent the precipitated chloride of sodium from coating the insoluble hydrochloric salt of hydroxylamine47 ,the methyl alcohol was driven off under a pressure #aej-bjj mm41 and after extracting a further portion of methyl alcohol by ether and several fractional distillations1 a solution was obtained containing #gj per cent4 of free hydroxylamine1 #h per cent4 water1 #ii per cent4 chloride of sodium1 and #aba per cent4 of the hydrochloric salt of hydroxylamine4 ,pure free hydroxylamine1 ,,nh#c,o1 is obtained by distilling under a pressure of #fj mm42 it then boils at #gj1 and solidifies in a condenser cooled to #j in the form of long needles4 ,it melts at #cc1 boils at #eh under a pressure of #bb mm41 and has a sp4 gr4 of about #abce 7,brhl74 ,under the action of ,na,h,o it gives ,,nh#c and ,,nho#b or ,n#b,o1 and forms nitric acid 7,kolotoff1 #ahic7 under the action of oxidising agents4 ,hydroxylamine is obtained in a great number of cases1 for instance by the action of tin on dilute nitric acid1 and also by the action of zinc on ethyl nitrate and dilute hydrochloric acid1 c4 ,the relation between hydroxylamine1 ,,nh#b7,,oh71 and nitrous acid1 ,,no7,,oh71 which is so clear in the sense of the law of substitutions1 becomes a reality in those cases when reducing agents act on salts of nitrous acid4 ,thus ,raschig 7#ahhh7 proposed the following method for the preparation of the hydroxylamine sulphate4 ,a mixture of strong solutions of potassium nitrite1 ,,kno#b1 and hydroxide1 ,,kho1 in molecular proportions1 is prepared and cooled4 ,an excess of sulphurous anhydride is then passed into the mixture1 and the solution boiled for a long time4 ,a mixture of the sulphates of potassium and hydroxylamine is thus obtained3 ,,kno#b ,,kho #b,,so#b #b;,h#b,o ,,nh#b7,,oh71,h#b,,so#d ,k#b,,so#d4 ,the salts may be separated from each other by crystallisation4 #be bis ,in order to illustrate the application of the law of substitution to a given case1 and to show the connection between ammonia and the oxides of nitrogen1 let us consider the possible products of an oxygen and hydroxyl substitution in caustic ammonia1 ,,nh#d7,,oh74 ,it is evident that the substitution of ,h by ,,oh can give3 7#a7 ,,nh#c7,,oh7#b2 7#b7 ,,nh#b7,,oh7#c2 7#c7 ,,nh7,,oh7#d2 and 7#d7 ,n7,,oh7#e4 ,they should all1 like caustic ammonia itself1 easily part with water and form products 7hydroxylic7 of the oxidation of ammonia4 ,the first of them is the hydrate of hydroxylamine1 ,,nh#b7,,oh7 ,h#b,o2 the second1 ,,nh7,,oh7#b ,h#b,o 7and also the substance ,,nh7,,oh7#d or ,,nh#c,o#b71 containing1 as it does1 both hydrogen and oxygen1 is able to part with all its hydrogen in the form of water 7which could not be done by the first product1 since it contained too little oxygen71 forming1 as the ultimate product1 #b,,nh#b7,,oh7#c-e;,h#b,o ,n#b,o--that is1 it corresponds with nitrous oxide1 or the lower degree of the oxidation of nitrogen4 ,so1 also1 nitrous anhydride corresponds with the third of the above products1 #b,,nh7,,oh7#d-e;,h#b,o ,n#b,o#c1 and nitric anhydride with the fourth1 #b,n7,,oh7#e-e;,h#b,o ,n#b,o#e4 ,as1 in these three equations1 two molecules of the substitution products 7-#e;,h#b,o7 are taken1 it is also possible to combine two different products in one equation4 ,for instance1 the third and fourth products3 ,,nh7,,oh7#d ,n7,,oh7#e-e;,h#b,o corresponds to ,n#b,o#d or #b,,no#b1 that is1 to peroxide of nitrogen4 ,thus all the five 7see later7 oxides of nitrogen1 ,n#b,o1 ,,no1 ,n#b,o#c1 ,,no#b1 and ,n#b,o#e1 may be deduced from ammonia4 ,the above may be expressed in a general form by the equation 7it should be remarked that the composition of all the substitution products of caustic ammonia may be expressed by ,,nh#c,o#e-;a1 where a varies between #j and #d73 ,,nh#e,o#e - a ,,nh#e,o#e - b - #e;,h#b,o ,n#b,o#e - 7a b71 where a b can evidently be not greater than #e2 when a b #e we have ,n#b--nitrogen1 when #d we have ,n#b,o nitrous oxide2 when a b #c we have ,n#b,o#b or ,,no--nitric oxide1 and so on to ,n#b,o#e1 when a b #j4 ,besides which it is evident that intermediate products may correspond with 7and hence also break up into7 different starting points2 for instance1 ,n#b,o is obtained when a b #b1 and this may occur either when a #j 7nitric acid71 and b #b 7hydroxylamine71 or when a b #a 7the third of the above substitution products74 #bf ,nitric acid corresponds with the anhydride ,n#b,o#e1 which will afterwards be described1 but which must be regarded as the highest saline oxide of nitrogen1 just as ,na#b,o 7and the hydroxide ,na,h,o7 in the case of sodium1 although sodium forms a peroxide possessing the property of parting with its oxygen with the same ease as hydrogen peroxide1 if not on heating1 at all events in reactions--for instance1 with acids4 ,so also nitric acid has its corresponding peroxide1 which may be called pernitric acid4 ,its composition is not well known--probably ,,nho#d--so that its corresponding anhydride would be ,n#b,o#g4 ,it is formed by the action of a silent discharge on a mixture of nitrogen and oxygen1 so that a portion of its oxygen is in a state similar to that in ozone4 ,the instability of this substance 7obtained by ,hautefeuille1 ,chappuis1 and ,berthelot71 which easily splits up with the formation of nitric peroxide1 and its resemblance to persulphuric acid1 which we shall afterwards describe1 will permit our passing over the consideration of the little that is further known concerning it4 #bg ,phosphorus 7,chapter ,,xix47 gives the hydride ,,ph#c1 corresponding with ammonia1 ,,nh#c1 and forms phosphorous acid1 ,,ph#c,o#c1 which is analogous to nitrous acid1 just as phosphoric acid is to nitric acid2 but phosphoric 7or1 better1 orthophosphoric7 acid1 ,,ph#c,o#d1 is able to lose water and give pyro-and meta-phosphoric acids4 ,the latter is equal to the ortho-acid minus water ,,pho#c1 and therefore nitric acid1 ,,nho#c1 is really meta-nitric acid4 ,so also nitrous acid1 ,,hno#b1 is meta-nitrous 7anhydrous7 acid1 and thus the ortho-acid is ,,nh#c,o#c ,n7,,oh7#c4 ,hence for nitric acid we should expect to find1 besides the ordinary or meta-nitric acid1 ,,hno#c 7 #a_/b,n#b,o#c1;,h#b,o71 and ortho-nitric acid1 ,h#c,,no#d 7 #a_/b,n#b,o#c1c;,h#b,o71 an intermediate pyro-nitric acid1 ,n#b;,h#d,o#g1 corresponding to pyrophosphoric acid1 ,p#b;,h#d,o#g4 ,we shall see 7for instance1 in ,chapter ,,xvi41 ,note #ba7 that in nitric acid there is indeed an inclination of the ordinary salts 7of the meta-acid71 ,,mno#c1 to combine with bases ,m#b,o1 and to approximate to the composition of ortho-compounds which are equal to meta-compound and bases 7,,mno#c ,m#b,o ,m#c,,no#d74 #a4 ,it is easy to prove the possibility of the oxidation of ammonia into nitric acid by passing a mixture of ammonia and air over heated spongy platinum4 ,this causes the oxidation of the ammonia1 nitric acid being formed1 which partially combines with the excess of ammonia4 ,the converse passage of nitric acid into ammonia is effected by the action of hydrogen at the moment of its evolution4#bh ,thus metallic aluminium1 evolving hydrogen from a solution of caustic soda1 is able to completely convert nitric acid added to the mixture 7as a salt1 because the alkali gives a salt with the nitric acid7 into ammonia1 ,,nho#c #h;,h ,,nh#c #c;,h#b,o4 #b4 ,in #ahij ,curtius in ,germany obtained a gaseous substance of the composition ,,hn#c 7hydrogen trinitride71 having the distinctive properties of an acid1 and giving1 like hydrochloric acid1 salts2 for example1 a sodium salt1 ,na,n#c2 ammonium salt1 ,,nh#d,n#c ,n#d;,h#d2 barium salt1 ,ba7,n#c7b1 c41 which he therefore named hydronitrous acid1 ,,hn#c4bh bis ,the extraordinary composition of the compound 7ammonia1 ,,nh#c1 contains one ,n atom and three ,h atoms2 in ,,hn#c1 on the contrary1 there are three ,n atoms and one ,h atom71 the facile decomposition of its salts with an explosion1 and above all its distinctly acid character 7an aqueous solution shows a strong acid reaction to litmus71 not only indicated the importance of this unexpected discovery1 but at first gave rise to some perplexity as to the nature of the substance obtained1 for the relations in which ,,hn#c stood to other simple compounds of nitrogen which had long been known was not at all evident1 and the scientific spirit especially requires that there should be a distinct bond between every innovation1 every fresh discovery1 and that which is already firmly established and known1 for upon this basis is founded that apparently paradoxical union in science of a conservative stability with an irresistible and never-ceasing improvement4 ,this missing1 connection between the newly discovered hydronitrous acid1 ,,hn#c1 and the long known ammonia1 ,,nh#c1 and nitric acid1 ,,hno#c1 may be found in the law of substitution1 starting from the well-known properties and composition of nitric acid and ammonia1 as ,i mentioned in the ',journal of the ,russian ,physico-,chemical ,society' 7#ahij74 ,the essence of the matter lies in the fact that to the hydrate of ammonium1 or caustic ammonia1 ,,nh#d,,oh1 there should correspond1 according to the law of substitution1 an ortho-nitric acid 7see ,note #bg71 ,h#c,,no#d ,,no7,,oh7#c1 which equals ,,nh#d7,,oh7 with the substitution in it of 7a7 two atoms of hydrogen by oxygen 7,o--,h#b7 and 7b7 two atoms of hydrogen by the aqueous radicle 7,,oh--,h74 ,ordinary or meta-nitric acid is merely this ortho-nitric acid minus water4 ,to ortho-nitric acid there should correspond the ammoniacal salts3 mono-substituted1 ,h#b,,nh#d,,no#d2 bi-substituted1 ,h7,,nh#d7b,,no#d2 and tri-substituted1 7,,nh#d7c,,no#d4 ,these salts1 containing as they do hydrogen and oxygen1 like many similar ammoniacal salts 7see1 for instance1 ,chapter ,,ix4--,cyanides71 are able to part with them in the form of water4 ,then from the first salt we have ,h#b,,nh#d,,no#d-d;,h#b,o ,n#b,o--nitrous oxide1 and from the second ,h7,,nh#d7b,,no#d-d;,h#b,o ,,hn#c--;hydronitrous acid1 and from the third 7,,nh#d7c,,no-#d;,h#b,o ,n#d;,h#d--the ammonium salt of the same acid4 ,the composition of ,,hn#c should be thus understood1 whilst its acid properties are explained by the fact that the water 7#d;,h#b,o7 from ,h7,,nh#d7b,n,o#d is formed at the expense of the hydrogen of the ammonium and oxygen of the nitric acid1 so that there remains the same hydrogen as in nitric acid1 or that which may be replaced by metals and give salts4 ,moreover1 nitrogen undoubtedly belongs to that category of metalloids which give acids1 like chlorine and carbon1 and therefore1 under the influence of three of its atoms1 one atom of hydrogen acquires those properties which it has in acids1 just as in ,,hcn 7hydrocyanic acid7 the hydrogen has received these properties under the influence of the carbon and nitrogen 7and ,,hn#c may be regarded as ,,hcn where ,c has been replaced by ,n#b74 ,moreover1 besides explaining the composition and acid properties of ,,hn#c1 the above method gives the possibility of foretelling the closeness of the bond between hydronitrous acid and nitrous oxide1 for ,n#b,o ,,nh#c ,,hn#c ,h#b,o4 ,this reaction1 which was foreseen from the above considerations1 was accomplished by ,wislicenus 7#ahib7 by the synthesis of the sodium salt1 by taking the amide of sodium1 ,,nh#b,na 7obtained by heating ,na in a current of ,,nh#c71 and acting upon it 7when heated7 with nitrous oxide1 ,n#b,o1 when #b,,nh#b,na ,n#b,o ,na,n#c ,na,h,o ,,nh#c4 ,the resultant salt1 ,na,n#c1 gives hydronitrous acid when acted upon by sulphuric acid1 ,na,n#c ,h#b,,so#d ,na,h,s,o#d ,,hn#c4 ,the latter gives1 with the corresponding solutions of their salts1 the insoluble 7and easily explosive7 salts of silver1 ,ag,n#c 7insoluble1 like ,ag,cl or ,ag,c,n71 and lead1 ,pb7,n#c7b4 #bh ,the formation of ammonia is observed in many cases of oxidation by means of nitric acid4 ,this substance is even formed in the action of nitric acid on tin1 especially if dilute acid be employed in the cold4 ,a still more considerable amount of ammonia is obtained if1 in the action of nitric acid1 there are conditions directly tending to the evolution of hydrogen1 which then reduces the acid to ammonia2 for instance1 in the action of zinc on a mixture of nitric and sulphuric acids4 #bh bis ,curtius started with benzoylhydrazine1 ,c#f;,h#e;,,conhnh#b 7hydrazine1 see ,note #bj bis74 7,this substance is obtained by the action of hydrated hydrazine on the compound ether of benzoic acid74 ,benzoylhydrazine under the action of nitrous acid gives benzoylazoimide and water3 ,c#f;,h#e;,,conhnh#b ,,no#b;,h ,c#f;,h#e;,,con#c #b;,h#b,o4 ,benzoylazoimide when treated with sodium alcoholate gives the sodium salt of hydronitrous acid3 ,c#f;,h#e;,,con#c ,c#b;,h#c,o,na ,c#f;,h#e,o#b;,c#b;,h#c ,na,n#c4 ,the addition of ether to the resultant solution precipitates the ,na,n#c1 and this salt when treated with sulphuric acid gives gaseous hydronitrous acid1 ,,hn#c4 ,it has an acrid smell1 and is easily soluble in water4 ,the aqueous solution exhibits a strongly acid reaction4 ,metals dissolve in this solution and give the corresponding salts4 ,with hydronitrous acid gaseous ammonia forms a white cloud1 consisting of the salt of ammonium1 ,,nh#d,n#c4 ,this salt separates out from an alcoholic solution in the form of white lustrous scales4 ,the salts of hydronitrous acid are obtained by a reaction of substitution with the sodium or ammonium salts4 ,in this manner ,curtius obtained and studied the salts of silver 7,ag,n#c71 mercury 7,hg,n#c71 lead 7,pb,n#f71 barium 7,ba,n#f74 ,with hydrazine1 ,n#b;,h#d1 hydronitrous acid forms saline compounds in the composition of which there are one or two particles of ,n#c;,h per one particle of hydrazine2 thus ,n#e;,h#e and ,n#h;,h#f4 ,the first was obtained in an almost pure form4 ,it crystallises from an aqueous solution in dense1 volatile1 lustrous prisms 7up to #a in4 long71 which fuse at #ej1 and deliquesce in the air2 from a solution in boiling alcohol it separates out in bright crystalline plates4 ,this salt1 ,n#e;,h#e1 has the same empirical composition1 ,,nh1 as the ammonium salt of hydronitrous acid1 ,n#d;,h#d1 and imide2 but their molecules and structure are different4 ,curtius also obtained 7#ahic7 hydronitrous acid by passing the vapour of ,n#b,o#e 7evolved by the action of ,,hno#c on ,as#b,o#c7 into a solution of hydrazine1 ,n#b;,h#d4 ,similarly ,angeli1 by acting upon a saturated solution of silver nitrite with a strong solution of hydrazine1 obtained the explosive ,ag,n#c in the form of a precipitate1 and this reaction1 which is based upon the equation ,n#b;,h#d ,,nho#b ,,hn#c #b;,h#b,o1 proceeds so easily that it forms an experiment for the lecture table4 ,a thermal investigation of hydronitrous acid by ,berthelot and ,matignon gave the following figures for the heat of solution of the ammonium salt ,n#c;,,hnh#c 7#a grm4 in #ajj parts of water7-#gjh ,c41 and for the heat of neutralisation by barium hydrate #ajj ,c41 and by ammonia #hb ,c4 ,the heat of combustion of ,n#d;,h#d 7#afch ,c4 at a constant vol47 gives the heat of formation of the salt ,n#d;,h#d 7solid7 as -#bec ,c4 and 7solution7-#cbc ,c42 this explains the explosive nature of this compound4 ,in its heat of formation from the elements ,n#c;,h -#fbf ,c41 this compound differs from all the hydrogen compounds of nitrogen in having a maximum absorption of heat1 which explains its instability4 ,the compounds of nitrogen with oxygen present an excellent example of the law of multiple proportions1 because they contain1 for #ad parts by weight of nitrogen1 #h1 #af1 #bd1 #cb1 and #dj parts respectively by weight of oxygen4 ,the composition of these compounds is as follows3-- ,n#b,o1 nitrous oxide2 hydrate ,,nho4 ,n#b,o#b1 nitric oxide1 ,,no4 ,n#b,o#c1 nitrous anhydride2 hydrate ,,nho#b4 ,n#b,o#d1 peroxide of nitrogen1 ,,no#b4 ,n#b,o#e1 nitric anhydride2 hydrate ,,nho#c4 ,of these compounds1#bi nitrous and nitric oxides1 peroxide of nitrogen1 and nitric acid1 ,,nho#c1 are characterised as being the most stable4 ,the lower oxides1 when coming into contact with the higher1 may give the intermediate forms2 for instance1 ,,no and ,,no#b form ,n#b,o#c1 and the intermediate oxides may1 in splitting up1 give a higher and lower oxide4 ,so ,n#b,o#d gives ,n#b,o#c and ,n#b,o#e1 or1 in the presence of water1 their hydrates4 #bi ,according to the thermochemical determinations of ,favre1 ,thomsen1 and more especially of ,berthelot1 it follows that1 in the formation of such quantities of the oxides of nitrogen as express their formul1 if gaseous nitrogen and oxygen be taken as the starting points1 and if the compounds formed be also gaseous1 the following amounts of heat1 expressed in thousands of heat units1 are absorbed 7hence a minus sign73-- ,n#b,o ,n#b,o#b ,n#b,o#c ,n#b,o#d ,n#b,o#e -#ba -#dc -#bb -#e -#a -#bb #ba #ag #d ,the difference is given in the lower line4 ,for example1 if ,n#b1 or #bh grams of nitrogen1 combine with ,o--that is1 with #af grams of oxygen--then #ba1jjj units of heat are absorbed1 that is1 sufficient heat to raise #ba1jjj grams of water through #a4 ,naturally1 direct observations are impossible in this case2 but if charcoal1 phosphorus1 or similar substances are burnt both in nitrous oxide and in oxygen1 and the heat evolved is observed in both cases1 then the difference 7more heat will be evolved in burning in nitrous oxide7 gives the figures required4 ,if ,n#b,o#b1 by combining with ,o#b1 gives ,n#b,o#d1 then1 as is seen from the table1 heat should be developed1 namely1 #ch1jjj units of heat1 or ,,no ,o #ai1jjj units of heat4 ,the differences given in the table show that the maximum absorption of heat corresponds with nitric oxide1 and that the higher oxides are formed from it with evolution of heat4 ,if liquid nitric acid1 ,,nho#c1 were decomposed into ,n ,o#c ,h1 then #da1jjj heat units would be required2 that is1 an evolution of heat takes place in its formation from the gases4 ,it should be observed that the formation of ammonia1 ,,nh#c1 from the gases ,n ,h#c evolves #abb thousand heat units4 ,we have already seen that1 under certain conditions1 nitrogen combines with oxygen1 and we know that ammonia may he oxidised4 ,in these cases various oxidation products of nitrogen are formed1 but in the presence of water and an excess of oxygen they always give nitric acid4 ,nitric acid1 as corresponding with the highest oxide1 is able1 in deoxidising1 to give the lower oxides2 it is the only nitrogen acid whose salts occur somewhat widely in nature1 and it has many technical uses1 for which reason we will begin with it4 ,nitric acid1 ,,nho#c1 is likewise known as aqua fortis4 ,in a free state it is only met with in nature in small quantities1 in the air and in rain-water after storms2 but even in the atmosphere nitric acid does not long remain free1 but combines with ammonia1 traces of which are always found in air4 ,on falling on the soil and into running water1 c41 the nitric acid everywhere comes into contact with bases 7or their carbonates71 which easily act on it1 and therefore it is converted into the nitrates of these bases4 ,hence nitric acid is always met with in the form of salts in nature4 ,the soluble salts of nitric acid are called nitres4 ,this name is derived from the ,latin sal nitri4 ,the potassium salt1 ,,kno#c1 is common nitre1 and the sodium salt1 ,na,n,o#c1 ,chili saltpetre1 or cubic nitre4 ,nitres are formed in the soil when a nitrogenous substance is slowly oxidised in the presence of an alkali by means of the oxygen of the atmosphere4 ,in nature there are very frequent instances of such oxidation4 ,for this reason certain soils and rubbish heaps--for instance1 lime rubbish 7in the presence of a base7--lime contain a more or less considerable amount of nitre4 ,one of these nitres--sodium nitrate--is extracted from the earth in large quantities in ,chili1 where it was probably formed by the oxidation of animal refuse4 ,this kind of nitre is employed in practice for the manufacture of nitric acid and the other oxygen compounds of nitrogen4 ,nitric acid is obtained from ,chili saltpetre by heating it with sulphuric acid4 ,the hydrogen of the sulphuric acid replaces the sodium in the nitre4 ,the sulphuric acid then forms either an acid salt1 ,na,h,s,o#d1 or a normal salt1 ,na#b,,so#d1 whilst nitric acid is formed from the nitre and is volatilised4 ,the decomposition is expressed by the equations3 7#a7 ,na,n,o#c ,h#b,,so#d ,,hno#c ,na,h,s,o#d1 if the acid salt be formed1 and 7#b7 #b,na,n,o#c ,h#b,,so#d ,na#b,,so#d #b;,,hno#c1 if the normal sodium sulphate is formed4 ,with an excess of sulphuric acid1 at a moderate heat1 and at the commencement of the reaction1 the decomposition proceeds according to the first equation2 and on further heating with a sufficient amount of nitre according to the second1 because the acid salt ,na,h,s,o#d itself acts like an acid 7its hydrogen being replaceable as in acids71 according to the equation ,na,n,o#c ,na,h,s,o#d ,na#b,,so#d ,,hno#c4 ,illustration3 ,,fig4 #dg4--,method of preparing nitric acid on a large scale4 ,a cast-iron retort1 ,c1 is fixed into the furnace1 and heated by the fire1 ,b4 ,the flame and products of combustion are at first led along the flue1 ,m 7in order to heat the receivers71 and afterwards into ,l4 ,the retort is charged with ,chili saltpetre and sulphuric acid1 and the cover is luted on with clay and gypsum4 ,a clay tube1 a1 is fixed into the neck of the retort 7in order to prevent the nitric acid from corroding the cast iron71 and a bent glass tube1 ,d1 is luted on to it4 ,this tube carries the vapours into a series of earthenware receivers1 ,e4 ,nitric acid mixed with sulphuric acid collects in the first4 ,the purest nitric acid is procured from the second1 whilst that which condenses in the third receiver contains hydrochloric acid1 and that in the fourth nitrous oxide4 ,water is poured into the last receiver in order to condense the residual vapours4 ,the sulphuric acid1 as it is said1 here displaces the nitric acid from its compound with the base4#bi bis ,thus1 in the reaction of sulphuric acid on nitre there is formed a non-volatile salt of sulphuric acid1 which remains1 together with an excess of this acid1 in the distilling apparatus1 and nitric acid1 which is converted into vapour1 and may be condensed1 because it is a liquid and volatile substance4 ,on a small scale1 this reaction may be carried on in a glass retort with a glass condenser4 ,on a large scale1 in chemical works1 the process is exactly similar1 only iron retorts are employed for holding the mixture of nitre and sulphuric acid1 and earthenware three-necked bottles are used instead of a condenser1#cj as shown in fig4 #dg4 #bi bis ,this often gives rise to the supposition that sulphuric acid possesses a considerable degree of affinity or energy compared with nitric acid1 but we shall afterwards see that the idea of the relative degree of affinity of acids and bases is1 in many cases1 exceedingly unbiassed2 it need not be accepted so long as it is possible to explain the observed phenomena without admitting any supposition whatever of the degree of the force of affinity1 because the latter cannot be measured4 ,the action of sulphuric acid upon nitre may be explained by the fact alone that the resultant nitric acid is volatile4 ,the nitric acid is the only one of all the substances partaking in the reaction which is able to pass into vapour2 it alone is volatile1 while the remainder are non-volatile1 or1 more strictly speaking1 exceedingly difficultly volatile substances4 ,let us imagine that the sulphuric acid is only able to set free a small quantity of nitric acid from its salt1 and this will suffice to explain the decomposition of the whole of the nitre by the sulphuric acid1 because once the nitric acid is separated it passes into vapour when heated1 and passes away from the sphere of action of the remaining substances2 then the free sulphuric acid will set free a fresh small quantity of nitric acid1 and so on until it drives off the entire quantity4 ,it is evident that1 in this explanation1 it is essential that the sulphuric acid should be in excess 7although not greatly7 throughout the reaction2 according to the equation expressing the reaction1 #ih parts of sulphuric acid are required per #he parts of ,chili nitre2 but if this proportion be maintained in practice the nitric acid is not all disengaged by the sulphuric acid2 an excess of the latter must be taken1 and generally #hj parts of ,chili nitre are taken per #ih parts of acid1 so that a portion of the sulphuric acid remains free to the very end of the reaction4 #cj ,it must be observed that sulphuric acid1 at least when undiluted 7#fj ,baum71 corrodes cast iron with difficulty1 so that the acid may be heated in cast-iron retorts4 ,nevertheless1 both sulphuric and nitric acids have a certain action on cast iron1 and therefore the acid obtained will contain traces of iron4 ,in practice sodium nitrate 7,chili saltpetre7 is usually employed because it is cheaper1 but in the laboratory it is best to take potassium nitrate1 because it is purer and does not froth up so much as sodium nitrate when heated with sulphuric acid4 ,in the action of an excess of sulphuric acid on nitre and nitric acid a portion of the latter is decomposed1 forming lower oxides of nitrogen1 which are dissolved in the nitric acid4 ,a portion of the sulphuric acid itself is also carried over as spray by the vapours of the nitric acid4 ,hence sulphuric acid occurs as an impurity in commercial nitric acid4 ,a certain amount of hydrochloric acid will also be found to be present in it1 because sodium chloride is generally found as an impurity in nitre1 and under the action of sulphuric acid it forms hydrochloric acid4 ,commercial acid further contains a considerable excess of water above that necessary for the formation of the hydrate1 because water is first poured into the earthenware vessels employed for condensing the nitric acid in order to facilitate its cooling and condensation4 ,further1 the acid of composition ,,hno#c decomposes with great ease1 with the evolution of oxides of nitrogen4 ,thus the commercial acid contains a great number of impurities1 and is frequently purified in the following manner3--,lead nitrate is first added to the acid because it forms non-volatile and almost insoluble 7precipitated7 substances with the free sulphuric and hydrochloric acids1 and liberates nitric acid in so doing1 according to the equations ,pb7,,no#c7b #b;,h,cl ,pb,cl#b #b,,nho#c and ,pb7,,no#c7b ,h#b,,so#d ,pb,s,o#d #b,,nho#c4 ,potassium chromate is then added to the impure nitric acid1 by which means oxygen is liberated from the chromic acid1 and this oxygen1 at the moment of its evolution1 oxidises the lower oxides of nitrogen and converts them into nitric acid4 ,a pure nitric acid1 containing no impurities other than water1 may be then obtained by carefully distilling the acid1 treated as above described1 and particularly if only the middle portions of the distillate are collected4 ,such acid should give no precipitate1 either with a solution of barium chloride 7a precipitate shows the presence of sulphuric acid7 or with a solution of silver nitrate 7a precipitate shows the presence of hydrochloric acid71 nor should it1 after being diluted with water1 give a coloration with starch containing potassium iodide 7a coloration shows the admixture of other oxides of nitrogen74 ,the oxides of nitrogen may be most easily removed from impure nitric acid by heating for a certain time with a small quantity of pure charcoal4 ,by the action of nitric acid on the charcoal carbonic anhydride is evolved1 which carries off the lower oxides of nitrogen4 ,on redistilling1 pure acid is obtained4 ,the oxides of nitrogen occurring in solution may also be removed by passing air through the nitric acid4 ,nitric acid so obtained always contains water4 ,it is extremely difficult to deprive it of all the admixed water without destroying a portion of the acid itself and partially converting it into lower oxides1 because without the presence of an excess of water it is very unstable4 ,when rapidly distilled a portion is decomposed1 and there are obtained free oxygen and lower oxides of nitrogen1 which1 together with the water1 remain in solution with the nitric acid4 ,therefore it is necessary to work with great care in order to obtain a pure hydrate of nitric acid1 ,,hno#c1 and especially to mix the nitric acid obtained from nitre1 as above described1 with sulphuric acid1 which takes up the water1 and to distil it at the lowest possible temperature--that is1 by placing the retort holding the mixture in a water or oil bath and carefully heating it4 ,the first portion of the nitric acid thus distilled boils at #hf1 has a specific gravity at #ae of #aebf1 and solidifies at -#ej2 it is very unstable at higher temperatures4 ,this is the normal hydrate1 ,,hno#c1 which corresponds with the salts1 ,,nmo#c1 of nitric acid4 ,when diluted with water nitric acid presents a higher boiling point1 not only as compared with that of the nitric acid itself1 but also with that of water2 so that1 if very dilute nitric acid be distilled1 the first portions passing over will consist of almost pure water1 until the boiling point in the vapours reaches #aba4 ,at this temperature a compound of nitric acid with water1 containing about #gj p4c4 of nitric acid1#ca distils over2 its specific gravity at #ae #adba4 ,if the solution contain less than #be p4c4 of water1 then1 the specific gravity of the solution being above #add1 ,,hno#c evaporates off and fumes in the air1 forming the above hydrate1 whose vapour tension is less than that of water4 ,such solutions form fuming nitric acid4 ,on distilling it gives monohydrated acid1#cb ,,hno#c2 it is a hydrate boiling at #aba1 so that it is obtained from both weak and strong solutions4 ,fuming nitric acid1 under the action not only of organic substances1 but even of heat1 loses a portion of its oxygen1 forming lower oxides of nitrogen1 which impart a red-brown colour to it2#cc the pure acid is colourless4 #ca ,dalton1 ,smith1 ,bineau1 and others considered that the hydrate of constant boiling point 7see ,chapter ,i41 ,note #fj7 for nitric acid was the compound #b;,,hno#c1c;,h#b,o1 but ,roscoe showed that its composition changes with a variation of the pressure and temperature under which the distillation proceeds4 ,thus1 at a pressure of #a atmosphere the solution of constant boiling point contains #fhf p4c41 and at one-tenth of an atmosphere #ffh p4c4 ,judging from what has been said concerning solutions of hydrochloric acid1 and from the variation of specific gravity1 ,i think that the comparatively large decrease in the tensions of the vapours depends on the formation of a hydrate1 ,,nho#c1b;,h#b,o 7 #fcf p4c474 ,such a hydrate may be expressed by ,n7,,ho7#e1 that is1 as ,,nh#d7;,,ho71 in which all the equivalents of hydrogen are replaced by hydroxyl4 ,the constant boiling point will then be the temperature of the decomposition of this hydrate4 ,the variation of the specific gravity at #ae from water 7p #j7 to the hydrate ,,nho#c1e;,h#b,o 7#dab p4c4 ,,hno#c7 is expressed by s #iiib #egdp #jafp#b1 if water #aj1jjj at #d4 ,for example1 when p #cj p4c41 s #aa1hfj4 ,for more concentrated solutions1 at least1 the above-mentioned hydrate1 ,,hno#c1b;,h#b,o1 must be taken1 up to which the specific gravity s #iegj #hdahp-#jbdjp#b2 but perhaps 7since the results of observations of the specific gravity of the solutions are not in sufficient agreement to arrive at a conclusion7 the hydrate ,,hno#c1c;,h#b,o should be recognised1 as is indicated by many nitrates 7,al1 ,mg1 ,co1 c471 which crystallise with this amount of water of crystallisation4 ,from ,,hno#c1b;,h#b,o to ,,hno#c the specific gravity of the solutions 7at #ae7 s #aj1feb #fbjhp-#jafjp#b4 ,the hydrate ,,hno#c1b;,h#b,o is recognised by ,berthelot on the basis of the thermochemical data for solutions of nitric acid1 because on approaching to this composition there is a rapid change in the amount of heat evolved by mixing nitric acid with water4 ,pickering 7#ahib7 by refrigeration obtained the crystalline hydrates3 ,,hno#c1;,h#b,o1 melting at -#cg and ,,hno#c1c;,h#b,o1 melting at -#ah4 ,a more detailed study of the reactions of hydrated nitric acid would no doubt show the existence of change in the process and rapidity of reaction in approaching these hydrates4 #cb ,the normal hydrate ,,hno#c1 corresponding with the ordinary salts1 may be termed the monohydrated acid1 because the anhydride ,n#b,o#e with water forms this normal nitric acid4 ,in this sense the hydrate ,,hno#c1b;,h#b,o is the pentahydrated acid4 #cc ,for technical and laboratory purposes recourse is frequently had to red fuming nitric acid--that is1 the normal nitric acid1 ,,hno#c1 containing lower oxides of nitrogen in solution4 ,this acid is prepared by decomposing nitre with half its weight of strong sulphuric acid1 or by distilling nitric acid with an excess of sulphuric acid4 ,the normal nitric acid is first obtained1 but it partially decomposes1 and gives the lower oxidation products of nitrogen1 which are dissolved by the nitric acid1 to which they impart its usual pale-brown or reddish colour4 ,this acid fumes in the air1 from which it attracts moisture1 forming a less volatile hydrate4 ,if carbonic anhydride be passed through the red-brown fuming nitric acid for a long period of time1 especially if1 assisted by a moderate heat1 it expels all the lower oxides1 and leaves a colourless acid free from these oxides4 ,it is necessary1 in the preparation of the red acid1 that the receivers should be kept quite cool1 because it is only when cold that nitric acid is able to dissolve a large proportion of the oxides of nitrogen4 ,the strong red fuming acid has a specific gravity #aef at #bj1 and has a suffocating smell of the oxides of nitrogen4 ,when the red acid is mixed with water it turns green1 and then of a bluish colour1 and with an excess of water ultimately becomes colourless4 ,this is owing to the fact that the oxides of nitrogen in the presence of water and nitric acid are changed1 and give coloured solutions4 ,markleffsky 7#ahib7 showed that the green solutions contain 7besides ,,hno#c7 ,,hno#b and ,n#b,o#d1 whilst the blue solutions only contain ,,hno#b 7see ,note #dh74 ,the action of red fuming nitric acid 7or a mixture with sulphuric acid7 is in many cases very powerful and rapid1 and it sometimes acts differently from pure nitric acid4 ,thus iron becomes covered with a coating of oxides1 and insoluble in acids2 it becomes1 as is said1 passive4 ,thus chromic acid 7and potassium dichromate7 gives oxide of chromium in this red acid--that is1 it is deoxidised4 ,this is owing to the presence of the lower oxides of nitrogen1 which are capable of being oxidised--that is1 of passing into nitric acid like the higher oxides4 ,but1 generally1 the action of fuming nitric acid1 both red and colourless1 is powerfully oxidising4 ,nitric acid1 as an acid hydrate1 enters into reactions of double decomposition with bases1 basic hydrates 7alkalis71 and with salts4 ,in all these cases a salt of nitric acid is obtained4 ,an alkali and nitric acid give water and a salt2 so1 also1 a basic oxide with nitric acid gives a salt and water2 for instance1 lime1 ,ca,o #b;,,hno#c ,ca7,,no#c7b ,h#b,o4 ,many of these salts are termed nitres4#cd ,the composition of the ordinary salts of nitric acid may be expressed by the general formula ,m7,,no#c7n1 where ,m indicates a metal replacing the hydrogen in one or several 7n7 equivalents of nitric acid4 ,we shall find afterwards that the atoms ,m of metals are equivalent to one 7,k1 ,na1 ,ag7 atom of hydrogen1 or two 7,ca1 ,mg1 ,ba71 or three 7,al1 ,in71 or1 in general1 n atoms of hydrogen4 ,the salts of nitric acid are especially characterised by being all soluble in water4#ce ,from the property common to all these salts of entering into double decompositions1 and owing to the volatility of nitric acid1 they evolve nitric acid when heated with sulphuric acid4 ,they all1 like the acid itself1 are capable of evolving oxygen when heated1 and consequently of acting as oxidising substances2 they therefore1 for instance1 deflagrate with ignited carbon1 the carbon burning at the expense of the oxygen of the salt and forming gaseous products of combustion4#cf #cd ,hydrogen is not evolved in the action of nitric acid 7especially strong7 on metals1 even with those metals which evolve hydrogen under the action of other acids4 ,this is because the hydrogen at the moment of its separation reduces the nitric acid1 with formation of the lower oxides of nitrogen1 as we shall afterwards see4 #ce ,certain basic salts of nitric acid1 however 7for example1 the basic salt of bismuth71 are insoluble in water2 whilst1 on the other hand1 all the normal salts are soluble1 and this forms an exceptional phenomenon among acids1 because all the ordinary acids form insoluble salts with one or another base4 ,thus1 for sulphuric acid the salts of barium1 lead1 c41 for hydrochloric acid the salts of silver1 c41 are insoluble in water4 ,however1 the normal salts of acetic and certain other acids are all soluble4 #cf ,ammonium nitrate1 ,,nh#d,,no#c1 is easily obtained by adding a solution of ammonia or of ammonium carbonate to nitric acid until it becomes neutral4 ,on evaporating this solution1 crystals of the salt are formed which contain no water of crystallisation4 ,it crystallises in prisms like those formed by common nitre1 and has a refreshing taste2 #ajj parts of water at t dissolve #ed #jfat parts by weight of the salt4 ,it is soluble in alcohol1 melts at #afj1 and is decomposed at about #ahj1 forming water and nitrous oxide1 ,,nh#d,,no#c #b;,h#b,o ,n#b,o4 ,if ammonium nitrate be mixed with sulphuric acid1 and the mixture be heated to about the boiling point of water1 then nitric acid is evolved1 and ammonium hydrogen sulphate remains in solution2 but if the mixture be heated rapidly to #af,o1 then nitrous oxide is evolved4 ,in the first case the sulphuric acid takes up ammonia1 and in the second place water4 ,ammonium nitrate is employed in practice for the artificial production of cold1 because in dissolving in water it lowers the temperature very considerably4 ,for this purpose it is best to take equal parts by weight of the salt and water4 ,the salt must first be reduced to a powder and then rapidly stirred up in the water1 when the temperature will fall from #ae to -#aj1 so that the water freezes4 ,ammonium nitrate absorbs ammonia1 with which it forms unstable compounds resembling compounds containing water of crystallisation4 7,divers #ahgb1 ,raoult #ahgc47 ,at -#aj ,,nh#d,,no#c1b,,nh#c is formed3 it is a liquid of sp4 gr4 #aae1 which loses all its ammonia under the influence of heat4 ,at #bh ,,nh#d,,no#c1,,nh#c is formed3 it is a solid which easily parts with its ammonia when heated1 especially in solution4 ,troost 7#ahhb7 investigated the tension of the dissociation of the compounds formed1 and came to the conclusion that a definite compound corresponding to the formula #b,,nh#d,,no#c1c,,nh#c is formed1 because the tension of dissociation remains constant in the decomposition of such a compound at #j4 ,y4 ,kouriloff 7#ahic71 however1 considers that the constancy of the tension of the ammonia evolved is due to the decomposition of a saturated solution1 and not of a definite compound4 ,during decomposition the system is composed of a liquid and a solid2 the tension only becomes constant from the moment the solid falls down4 ,the composition #b,,nh#d,,no#c1c,,nh#c corresponds to a saturated solution at #j1 and the solubility of ,,nh#d,,no#c in ,,nh#c increases with a rise of temperature4 ,nitric acid also enters into double decompositions with a number of hydrocarbons not in any way possessing alkaline characters and not reacting with other acids4 ,under these circumstances the nitric acid gives water and a new substance termed a nitro-compound4 ,the chemical character of the nitro-compound is the same as that of the original substance2 for example1 if an indifferent substance be taken1 then the nitro compound obtained from it will also be indifferent2 if an acid be taken1 then an acid is obtained also4#cf bis ,benzene1 ,c#f;,h#f1 for instance1 acts according to the equation ,c#f;,h#f ,,hno#c ,h#b,o ,c#f;,h#e,,no#b4 ,nitrobenzene is produced4 ,the substance taken1 ,c#f;,h#f1 is a liquid hydrocarbon having a faint tarry smell1 boiling at #hj1 and lighter than water2 by the action of nitric acid nitrobenzene is obtained1 which is a substance boiling at about #baj1 heavier than water1 and having an almond-like odour3 it is employed in large quantities for the preparation of aniline and aniline dyes4#cg ,as the nitro-compounds contain both combustible elements 7hydrogen and carbon71 as well as oxygen in unstable combination with nitrogen1 in the form of the radicle ,,no#b of nitric acid1 they decompose with an explosion when ignited or even struck1 owing to the pressure of the vapours and gases formed--free nitrogen1 carbonic anhydride1 ,,co#b1 carbonic oxide1 ,,co1 and aqueous vapour4 ,in the explosion of nitro compounds#cg bis much heat is evolved1 as in the combustion of gunpowder or detonating gas1 and in this case the force of explosion in a closed space is great1 because from a solid or liquid nitro-compound occupying a small space there proceed vapours and gases whose elasticity is great not only from the small space in which they are formed1 but owing to the high temperature corresponding to the combustion of the nitro-compound4#ch #cf bis ,this is explained by saying that in true nitro-compounds the residue of nitric acid ,,no#b takes the place of the hydrogen in the hydrocarbon group4 ,for example1 if ,c#f;,h#e,,oh be given1 then ,c#f;,h#d7,,no#b7,,oh will be a true nitro-compound having the radical properties of ,c#f;,h#e,,oh4 ,if1 on the other hand1 the ,,no#b replace the hydrogen of the aqueous radicle 7,c#f;,h#e,,ono#b71 then the chemical character varies1 as in the passage of ,,koh into ,,kono#b 7nitre7 7see ,note #cg and ,organic ,chemistry74 #cg ,the compound ethers of nitric acid in which the hydrogen of the aqueous radicle 7,,oh7 is replaced by the residue of nitric acid 7,,no#b7 are frequently called nitro-compounds4 ,but in their chemical character they differ from true nitro-compounds 7for details see ,organic ,chemistry7 and do not burn like them4 ,the action of nitric acid on cellulose1 ,c#f;,h#aj,o#e1 is an example4 ,this substance1 which forms the outer coating of all plant cells1 occurs in an almost pure state in cotton1 in common writing-paper1 and in flax1 c42 under the action of nitric acid it forms water and nitrocellulose 7like water and ,,kno#c from ,,kho71 which1 although it has the same appearance as the cotton originally taken1 differs from it entirely in properties4 ,it explodes when struck1 bursts into flame very easily under the action of sparks1 and acts like gunpowder1 whence its name of pyroxylin1 or gun-cotton4 ,the composition of gun-cotton is ,c#f;,h#g,n#c,o#aa ,c#f;,h#aj,o#e #c,,nho#c-c;,h#b,o4 ,the proportion of the group ,,no#b in nitrocellulose may be decreased by limiting the action of the nitric acid and compounds obtained with different properties2 for instance1 the 7impure7 well-known collodion cotton1 containing from #aa to #ab per cent4 of nitrogen1 and pyro-collodion 7,mendeleff1 #ahij71 containing #abd per cent4 of nitrogen4 ,both these products are soluble in a mixture of alcohol and ether 7in collodion a portion of the substance is soluble in alcohol71 and the solution when evaporated gives a transparent film1 which is insoluble in water4 ,a solution of collodion is employed in medicine for covering wounds1 and in wet-plate photography for giving on glass an even coating of a substance into which the various reagents employed in the process are introduced4 ,extremely fine threads 7obtained by forcing a gelatinous mixture of collodion1 ether1 and alcohol through capillary tubes in water7 of collodion form artificial silk4 #cg bis ,the property possessed by nitroglycerin 7occurring in dynamite71 nitrocellulose1 and the other nitro-compounds1 of burning with an explosion1 and their employment for smokeless powder and as explosives in general1 depends on the reasons in virtue of which a mixture of nitre and charcoal deflagrates and explodes2 in both cases the elements of the nitric acid occurring in the compound are decomposed1 the oxygen in burning unites with the carbon1 and the nitrogen is set free2 thus a very large volume of gaseous substances 7nitrogen and oxides of carbon7 is rapidly formed from the solid substances originally taken4 ,these gases occupy an incomparably larger volume than the original substance1 and therefore produce a powerful pressure and explosion4 ,it is evident that in exploding with the development of heat 7that is1 in decomposing1 not with the absorption of energy1 as is generally the case1 but with the evolution of energy7 the nitro-compounds form stores of energy which are easily set free1 and that consequently their elements occur in a state of particularly energetic motion1 which is especially strong in the group ,,no#b3 this group is common to all nitro-compounds1 and all the oxygen compounds of nitrogen are unstable1 easily decomposable1 and 7,note #bi7 absorb heat in their formation4 ,on the other hand1 the nitro-compounds are instructive as an example and proof of the fact that the elements and groups forming compounds are united in definite order in the molecules of a compound4 ,a blow1 concussion1 or rise of temperature is necessary to bring the combustible elements ,c and ,h into the most intimate contact with ,,no#b1 and to distribute the elements in a new order in new compounds4 ,as regards the composition of the nitro-compounds1 it will be seen that the hydrogen of a given substance is replaced by the complex group ,,no#b of the nitric acid4 ,the same is observed in the passage of alkalis into nitrates1 so that the reactions of substitution of nitric acid--that is1 the formation of salts and nitro-compounds--may be expressed in the following manner4 ,in these cases the hydrogen is replaced by the so-called radicle of nitric acid ,,no#b1 as is evident from the following table3-- ,caustic potash ,,kho4 ,nitre ,k7,,no#b7,o4 ,hydrate of lime ,ca,h#b,o#b4 ,calcium nitrate ,ca7,,no#b7b,o#b4 ,glycerin ,c#c;,h#e;,h#c,o#c4 ,nitroglycerin ,c#c;,h#e7,,no#b7c,o#c4 ,phenol ,c#f;,h#e,,oh4 ,picric acid ,c#f;,h#b7,,no#b7c,,oh1 c4 ,the difference between the salts formed by nitric acid and the nitro-compounds consists in the fact that nitric acid is very easily separated from the salts of nitric acid by means of sulphuric acid 7that is1 by a method of double saline decomposition71 whilst nitric acid is not displaced by sulphuric acid from true nitro-compounds2 for instance1 nitrobenzene1 ,c#f;,h#e,,no#b4 ,as nitro-compounds are formed exclusively from hydrocarbons1 they are described with them in organic chemistry4 ,the group ,,no#b of nitro-compounds in many cases 7like all the oxidised compounds of nitrogen7 passes into the ammonia group or into the ammonia radicle ,,nh#b4 ,this requires the action of reducing substances evolving hydrogen3 ,,rno#b #f;,h ,,rnh#c #b;,h#b,o4 ,thus ,zinin converted nitrobenzene1 ,c#f;,h#e,,no#b1 into aniline1 ,c#f;,h#e,,nh#b1 by the action of hydrogen sulphide4 ,admitting the existence of the group ,,no#b1 as replacing hydrogen in various compounds1 then nitric acid may be considered as water in which half the hydrogen is replaced by the radical of nitric acid4 ,in this sense nitric acid is nitro-water1 ,,no#b,,oh1 and its anhydride dinitro-water1 7,,no#b7b,o4 ,in nitric acid the radical of nitric acid is combined with hydroxyl1 just as in nitrobenzene it is combined with the radical of benzene4 ,it should here be remarked that the group ,,no#c may be recognised in the salts of nitric acid1 because the salts have the composition ,m7,,no#c7n1 just as the metallic chlorides have the composition ,m,cln4 ,but the group ,,no#c does not form any other compounds beyond the salts1 and therefore it should he considered as hydroxyl1 ,,ho1 in which ,h is replaced by ,,no#b4 #ch ,the nitro-compounds play a very important part in mining and artillery4 ,detailed accounts of them must be looked for in special works1 among which the works of ,a4 ,r4 ,shuliachenke and ,t4 ,m4 ,chelletsoff occupy an important place in the ,russian literature on this subject1 although historically the scientific works of ,abel in ,england and ,berthelot in ,france stand pre-eminent4 ,the latter elucidated much in connection with explosive compounds by a series of both experimental and theoretical researches4 ,among explosives a particularly important place from a practical point of view is occupied by ordinary or black gunpowder 7,chapter ,,xiii41 ,note #af71 fulminating mercury 7,chapter ,,xvi41 ,note #bf71 the different forms of gun-cotton 7,chapter ,,vi41 ,note #cg71 and nitro-glycerine 7,chapter ,,viii41 ,note #de1 and ,chapter ,,xii41 ,note #cc74 ,the latter when mixed with solid pulverulent substances1 like magnesia1 tripoli1 c41 forms dynamite1 which is so largely used in quarries and mines in driving tunnels1 c4 ,we may add that the simplest true nitro-compound1 or marsh gas1 ,,ch#d1 in which all the hydrogens are replaced by ,,no#b groups has been obtained by ,l4 ,n4 ,shishkoff1 ,c7,,no#b7d1 as well as nitroform1 ,,ch7,,no#b7c4 ,illustration3 ,,fig4 #dh4--,the method of decomposition of nitrous anhydride1 also applicable to the other oxides of nitrogen1 and to their analysis4 ,,no#b is generated from nitrate of lead in the retort ,a4 ,nitric acid and other less volatile products are condensed in ,b4 ,the tube ,c ,c contains copper1 and is heated from below4 ,undecomposed volatile products 7if any are formed7 are condensed in ,d1 which is cooled4 ,if the decomposition be incomplete1 brown fumes make their appearance in this receiver4 ,the gaseous nitrogen is collected in the cylinder ,e4 ,if the vapour of nitric acid is passed through an even moderately heated glass tube1 the formation of dark-brown fumes of the lower oxides of nitrogen and the separation of free oxygen may be observed1 #b,,nho#c ,h#b,o #b,,no#b ,o4 ,the decomposition is complete at a white heat--that is1 nitrogen is formed1 #b,,nho#c ,h#b,o ,n#b ,o#e4 ,hence it is easily understood that nitric acid may part with its oxygen to a number of substances capable of being oxidised4#ci ,it is consequently an oxidising agent4 ,charcoal1 as we have already seen1 burns in nitric acid2 phosphorus1 sulphur1 iodine1 and the majority of metals also decompose nitric acid1 some on heating and others even at the ordinary temperature3 the substances taken are oxidised and the nitric acid is deoxidised1 yielding compounds containing less oxygen4 ,only a few metals1 such as gold and platinum1 do not act on nitric acid1 but the majority decompose it2 in so doing1 an oxide of the metal is formed1 which1 if it has the character of a base1 acts on the remaining nitric acid2 hence1 with the majority of metals the result of the reaction is usually not an oxide of the metal1 but the corresponding salt of nitric acid1 and1 at the same time1 one of the lower oxides of nitrogen4 ,the resulting salts of the metals are soluble1 and hence it is said that nitric acid dissolves nearly all metals4#dj ,this case is termed the solution of metals by acids1 although it is not a case of simple solution1 but a complex chemical change of the substances taken4 ,when treated with this acid1 those metals whose oxides do not combine with nitric acid yield the oxide itself1 and not a salt2 for example1 tin acts in this manner on nitric acid1 forming a hydrated oxide1 ,sn,h#b,o#c1 which is obtained in the form of a white powder1 ,sn #d,,nho#c ,h#b,sn,o#c #d,,no#b ,h#b,o4 ,silver is able to take up still more oxygen1 and to convert a large portion of nitric acid into nitrous anhydride1 #d;,ag #f;,,hno#c #d;,ag,n,o#c ,n#b,o#c #c;,h#b,o4 ,copper takes up still more oxygen from nitric acid1 converting it into nitric oxide1 and1 by the action of zinc1 nitric acid is able to give up a still further quantity of nitrogen1 forming nitrous oxide1 #d,zn #aj,,nho#c #d,zn7,,no#c7b ,n#b,o #e;,h#b,o4#da ,sometimes1 and especially with dilute solutions of nitric acid1 the deoxidation proceeds as far as the formation of hydroxylamine and ammonia1 and sometimes it leads to the formation of nitrogen itself4 ,the formation of one or other nitrogenous substance from nitric acid is determined1 not only by the nature of the reacting substances1 but also by the relative mass of water and nitric acid1 and also by the temperature and pressure1 or the sum total of the conditions of reaction2 and as in a given mixture even these conditions vary 7the temperature and the relative mass vary71 it not unfrequently happens that a mixture of different products of the deoxidation of nitric acid is formed4 #ci ,illustration3 ,,fig4 #di4--;,decomposition of nitrous oxide by sodium4 ,nitric acid may be entirely decomposed by passing its vapour over highly incandescent copper1 because the oxides of nitrogen first formed give up their oxygen to the red-hot metallic copper1 so that water and nitrogen gas alone are obtained4 ,this forms a means for determining the composition both of nitric acid and of all the other compounds of nitrogen with oxygen1 because by collecting the gaseous nitrogen formed it is possible to calculate1 from its volume1 its weight and consequently its amount in a given quantity of a nitrogenous substance1 and by weighing the copper before and after the decomposition it is possible to determine the amount of oxygen by the increase in weight4 ,the complete decomposition of nitric acid is also accomplished by passing a mixture of hydrogen and nitric acid vapours through a red-hot tube4 ,sodium also decomposes the oxides of nitrogen at a red-heat1 taking up all the oxygen4 ,this method is sometimes used for determining the composition of the oxides of nitrogen4 #dj ,the application of this acid for etching copper or steel in engraving is based on this fact4 ,the copper is covered with a coating of wax1 resin1 c4 7etching ground71 on which nitric acid does not act1 and then the ground is removed in certain parts with a needle1 and the whole is washed in nitric acid4 ,the parts coated remain untouched1 whilst the uncovered portions are eaten into by the acid4 ,copper plates for etchings1 aquatints1 c41 are prepared in this manner4 #da ,the formation of such complex equations as the above often presents some difficulty to the beginner4 ,it should be observed that if the reacting and resultant substances be known1 it is easy to form an equation for the reaction4 ,thus1 if we wish to form an equation expressing the reaction that nitric acid acting on zinc gives nitrous oxide1 ,n#b,o1 and zinc nitrate1 ,zn7,,no#c7b1 we must reason as follows3--,nitric acid contains hydrogen1 whilst the salt and nitrous oxide do not2 hence water is formed1 and therefore it is as though anhydrous nitric acid1 ,n#b,o#e1 were acting4 ,for its conversion into nitrous oxide it parts with four equivalents of oxygen1 and hence it is able to oxidise four equivalents of zinc and to convert it into zinc oxide1 ,zn,o4 ,these four equivalents of zinc oxide require for their conversion into the salt four more equivalents of nitric anhydride2 consequently five equivalents in all of the latter are required1 or ten equivalents of nitric acid4 ,thus ten equivalents of nitric acid are necessary for four equivalents of zinc in order to express the reaction in whole equivalents4 ,it must not be forgotten1 however1 that there are very few such reactions which can be entirely expressed by simple equations4 ,the majority of equations of reactions only express the chief and ultimate products of reaction1 and thus none of the three preceding equations express all that in reality occurs in the action of metals on nitric acid4 ,in no one of them is only one oxide of nitrogen formed1 but always several together or consecutively--one after the other1 according to the temperature and strength of the acid4 ,and this is easily intelligible4 ,the resulting oxide is itself capable of acting on metals and of being deoxidised1 and in the presence of the nitric acid it may change the acid and be itself changed4 ,the equations given must be looked on as a systematic expression of the main features of reactions1 or as a limit towards which they tend1 but to which they only attain in the absence of disturbing influences4 ,thus the action of nitric acid on metals consists in their being oxidised1 whilst the acid itself is converted1 according to the temperature1 concentration in which it is taken1 and the nature of the metal1 c41 into lower oxides1 ammonia1 or even into nitrogen4#db ,many compounds are oxidised by nitric acid like metals and other elements2 for instance1 lower oxides are converted into higher oxides4 ,thus1 arsenious acid is converted into arsenic acid1 suboxide of iron into oxide1 sulphurous acid into sulphuric acid1 the sulphides of the metals1 ,m#b,s1 into sulphates1 ,m#b,,so#d1 c42 in a word1 nitric acid brings about oxidation1 its oxygen is taken up and transferred to many other substances4 ,certain substances are oxidised by strong nitric acid so rapidly and with so great an evolution of heat that they deflagrate and burst into flame4 ,thus turpentine1 ,c#aj;,h#af1 bursts into flame when poured into fuming nitric acid4 ,in virtue of its oxidising property1 nitric acid removes the hydrogen from many substances4 ,thus it decomposes hydriodic acid1 separating the iodine and forming water2 and if fuming nitric acid be poured into a flask containing gaseous hydriodic acid1 then a rapid reaction takes place1 accompanied by flame and the separation of violet vapours of iodine and brown fumes of oxides of nitrogen4#dc #db ,montemartini endeavours to show that the products evolved in the action of nitric acid upon metals 7and their amount7 is in direct connection with both the concentration of the acid and the capacity of the metals to decompose water4 ,those metals which only decompose water at a high temperature give1 under the action of nitric acid1 ,,no#b1 ,n#b,o#d1 and ,,no2 whilst those metals which decompose water at a lower temperature give1 besides the above products1 ,n#b,o1 ,n1 and ,,nh#c2 and1 lastly1 the metals which decompose water at the ordinary temperature also evolve hydrogen4 ,it is observed that concentrated nitric acid oxidises many metals with much greater difficulty than when diluted with water2 iron1 copper1 and tin are very easily oxidised by dilute nitric acid1 but remain unaltered under the influence of monohydrated nitric acid or of the pure hydrate ,,nho#c4 ,nitric acid diluted with a large quantity of water does not oxidise copper1 but it oxidises tin2 dilute nitric acid also does not oxidise either silver or mercury2 but1 on the addition of nitrous acid1 even dilute acid acts on the above metals4 ,this naturally depends on the smaller stability of nitrous acid1 and on the fact that after the commencement of the action the nitric acid is itself converted into nitrous acid1 which continues to act on the silver and mercury4 ,veley 7,oxford #ahia7 made detailed researches on the action of nitric acid upon ,cu1 ,hg1 and ,bi1 and showed that nitric acid of #cj p4c4 strength does not act upon these metals at the ordinary temperature if nitrous acid 7traces are destroyed by urea7 and oxidising agents such as ,h#b,o#b1 ,k,cl,o#c1 c4 be entirely absent2 but in the presence of even a small amount of nitrous acid the metals form nitrites1 which1 with ,,hno#c1 form nitrates and the oxides of nitrogen1 which re-form the nitrous acid necessary for starting the reaction1 because the reaction #b,,no ,,hno#c ,h#b,o #c;,,hno#b is reversible4 ,the above metals are quickly dissolved in a #a p4c4 solution of nitrous acid4 ,moreover1 ,veley observed that nitric acid is partially converted into nitrous acid by gaseous hydrogen in the presence of the nitrates of ,cu and ,pb4 #dc ,when nitric acid acts on many organic substances it often happens that not only is hydrogen removed1 but also oxygen is combined2 thus1 for example1 nitric acid converts toluene1 ,c#g;,h#h1 into benzoic acid1 ,c#g;,h#f,o#b4 ,in certain cases1 also1 a portion of the carbon contained in an organic substance burns at the expense of the oxygen of the nitric acid4 ,so1 for instance1 phthalic acid1 ,c#h;,h#f,o#d1 is obtained from naphthalene1 #aj;,h#h4 ,thus the action of nitric acid on the hydrocarbons is often most complex2 not only does nitrification take place1 but also separation of carbon1 displacement of hydrogen1 and combination of oxygen4 ,there are few organic substances which can withstand the action of nitric acid1 and it causes fundamental changes in a number of them4 ,it leaves a yellow stain on the skin1 and in a large quantity causes a wound and entirely eats away the membranes of the body4 ,the membranes of plants are eaten into with the greatest ease by strong nitric acid in just the same manner4 ,one of the most durable blue vegetable dyes employed in dyeing tissues is indigo2 yet it is easily converted into a yellow substance by the action of nitric acid1 and small traces of free nitric acid may be recognised by this means4 ,as nitric acid is very easily decomposed with the separation of oxygen1 it was for a long time supposed that it was not capable of forming the corresponding nitric anhydride1 ,n#b,o#e2 but ,deville first and subsequently ,weber and others1 discovered the methods of its formation4 ,deville obtained nitric anhydride by decomposing silver nitrate by chlorine under the influence of a moderate heat4 ,chlorine acts on the above salt at a temperature of #ie 7#b;,ag,n,o#c ,cl#b #b;,ag,cl ,n#b,o#e ,o71 and when once the reaction is started1 it continues by itself without further heating4 ,brown fumes are given off1 which are condensed in a tube surrounded by a freezing-mixture4 ,a portion condenses in this tube and a portion remains in a gaseous state4 ,the latter contains free oxygen4 ,a crystalline mass and a liquid substance are obtained in the tube2 the liquid is poured off1 and a current of dry carbonic acid gas is passed through the apparatus in order to remove all traces of volatile substances 7liquid oxides of nitrogen7 adhering to the crystals of nitric anhydride4 ,these form a voluminous mass of rhombic crystals 7density #afd71 which sometimes are of rather large size2 they melt at about #cj and distil at about #dg4 ,in distilling1 a portion of the substance is decomposed4 ,with water these crystals give nitric acid4 ,nitric anhydride is also obtained by the action of phosphoric anhydride1 ,p#b,o#e1 on cold pure nitric acid 7below #j74 ,during the very careful distillation of equal parts by weight of these two substances a portion of the acid decomposes1 giving a liquid compound1 ,h#b,o1#b,n#b,o#e ,n#b,o#e1b;,,hno#c1 whilst the greater part of the nitric acid gives the anhydride according to the equation #b,,nho#c ,p#b,o#e #b,,pho#c ,n#b,o#e4 ,on heating1 nitric anhydride decomposes with an explosion1 or gradually1 into nitric peroxide and oxygen1 ,n#b,o#e ,n#b,o#d ,o4 ,nitrogen peroxide1 ,n#b,o#d1 and nitrogen dioxide1 ,,no#b1 express one and the same composition1 but they should be distinguished like ordinary oxygen and ozone1 although in this case their mutual conversion is more easily effected and takes place on vaporisation2 also1 ,o#c loses heat in passing into ,o#b1 whilst ,n#b,o#d absorbs heat in forming ,,no#b4 ,nitric acid in acting on tin and on many organic substances 7for example1 starch7 gives brown vapours1 consisting of a mixture of ,n#b,o#c and ,,no#b4 ,a purer product is obtained by the decomposition of lead nitrate by heat1 ,pb7,,no#c7b #b,,no#b ,o ,pb,o1 when non-volatile lead oxide1 oxygen gas1 and nitrogen peroxide are formed4 ,the latter condenses1 in a well-cooled vessel1 to a brown liquid1 which boils at about #bb4 ,the purest peroxide of nitrogen1 solidifying at -#i1 is obtained by mixing dry oxygen in a freezing-mixture with twice its volume of dry nitric oxide1 ,,no1 when transparent prisms of nitrogen peroxide are formed in the receiver3 they melt into a colourless liquid at about -#aj4 ,when the temperature of the receiver is above -#i1 the crystals melt1#dd and at #j give a reddish yellow liquid1 like that obtained in the decomposition of lead nitrate4 ,the vapours of nitrogen peroxide have a characteristic odour1 and at the ordinary temperature are of a dark-brown colour1 but at lower temperatures the colour of the vapour is much fainter4 ,when heated1 especially above #ej1 the colour becomes a very dark brown1 so that the vapours almost lose their transparency4 #dd ,according to certain investigations1 if a brown liquid is formed from the melted crystals by beating above -#i1 then they no longer solidify at -#aj1 probably because a certain amount of ,n#b,o#c 7and oxygen7 is formed1 and this substance remains liquid at -#cj1 or it may be that the passage from #b,,no#b into ,n#b,o#d is not so easily accomplished as the passage from ,n#b,o#d into #b,,no#b4 ,liquid nitrogen peroxide 7that is1 a mixture of ,,no#b and ,n#b,o#d7 is employed in admixture with hydrocarbons as an explosive4 ,the causes of these peculiarities of nitrogen peroxide were not clearly understood until ,deville and ,troost determined the density and dissociation of the vapour of this substance at different temperatures1 and showed that the density varies4 ,if the density be referred to that of hydrogen at the same temperature and pressure1 then it is found to vary from #ch at the boiling point1 or about #bg1 to #bc at #ace1 after which the density remains constant up to those high temperatures at which the oxides of nitrogen are decomposed4 ,as on the basis of the laws enunciated in the following chapter1 the density #bc corresponds with the compound ,,no#b 7because the weight corresponding with this molecular formula #df1 and the density referred to hydrogen as unity is equal to half the molecular weight72 therefore at temperatures above #ace the existence of nitrogen dioxide only must be recognised4 ,it is this gas which is of a brown colour4 ,at a lower temperature it forms nitrogen peroxide1 ,n#b,o#d1 whose molecular weight1 and therefore density1 is twice that of the dioxide4 ,this substance1 which is isomeric with nitrogen dioxide1 as ozone is isomeric with oxygen1 and has twice as great a vapour density 7#df referred to hydrogen71 is formed in greater quantity the lower the temperature1 and crystallises at -#aj4 ,the reasons both of the variation of the colour of the gas 7,n#b,o#d gives colourless and transparent vapours1 whilst those of ,,no#b are brown and opaque7 and the variation of the vapour density with the variation of temperature are thus made quite clear2 and as at the boiling point a density #ch was obtained1 therefore at that temperature the vapours consist of a mixture of #gi parts by weight of ,n#b,o#d with #ba parts by weight of ,,no#b4de ,it is evident that a decomposition here takes place the peculiarity of which consists in the fact that the product of decomposition1 ,,no#b1 is polymerised 7i4e4 becomes denser1 combines with itself7 at a lower temperature2 that is1 the reaction ,n#b,o#d ,,no#b ,,no#b is a reversible reaction1 and consequently the whole phenomenon represents a dissociation in a homogeneous gaseous medium1 where the original substance1 ,n#b,o#d1 and the resultant1 ,,no#b1 are both gases4 ,the measure of dissociation will be expressed if we find the proportion of the quantity of the substance decomposed to the whole amount of the substance4 ,at the boiling point1 therefore1 the measure of the decomposition of nitrogen peroxide will be #ba p4c42 at #ace it #a1 and at #aj it #j2 that is1 the ,n#b,o#d is not then decomposable4 ,consequently the limits of dissociation here are -#aj and #ace at the atmospheric pressure4#df ,within the limits of these temperatures the vapours of nitrogen peroxide have not a constant density1 but1 on the other hand1 above and below these limits definite substances exist4 ,thus above #ace ,n#b,o#d has ceased to exist and ,,no#b alone remains4 ,it is evident that at the ordinary temperature there is a partially dissociated system or mixture of nitrogen peroxide1 ,n#b,o#d1 and nitrogen dioxide1 ,,no#b4 ,in the brown liquid boiling at #bb probably a portion of the ,n#b,o#d has already passed into ,,no#b1 and it is only the colourless liquid and crystalline substance at -#aj that can be considered as pure nitrogen peroxide4#dg #de ,because if x equal the amount by weight of ,n#b,o#d1 its volume will x_/#df1 and the amount of ,,no#b will #ajj-x1 and consequently its volume will 7#ajj-x7_/#bc4 ,but the mixture1 having a density #ch1 will weigh #ajj2 consequently its volume will #ajj_/ch4 ,hence x_/#df 7#ajj-x7_/#bc #ajj_/ch1 or x #gi,o4 #df ,the phenomena and laws of dissociation1 which we shall consider only in particular instances1 are discussed in detail in works on theoretical chemistry4 ,nevertheless1 in respect to nitrogen peroxide1 as an historically important example of dissociation in a homogeneous gaseous medium1 we will cite the results of the careful investigations 7#ahhe-ahhj7 of ,e4 and ,l4 ,natanson1 who determined the densities under various conditions of temperature and pressure4 ,the degree of dissociation1 expressed as above 7it may also he expressed otherwise--for example1 by the ratio of the quantity of substance decomposed to that unaltered71 proves to increase at all temperatures as the pressure diminishes1 which would he expected for a homogeneous gaseous medium1 as a decreasing pressure aids the formation of the lightest product of dissociation 7that having the least density or largest volume74 ,thus1 in the ,natansons' experiments the degree of dissociation at #j increases from #aj p4c4 to #cj p4c41 with a decrease of pressure of from #bea to #ch mm42 at #dig it increases from #di p4c4 to #ic p4c41 with a fall of pressure of from #dih to #bg mm41 and at #ajj it increases from #hib p4c4 to #iig p4c41 with a fall of pressure of from #gcbe to #aag mm4 ,at #acj and #aej the decomposition is complete--that is1 only ,,no#b remains at the low pressures 7less than the atmospheric7 at which the ,natansons made their determinations2 but it is probable that at higher pressures 7of several atmospheres7 molecules of ,n#b,o#d would still be formed1 and it would be exceedingly interesting to trace the phenomena under the conditions of both very considerable pressures and of relatively large volumes4 #dg ,liquid nitrogen peroxide is said by ,geuther to boil at #bb-bf1 and to have a sp4 gr4 at #j #adid and at #ae #adgd4 ,it is evident that1 in the liquid as in the gaseous state1 the variation of density with the temperature depends1 not only on physical1 but also on chemical changes1 as the amount of ,n#b,o#d decreases and the amount of ,,no#b increases with the temperature1 and they 7as polymeric substances7 should have different densities1 as we find1 for instance1 in the hydrocarbons ,c#e;,h#aj and ,c#aj;,h#bj4 ,it may not be superfluous to mention here that the measurement of the specific heat of a mixture of the vapours of ,n#b,o#d and ,,no#b enabled ,berthelot to determine that the transformation of #b,,no#b into ,n#b,o#d is accompanied by the evolution of about #ac1jjj units of heat1 and as the reaction proceeds with equal facility in either direction1 it will be exothermal in the one direction and endothermal in the other2 and this clearly demonstrates the possibility of reactions taking place in either direction1 although1 as a rule1 reactions evolving heat proceed with greater ease4 ,the above explains the action of nitrogen peroxide on water at low temperatures4 ,n#b,o#d then acts on water like a mixture of the anhydrides of nitrous and nitric acids4 ,the first1 ,n#b,o#c1 may be looked on as water in which each of the two atoms of hydrogen is replaced by the radicle ,,no1 while in the second each hydrogen is replaced by the radicle ,,no#b1 proper to nitric acid2 and in nitrogen peroxide one atom of the hydrogen of water is replaced by ,,no and the other by ,,no#b1 as is seen from the formul-- ,h ,,no ,,no ,,no#b ,h ,o2 ,,no ,o2 ,,no#b ,o2 ,,no#b ,o2 or ,h#b,o2 ,n#b,o#c2 ,n#b,o#d2 ,n#b,o#e4 ,in fact1 nitrogen peroxide at low temperatures gives with water 7ice7 both nitric1 ,,hno#c1 and nitrous1 ,,hno#b1 acids4 ,the latter1 as we shall afterwards see1 splits up into water and the anhydride1 ,n#b,o#c4 ,if1 however1 warm water act on nitrogen peroxide1 only nitric acid and monoxide of nitrogen are formed3 #c,,no#b ,h#b,o ,,no #b,,nho#c4 ,although ,,no#b is not decomposed into ,n and ,o even at #ejj1 still in many cases it acts as an oxidising agent4 ,thus1 for instance1 it oxidises mercury1 converting it into mercurous nitrate1 #b,,no#b ,hg ,hg,n,o#c ,,no1 being itself deoxidised into nitric oxide1 into which the dioxide in many other instances passes1 and from which it is easily formed4#dh #dh ,nitric acid of sp4 gr4 #aea in dissolving nitrogen peroxide becomes brown1 whilst nitric acid of sp4 gr4 #acb is coloured greenish blue1 and acid of sp4 gr4 below #aae remains colourless after absorbing nitrogen peroxide 7,note #cc74 ,nitrous anhydride1 ,n#b,o#c1 corresponds#di to nitrous acid1 ,,nho#b1 which forms a series of salts1 the nitrites--for example1 the sodium salt ,na,n,o#b1 the potassium salt ,,kno#b1 the ammonium salt 7,,nh#d7,,no#b1ej the silver salt ,ag,n,o#b1ea c4 ,neither the anhydride nor the hydrate of the acid is known in a perfectly pure state4 ,the anhydride has only been obtained as a very unstable substance1 and has not yet been fully investigated2 and on attempting to obtain the acid ,,nho#b from its salts1 it always gives water and the anhydride1 whilst the latter1 as an intermediate oxide1 partially or wholly splits up into ,,no ,,no#b4 ,but the salts of nitrous acid are distinguished for their great stability4 ,potassium nitrate1 ,,kno#c1 may be converted into potassium nitrite by depriving it of a portion of its oxygen2 for instance1 by fusing it 7at not too high a temperature7 with metals1 such as lead1 ,,kno#c ,pb ,,kno#b ,pb,o4#ea bis ,the resultant salt is soluble in water1 whilst the oxide of lead is insoluble4 ,with sulphuric and other acids the solution of potassium nitrite#eb immediately evolves a brown gas1 nitrous anhydride3 #b,,kno#b ,h#b,,so#d ,k#b,,so#d ,n#b,o#c ,h#b,o4 ,the same gas 7,n#b,o#c7 is obtained by passing nitric oxide at #j through liquid peroxide of nitrogen1#ec or by heating starch with nitric acid of sp4 gr4 #ac4 ,at a very low temperature it condenses into a blue liquid boiling below #j1ed but then partially decomposing into ,,no ,,no#b4 ,nitrous anhydride possesses a remarkable capacity for oxidising4 ,ignited bodies burn in it1 nitric acid absorbs it1 and then acquires the property of acting on silver and other metals1 even when diluted4 ,potassium iodide is oxidised by this gas just as it is by ozone 7and by peroxide of hydrogen1 chromic and other acids1 but not by dilute nitric acid nor by sulphuric acid71 with the separation of iodine4 ,this iodine may he recognised 7see ,ozone1 ,chapter ,,iv47 by its turning starch blue4 ,very small traces of nitrites may be easily detected by this method4 ,if1 for example1 starch and potassium iodide are added to a solution of potassium nitrite 7at first there will be no change1 there being no free nitrous acid71 and then sulphuric acid be added1 the nitrous acid 7or its anhydride7 immediately set free liberates iodine1 which produces a blue colour with the starch4 ,nitric acid does not act in this manner1 but in the presence of zinc the coloration takes place1 which proves the formation of nitrous acid in the deoxidation of nitric acid4#ee ,nitrous acid acts directly on ammonia1 forming nitrogen and water1 ,,hno#b ,,nh#c ,n#b #b;,h#b,o4#ef #di ,nitrogen peroxide as a mixed substance has no corresponding independent salts1 but ,sabatier and ,senderens 7#ahib7 showed that under certain conditions ,,no#b combines directly with some metals--for instance1 copper and cobalt--forming ,cu#b,,no#b and ,co,n,o#b as dark brown powders1 which do not1 however1 exhibit the reactions of salts4 ,thus by passing gaseous nitrogen dioxide over freshly reduced 7from the oxides by heating with hydrogen7 copper at #be-cj1 ,cu#b,,no#b is directly formed4 ,with water it partly gives off ,,no#b and partly forms nitrite of copper1 leaving metallic copper and its suboxide4 ,the nature of these compounds has not yet been sufficiently investigated4 #ej ,ammonium nitrite may be easily obtained in solution by a similar method of double decomposition 7for instance1 of the barium salt with ammonium sulphate7 to the other salts of nitrous acid1 but it decomposes with great ease when evaporated1 with evolution of gaseous nitrogen1 as already mentioned 7,chapter ,v47 ,if the solution1 however1 be evaporated at the ordinary temperature under the receiver of an air-pump1 a solid saline mass is obtained1 which is easily decomposed when heated4 ,the dry salt even decomposes with an explosion when struck1 or when heated to about #gj--,,nh#d,,no#b #b;,h#b,o ,n#b4 ,it is also formed by the action of aqueous ammonia on a mixture of nitric oxide and oxygen1 or by the action of ozone on ammonia1 and in many other instances4 ,zrensen 7#ahid7 prepared ,,nh#d,,no#b by the action of a mixture of ,n#b,o#c and other oxides of nitrogen on lumps of ammonium carbonate1 extracting the nitrite of ammonium formed with absolute alcohol1 and precipitating it from this solution by ether4 ,this salt is crystalline1 dissolves in water with absorption of heat1 and attracts moisture from the air4 ,the solid salt and its concentrated solutions decompose with an explosion when heated to #ej-hj1 especially in the presence of traces of foreign acids4 ,decomposition also proceeds at the ordinary temperature1 but more slowly2 and in order to preserve the salt it should be covered with a layer of pure dry ether4 #ea ,silver nitrite1 ,ag,n,o#b1 is obtained as a very slightly soluble substance1 as a precipitate1 on mixing solutions of silver nitrate1 ,ag,n,o#c1 and potassium nitrite1 ,,kno#b4 ,it is soluble in a large volume of water1 and this is taken advantage of to free it from silver oxide1 which is also present in the precipitate1 owing to the fact that potassium nitrite always contains a certain amount of oxide1 which with water gives the hydroxide1 forming oxide of silver with silver nitrate4 ,the solution of silver nitrite gives1 by double decomposition with metallic chlorides 7for instance1 barium chloride71 insoluble silver chloride and the nitrite of the metal taken 7in this case1 barium nitrite1 ,ba7,,no#b7b74 #ea bis ,leroy 7#ahhi7 obtained ,,kno#b by mixing powdered ,,kno#c with ,ba,s1 igniting the mixture in a crucible and washing the fused salts2 ,ba,s,o#d is then left as an insoluble residue1 and ,,kno#b passes into solution3 #d,,kno#c ,ba,s #d,,kno#b ,ba,s,o#d4 #eb ,probably potassium nitrite1 ,,kno#b1 when strongly heated1 especially with metallic oxides1 evolves ,n and ,o1 and gives potassium oxide1 ,k#b,o1 because nitre is liable to such a decomposition2 but it has1 as yet1 been but little investigated4 #ec ,there are many researches which lead to the conclusion that the reaction ,n#b,o#c ,,no#b-,,no is reversible1 i4e4 resembles the conversion of ,n#b,o#d into ,,no#b4 ,the brown colour of the fumes of ,n#b,o#c is due to the formation of ,,no#b4 ,if nitrogen peroxide be cooled to -#bj1 and half its weight of water be added to it drop by drop1 then the peroxide is decomposed1 as we have already said1 into nitrous and nitric acids2 the former does not then remain as a hydrate1 but straightway passes into the anhydride1 and1 hence1 if the resultant liquid be slightly warmed vapours of nitrous anhydride1 ,n#b,o#c1 are evolved1 and condense into a blue liquid1 as ,fritzsche showed4 ,this method of preparing nitrous anhydride apparently gives the purest product1 but it easily dissociates1 forming ,,no and ,,no#b 7and therefore also nitric acid in the presence of water74 #ed ,according to ,thorpe1 ,n#b,o#c boils at #ah4 ,according to ,geuther1 at #ce1 and its sp4 gr4 at #j #addi4 #ee ,in its oxidising action nitrous anhydride gives nitric oxide1 ,n#b,o#c #b,,no ,o4 ,thus its analogy to ozone becomes still more marked1 because in ozone it is only one-third of the oxygen that acts in oxidising2 from ,o#c there is obtained ,o1 which acts as an oxidiser1 and common oxygen ,o#b4 ,in a physical aspect the relation between ,n#b,o#c and ,o#c is revealed in the fact that both substances are of a blue colour when in the liquid state4 #ef ,this reaction is taken advantage of for converting the amides1 ,,nh#b,r 7where ,r is an element or a complex group7 into hydroxides1 ,,rho4 ,in this case ,,nh#b,r ,,nho#b forms #b,n ,h#b,o ,,rho2 ,,nh#b1 is replaced by ,,ho1 the radicle of ammonia by the radicle of water4 ,this reaction is employed for transforming many nitrogenous organic substances having the properties of amides into their corresponding hydroxides4 ,thus aniline1 ,c#f;,h#e,,nh#b1 which is obtained from nitrobenzene1 ,c#f;,h#e,,no#b 7,note #cg71 is converted by nitrous anhydride into phenol1 ,c#f;,h#e,,oh1 which occurs in the creosote extracted from coal tar4 ,thus the ,h of the benzene is successively replaced by ,,no#b1 ,,nh#b1 and ,,ho2 a method which is suitable for other cases also4 ,as nitrous anhydride easily splits up into ,,no#b ,,no1 so1 like ,,no#b1 with warm water it gives nitric acid and nitric oxide1 according to the equation #c,n#b,o#c ,h#b,o #d,,no #b,,nho#c4 ,being in a lower degree of oxidation than nitric acid1 nitrous acid and its anhydride are oxidised in solutions by many oxidising substances--for example1 by potassium permanganate--into nitric acid4#eg #eg ,the action of a solution of potassium permanganate1 ,k,mn,o#d1 on nitrous acid in the presence of sulphuric acid is determined by the fact that the higher oxide of manganese1 ,mn#b,o#g1 contained in the permanganate is converted into the lower oxide1 ,mn,o1 which as a base forms manganese sulphate1 ,mn,s,o#d1 and the oxygen serves for the oxidation of the ,n#b,o#c into ,n#b,o#e1 or its hydrate4 ,as the solution of the permanganate is of a red colour1 whilst that of manganese sulphate is almost colourless1 this reaction is clearly seen1 and may be employed for the detection and determination of nitrous acid and its salts4 ,nitric oxide1 ,,no4--,this permanent gas#eh 7that is1 unliquefiable by pressure without the aid of cold7 may be obtained from all the above-described compounds of nitrogen with oxygen4 ,the deoxidation of nitric acid by metals is the usual method employed for its preparation4 ,dilute nitric acid 7sp4 gr4 #aah1 but not stronger1 as then ,n#b,o#c and ,,no#b are produced7 is poured into a flask containing metallic copper4#ei ,the reaction commences at the ordinary temperature4 ,mercury and silver also give nitric oxide with nitric acid4 ,in these reactions with metals one portion of the nitric acid is employed in the oxidation of the metal1 whilst the other1 and by far the greater1 portion combines with the metallic oxide so obtained1 with formation of the nitrate corresponding with the metal taken4 ,the first action of the copper on the nitric acid is thus expressed by the equation #b,,nho#c #c;,cu ,h#b,o #c;,cu,o #b,,no4 ,the second reaction consists in the formation of copper nitrate-- #f,,nho#c #c;,cu,o #c;,h#b,o #c;,cu7,,no#c7b4 #eh ,the absolute boiling point -#ic 7see ,chapter ,,ii41 ,note #bi74 #ei ,kammerer proposed preparing nitric oxide1 ,,no1 by pouring a solution of sodium nitrate over copper shavings1 and adding sulphuric acid drop by drop4 ,the oxidation of ferrous salts by nitric acid also gives ,,no4 ,one part of strong hydrochloric acid is taken and iron is dissolved in it 7,fe,cl#b71 and then an equal quantity of hydrochloric acid and nitre is added to the solution4 ,on heating1 nitric oxide is evolved4 ,in the presence of an excess of sulphuric acid and mercury the conversion of nitric acid into nitric oxide is complete 7that is1 the reaction proceeds to the end and the nitric oxide is obtained without other products71 and upon this is founded one of the methods for determining nitric acid 7in nitrometers of various kinds1 described in text-books of analytical chemistry71 as the amount of ,,no can be easily and accurately measured volumetrically4 ,the amount of nitrogen in gun-cotton1 for instance1 is determined by dissolving it in sulphuric acid4 ,nitrous acid acts in the same manner4 ,upon this property ,emich 7#ahib7 founds his method for preparing pure ,,no4 ,he pours mercury into a flask1 and then covers it with sulphuric acid1 in which a certain amount of ,na,n,o#b or other substance corresponding to ,,hno#b or ,,hno#c has been dissolved4 ,the evolution of ,,no proceeds at the ordinary temperature1 being more rapid as the surface of the mercury is increased 7if shaken1 the reaction proceeds very rapidly74 ,if the gas be passed over ,,kho1 it is obtained quite pure1 because ,,kho does not act upon ,,no at the ordinary temperature 7if heated1 ,,kno#b and ,n#b,o or ,n#b1 are formed74 ,nitric oxide is a colourless gas which is only slightly soluble in water 7#a_/bj of a volume at the ordinary temperature74 ,reactions of double decomposition in which nitric oxide readily takes part are not known--that is to say1 it is an indifferent1 not a saline1 oxide4 ,like the other oxides of nitrogen1 it is decomposed into its elements at a red heat 7starting from #ijj1 at #a1bjj #fj per cent4 give ,n#b and #b,n#b,o#c1 but complete decomposition into ,n#b and ,o#b only takes place at the melting point of platinum1 ,emich #ahib74 ,the most characteristic property of nitric oxide is its capacity for directly and easily combining with oxygen 7owing to the evolution of heat in the combination74 ,with oxygen it forms nitrous anhydride and nitrogen peroxide1 #b,,no ,o ,n#b,o#c1 #b,,no ,o#b #b,,no#b4 ,if nitric oxide is mixed with oxygen and immediately shaken up with caustic potash1 it is almost entirely converted into potassium nitrite2 whilst after a certain time1 when the formation of nitric peroxide has already commenced1 a mixture of potassium nitrite and nitrate is obtained4 ,if oxygen is passed into a bell jar filled with nitric oxide1 brown fumes of nitrous anhydride and nitric peroxide are formed1 even in the absence of moisture2 these in the presence of water give1 as we already know1 nitric acid and nitric oxide1 so that in the presence of an excess of water and oxygen the whole of the nitric oxide is easily and directly converted into nitric acid4 ,this reaction of the re-formation of nitric acid from nitric oxide1 air1 and water1 #b,,no ,h#b,o ,o#c #b;,,hno#c1 is frequently made use of in practice4 ,the experiment showing the conversion of nitric oxide into nitric acid is very striking and instructive4 ,as the intermixture of the oxygen with the oxide of nitrogen proceeds1 the nitric acid formed dissolves in water1 and if an excess of oxygen has not been added the whole of the gas 7nitric oxide71 being converted into ,,hno#c1 is absorbed1 and the water entirely fills the bell jar previously containing the gas4#fj ,it is evident that nitric oxide#fa in combining with oxygen has a strong tendency to give the higher types of nitrogen compounds1 which we see in nitric acid1 ,,hno#c or ,,no#b7,,oh71 in nitric anhydride1 ,n#b,o#e or 7,,no#b7b,o1 and in ammonium chloride1 ,,nh#d;,cl4 ,if ,x stand for an atom of hydrogen1 or its equivalents1 chlorine1 hydroxyl1 c41 and if ,o1 which is1 according to the law of substitution1 equivalent to ,h#b1 be indicated by ,x#b1 then the three compounds of nitrogen above named should be considered as compounds of the type or form ,,nx#e4 ,for example1 in nitric acid ,x#e ,o#b 7,,oh71 where ,o#b ,x#d1 and ,,oh ,x2 whilst nitric oxide is a compound of the form ,,nx#b4 ,hence this lower form1 like lower forms in general1 strives by combination to attain to the higher forms proper to the compounds of a given element4 ,,nx#b passes consecutively into ,,nx#c--namely1 into ,n#b,o#c and ,,nho#b1 ,,nx#d 7for instance ,,no#b7 and ,,nx#e4 #fj ,this transformation of the permanent gases nitric oxide and oxygen into liquid nitric acid in the presence of water1 and with the evolution of heat1 presents a most striking instance of liquefaction produced by the action of chemical forces4 ,they perform with ease the work which physical 7cooling7 and mechanical 7pressure7 forces effect with difficulty4 ,in this the motion1 which is so distinctively the property of the gaseous molecules1 is apparently destroyed4 ,in other cases of chemical action it is apparently created1 arising1 no doubt1 from latent energy--that is1 from the internal motion of the atoms in the molecules4 #fa ,nitric oxide is capable of entering into many characteristic combinations2 it is absorbed by the solutions of many acids1 for instance1 tartaric1 acetic1 phosphoric1 sulphuric1 and metallic chlorides 7for example1 ,sb,cl#e1 ,bi,cl#c1 c41 with which it forms definite compounds2 ,besson #ahhi71 and also by the solutions of many salts1 especially those formed by suboxide of iron 7for instance1 ferrous sulphate74 ,in this case a brown compound is formed which is exceedingly unstable1 like all the analogous compounds of nitric oxide4 ,the amount of nitric oxide combined in this manner is in atomic proportion with the amount of the substance taken2 thus ferrous sulphate1 ,fe,s,o#d1 absorbs it in the proportion of ,,no to #b;,fe,s,o#d4 ,ammonia is obtained by the action of a caustic alkali on the resultant compound1 because the oxygen of the nitric oxide and water are transferred to the ferrous oxide1 forming ferric oxide1 whilst the nitrogen combines with the hydrogen of the water4 ,according to the investigations of ,gay 7#ahhe71 the compound is formed with the evolution of a large quantity of heat1 and is easily dissociated1 like a solution of ammonia in water4 ,it is evident that oxidising substances 7for example1 potassium permanganate1 ,k,mn,o#d1 ,note #eg7 are able to convert it into nitric acid4 ,if the presence of a radicle ,,no#b1 composed like nitrogen peroxide1 must be recognised in the compounds of nitric acid1 then a radicle ,,no1 having the composition of nitric oxide1 may be admitted in the compounds of nitrous acid4 ,the compounds in which the radicle ,,no is recognised are called nitroso-compounds4 ,these substances are described in ,prof4 ,bunge's work 7,kief1 #ahfh74 ,as the decomposition of nitric oxide begins at temperatures above #ijj1 many substances burn in it2 thus1 ignited phosphorus continues to burn in nitric oxide1 but sulphur and charcoal are extinguished in it4 ,this is due to the fact that the heat evolved in the combustion of these two substances is insufficient for the decomposition of the nitric oxide1 whilst the heat developed by burning phosphorus suffices to produce this decomposition4 ,that nitric oxide really supports combustion1 owing to its being decomposed by the action of heat1 is proved by the fact that strongly ignited charcoal continues to burn in the same nitric oxide#fb in which a feebly incandescent piece of charcoal is extinguished4 #fb ,a mixture of nitric oxide and hydrogen is inflammable4 ,if a mixture of the two gases be passed over spongy platinum the nitrogen and hydrogen even combine1 forming ammonia4 ,a mixture of nitric oxide with many combustible vapours and gases is very inflammable4 ,a very characteristic flame is obtained in burning a mixture of nitric oxide and the vapour of the combustible carbon bisulphide1 ,,cs#b4 ,the latter substance is very volatile1 so that it is sufficient to pass the nitric oxide through a layer of the carbon bisulphide 7for instance1 in a ,woulfe's bottle7 in order that the gas escaping should contain a considerable amount of the vapours of this substance4 ,this mixture continues to burn when ignited1 and the flame emits a large quantity of the so-called ultra-violet rays1 which are capable of inducing chemical combinations and decompositions1 and therefore the flame may be employed in photography in the absence of sufficient daylight 7magnesium light and electric light have the same property74 ,there are many gases 7for instance1 ammonia7 which when mixed with nitric oxide explode in a eudiometer4 ,the compounds of nitrogen with oxygen which we have so far considered may all be prepared from nitric oxide1 and may themselves be converted into it4 ,thus nitric oxide stands in intimate connection with them4#fc ,the passage of nitric oxide into the higher degrees of oxidation and the converse reaction is employed in practice as a means for transferring the oxygen of the air to substances capable of being oxidised4 ,starting with nitric oxide1 it may easily be converted1 with the aid of the oxygen of the atmosphere and water1 into nitric acid1 nitrous anhydride1 and nitric peroxide1 and by their means employed to oxidise other substances4 ,in this oxidising action nitric oxide is again formed1 and it may again be converted into nitric acid1 and so on continuously1 if only oxygen and water be present4 ,hence the fact1 which at first appears to be a paradox1 that by means of a small quantity of nitric oxide in the presence of oxygen and water it is possible to oxidise an indefinitely large quantity of substances which cannot be directly oxidised either by the action of the atmospheric oxygen or by the action of nitric oxide itself4 ,the sulphurous anhydride1 ,,so#b1 which is obtained in the combustion of sulphur and in roasting many metallic sulphides in the air is an example of this kind4 ,in practice this gas is obtained by burning sulphur or iron pyrites1 the latter being thereby converted into oxide of iron and sulphurous anhydride4 ,in contact with the oxygen of the atmosphere this gas does not pass into the higher degree of oxidation1 sulphuric anhydride1 ,,so#c1 and if it does form sulphuric acid with water and the oxygen of the atmosphere1 ,,so#b ,h#b,o ,o ,h#b,,so#d1 it does so very slowly4 ,with nitric acid 7and especially with nitrous acid1 but not with nitrogen peroxide7 and water1 sulphurous anhydride1 on the contrary1 very easily forms sulphuric acid1 and especially so when slightly heated 7about #dj71 the nitric acid 7or1 better still1 nitrous acid7 being converted into nitric oxide-- #c,,so#b #b,,nho#c #b;,h#b,o #b;,h#b,,so#d #b,,no4 #fc ,the oxides of nitrogen naturally do not proceed directly from oxygen and nitrogen by contact alone1 because their formation is accompanied by the absorption of a large quantity of heat1 for 7see ,note #bi7 about #ba1ejj heat units are absorbed when #af parts of oxygen and #ad parts of nitrogen combine2 consequently the decomposition of nitric oxide into oxygen and nitrogen is accompanied by the evolution of this amount of heat2 and therefore with nitric oxide1 as with all explosive substances and mixtures1 the reaction once started is able to proceed by itself4 ,in fact1 ,berthelot remarked the decomposition of nitric oxide in the explosion of fulminate of mercury4 ,this decomposition does not take place spontaneously2 substances even burn with difficulty in nitric oxide1 probably because a certain portion of the nitric oxide in decomposing gives oxygen1 which combines with another portion of nitric oxide1 and forms nitric peroxide1 a somewhat more stable compound of nitrogen and oxygen4 ,the further combinations of nitric oxide with oxygen all proceed with the evolution of heat1 and take place spontaneously by contact with air alone4 ,it is evident from these examples that the application of thermochemical data is limited4 ,the presence of water is absolutely indispensable here1 otherwise sulphuric anhydride is formed1 which combines with the oxides of nitrogen 7nitrous anhydride71 forming a crystalline substance containing oxides of nitrogen 7chamber crystals1 which will be described in ,chapter ,,xx47 ,water destroys this compound1 forming sulphuric acid and separating the oxides of nitrogen4 ,the water must be taken in a greater quantity than that required for the formation of the hydrate ,h#b,,so#d1 because the latter absorbs oxides of nitrogen4 ,with an excess of water1 however1 solution does not take place4 ,if1 in the above reaction1 only water1 sulphurous anhydride1 and nitric or nitrous acid be taken in a definite quantity1 then a definite quantity of sulphuric acid and nitric oxide will be formed1 according to the preceding equation2 but there the reaction ends and the excess of sulphurous anhydride1 if there be any1 will remain unchanged4 ,but if we add air and water1 then the nitric oxide will unite with the oxygen to form nitrogen peroxide1 and the latter with water to form nitric and nitrous acids1 which again give sulphuric acid from a fresh quantity of sulphurous anhydride4 ,nitric oxide is again formed1 which is able to start the oxidation afresh if there be sufficient air4 ,thus it is possible with a definite quantity of nitric oxide to convert an indefinitely large quantity of sulphurous anhydride into sulphuric acid1 water and oxygen only being required4#fd ,this may be easily demonstrated by an experiment on a small scale1 if a certain quantity of nitric oxide be first introduced into a flask1 and sulphurous anhydride1 steam1 and oxygen be then continually passed in4 ,thus the above-described reaction may be expressed in the following manner3-- n,,so#b n,o 7n m7,h#b,o ,,no n,h#b,,so#d1m,h#b,o ,,no if we consider only the original substances and those finally formed4 ,in this way a definite quantity of nitric oxide may serve for the conversion of an indefinite quantity of sulphurous anhydride1 oxygen1 and water into sulphuric acid4 ,in reality1 however1 there is a limit to this1 because air1 and not pure oxygen1 is employed for the oxidation1 so that it is necessary to remove the nitrogen of the air and to introduce a fresh quantity of air4 ,a certain quantity of nitric oxide will pass away with this nitrogen1 and will in this way be lost4#fe #fd ,the instance of the action of a small quantity of ,,no in inducing a definite chemical reaction between large masses 7,,so#b ,o ,h#b,o ,h#b,,so#d7 is very instructive1 because the particulars relating to it have been studied1 and show that intermediate forms of reaction may be discovered in the so-called contact or catalytic phenomena4 ,the essence of the matter here is that ,a 7 ,,so#b7 reacts upon ,b 7 ,o and ,h#b,o7 in the presence of ,c1 because it gives ,,bc1 a substance which forms ,,ab with ,a1 and again liberates ,c4 ,consequently ,c is a medium1 a transferring substance1 without which the reaction does not proceed4 ,many similar phenomena may be found in other departments of life4 ,thus the merchant is an indispensable medium between the producer and the consumer2 experiment is a medium between the phenomena of nature and the cognisant faculties1 and language1 customs1 and laws are media which are as necessary for the exchanges of social intercourse as nitric oxide for those between sulphurous anhydride and oxygen and water4 #fe ,if the sulphurous anhydride be prepared by roasting iron pyrites1 ,fe,s#b1 then each equivalent of pyrites 7equivalent of iron1 #ef1 of sulphur #cb1 of pyrites #abj7 requires six equivalents of oxygen 7that is #if parts7 for the conversion of its sulphur into sulphuric acid 7for forming #b;,h#b,,so#d with water71 besides #a-a_/b equivalents 7#bd parts7 for converting the iron into oxide1 ,fe#b,o#c2 hence the combustion of the pyrites for the formation of sulphuric acid and ferric oxide requires the introduction of an equal weight of oxygen 7#abj parts of oxygen to #abj parts of pyrites71 or five times its weight of air1 whilst four parts by weight of nitrogen will remain inactive1 and in the removal of the exhausted air will carry off the remaining nitric oxide4 ,if not all1 at least a large portion of the nitric oxide may be collected by passing the escaping air1 still containing some oxygen1 through substances which absorb oxides of nitrogen4 ,sulphuric acid itself may be employed for this purpose if it be used in the form of the hydrate ,h#b,,so#d1 or containing only a small amount of water1 because such sulphuric acid dissolves the oxides of nitrogen4 ,they may be easily expelled from this solution by heating or by dilution with water1 as they are only slightly soluble in aqueous sulphuric acid4 ,besides which1 sulphurous anhydride acts on such sulphuric acid1 being oxidised at the expense of the nitrous anhydride1 and forming nitric oxide from it1 which again enters into the cycle of action4 ,for this reason the sulphuric acid which has absorbed the oxides of nitrogen escaping from the chambers in the tower ,k 7see fig4 #ej7 is led back into the first chamber1 where it comes into contact with sulphurous anhydride1 by which means the oxides of nitrogen are reintroduced into the reaction which proceeds in the chambers4 ,this is the use of the towers 7,gay-,lussac's and ,glover's7 which are erected at either end of the chambers4 ,the preceding series of changes serve as the basis of the manufacture of sulphuric acid or so-called chamber acid4 ,this acid is prepared on a very large scale in chemical works because it is the cheapest acid whose action can be applied in a great number of cases4 ,it is therefore used in immense quantities4 ,illustration3 ,,fig4 #ej4--,section of sulphuric acid chambers1 the first and last chambers only being represented4 ,the tower to the left is called the ,glover's tower1 and that on the right the ,gay-,lussac's tower4 ,less than #a_/ajth of the natural size4 ,the process is carried on in a series of chambers 7or in one divided by partitions as in fig4 #ej1 which shows the beginning and end of a chamber7 constructed of sheet lead4 ,these chambers are placed one against the other1 and communicate by tubes or special orifices so placed that the inlet tubes are in the upper portion of the chamber1 and the outlet in the lower and opposite end4 ,the current of steam and gases necessary for the preparation of the sulphuric acid passes through these chambers and tubes4 ,the acid as it is formed falls to the bottom of the chambers or runs down their walls1 and flows from chamber to chamber 7from the last towards the first71 to permit of which the partitions do not reach to the bottom4 ,the floor and walls of the chambers should therefore be made of a material on which the sulphuric acid will not act4 ,among the ordinary metals lead is the only one suitable4#fe bis #fe bis ,other metals1 iron1 copper1 zinc1 are corroded by it2 glass and china are not acted upon1 but they crack from the variations of temperature taking place in the chambers1 and besides they are more difficult to join properly than lead2 wood1 c41 becomes charred4 ,for the formation of the sulphuric acid it is necessary to introduce sulphurous anhydride1 steam1 air1 and nitric acid1 or some oxide of nitrogen1 into the chambers4 ,the sulphurous anhydride is produced by burning sulphur or iron pyrites4 ,this is carried on in the furnace with four hearths to the left of the drawing4 ,air is led into the chambers and furnace through orifices in the furnace doors4 ,the current of air and oxygen is regulated by opening or closing these orifices to a greater or less extent4 ,the ingoing draught in the chambers is brought about by the fact that heated gases and vapours pass into the chambers1 whose temperature is further raised by the reaction itself1 and also by the remaining nitrogen being continually withdrawn from the outlet 7above the tower ,k7 by a tall chimney situated near the chambers4 ,nitric acid is prepared from a mixture of sulphuric acid and ,chili saltpetre1 in the same furnaces in which the sulphurous anhydride is evolved 7or in special furnaces74 ,not more than #h parts of nitre are taken to #ajj parts of sulphur burnt4 ,on leaving the furnace the vapours of nitric acid and oxides of nitrogen mixed with air and sulphurous anhydride first pass along the horizontal tubes ,t into the receiver ,b ,b1 which is partially cooled by water flowing in on the right-hand side and running out on the left by o1 in order to reduce the temperature of the gases entering the chamber4 ,the gases then pass up a tower filled with coke1 and shown to the left of the drawing4 ,in this tower are placed lumps of coke 7the residue from the dry distillation of coal71 over which sulphuric acid trickles from the reservoir ,m4 ,this acid has absorbed in the end tower ,k the oxides of nitrogen escaping from the chamber4 ,this end tower is also filled with coke1 over which a stream of strong sulphuric acid trickles from the reservoir ,m4 ,the acid spreads over the coke1 and1 owing to the large surface offered by the latter1 absorbs the greater part of the oxides of nitrogen escaping from the chambers4 ,the sulphuric acid in passing down the tower becomes saturated with the oxides of nitrogen1 and flows out at h into a special receiver 7in the drawing situated by the side of the furnaces71 from which it is forced up the tubes h h by steam pressure into the reservoir ,m1 situated above the first tower4 ,the gases passing through this tower 7hot7 from the furnace on coming into contact with the sulphuric acid take up the oxides of nitrogen contained in it1 and these are thus returned to the chamber and again participate in the reaction4 ,the sulphuric acid left after their extraction flows into the chambers4 ,thus1 on leaving the first coke tower the sulphurous anhydride1 air1 and vapours of nitric acid and of the oxides of nitrogen pass through the upper tube m into the chamber4 ,here they come into contact with steam introduced by lead tubes into various parts of the chamber4 ,the reaction takes place in the presence of water1 the sulphuric acid falls to the bottom of the chamber1 and the same process takes place in the following chambers until the whole of the sulphurous anhydride is consumed4 ,a somewhat greater proportion of air than is strictly necessary is passed in1 in order that no sulphurous anhydride should be left unaltered for want of sufficient oxygen4 ,the presence of an excess of oxygen is shown by the colour of the gases escaping from the last chamber4 ,if they be of a pale colour it indicates an insufficiency of air 7and the presence of sulphurous anhydride71 as otherwise peroxide of nitrogen would be formed4 ,a very dark colour shows an excess of air1 which is also disadvantageous1 because it increases the inevitable loss of nitric oxide by increasing the mass of escaping gases4#ff #ff ,by this means as much as #b1ejj1jjj kilograms of chamber acid1 containing about #fj per cent4 of the hydrate ,h#b,,so#d and about #dj per cent4 of water1 may be manufactured per year in one plant of #e1jjj cubic metres capacity 7without stoppages74 ,this process has been brought to such a degree of perfection that as much as #cjj parts of the hydrate ,h#b,,so#d are obtained from #ajj parts of sulphur1 whilst the theoretical amount is not greater than #cjf parts4 ,the acid parts with its excess of water on heating4 ,for this purpose it is heated in lead vessels4 ,however1 the acid containing about #ge per cent4 of the hydrate 7#fj ,baum7 already begins to act on the lead when heated1 and therefore the further removal of water is conducted by evaporating in glass or platinum vessels1 as will he described in ,chapter ,,xx4 ,the aqueous acid 7#ej ,baum7 obtained in the chambers is termed chamber acid4 ,the acid concentrated to #fj ,baum is more generally employed1 and sometimes the hydrate 7#ff ,baum7 termed vitriol acid is also used4 ,in ,england alone more than #a1jjj million kilograms of chamber acid are produced by this method4 ,the formation of sulphuric acid by the action of nitric acid was discovered by ,drebbel1 and the first lead chamber was erected by ,roebuck1 in ,scotland1 in the middle of the last century4 ,the essence of the process was only brought to light at the beginning of this century1 when many improvements were introduced into practice4 ,nitrous oxide1 ,n#b,o1#fg is similar to water in its volumetric composition4 ,two volumes of nitrous oxide are formed from two volumes of nitrogen and one volume of oxygen1 which may be shown by the ordinary method for the analysis of the oxides of nitrogen 7by passing them over red-hot copper or sodium74 ,in contradistinction to the other oxides of nitrogen1 it is not directly oxidised by oxygen1 but it may be obtained from the higher oxides of nitrogen by the action of certain deoxidising substances2 thus1 for example1 a mixture of two volumes of nitric oxide and one volume of sulphurous anhydride if left in contact with water and spongy platinum is converted into sulphuric acid and nitrous oxide1 #b,,no ,,so#b ,h#b,o ,h#b,,so#d ,n#b,o4 ,nitric acid1 also1 under the action of certain metals--for instance1 of zinc#fh--;gives nitrous oxide1 although in this case mixed with nitric oxide4 ,the usual method of preparing nitrous oxide consists in the decomposition of ammonium nitrate by the aid of heat1 because in this case only water and nitrous oxide are formed1 ,,nh#d,,no#c #b;,h#b,o ,n#b,o 7a mixture of ,,nh#d;,cl and ,,kno#c is sometimes taken74 ,the decomposition#fi proceeds very easily in an apparatus like that used for the preparation of ammonia or oxygen--that is1 in a retort or flask with a gas-conducting tube4 ,the decomposition must1 however1 be carried on carefully1 as otherwise nitrogen is formed from the decomposition of the nitrous oxide4#gj #fg ,if the hydrate ,,hno#c corresponds to ,n#b,o#d1 the hydrate ,,hno1 hyponitrous acid1 corresponds to ,n#b,o1 and in this sense ,n#b,o is hyponitrous anhydride4 ,hyponitrous acid1 corresponding with nitrous oxide 7as its anhydride71 is not known in a pure state1 but its salts 7,divers7 are known4 ,they are prepared by the reduction of nitrous 7and consequently of nitric7 salts by sodium amalgam4 ,if this amalgam he added to a cold solution of an alkaline nitrite until the evolution of gas ceases1 and the excess of alkali saturated with acetic acid1 an insoluble yellow precipitate of silver hyponitrite1 ,n,ag,o1 will he obtained on adding a solution of silver nitrate4 ,this hyponitrite is insoluble in cold acetic acid1 and decomposes when heated1 with the evolution of nitrous oxide4 ,if rapidly heated it decomposes with an explosion4 ,it is dissolved unchanged by weak mineral acids1 whilst the stronger acids 7for example1 sulphuric and hydrochloric acids7 decompose it1 with the evolution of nitrogen1 nitric and nitrous acids remaining in solution4 ,among the other salts of hyponitrous acid1 ,,hno1 the salts of lead1 copper1 and mercury are insoluble in water4 ,judging by the bond between hyponitrous acid and the other compounds of nitrogen1 there is reason for thinking that its formula should he doubled1 ,n#b;,h#b,o#b4 ,for instance1 ,thoune 7#ahic7 on gradually oxidising hydroxylamine1 ,,nh#b7,,oh71 into nitrous acid1 ,,no7,,oh7 7,note #be71 by means of an alkaline solution of ,k,mn,o#d1 first obtained hyponitrous acid1 ,n#b;,h#b,o#b1 and then a peculiar intermediate acid1 ,n#b;,h#b,o#c1 which1 by further oxidation1 gave nitrous acid4 ,on the other hand1 ,wislicenus 7#ahic7 showed that in the action of the sulphuric acid salt of hydroxylamine upon nitrite of sodium1 there is formed1 besides1 nitrous oxide 7according to ,v4 ,meyer1 ,,nh#c,o1,h#b,,so#d ,na,n,o#b ,na,h,s,o#d #b;,h#b,o ,n#b,o71 a small amount of hyponitrous acid which may be precipitated in the form of the silver salt2 and this reaction is most simply expressed by taking the doubled formula of hyponitrous acid1 ,,nh#b7,,oh7 ,,no7,,oh7 ,h#b,o ,n#b;,h#b,o#b4 ,the best argument in favour of the doubled formula is the property possessed by hyponitrous acid of forming acid salts1 ,h,na,n#b,o#b 7,zorn74 ,according to ,thoune1 the following are the properties of hyponitrous acid4 ,when liberated from the dry silver salt by the action of dry sulphuretted hydrogen1 hyponitrous acid is unstable1 and easily explodes even at low temperatures4 ,but when dissolved in water 7having been formed by the action of hydrochloric acid upon the silver salt71 it is stable even when boiled with dilute acids and alkalis4 ,the solution is colourless and has a strongly acid reaction4 ,in the course of time1 however1 the aqueous solution also decomposes into nitrous oxide and water4 ,the complete oxidation by permanganate of potash proceeds according to the following equation3 #e;,h#b,n#b,o#b #h,k,mn,o#d #ab;,h#b,,so#d #aj;,,hno#c #d,k#b,,so#d #h,mn,s,o#d #ab;,h#b,o4 ,in an alkaline solution1 ,k,mn,o#d only oxidises hyponitrous acid into nitrous and not into nitric acid4 ,nitrous acid has a decomposing action upon hyponitrous acid1 and if the aqueous solutions of the two acids be mixed together they immediately give off oxides of nitrogen4 ,hyponitrous acid does not liberate ,,co#b from its salts1 but on the other hand it is not displaced by ,,co#b4 #fh ,it is remarkable that electro-deposited copper powder gives nitrous oxide with a #aj p4c4 solution of nitric acid1 whilst ordinary copper gives nitric oxide4 ,it is here evident that the physical and mechanical structure of the substance affects the course of the reaction--that is to say1 it is a case of contact-action4 #fi ,this decomposition is accompanied by the evolution of about #be1jjj calories per molecular quantity1 ,,nh#d,,no#c1 and therefore takes place with ease1 and sometimes with an explosion4 #gj ,in order to remove any nitric oxide that might be present1 the gas obtained is passed through a solution of ferrous sulphate4 ,as nitrous oxide is very soluble in cold water 7at #j1 #ajj volumes of water dissolve #acj volumes of ,n#b,o2 at #bj1 #fg volumes71 it must be collected over warm water4 ,the nitrous oxide is much more soluble than nitric oxide1 which is in agreement with the fact that nitrous oxide is much more easily liquefied than nitric oxide4 ,villard obtained a crystallohydrate1 ,n#b,o1#f;,h#b,o1 which was tolerably stable at #j4 ,illustration3 ,,fig4 #ea4--,natterer's apparatus for the preparation of liquid nitrous oxide and carbonic anhydride4 ,the gas first passes though the vessel ,v1 for drying1 and then into the pump 7a section of the upper part of the apparatus is given on the left74 ,the piston t of the force pump is moved by the crank ,e and fly-wheel turned by hand4 ,the gas is pumped into the iron chamber ,a1 where it is liquefied4 ,the valve ,s allows the gas to enter ,a1 but not to escape from it4 ,the chamber and pump are cooled by the jacket ,b1 filled with ice4 ,when the gas is liquefied the vessel ,a is unscrewed from the pump1 and the liquid may be poured from it by inverting it and unscrewing the valve v1 when the liquid runs out of the tube x4 ,nitrous oxide is not a permanent gas 7absolute boiling point #cf72 it is easily liquefied by the action of cold under a high pressure2 at #ae it may be liquefied by a pressure of about #dj atmospheres4 ,this gas is usually liquefied by means of the force pump#ga shown in fig4 #ea4 ,as it is liquefied with comparative ease1 and as the cold produced by its vaporisation is very considerable1#gb it 7as also liquid carbonic anhydride7 is often employed in investigations requiring a low temperature4 ,nitrous oxide forms a very mobile1 colourless liquid1 which acts on the skin1 and is incapable in a cold state of oxidising either metallic potassium1 phosphorus1 or carbon2 its specific gravity is slightly less than that of water 7#j #jiaj1 #aj #jhef1 #ce #jfj1 #ci #jde1 ,villard1 #ahid74 ,when evaporated under the receiver of an air-pump1 the temperature falls to -#ajj1 and the liquid solidifies into a snow-like mass1 and partially forms transparent crystals4 ,both these substances are solid nitrous oxide4 ,mercury is immediately solidified in contact with evaporating liquid nitrous oxide4#gc #ga ,faraday obtained liquid nitrous oxide by the same method as liquid ammonia1 by beating dry ammonium nitrate in a closed bent tube1 one arm of which was immersed in a freezing mixture4 ,in this case two layers of liquid are obtained at the cooled end1 a lower layer of water and an upper layer of nitrous oxide4 ,this experiment should be conducted with great care1 as the pressure of the nitrous oxide in a liquid state is considerable1 namely 7according to ,regnault71 at #aj #de atmospheres1 at #j #cf atmospheres1 at -#aj #bi atmospheres1 and at -#bj #bc atmospheres4 ,it boils at -#ib1 and the pressure is then therefore #a atmosphere 7see ,chapter ,,ii41 ,note #bg74 #gb ,liquid nitrous oxide1 in vaporising at the same pressure as liquid carbonic anhydride1 gives rise to almost equal or even slightly lower temperatures4 ,thus at a pressure of #be mm4 carbonic anhydride gives a temperature as low as -#aae1 and nitrous oxide of -#abe 7,dewar74 ,the similarity of these properties and even of the absolute boiling point 7,,co#b #cb1 ,n#b,o #cf7 is all the more remarkable because these gases have the same molecular weight #dd 7,chapter ,,vii47 #gc ,a very characteristic experiment of simultaneous combustion and intense cold may be performed by means of liquid nitrous oxide2 if liquid nitrous oxide be poured into a test tube containing some mercury the mercury will solidify1 and if a piece of red-hot charcoal be thrown upon the surface of the nitrous oxide it will continue to burn very brilliantly1 giving rise to a high temperature4 ,when introduced into the respiratory organs 7and consequently into the blood also7 nitrous oxide produces a peculiar kind of intoxication accompanied by spasmodic movements1 and hence this gas1 discovered by ,priestley in #aggf1 received the name of 'laughing gas4' ,on a prolonged respiration it produces a state of insensibility 7it is an ansthetic like chloroform71 and is therefore employed in dental and surgical operations4 ,nitrous oxide is easily decomposed into nitrogen and oxygen by the action of heat1 or a series of electric sparks2 and this explains why a number of substances which cannot burn in nitric oxide do so with great ease in nitrous oxide4 ,in fact1 when nitric oxide gives some oxygen on decomposition1 this oxygen immediately unites with a fresh portion of the gas to form nitric peroxide1 whilst nitrous oxide does not possess this capacity for further combination with oxygen4#gd ,a mixture of nitrous oxide with hydrogen explodes like detonating gas1 gaseous nitrogen being formed1 ,n#b,o ,h#b ,h#b,o ,n#b4 ,the volume of the remaining nitrogen is equal to the original volume of nitrous oxide1 and is equal to the volume of hydrogen entering into combination with the oxygen2 hence in this reaction equal volumes of nitrogen and hydrogen replace each other4 ,nitrous oxide is also very easily decomposed by red-hot metals2 and sulphur1 phosphorus1 and charcoal burn in it1 although not so brilliantly as in oxygen4 ,a substance in burning in nitrous oxide evolves more heat than an equal quantity burning in oxygen2 which most clearly shows that in the formation of nitrous oxide by the combination of nitrogen with oxygen there was not an evolution but an absorption of heat1 there being no other source for the excess of heat in the combustion of substances in nitrous oxide 7see ,note #bi74 ,if a given volume of nitrous oxide be decomposed by a metal--for instance1 sodium--then there remains1 after cooling and total decomposition1 a volume of nitrogen1 exactly equal to that of the nitrous oxide taken2 consequently the oxygen is1 so to say1 distributed between the atoms of nitrogen without producing an increase in the volume of the nitrogen4 #gd ,in the following chapter we shall consider the volumetric composition of the oxides of nitrogen4 ,it explains the difference between nitric and nitrous oxide4 ,nitrous oxide is formed with a diminution of volumes 7contraction71 nitric oxide without contraction1 its volume being equal to the sum of the volumes of the nitrogen and oxygen of which it is composed4 ,by oxidation1 if it could be directly accomplished1 two volumes of nitrous oxide and one volume of oxygen would not give three but four volumes of nitric oxide4 ,these facts must be taken into consideration in comparing the calorific equivalents of formation1 the capacity for supporting combustion1 and other properties of nitrous and nitric oxides1 ,n#b,o and ,,no4 ,,chapter ,,vii ,,molecules ,,and ,,atoms4 ,,the ,,laws ,,of ,,gay-,,lussac ,,and ,,avogadro-,,gerhardt ,hydrogen combines with oxygen in the proportion of two volumes to one4 ,the composition by volume of nitrous oxide is exactly similar--it is composed of two volumes of nitrogen and one volume of oxygen4 ,by decomposing ammonia by the action of an electric spark it is easy to prove that it contains one volume of nitrogen to three volumes of hydrogen4 ,so1 similarly1 it is found1 whenever a compound is decomposed and the volumes of the gases proceeding from it are measured1 that the volumes of the gases or vapours entering into combination are in a very simple proportion to one another4 ,with water1 nitrous oxide1 c41 this may be proved by direct observation2 but in the majority of cases1 and especially with substances which1 although volatile--that is1 capable of passing into a gaseous 7or vaporous7 state--are liquid at the ordinary temperature1 such a direct method of observation presents many difficulties4 ,but1 then1 if the densities of the vapours and gases be known1 the same simplicity in their ratio is shown by calculation4 ,the volume of a substance is proportional to its weight1 and inversely proportional to its density1 and therefore by dividing the amount by weight of each substance entering into the composition of a compound by its density in the gaseous or vaporous state we shall obtain factors which will be in the same proportion as the volumes of the substances entering into the composition of the compound4#a ,so1 for example1 water contains eight parts by weight of oxygen to one part by weight of hydrogen1 and their densities are #af and #a1 consequently their volumes 7or the above-mentioned factors7 are #a and #a_/b1 and therefore it is seen without direct experiment that water contains two volumes of hydrogen for every one volume of oxygen4 ,so also1 knowing that nitric oxide contains fourteen parts of nitrogen and sixteen parts of oxygen1 and knowing that the specific gravities of these last two gases are fourteen and sixteen1 we find that the volumes in which nitrogen and oxygen combine for the formation of nitric oxide are in the proportion of #a 3 #a4 ,we will cite another example4 ,in the last chapter we saw that the density of ,,no#b only becomes constant and equal to twenty-three 7referred to hydrogen7 above #ace1 and as a matter of fact a method of direct observation of the volumetric composition of this substance would be very difficult at so high a temperature4 ,but it may be easily calculated4 ,,no#b1 as is seen from its formula and analysis1 contains thirty-two parts by weight of oxygen to fourteen parts by weight of nitrogen1 forming forty-six parts by weight of ,,no#b1 and knowing the densities of these gases we find that one volume of nitrogen with two volumes of oxygen gives two volumes of nitrogen peroxide4 ,therefore1 knowing the amounts by weight of the substances participating in a reaction or forming a given substance1 and knowing the density of the gas or vapour1#b the volumetric relations of the substances acting in a reaction or entering into the composition of a compound1 may be also determined4 #a ,if the weight be indicated by ,p1 the density by ,d1 and the volume by ,v1 then ,p_/,d ,k,v where ,k is a coefficient depending on the system of the expressions ,p1 ,d1 and ,v4 ,if ,d be the weight of a cubic measure of a substance referred to the weight of the same measure of water--if1 as in the metrical system 7,chapter ,i41 ,note #i71 the cubic measure of one part by weight of water be taken as a unit of volume--then ,k #a4 ,but1 whatever it be1 it is cancelled in dealing with the comparison of volumes1 because comparative and not absolute measures of volumes are taken4 ,in this chapter1 as throughout the book1 the weight ,p is given in grams in dealing with absolute weights2 and if comparative1 as in the expression of chemical composition1 then the weight of an atom is taken as unity4 ,the density of gases1 ,d1 is also taken in reference to the density of hydrogen1 and the volume ,v in metrical units 7cubic centimetres71 if it be a matter of absolute magnitudes of volumes1 and if it be a matter of chemical transformations--that is1 of relative volumes--then the volume of an atom of hydrogen1 or of one part by weight of hydrogen1 is taken as unity1 and all volumes are expressed according to these units4 #b ,as the volumetric relations of vapours and gases1 next to the relations of substances by weight1 form the most important province of chemistry1 and a most important means for the attainment of chemical conclusions1 and inasmuch as these volumetric relations are determined by the densities of gases and vapours1 necessarily the methods of determining the densities of vapours 7and also of gases7 are important factors in chemical research4 ,these methods are described in detail in works on physics and physical and analytical chemistry1 and therefore we here only touch on the general principles of the subject4 ,if we know the weight p and volume v1 occupied by the vapour of a given substance at a temperature t and pressure h1 then its density may be directly obtained by dividing p by the weight of a volume v of hydrogen 7if the density be expressed according to hydrogen1 see ,chapter ,,ii41 ,note #bc7 at t and h4 ,hence1 the methods of determining the density of vapours and gases are based on the determination of p1 v1 t1 and h4 ,the two last data 7the temperature t and pressure h7 are given by the thermometer and barometer and the heights of mercury or other liquid confining the gas1 and therefore do not require further explanation4 ,it need only be remarked that3 7#a7 ,in the case of easily volatile liquids there is no difficulty in procuring a bath with a constant temperature1 but that it is nevertheless best 7especially considering the inaccuracy of thermometers7 to have a medium of absolutely constant temperature1 and therefore to take either a bath in which some substance is melting--such as melting ice at #j or crystals of sodium acetate1 melting at #ef--or1 as is more generally practised1 to place the vessel containing the substance to be experimented with in the vapour of a liquid boiling at a definite temperature1 and knowing the pressure under which it is boiling1 to determine the temperature of the vapour4 ,for this purpose the boiling points of water at different pressures are given in ,chapter ,i41 ,note #aa1 and the boiling points of certain easily procurable liquids at various pressures are given in ,chapter ,,ii41 ,note #bg4 7#b7 ,with respect to temperatures above #cjj 7below which mercurial thermometers may be conveniently employed71 they are most simply obtained constant 7to give time for the weight and volume of a substance being observed in a given space1 and to allow that space to attain the calculated temperature t7 by means of substances boiling at a high temperature4 ,thus1 for instance1 at the ordinary atmospheric pressure the temperature t of the vapour of sulphur is about #dde1 of phosphorus pentasulphide #eah1 of tin chloride #fjf1 of cadmium #ggj1 of zinc #icj 7according to ,violle and others71 or #ajdj 7according to ,deville71 c4 7#c7 ,the indications of the hydrogen thermometer must be considered as the most exact 7but as hydrogen diffuses through incandescent platinum1 nitrogen is usually employed74 7#d7 ,the temperature of the vapours used as the bath should in every case be several degrees higher than the boiling point of the liquid whose density is to be determined1 in order that no portion should remain in a liquid state4 ,but even in this case1 as is seen from the example of nitric peroxide 7,chapter ,,vi471 the vapour density does not always remain constant with a change of t1 as it should were the law of the expansion of gases and vapours absolutely exact 7,chapter ,,ii41 ,note #bf74 ,if variations of a chemical and physical nature similar to that which we saw in nitric peroxide take place in the vapours1 the main interest is centred in constant densities1 which do not vary with t1 and therefore the possible effect of t on the density must always be kept in mind in having recourse to this means of investigation4 7#e7 ,usually1 for the sake of convenience of observation1 the vapour density is determined at the atmospheric pressure which is read on the barometer2 but in the case of substances which are volatilised with difficulty1 and also of substances which decompose1 or1 in general1 vary at temperatures near their boiling points1 it is best or even indispensable to conduct the determination at low pressures1 whilst for substances which decompose at low pressures the observations have to be conducted under a more or less considerably increased pressure4 7#f7 ,in many cases it is convenient to determine the vapour density of a substance in admixture with other gases1 and consequently under the partial pressure1 which may be calculated from the volume of the mixture and that of the intermixed gas 7see ,chapter ,i41 ,note #a74 ,this method is especially important for substances which are easily decomposable1 because1 as shown by the phenomena of dissociation1 a substance is able to remain unchanged in the atmosphere of one of its products of decomposition4 ,thus1 ,wurtz determined the density of phosphoric chloride1 ,p,cl#e1 in admixture with the vapour of phosphorous chloride1 ,p,cl#c4 7#g7 ,it is evident1 from the example of nitric peroxide1 that a change of pressure may alter the density and aid decomposition1 and therefore identical results are sometimes obtained 7if the density be variable7 by raising t and lowering h2 but if the density does not vary under these variable conditions 7at least1 to an extent appreciably exceeding the limits of experimental error71 then this constant density indicates the gaseous and invariable state of a substance4 ,the laws hereafter laid down refer only to such vapour densities4 ,but the majority of volatile substances show such a constant density at a certain degree above their boiling points up to the starting point of decomposition4 ,thus1 the density of aqueous vapour does not vary for t between the ordinary temperature and #ajjj 7there are no trustworthy determinations beyond this7 and for pressures varying from fractions of an atmosphere up to several atmospheres4 ,if1 however1 the density does vary considerably with a variation of h and t1 the fact may serve as a guide for the investigation of the chemical changes which are undergone by the substance in a state of vapour1 or at least as an indication of a deviation from the laws of ,boyle1 ,mariotte1 and ,gay-,lussac 7for the expansion of gases with t74 ,in certain cases the separation of one form of deviation from the other may be explained by special hypotheses4 ,with respect to the means of determining p and v1 with a view to finding the vapour density1 we may distinguish three chief methods3 7a7 by weight1 by ascertaining the weight of a definite volume of vapour2 7b7 by volume1 by measuring the volume occupied by the vapour of a definite weight of a substance2 and 7c7 by displacement4 ,the last-mentioned is essentially volumetric1 because a known weight of a substance is taken1 and the volume of the air displaced by the vapour at a given t and h is determined4 ,illustration3 ,,fig4 #eb4--;,apparatus for determining the vapour density by ,dumas' method4 ,a small quantity of the liquid whose vapour density is to be determined is placed in the glass globe1 and heated in a water or oil bath to a temperature above the boiling point of the liquid4 ,when all the liquid has been converted into vapour and has displaced all the air from the globe1 the latter is sealed up and weighed4 ,the capacity of the globe is then measured1 and in this manner the volume occupied by a known weight of vapour at a known temperature is determined4 ,illustration3 ,,fig4 #ec4--;,deville and ,troost's apparatus for determining the vapour densities1 according to ,dumas' method1 of substances which boil at high temperatures4 ,a porcelain globe containing the substance whose vapour density is to be determined is heated in the vapour of mercury 7#cej71 sulphur 7#daj71 cadmium 7#hej71 or zinc 7#a1jdj74 ,the globe is sealed up in an oxyhydrogen flame4 ,the method by weight 7a7 is the most trustworthy and historically important4 ,dumas' method is typical4 ,an ordinary spherical glass or porcelain vessel1 like those shown respectively in figs4 #eb and #ec1 is taken1 and an excess of the substance to be experimented upon is introduced into it4 ,the vessel is heated to a temperature t higher than the boiling point of the liquid3 this gives a vapour which displaces the air1 and fills the spherical space4 ,when the air and vapour cease escaping from the sphere1 it is fused up or closed by some means2 and when cool1 the weight of the vapour remaining in the sphere is determined 7either by direct weighing of the vessel with the vapour and introducing the necessary corrections for the weight of the air and of the vapour itself1 or the weight of the volatilised substance is determined by chemical methods71 and the volume of the vapour at t and the barometric pressure h are then calculated4 ,the volumetric method 7b7 originally employed by ,gay-,lussac and then modified by ,hofmann and others is based on the principle that a weighed quantity of the liquid to be experimented with 7placed in a small closed vessel1 which is sometimes fused up before weighing1 and1 if quite full of the liquid1 breaks when heated in a vacuum7 is introduced into a graduated cylinder heated to t1 or simply into a ,torricellian vacuum1 as shown in fig4 #ed1 and the number of volumes occupied by the vapour noted when the space holding it is heated to the desired temperature t4 ,illustration3 ,,fig4 #ed4--;,hofmann's apparatus for determining vapour densities4 ,the internal tube1 about one metre long1 which is calibrated and graduated1 is filled with mercury and inverted in a mercury bath4 ,a small bottle 7depicted in its natural size on the left7 containing a weighed quantity of the liquid whose vapour density is to be determined1 is introduced into the ,torricellian vacuum4 ,steam1 or the vapour of amyl alcohol1 c41 is passed through the outer tube1 and heats the internal tube to the temperature t1 at which the volume of vapour is measured4 ,illustration3 ,,fig4 #ee4--,victor ,meyer's apparatus for determining vapour densities4 ,the tube b is heated in the vapour of a liquid of constant boiling point4 ,a glass tube1 containing the liquid to be experimented upon1 is caused to fall from d4 ,the air displaced is collected in the cylinder e1 in the trough f4 ,the method of displacement 7c7 proposed by ,victor ,meyer is based on the fact that a space b is heated to a constant temperature t 7by the surrounding vapours of a liquid of constant boiling point71 and the air 7or other gas enclosed in this space7 is allowed to attain this temperature1 and when it has done so a glass bulb containing a weighed quantity of the substance to be experimented with is dropped into the space4 ,the substance is immediately converted into vapour1 and displaces the air into the graduated cylinder e4 ,the amount of this air is calculated from its volume1 and hence the volume at t1 and therefore also the volume occupied by the vapour1 is found4 ,the general arrangement of the apparatus is given in fig4 #ee4 ,such an investigation 7either direct1 or by calculation from the densities and composition7 of every chemical reaction1 resulting in the formation of definite chemical compounds1 shows that the volumes of the reacting substances in a gaseous or vaporous state are either equal or are in simple multiple proportion4#c ,this forms the first law of those discovered by ,gay-,lussac4 ,it may be formulated as follows3 ,the amounts of substances entering into chemical reaction occupy under similar physical conditions1 in a gaseous or vaporous state1 equal or simple multiple volumes4 ,this law refers not only to elements1 but also to compounds entering into mutual chemical combination2 thus1 for example1 one volume of ammonia gas combines with one volume of hydrogen chloride4 ,for in the formation of sal-ammoniac1 ,,nh#d;,cl1 there enter into reaction #ag parts by weight of ammonia1 ,,nh#c1 which is #he times denser than hydrogen1 and #cfe parts by weight of hydrogen chloride1 whose vapour density is #ahbe times that of hydrogen1 as has been proved by direct experiment4 ,by dividing the weights by the respective densities we find that the volume of ammonia1 ,,nh#c1 is equal to two1 and so also the volume of hydrogen chloride4 ,hence the volumes of the compounds which here combine together are equal to each other4 ,taking into consideration that the law of ,gay-,lussac holds good1 not only for elements1 but also for compounds1 it should be expressed as follows3 ,substances interact with one another in commensurable volumes of their vapours4#d #c ,vapours and gases1 as already explained in the second chapter1 are subject to the same laws1 which are1 however1 only approximate4 ,it is evident that for the deduction of the laws which will presently be enunciated it is only possible to take into consideration a perfect gaseous state 7far removed from the liquid state7 and chemical invariability in which the vapour density is constant--that is1 the volume of a given gas or vapour varies like a volume of hydrogen1 air1 or other gas1 with the pressure and temperature4 ,it is necessary to make this statement in order that it may be clearly seen that the laws of gaseous volumes1 which we shall describe presently1 are in the most intimate connection with the laws of the variations of volumes with pressure and temperature4 ,and as these latter laws 7,chapter ,,ii47 are not infallible1 but only approximately exact1 the same1 therefore1 applies to the laws about to be described4 ,and as it is possible to find more exact laws 7a second approximation7 for the variation of v with p and t 7for example1 van der ,waals' formula1 ,chapter ,,ii41 ,note #cc71 so also a more exact expression of the relation between the composition and the density of vapours and gases is also possible4 ,but to prevent any doubt arising at the very beginning as to the breadth and general application of the laws of volumes1 it will be sufficient to mention that the density of such gases as oxygen1 nitrogen1 and carbonic anhydride is already known to remain constant 7within the limits of experimental error7 between the ordinary temperature and a white heat2 whilst1 judging from what is said in my work on the ',tension of ,gases' 7vol4 i4 p4 #i71 it may be said that1 as regards pressure1 the relative density remains very constant1 even when the deviations from ,mariotte's law are very considerable4 ,however1 in this respect the number of data is as yet too small to arrive at an exact conclusion4 #d ,we must recollect that this law is only approximate1 like ,boyle and ,mariotte's law1 and that1 therefore1 like the latter1 a more exact expression may be found for the exceptions4 ,the law of combining volumes and the law of multiple proportion were discovered independently of each other--the one in ,france by ,gay-,lussac1 the other in ,england by ,dalton--almost simultaneously4 ,in the language of the atomic hypothesis it may be said that atomic quantities of elements occupy equal or multiple volumes4 ,the first law of ,gay-,lussac expresses the relation between the volumes of the component parts of a compound4 ,let us now consider the relation existing between the volumes of the component parts and of the compounds which proceed from them4 ,this may sometimes be determined by direct observation4 ,thus the volume occupied by water1 formed by two volumes of hydrogen and one volume of oxygen1 may be determined by the aid of the apparatus shown in fig4 #ef4 ,the long glass tube is closed at the top and open at the bottom1 which is immersed in a cylinder containing mercury4 ,the closed end is furnished with wires like a eudiometer4 ,the tube is filled with mercury1 and then a certain volume of detonating gas is introduced4 ,this gas is obtained from the decomposition of water1 and therefore in every three volumes contains two volumes of hydrogen and one volume of oxygen4 ,the tube is surrounded by a second and wider glass tube1 and the vapour of a substance boiling above #ajj--that is1 whose boiling point is higher than that of water--is passed through the annular space between them4 ,amyl alcohol1 whose boiling point is #acb1 may be taken for this purpose4 ,the amyl alcohol is boiled in the vessel to the right hand and its vapour passed between the walls of the two tubes4 ,in the case of amyl alcohol the outer glass tube should be connected with a condenser to prevent the escape into the air of the unpleasant-smelling vapour4 ,the detonating gas is thus heated up to a temperature of #acb4 ,when its volume becomes constant it is measured1 the height of the column of mercury in the tube above the level of the mercury in the cylinder being noted4 ,let this volume equal v2 it will therefore contain #a_/c v of oxygen and #b_/c v of hydrogen4 ,the current of vapour is then stopped1 and the gas exploded2 water is formed1 which condenses into a liquid4 ,the volume occupied by the vapour of the water formed has now to be determined4 ,for this purpose the vapour of the amyl alcohol is again passed between the tubes1 and thus the whole of the water formed is converted into vapour at the same temperature as that at which the detonating gas was measured2 and the cylinder of mercury being raised until the column of mercury in the tube stands at the same height above the surface of the mercury in the cylinder as it did before the explosion1 it is found that the volume of the water formed is equal to #b_/c v--that is1 it is equal to the volume of the hydrogen contained in it4 ,consequently the volumetric composition of water is expressed in the following terms3 ,two volumes of hydrogen combine with one volume of oxygen to form two volumes of aqueous vapour4 ,for substances which are gaseous at the ordinary temperature1 this direct method of observation is sometimes very easily conducted2 for instance1 with ammonia1 nitric and nitrous oxides4 ,thus to determine the composition by volume of nitrous oxide1 the above-described apparatus may be employed4 ,nitrous oxide is introduced into the tube1 and after measuring its volume electric sparks are passed through the gas2 it is then found that two volumes of nitrous oxide have given three volumes of gases--namely1 two volumes of nitrogen and one volume of oxygen4 ,consequently the composition of nitrous oxide is similar to that of water2 two volumes of nitrogen and one volume of oxygen give two volumes of nitrous oxide4 ,by decomposing ammonia it is found to be composed in such a manner that two volumes give one volume of nitrogen and three volumes of hydrogen2 also two volumes of nitric oxide are formed by the union of one volume of oxygen with one volume of nitrogen4 ,the same relations may be proved by calculation from the vapour densities1 as was described above4 ,illustration3 ,,fig4 #ef4--;,apparatus for demonstrating the volume occupied by the steam formed from the explosion of detonating gas4 ,comparisons of various results made by the aid of direct observations or calculation1 an example of which has just been cited1 led ,gay-,lussac to the conclusion that the volume of a compound in a gaseous or vaporous state is always in simple multiple proportion to the volume of each of the component parts of which it is formed 7and consequently to the sum of the volumes of the elements of which it is formed74 ,this is the second law of ,gay-,lussac2 it extends the simplicity of the volumetric relations to compounds1 and is of the same nature as that presented by the elements entering into mutual combination4 ,hence not only the substances forming a given compound1 but also the substances formed1 exhibit a simple relation of volume when measured as vapour or gas4#e #e ,this second law of volumes may be considered as a consequence of the first law4 ,the first law requires simple ratios between the volumes of the combining substances ,a and ,b4 ,a substance ,,ab is produced by their combination4 ,it may1 according to the law of multiple proportion1 combine1 not only with substances ,c1 ,d1 c41 but also with ,a and with ,b4 ,in this new combination the volume of ,,ab1 combining with the volume of ,a1 should be in simple multiple proportion with the volume of ,a2 hence the volume of the compound ,,ab is in simple proportion to the volume of its component parts4 ,therefore only one law of volumes need be accepted4 ,we shall afterwards see that there is a third law of volumes embracing also the two first laws4 ,when a compound is formed from two or more components1 there may or may not be a contraction2 the volume of the reacting substances is in this case either equal to or greater than the volume of the resultant compound4 ,the reverse is naturally observed in the case of decompositions1 when from one substance there are produced several of simpler nature4 ,therefore in the future we shall term combination a reaction in which a contraction is observed--that is1 a diminution in the volume of the component bodies in a state of vapour or gas2 and we shall term decomposition a reaction in which an expansion is produced2 while those reactions in which the volumes in a gaseous or vaporous state remain constant 7the volumes being naturally compared at the same temperature and pressure7 we shall term reactions of substitution or of double decomposition4 ,thus the transition of oxygen into ozone is a reaction of combination1 the formation of nitrous oxide from oxygen and nitrogen will also be a combination1 the formation of nitric oxide from the same will be a reaction of substitution1 the action of oxygen on nitric oxide a combination1 and so on4 ,the degree of contraction produced in the formation of chemical compounds not unfrequently leads to the possibility of distinguishing the degree of change which takes place in the chemical character of the components when combined4 ,in those cases in which a contraction occurs1 the properties of the resultant compound are very different from the properties of the substances of which it is composed4 ,thus ammonia bears no resemblance in its physical or chemical properties to the elements from which it is derived2 a contraction takes place in a state of vapour1 indicating a proximation of the elements--the distance between the atoms is diminished1 and from gaseous substances there is formed a liquid substance1 or at any rate one which is easily liquefied4 ,for this reason nitrous oxide formed by the condensation of two permanent gases is a substance which is somewhat easily converted into a liquid2 again1 nitric acid1 which is formed from elements which are permanent gases1 is a liquid1 whilst1 on the contrary1 nitric oxide1 which is formed without contraction and is decomposed without expansion1 remains a gas which is as difficult to liquefy as nitrogen and oxygen4 ,in order to obtain a still more complete idea of the dependence of the properties of a compound on the properties of the component substances1 it is further necessary to know the quantity of heat which is developed in the formation of the compound4 ,if this quantity be large--as1 for example1 in the formation of water--then the amount of energy in the resultant compound will be considerably less than the energy of the elements entering into its composition2 whilst1 on the contrary1 if the amount of heat evolved in the formation of a compound be small1 or if there even be an absorption of heat1 as in the formation of nitrous oxide1 then the energy of the elements is not destroyed1 or is only altered to a slight extent2 hence1 notwithstanding the contraction 7compression7 involved in its formation1 nitrous oxide supports combustion4 ,the preceding laws were deduced from purely experimental and empirical data and as such evoke further consequences1 as the law of multiple proportions gave rise to the atomic theory and the law of equivalents 7,chapter ,,iv47 ,in view of the atomic conception of the constitution of substances1 the question naturally arises as to what1 then1 are the relative volumes proper to those physically indivisible molecules which chemically react on each other and consist of the atoms of elements4 ,the simplest possible hypothesis in this respect would be that the volumes of the molecules of substances are equal2 or1 what is the same thing1 to suppose that equal volumes of vapours and gases contain an equal number of molecules4 ,this proposition was first enunciated by the ,italian savant ,avogadro in #ahaj4 ,it was also admitted by the ,french physico-mathematician ,ampre 7#ahae7 for the sake of simplifying all kinds of physico-mathematical conceptions respecting gases4 ,but ,avogadro and ,ampre's propositions were not generally received in science until ,gerhardt in the forties had applied them to the generalisation of chemical reactions1 and had demonstrated1 by aid of a series of phenomena1 that the reactions of substances actually take place with the greatest simplicity1 and more especially that such reactions take place between those quantities of substances which occupy equal volumes1 and until he had stated the hypothesis in an exact manner and deduced the consequences that necessarily follow from it4 ,following ,gerhardt1 ,clausius1 in the fifties1 placed this hypothesis of the equality of the number of molecules in equal volumes of gases and vapours on the basis of the kinetic theory of gases4 ,at the present day the hypothesis of ,avogadro and ,gerhardt lies at the basis of contemporary physical1 mechanical1 and chemical conceptions2 the consequences arising from it have often been subject to doubt1 but in the end have been verified by the most diverse methods2 and now1 when all efforts to refute those consequences have proved fruitless1 the hypothesis must be considered as verified1#f and the law of ,avogadro-,gerhardt must be spoken of as fundamental1 and as of great importance for the comprehension of the phenomena of nature4 ,the law may now be formulated from two points of view4 ,in the first place1 from a physical aspect3 equal volumes of gases 7or vapours7 at equal temperatures and pressures contain the same number of molecules--or of particles of matter which are neither mechanically nor physically divisible--previous to chemical change4 ,in the second place1 from a chemical aspect1 the same law may be expressed thus3 the quantities of substances entering into chemical reactions occupy1 in a state of vapour1 equal volumes4 ,for our purpose the chemical aspect is the most important1 and therefore1 before developing the law and its consequences1 we will consider the chemical phenomena from which the law is deduced or which it serves to explain4 #f ,it must not be forgotten that ,newton's law of gravity was first a hypothesis1 but it became a trustworthy1 perfect theory1 and acquired the qualities of a fundamental law owing to the concord between its deductions and actual facts4 ,all laws1 all theories1 of natural phenomena1 are at first hypotheses4 ,some are rapidly established by their consequences exactly agreeing with facts2 others only take root by slow degrees2 and there are many which are destined to be refuted owing to their consequences being found to be at variance with facts4 ,when two isolated substances interact with each other directly and easily--as1 for instance1 an alkali and an acid--then it is found that the reaction is accomplished between quantities which in a gaseous state occupy equal volumes4 ,thus ammonia1 ,,nh#c1 reacts directly with hydrochloric acid1 ,h,cl1 forming sal-ammoniac1 ,,nh#d;,cl1 and in this case the #ag parts by weight of ammonia occupy the same volume as the #cfe parts by weight of hydrochloric acid4#g ,ethylene1 ,c#b;,h#d1 combines with chlorine1 ,cl#b1 in only one proportion1 forming ethylene dichloride1 ,c#b;,h#d;,cl#b1 and this combination proceeds directly and with great facility1 the reacting quantities occupying equal volumes4 ,chlorine reacts with hydrogen in only one proportion1 forming hydrochloric acid1 ,h,cl1 and in this case equal volumes interact with each other4 ,if an equality of volumes is observed in cases of combination1 it should be even more frequently encountered in cases of decomposition1 taking place in substances which split up into two others4 ,indeed1 acetic acid breaks up into marsh gas1 ,,ch#d1 and carbonic anhydride1 ,,co#b1 and in the proportions in which they are formed from acetic acid they occupy equal volumes4 ,also from phthalic acid1 ,c#h;,h#f,o#d1 there may be obtained benzoic acid1 ,c#g;,h#f,o#b1 and carbonic anhydride1 ,,co#b1 and as all the elements of phthalic acid enter into the composition of these substances1 it follows that1 although they cannot re-form it by their direct action on each other 7the reaction is not reversible71 still they form the direct products of its decomposition1 and they occupy equal volumes4 ,but benzoic acid1 ,c#g;,h#f,o#b1 is itself composed of benzene1 ,c#f;,h#f1 and carbonic anhydride1 ,,co#b1 which also occupy equal volumes4#h ,there is an immense number of similar examples among those organic substances to whose study ,gerhardt consecrated his whole life and work1 and he did not allow such facts as these to escape his attention4 ,still more frequently in the phenomena of substitution1 when two substances react on one another1 and two are produced without a change of volume1 it is found that the two substances acting on each other occupy equal volumes as well as each of the two resultant substances4 ,thus1 in general1 reactions of substitution take place between volatile acids1 ,,hx1 and volatile alcohols1 ,r7,,oh71 with the formation of ethereal salts1 ,,rx1 and water1 ,h7,,oh71 and the volume of the vapour of the reacting quantities1 ,,hx1 ,r7,,oh71 and ,,rx1 is the same as that of water ,h7,,oh71 whose weight1 corresponding with the formula1 #ah1 occupies #b volumes1 if #a part by weight of hydrogen occupy #a volume and the density of aqueous vapour referred to hydrogen is #i4 ,such general examples1 of which there are many1#i show that the reaction of equal volumes forms a chemical phenomenon of frequent occurrence1 indicating the necessity for acknowledging the law of ,avogadro-,gerhardt4 #g ,this is not only seen from the above calculations1 but may be proved by experiment4 ,a glass tube1 divided in the middle by a stopcock1 is taken and one portion filled with dry hydrogen chloride 7the dryness of the gases is very necessary1 because ammonia and hydrogen chloride are both very soluble in water1 so that a small trace of water may contain a large amount of these gases in solution7 and the other with dry ammonia1 under the atmospheric pressure4 ,one orifice 7for instance1 of that portion which contains the ammonia7 is firmly closed1 and the other is immersed under mercury1 and the cock is then opened4 ,solid sal-ammoniac is formed1 but if the volume of one gas be greater than that of the other1 some of the first gas will remain4 ,by immersing the tube in the mercury in order that the internal pressure shall equal the atmospheric pressure1 it may easily be shown that the volume of the remaining gas is equal to the difference between the volumes of the two portions of the tube1 and that this remaining gas is part of that whose volume was the greater4 #h ,let us demonstrate this by figures4 ,from #abb grams of benzoic acid there are obtained 7a7 #gh grams of benzene1 whose density referred to hydrogen #ci1 hence the relative volume #b2 and 7b7 #dd grams of carbonic anhydride1 whose density #bb1 and hence the volume #b4 ,it is the same in other cases4 #i ,a large number of such generalised reactions1 showing reaction by equal volumes1 occur in the case of the hydrocarbon derivatives1 because many of these compounds are volatile4 ,the reactions of alkalis on acids1 or anhydrides on water1 c41 which are so frequent between mineral substances1 present but few such examples1 because many of these substances are not volatile and their vapour densities are unknown4 ,but essentially the same is seen in these cases also2 for instance1 sulphuric acid1 ,h#b,,so#d1 breaks up into the anhydride1 ,,so#c1 and water1 ,h#b,o1 which exhibit an equality of volumes4 ,let us take another example where three substances combine in equal volumes3 carbonic anhydride1 ,,co#b1 ammonia1 ,,nh#c1 and water1 ,h#b,o 7the volumes of all are equal to #b71 form acid ammonium carbonate1 7,,nh#d7;,,hco#c4 ,but the question arises1 ,what is the relation of volumes if the reaction of two substances takes place in more than one proportion1 according to the law of multiple proportions8 ,a definite answer can only be given in cases which have been very thoroughly studied4 ,thus chlorine1 in acting on marsh gas1 ,,ch#d1 forms four compounds1 ,,ch#c;,cl1 ,,ch#b;,cl#b1 ,c,h,cl#c1 and ,c,cl#d1 and it may be established by direct experiment that the substance ,,ch#c;,cl 7methylic chloride7 precedes the remainder1 and that the latter proceed from it by the further action of chlorine4 ,and this substance1 ,,ch#c;,cl1 is formed by the reaction of equal volumes of marsh gas1 ,,ch#d1 and chlorine1 ,cl#b1 according to the equation ,,ch#d ,cl#b ,,ch#c;,cl ,h,cl4 ,a great number of similar cases are met with amongst organic--that is1 carbon--compounds4 ,gerhardt was led to the discovery of his law by investigating many such reactions1 and by observing that in them the reaction of equal volumes precedes all others4 ,but if nitrogen or hydrogen give several compounds with oxygen1 the question proposed above cannot be answered with complete clearness1 because the successive formations of the different combinations cannot be so strictly defined4 ,it may be supposed1 but neither definitely affirmed nor experimentally confirmed1 that nitrogen and oxygen first give nitric oxide1 ,,no1 and only subsequently the brown vapours ,n#b,o#c and ,,no#b4 ,such a sequence in the combination of nitrogen with oxygen can only be supposed on the basis of the fact that ,,no forms ,n#b,o#c and ,,no#b directly with oxygen4 ,if it be admitted that ,,no 7and not ,n#b,o or ,,no#b7 be first formed1 then this instance would also confirm the law of ,avogadro-,gerhardt1 because nitric oxide contains equal volumes of nitrogen and oxygen4 ,so1 also1 it may be admitted that1 in the combination of hydrogen with oxygen1 hydrogen peroxide is first formed 7equal volumes of hydrogen and oxygen71 which is decomposed by the heat evolved into water and oxygen4 ,this explains the presence of traces of hydrogen peroxide 7,chapter ,,iv47 in almost all cases of the combustion or oxidation of hydrogenous substances2 for it cannot be supposed that water is first formed and then the peroxide of hydrogen1 because up to now such a reaction has not been observed1 whilst the formation of ,h#b,o from ,h#b,o#b is very easily reproduced4#aj #aj ,this opinion which ,i have always held 7since the first editions of this work71 as to the primary origin of hydrogen peroxide and of the formation of water by means of its decomposition1 has in latter days become more generally accepted1 thanks more especially to the work of ,traube4 ,probably it explains most simply the necessity for the presence of traces of water in many reactions1 as1 for instance1 in the explosion of carbonic oxide with oxygen1 and perhaps the theory of the explosion of detonating gas itself and of the combustion of hydrogen will gain in clearness and truth if we take into consideration the preliminary formation of hydrogen peroxide and its decomposition4 ,we may here point out the fact that ,ettingen 7at ,dorpat1 #ahhh7 observed the existence of currents and waves in the explosion of detonating gas by taking photographs1 which showed the periods of combustion and the waves of explosion1 which should be taken into consideration in the theory of this subject4 ,as the formation of ,h#b,o#b from ,o#b and ,h#b corresponds with a less amount of heat than the formation of water from ,h#b and ,o1 it may be that the temperature of the flame of detonating gas depends on the pre-formation of hydrogen peroxide4 ,thus a whole series of phenomena show that the chemical reaction of substances actually takes place1 as a rule1 between equal volumes1 but this does not preclude the possibility of the frequent reaction of unequal volumes1 although1 in this case1 it is often possible to discover a preceding reaction between equal volumes4#aa #aa ,the possibility of reactions between unequal volumes1 notwithstanding the general application of the law of ,avogadro-,gerhardt1 may1 in addition to what has been said above1 depend on the fact that the participating substances1 at the moment of reaction1 undergo a preliminary modification1 decomposition1 isomeric 7polymeric7 transformation1 c4 ,thus1 if ,,no#b1 seems to proceed from ,n#b,o#d1 if ,o#b is formed from ,o#c1 and the converse1 then it cannot be denied that the production of molecules containing only one atom is also possible--for instance1 of oxygen--as also of higher polymeric forms--as the molecule ,n from ,n#b1 or ,h#c from ,h#b4 ,in this manner it is obviously possible1 by means of a series of hypotheses1 to explain the cases of the formation of ammonia1 ,,nh#c1 from #c vols4 of hydrogen and #a vol4 of nitrogen4 ,but it must be observed that perhaps our information in similar instances is1 as yet1 far from being complete4 ,if hydrazine or diamide ,n#b;,h#d 7,chapter ,,vi4 ,note #bj bis7 is formed and the imide ,n#b;,h#b in which #b vols4 of hydrogen are combined with #b vols4 of nitrogen1 then the reaction here perhaps first takes place between equal volumes4 ,if it be shown that diamide gives nitrogen and ammonia 7#c,n#b;,h#d ,n#b #d,,nh#c7 under the action of sparks1 heat1 or the silent discharge1 c41 then it will be possible to admit that it is formed before ammonia4 ,and perhaps the still less stable imide ,n#b;,h#b1 which may also decompose with the formation of ammonia1 is produced before the amide ,n#b;,h#d4 ,i mention this to show that the fact of apparent exceptions existing to the law of reactions between equal volumes does not prove the impossibility of their being included under the law on further study of the subject4 ,having put forward a certain law or hypothesis1 consequences must be deduced from it1 and if by their means clearness and consistency are attained--and especially1 if by their means that which could not otherwise be known can be predicted--then the consequences verify the hypothesis4 ,this was the case with the law now under discussion4 ,the mere simplicity of the deduction of the weights proper to the atoms of the elements1 or the mere fact that having admitted the law it follows 7as will afterwards be shown7 that the vis viva of the molecules of all gases is a constant quantity1 is quite sufficient reason for retaining the hypothesis1 if not for believing in it as a fact beyond doubt4 ,and such is the whole doctrine of atoms4 ,and since by the acceptance of the law it became possible to foretell even the properties and atomic weights of elements which had not yet been discovered1 and these predictions afterwards proved to be in agreement with the actual facts1 it is evident that the law of ,avogadro-,gerhardt penetrates deeply into the nature of the chemical relation of substances4 ,this being granted1 it is possible at the present time to exhibit and deduce the truth under consideration in many ways1 and in every case1 like all that is highest in science 7for example1 the laws of the indestructibility of matter1 of the conservation of energy1 of gravity1 c471 it proves to be not an empirical conclusion from direct observation and experiment1 not a direct result of analysis1 but a creation1 or instinctive penetration1 of the inquiring mind1 guided and directed by experiment and observation--a synthesis of which the exact sciences are capable equally with the highest forms of art4 ,without such a synthetical process of reasoning1 science would only be a mass of disconnected results of arduous labour1 and would not be distinguished by that vitality with which it is really endowed when once it succeeds in attaining a synthesis1 or concordance of outward form with the inner nature of things1 without losing sight of the diversities of individual parts2 in short1 when it discovers by means of outward phenomena1 which are apparent to the sense of touch1 to observation1 and to the common mind1 the internal signification of things--discovering simplicity in complexity and uniformity in diversity4 ,and this is the highest problem of science4 ,the law of ,avogadro-,gerhardt may also be easily expressed in an algebraical form4 ,if the weight of a molecule1 or of that quantity of a substance which enters into chemical reaction and occupies in a state of vapour1 according to the law1 a volume equal to that occupied by the molecules of other bodies1 be indicated by the letters ,m#a1 ,m#b 444 or1 in general1 ,m1 and if the letters ,d#a1 ,d#b1 444 or1 in general1 ,d1 stand for the density or weight of a given volume of the gases or vapours of the corresponding substances under certain definite conditions of temperature and pressure1 then the law requires that ,m#a_/;,d#a ,m#b_/;,d#b 444 ,m_/,d ,c where ,c is a certain constant4 ,this expression shows directly that the volumes corresponding with the weights ,m#a1 ,m#b 444 ,m1 are equal to a certain constant1 because the volume is proportional to the weight and inversely proportional to the density4 ,the magnitude of ,c is naturally conditioned by and dependent on the units taken for the expression of the weights of the molecules and the densities4 ,the weight of a molecule 7equal to the sum of the atomic weights of the elements forming it7 is usually expressed by taking the weight of an atom of hydrogen as unity1 and hydrogen is now also chosen as the unit for the expression of the densities of gases and vapours2 it is therefore only necessary to find the magnitude of the constant for any one compound1 as it will be the same for all others4 ,let us take water4 ,its reacting mass is expressed 7conditionally and relatively7 by the formula or molecule ,h#b,o1 for which ,m #ah1 if ,h #a1 as we already know from the composition of water4 ,its vapour density1 or ,d1 compared to hydrogen #i1 and consequently for water ,c #b1 and therefore and in general for the molecules of all substances ,m_/,d #b4 ,consequently the weight of a molecule is equal to twice its vapour density expressed in relation to hydrogen1 and conversely the density of a gas is equal to half the molecular weight referred to hydrogen4 ,the truth of this may be seen from a very large number of observed vapour densities by comparing them with the results obtained by calculation4 ,as an illustration1 we may point out that for ammonia1 ,,nh#c1 the weight of the molecule or quantity of the reacting substance1 as well as the composition and weight corresponding with the formula1 is expressed by the figures #ad #c #ag4 ,consequently ,m #ag4 ,hence1 according to the law1 ,d #he4 ,and this result is also obtained by experiment4 ,the density1 according to both formula and experiment1 of nitrous oxide1 ,n#b,o1 is #bb1 of nitric acid #ae1 and of nitric peroxide #bc4 ,in the case of nitrous anhydride1 ,n#b,o#c1 as a substance which dissociates into ,,no ,,no#b1 the density should vary between #ch 7so long as the ,n#b,o#c remains unchanged7 and #ai 7when ,,no ,,no#b is obtained74 ,there are no figures of constant density for ,h#b,o#b1 ,,nho#c1 ,n#b,o#d1 and many similar compounds which are either wholly or partially decomposed in passing into vapour4 ,salts and similar substances either have no vapour density because they do not pass into vapour 7for instance1 potassium nitrate1 ,,kno#c7 without decomposition1 or1 if they pass into vapour without decomposing1 their vapour density is observed with difficulty only at very high temperatures4 ,the practical determination of the vapour density at these high temperatures 7for example1 for sodium chloride1 ferrous chloride1 stannous chloride1 c47 requires special methods which have been worked out by ,sainte-,claire ,deville1 ,crafts1 ,nilson and ,pettersson1 ,meyer1 ,scott1 and others4 ,having overcome the difficulties of experiment1 it is found that the law of ,avogadro-,gerhardt holds good for such salts as potassium iodide1 beryllium chloride1 aluminium chloride1 ferrous chloride1 c4--that is1 the density obtained by experiment proves to be equal to half the molecular weight--naturally within the limits of experimental error or of possible deviation from the law4 ,gerhardt deduced his law from a great number of examples of volatile carbon compounds4 ,we shall become acquainted with certain of them in the following chapters2 their entire study1 from the complexity of the subject1 and from long-established custom1 forms the subject of a special branch of chemistry termed 'organic' chemistry4 ,with all these substances the observed and calculated densities are very similar4 ,when the consequences of a law are verified by a great number of observations1 it should be considered as confirmed by experiment4 ,but this does not exclude the possibility of apparent deviations4 ,they may evidently be of two kinds3 the fraction ,m_/,d may be found to be either greater or less than #b--that is1 the calculated density may be either greater or less than the observed density4 ,when the difference between the results of experiment and calculation falls within the possible errors of experiment 7for example1 equal to hundredths of the density71 or within a possible error owing to the laws of gases having an only approximate application 7as is seen from the deviations1 for instance1 from the law of ,boyle and ,mariotte71 then the fraction ,m_/,d proves but slightly different from #b 7between #ai and #bb71 and such cases as these may be classed among those which ought to be expected from the nature of the subject4 ,it is a different matter if the quotient of ,m_/,d be several times1 and in general a multiple1 greater or less than #b4 ,the application of the law must then be explained or it must be laid aside1 because the laws of nature admit of no exceptions4 ,we will therefore take two such cases1 and first one in which the quotient ,m_/,d is greater than #b1 or the density obtained by experiment is less than is in accordance with the law4 ,it must be admitted1 as a consequence of the law of ,avogadro-,gerhardt1 that there is a decomposition in those cases where the volume of the vapour corresponding with the weight of the amount of a substance entering into reaction is greater than the volume of two parts by weight of hydrogen4 ,suppose the density of the vapour of water to be determined at a temperature above that at which it is decomposed1 then1 if not all1 at any rate a large proportion of the water will be decomposed into hydrogen and oxygen4 ,the density of such a mixture of gases1 or of detonating gas1 will be less than that of aqueous vapour2 it will be equal to #f 7compared with hydrogen71 because #a volume of oxygen weighs #af1 and #b volumes of hydrogen #b2 and1 consequently1 #c volumes of detonating gas weigh #ah and #a volume #f1 while the density of aqueous vapour #i4 ,hence1 if the density of aqueous vapour be determined after its decomposition1 the quotient ,m_/,d would be found to be #c and not #b4 ,this phenomenon might be considered as a deviation from ,gerhardt's law1 but this would not be correct1 because it may be shown by means of diffusion through porous substances1 as described in ,chapter ,,ii41 that water is decomposed at such high temperatures4 ,in the case of water itself there can naturally be no doubt1 because its vapour density agrees with the law at all temperatures at which it has been determined4#ab ,but there are many substances which decompose with great ease directly they are volatilised1 and therefore only exist as solids or liquids1 and not in a state of vapour4 ,there are1 for example1 many salts of this kind1 besides all definite solutions having a constant boiling point1 all the compounds of ammonia for example1 all ammonium salts--c4 ,their vapour densities1 determined by ,bineau1 ,deville1 and others1 show that they do not agree with ,gerhardt's law4 ,thus the vapour density of sal-ammoniac1 ,,nh#d;,cl1 is nearly #ad 7compared with hydrogen71 whilst its molecular weight is not less than #ece1 whence the vapour density should be nearly #bg1 according to the law4 ,the molecule of sal-ammoniac cannot be less than ,,nh#d;,cl1 because it is formed from the molecules ,,nh#c and ,h,cl1 and contains single atoms of nitrogen and chlorine1 and therefore cannot be divided2 it further never enters into reactions with the molecules of other substances 7for instance1 potassium hydroxide1 or nitric acid7 in quantities of less than #ece parts by weight1 c4 ,the calculated density 7about #bg7 is here double the observed density 7about #acd72 hence ,m_/,d #d and not #b4 ,for this reason the vapour density of sal-ammoniac for a long time served as an argument for doubting the truth of the law4 ,but it proved otherwise1 after the matter had been fully investigated4 ,the low density depends on the decomposition of sal-ammoniac1 on volatilising1 into ammonia and hydrogen chloride4 ,the observed density is not that of sal-ammoniac1 but of a mixture of ,,nh#c and ,h,cl1 which should be nearly #ad1 because the density of ,,nh#c #he and of ,h,cl #ahb1 and therefore the density of their mixture 7in equal volumes7 should be about #acd4ac ,the actual decomposition of the vapours of sal-ammoniac was demonstrated by ,pebal and ,than by the same method as the decomposition of water1 by passing the vapour of sal-ammoniac through a porous substance4 ,the experiment demonstrating the decomposition during volatilisation of sal-ammoniac may be made very easily1 and is a very instructive point in the history of the law of ,avogadro-,gerhardt1 because without its aid it would never have been imagined that sal-ammoniac decomposed in volatilising1 as this decomposition bears all the signs of simple sublimation2 consequently the knowledge of the decomposition itself was forestalled by the law4 ,the whole aim and practical use of the discovery of the laws of nature consists in1 and is shown by1 the fact that they enable the unknown to be foretold1 the unobserved to be foreseen4 ,the arrangement of the experiment is based on the following reasoning4#ad ,according to the law and to experiment1 the density of ammonia1 ,,nh#c1 is #h-a_/b1 and of hydrochloric acid1 ,h,cl1 #ah-a_/d1 if the density of hydrogen #a4 ,consequently1 in a mixture of ,,nh#c and ,h,cl1 the ammonia will penetrate much more rapidly through a porous mass1 or a fine orifice1 than the heavier hydrochloric acid1 just as in a former experiment the hydrogen penetrated more rapidly than the oxygen4 ,therefore1 if the vapour of sal-ammoniac comes into contact with a porous mass1 the ammonia will pass through it in greater quantities than the hydrochloric acid1 and this excess of ammonia may be detected by means of moist red litmus paper1 which should be turned blue4 ,if the vapour of sal-ammoniac were not decomposed1 it would pass through the porous mass as a whole1 and the colour of the litmus paper would not be altered1 because sal-ammoniac is a neutral salt4 ,thus1 by testing with litmus the substances passing through the porous mass1 it may be decided whether the sal-ammoniac is decomposed or not when passing into vapour4 ,sal-ammoniac volatilises at so moderate a temperature that the experiment may be conducted in a glass tube heated by means of a lamp1 an asbestos plug being placed near the centre of the tube4#ae ,the asbestos forms a porous mass1 which is unaltered at a high temperature4 ,a piece of dry sal-ammoniac is placed at one side of the asbestos plug1 and is heated by a ,bunsen burner4 ,the vapours formed are driven by a current of air forced from a gasometer or bag through two tubes containing pieces of moist litmus paper1 one blue and one red paper in each4 ,if the sal-ammoniac be heated1 then the ammonia appears on the opposite side of the asbestos plug1 and the litmus there turns blue4 ,and as an excess of hydrochloric acid remains on the side where the sal-ammoniac is heated1 it turns the litmus at that end red4 ,this proves that the sal-ammoniac1 when converted into vapour1 splits up into ammonia and hydrochloric acid1 and at the same time gives an instance of the possibility of correctly conjecturing a fact on the basis of the law of ,avogadro-,gerhardt4#ae bis #ab ,as the density of aqueous vapour remains constant within the limits of experimental accuracy1 even at #a1jjj1 when dissociation has certainly commenced1 it would appear that only a very small amount of water is decomposed at these temperatures4 ,if even #aj p4c4 of water were decomposed1 the density would be #heg and the quotient ,m_/,d #ba1 but at the high temperatures here concerned the error of experiment is not greater than the difference between this quantity and #b4 ,and probably at #a1jjj the dissociation is far from being equal to #aj p4c4 ,hence the variation in the vapour density of water does not give us the means of ascertaining the amount of its dissociation4 #ac ,this explanation of the vapour density of sal-ammoniac1 sulphuric acid1 and similar substances which decompose in being distilled was the most natural to resort to as soon as the application of the law of ,avogadro-,gerhardt to chemical relations was begun2 it was1 for instance1 given in my work on ,specific ,volumes1 #ahef1 p4 #ii4 ,the formula1 ,m_/,d #b1 which was applied later by many other investigators1 had already been made use of in that work4 #ad ,the beginner must remember that an experiment and the mode in which it is carried out must be determined by the principle or fact which it is intended to illustrate1 and not vice versa1 as some suppose4 ,the idea which determines the necessity of an experiment is the chief consideration4 #ae ,it is important that the tubes1 asbestos1 and sal-ammoniac should be dry1 as otherwise the moisture retains the ammonia and hydrogen chloride4 #ae bis ,baker 7#ahid7 showed that the decomposition of ,,nh#d;,cl in the act of volatilising only takes place in the presence of water1 traces of which are amply sufficient1 but that in the total absence of moisture 7attained by carefully drying with ,p#b,o#e7 there is no decomposition1 and the vapour density of the sal-ammoniac is found to be normal1 i4e41 nearly #bg4 ,it is not yet quite clear what part the trace of moisture plays here1 and it must be presumed that the phenomenon belongs to the category of electrical and contact phenomena1 which have not yet been fully explained 7see ,chapter ,,ix41 ,note #bi74 ,so also the fact of a decomposition may be proved in the other instances where ,m_/,d proved greater than #b1 and hence the apparent deviations appear in reality as an excellent proof of the general application and significance of the law of ,avogadro-,gerhardt4 ,in those cases where the quotient ,m_/,d proves to be less than #b1 or the observed density greater than that calculated1 by a multiple number of times1 the matter is evidently more simple1 and the fact observed only indicates that the weight of the molecule is as many times greater as that taken as the quotient obtained is less than #b4 ,so1 for instance1 in the case of ethylene1 whose composition is expressed by ,,ch#b1 the density was found by experiment to be #ad1 and in the case of amylene1 whose composition is also ,,ch#b1 the density proved to be #ce1 and consequently the quotient for ethylene #a1 and for amylene #b_/e4 ,if the molecular weight of ethylene be taken1 not as #ad1 as might be imagined from its composition1 but as twice as great--namely1 as #bh--;and for amylene as five times greater--that is as #gj--then the molecular composition of the first will be ,c#b;,h#d1 and of the second ,c#e;,h#aj1 and for both of them ,m_/,d will be equal to #b4 ,this application of the law1 which at first sight may appear perfectly arbitrary1 is nevertheless strictly correct1 because the amount of ethylene which reacts--for example1 with sulphuric and other acids--is not equal to #ad1 but to #bh parts by weight4 ,thus with ,h#b,,so#d1 ,br#b1 or ,,hi1 c41 ethylene combines in a quantity ,c#b;,h#d1 and amylene in a quantity ,c#e;,h#aj1 and not ,,ch#b4 ,on the other hand1 ethylene is a gas which liquefies with difficulty 7absolute boiling point #aj71 whilst amylene is a liquid boiling at #ce 7absolute boiling point #aib71 and by admitting the greater density of the molecules of amylene 7,m #gj7 its difference from the lighter molecules of ethylene 7,m #bh7 becomes clear4 ,thus1 the smaller quotient ,m_/,d is an indication of polymerisation1 as the larger quotient is of decomposition4 ,the difference between the densities of oxygen and ozone is a case in point4 ,on turning to the elements1 it is found in certain cases1 especially with metals--for instance1 mercury1 zinc1 and cadmium--that that weight of the atoms which must be acknowledged in their compounds 7of which mention will be afterwards made7 appears to be also the molecular weight4 ,thus1 the atomic weight of mercury must be taken as #bjj1 but the vapour density #ajj1 and the quotient #b4 ,consequently the molecule of mercury contains one atom1 ,hg4 ,it is the same with sodium1 cadmium1 and zinc4 ,this is the simplest possible molecule1 which necessarily is only possible in the case of elements1 as the molecule of a compound must contain at least two atoms4 ,however1 the molecules of many of the elements prove to be complex--for instance1 the weight of an atom of oxygen #af1 and its density #af1 so that its molecule must contain two atoms1 ,o#b1 which might already be concluded by comparing its density with that of ozone1 whose molecule contains ,o#c 7,chapter ,,iv47 ,so also the molecule of hydrogen equals ,h#b1 of chlorine ,cl#b1 of nitrogen ,n#b1 c4 ,if chlorine react with hydrogen1 the volume remains unaltered after the formation of hydrochloric acid1 ,h#b ,cl#b ,h,cl ,h,cl4 ,it is a case of substitution between the one and the other1 and therefore the volumes remain constant4 ,there are elements whose molecules are much more complex--for instance1 sulphur1 ,s#f--;although1 by heating1 the density is reduced to a third1 and ,s#b is formed4 ,judging from the vapour density of phosphorus 7,d #fb7 the molecule contains four atoms ,p#d4 ,hence many elements when polymerised appear in molecules which are more complex than the simplest possible4 ,in carbon1 as we shall afterwards find1 a very complex molecule must be admitted1 as otherwise its non-volatility and other properties cannot be understood4 ,and if compounds are decomposed by a more or less powerful heat1 and if polymeric substances are depolymerised 7that is1 the weight of the molecule diminishes7 by a rise of temperature1 as ,n#b,o#d passes into ,,no#b1 or ozone1 ,o#c1 into ordinary oxygen1 ,o#b1 then we might expect to find the splitting-up of the complex molecules of elements into the simplest molecule containing a single atom only--that is to say1 if ,o#b be obtained from ,o#c1 then the formation of ,o might also be looked for4 ,the possibility but not proof of such a proposition is indicated by the vapour of iodine4 ,its normal density #abg 7,dumas1 ,deville1 and others71 which corresponds with the molecule ,i#b4 ,at temperatures above #hjj 7up to which the density remains almost constant71 this density distinctly decreases1 as is seen from the verified results obtained by ,victor ,meyer1 ,crafts1 and ,troost4 ,at the ordinary pressure and #a1jjj it is about #ajj1 at #a1bej about #hj1 at #a1djj about #ge1 and apparently it strives to reduce itself to one-half--that is1 to #fc4 ,under a reduced pressure this splitting-up1 or depolymerisation1 of iodine vapour actually reaches a density#af of #ff1 as ,crafts demonstrated by reducing the pressure to #ajj mm4 and raising the temperature to #a1ejj4 ,from this it may be concluded that at high temperatures and low pressures the molecule ,i#b gradually passes into the molecule ,i containing one atom like mercury1 and that something similar occurs with other elements at a considerable rise of temperature1 which tends to bring about the disunion of compounds and the decomposition of complex molecules4#ag #af ,just as we saw 7,chapter ,,vi4 ,note #df7 an increase of the dissociation of ,n#b,o#d and the formation of a large proportion of ,,no#b1 with a decrease of pressure4 ,the decomposition of ,i#b into ,i ,i is a similar dissociation4 #ag ,although at first there appeared to be a similar phenomenon in the case of chlorine1 it was afterwards proved that if there is a decrease of density it is only a small one4 ,in the case of bromine it is not much greater1 and is far from being equal to that for iodine4 ,as in general we very often involuntarily confuse chemical processes with physical1 it may be that a physical process of change in the coefficient of expansion with a change of temperature participates with a change in molecular weight1 and partially1 if not wholly1 accounts for the decrease of the density of chlorine1 bromine1 and iodine4 ,thus1 ,i have remarked 7,comptes ,rendus1 #ahgf7 that the coefficient of expansion of gases increases with their molecular weight1 and 7,chapter ,,ii41 ,note #bf7 the results of direct experiment show the coefficient of expansion of hydrobromic acid 7,m #ha7 to be #jjjchf instead of #jjjcfg1 which is that of hydrogen 7,m #b74 ,hence1 in the case of the vapour of iodine 7,m #bed7 a very large coefficient of expansion is to be expected1 and from this cause alone the relative density would fall4 ,as the molecule of chlorine ,cl#b is lighter 7 #ga7 than that of bromine 7 #afj71 which is lighter than that of iodine 7 #bed71 we see that the order in which the decomposability of the vapours of these haloids is observed corresponds with the expected rise in the coefficient of expansion4 ,taking the coefficient of expansion of iodine vapour as #jjjd1 then at #a1jjj its density would be #aaf4 ,therefore the dissociation of iodine may be only an apparent phenomenon4 ,however1 on the other hand1 the heavy vapour of mercury 7,m #bjj1 ,d #ajj7 scarcely decreases in density at a temperature of #a1ejj 7,d #ih1 according to ,victor ,meyer72 but it must not be forgotten that the molecule of mercury contains only one atom1 whilst that of iodine contains two1 and this is very important4 ,questions of this kind which are difficult to decide by experimental methods must long remain without a certain explanation1 owing to the difficulty1 and sometimes impossibility1 of distinguishing between physical and chemical changes4 ,besides these cases of apparent discrepancy from the law of ,avogadro-,gerhardt there is yet a third1 which is the last1 and is very instructive4 ,in the investigation of separate substances they have to be isolated in the purest possible form1 and their chemical and physical properties1 and among them the vapour density1 then determined4 ,if it be normal--that is1 if ,d ,m_/#b--;it often serves as a proof of the purity of the substance1 i4e4 of its freedom from all foreign matter4 ,if it be abnormal--that is1 if ,d be not equal to ,m_/#b--then for those who do not believe in the law it appears as a new argument against it and nothing more2 but to those who have already grasped the important significance of the law it becomes clear that there is some error in the observation1 or that the density was determined under conditions in which the vapour does not follow the laws of ,boyle or ,gay-,lussac1 or else that the substance has not been sufficiently purified1 and contains other substances4 ,the law of ,avogadro-,gerhardt in that case furnishes convincing evidence of the necessity of a fresh and more exact research4 ,and as yet the causes of error have always been found4 ,there are not a few examples in point in the recent history of chemistry4 ,we will cite one instance4 ,in the case of pyrosulphuryl chloride1 ,s#b,o#e;,cl#b1 ,m #bae1 and consequently ,d should #ajge1 instead of which ,ogier and others obtained #ech--that is1 a density half as great2 and further1 ,ogier 7#ahhb7 demonstrated clearly that the substance is not dissociated by distillation into ,,so#c and ,,so#b;,cl#b1 or any other two products1 and thus the abnormal density of ,s#b,o#e;,cl#b remained unexplained until ,d4 ,p4 ,konovaloff 7#ahhe7 showed that the previous investigators were working with a mixture 7containing ,,so#c;,h,cl71 and that pyrosulphuryl chloride has a normal density of approximately #ajg4 ,had not the law of ,avogadro-,gerhardt served as a guide1 the impure liquid would have still passed as pure2 the more so since the determination of the amount of chlorine could not aid in the discovery of the impurity4 ,thus1 by following a true law of nature we are led to true deductions4 ,all cases which have been studied confirm the law of ,avogadro-,gerhardt1 and as by it a deduction is obtained1 from the determination of the vapour density 7a purely physical property71 as to the weight of the molecule or quantity of a substance entering into chemical reaction1 this law links together the two provinces of learning--physics and chemistry--in the most intimate manner4 ,besides which1 the law of ,avogadro-,gerhardt places the conceptions of molecules and atoms on a firm foundation1 which was previously wanting4 ,although since the days of ,dalton it had become evident that it was necessary to admit the existence of the elementary atom 7the chemical individual indivisible by chemical or other forces71 and of the groups of atoms 7or molecules7 of compounds1 indivisible by mechanical and physical forces2 still the relative magnitude of the molecule and atom was not defined with sufficient clearness4 ,thus1 for instance1 the atomic weight of oxygen might be taken as #h or #af1 or any multiple of these numbers1 and nothing indicated a reason for the acceptation of one rather than another of these magnitudes2#ah whilst as regards the weights of the molecules of elements and compounds there was no trustworthy knowledge whatever4 ,with the establishment of ,gerhardt's law the idea of the molecule was fully defined1 as well as the relative magnitude of the elementary atom4 #ah ,and so it was in the fifties4 ,some took ,o #h1 others ,o #af4 ,water in the first case would be ,,ho and hydrogen peroxide ,,ho#b1 and in the second case1 as is now generally accepted1 water ,h#b,o and hydrogen peroxide ,h#b,o#b or ,,ho4 ,disagreement and confusion reigned4 ,in #ahfj the chemists of the whole world met at ,carlsruhe for the purpose of arriving at some agreement and uniformity of opinion4 ,i was present at this ,congress1 and well remember how great was the difference of opinion1 and how a compromise was advocated with great acumen by many scientific men1 and with what warmth the followers of ,gerhardt1 at whose head stood the ,italian professor1 ,canizzaro1 followed up the consequences of the law of ,avogadro4 ,in the spirit of scientific freedom1 without which science would make no progress1 and would remain petrified as in the middle ages1 and with the simultaneous necessity of scientific conservatism1 without which the roots of past study could give no fruit1 a compromise was not arrived at1 nor ought it to have been1 but instead of it truth1 in the form of the law of ,avogadro-,gerhardt1 received by means of the ,congress a wider development1 and soon afterwards conquered all minds4 ,then the new so-called ,gerhardt atomic weights established themselves1 and in the seventies they were already in general use4 ,the chemical particle or molecule must be considered as the quantity of a substance which enters into chemical reaction with other molecules1 and occupies in a state of vapour the same volume as two parts by weight of hydrogen4 ,the molecular weight 7which has been indicated by ,m7 of a substance is determined by its composition1 transformations1 and vapour density4 ,the molecule is not divisible by the mechanical and physical changes of substances1 but in chemical reaction it is either altered in its properties1 or quantity1 or structure1 or in the nature of the motion of its parts4 ,an agglomeration of molecules1 which are alike in all chemical respects1 makes up the masses of homogeneous substances in all states4#ai #ai ,a bubble of gas1 a drop of liquid1 or the smallest crystal1 presents an agglomeration of a number of molecules1 in a state of continual motion 7like the stars of the ,milky ,way71 distributing themselves evenly or forming new systems4 ,if the aggregation of all kinds of heterogeneous molecules be possible in a gaseous state1 where the molecules are considerably removed from each other1 then in a liquid state1 where they are already close together1 such an aggregation becomes possible only in the sense of the mutual reaction between them which results from their chemical attraction1 and especially in the aptitude of heterogeneous molecules for combining together4 ,solutions and other so-called indefinite chemical compounds should be regarded in this light4 ,according to the principles developed in this work we should regard them as containing both the compounds of the heterogeneous molecules themselves and the products of their decomposition1 as in peroxide of nitrogen1 ,n#b,o#d and ,,no#b4 ,and we must consider that those molecules ,a1 which at a given moment are combined with ,b in ,,ab1 will in the following moment become free in order to again enter into a combined form4 ,the laws of chemical equilibrium proper to dissociated systems cannot be regarded in any other light4 ,molecules consist of atoms in a certain state of distribution and motion1 just as the solar system#bj is made up of inseparable parts 7the sun1 planets1 satellites1 comets1 c47 ,the greater the number of atoms in a molecule1 the more complex is the resultant substance4 ,the equilibrium between the dissimilar atoms may be more or less stable1 and may for this reason give more or less stable substances4 ,physical and mechanical transformations alter the velocity of the motion and the distances between the individual molecules1 or of the atoms in the molecules1 or of their sum total1 but they do not alter the original equilibrium of the system2 whilst chemical changes1 on the other hand1 alter the molecules themselves1 that is1 the velocity of motion1 the relative distribution1 and the quality and quantity of the atoms in the molecules4 #bj ,this strengthens the fundamental idea of the unity and harmony of type of all creation and is one of those ideas which impress themselves on man in all ages1 and give rise to a hope of arriving in time1 by means of a laborious series of discoveries1 observations1 experiments1 laws1 hypotheses1 and theories1 at a comprehension of the internal and invisible structure of concrete substances with that same degree of clearness and exactitude which has been attained in the visible structure of the heavenly bodies4 ,it is not many years ago since the law of ,avogadro-,gerhardt took root in science4 ,it is within the memory of many living scientific men1 and of mine amongst others4 ,it is not surprising1 therefore1 that as yet little progress has been made in the province of molecular mechanics2 but the theory of gases alone1 which is intimately connected with the conception of molecules1 shows by its success that the time is approaching when our knowledge of the internal structure of matter will be defined and established4 ,atoms are the smallest quantities or chemically indivisible masses of the elements forming the molecules of elements and compounds4 ,atoms have weight1 the sum of their weights forms the weight of the molecule1 and the sum of the weights of the molecules forms the weight of masses1 and is the cause of gravity1 and of all the phenomena which depend on the mass of a substance4 ,the elements are characterised1 not only by their independent existence1 their incapacity of being converted into each other1 c41 but also by the weight of their atoms4 ,chemical and physical properties depend on the weight1 composition1 and properties of the molecules forming a substance1 and on the weight and properties of the atoms forming the molecules4 ,this is the substance of those principles of molecular mechanics which lie at the basis of all contemporary physical and chemical constructions since the establishment of the law of ,avogadro-,gerhardt4 ,the fecundity of the principles enunciated is seen at every step in all the particular cases forming the present store of chemical data4 ,we will here cite a few examples of the application of the law4 ,as the weight of an atom must be understood as the minimum quantity of an element entering into the composition of all the molecules formed by it1 therefore1 in order to find the weight of an atom of oxygen1 let us take the molecules of those of its compounds which have already been described1 together with the molecules of certain of those carbon compounds which will be described in the following chapter3 ,molecular ,amount of ,molecular ,amount of ,weight ,oxygen ,weight ,oxygen ,h#b,o #ah #af ,,hno#c #fc #dh ,n#b,o #dd #af ,,co #bh #af ,,no #cj #af ,,co#b #dd #cb ,,no#b #df #cb ,the number of substances taken might be considerably increased1 but the result would be the same--that is1 the molecules of the compounds of oxygen would never be found to contain less than #af parts by weight of this element1 but always n#af1 where n is a whole number4 ,the molecular weights of the above compounds are found either directly from the density of their vapour or gas1 or from their reactions4 ,thus1 the vapour density of nitric acid 7as a substance which easily decomposes above its boiling point7 cannot be accurately determined1 but the fact of its containing one part by weight of hydrogen1 and all its properties and reactions1 indicate the above molecular composition and no other4 ,in this manner it is very easy to find the atomic weight of all the elements1 knowing the molecular weight and composition of their compounds4 ,it may1 for instance1 be easily proved that less than n#ab parts of carbon never enters into the molecules of carbon compounds1 and therefore ,c must be taken as #ab1 and not as #f which was the number in use before ,gerhardt4 ,in a similar manner the atomic weights now accepted for the elements oxygen1 nitrogen1 carbon1 chlorine1 sulphur1 c41 were found and indubitably established1 and they are even now termed the ,gerhardt atomic weights4 ,as regards the metals1 many of which do not give a single volatile compound1 we shall afterwards see that there are also methods by which their atomic weights may be established1 but nevertheless the law of ,avogadro-,gerhardt is here also ultimately resorted to1 in order to remove any doubt which may be encountered4 ,thus1 for instance1 although much that was known concerning the compounds of beryllium necessitated its atomic weight being taken as ,be #i--that is1 the oxide as ,be,o and the chloride ,be,cl#b--still certain analogies gave reason for considering its atomic weight to be ,be #ace1 in which case its oxide would be expressed by the composition ,be#b,o#c1 and the chloride by ,be,cl#c4ba ,it was then found that the vapour density of beryllium chloride was approximately #dj1 when it became quite clear that its molecular weight was #hj1 and as this satisfies the formula ,be,cl#b1 but does not suit the formula ,be,cl#c1 it therefore became necessary to regard the atomic weight of ,be as #i and not as #ac-a_/b4 #ba ,if ,be #i1 and beryllium chloride be ,be,cl#b1 then for every #i parts of beryllium there are #ga parts of chlorine1 and the molecular weight of ,be,cl#b #hj2 hence the vapour density should be #dj or n#dj4 ,if ,be #ace1 and beryllium chloride be ,be,cl#c1 then to #ace of beryllium there are #ajfe of chlorine2 hence the molecular weight would be #abj1 and the vapour density #fj or n#fj4 ,the composition is evidently the same in both cases1 because #i 3 #ga 33 #ace 3 #ajfe4 ,thus1 if the symbol of an element designate different atomic weights1 apparently very different formul may equally well express both the percentage composition of compounds1 and those properties which are required by the laws of multiple proportions and equivalents4 ,the chemists of former days accurately expressed the composition of substances1 and accurately applied ,dalton's laws1 by taking ,h #a1 ,o #h1 ,c #f1 ,si #ad1 c4 ,the ,gerhardt equivalents are also satisfied by them1 because ,o #af1 ,c #ab1 ,si #bh1 c41 are multiples of them4 ,the choice of one or the other multiple quantity for the atomic weight is impossible without a firm and concrete conception of the molecule and atom1 and this is only obtained as a consequence of the law of ,avogadro-,gerhardt1 and hence the modern atomic weights are the results of this law 7see ,note #bh74 ,with the establishment of a true conception of molecules and atoms1 chemical formul became direct expressions1 not only of composition1#bb but also of molecular weight or vapour density1 and consequently of a series of fundamental chemical and physical data1 inasmuch as a number of the properties of substances are dependent on their vapour density1 or molecular weight and composition4 ,the vapour density ,d ,m_/#b4 ,for instance1 the formula of ethyl ether is ,c#d;,h#aj,o1 corresponding with the molecular weight #gd1 and the vapour density #cg1 which is the fact4 ,therefore1 the density of vapours and gases has ceased to be an empirical magnitude obtained by experiment only1 and has acquired a rational meaning4 ,it is only necessary to remember that #b grams of hydrogen1 or the molecular weight of this primary gas in grams1 occupies1 at #j and #gfj mm4 pressure1 a volume of #bbc litres 7or #bb1cjj cubic centimetres71 in order to directly determine the weights of cubical measures of gases and vapours from their formul1 because the molecular weights in grams of all other vapours at #j and #gfj mm4 occupy the same volume1 #bbc litres4 ,thus1 for example1 in the case of carbonic anhydride1 ,,co#b1 the molecular weight ,m #dd1 hence #dd grams of carbonic anhydride at #j and #gfj mm4 occupy a volume of #bbc litres--consequently1 a litre weighs #aig gram4 ,by combining the laws of gases--,gay-,lussac's1 ,mariotte's1 and ,avogadro-,gerhardt's--we obtain#bc a general formula for gases #fbjjs7#bgc t7 ,mp where s is the weight in grams of a cubic centimetre of a vapour or gas at a temperature t and pressure p 7expressed in centimetres of mercury7 if the molecular weight of the gas ,m4 ,thus1 for instance1 at #ajj and #gfj millimetres pressure 7i4e4 at the atmospheric pressure7 the weight of a cubic centimetre of the vapour of ether 7,m #gd7 is s #jjjbd4bd #bb ,the percentage amounts of the elements contained in a given compound may be calculated from its formula by a simple proportion4 ,thus1 for example1 to find the percentage amount of hydrogen in hydrochloric acid we reason as follows3--,h,cl shows that hydrochloric acid contains #cee of chlorine and #a part of hydrogen4 ,hence1 in #cfe parts of hydrochloric acid there is #a part by weight of hydrogen1 consequently #ajj parts by weight of hydrochloric acid will contain as many more units of hydrogen as #ajj is greater than #cfe2 therefore1 the proportion is as follows--x 3 #a 33 #ajj 3 #cfe or x #ajj_/cfe #bgci4 ,therefore #ajj parts of hydrochloric acid contain #bgci parts of hydrogen4 ,in general1 when it is required to transfer a formula into its percentage composition1 we must replace the symbols by their corresponding atomic weights and find their sum1 and knowing the amount by weight of a given element in it1 it is easy by proportion to find the amount of this element in #ajj or any other quantity of parts by weight4 ,if1 on the contrary1 it be required to find the formula from a given percentage composition1 we must proceed as follows3 ,divide the percentage amount of each element entering into the composition of a substance by its atomic weight1 and compare the figures thus obtained--they should be in simple multiple proportion to each other4 ,thus1 for instance1 from the percentage composition of hydrogen peroxide1 #ehh of hydrogen and #idab of oxygen1 it is easy to find its formula2 it is only necessary to divide the amount of hydrogen by unity and the amount of oxygen by #af4 ,the numbers #ehh and #ehh are thus obtained1 which are in the ratio #a 3 #a1 which means that in hydrogen peroxide there is one atom of hydrogen to one atom of oxygen4 ,the following is a proof of the practical rule given above that to find the ratio of the number of atoms from the percentage composition1 it is necessary to divide the percentage amounts by the atomic weights of the corresponding substances1 and to find the ratio which these numbers bear to each other4 ,let us suppose that two radicles 7simple or compound71 whose symbols and combining weights are ,a and ,b1 combine together1 forming a compound composed of x atoms of ,a and y atoms of ,b4 ,the formula of the substance will be ,ax,by4 ,from this formula we know that our compound contains x,a parts by weight of the first element1 and y,b of the second4 ,in #ajj parts of our compound there will be 7by proportion7 7#ajj4 x,a7_/7x,a y,b7 of the first element1 and 7#ajj4 y,b7_/7x,a y,b7 of the second4 ,let us divide these quantities1 expressing the percentage amounts by the corresponding combining weights2 we then obtain #ajjx_/7x,a y,b7 for the first element and #ajjy_/7x,a y,b7 for the second element4 ,and these numbers are in the ratio x 3 y--that is1 in the ratio of the number of atoms of the two substances4 ,it may be further observed that even the very language or nomenclature of chemistry acquires a particular clearness and conciseness by means of the conception of molecules1 because then the names of substances may directly indicate their composition4 ,thus the term 'carbon dioxide' tells more about and expresses ,,co#b better than carbonic acid gas1 or even carbonic anhydride4 ,such nomenclature is already employed by many4 ,but expressing the composition without an indication or even hint as to the properties1 would be neglecting the advantageous side of the present nomenclature4 ,sulphur dioxide1 ,,so#b1 expresses the same as barium dioxide1 ,ba,o#b1 but sulphurous anhydride indicates the acid properties of ,,so#b4 ,probably in time one harmonious chemical language will succeed in embracing both advantages4 #bc ,this formula 7which is given in my work on ',the ,tension of ,gases1' and in a somewhat modified form in the ',comptes ,rendus1' ,feb4 #ahgf7 is deduced in the following manner4 ,according to the law of ,avogadro-,gerhardt1 ,m #b;,d for all gases1 where ,m is the molecular weight and ,d the density referred to hydrogen4 ,but it is equal to the weight s#j of a cubic centimetre of a gas in grams at #j and #gf cm4 pressure1 divided by #jjjjjhih1 for this is the weight in grams of a cubic centimetre of hydrogen4 ,but the weight s of a cubic centimetre of a gas at a temperature t and under a pressure p 7in centimetres7 is equal to s#jp_/#gf7a at74 ,therefore1 s#j s4#gf7a at7_/p2 hence ,d #gf4s7#a at7_/#jjjjjhihp1 whence ,m #aebs7#a at7_/#jjjjjhihp1 which gives the above expression1 because #a_/;a #bgc1 and #aeb multiplied by #bgc and divided by #jjjjjhih is nearly #fbjj4 ,in place of s1 m_/v may be taken1 where m is the weight and v the volume of a vapour4 #bd ,the above formula may be directly applied in order to ascertain the molecular weight from the data2 weight of vapour m grms41 its volume v c4c41 pressure p cm41 and temperature t2 for s the weight of vapour m1 divided by the volume v1 and consequently ,m #f1bjjm7#bgc t7_/pv4 ,therefore1 instead of the formula 7see ,chapter ,,ii41 ,note #cd71 pv ,r7#bgc t71 where ,r varies with the mass and nature of a gas1 we may apply the formula pv #f1bjj7m_/,m77#bgc t74 ,these formul simplify the calculations in many cases4 ,for example1 required the volume v occupied by #e grms4 of aqueous vapour at a temperature t #abg and under a pressure p #gf cm4 ,according to the formula ,m #f1bjjm7#bgc t7_/pv1 we find that v #i1jfd c4c41 as in the case of water ,m #ah1 m in this instance #e grms4 7,these formul1 however1 like the laws of gases1 are only approximate47 ,as the molecules of many elements 7hydrogen1 oxygen1 nitrogen1 chlorine1 bromine1 sulphur--at least at high temperatures7 are of uniform composition1 the formul of the compounds formed by them directly indicate the composition by volume4 ,so1 for example1 the formula ,,hno#c directly shows that in the decomposition of nitric acid there is obtained #a vol4 of hydrogen1 #a vol4 of nitrogen1 and #c vols4 of oxygen4 ,and since a great number of mechanical1 physical1 and chemical properties are directly dependent on the elementary and volumetric composition1 and on the vapour density1 the accepted system of atoms and molecules gives the possibility of simplifying a number of most complex relations4 ,for instance1 it may be easily demonstrated that the vis viva of the molecules of all vapours and gases is alike4 ,for it is proved by mechanics that the vis viva of a moving mass 7#a_/b7 mv#b1 where m is the mass and v the velocity4 ,for a molecule1 m ,m1 or the molecular weight1 and the velocity of the motion of gaseous molecules a constant which we will designate by ,c1 divided by the square root of the density of the gas#be ,c_/sqrt,d1 and as ,d ,m_/#b1 the vis viva of molecules ,c#b--that is1 a constant for all molecules4 ,q4,e4,d4#bf ,the specific heat of gases 7,chapter ,,xiv471 and many other of their properties1 are determined by their density1 and consequently by their molecular weight4 ,gases and vapours in passing into a liquid state evolve the so-called latent heat1 which also proves to be in connection with the molecular weight4 ,the observed latent heats of carbon bisulphide1 ,,cs#b #ij1 of ether1 ,c#d;,h#aj,o1 #id1 of benzene1 ,c#f;,h#f1 #aji1 of alcohol1 ,c#b;,h#f,o1 #bjj1 of chloroform1 ,c,h,cl#c1 #fg1 c41 show the amount of heat expended in converting one part by weight of the above substances into vapour4 ,a great uniformity is observed if the measure of this heat he referred to the weight of the molecule4 ,for carbon bisulphide the formula ,,cs#b expresses a weight #gf1 hence the latent heat of evaporation referred to the molecular quantity ,,cs#b #gf x #ij #f1hdj1 for ether #i1fef1 for benzene #h1ejb1 for alcohol #i1bjj1 for chloroform #h1jjg1 for water #i1fbj1 c4 ,that is1 for molecular quantities1 the latent heat varies comparatively little1 from #g1jjj to #aj1jjj heat units1 whilst for equal parts by weight it is ten times greater for water than for chloroform and many other substances4#bg #be ,chapter ,i41 ,note #cd4 #bf ,the velocity of the transmission of sound through gases and vapours closely bears on this4 ,it sqrt7,kpg7_/,d7#a ,greek3 at71 where ,k is the ratio between the two specific heats 7it is approximately #ad for gases containing two atoms in a molecule71 p the pressure of the gas expressed by weight 7that is1 the pressure expressed by the height of a column of mercury multiplied by the density of mercury71 g the acceleration of gravity1 ,d the weight of a cubic measure of the gas1 ,greek3 a #jjjcfg1 and t the temperature4 ,hence1 if ,k be known1 and as ,d can he found from the composition of a gas1 we can calculate the velocity of the transmission of sound in that gas4 ,or if this velocity be known1 we can find ,k4 ,the relative velocities of sound in two gases can he easily determined 7,kundt74 ,if a horizontal glass tube 7about #a metre long and closed at both ends7 be full of a gas1 and be firmly fixed at its middle point1 then it is easy to bring the tube and gas into a state of vibration1 by rubbing it from centre to end with a damp cloth4 ,the vibration of the gas is easily rendered visible1 if the interior of the tube be dusted with lycopodium 7the yellow powder-dust or spores of the lycopodium plant is often employed in medicine71 before the gas is introduced and the tube fused up4 ,the fine lycopodium powder arranges itself in patches1 whose number depends on the velocity of sound in the gas4 ,if there be #aj patches1 then the velocity of sound in the gas is ten times slower than in glass4 ,it is evident that this is an easy method of comparing the velocity of sound in gases4 ,it has been demonstrated by experiment that the velocity of sound in oxygen is four times less than in hydrogen1 and the square roots of the densities and molecular weights of hydrogen and oxygen stand in this ratio4 #bg ,if the conception of the molecular weights of substances does not give an exact law when applied to the latent heat of evaporation1 at all events it brings to light a certain uniformity in figures1 which otherwise only represent the simple result of observation4 ,molecular quantities of liquids appear to expend almost equal amounts of heat in their evaporation4 ,it may be said that the latent heat of evaporation of molecular quantities is approximately constant1 because the vis viva of the motion of the molecules is1 as we saw above1 a constant quantity4 ,according to thermodynamics the latent heat of evaporation is equal to 77t #bgc7_/;,e77n-n7dp_/d,t #acei1 where t is the boiling point1 n the specific volume 7i4e4 the volume of a unit of weight7 of the vapour1 and n the specific volume of the liquid1 dp_/d,t the variation of the tension with a rise of temperature per #a1 and #achi the density of the mercury according to which the pressure is measured4 ,thus the latent heat of evaporation increases not only with a decrease in the vapour density 7i4e4 the molecular weight71 but also with an increase in the boiling point1 and therefore depends on different factors4 ,generalising from the above1 the weight of the molecule determines the properties of a substance independently of its composition--i4e4 of the number and quality of the atoms entering into the molecule--whenever the substance is in a gaseous state 7for instance1 the density of gases and vapours1 the velocity of sound in them1 their specific heat1 c471 or passes into that state1 as we see in the latent heat of evaporation4 ,this is intelligible from the point of view of the atomic theory in its present form1 for1 besides a rapid motion proper to the molecules of gaseous bodies1 it is further necessary to postulate that these molecules are dispersed in space 7filled throughout with the luminiferous ether7 like the heavenly bodies distributed throughout the universe4 ,here1 as there1 it is only the degree of removal 7the distance7 and the masses of substances which take effect1 while those peculiarities of a substance which are expressed in chemical transformations1 and only come into action on near approach or on contact1 are in abeyance by reason of the dispersal4 ,hence it is at once obvious1 in the first place1 that in the case of solids and liquids1 in which the molecules are closer together than in gases and vapours1 a greater complexity is to be expected1 i4e4 a dependence of all the properties not only upon the weight of the molecule but also upon its composition and quality1 or upon the properties of the individual chemical atoms forming the molecule2 and1 in the second place1 that1 in the case of a small number of molecules of any substance being disseminated through a mass of another substance--for example1 in the formation of weak 7dilute7 solutions 7although in this case there is an act of chemical reaction--i4e4 a combination1 decomposition1 or substitution7--the dispersed molecules will alter the properties of the medium in which they are dissolved1 almost in proportion to the molecular weight and almost independently of their composition4 ,the greater the number of molecules disseminated--i4e4 the stronger the solution--the more clearly defined will those properties become which depend upon the composition of the dissolved substance and its relation to the molecules of the solvent1 for the distribution of one kind of molecules in the sphere of attraction of others cannot but be influenced by their mutual chemical reaction4 ,these general considerations give a starting point for explaining why1 since the appearance of ,van't ,hoff's memoir 7#ahhf71 ',the ,laws of ,chemical ,equilibrium in a ,diffused ,gaseous or ,liquid ,state' 7see ,chapter ,i41 ,note #ai71 it has been found more and more that dilute 7weak7 solutions exhibit such variations of properties as depend wholly upon the weight and number of the molecules and not upon their composition1 and even give the means of determining the weight of molecules by studying the variations of the properties of a solvent on the introduction of a small quantity of a substance passing into solution4 ,although this subject has been already partially considered in the first chapter 7in speaking of solutions71 and properly belongs to a special 7physical7 branch of chemistry1 we touch upon it here because the meaning and importance of molecular weights are seen in it in a new and peculiar light1 and because it gives a method for determining them whenever it is possible to obtain dilute solutions4 ,among the numerous properties of dilute solutions which have been investigated 7for instance1 the osmotic pressure1 vapour tension1 boiling point1 internal friction1 capillarity1 variation with change of temperature1 specific heat1 electroconductivity1 index of refraction1 c47 we will select one--the 'depression' or fall of the temperature of freezing 7,raoult's cryoscopic method71 not only because this method has been the most studied1 but also because it is the most easily carried out and most frequently applied for determining the weight of the molecules of substances in solution1 although here1 owing to the novelty of the subject there are also many experimental discrepancies which cannot as yet be explained by theory4#bg bis #bg bis ,the osmotic pressure1 vapour tension of the solvent1 and several other means applied like the cryoscopic method to dilute solutions for determining the molecular weight of a substance in solution1 are more difficult to carry out in practice1 and only the method of determining the rise of the boiling point of dilute solutions can from its facility be placed parallel with the cryoscopic method1 to which it bears a strong resemblance1 as in both the solvent changes its state and is partially separated4 ,in the boiling point method it passes off in the form of a vapour1 while in cryoscopic determinations it separates out in the form of a solid body4 ,van't ,hoff1 starting from the second law of thermodynamics1 showed that the dependence of the rise of pressure 7dp7 upon a rise of temperature 7d,t7 is determined by the equation dp 7kmp_/#b,t#b7;d,t1 where k is the latent heat of evaporation of the solvent1 m its molecular weight1 p the tension of the saturated vapour of the solvent at ,t1 and ,t the absolute temperature 7,t #bgc t71 while ,raoult found that the quantity 7p-p7_/p 7,chapter ,i41 ,note #ej7 or the measure of the relative fall of tension 7p the tension of the solvent or water1 and p of the solution7 is found by the ratio of the number of molecules1 n of the substance dissolved1 and ,n of the solvent1 so that 7p-p7_/p ,cn_/7,n n7 where ,c is a constant4 ,with very dilute solutions p -p may be taken as equal to dp1 and the fraction n_/7,n n7 as equal to n_/,n 7because in that case the value of ,n is very much greater than n71 and then1 judging from experiment1 ,c is nearly unity--hence3 dp_/p n_/,n or dp np_/,n1 and on substituting this in the above equation we have 7kmp_/#b,t#b7;d,t np_/,n4 ,taking a weight of the solvent m_/,n #ajj1 and of the substance dissolved 7per #ajj of the solvent7 q1 where q evidently n,m1 if ,m be the molecular weight of the substance dissolved1 we find that n_/,n qm_/#ajj,m1 and hence1 according to the preceding equation1 we have ,m 7#jjb,t#b_/k77q_/d,t71 that is1 by taking a solution of q grms4 of a substance in #ajj grms4 of a solvent1 and determining by experiment the rise of the boiling point d,t1 we find the molecular weight ,m of the substance dissolved1 because the fraction #jjb,t#b_/k is 7for a given pressure and solvent7 a constant2 for water at #ajj 7,t #cgc7 when k #ecd 7,chapter ,i41 ,note #aa71 it is nearly #eb1 for ether nearly #ba1 for bisulphide of carbon nearly #bd1 for alcohol nearly #aae1 c4 ,as an example1 we will cite from the determinations made by ,professor ,sakurai1 of ,japan 7#ahic71 that when water was the solvent and the substance dissolved1 corrosive sublimate1 ,hg,cl#b1 was taken in the quantity q #high and #dbec grms41 the rise in the boiling point d,t was ,o#agi and #jjhd1 whence ,m #bfa and #bfc1 and when alcohol was the solvent1 q #ajhgc and #hgfe and d,t #jdga and #jchj1 whence ,m #bff and #bfe1 whilst the actual molecular weight of corrosive sublimate #bga1 which is very near to that given by this method4 ,in the same manner for aqueous solutions of sugar 7,m #cdb71 when q varied from #ad to #bd1 and the rise of the boiling point from #jba to #jjce1 ,m was found to vary between #cci and #cfd4 ,for solutions of iodine ,i#b in ether1 the molecular weight was found by this method to be between #bee and #bfb1 and ,i#b #bed4 ,sakurai obtained similar results 7between #bdg and #bfb7 for solutions of iodine in bisulphide of carbon4 ,we will here remark that in determining ,m 7the molecular weight of the substance dissolved7 at small but increasing concentrations 7per #ajj grms4 of water71 the results obtained by ,julio ,baroni 7#ahic7 show that the value of ,m found by the formula may either increase or decrease4 ,an increase1 for instance1 takes place in aqueous solutions of ,hg,cl#b 7from #bee to #ccd instead of #bga71 ,,kno#c 7#eg-ff instead of #aja71 ,ag,n,o#c 7#ajd-ajg instead of #agj71 ,k#b,,so#d 7#ee-hi instead of #agd71 sugar 7#cbh-cdh instead of #cdb71 c4 ,on the contrary the calculated value of ,m decreases as the concentration increases1 for solutions of ,k,cl 7#dj-ci instead of #gde71 ,na,cl 7#cc-bh instead of #ehe71 ,na,br 7#fj-di instead of #ajc71 c4 ,in this case 7as also for ,li,cl1 ,na,i1 ,c#b;,h#c,na,o#b1 c47 the value of i 7,chapter ,i41 ,note #di71 or the ratio between the actual molecular weight and that found by the rise of the boiling point1 was found to increase with the concentration1 i4e4 to be greater than #a1 and to differ more and more from unity as the strength of the solution becomes greater4 ,for example1 according to ,schlamp 7#ahid71 for ,li,cl1 with a variation of from #aa to #fg grm4 ,li,cl per #ajj of water1 i varies from #afc to #ahi4 ,but for substances of the first series 7,hg,cl#b1 c471 although in very dilute solutions i is greater than #a1 it approximates to #a as the concentration increases1 and this is the normal phenomenon for solutions which do not conduct an electric current1 as1 for instance1 of sugar4 ,and with certain electrolytes1 such as ,hg,cl#b1 ,mg,s,o#d1 c41 i exhibits a similar variation2 thus1 for ,hg,cl#b the value of ,m is found to vary between #bee and #ccd2 that is1 i 7as the molecular weight #bga7 varies between #ajf and #jha4 ,hence ,i do not believe that the difference between i and unity 7for instance1 for ,ca,cl#b1 i is about #c1 for ,,ki about #b1 and decreases with the concentration7 can at present be placed at the basis of any general chemical conclusions1 and it requires further experimental research4 ,among other methods by which the value of i is now determined for dilute solutions is the study of their electroconductivity1 admitting that i #a a7k-#a71 where a the ratio of the molecular conductivity to the limiting conductivity corresponding to an infinitely large dilution 7see ,physical ,chemistry71 and k is the number of ions into which the substance dissolved can split up4 ,without entering upon a criticism of this method of determining i1 ,i will only remark that it frequently gives values of i very close to those found by the depression of the freezing point and rise of the boiling point2 but that this accordance of results is sometimes very doubtful4 ,thus for a solution containing #efg grms4 ,ca,cl#b per #ajj grms4 of water1 i1 according to the vapour tension #beb1 according to the boiling point #bga1 according to the electroconductivity #bbh1 while for solutions in propyl alcohol 7,schlamp #ahid7 i is near to #acc4 ,in a word1 although these methods of determining the molecular weight of substances in solution show an undoubted progress in the general chemical principles of the molecular theory1 there are still many points which require explanation4 ,we will add certain general relations which apply to these problems4 ,isotonic 7,chapter ,i41 ,note #ai7 solutions exhibit not only similar osmotic pressures1 but also the same vapour tension1 boiling point and freezing temperature4 ,the osmotic pressure bears the same relation to the fall of the vapour tension as the specific gravity of a solution does to the specific gravity of the vapour of the solvent4 ,the general formul underlying the whole doctrine of the influence of the molecular weight upon the properties of solutions considered above1 are3 #a4 ,raoult in #ahhf-ahij showed that 77p-p7_/p7 7#ajj_/;a7 7,m_/m7 a constant ,c where p and p are the vapour tensions of the solvent and substance dissolved1 a the amount in grms4 of the substance dissolved per #ajj grms4 of solvent1 ,m and m the molecular weights of the substance dissolved and solvent4 #b4 ,raoult and ,recoura in #ahij showed that the constant above ,c the ratio of the actual vapour density d of the solvent to the theoretical density d calculated according to the molecular weight4 ,this deduction may now be considered proved1 because both the fall of tension and the ratio of the vapour densities d_/d give1 for water #ajc1 for alcohol #ajb1 for ether #ajd1 for bisulphide of carbon #ajj1 for benzene #ajb1 for acetic acid #afc4 #c4 ,by applying the principles of thermodynamics and calling ,l#a the latent heat of fusion and ,t#a the absolute 7 t #bgc7 temperature of fusion of the solvent1 and ,l#b and ,t#b the corresponding values for the boiling point1 ,van't ,hoff in #ahhf-ahij deduced3-- 7,depression of freezing point7_/7,rise of boiling point7 7,l#b_/,l#a7 7,t#ab_/,t#bb7 ,depression of freezing point 7,,at#ab;a7_/7,l#a,m#a7 ,rise of boiling point 7,,at#bb;a7_/7,l#b,m#a7 where ,a #jjaihh 7or nearly #jjb as we took it above71 a is the weight in grms4 of the substance dissolved per #ajj grms4 of the solvent1 ,m#a the molecular weight of the dissolved substance 7in the solution71 and ,m the molecular weight of this substance according to its composition and vapour density1 then i ,m_/,m#a4 ,the experimental data and theoretical considerations upon which these formul are based will be found in text-books of physical and theoretical chemistry4 ,if #ajj gram-molecules of water1 i4e4 #a1hjj grms1 be taken and n gram-molecules of sugar1 ,c#ab;,h#bb,o#aa1 i4e4 n #cdb grms41 be dissolved in them1 then the depression d1 or fall 7counting from #j7 of the temperature of the formation of ice will be 7according to ,pickering7 n #j #jjaj #jjbe #jajj #jbej #ajjj d #j #jjajc #jjbhj #jaaae #jbgeh #aadab which shows that for high degrees of dilution 7up to #jben7 d approximately 7estimating the possible errors of experiment at #jjje7 n#aaj1 because then d #j1 #jjaaj1 #jjbge1 #jaajj1 #jbgej1 #aajjj1 and the difference between these figures and the results of experiment for very dilute solutions is less than the possible errors of experiment 7for n #a the difference is already greater7 and therefore for dilute solutions of sugar it may be said that n molecules of sugar in dissolving in #ajj molecules of water give a depression of about #aan4 ,similar data for acetone 7,chapter ,i41 ,note #di7 give a depression of #ajjfn for n molecules of acetone per #ajj molecules of water4 ,and in general1 for indifferent substances 7the majority of organic bodies7 the depression per #ajj;,h#b,o is nearly n#aa to n#aj 7ether1 for instance1 gives the last number71 and consequently in dissolving in #ajj grms4 of water it is about #ahjn to #aijn1 taking this rule to apply to the case of a small number of n 7not over #jbn74 ,if instead of water1 other liquid or fused solvents 7for example1 benzene1 acetic acid1 acetone1 nitrobenzene or molten naphthaline1 metals1 c47 be taken and in the proportion of #ajj molecules of the solvent to n molecules of a dissolved indifferent 7neither acid nor saline7 substance1 then the depression is found to be equal to from #jfbn to #jfen and in general ,kn4 ,if the molecular weight of the solvent m1 then #ajj gram-molecules will weigh #ajjm grms41 and the depression will be approximately 7taking #jfcn7 equal to m#jfcn degrees for n molecules of the substance dissolved in #ajj grms4 of the solvent1 or in general the depression for #ajj grms4 of a given solvent kn where k is almost a constant quantity 7for water nearly #ah1 for acetone nearly #cg1 c47 for all dilute solutions4 ,thus1 having found a convenient solvent for a given substance and prepared a definite 7by weight7 solution 7i4e4 knowing how many grms4 r of the solvent there are to q grms4 of the substance dissolved7 and having determined the depression d--i4e4 the fall in temperature of freezing for the solvent--it is possible to determine the molecular weight of the substance dissolved1 because d kn where d is found by experiment and k is determined by the nature of the solvent1 and therefore n or the number of molecules of the substance dissolved can be found4 ,but if r grms4 of the solvent and q grms4 of the substance dissolved are taken1 then there are #ajjq_/r of the latter per #ajj grms4 of the former1 and this quantity n,x1 where n is found from the depression and d_/k and ,x is the molecular weight of the substance dissolved4 ,hence ,x #ajjqk_/rd1 which gives the molecular weight1 naturally only approximately1 but still with sufficient accuracy to easily indicate1 for instance1 whether in peroxide of hydrogen the molecule contains ,,ho or ,h#b,o#b or ,h#c,o#c1 c4 7,h#b,o#b is obtained74 ,moreover1 attention should be drawn to the fact that a great many substances taken as solvents give per #ajj molecules a depression of about #jfcn1 whilst water gives about #ajen1 i4e4 a larger quantity1 as though the molecules of liquid water were more complex than is expressed by the formula ,h#b,o4#bh ,a similar phenomenon which repeats itself in the osmotic pressure1 vapour tension of the solvent1 c4 7see ,chapter ,i41 ,notes #ai and #di71 i4e4 a variation of the constant 7k for #ajj grms4 of the solvent or ,k for #ajj molecules of it71 is also observed in passing from indifferent substances to saline 7to acids1 alkalis and salts7 both in aqueous and other solutions as we will show 7according to ,pickering's data #ahib7 for solutions of ,na,cl and ,cu,s,o#d in water4 ,for n #jja #jjc #jje #ja #je molecules of ,na,cl the depression is d #jjagg #jjeih #jjiib #jaieh #jiedd which corresponds to a depression per molecule ,k #agg #aif #aih #aif #aia i4e4 here in the most dilute solutions 7when n is nearly #j7 d is obtained about #agn1 while in the case of sugar it was about #aan4 ,for ,cu,s,o#d for the same values of n1 experiment gave3 d #jjafd #jjdea #jjfba #jacba #jebde ,k #afd #aej #add #acb #aje i4e4 here again d for very dilute solutions is nearly #agn1 but the value of ,k falls as the solution becomes more concentrated1 while for ,na,cl it at first increased and only fell for the more concentrated solutions4 ,the value of ,k in the solution of n molecules of a body in #ajj;,h#b,o1 when d ,kn1 for very dilute solutions of ,ca,cl#b is nearly #bf1 for ,ca7,,no#c7b nearly #be1 for ,,hno#c1 ,,ki and ,,kho nearly #ai-b,o1 for borax ,na#b;,b#d,o#g nearly #cg1 c41 while for sugar and similar substances it is1 as has been already mentioned1 nearly #aj-aa4 ,although these figures are very different#bh bis still k and ,k may be considered constant for analogous substances1 and therefore the weight of the molecule of the body in solution can be found from d4 ,and as the vapour tension of solutions and their boiling points 7see ,note #bg bis and ,chapter ,i41 ,note #ea7 vary in the same manner as the freezing point depression1 so they also may serve as means for determining the molecular weight of a substance in solution4#bi #bh ,a similar conclusion respecting the molecular weight of liquid water 7i4e4 that its molecule in a liquid state is more complex than in a gaseous state1 or polymerized into ,h#h,o#d1 ,h#f,o#c or in general into n,h#b,o7 is frequently met in chemico-physical literature1 but as yet there is no basis for its being fully admitted1 although it is possible that a polymerization or aggregation of several molecules into one takes place in the passage of water into a liquid or solid state1 and that there is a converse depolymerization in the act of evaporation4 ,recently1 particular attention has been drawn to this subject owing to the researches of ,etvs 7#ahhf7 and ,ramsay and ,shields 7#ahic7 on the variation of the surface tension ,n with the temperature 7,n the capillary constant a#b multiplied by the specific gravity and divided by #b1 for example1 for water at #j and #ajj the value of a#b #aeda and #abeh sq4 mm41 and the surface tension #gib and #fjd74 ,starting from the absolute boiling point 7,chapter ,,ii41 ,note #bi7 and adding #f1 as was necessary from all the data obtained1 and calling this temperature ,t1 it is found that ,,as k,t1 where ,s is the surface of a gram-molecule of the liquid 7if ,m is its weight in grams1 s its sp4 gr41 then its sp4 volume ,m_/s1 and the surface ,s #croot7,m_/s7#b71 ,a the surface tension 7determined by experiment at ,t71 and k a constant which is independent of the composition of the molecule4 ,the equation ,,as k,t is in complete agreement with the well-known equation for gases vp ,,rt 7p4 #adj7 which serves for deducing the molecular weight from the vapour density4 ,ramsay's researches led him to the conclusion that the liquid molecules of ,,cs#b1 ether1 benzene1 and of many other substances1 have the same value as in a state of vapour1 whilst with other liquids this is not the case1 and that to obtain an accordance1 that is1 that k shall be a constant1 it is necessary to assume the molecular weight in the liquid state to be n times as great4 ,for the fatty alcohols and acids n varies from #a-a_/b to #c-a_/b1 for water from #b-a_/d to #d1 according to the temperature 7at which the depolymerization takes place74 ,hence1 although this subject offers a great theoretical interest1 it cannot be regarded as firmly established1 the more so since the fundamental observations are difficult to make and not sufficiently numerous2 should1 however1 further experiments confirm the conclusions arrived at by ,professor ,ramsay1 this will give another method of determining molecular weights4 #bh bis ,their variance is expressed in the same manner as was done by ,van't ,hoff 7,chapter ,i41 ,notes #ai and #di7 by the quantity i1 taking it as #a when k #aje1 in that case for ,,ki1 i is nearly #b1 for borax about #d1 c4 #bi ,we will cite one more example1 showing the direct dependence of the properties of a substance on the molecular weight4 ,if one molecular part by weight of the various chlorides--for instance1 of sodium1 calcium1 barium1 c4--be dissolved in #bjj molecular parts by weight of water 7for instance1 in #c1fjj grams7 then it is found that the greater the molecular weight of the salt dissolved1 the greater is the specific gravity of the resultant solution4 ,molecular ,sp4 gr4 ,molecular ,sp4 gr4 weight at #ae weight at #ae ,h,cl #cfe #ajjda ,ca,cl#b #aaa #ajbcf ,na,cl #ehe #ajajf ,ni,cl#b #acj #ajcbh ,k,cl #gde #ajaba ,zn,cl#b #acf #ajcca ,be,cl#b #hj #ajach ,ba,cl#b #bjh #ajdhi ,mg,cl#b #ie #ajbjc ,thus not only in vapours and gases1 but also in dilute solutions of solid and liquid substances1 we see that if not all1 still many properties are wholly dependent upon the molecular weight and not upon the quality of a substance1 and that this gives the possibility of determining the weight of molecules by studying these properties 7for instance1 the vapour density1 depression of the freezing point1 c47 ,it is apparent from the foregoing that the physical and even more so the chemical properties of homogeneous substances1 more especially solid and liquid1 do not depend exclusively upon the weights of their molecules1 but that many are in definite 7see ,chapter ,,xv47 dependence upon the weights of the atoms of the elements entering into their composition1 and are determined by their quantitative and individual peculiarities4 ,thus the density of solids and liquids 7as will afterwards be shown7 is chiefly determined by the weights of the atoms of the elements entering into their composition1 inasmuch as dense elements 7in a free state7 and compounds are only met with among substances containing elements with large atomic weights1 such as gold1 platinum1 and uranium4 ,and these elements themselves1 in a free state1 are the heaviest of all elements4 ,substances containing such light elements as hydrogen1 carbon1 oxygen and nitrogen 7like many organic substances7 never have a high specific gravity2 in the majority of cases it scarcely exceeds that of water4 ,the density generally decreases with the increase of the amount of hydrogen1 as the lightest element1 and a substance is often obtained lighter than water4 ,the refractive power of substances also entirely depends on the composition and the properties of the component elements4#bi bis ,the history of chemistry presents a striking example in point--,newton foresaw from the high refractive index of the diamond that it would contain a combustible substance since so many combustible oils have a high refractive power4 ,we shall afterwards see 7,chapter ,,xv47 that many of those properties of substances which are in direct dependence not upon the weight of the molecules but upon their composition1 or1 in other words1 upon the properties and quantities of the elements entering into them1 stand in a peculiar 7periodic7 dependence upon the atomic weight of the elements2 that is1 the mass 7of molecules and atoms71 proportional to the weight1 determines the properties of substances as it also determines 7with the distance7 the motions of the heavenly bodies4 #bi bis ,with respect to the optical refractive power of substances1 it must first be observed that the coefficient of refraction is determined by two methods3 7a7 either all the data are referred to one definite ray--for instance1 to the ,fraunhofer 7sodium7 line ,d of the solar spectrum--that is1 to a ray of definite wave length1 and often to that red ray 7of the hydrogen spectrum7 whose wave length is #fef millionths of a millimetre2 7b7 or ,cauchy's formula is used1 showing the relation between the coefficient of refraction and dispersion to the wave length n ,a 7,b_/7,greek3 l#b771 where ,a and ,b are two constants varying for every substance but constant for all rays of the spectrum1 and ,greek3 l is the wave length of that ray whose coefficient of refraction is n4 ,in the latter method the investigation usually concerns the magnitudes of ,a1 which are independent of dispersion4 ,we shall afterwards cite the data1 investigated by the first method1 by which ,gladstone1 ,landolt1 and others established the conception of the refraction equivalent4 ,it has long been known that the coefficient of refraction n for a given substance decreases with the density of a substance ,d1 so that the magnitude 7n-#a7 ,d ,c is almost constant for a given ray 7having a definite wave length7 and for a given substance4 ,this constant is called the refractive energy1 and its product with the atomic or molecular weight of a substance the refraction equivalent4 ,the coefficient of refraction of oxygen is #ajjjba1 of hydrogen #ajjjad1 their densities 7referred to water7 are #jjjadc and #jjjjji1 and their atomic weights1 ,o #af1 ,h #a2 hence their refraction equivalents are #c and #ae4 ,water contains ,h#b,o1 consequently the sum of the equivalents of refraction is 7#b #ae7 #c #f4 ,but as the coefficient of refraction of water #acca1 its refraction equivalent #eieh1 or nearly #f4 ,comparison shows that1 approximately1 the sum of the refraction equivalents of the atoms forming compounds 7or mixtures7 is equal to the refraction equivalent of the compound4 ,according to the researches of ,gladstone1 ,landolt1 ,hagen1 ,brhl and others1 the refraction equivalents of the elements are--,h #ac1 ,li #ch1 ,b #dj1 ,c #ej1 ,n #da 7in its highest state of oxidation1 #ec71 ,o #bi1 ,f #ad1 ,na #dh1 ,mg #gj1 ,al #hd1 ,si #fh1 ,p #ahc1 ,s #afj1 ,cl #ii1 ,k #ha1 ,ca #ajd1 ,mn #abb1 ,fe #abj 7in the salts of its higher oxides1 #bja71 ,co #ajh1 ,cu #aaf1 ,zn #ajb1 ,as #aed1 ,bi #aec1 ,ag #aeg1 ,cd #acf1 ,i #bde1 ,pt #bfj1 ,hg #bjb1 ,pb #bdh1 c4 ,the refraction equivalents of many elements could only be calculated from the solutions of their compounds4 ,the composition of a solution being known it is possible to calculate the refraction equivalent of one of its component parts1 those for all its other components being known4 ,the results are founded on the acceptance of a law which cannot be strictly applied4 ,nevertheless the representation of the refraction equivalents gives an easy means for directly1 although only approximately1 obtaining the coefficient of refraction from the chemical composition of a substance4 ,for instance1 the composition of carbon bisulphide is ,,cs#b #gf1 and from its density1 #abg1 we find its coefficient of refraction to be #afah 7because the refraction equivalent #e #b #af #cg71 which is very near the actual figure4 ,it is evident that in the above representation compounds are looked on as simple mixtures of atoms1 and the physical properties of a compound as the sum of the properties present in the elementary atoms forming it4 ,if this representation of the presence of simple atoms in compounds had not existed1 the idea of combining by a few figures a whole mass of data relating to the coefficient of refraction of different substances could hardly have arisen4 ,for further details on this subject1 see works on ,physical ,chemistry4 ,,chapter ,,viii ,,carbon ,,and ,,the ,,hydrocarbons ,it is necessary to clearly distinguish between the two closely-allied terms1 charcoal and carbon4 ,charcoal is well known to everybody1 although it is no easy matter to obtain it in a chemically pure state4 ,pure charcoal is a simple1 insoluble1 infusible1 combustible substance produced by heating organic matter1 and has the familiar aspect of a black mass1 devoid of any crystalline structure1 and completely insoluble4 ,charcoal is a substance possessing a peculiar combination of physical and chemical properties4 ,this substance1 whilst in a state of ignition1 combines directly with oxygen2 in organic substances it is found in combination with hydrogen1 oxygen1 nitrogen1 and sulphur4 ,but in all these combinations there is no real charcoal1 as in the same sense there is no ice in steam4 ,what is found in such combinations is termed 'carbon'--that is1 an element common to charcoal1 to those substances which can be formed from it1 and also to those substances from which it can be obtained4 ,carbon may take the form of charcoal1 but occurs also as diamond and as graphite4 ,truly no other element has such a wide terminology4 ,oxygen is always called 'oxygen1' whether it is in a free gaseous state1 or in the form of ozone1 or oxygen in water1 or in nitric acid or in carbonic anhydride4 ,but here there is some confusion4 ,in water it is evident that there is no oxygen in a gaseous form1 such as can be obtained in a free state1 no oxygen in the form of ozone1 but a substance which is capable of producing both oxygen1 ozone1 and water4 ,as an element1 oxygen possesses a known chemical individuality1 and an influence on the properties of those combinations into which it enters4 ,hydrogen gas is a substance which reacts with difficulty1 but the element hydrogen represents in its combinations an easily displaceable component part4 ,carbon may be considered as an atom of carbon matter1 and charcoal as a collection of such atoms forming a whole substance1 or mass of molecules of the substance4 ,the accepted atomic weight of carbon is #ab1 because that is the least quantity of carbon which enters into combination in molecules of its compounds2 but the weight of the molecules of charcoal is probably very much greater4 ,this weight remains unknown because charcoal is capable of but few direct reactions and those only at a high temperature 7when the weight of its molecules probably changes1 as when ozone changes into oxygen71 and it does not turn into vapour4 ,carbon exists in nature1 both in a free and combined state1 in most varied forms and aspects4 ,carbon in a free state is found in at least three different forms1 as charcoal1 graphite1 and the diamond4 ,in a combined state it enters into the composition of what are called organic substances--a multitude of substances which are found in all plants and animals4 ,it exists as carbonic anhydride both in air and in water1 and in the soil and crust of the earth as salts of carbonic acid and as organic remains4 ,the variety of the substances of which the structure of plants and animals is built up is familiar to all4 ,wax1 oil1 turpentine1 and tar1 cotton and albumin1 the tissue of plants and the muscular fibre of animals1 vinegar and starch1 are all vegetable and animal matters1 and all carbon compounds4#a ,the class of carbon compounds is so vast that it forms a separate branch of chemistry1 known under the name of organic chemistry--that is1 the chemistry of carbon compounds1 or1 more strictly1 of the hydrocarbons and their derivatives4 #a ,wood is the non-vital part of ligneous plants3 the vital part of ordinary trees is situated between the bark and the lignin4 ,every year a layer of lignin is deposited on this part by the juices which are absorbed by the roots and drawn up by the leaves2 for this reason the age of trees may be determined by the number of lignin layers deposited4 ,the woody matter consists principally of fibrous tissue on to which the lignin or so-called incrusting matter has been deposited4 ,the tissue has the composition ,c#f;,h#aj,o#e1 the substance deposited on it contains more carbon and hydrogen and less oxygen4 ,this matter is saturated with moisture when the wood is in a fresh state4 ,fresh birch wood contains about #ca p4c4 of water1 lime wood #dg p4c41 oak #ce p4c41 pine and fir about #cg p4c4 ,when dried in the air the wood loses a considerable quantity of water and not more than #ai p4c4 remains4 ,by artificial means this loss of water may be increased4 ,if water be driven into the pores of wood the latter becomes heavier than water1 as the lignin of which it is composed has a density of about #af4 ,one cubic centimetre of birch wood does not weigh more than #jija gram1 fir #jhid1 lime tree #jhag1 poplar #jgfe when in a fresh state2 when in a dry state birch weighs #jfbb1 pine #jeej1 fir #jcee1 lime #jdcj1 guaiacum #acdb1 ebony #abbf4 ,on one hectare 7#bg acres7 of woodland the yearly growth averages the amount of #c1jjj kilograms 7or about #c tons7 of wood1 but rarely reaches as much as #e1jjj kilos4 ,the average chemical composition of wood dried in air may be expressed as follows3--,hygroscopic water #ae p4c41 carbon #db p4c41 hydrogen #e p4c41 oxygen and nitrogen #cg p4c41 ash #a p4c4 ,wood parts with its hygroscopic water at #aej1 and decomposes at about #cjj1 giving a brown1 brittle1 so-called red charcoal2 above #cej black charcoal is produced4 ,as the hydrogen contained in wood requires for its combustion about forty parts by weight of oxygen1 which is present to the amount of about #cf p4c41 all that burns of the wood is the carbon which it contains1 #ajj parts of wood only giving out as much heat as forty parts of charcoal1 and therefore it would be far more profitable to use charcoal for heating purposes than wood1 if it were possible to obtain it in such quantities as correspond with its percentage ratio--that is forty parts per #ajj parts of wood4 ,generally1 however1 the quantity produced is far less1 not more than #cj p4c41 because part of the carbon is given off as gas1 tar1 c4 ,if wood has to be transported great distances1 or if it is necessary to obtain a very high temperature by burning it1 then even as little as #be p4c4 of charcoal from #ajj parts of wood may be advantageous4 ,charcoal 7from wood7 develops on burning #h1jjj heat units1 whilst wood dried in air does not develop more than #b1hjj units of heat2 therefore seven parts of charcoal give as much heat as twenty parts of wood4 ,as regards the temperature of combustion1 it is far higher with charcoal than with wood1 because twenty parts of burning wood give1 besides the carbonic anhydride which is also formed together with charcoal1 eleven parts of water1 the evaporation of which requires a considerable amount of heat4 ,illustration3 ,,fig4 #eg4--;,apparatus for the dry distillation of wood4 ,the retort a containing the wood is heated by the flues c e4 ,the steam and volatile products of distillation pass along the tube g through the condenser m1 where they are condensed4 ,the form1 distribution1 and dimensions of the apparatus vary4 ,the composition of the growing parts of plants1 the leaves1 young branches1 shoots1 c41 differs from the composition of the wood in that these vital parts contain a considerable quantity of sap which contains much nitrogenous matter 7in the wood itself there is very little71 mineral salts1 and a large amount of water4 ,taking1 for example1 the composition of clover and pasture hay in the green and dry state2 in #ajj parts of green clover there is about #hj p4c4 of water and #bj p4c4 of dry matter1 in which there are about #ce parts of nitrogenous albuminous matter1 about #ie parts of soluble and about #e parts of insoluble non-nitrogenous matter1 and about #b p4c4 of ash4 ,in dry clover or clover-hay there is about #ae p4c4 of water1 #ac p4c4 of nitrogenous matter1 and #g p4c4 of ash4 ,this composition of grassy substances shows that they are capable of forming the same sort of charcoal as wood itself4 ,it also shows the difference of nutritive properties existing between wood and the substances mentioned4 ,these latter serve as food for animals1 because they contain those substances which are capable of being dissolved 7entering into the blood7 and forming the body of animals2 such substances are proteids1 starch1 c4 ,let us remark here that with a good harvest an acre of land gives in the form of grass as much organic substance as it yields in the form of wood4 ,one hundred parts of dry wood are capable of giving1 on dry distillation1 besides #be p4c4 of charcoal and #aj p4c4 or more of tar1 #dj p4c4 of watery liquid1 containing acetic acid and wood spirit1 and about #be p4c4 of gases1 which may be used for heating or lighting purposes1 because they do not differ from ordinary illuminating gas1 which can indeed be obtained from wood4 ,as wood-charcoal and tar are valuable products1 in some cases the dry distillation of wood is carried on principally for producing them4 ,for this purpose those kinds of woods are particularly advantageous which contain resinous substances1 especially coniferous trees1 such as fir1 pine1 c42 birch1 oak1 and ash give much less tar1 but on the other hand they yield more aqueous liquor4 ,the latter is used for the manufacture of wood spirit1 ,,ch#d,o1 and acetic acid1 ,c#b;,h#d,o#b4 ,in such cases1 the dry distillation is carried on in stills4 ,the stills are nothing more than horizontal or vertical cylindrical retorts1 made of boiler plate1 heated with fuel and having apertures at the top and sometimes also at the bottom for the exit of the light and heavy products of distillation4 ,the dry distillation of wood in stoves is carried on in two ways1 either by burning a portion of the wood inside the stove in order to submit the remainder to dry distillation by means of the heat obtained in this manner1 or by placing the wood in a stove the thin sides of which are surrounded with a flue leading from the fuel1 placed in a space below4 ,the first method does not give such a large amount of liquid products of the dry distillation as the latter4 ,in the latter process there is generally an outlet below for emptying out the charcoal at the close of the operation4 ,for the dry distillation of #ajj parts of wood from forty to twenty parts of fuel are used4 ,in the north of ,russia wood is so plentiful and cheap that this locality is admirably fitted to become the centre of a general trade in the products of its dry distillation4 ,coal 7,note #f71 sea-weed1 turf1 animal substances 7,chapter ,,vi471 c41 are also submitted to the process of dry distillation4 ,if any one of these organic compounds be strongly heated without free access of air--or1 better still1 in a vacuum--it decomposes with more or less facility4 ,if the supply of air be insufficient1 or the temperature be too low for combustion 7see ,chapter ,,iii471 and if the first volatile products of transformation of the organic matter are subjected to condensation 7for example1 if the door of a stove be opened71 an imperfect combustion takes place1 and smoke1 with charcoal or soot1 is formed4#b ,the nature of the phenomenon1 and the products arising from it1 are the same as those produced by heating alone1 since that part which is in a state of combustion serves to heat the remainder of the fuel4 ,the decomposition which takes place on heating a compound composed of carbon1 hydrogen1 and oxygen is as follows3--,a part of the hydrogen is separated in a gaseous state1 another part in combination with oxygen1 and a third part separates in combination with carbon1 and sometimes in combination with carbon and oxygen in the form of gaseous or volatile products1 or1 as they are also called1 the products of dry distillation4 ,if the vapours of these products are passed through a strongly heated tube1 they are changed again in a similar manner and finally resolve themselves into hydrogen and charcoal4 ,altogether these various products of decomposition contain a smaller amount of carbon than the original organic matter2 part of the carbon remains in a free state1 forming charcoal4#c ,it remains in that space where the decomposition took place1 in the shape of the black1 infusible1 non-volatile charcoal familiar to all4 ,the earthy matter and all non-volatile substances 7ash7 forming a part of the organic matter1 remain behind with the charcoal4 ,the tar-like substances1 which require a high temperature in order to decompose them1 also remain mixed with charcoal4 ,if a volatile organic substance1 such as a gaseous compound containing oxygen and hydrogen1 be taken1 the carbon separates on passing the vapour through a tube heated to a high temperature4 ,organic substances when burning with an insufficient supply of air give off soot--that is1 charcoal--proceeding from carbon compounds in a state of vapour1 the hydrogen of which has1 by combustion1 been converted into water2 so1 for instance1 turpentine1 naphthalene1 and other hydrocarbons which are with difficulty decomposed by heat1 easily yield carbon in the form of soot during combustion4 ,chlorine and other substances which1 like oxygen1 are capable of taking up hydrogen1 and also substances which are capable of taking up water1 can also separate carbon from 7or char7 most organic substances4 #b ,the result of imperfect combustion is not only the loss of a part of the fuel and the production of smoke1 which in some respects is inconvenient and injurious to health1 but also a low flame temperature1 which means that a less amount of heat is transmitted to the object heated4 ,imperfect combustion is not only always accompanied by the formation of soot or unburnt particles of charcoal1 but also by that of carbonic oxide1 ,,co1 in the smoke 7,chapter ,,ix47 which burns1 emitting much heat4 ,in works and factories where large quantities of fuel are consumed1 many appliances are adopted to ensure perfect combustion1 and to combat against such a ruinous practice as the imperfect combustion of fuel4 ,the most effective and radical means consists in employing combustible gases 7producer and water gases71 because by their aid perfect combustion can be easily realised without a loss of heat-producing power and the highest temperature can be reached4 ,when solid fuel is used 7such as coal1 wood1 and turf71 imperfect combustion is most liable to occur when the furnace doors are opened for the introduction of fresh fuel4 ,the step furnace may often prove a remedy for this defect4 ,in the ordinary furnace fresh fuel is placed on the burning fuel1 and the products of dry distillation of the fresh fuel have to burn at the expense of the oxygen remaining uncombined with the burnt fuel4 ,imperfect combustion is observed in this case also from the fact that the dry distillation and evaporation of the water of the fresh fuel lying on the top of that burnt1 lowers the temperature of the flame1 because part of the heat becomes latent4 ,on this account a large amount of smoke 7imperfect combustion7 is observed when a fresh quantity of fuel is introduced into the furnace4 ,this may be obviated by constructing the furnace 7or managing the stoking7 in such a way that the products of distillation pass through the red-hot charcoal remaining from the burnt fuel4 ,it is only necessary in order to ensure this to allow a sufficient quantity of air for perfect combustion4 ,all this may be easily attained by the use of step fire-bars4 ,the fuel is fed into a hopper and falls on to the fire-bars1 which are arranged in the form of a staircase4 ,the burning charcoal is below1 and hence the flame formed by the fresh fuel is heated by the contact of the red-hot burning charcoal4 ,an air supply through the fire grate1 an equal distribution of the fuel on the fire-bars 7otherwise the air will blow through empty spaces and lower the temperature71 a proper proportion between the supply of air and the chimney draught1 and a perfect admixture of air with the flame 7without an undue excess of air71 are the means by which we can contend against the imperfect combustion of such kinds of fuel as wood1 peat1 and ordinary 7smoky7 coal4 ,coke1 charcoal1 anthracite1 burn without smoke1 because they do not contain hydrogenous substances which furnish the products of dry distillation1 but imperfect combustion may occur with them also2 in that case the smoke contains carbonic oxide4 #c ,under the action of air1 organic substances are capable of oxidising to such an extent that all the carbon and all the hydrogen they contain will be transformed into carbonic anhydride and water4 ,the refuse of plants and that of animals are subjected to such a change whether they slowly decompose and putrefy1 or rapidly burn with direct access of air4 ,but if the supply of air be limited1 there can be no complete transformation into water and carbonic anhydride1 there will be other volatile matters 7rich in hydrogen71 while charcoal must remain as a non-volatile substance4 ,all organic substances are unstable1 they do not resist heat1 and change even at ordinary temperatures1 particularly if water be present4 ,it is therefore easy to understand that charcoal may in many cases be obtained through the transformation of substances entering into the composition of organisms1 but that it is never found in a pure state4 ,however1 water and carbonic anhydride are not the only products separated from organic substances4 ,carbon1 hydrogen1 and oxygen are capable of giving a multitude of compounds2 some of these are volatile compounds1 gaseous1 soluble in water--they are carried off from organic matter1 undergoing change without access of air4 ,others1 on the contrary1 are non-volatile1 rich in carbon1 unaffected by heat and other agents4 ,the latter remain in admixture with charcoal in the place where the decomposition takes place2 such1 for example1 are tarry substances4 ,the quantity of those bodies which are found mixed with the charcoal is very varied1 and depends on the energy and duration of the decomposing agent4 ,the annexed table shows1 according to the data of ,violette1 those changes which wood undergoes at various temperatures when submitted to dry distillation by means of superheated steam3-- ---------------------------------------- ------------------ ,residue ,temperature from #ajj parts ,in #ajj parts of the residual of alder wood charcoal ---------------------------------------- ----------------- ,c ,h ,o and ,n ,ash #aej #ajjj #dge #fa #dfc #ja #cej #big #gff #da #ahd #jf #ajcb #ahg #hai #bc #ada #af #aejj #agc #iej #jg #ch #ag ---------------------------------------- ----------------- ,wood charcoal is prepared in large quantities in a similar manner--that is1 by the partial combustion of wood4#d ,in nature a similar process of carbonisation of vegetable refuse takes place in its transformation under water1 as shown by the marshy vegetation which forms peat4#e ,in this manner doubtless the enormous masses of coal were formed#f which1 following the example set by ,england1 are now utilised everywhere as the principal material for heating steam boilers1 and in general for all purposes of heating and burning4#g ,russia possesses many very rich coalfields1 amongst which the ,donetz district is most worthy of remark4#h #d ,the object of producing charcoal from wood has been explained in ,note #a4 ,wood charcoal is obtained in so-called stacks by partially burning the wood1 or by means of dry distillation 7,note #a7 without the access of air4 ,it is principally manufactured for metallurgical processes1 especially for smelting and forging iron4 ,the preparation of charcoal in stacks has one advantage1 and that is that it may be done on any spot in the forest4 ,but in this way all the products of dry distillation are lost4 ,for charcoal burning1 a pile or stack is generally built1 in which the logs are placed close together1 either horizontally1 vertically1 or inclined1 forming a stack of from six to fifty feet in diameter and even larger4 ,under the stack are several horizontal air passages1 and an opening in the middle to let out the smoke4 ,the surface of the stack is covered with earth and sods to a considerable thickness1 especially the upper part1 in order to hinder the free passage of air and to concentrate the heat inside4 ,when the stack is kindled1 the pile begins to settle down by degrees1 and it is then necessary to look after the turf casing and keep it in repair4 ,as the combustion spreads throughout the whole pile1 the temperature rises and real dry distillation commences4 ,it is then necessary to stop the air holes1 in order as much as possible to prevent unnecessary combustion4 ,the nature of the process is1 that part of the fuel burns and develops the heat required for subjecting the remainder to dry distillation4 ,the charring is stopped when the products of dry distillation1 which are emitted1 no longer burn with a brilliant flame1 but the pale blue flame of carbonic oxide appears4 ,dry wood in stacks yields about one-fourth of its weight of charcoal4 #e ,when dead vegetable matter undergoes transformation in air1 in the presence of moisture and lower organisms1 there remains a substance much richer in carbon--namely1 humus1 black earth or mould4 #ajj parts of humus in a dry state contain about #gj p4c4 of carbon4 ,the roots1 leaves1 and stems of plants which wither and fall to the ground form a soil rich in humus4 ,the non-vital vegetable substances 7ligneous tissue7 first form brown matter 7ulmic compounds71 and then black matter 7humic substances71 which are both insoluble in water2 after this a brown acid is produced1 which is soluble in water 7apocrenic acid71 and lastly a colourless acid also soluble in water 7crenic acid74 ,alkali dissolves a part of the original brown and black substances1 forming solutions of a brown tint 7ulmic and humic acids7 which sometimes communicate their colour to springs and rivers4 ,the proportion of humus in soil generally has a direct influence on its fertility2 firstly1 because putrefying plants develop carbonic anhydride and ammonia1 and yield the substances forming the ashes of plants1 which are necessary to vegetation2 secondly1 because humus is capable of attracting the moisture of the air and of absorbing water 7twice its weight7 and in this way keeps the soil in a damp condition1 which is indispensable for nourishment2 thirdly1 humus renders the soil porous1 and1 fourthly1 it renders it more capable of absorbing the heat of the sun's rays4 ,on this account black earth is often most remarkable for its fertility4 ,one object of manuring is to increase the quantity of humus in the soil1 and any easily changeable vegetable or any animal matter 7composts7 may be used4 ,the boundless tracts of black earth soil in ,russia are capable of bestowing countless wealth on the country4 ,the origin and extent of black earth soil are treated in detail in ,professor ,dokouchaeff's works4 ,if those substances which produce humus undergo decomposition under water1 less carbonic anhydride is formed1 a quantity of marsh gas1 ,,ch#d1 is evolved1 and the solid residue forms an acid humus found in great quantities in marshy places and called peat4 ,peat is especially abundant in the lowlands of ,holland1 ,north ,germany1 ,ireland1 and ,bavaria4 ,in ,russia it is likewise found in large quantities1 especially in the ,north-,west districts4 ,the old hard forms of peat resemble in composition and properties brown coal2 the newest formations1 as yet unhardened by pressure1 form very porous masses which retain traces of the vegetable matter from which they have been formed4 ,dried 7and sometimes pressed7 peat is used as fuel4 ,the composition of peat varies considerably with the locality in which it is found4 ,when dried in air it does not contain less than #ae p4c4 of water and #h p4c4 of ash2 the remainder consists of #de p4c4 of carbon1 #d p4c4 of hydrogen1 #a p4c4 of nitrogen1 and #bh p4c4 of oxygen4 ,its heating power is about equivalent to that of wood4 ,the brown earthy varieties of coal were probably formed from peat4 ,in other cases they have a marked woody structure1 and are then known as lignites4 ,the composition of the brown sorts of coal resembles in a marked degree that of peat--namely1 in a dried state brown coal contains on an average #fj p4c4 of carbon1 #e p4c4 of hydrogen1 #bf p4c4 of oxygen and nitrogen1 and #i p4c4 of ash4 ,in ,russia brown coal is met with in many districts near ,moscow1 in the ,governments of ,toula and ,tver and the neighbourhood2 it is very usually used as fuel1 particularly when found in thick seams4 ,the brown coals usually burn with a flame like wood and peat1 and are akin to them in heating power1 which is half or a third that of the best coal4 #f ,grass and wood1 the vegetation of primval seas and similar refuse of all geological periods1 must have been in many cases subjected to the same changes they now undergo--that is1 under water they formed peat and lignites4 ,such substances1 preserved or a long time underground1 subjected to the action of water1 compressed by the new strata formed above them1 transformed by the separation of their more volatile component parts 7peat and lignites1 even in their last condition1 still continue to evolve nitrogen1 carbonic anhydride1 and marsh gases7 form coal4 ,coal is a dense homogeneous mass1 black1 with an oily or glassy lustre1 or more rarely dull without any evident vegetable structure2 this distinguishes it in appearance from the majority of lignites4 ,the density of coal 7not counting the admixture of pyrites1 c47 varies from #abe 7dry bituminous coal7 to #af 7anthracite1 flameless71 and even reaches #ai in the very dense variety of coal found in the ,olonetzky government 7termed shungite71 which according to the investigations of ,professor ,inostrantzeff may be regarded as the extreme member of the various forms of coal4 ,in order to explain the formation of coal from vegetable matter1 ,cagniard de la ,tour enclosed pieces of dried wood in a tube and heated them to the boiling point of mercury1 when the wood was changed into a semi-liquid black mass from which a substance exceedingly like coal separated4 ,in this manner some kinds of wood formed coal which on being heated left caking coke1 others non-caking2 precisely as we find with the natural varieties of coal4 ,violette repeated these experiments with wood dried at #aej1 and showed that when wood is decomposed in this way1 a gas1 an aqueous liquor1 and a residue are formed4 ,the latter at a temperature of #bjj has the properties of wood charcoal incompletely burnt2 at #cjj and higher a homogeneous mass like coal is formed which at #cdj is dense and without cavities4 ,at #djj the residue resembles anthracite4 ,in nature probably the decomposition was in rare cases effected by heat alone2 more generally it was effected by means of water and heat1 but in either case the result ought to be almost the same4 ,the average composition of coal compiled from many analyses1 disregarding the ash1 is as follows3 #hd parts of carbon1 #e parts of hydrogen1 #a part of nitrogen1 #h parts of oxygen1 #b of sulphur4 ,the quantity of ash is on an average #e p4c41 but there are coals which contain a larger quantity1 and naturally they are not so advantageous for use as fuel4 ,the amount of water does not usually exceed more than #aj p4c4 ,the anthracites form a remarkable variety of coals1 they do not give any volatile products1 or but a very small amount1 as they contain but little hydrogen compared to oxygen4 ,in the average composition of coal we saw that for #e parts of hydrogen there were #h parts of oxygen2 therefore #d parts by weight of hydrogen are capable of forming hydrocarbons1 because #a part of hydrogen is necessary in order to form water with the #h parts of oxygen4 ,these #d parts by weight of hydrogen can convert #dh parts of carbon into volatile products1 because #a part of hydrogen by weight in these substances combines with #ab parts of carbon4 ,the anthracites differ essentially from this3 neglecting the ash1 their average composition is as follows3 #id parts of carbon1 #c of hydrogen1 and #c of oxygen and nitrogen4 ,according to the analyses of ,a4 ,a4 ,voskresensky1 the ,grousheffsky anthracite 7,don district7 contains3 ,c #ich1 ,h #ag1 ash #ae4 ,therefore the anthracites contain but little hydrogen capable of combining with the carbon to form hydrocarbons which burn with a flame4 ,anthracites are the oldest forms of coal4 ,the newest and least transformed coals1 which resemble some of the brown varieties1 are the dry coals4 ,they burn with a flame like wood1 and leave a coke having the appearance of lumps of coal1 half their component parts being absorbed by the flame 7they contain much hydrogen and oxygen74 ,the remaining varieties of coal 7gas coal1 smithy coal1 coking1 and anthracite7 according to ,grner in all respects form connecting links between the dry coals and the anthracites4 ,these coals burn with a very smoky flame1 and on being heated leave coke1 which bears the same relation to coal that charcoal does to wood4 ,the quantity and quality of coke vary considerably with the different sorts of coal from which it is formed4 ,in practice coals are most often distinguished by the properties and quantity of the coke which they give4 ,in this particular the so-called bituminous coals are especially valuable1 as even the slack of this kind gives on dry distillation large spongy masses of coke4 ,if large pieces of these kinds of coal are subjected to dry distillation1 they1 as it were1 melt1 flow together1 and form caking masses of coke4 ,the best coking coals give #fe p4c4 of dense caking coke4 ,such coal is very valuable for metallurgical purposes 7see ,note #h74 ,besides coke1 the dry distillation of coal produces gas 7see further1 illuminating gas1 p4 #cfa71 coal-tar 7which gives benzene1 carbolic acid1 naphthalene1 tar for artificial asphalt1 c47 and also an aqueous alkaline liquor 7with wood and lignites the liquid is acid from acetic acid7 which contains ammonium carbonate 7see ,note #f74 #g ,in ,england in #ahej the output of coal was as much as #dh million tons1 and in latter years it has risen to about #aij millions4 ,besides this other countries contribute #cjj millions--,russia about #f millions4 ,the ,united ,states of ,america come next to ,england with an output of #afj million tons1 then ,germany #ij millions2 ,france produces but little 7#be millions71 and takes about #e million tons from ,england4 ,thus the world consumes about #ejj million tons of coal yearly4 ,besides household purposes1 coal is chiefly used as fuel for steam-engines4 ,as every horse-power 7 #ge kilogrammetres per second7 of a steam-engine expends on the average more than #be kilograms in #bd hours1 or in a year 7counting stoppages7 not less than #e tons per horse-power1 and there are not less than #dj million horse-power at work in the world1 the consumption of coal for motive-power is at least equal to half the whole production4 ,for this reason coal serves as a criterion of the industrial development of a country4 ,about #ae p4c4 of coal is used for the manufacture of cast iron1 wrought iron1 steel1 and articles made of them4 #h ,the principal coal beds of ,russia under exploitation are3 ,the ,don basin 7#aej million poods per annum1 #fb poods #a ton71 the ,polish basin 7,dombrovo and others #abj million poods per annum71 the ,toula and ,riazan beds of the ,moscow basin 7up to #be million poods71 the ,ural basin 7#aj million poods71 the ,caucasian 7,kviboul1 near ,kutais71 the ,khirjhis steppes1 the smithy coal basin 7,gov4 of ,tomsk71 the ,sahaline1 c4 ,the ,polish and ,moscow basins do not give any coking coals4 ,the presence of every variety of coal 7from the dry coal near ,lisichansk on the ,donetz to the anthracites of the entire south-east basin71 the great abundance of excellent metallurgical coal 7coking1 see ,note #f7 in the western part of the basin1 its vast extent 7as much as #be1jjj sq4 versts71 the proximity of the seams to the surface 7the shafts are now from #bj to #ajj fathoms deep1 and in ,england and ,belgium as deep as #ejj fathoms71 the fertility of the soil 7black earth71 the proximity of the sea 7about #ajj versts from the ,sea of ,azoff7 and of the rivers ,donetz1 ,don1 and ,dneiper1 the most abundant seams of excellent iron ore 7,korsan ,mogila1 ,krivoy ,rog1 ,soulin1 c41 c471 copper ore1 mercury ore 7near ,nikitovka1 in the ,bakhmouth district of the ,ekaterinoslav ,gov471 and other ores1 the richest probably in the whole world1 the beds of rock-salt 7near the stations of the ,stoupka and ,brianzovka7 the excellent clay of all kinds 7china1 fire-clay71 gypsum1 slate1 sandstone1 and other wealth of the ,don coal basin1 give complete assurance of the fact that with the growth of industrial activity in ,russia this bountiful land of the ,cossacks and ,new ,russia will become the centre of the most extensive productive enterprise1 not for the requirements of ,russia alone1 but of the whole world1 because in no other place can be found such a concentration of favourable conditions4 ,the growth of enterprise and knowledge1 together with the extinction of the forests which compels ,russia to foster the production of coal1 will help to bring about this desired result4 ,england with a whole fleet of merchant vessels exports annually about #be million tons of coal1 the price of which is higher than on the ,donetz 7where a pood of worked coal costs less than #e copecks on the average71 where anthracites and semi-anthracites 7like ,cardiff or steam coal1 which burns without smoke7 and coking and metallurgical coals are able both in quantity and quality to satisfy the most fastidious requirements of the industry already existing and rapidly increasing everywhere4 ,the coal mines of ,england and ,belgium are approaching a state of exhaustion1 whilst in those of the ,don basin1 only at a depth of #ajj fathoms1 #a1bjj1jjj million poods of coal lie waiting to be worked4 ,during the imperfect combustion of volatile substances containing carbon and hydrogen1 the hydrogen and part of the carbon first burn1 and the remainder of the carbon forms soot4 ,tar1 pitch1 and similar substances for this reason burn with a smoky flame4 ,thus soot is finely-divided charcoal separated during the imperfect combustion of the vapours and gases of carbonaceous substances rich in carbon4 ,specially-prepared soot 7lampblack7 is very largely used as a black paint and a large quantity goes for the manufacture of printers' ink4 ,it is prepared by burning tar1 oil1 natural gas1 naphtha1 c4 ,the quantity of organic matter remaining undecomposed in the charcoal depends on the temperature to which it has been submitted4 ,charcoal prepared at the lowest temperature still contains a considerable quantity of hydrogen and oxygen--even as much as #d p4c4 of hydrogen and #bj p4c4 of oxygen4 ,such charcoal still preserves the structure of the substance from which it was obtained4 ,ordinary charcoal1 for instance1 in which the structure of the tree is still visible1 is of this kind4 ,on submitting it to further heating1 a fresh quantity of hydrogen with carbon and oxygen 7in the form of gases or volatile matter7 may be separated1 and the purest charcoal will be obtained on submitting it to the greatest heat4#i ,if it be required to prepare pure charcoal from soot it is necessary first to wash it with alcohol and ether in order to remove the soluble tarry products1 and then submit it to a powerful heat to drive off the impurities containing hydrogen and oxygen4 ,charcoal however when completely purified does not change in appearance4 ,its porosity1#aj bad conducting power for heat1 capability of absorbing the luminous rays 7hence its blackness and opacity71 and many other qualities1 are familiar from everyday experience4#aa ,the specific gravity of charcoal varies from #ad to #ai1 and that it floats on water is due to the air contained in its pores4 ,if charcoal is reduced to a powder and moistened with spirit1 it immediately sinks in water4 ,it is infusible in the furnace and even at the temperature of the oxyhydrogen flame4 ,in the heat generated by means of a powerful galvanic current charcoal only softens but does not completely melt1 and on cooling it is found to have undergone a complete change both in properties and appearance1 and is more or less transformed into graphite4 ,the physical stability of charcoal is without doubt allied to its chemical stability4 ,it is evidently a substance devoid of energy1 for it is insoluble in all known liquids1 and at an ordinary temperature does not combine with anything2 it is an inactive substance1 like nitrogen4#ab ,but these properties of charcoal change with a rise of temperature2 thus1 unlike nitrogen1 charcoal1 at a high temperature1 combines directly with oxygen4 ,this is well known1 as charcoal burns in air4 ,indeed1 not only does oxygen combine with charcoal at a red heat1 but sulphur1 hydrogen1 silicon1 and also iron and some other metals#ab bis do so at a very high temperature--that is1 when the molecules of the charcoal have reached a state of great instability--whilst at ordinary temperatures neither oxygen1 sulphur1 nor metals act on charcoal in any way4 ,when burning in oxygen1 charcoal forms carbonic anhydride1 ,,co#b1 whilst in the vapours of sulphur1 carbon bisulphide1 ,,cs#b1 is formed1 and wrought iron1 when acted on by carbon1 becomes cast iron4 ,at the great heat obtained by passing the galvanic current through carbon electrodes1 charcoal combines with hydrogen1 forming acetylene1 ,c#b;,h#b4 ,charcoal does not combine directly with nitrogen1 but in the presence of metals and alkaline oxides1 nitrogen is absorbed1 forming a metallic cyanide1 as1 for instance1 potassium cyanide1 ,,kcn4 ,from these few direct combinations which charcoal is capable of entering into1 may be derived those numerous carbonaceous compounds which enter into the composition of plants and animals1 and can be thus obtained artificially4 ,certain substances containing oxygen give up a part of it to charcoal at a relatively low temperature4 ,for instance1 nitric acid when boiled with charcoal gives carbonic anhydride and nitric peroxide4 ,sulphuric acid is reduced to sulphurous anhydride when heated with carbon4 ,when heated to redness charcoal absorbs oxygen from a large number of the oxides4 ,even such oxides as those of sodium and potassium1 when heated to redness1 yield their oxygen to charcoal although they do not part with it to hydrogen4 ,only a few of the oxides1 like silica 7oxide of silicon7 and lime 7calcium oxide7 resist the reducing action of charcoal4 ,charcoal is capable of changing its physical condition without undergoing any alteration in its essential chemical properties--that is1 it passes into isomeric or allotropic forms4 ,the two other particular forms in which carbon appears are the diamond and graphite4 ,the identity of composition of these with charcoal is proved by burning an equal quantity of all three separately in oxygen 7at a very high temperature71 when each gives the same quantity of carbonic anhydride--namely1 #ab parts of charcoal1 diamond1 or graphite in a pure state1 yield on burning #dd parts by weight of carbonic anhydride4 ,the physical properties present a marked contrast2 the densest sorts of charcoal have a density of only #ai1 whilst the density of graphite is about #bc1 and that of the diamond #ce4 ,a great many other properties depend on the density1 for instance combustibility4 ,the lighter charcoal is1 the more easily it burns2 graphite burns with considerable difficulty even in oxygen1 and the diamond burns only in oxygen and at a very high temperature4 ,on burning1 charcoal1 the diamond1 and graphite develop different quantities of heat4 ,one part by weight of wood charcoal converted by burning into carbonic anhydride develops #h1jhj heat units2 dense charcoal separated in gas retorts develops #h1jej heat units2 natural graphite1 #g1hjj heat units2 and the diamond #g1ggj4 ,the greater the density the less the heat evolved by the combustion of the carbon4#ac #i ,as it is difficult to separate from the charcoal the admixture of ash--that is1 the earthy matter contained in the vegetable substance used for producing charcoal--in order to obtain it in its purest condition it is necessary to use such organic substances as do not contain any ash1 for example completely refined or purified crystallised sugar1 crystallised tartaric acid1 c4 #aj ,the cavities in charcoal are the passages through which those volatile products formed at the same time as the charcoal have passed4 ,the degree of porosity of charcoal varies considerably1 and has a technical significance1 in different kinds of charcoal4 ,the most porous charcoal is very light2 a cubic metre of wood charcoal weighs about #bjj kilograms4 ,many of the properties of charcoal which depend exclusively on its porosity are shared by many other porous substances1 and vary with the density of the charcoal and depend on the way it was prepared4 ,the property which charcoal has of absorbing gases1 liquids1 and many substances in solution1 is a case in point4 ,the densest kind of charcoal is formed by the action of great heat on sugar and other fusible substances4 ,the lustrous grey dense coke formed in gas retorts is also of this character4 ,this dense coke collects on the internal walls of the retorts subjected to great heat1 and is produced by the vapours and gases separated from the heated coal in the retorts4 ,in virtue of its density such coke becomes a good conductor of the galvanic current and approaches graphite4 ,it is principally used in galvanic batteries4 ,coke1 or the charcoal remaining from the imperfect combustion of coal and tarry substances1 is also but slightly porous1 brilliant1 does not soil or mark paper1 is dense1 almost devoid of the faculty of retaining liquids and solids1 and does not absorb gases4 ,the light sorts of charcoal produced from charred wood1 on the other hand1 show this absorptive power in a most marked degree4 ,this property is particularly developed in that very fine and friable charcoal prepared by heating animal substances such as hides and bones4 ,the absorptive power of charcoal for gases is similar to the condensation of gases in spongy platinum4 ,here evidently there is a case of the adherence of gases to a solid1 precisely as liquids have the property of adhering to various solids4 ,one volume of charcoal will absorb the following volumes of gases 7charcoal is capable of absorbing an immense amount of chlorine1 almost equal to its own weight73-- ---------------------------------------- ---------------- ,saussure4 ,favre4 ,heat emitted ,boxwood ,charcoal ,cocoanut ,charcoal per gram of gas ---------------------------------------- --------------- ,,nh#c #ij #agb vols4 #did units ,,co#b #ce #ig #aeh ,n#b,o #dj #ii #afi ,h,cl #he #afe #bgd ---------------------------------------- --------------- ,the quantity of gas absorbed by the charcoal increases with the pressure1 and is approximately proportional to it4 ,the quantity of heat given out by the absorption nearly approaches that set free on dissolving1 or passing into a liquid condition4 ,charcoal absorbs not only gases1 but a number of other substances4 ,for instance1 alcohol which contains disagreeably smelling fusel oil1 on being mixed with charcoal or filtered through it1 loses most of the fusel oil4 ,the practice of filtering substances through charcoal in order to get rid of foreign matters is often applied in chemical and manufacturing processes4 ,oils1 spirits1 various extracts1 and vegetable and other solutions are filtered through charcoal in order to purify them4 ,the bleaching power of charcoal may be tested by using various coloured solutions--such as aniline dyes1 litmus1 c4 ,charcoal1 which has absorbed one substance to saturation is still capable of absorbing certain other substances4 ,animal charcoal1 produced in a very finely-divided state1 especially by heating bones1 makes the best sort for the purposes of absorption4 ,bone charcoal is used in large quantities in sugar works for filtering syrups and all saccharine solutions1 in order to purify them1 not only from colouring and odorous matter1 but also from the lime which is mixed with the syrups in order to render them less unstable during boiling4 ,the absorption of lime by animal charcoal depends1 in all probability1 in a great degree on the mineral component parts of bone charcoal4 #aa ,charcoal is a very bad conductor of heat1 and therefore forms an excellent insulator or packing to prevent the transmission of heat4 ,a charcoal lining is often used in crucibles for heating many substances1 as it does not melt and resists a far greater heat than many other substances4 #ab ,the unalterability of charcoal under the action of atmospheric agencies1 which produce changes in the majority of stony and metallic substances1 is often made use of in practice4 ,for example1 charcoal is frequently strewn in boundary ditches4 ,the surface of wood is often charred to render it durable in those places where the soil is damp and wood itself would soon rot4 ,the chambers 7or in some works towers7 through which acids pass 7for example1 sulphuric and hydrochloric7 in order to bring them into contact with gases or liquids1 are filled with charcoal or coke1 because at ordinary temperatures it resists the action of even the strongest acids4 #ab bis ,maquenne 7#ahib7 discovered that carbon is capable of combining with the alkali metals4 ,a #bj p4c4 amalgam of the metals was heated to a red heat with charcoal powder in a stream of hydrogen4 ,the compounds so obtained possessed1 after the mercury had been driven off1 the compositions ,ba,c#b1 ,sr,c#b1 ,ca,c#b4 ,all these compounds react with water forming acetylene1 for example3 ,ba,c#b #b;,h#b,o ,c#b;,h#b ,ba7,,oh7#b ,maquenne proposes the barium carbide as a source of acetylene4 ,he obtained this compound by heating carbonate of barium1 magnesium powder1 and retort carbon in a ,perreau furnace 7,ba,c,o#c #c,mg ,c #c,mg,o ,ba,c#b74 ,one hundred grams of ,ba,c#b evolve #e1bjj to #e1djj c4c4 of acetylene1 mixed with about #b-c p4c4 of hydrogen4 ,the relation of acetylene1 ,c#b;,h#b1 to these metallic carbides is evident from the fact that these metals 7,ca1 ,sr1 ,ba7 replace #b atoms of hydrogen1 and therefore ,c#b;,ba corresponds to ,c#b;,h#b1 so that they may be regarded as metallic derivatives of acetylene4 ,moissan 7#ahic7 obtained similar carbides directly from the oxides by subjecting them to the action of the voltaic arc1 in the presence of carbon1 for instance1 ,ba,o #c;,c ,,co ,c#b;,ba1 although at a furnace heat carbon has no action on the oxides ,ca,o1 ,ba,o1 ,sr,o4 ,concerning ,al#d;,c#e1 see ,chapter ,,xvii4 ,note #ch4 #ac ,when subjected to pressure1 charcoal loses heat1 hence the densest form stands to the less dense as a solid to a liquid1 or as a compound to an element4 ,from this the conclusion may be drawn that the molecules of graphite are more complex than those of charcoal1 and those of the diamond still more so4 ,the specific heat shows the same variation1 and as we shall see further on1 the increased complexity of a molecule leads to a diminution of the specific heat4 ,at ordinary temperatures the specific heat of charcoal is #jbd1 graphite #jbj1 the diamond #jadg4 ,for retort carbon ,le ,chatelier 7#ahic7 found that the product of the sp4 heat and atomic weight varies1 between #j and #bej1 according to the formula3 #aib #jjjggt1 and between #bej and #ajjj1 #ced #jjjbdft 7see ,chapter ,,xiv4 ,note #d74 ,by means of intense heat charcoal may be transformed into graphite4 ,if a charcoal rod #d mm4 in diameter and #e mm4 long be enclosed in an exhausted receiver and the current from #fjj ,bunsen's elements1 placed in parallel series of #ajj1 be passed through it1 the charcoal becomes strongly incandescent1 partially volatilises1 and is deposited in the form of graphite4 ,if sugar be placed in a charcoal crucible and a powerful galvanic current passed through it1 it is baked into a mass similar to graphite4 ,if charcoal be mixed with wrought iron and heated1 cast iron is formed1 which contains as much as five per cent4 of charcoal4 ,if molten cast iron be suddenly chilled1 the carbon remains in combination with the iron1 forming so called white cast iron2 but if the cooling proceeds slowly1 the greater part of the carbon separates in the form of graphite1 and if such cast iron 7so called grey cast iron7 be dissolved in acid1 the carbon remains in the form of graphite4 ,graphite is met with in nature1 sometimes in the form of large compact masses1 sometimes permeating rocky formations like the schists or slates1 and in fact is met with in those places which1 in all probability1 have been subjected to the action of subterranean heat4#ad ,the graphite in cast iron1 and sometimes also natural graphite1 occasionally appears in a crystalline form in the shape of six-sided plates1 but more often it occurs as a compact amorphous mass having the characteristic properties of the familiar black-lead pencil4#ae #ad ,there are places where anthracite gradually changes into graphite as the strata sink4 ,i myself had the opportunity of observing this gradual transformation in the valley of ,aosta4 #ae ,pencils are made of graphite worked up into a homogeneous mass by disintegrating1 powdering1 and cleansing it from earthy impurities2 the best kinds are made of completely homogeneous graphite sawn up into the requisite sticks4 ,graphite is found in many places4 ,in ,russia the so-called ,aliberoffsky graphite is particularly renowned2 it is found in the ,altai mountains near the ,chinese frontier2 in many places in ,finland and likewise on the banks of the ,little ,tungouska1 ,sidoroff also found a considerable quantity of graphite4 ,when mixed with clay1 graphite is used for making crucibles and pots for melting metals4 ,graphite1 like most forms of charcoal1 still contains a certain quantity of hydrogen1 oxygen1 and ash1 so that in its natural state it does not contain more than #ih p4c4 of carbon4 ,in practice1 graphite is purified simply by washing it when in a finely-ground state1 by which means the bulk of the earthy matter may be separated4 ,the following process1 proposed by ,brodie1 consists in mixing the powdered graphite with #a_/ad part of its weight of potassium chlorate4 ,the mixture is then heated with twice its weight of strong sulphuric acid until no more odoriferous gases are emitted2 on cooling1 the mixture is thrown into water and washed2 the graphite is then dried and heated to a red heat2 after this it shrinks considerably in volume and forms a very fine powder1 which is then washed4 ,by acting on graphite several times with a mixture of potassium chlorate and nitric acid heated up to #fj1 ,brodie transformed it into a yellow insoluble acid substance which he called graphitic acid1 ,c#aa;,h#d,o#e4 ,the diamond remains unchanged when subjected to this treatment1 whilst amorphous charcoal is completely oxidised4 ,availing himself of this possibility of distinguishing graphite from the diamond or amorphous charcoal1 ,berthelot showed that when compounds of carbon and hydrogen are decomposed by heat1 amorphous charcoal is mainly formed1 whilst when compounds of carbon with chlorine1 sulphur1 and boron are decomposed1 graphite is principally deposited4 ,the diamond is a crystalline and transparent form of carbon4 ,it is of rare occurrence in nature1 and is found in the alluvial deposits of the diamond mines of ,brazil1 ,india1 ,south ,africa1 c4 ,it has also been found in meteorites4#ae bis ,it crystallises in octahedra1 dodecahedra1 cubes1 and other forms of the regular system4#af ,the efforts which have been made to produce diamonds artificially1 although they have not been fruitless1 have not as yet led to the production of large-sized crystals1 because those means by which crystals are generally formed are inapplicable to carbon4 ,indeed1 carbon in all its forms being insoluble and infusible does not pass into a liquid condition by means of which crystallisation could take place4 ,diamonds have several times been successfully produced in the shape of minute crystals having the appearance of a black powder1 but when viewed under the microscope appearing transparent1 and possessing that hardness which is the peculiar characteristic of the diamond4 ,this diamond powder is deposited on the negative electrode1 when a weak galvanic current is passed through liquid chloride of carbon4#af bis #ae bis ,diamonds are found in a particular dense rock1 known by the name of itacolumite1 and are dug out of the dbris produced by the destruction of the itacolumite by water4 ,when the dbris is washed the diamonds remain behind2 they are principally found in ,brazil1 in the provinces of ,rio and ,bahia1 and at the ,cape of ,good ,hope4 ,the dbris gives the black or amorphous diamond1 carbonado1 and the ordinary colourless or yellow translucent diamond4 ,as the diamond possesses a very marked cleavage1 the first operation consists in splitting it1 and then roughly and finely polishing it with diamond powder4 ,it is very remarkable that ,professors ,p4 ,a4 ,latchinoff and ,erofeff found 7#ahhg7 diamond powder in a meteoric stone which fell in the ,government of ,penza1 in the district of ,krasnoslobodsk1 near the settlement of ,novo ,urei 7,sept4 #aj1 #ahhf74 ,up to that time charcoal and graphite 7a special variety1 cliftonite7 had been found in meteorites and the diamond only conjectured to occur therein4 ,the ,novo ,urei meteorite was composed of siliceous matter and metallic iron 7with nickel7 like many other meteorites4 #af ,diamonds are sometimes found in the shape of small balls1 and in that case it is impossible to cut them because directly the surface is ground or broken they fall into minute pieces4 ,sometimes minute diamond crystals form a dense mass like sugar1 and this is generally reduced to diamond powder and used for grinding4 ,some known varieties of the diamond are almost opaque and of a black colour4 ,such diamonds are as hard as the ordinary ones1 and are used for polishing diamonds and other precious stones1 and also for rock boring and tunnelling4 #af bis ,hannay1 in #ahhj1 obtained diamonds by heating a mixture of heavy liquid hydrocarbons 7paraffin oils7 with magnesium in a thick iron tube4 ,this investigation1 however1 was not repeated4 ,moissan 7,paris1 #ahic7 produced diamonds artificially by means of the high temperature attained in the electrical furnace#ag by dissolving carbon in molten cast iron1 and allowing the solution with an excess of carbon1 to cool under the powerful pressure exerted by rapidly cooling the metal4#ag bis ,k4 ,chroustchoff attained the same end by means of silver1 which dissolves carbon to the extent of #f p4c4 at a high temperature4 ,rousseau1 for the same purpose1 heated carbide of calcium in the electric furnace4 ,there is no doubt that all these investigators obtained the diamond as a transparent body1 which burnt into ,,co#b1 and possessed an exceptional hardness1 but only in the form of a fine powder4 #ag ,the electrical furnace is an invention of recent times1 and gives the possibility of obtaining a temperature of #c1ejj1 which is not only not obtainable in ordinary furnaces1 but even in the oxyhydrogen flame1 whose temperature does not exceed #b1jjj4 ,the electrical furnace consists of two pieces of lime1 laid one on the other4 ,a cavity is made in the lower piece for the reception of the substance to be melted between two thick electrodes of dense carbon4 ,on passing a current of #gj volts and #dej ampres a temperature of #c1jjj is easily obtained4 ,at a temperature of #b1ejj 7#ajj ampres and #dj volts7 not only do all metals melt1 but even lime and magnesia 7when placed in the space between the carbon electrodes1 i4e4 in the voltaic arc7 become soft and crystallise on cooling4 ,at #c1jjj lime becomes very fluid1 metallic calcium partially separates out and a carbon compound1 which remains liquid for a long time4 ,at this temperature oxide of uranium is reduced to the suboxide and metal1 zirconia and rock crystal fuse and partially volatilise1 as also does alumina2 platinum1 gold1 and even carbon distinctly volatilise2 the majority of the metals form carbides4 ,at such a temperature also cast iron and carbon give graphite1 while according to ,rousseau1 between #b1jjj and #c1jjj the diamond passes into graphite and conversely graphite into the diamond1 so that this is a kind of reversible reaction4 #ag bis ,moissan first investigated the solution of carbon in molten metals 7and the formation of the carbides7 such as magnesium1 aluminium1 iron1 manganese1 chromium1 uranium1 silver1 platinum1 and silicon4 ,at the same time ,friedel1 owing to the discovery of the diamond in meteoric iron1 admitted that the formation of the diamond is dependent upon the influence of iron and sulphur4 ,with this object1 that is to obtain the diamond1 ,friedel caused sulphur to react upon samples of cast iron rich in carbon1 in a closed vessel at a maximum temperature of #ejj1 and after dissolving the sulphide of iron formed1 he obtained a small quantity of a black powder which scratched corundum1 i4e4 diamond4 ,moissan's experiments 7#ahic7 were more successful1 probably owing to his having employed the electrical furnace4 ,if iron be saturated with carbon at a temperature between #a1ajj and #c1jjj1 then at #a1ajj-a1bjj a mixture of amorphous carbon and graphite is formed1 while at #c1jjj graphite alone is obtained in very beautiful crystals4 ,thus under these conditions the diamond is not formed1 and it can only be obtained if the high temperature be aided by powerful pressures4 ,for this purpose ,moissan took advantage of the pressure produced in the passage of a mass of molten cast iron from a liquid into a solid state4 ,he first melted #aej-bjj grams of iron in the electrical furnace1 and quickly introduced a cylinder of carbon into the molten iron4 ,he then removed the crucible with the molten iron from the furnace and plunged it into a reservoir containing water4 ,after treating with boiling hydrochloric acid1 three varieties of carbon were obtained3 7#a7 a small amount of graphite 7if the cooling be rapid72 7#b7 carbon of a chestnut colour in very fine twisted threads1 showing that it had been subjected to a very high pressure 7a similar variety was met with in various samples of the ,canon ,diabolo71 and lastly 7#c7 an inconsiderable quantity of an exceeding dense mass which was freed from the admixture of the lighter modifications by treatment with aqua regia1 sulphuric and hydrofluoric acids1 and from which ,moissan1 by means of liquid bromoform 7sp4 gr4 #bijj71 succeeded in separating some small pieces1 having a greater density than bromoform1 which scratched the ruby and had the properties of the diamond4 ,some of these pieces were black1 others were transparent and refracted light strongly4 ,the dark grey tint of the former resembled that of the black diamonds 7carbonado74 ,their density was between #c and #ce4 ,the transparent specimens had a greasy appearance and seemed to be1 as it were1 surrounded by an envelope of carbon4 ,at #a1jej they did not burn entirely in a current of air1 so that the imperfectly burnt particles1 and a peculiar form of grains of a light ochre colour1 which retained their crystalline form1 could be examined under the microscope4 ,similar grains also remain after the imperfect combustion of the ordinary diamond4 ,moissan obtained the same results by rapidly cooling in a stream of coal gas a piece of cast iron1 saturated with carbon obtained from sugar and first heated to #b1jjj4 ,in this instance he obtained small crystals of diamonds4 ,k4 ,chroustchoff showed that at its boiling point silver dissolves #f p4c4 of carbon4 ,this silver was rapidly cooled1 so that a crust formed on the surface and prevented the metal expanding1 and so produced a powerful pressure4 ,a portion of the carbon which separates out under these conditions exhibits the properties of the diamond4 ,judging from the fact that carbon forms a number of gaseous bodies 7carbonic oxide1 carbonic anhydride1 methane1 ethylene1 acetylene1 c47 and volatile substances 7for example1 many hydrocarbons and their most simple derivatives71 and considering that the atomic weight of carbon1 ,c #ab1 approaches that of nitrogen1 ,n #ad1 and that of oxygen1 ,o #af1 and that the compounds ,,co 7carbonic oxide7 and ,n#b;,c#b 7cyanogen7 are gases1 it may be argued that if carbon formed the molecule ,c#b1 like ,n#b and ,o#b1 it would be a gas4 ,and as through polymerism or the combination of like molecules 7as ,o#b passes into ,o#c or ,,no#b into ,n#b,o#d7 the temperatures of ebullition and fusion rise 7which is particularly clearly proved with the hydrocarbons of the ,cn,h#bn series71 it ought to be considered that the molecules of charcoal1 graphite1 and the diamond are very complex1 seeing that they are insoluble1 non-volatile1 and infusible4 ,the aptitude which the atoms of carbon show for combining together and forming complex molecules appears in all carbon compounds4 ,among the volatile compounds of carbon many are well known the molecules of which contain ,c#e 444 ,c#aj 444 ,c#bj 444 ,c#cj1 c41 in general ,cn where n may be very large1 and in none of the other elements is this faculty of complexity so developed as in carbon4#ah ,up to the present time there are no grounds for determining the degree of polymerism of the charcoal1 graphite1 or diamond molecules1 and it can only be supposed that they contain ,cn where n is a large quantity4 ,charcoal and those complex non-volatile organic substances which represent the gradual transitions to charcoal#ai and form the principal solid substances of organisms1 contain a store or accumulation of internal power in the form of the energy binding the atoms into complex molecules4 ,when charcoal or complex compounds of carbon burn1 the energy of the carbon and oxygen is turned into heat1 and this fact is taken advantage of at every turn for the generation of heat from fuel4#bj #ah ,the existence of a molecule ,s#f is known 7up to #fjj71 and it must he beld that this accounts for the formation of hydrogen persulphide1 ,h#b,s#e4 ,phosphorus appears in the molecule ,p#d and gives ,p#d;,h#b4 ,when expounding the data on specific heat we shall have occasion to return to the question of the complexity of the carbon molecule4 #ai ,the hydrocarbons poor in hydrogen and containing many atoms of carbon1 like chrysene and carbopetrocene1 c41 ,cn,h#b7n-m71 are solids1 and less fusible as n and m increase4 ,they present a marked approach to the properties of the diamond4 ,and in proportion to the diminution of the water in the carbohydrates ,cn,h#bm,om--for example in the humic compounds 7,note #e7--the transition of complex organic substances to charcoal is very evident4 ,that residue resembling charcoal and graphite which is obtained by the separation 7by means of copper sulphate and sodium chloride7 of iron from white cast-iron containing carbon chemically combined with the iron1 also seems1 especially after the researches of ,g4 ,a4 ,zaboudsky1 to be a complex substance containing ,c#ab;,h#f,o#c4 ,the endeavours which have been directed towards determining the measure of complexity of the molecules of charcoal1 graphite1 and the diamond will probably at some period lead to the solution of this problem and will most likely prove that the various forms of charcoal1 graphite1 and the diamond contain molecules of different and very considerable complexity4 ,the constancy of the grouping of benzene1 ,c#f;,h#f1 and the wide diffusion and facility of formation of the carbohydrates containing ,c#f 7for example1 cellulose1 ,c#f;,h#aj,o#e1 glucose1 ,c#f;,h#ab,o#f7 give reason for thinking that the group ,c#f is the first and simplest of those possible to free carbon1 and it may be hoped that some time or other it may be possible to get carbon in this form4 ,perhaps in the diamond there may be found such a relation between the atoms as in the benzene group1 and in charcoal such as in carbohydrates4 #bj ,when charcoal burns1 the complex molecule ,cn is resolved into the simple molecules n,,co#b1 and therefore part of the heat--probably no small amount--is expended in the destruction of the complex molecule ,cn4 ,perhaps by burning the most complex substances1 which are the poorest as regards hydrogen1 it may be possible to form an idea of the work required to split up ,cn into separate atoms4 ,no other two elements are capable of combining together in such variety as carbon and hydrogen4 ,the hydrocarbons of the ,cn,h#bm series in many cases differ widely from each other1 although they have some properties in common4 ,all hydrocarbons1 whether gaseous1 liquid or solid1 are combustible substances sparingly soluble or insoluble in water4 ,the liquefied gaseous hydrocarbons1 as well as those which are liquid at ordinary temperatures1 and those solid hydrocarbons which have been liquefied by fusion1 have the appearance and property of oily liquors1 more or less viscid1 or fluid4#ba ,the solid hydrocarbons more or less resemble wax in their properties1 although ordinary oils and wax generally contain oxygen in addition to carbon and hydrogen1 but in relatively small proportion4 ,there are also many hydrocarbons which have the appearance of tar--as1 for instance1 metacinnamene and gutta-percha4 ,those liquid hydrocarbons which boil at a high temperature are like oils1 and those which have a low boiling point resemble ether1 whilst the gaseous hydrocarbons in many of their properties are akin to hydrogen4 ,all this tends to show that in hydrocarbons physically considered the properties of solid non-volatile charcoal are strongly modified and hidden1 whilst those of the hydrogen predominate4 ,all hydrocarbons are neutral substances 7neither basic nor acid71 but under certain conditions they enter into peculiar reactions4 ,it has been seen in those hydrogen compounds which have been already considered 7water1 nitric acid1 ammonia7 that the hydrogen in almost all cases enters into reaction1 being displaced by metals4 ,the hydrogen of the hydrocarbons1 it may be said1 has no metallic character that is to say1 it is not directly#bb displaced by metals1 even by such as sodium and potassium4 ,on the application of more or less heat all hydrocarbons decompose#bc forming charcoal and hydrogen4 ,the majority of hydrocarbons do not combine with the oxygen of the air or oxidise at ordinary temperatures1 but under the action of nitric acid and many other oxidising substances most of them undergo oxidation1 in which either a portion of the hydrogen and carbon is separated1 or the oxygen enters into combination1 or else the elements of hydrogen peroxide enter into combination with the hydrocarbon4#bd ,when heated in air1 hydrocarbons burn1 and1 according to the amount of carbon they contain1 their combustion is attended more or less with a separation of soot--that is1 finely divided charcoal--which imparts great brilliancy to the flame1 and on this account many of them are used for the purposes of illumination--as1 for instance1 kerosene1 coal gas1 oil of turpentine4 ,as hydrocarbons contain reducing elements 7that is1 those capable of combining with oxygen71 they often act as reducing agents--as1 for instance1 when heated with oxide of copper1 they burn1 forming carbonic anhydride and water1 and leave metallic copper4 ,gerhardt proved that all hydrocarbons contain an even number of hydrogen atoms4 ,therefore1 the general formula for all hydrocarbons is ,cn,h#bm where n and m are whole numbers4 ,this fact is known as the law of even numbers4 ,hence1 the simplest possible hydrocarbons ought to be3 ,,ch#b1 ,,ch#d1 ,,ch#f 444 ,c#b;,h#b1 ,c#b;,h#d1 ,c#b;,h#f1 ,c#b;,h#h 444 but they do not all exist1 since the quantity of ,h which can combine with a certain amount of carbon is limited1 as we shall learn directly4 #ba ,the viscosity1 or degree of mobility1 of liquids is determined by their internal friction4 ,it is estimated by passing the liquids through narrow 7capillary7 tubes1 the mobile liquids passing through with greater facility and speed than the viscid ones4 ,the viscosity varies with the temperature and nature of the liquids1 and in the case of solutions changes with the amount of the substance dissolved1 but is not proportional to it4 ,so that1 for example1 with alcohol at #bj the viscosity will be #fi1 and for a #ej p4c4 solution #afj1 the viscosity of water being taken as #ajj4 ,the volume of the liquid which passes through by experiment 7,poiseuille7 and theory 7,stokes7 is proportional to the time1 the pressure1 and the fourth power of the diameter of the 7capillary7 tube1 and inversely proportional to the length of the tube2 this renders it possible to form comparative estimates of the coefficients of internal friction and viscosity4 ,as the complexity of the molecules of hydrocarbons and their derivatives increases by the addition of carbon 7or ,,ch#b71 so does the degree of viscosity also rise4 ,the extensive series of investigations referring to this subject still await the necessary generalisation4 ,that connection which 7already partly observed7 ought to exist between the viscosity and the other physical and chemical properties1 forces us to conclude that the magnitude of internal friction plays an important part in molecular mechanics4 ,in investigating organic compounds and solutions1 similar researches ought to stand foremost4 ,many observations have already been made1 but not much has yet been done with them2 the bare facts and some mechanical data exist1 but their relation to molecular mechanics has not been cleared up in the requisite degree4 ,it has already been seen from existing data that the viscosity at the temperature of the absolute boiling point becomes as small as in gases4 #bb ,in a number of hydrocarbons and their derivatives such a substitution of metals for the hydrogen may be attained by indirect means4 ,the property shown by acetylene1 ,c#b;,h#b1 and its analogues1 of forming metallic derivatives is in this respect particularly characteristic4 ,judging from the fact that carbon is an acid element 7that is1 gives an acid anhydride with oxygen71 though comparatively slightly acid 7for carbonic acid is not at all a strong acid and compounds of chlorine and carbon1 even ,c,cl#d1 are not decomposed by water as is the case with phosphorus chloride and even silicic chloride and boric chloride1 although they correspond with acids of but little energy71 one might expect to find in the hydrogen of hydrocarbons this faculty for being substituted by metals4 ,the metallic compounds which correspond with hydrocarbons are known under the name of organo-metallic compounds4 ,such1 for instance1 is zinc ethyl1 ,zn7,c#b;,h#e7b1 which corresponds with ethyl hydride or ethane1 ,c#b;,h#f1 in which two atoms of hydrogen have been exchanged for one of zinc4 #bc ,gaseous and volatile hydrocarbons decompose when passed through a heated tube4 ,when hydrocarbons are decomposed by heating1 the primary products are generally other more stable hydrocarbons1 among which are acetylene1 ,c#b;,h#b1 benzene1 ,c#f;,h#f1 naphthalene1 ,c#aj;,h#h1 c4 #bd ,wagner 7#ahhh7 showed that when unsaturated hydrocarbons are shaken with a weak 7#a p4c47 solution of potassium permanganate1 ,k,mn,o#d1 at ordinary temperatures1 they form glycols--for example1 ,c#b;,h#d yields ,c#b;,h#f,o#b4 ,some of the hydrocarbons are capable of combination1 whilst others do not show that power4 ,those which contain less hydrogen belong to the former category1 and those which1 for a given quantity of carbon1 contain the maximum amount of hydrogen1 belong to the latter4 ,the composition of those last mentioned is expressed by the general formula ,cn,h#bn #b4 ,these so-called saturated hydrocarbons are incapable of combination4#be ,the hydrocarbons ,,ch#f1 ,c#b;,h#h1 ,c#c;,h#aj1 c4444 do not exist4 ,those containing the maximum amount of hydrogen will be represented by ,,ch#d 7n #a1 #bn #b #d71 ,c#b;,h#f 7n #b71 ,c#c;,h#h 7n #c71 ,c#d;,h#aj1 c4 ,this may be termed the law of limits4 ,placing this in juxtaposition with the law of even numbers1 it is easy to perceive that the possible hydrocarbons can be ranged in series1 the terms of which may be expressed by the general formul ,cn,h#bn#b1 ,cn,h#bn1 ,cn,h#bn-#b1 c4444 ,those hydrocarbons which belong to any one of the series expressible by a general formula are said to be homologous#j with one another4 ,thus1 the hydrocarbons ,,ch#d1 ,c#b;,h#f1 ,c#c;,h#h1 ,c#d;,h#aj1 c4444 are members of the limiting 7saturated7 homologous series ,cn,h#bn#b4 ,that is1 the difference between the members of the series is ,,ch#b4bf ,not only the composition but also the properties of the members of a series tend to classification in one group4 ,for instance1 the members of the series ,cn,h#bn#b are not capable of forming additive compounds1 whilst those of the series ,cn,h#bn are capable of combining with chlorine1 sulphuric anhydride1 c42 and the members of the ,cn,h#bn-#f group1 belonging to the coal tar series1 are easily nitrated 7give nitro-compounds1 ,chapter ,,vi471 and have other properties in common4 ,the physical properties of the members of a given homologous series vary in some such manner as this2 the boiling point generally rises and the internal friction increases as n increases#bg--that is1 with an increase in the relative amount of carbon and the atomic weight2 the specific gravity also regularly changes as n becomes greater4#bh #be ,my article on this subject appeared in the ,journal of the ,st4 ,petersburg ,academy of ,sciences in #ahfa4 ,up to that time1 although many additive combinations with hydrocarbons and their derivatives were known1 they had not been generalised1 and were even continually quoted as cases of substitution4 ,thus the combination of ethylene1 ,c#b;,h#d1 with chlorine1 ,cl#b1 was often regarded as a formation of the products of the substitution of ,c#b;,h#e;,cl and ,h,cl1 which it was supposed were held together as the water of crystallisation is in salts4 ,even earlier than this 7#aheg1 ,journal of the ,petroffsky ,academy7 ,i considered similar cases as true compounds4 ,in general1 according to the law of limits1 an unsaturated hydrocarbon1 or its derivative1 on combining with r,x#b1 gives a substance which is saturated or else approaching the limit4 ,the investigations of ,frankland with many organo-metallic compounds clearly showed the limit in the case of metallic compounds1 which we shall constantly refer to later on4 #bf ,the conception of homology has been applied by ,gerhardt to all organic compounds in his classical work1 ',trait de ,chimie ,organique1' finished in #ahee 7#d vols471 in which he divided all organic compounds into fatty and aromatic1 which is in principle still adhered to at the present time1 although the latter are more often called benzene derivatives1 on account of the fact that ,kekul1 in his beautiful investigations on the structure of aromatic compounds1 showed the presence in them all of the 'benzene nucleus1' ,c#f;,h#f4 #bg ,this is always true for hydrocarbons1 but for derivatives of the lower homologues the law is sometimes different2 for instance1 in the series of saturated alcohols1 ,cn,h#bn#a7,,oh71 when n #j1 we obtain water1 ,h7,,oh71 which boils at #ajj1 and whose specific gravity at #ae #jiiib2 when n #a1 wood spirit ,,ch#c7,,oh71 which boils at #ff1 and at #ae has a specific gravity #jgifd2 when n #b1 ordinary alcohol1 ,c#b;,h#e7,,oh71 boiling at #gh1 specific gravity at #ae #jgicf1 and with further increase of ,,ch#b the specific gravity increases4 ,for the glycols ,cn,h#bn7,,oh7#b the phenomenon of a similar kind is still more striking2 at first the temperature of the boiling point and the density increase1 and then for higher 7more complex7 members of the series diminish4 ,the reason for this phenomenon1 it is evident1 must be sought for in the influence and properties of water1 and that strong affinity which1 acting between hydrogen and oxygen1 determines many of the exceptional properties of water 7,chapter ,i474 #bh ,as1 for example1 in the saturated series of hydrocarbons ,cn,h#bn#b1 the lowest member 7n #j7 must be taken as hydrogen ,h#b1 a gas which 7t4c4 below -#aij7 is liquefied with great difficulty1 and when in a liquid state has doubtless a very small density4 ,where n #a1 #b1 #c1 the hydrocarbons ,,ch#d1 ,c#b;,h#f1 ,c#c;,h#h are gases1 more and more readily liquefiable4 ,the temperature of the absolute boiling point for ,,ch#d -#ajj1 and for ethane ,c#b;,h#f1 and in the higher members it rises4 ,the hydrocarbon ,c#d;,h#aj1 liquefies at about #j4 ,c#e;,h#ab 7there are several isomers7 boils at from #i 7,lvoff7 to #cg1 ,c#f;,h#ad from #eh to #gh1 c4 ,the specific gravities in a liquid state at #ae are3-- ,c#e;,h#ab ,c#f;,h#ad ,c#g;,h#af ,c#aj;,h#bb ,c#af;,h#cd #jfc #jff #jgj #jge #jhe ,many of the hydrocarbons met with in nature are the products of organisms1 and do not belong to the mineral kingdom4 ,a still greater number are produced artificially4 ,these are formed by what is termed the combination of residues4 ,for instance1 if a mixture of the vapours of hydrogen sulphide and carbon bisulphide be passed through a tube in which copper is heated1 this latter absorbs the sulphur from both the compounds1 and the liberated carbon and hydrogen combine to form a hydrocarbon1 methane4 ,if carbon be combined with any metal and this compound ,,mcn be treated with an acid ,,hx1 then the haloid ,x will give a salt with the metal and the residual carbon and hydrogen will give a hydrocarbon4 ,thus cast iron which contains a compound of iron and carbon gives liquid hydrocarbons like naphtha under the action of acids4 ,if a mixture of bromo-benzene1 ,c#f;,h#e;,br1 and ethyl bromide1 ,c#b;,h#e;,br1 be heated with metallic sodium1 the sodium combines with the bromine of both compounds1 forming sodium bromide1 ,na,br4 ,from the first combination the group ,c#f;,h#e remains1 and from the second ,c#b;,h#e4 ,having an odd number of hydrogen atoms1 they1 in virtue of the law of even numbers1 cannot exist alone1 and therefore combine together forming the compound ,c#f;,h#e4;,c#b;,h#e or ,c#h;,h#aj 7ethylbenzene74 ,hydrocarbons are also produced by the breaking up of more complex organic or hydrocarbon compounds1 especially by heating--that is1 by dry distillation4 ,for instance1 gum-benzoin contains an acid called benzoic acid1 ,c#g;,h#f,o#b1 the vapours of which1 when passed through a heated tube1 split up into carbonic anhydride1 ,,co#b1 and benzene1 ,c#f;,h#f4 ,carbon and hydrogen only unite directly in one ratio of combination--namely1 to form acetylene1 having the composition ,c#b;,h#b1 which1 as compared with other hydrocarbons1 exhibits a very great stability at a somewhat high temperature4#bi #bi ,if1 at the ordinary temperature 7assuming therefore that the water formed will be in a liquid state7 a gram molecule 7#bf grams7 of acetylene1 ,c#b;,h#b1 be burnt1 #caj thousand calories will be emitted 7,thomsen71 and as #ab grams of charcoal produce #ig thousand calories1 and #b grams of hydrogen #fi thousand calories1 it follows that1 if the hydrogen and carbon of the acetylene were burnt there would be only #b #ig #fi1 or #bfc thousand calories produced4 ,it is evident1 then1 that acetylene in its formation absorbs #caj-bfc1 or #dg thousand calories4 ,for considerations relative to the combustion of carbon compounds1 we will first enumerate the quantity of heat separated by the combustion of definite chemical carbon compounds1 and then give a few figures bearing on the kinds of fuel used in practice4 ,for molecular quantities in perfect combustion the following amounts of heat are given out 7when gaseous carbonic anhydride and liquid water are formed71 according to ,thomsen's data 7#a7 for gaseous ,cn,h#bn #b3 #ebh #aehhn thousand calories2 7#b7 for ,cn,h#bn3 #agg #aehan thousand calories2 7#c7 according to ,stohmann 7#ahhh7 for liquid saturated alcohols1 ,cn,h#bn #b,o3 #aah #aefcn1 and as the latent heat of evaporation about #hb #jfn1 in a gaseous state1 #bjj #aefin2 7#d7 for monobasic saturated liquid acids1 ,cn,h#bn,o#b3--iec #aedcn1 and as their latent heat of evaporation is about #ej #abn1 in a gaseous form1 about--#ij #aeen2 7#e7 for solid saturated bibasic acids1 ,cn,h#bn-#b,o#d3--bech #aebfn1 if they are expressed as ,cn,h#bn,c#b;,h#b,o#d1 then #ead #aebfn2 7#f7 for benzene and its liquid homologues 7still according to ,stohmann7 ,cn,h#bn-#f3--aehf #aefcn1 and in a gaseous form about--#aee #aegn2 7#g7 for the gaseous homologues of acetylene1 ,cn,h#bn-#b 7according to ,thomsen7--#e #aegn4 ,it is evident from the preceding figures that the group ,,ch#b1 or ,,ch#c substituted for ,h1 on burning gives out from #aeb to #aei thousand calories4 ,this is less than that given out by ,c ,h#b1 which is #ig #fi or #aff thousand2 the reason for this difference 7it would be still greater if carbon were gaseous7 is the amount of heat separated during the formation of ,,ch#b4 ,according to ,stohmann1 for dextroglucose1 ,c#f;,h#ab,o#f1 it is #fgcg2 for common sugar1 ,c#ab;,h#bb,o#aa1 #acbeg2 for cellulose1 ,c#f;,h#aj,o#e1 #fghj2 starch1 #fgge2 dextrin1 #fffb2 glycol1 ,c#b;,h#f,o#b1 #bhag2 glycerine1 #cigb1 c4 ,the heat of combustion of the following solids 7determined by ,stohmann7 is expressed per unit of weight3 naphthalene1 ,c#aj;,h#h1 #i1fba2 urea1 ,,cn#b;,h#d,o1 #b1dfe2 white of egg1 #e1egi2 dry rye bread1 #d1dba2 wheaten bread1 #d1cjb2 tallow1 #i1cfe2 butter1 #i1aib2 linseed oil1 #i1cbc4 ,the most complete collection of arithmetical data for the heats of combustion will be found in ,v4 ,f4 ,longinin's work1 ',description of the ,various ,methods of ,determining the ,heats of ,combustion of ,organic ,compounds' 7,moscow1 #ahid74 ,the number of units of heat given out by unit weight during the complete combustion and cooling of the following ordinary kinds of fuel in their usual state of dryness and purity are3--7#a7 for wood charcoal1 anthracite1 semi-anthracite1 bituminous coal and coke1 from #g1bjj to #h1bjj2 7#b7 dry1 long flaming coals1 and the best brown coals1 from #f1bjj to #f1hjj2 7#c7 perfectly dry wood1 #c1ejj2 hardly dry1 #b1ejj2 7#d7 perfectly dry peat1 best kind1 #d1ejj2 compressed and dried1 #c1jjj2 7#e7 petroleum refuse and similar liquid hydrocarbons1 about #aa1jjj2 7#f7 illuminating gas of the ordinary composition 7about #de vols4 ,h1 #dj vols4 ,,ch#d1 #e vols4 ,,co1 and #e vols4 ,n71 about #ab1jjj2 7#g7 producer gas 7see next ,chapter71 containing #b vols4 carbonic anhydride1 #cj vols4 carbonic oxide1 and #fh vols4 nitrogen for one part by weight of the whole carbon burnt1 #e1cjj1 and for one part by weight of the gas1 #iaj1 units of heat2 and 7#h7 water gas 7see next chapter7 containing #d vols4 carbonic anhydride1 #h vols4 ,n#b1 #bd vols4 carbonic oxide1 and #df vols4 ,h#b1 for one part by weight of the carbon consumed in the generator #aj1ijj1 and for one part by weight of the gas1 #c1fjj units of heat4 ,in these figures1 as in all calorimetric observations1 the water produced by the combustion of the fuel is supposed to be liquid4 ,as regards the temperature reached by the fuel1 it is important to remark that for solid fuel it is indispensable to admit 7to ensure complete combustion7 twice the amount of air required1 but liquid1 or pulverised fuel1 and especially gaseous fuel1 does not require an excess of air2 therefore1 a kilogram of charcoal1 giving #h1jjj units of heat1 requires about #bd kilograms of air 7#c kilograms of air per thousand calories7 and a kilogram of producer gas requires only #jgg kilogram of air 7#jhe kilo4 of air per #a1jjj calories71 #a kilogram of water gas about #de of air 7#abe kilo4 of air per #a1jjj calories74 ,there is one substance known among the saturated hydrocarbons composed of #a atom of carbon and #d atoms of hydrogen2 this is a compound containing the highest percentage of hydrogen 7,,ch#d contains #be per cent4 of hydrogen71 and at the same time it is the only hydrocarbon whose molecule contains but a single atom of carbon4 ,this saturated hydrocarbon1 ,,ch#d1 is called marsh gas or methane4 ,if vegetable or animal refuse suffers decomposition in a space where the air has not free access1 or no access at all1 then the decomposition is accompanied with the formation of marsh gas1 and this either at the ordinary temperature1 or at a comparatively much higher one4 ,on this account plants1 when decomposing under water in marshes1 give out this gas4#bi bis ,it is well known that if the mud in bogs be stirred up1 the act is accompanied with the evolution of a large quantity of gas bubbles2 these may1 although slowly1 also separate of their own accord4 ,the gas which is evolved consists principally of marsh gas4#cj ,if wood1 coal1 or many other vegetable or animal substances are decomposed by the action of heat without access of air--that is1 are subjected to dry distillation--they1 in addition to many other gaseous products of decomposition 7carbonic anhydride1 hydrogen1 and various other substances71 evolve a great deal of methane4 ,generally the gas which is used for lighting purposes is obtained by this means and therefore always contains marsh gas1 mixed with dry hydrogen and other vapours and gases1 although it is subsequently purified from many of them4#ca ,as the decomposition of the organic matter1 which forms coal1 is still going on underground1 the evolution of large quantities of marsh gas frequently occurs in coal-mines4#cb ,when mixed with air it forms an explosive mixture1 which forms one of the great dangers of coal mining1 as subterranean work has always to be carried on by lamp-light4 ,this danger is1 however1 overcome by the use of ,humphry ,davy's safety lamp4#cc ,sir ,humphry ,davy observed that on introducing a piece of wire gauze into a flame1 it absorbs so much heat that combustion does not proceed beyond it 7the unburnt gases which pass through it may be ignited on the other side74 ,in accordance with this1 the flame of the ,davy lamp is surrounded with a thick glass 7as shown in the drawing71 and has no communication whatever with the explosive mixture except through a wire gauze which prevents it igniting the mixture of the marsh-gas issuing from the coal with air4 ,in some districts1 particularly in those where petroleum is found--as1 for instance1 near ,baku1 where a temple of the ,indian fire-worshippers was built1 and in ,pennsylvania1 and other places--marsh gas in abundance issues from the earth1 and it is used1 like coal gas1 for the purposes of lighting and warming4#cd ,tolerably pure marsh gas#ce may be obtained by heating a mixture of an acetate with an alkali4 ,acetic acid1 ,c#b;,h#d,o#b1 on being heated is decomposed into marsh gas and carbonic anhydride1 ,c#b;,h#d,o#b ,,ch#d ,,co#b4 #bi bis ,manure which decomposes under the action of bacteria gives off ,,co#b and ,,ch#d4 #cj ,it is easy to collect the gas which is evolved in marshy places if a glass bottle be inverted in the water and a funnel put into it 7both filled with water72 if the mud of the bottom be now agitated1 the bubbles which rise may be easily caught by the inverted funnel4 #ca ,illustration3 ,,fig4 #eh4--;,general view of gas works4 ,b1 retorts2 f1 hydraulic main2 ,h and ,i1 tar well2 i1 condensers2 ,l1 purifiers2 ,p1 gasholder4 ,illuminating gas is generally prepared by heating gas coal 7see ,note #f7 in oval cylindrical horizontal cast-iron or clay retorts4 ,several such retorts ,,bb 7fig4 #eh7 are disposed in the furnace ,a1 and heated together4 ,when the retorts are heated to a red heat1 lumps of coal are thrown into them1 and they are then closed with a closely fitting cover4 ,the illustration shows the furnace1 with five retorts4 ,coke 7see ,note #a1 dry distillation7 remains in the retorts1 and the volatile products in the form of vapours and gases travel along the pipe d1 rising from each retort4 ,these pipes branch above the stove1 and communicate with the receiver f 7hydraulic main7 placed above the furnace4 ,those products of the dry distillation which most easily pass from the gaseous into the liquid and solid states collect in the hydraulic main4 ,from the hydraulic main the vapours and gases travel along the pipe g and the series of vertical pipes j 7which are sometimes cooled by water trickling over the surface71 where the vapours and gases cool from the contact of the colder surface1 and a fresh quantity of vapour condenses4 ,the condensed liquids pass from the pipes g and j and into the troughs ,h4 ,these troughs always contain liquid at a constant level 7the excess flowing away7 so that the gas cannot escape1 and thus they form1 as it is termed1 a hydraulic joint4 ,in the state in which it leaves the condensers the gas consists principally of the following vapours and gases3 7#a7 vapour of water1 7#b7 ammonium carbonate1 7#c7 liquid hydrocarbons1 7#d7 hydrogen sulphide1 ,h#b,s1 7#e7 carbonic anhydride1 ,,co#b1 7#f7 carbonic oxide1 ,,co1 7#g7 sulphurous anhydride1 ,,so#b1 but a great part of the illuminating gas consists of 7#h7 hydrogen1 7#i7 marsh gas1 7#aj7 olefiant gas1 ,c#b;,h#d1 and other gaseous hydrocarbons4 ,the hydrocarbons 7#c1 #i1 and #aj71 the hydrogen1 and carbonic oxide are capable of combustion1 and are useful component parts1 but the carbonic anhydride1 the hydrogen sulphide1 and sulphurous anhydride1 as well as the vapours of ammonium carbonate1 form an injurious admixture1 because they do not burn 7,,co#b1 ,,so#b7 and lower the temperature and brilliancy of the flame1 or else1 although capable of burning 7for example1 ,h#b,s1 ,,cs#b1 and others71 they give out during combustion sulphurous anhydride which has a disagreeable smell1 is injurious when inhaled1 and spoils many surrounding objects4 ,in order to separate the injurious products1 the gas is washed with water1 a cylinder 7not shown in the illustration7 filled with coke continually moistened with water serving for this purpose4 ,the water coming into contact with the gas dissolves the ammonium carbonate2 hydrogen sulphide1 carbonic anhydride1 and sulphurous anhydride1 being only partly soluble in water1 have to be got rid of by a special means4 ,for this purpose the gas is passed through moist lime or other alkaline liquid1 as the above-mentioned gases have acid properties and are therefore retained by the alkali4 ,in the case of lime1 calcium carbonate1 sulphite and sulphide1 all solid substances1 are formed4 ,it is necessary to renew the purifying material as its absorbing power decreases4 ,a mixture of lime and sulphate of iron1 ,fe,s,o#d1 acts still better1 because the latter1 with lime1 ,ca7,,ho7#b1 forms ferrous hydroxide1 ,fe7,,ho7#b and gypsum1 ,ca,s,o#d4 ,the suboxide 7partly turning into oxide7 of iron absorbs ,h#b,s1 forming ,fe,s and ,h#b,o1 and the gypsum retains the remainder of the ammonia1 the excess of lime absorbing carbonic anhydride and sulphuric anhydride4 ,in ,english works a native hydrated ferric hydroxide is used for removing hydrogen sulphide4 ,this purification of the gas takes place in the apparatus ,l1 where the gas passes through perforated trays m1 covered with sawdust mixed with lime and sulphate of iron4 ,it is necessary to remark that in the manufacture of gas it is indispensable to draw off the vapours from the retorts1 so that they should not remain there long 7otherwise the hydrocarbons would in a considerable degree be resolved into charcoal and hydrogen71 and also to avoid a great pressure of gas in the apparatus1 otherwise a quantity of gas would escape at all cracks such as must inevitably exist in such a complicated arrangement4 ,for this purpose there are special pumps 7exhausters7 so regulated that they only pump off the quantity of gas formed 7the pump is not shown in the illustration74 ,the purified gas passes through the pipe n into the gasometer 7gasholder7 ,p1 a dome made of iron plate4 ,the edges of the dome dip into water poured into a ring-shaped channel g1 in which the sides of the dome rise and fall4 ,the gas is collected in this holder1 and distributed to its destination by pipes communicating with the pipe o1 issuing from the dome4 ,the pressure of the dome on the gas enables it1 on issuing from a long pipe1 to penetrate through the small aperture of the burner4 ,a hundred kilograms of coal give about #bj to #cj cubic metres of gas1 having a density from four to nine times greater than that of hydrogen4 ,a cubic metre 7#a1jjj litres7 of hydrogen weighs about #hg grams2 therefore #ajj kilograms of coal give about #ah kilograms of gas1 or about one-sixth of its weight4 ,illuminating gas is generally lighter than marsh gas1 as it contains a considerable amount of hydrogen1 and is only heavier than marsh gas when it contains much of the heavier hydrocarbons4 ,thus olefiant gas1 ,c#b;,h#d1 is fourteen times1 and the vapours of benzene thirty-nine times1 heavier than hydrogen1 and illuminating gas sometimes contains #ae p4c4 of its volume of them4 ,the brilliancy of the flame of the gas increases with the quantity of olefiant gas and similar heavy hydrocarbons1 as it then contains more carbon for a given volume and a greater number of carbon particles are separated4 ,gas usually contains from #ce to #fj p4c4 of its volume of marsh gas1 from #cj to #ej p4c4 of hydrogen1 from #c to #e p4c4 of carbonic oxide1 from #b to #aj p4c4 heavy hydrocarbons1 and from #c to #aj p4c4 of nitrogen4 ,wood gives almost the same sort of gas as coal and almost the same quantity1 but the wood gas contains a great deal of carbonic anhydride1 although on the other hand there is an almost complete absence of sulphur compounds4 ,tar1 oils1 naphtha1 and such materials furnish a large quantity of good illuminating gas4 ,an ordinary burner of #h to #ab candle-power burns #e to #f cubic feet of coal gas per hour1 but only #a cubic foot of naphtha gas4 ,one pood 7#cf lbs4 ,eng47 of naphtha gives #ejj cubic feet of gas--that is1 one kilogram of naphtha produces about one cubic metre of gas4 ,the formation of combustible gas by heating coal was discovered in the beginning of the last century1 but only put into practice towards the end by ,le-,bon in ,france and ,murdoch in ,england4 ,in ,england1 ,murdoch1 together with the renowned ,watt1 built the first gas works in #ahje4 ,illustration3 ,,fig4 #ei4--;,blowpipe4 ,air is blown in at the trumpet-shaped mouthpiece1 and escapes in a fine stream from the platinum jet placed at the extremity of the side tube4 ,illustration3 ,,fig4 #fj4--;,davy safety-lamp4 ,modern form4 ,in practice illuminating gas is not only used for lighting 7electricity and kerosene are cheaper in ,russia71 but also as the motive power for gas engines 7see p4 #age71 which consume about half a cubic metre per horse-power per hour2 gas is also used in laboratories for heating purposes4 ,when it is necessary to concentrate the heat1 either the ordinary blowpipe 7fig4 #ei7 is applied1 placing the end in the flame and blowing through the mouthpiece2 or1 in other forms1 gas is passed through the blowpipe2 when a large1 hot1 smokeless flame is required for heating crucibles or glass-blowing1 a foot-blower is used4 ,high temperatures1 which are often required for laboratory and manufacturing purposes1 are most easily attained by the use of gaseous fuel 7illuminating gas1 producer gas1 and water gas1 which will be treated of in the following chapter71 because complete combustion may be effected without an access of air4 ,it is evident that in order to obtain high temperatures means must be taken to diminish the loss of heat by radiation1 and to ensure perfect combustion4 #cb ,the gas which is set free in coal mines contains a good deal of nitrogen1 some carbonic anhydride1 and a large quantity of marsh gas4 ,the best means of avoiding an explosion consists in efficient ventilation4 ,it is best to light coal mines with electric lamps4 #cc ,the ,davy lamp1 of which an improved form is represented in the accompanying figure1 is used for lighting coal and other mines where combustible gas is found4 ,the wick of the lamp is enclosed in a thick glass cylinder which is firmly held in a metallic holder4 ,over this a metallic cylinder and the wire gauze are placed4 ,the products of combustion pass through the gauze1 and the air enters through the space between the cylinder and the wire gauze4 ,to ensure greater safety the lamp cannot be opened without extinguishing the flame4 #cd ,in ,pennsylvania 7beyond the ,alleghany mountains7 many of the shafts sunk for petroleum only emitted gas1 but many useful applications for it were found and it was conducted in metallic pipes to works hundreds of miles distant1 principally for metallurgical purposes4 #ce ,the purest gas is prepared by mixing the liquid substance called zinc methyl1 ,zn7,,ch#c7b1 with water1 when the following reaction occurs3 ,zn7,,ch#c7b #b;,,hoh ,zn7,,ho7#b #b;,,ch#c;,h4 ,an alkali--for instance1 ,na,h,o--gives with acetic acid a salt1 ,c#b;,h#c,na,o#b1 which on decomposition retains carbonic anhydride1 forming a carbonate1 ,na#b;,,co#c1 and marsh gas is given off3 ,c#b;,h#c,na,o#b ,na,h,o ,na#b;,,co#c ,,ch#d ,marsh gas is difficult to liquefy2 it is almost insoluble in water1 and is without taste or smell4 ,the most important point in connection with its chemical reactions is that it does not combine directly with anything1 whilst the other hydrocarbons which contain less hydrogen than expressed by the formula ,cn,h#bn #b are capable of combining with hydrogen1 chlorine1 certain acids1 c4 ,if the law of substitution gives a very simple explanation of the formation of hydrogen peroxide as a compound containing two aqueous residues 7,,oh77,,oh71 then on the basis of this law all hydrocarbons ought to be derived from methane1 ,,ch#d1 as being the simplest hydrocarbon4#cf ,the increase in complexity of a molecule of methane is brought about by the faculty of mutual combination which exists in the atoms of carbon1 and1 as a consequence of the most detailed study of the subject1 much that might have been foreseen and conjectured from the law of substitution has been actually brought about in such a manner as might have been predicted1 and although this subject on account of its magnitude really belongs1 as has been already stated1 to the sphere of organic chemistry1 it has been alluded to here in order to show1 although only in part1 the best investigated example of the application of the law of substitution4 ,according to this law1 a molecule of methane1 ,,ch#d1 is capable of undergoing substitution in the four following ways3--7#a7 ,methyl substitution1 when the radicle1 equivalent to hydrogen1 called methyl ,,ch#c1 replaces hydrogen4 ,in ,,ch#d this radicle is combined with ,h and therefore can replace it1 as 7,,oh7 replaces ,h because with it it gives water2 7#b7 methylene substitution1 or the exchange between ,h#b and ,,ch#b 7this radicle is called methylene71 is founded on a similar division of the molecule ,,ch#d into two equivalent parts1 ,h#b and ,,ch#b2 7#c7 acetylene substitution1 or the exchange between ,,ch on the one hand and ,h#c on the other2 and 7#d7 carbon substitution--that is1 the substitution of ,h#d by an atom of carbon ,c1 which is founded on the law of substitution just as is the methyl substitution4 ,these four cases of substitution render it possible to understand the principal relations of the hydrocarbons4 ,for instance1 the law of even numbers is seen from the fact that in all the cases of substitution mentioned the hydrogen atoms increase or decrease by an even number2 but as in ,,ch#d they are likewise even1 it follows that no matter how many substitutions are effected there will always be obtained an even number of hydrogen atoms4 ,when ,h is replaced by ,,ch#c there is an increase of ,,ch#b2 when ,h#b is replaced by ,,ch#b there is no increase of hydrogen2 in the acetylene substitution ,,ch replaces ,h#c1 therefore there is an increase of ,c and a decrease of ,h#b2 in the carbon substitution there is a decrease of ,h#d4 ,in a similar way the law of limit may be deduced as a corollary of the law of substitution4 ,for the largest possible quantity of hydrogen is introduced by the methyl substitution1 since it leads to the addition of ,,ch#b2 starting from ,,ch#d we obtain ,c#b;,h#f1 ,c#c;,h#h1 and in general1 ,cn,h#bn#b1 and these contain the greatest possible amount of hydrogen4 ,unsaturated hydrocarbons1 containing less hydrogen1 are evidently only formed when the increase of the new molecule derived from methane proceeds from one of the other forms of substitution4 ,when the methyl substitution alone takes place in methane1 ,,ch#d1 it is evident that the saturated hydrocarbon formed is ,c#b;,h#f or 7,,ch#c77;,,ch#c74cg ,this is called ethane4 ,by means of the methylene substitution alone1 ethylene1 ,c#b;,h#d1 or 7,,ch#b77;,,ch#b7 may be directly obtained from ,,ch#d1 and by the acetylene substitution ,c#b;,h#b or 7,,ch77,,ch71 or acetylene1 both the latter being unsaturated hydrocarbons4 ,thus we have all the possible hydrocarbons with two atoms of carbon in the molecule1 ,c#b;,h#f1 ethane1 ,c#b;,h#d1 ethylene1 and ,c#b;,h#b1 acetylene4 ,but in them1 according to the law of substitution1 the same forms of substitution may be repeated--that is1 the methyl1 methylene1 acetylene1 and even carbon substitutions 7because ,c#b;,h#f will still contain hydrogen when ,c replaces ,h#d7 and therefore further substitutions will serve as a source for the production of a fresh series of saturated and unsaturated hydrocarbons1 containing more and more carbon in the molecule and1 in the case of the acetylene substitution and carbon substitution1 containing less and less hydrogen4 ,thus by means of the law of substitution we can foresee not only the limit ,cn,h#bn#b1 but an unlimited number of unsaturated hydrocarbons1 ,cn,h#bn1 ,cn,h#bn-#b 444 ,cn,h#b7n-m71 where m varies from #j to n-#a1ch and where n increases indefinitely4 ,from these facts not only does the existence of a multitude of polymeric hydrocarbons1 differing in molecular weight1 become intelligible1 but it is also seen that there is a possibility of cases of isomerism with the same molecular weight4 ,this polymerism so common to hydrocarbon compounds is already apparent in the first unsaturated series ,cn,h#bn1 because all the terms of this series ,c#b;,h#d1 ,c#c;,h#f1 ,c#d;,h#h 444 ,c#cj;,h#fj 444 have one and the same composition ,,ch#b1 but different molecular weights1 as has been already explained in ,chapter ,,vii4 ,the differences in the vapour density1 boiling points1 and melting points1 of the quantities entering into reactions1#ci and the methods of preparation#dj also so clearly tally with the conception of polymerism1 that this example will always be the clearest and most conclusive for the illustration of polymerism and molecular weight4 ,such a case is also met with among other hydrocarbons4 ,thus benzene1 ,c#f;,h#f1 and cinnamene1 ,c#h;,h#h1 correspond with the composition of acetylene or to a compound of the composition ,,ch4#da ,the first boils at #ha1 the second at #add2 the specific gravity of the first is #jhii2 that of the second1 #jibe1 at #j--that is1 here also the boiling point rises with the increase of molecular weight1 and so also1 as might be expected1 does the density4 #cf ,methylene1 ,,ch#b1 does not exist4 ,when attempts are made to obtain it 7for example1 by removing ,x#b from ,,ch#b,x#b71 ,c#b;,h#d or ,c#c;,h#f are produced--that is to say1 it undergoes polymerisation4 #cg ,although the methods of formation and the reactions connected with hydrocarbons are not described in this work1 because they are dealt with in organic chemistry1 yet in order to clearly show the mechanism of those transformations by which the carbon atoms are built up into the molecules of the carbon compounds1 we here give a general example of reactions of this kind4 ,from marsh gas1 ,,ch#d1 on the one hand the substitution of chlorine or iodine1 ,,ch#c;,cl1 ,,ch#c;,i1 for the hydrogen may be effected1 and on the other hand such metals as sodium may be substituted for the hydrogen1 e4g4 ,,ch#c,na4 ,these and similar products of substitution serve as a means of obtaining other more complex substances from given carbon compounds4 ,if we place the two above-named products of substitution of marsh gas 7metallic and haloid7 in mutual contact1 the metal combines with the halogen1 forming a very stable compound--namely1 common salt1 ,na,cl1 and the carbon groups which were in combination with them separate in mutual combination1 as shown by the equation3 ,,ch#c;,cl ,,ch#c,na ,na,cl ,c#b;,h#f4 ,this is the most simple example of the formation of a complex hydrocarbon from these radicles4 ,the cause of the reaction must be sought for in the property which the haloid 7chlorine7 and sodium have of entering into mutual combination4 #ch ,when m n-#a1 we have the series ,cn,h#b4 ,the lowest member is acetylene1 ,c#b;,h#b4 ,these are hydrocarbons containing a minimum amount of hydrogen4 #ci ,for instance1 ethylene1 ,c#b;,h#d1 combines with ,br#b1 ,,hi1 ,h#b,,so#d1 as a whole molecule1 as also does amylene1 ,c#e;,h#aj1 and1 in general1 ,cn,h#bn4 #dj ,for instance1 ethylene is obtained by removing the water from ethyl alcohol1 ,c#b;,h#e7,,oh71 and amylene1 ,c#e;,h#aj1 from amyl alcohol1 ,c#e;,h#aa7,,oh71 or in general ,cn,h#bn1 from ,cn,h#bn#a7,,oh74 #da ,acetylene and its polymerides have an empirical composition ,,ch1 ethylene and its homologues 7and polymerides7 ,,ch#b1 ethane ,,ch#c1 methane ,,ch#d4 ,this series presents a good example of the law of multiple proportions1 but such diverse proportions are met with between the number of atoms of the carbon and hydrogen in the hydrocarbons already known that the accuracy of ,dalton's law might be doubted4 ,thus the substances ,c#cj;,h#fb and ,c#cj;,h#fj differ so slightly in their composition by weight as to be within the limits of experimental error1 but their reactions and properties are so distinct that they can be distinguished beyond a doubt4 ,without ,dalton's law chemistry could not have been brought to its present condition1 but it cannot alone express all those gradations which are quite clearly understood and predicted by the law of ,avogadro-,gerhardt4 ,cases of isomerism in the restricted sense of the word--that is1 when with an identity of composition and of molecular weight1 the properties of the substances are different--are very numerous among the hydrocarbons and their derivatives4 ,such cases are particularly important for the comprehension of molecular structure and they also1 like the polymerides1 may be predicted from the above-mentioned conceptions1 expressing the principles of the structure of the carbon compounds#db based on the law of substitution4 ,according to it1 for example1 it is evident that there can be no isomerism in the cases of the saturated hydrocarbons ,c#b;,h#f and ,c#c;,h#h1 because the former is ,,ch#d1 in which methyl has taken the place of ,h1 and as all the hydrogen atoms of methane must be supposed to have the same relation to the carbon1 it is all the same which of them be subjected to the methyl substitution--the resulting product can only be ethane1 ,,ch#c;,,ch#c2dc the same argument also applies in the case of propane1 ,,ch#c;,,ch#b;,,ch#c1 where one compound only can be imagined4 ,it is to be expected1 however1 that there should be two butanes1 ,c#d;,h#aj1 and this is actually the case4 ,in one1 methyl may be considered as replacing the hydrogen of one of the methyls1 ,,ch#c;,,ch#b;,,ch#b;,,ch#c2 and in the other ,,ch#c may be considered as substituted for ,h in _/,,ch#c ,,ch#b1 and there it will consist of ,,ch#c;,,ch4 ,the latter may ,,ch#c also be regarded as methane in which three of hydrogen are exchanged for three of methyl4 ,on going further in the series it is evident that the number of possible isomerides will be still greater1 but we have limited ourselves to the simplest examples1 showing the possibility and actual existence of isomerides4 ,c#b;,h#d and ,,ch#b;,,ch#b are1 it is evident1 identical2 but there ought to be1 and are1 two hydrocarbons of the composition ,c#c;,h#f1 propylene and trimethylene2 the first is ethylene1 ,,ch#b;,,ch#b1 in which one atom of hydrogen is exchanged for methyl1 ,,ch#b;,,chch#c1 and trimethylene is ethane1 ,,ch#c;,,ch#c1 with the substitution of methylene for two hydrogen atoms from two methyl groups--that _/,,ch#b is1 ,,ch#b1dd where the methylene introduced is united to both ,,ch#b the atoms of carbon in ,,ch#c;,,ch#c4 ,it is evident that the cause of isomerism here is1 on the one hand1 the difference of the amount of hydrogen in union with the particular atoms of carbon1 and1 on the other1 the different connection between the several atoms of carbon4 ,in the first case they may be said to be chained together 7more usually to form an 'open chain'71 and in the second case1 to be locked together 7to form a 'closed chain' or 'ring'74 ,here also it is easily understood that on increasing the quantity of carbon atoms the number of possible and existing isomerides will greatly increase4 ,if1 at the same time1 in addition to the substitution of one of the radicles of methane for hydrogen a further exchange of part of the hydrogen for some of the other groups of elements ,x1 ,y 444 occurs1 the quantity of possible isomerides still further increases in a considerable degree4 ,for instance1 there are even two possible isomerides for the derivatives of ethane1 ,c#b;,h#f3 if two atoms of the hydrogen be exchanged for ,x#b1 one will have the ethylene structure1 ,,ch#b,,xch#b,x1 and the other an ethylidene structure1 ,,ch#c;,,chx#b2 such are1 for instance1 ethylene chloride1 ,,ch#b;,cl,c,h#b;,cl1 and ethylidene chloride1 ,,ch#c;,c,h,cl#b4 ,and as in the place of the first atom of hydrogen not only metals may be substituted1 but ,cl1 ,br1 ,i1 ,,oh 7the water radicle71 ,,nh#b 7the ammonia radicle71 ,,no#b 7the radicle of nitric acid71 c41 so also in exchange for two atoms of hydrogen ,o1 ,,nh1 ,s1 c41 may be substituted2 hence it will be understood that the quantity of isomerides is sometimes very great4 ,it is impossible here to describe how the isomerides are distinguished from each other1 in what reactions they occur1 how and when one changes into another1 c42 for this1 taken together with the description of the hydrocarbons already known1 and their derivatives1 forms a very extensive and very thoroughly investigated branch of chemistry1 called organic chemistry4 ,enriched with a mass of closely observed phenomena and strictly deduced generalisations1 this branch of chemistry has been treated separately for the reason that in it the hydrocarbon groups are subjected to transformations which are not met with in such quantity in dealing with any of the other elements or their hydrogen compounds4 ,it was important for us to show that notwithstanding the great variety of the hydrocarbons and their products1#de they are all of them governed by the law of substitution1 and referring our readers for detailed information to works on organic chemistry1 we will limit ourselves to a short exposition of the properties of the two simplest unsaturated hydrocarbons3 ethylene1 ,,ch#b;,,ch#b1 and acetylene1 ,,chch1 and a short acquaintance with petroleum as the natural source of a mass of hydrocarbons4 ,ethylene1 or olefiant gas1 ,c#b;,h#d1 is the lowest known member of the unsaturated hydrocarbon series of the composition ,cn,h#bn4 ,as in composition it is equal to two molecules of marsh gas deprived of two molecules of hydrogen1 it is evident that it might be1 and it actually can be1 produced1 although but in small quantities1 together with hydrogen1 by heating marsh gas4 ,on being heated1 however1 olefiant gas splits up1 first into acetylene and methane 7#c;,c#b;,h#d #b;,c#b;,h#b #b;,,ch#d1 ,lewes1 #ahid71 and at a higher temperature into carbon and hydrogen2 and therefore in those cases where marsh gas is produced by heating1 olefiant gas1 hydrogen1 and charcoal will also be formed1 although only in small quantities4 ,the lower the temperature at which complex organic substances are heated1 the greater the quantity of olefiant gas found in the gases given off2 at a white heat it is entirely decomposed into charcoal and marsh gas4 ,if coal1 wood1 and more particularly petroleum1 tars1 and fatty substances1 are subjected to dry distillation1 they give off illuminating gas1 which contains more or less olefiant gas4 #db ,the conception of the structure of carbon compounds--that is1 the expression of those unions and correlations which their atoms have in the molecules--was for a long time limited to the representation that organic substances contained complex radicles 7for instance1 ethyl ,c#b;,h#e1 methyl ,,ch#c1 phenyl ,c#f;,h#e1 c472 then about the year #ahdj the phenomena of substitution and the correspondence of the products of substitution with the primary bodies 7nuclei and types7 were observed1 but it was not until about the year #ahfj and later when on the one hand the teaching of ,gerhardt about molecules was spreading1 and on the other hand the materials had accumulated for discussing the transformations of the simplest hydrocarbon compounds1 that conjectures began to appear as to the mutual connection of the atoms of carbon in the molecules of the complex hydrocarbon compounds4 ,then ,kekul and ,a4 ,m4 ,butleroff began to formulate the connection between the separate atoms of carbon1 regarding it as a quadrivalent element4 ,although in their methods of expression and in some of their views they differ from each other and also from the way in which the subject is treated in this work1 yet the essence of the matter--namely1 the comprehension of the causes of isomerism and of the union between the separate atoms of carbon--remains the same4 ,in addition to this1 starting from the year #ahgj1 there appears a tendency which from year to year increases to discover the actual spacial distribution of the atoms in the molecules4 ,thanks to the endeavours of ,le-,bel 7#ahgd71 ,van't ,hoff 7#ahgd71 and ,wislicenus 7#ahhg7 in observing cases of isomerism--such as the effect of different isomerides on the direction of the rotation of the plane of polarisation of light--this tendency promises much for chemical mechanics1 but the details of the still imperfect knowledge in relation to this matter must be sought for in special works devoted to organic chemistry4 #dc ,direct experiment shows that however ,,ch#c,x is prepared 7where ,x for instance ,cl1 c47 it is always one and the same substance4 ,if1 for example1 in ,,cx#d1 ,x is gradually replaced by hydrogen until ,,ch#c,x is produced1 or in ,,ch#d1 the hydrogen by various means is replaced by ,x1 or else1 for instance1 if ,,ch#c,x be obtained by the decomposition of more complex compounds1 the same product is always obtained4 ,this was shown in the year #ahfj1 or thereabout1 by many methods1 and is the fundamental conception of the structure of hydrocarbon compounds4 ,if the atoms of hydrogen in methyl were not absolutely identical in value and position 7as they are not1 for instance1 in ,,ch#c;,,ch#b;,,ch#c or ,,ch#c;,,ch#b,x71 then there would be as many different forms of ,,ch#c,x as there were diversities in the atoms of hydrogen in ,,ch#d4 ,the scope of this work does not permit of a more detailed account of this matter4 ,it is given in works on organic chemistry4 #dd ,the union of carbon atoms in closed chains or rings was first suggested by ,kekul as an explanation of the structure and isomerism of the derivatives of benzene1 ,c#f;,h#f1 forming aromatic compounds 7,note #bf74 #de ,the following are the most generally known of the oxygenised but non-nitrogenous hydrocarbon derivatives4 7#a7 the alcohols4 ,these are hydrocarbons in which hydrogen is exchanged for hydroxyl 7,,oh74 ,the simplest of these is methyl alcohol1 ,,ch#c7,,oh71 or wood spirit obtained by the dry distillation of wood4 ,the common spirits of wine or ethyl alcohol1 ,c#b;,h#c7,,oh71 and glycol1 ,c#b;,h#d7,,oh7#b1 correspond with ethane4 ,normal propyl alcohol1 ,,ch#c;,,ch#b;,,ch#b7,,oh71 and isopropyl alcohol1 ,,ch#c;,,ch7,,oh7,,ch#c1 propylene-glycol1 ,c#c;,h#f7,,oh7#b1 and glycerol1 ,c#c;,h#c7,,oh7#c 7which1 with stearic and other acids1 forms fatty substances71 correspond with propane1 ,c#c;,h#h4 ,all alcohols are capable of forming water and ethereal salts with acids1 just as alkalis form ordinary salts4 7#b7 ,aldehydes are alcohols minus hydrogen2 for instance1 acetaldehyde1 ,c#b;,h#d,o1 corresponds with ethyl alcohol4 7#c7 ,it is simplest to regard organic acids as hydrocarbons in which hydrogen has been exchanged for carboxyl 7,,co#b;,h71 as will be explained in the following chapter4 ,there are a number of intermediate compounds2 for example1 the aldehyde-alcohols1 alcohol-acids 7or hydroxy-acids71 c4 ,thus the hydroxy-acids are hydrocarbons in which some of the hydrogen has been replaced by hydroxyl1 and some by carboxyl2 for instance1 lactic acid corresponds with ,c#b;,h#f1 and has the constitution ,c#b;,h#d7,,oh77,,co#b;,h74 ,if to these products we add the haloid salts 7where ,h is replaced by ,cl1 ,br1 ,i71 the nitro-compounds containing ,,no#b in place of ,h1 the amides1 cyanides1 ketones1 and other compounds1 it will be readily seen what an immense number of organic compounds there are and what a variety of properties these substances have2 this we see also from the composition of plants and animals4 ,olefiant gas1 almost free from other gases1#df may be obtained from ordinary alcohol 7if possible1 free from water7 if it be mixed with five parts of strong sulphuric acid and the mixture heated to slightly above #ajj4 ,under these conditions1 the sulphuric acid removes the elements of water from the alcohol1 ,c#b;,h#e7,,oh71 and gives olefiant gas2 ,c#b;,h#f,o ,h#b,o ,c#b;,h#d4 ,the greater molecular weight of olefiant gas compared with marsh gas indicates that it may be comparatively easily converted into a liquid by means of pressure or great cold2 this may be effected1 for example1 by the evaporation of liquid nitrous oxide4 ,its absolute boiling point is #aj1 it boils at -#ajc 7#a atmosphere71 liquefies at #j1 at a pressure of #dc atmospheres1 and solidifies at -#afj4 ,ethylene is colourless1 has a slight ethereal smell1 is slightly soluble in water1 and somewhat more soluble in alcohol and in ether 7in five volumes of spirit and six volumes of ether74#dg #df ,ethylene bromide1 ,c#b;,h#d;,br#b1 when gently heated in alcoholic solution with finely divided zinc1 yields pure ethylene1 the zinc merely taking up the bromine 7,sabaneyeff74 #dg ,ethylene decomposes somewhat easily under the influence of the electric spark1 or a high temperature4 ,in this case the volume of the gas formed may remain the same when olefiant gas is decomposed into carbon and marsh gas1 or may increase to double its volume when hydrogen and carbon are formed1 ,c#b;,h#d ,,ch#d ,c #b;,c #b;,h#b4 ,a mixture of olefiant gas and oxygen is highly explosive2 two volumes of this gas require six volumes of oxygen for its perfect combustion4 ,the eight volumes thus taken then resolve themselves into eight volumes of the products of combustion1 a mixture of water and carbonic anhydride1 ,c#b;,h#d #c,o#b #b;,,co#b #b;,h#b,o4 ,on cooling after the explosion diminution of volume occurs because the water becomes liquid4 ,for two volumes of the olefiant gas taken1 the diminution will be equal to four volumes1 and the same for marsh gas4 ,the quantity of carbonic anhydride formed by both gases is not the same4 ,two volumes of marsh gas give only two volumes of carbonic anhydride1 and two volumes of ethylene give four volumes of carbonic anhydride4 ,like other unsaturated hydrocarbons1 olefiant gas readily enters into combination with certain substances1 such as chlorine1 bromine1 iodine1 fuming sulphuric acid1 or sulphuric anhydride1 c4 ,if olefiant gas be sealed up with a small quantity of sulphuric acid in a glass vessel1 and constantly agitated 7as1 for instance1 by attaching it to the moving part of a machine71 the prolonged contact and repeated mixing causes the olefiant gas1 little by little1 to combine with the sulphuric acid1 forming ,c#b;,h#d;,h#b,,so#d4 ,if1 after this absorption1 the sulphuric acid be diluted with water and distilled1 alcohol separates1 which is produced in this case by the olefiant gas combining with the elements of water1 ,c#b;,h#d ,h#b,o ,c#b;,h#f,o4 ,in this reaction 7,berthelot7 we see an excellent example of the fact that if a given substance1 like olefiant gas1 is produced by the decomposition of another1 then in the reverse way this substance1 entering into combination1 is capable of forming the original substance--in our example1 alcohol4 ,in combination with various molecules1 ,x#b1 ethylene gives saturated compounds1 ,c#b;,h#d,x#b or ,,ch#b,,xch#b,x 7for example1 ,c#b;,h#d;,cl#b71 which correspond with ethane1 ,,ch#c;,,ch#c or ,c#b;,h#f4dh #dh ,the homologues of ethylene1 ,cn,h#bn1 are also capable of direct combination with halogens1 c41 but with various degrees of facility4 ,the composition of these homologues can be expressed thus3 7,,ch#c7x7,,ch#b7y7,,ch7z,cr1 where the sum of x z is always an even number1 and the sum of x z r is equal to half the sum of #cx z1 whence z #br x2 by this means the possible isomerides are determined4 ,for example1 for butylenes1 ,c#d;,h#h1 7,,ch#c7b7;,,ch7#b1 7,,ch#c7b7;,,ch#b7;,c1 7,,ch#b77;,,ch#b7b;,,ch1 and 7,,ch#b7d are possible4 ,acetylene1 ,c#b;,h#b ,,chch1 is a gas2 it was first prepared by ,berthelot 7#aheg74 ,it has a very pungent smell1 is characterised by its great stability under the action of heat1 and is obtained as the only product of the direct combination of carbon with hydrogen when a luminous arc 7voltaic7 is formed between carbon electrodes4 ,this arc contains particles of carbon passing from one pole to the other4 ,if the carbons be surrounded with an atmosphere of hydrogen1 the carbon in part combines with the hydrogen1 forming ,c#b;,h#b4dh bis ,acetylene may be formed from olefiant gas if two atoms of hydrogen be taken from it4 ,this may be effected in the following way3 the olefiant gas is first made to combine with bromine1 giving ,c#b;,h#d;,br#b2 from this the hydrobromic acid is removed by means of an alcoholic solution of caustic potash1 leaving the volatile product ,c#b;,h#c;,br2 and from this yet another part of hydrobromic acid is withdrawn by passing it through anhydrous alcohol in which metallic sodium has been dissolved1 or by heating it with a strong alcoholic solution of caustic potash4 ,under these circumstances 7,berthelot1 ,sawitsch1 ,miasnikoff7 the alkali takes up the hydrobromic acid from ,cn,h#bn-#a;,br1 forming ,cn,h#bn-#b4 #dh bis ,see also method of preparing ,c#b;,h#b in ,note #ab bis4 ,acetylene is also produced in all those cases where organic substances are decomposed by the action of a high temperature--for example1 by dry distillation4 ,on this account a certain quantity is always found in coal gas1 and gives to it1 at all events in part1 its peculiar smell1 but the quantity of acetylene in coal gas is very small4 ,if the vapour of alcohol be passed through a heated tube a certain quantity of acetylene is formed4 ,it is also produced by the imperfect combustion of olefiant and marsh gas--for example1 if the flame of coal gas has not free access to air4#di ,the inner part of every flame contains gases in imperfect combustion1 and in them some amount of acetylene4 #di ,this is easily accomplished with those gas burners which are used in laboratories and mentioned in the ,introduction4 ,in these burners the gas is first mixed with air in a long tube1 above which it is kindled4 ,but if it be lighted inside the pipe it does not burn completely1 but forms acetylene1 on account of the cooling effect of the walls of the metallic tube2 this is detected by the smell1 and may be shown by passing the issuing gas 7by aid of an aspirator7 into an ammoniacal solution of cuprous chloride4 ,acetylene1 being further removed than ethylene from the limit ,cn,h#bn#b of hydrocarbon compounds1 has a still greater faculty of combination than is shown by olefiant gas1 and therefore can be more readily separated from any mixture containing it4 ,actually1 acetylene not only combines with one and two molecules of ,i#b1 ,,hi1 ,h#b,,so#d1 ,cl#b1 ,br#b1 c4444 7many other unsaturated hydrocarbons combine with them71 but also with cuprous chloride1 ,cu,cl1 forming a red precipitate4 ,if a gaseous mixture containing acetylene be passed through an ammoniacal solution of cuprous chloride 7or silver nitrate71 the other gases do not combine1 but the acetylene gives a red precipitate 7or grey with silver71 which detonates when struck with a hammer4 ,this red precipitate gives off acetylene under the action of acids4 ,in this manner pure acetylene may be obtained4 ,acetylene and its homologues also readily react with corrosive sublimate1 ,hg,cl#b 7,koucheroff1 ,favorsky74 ,acetylene burns with a very brilliant flame1 which is accounted for by the comparatively large amount of carbon it contains4#ej #ej ,amongst the homologues of acetylene ,cn,h#bn-#b1 the lowest is ,c#c;,h#d2 allylene1 ,,ch#c;,,cch1 and allene1 ,,ch#b;,,cch#b1 are known1 but the closed structure1 ,,ch#b7;,,ch7#b1 is little investigated4 ,the formation and existence in nature of large masses of petroleum or a mixture of liquid hydrocarbons1 principally of the series ,cn,h#bn #b and ,cn,h#bn is in many respects remarkable4#ea ,in some mountainous districts--as1 for instance1 by the slopes of the ,caucasian chain1 on inclines lying in a direction parallel to the range--an oily liquid issues from the earth together with salt water and hot gases 7methane and others72 it has a tarry smell and dark brown colour1 and is lighter than water4 ,this liquid is called naphtha or rock oil 7petroleum7 and is obtained in large quantities by sinking wells and deep bore-holes in those places where traces of naphtha are observed1 the naphtha being sometimes thrown up from the wells in fountains of considerable height4#eb ,the evolution of naphtha is always accompanied by salt water and marsh gas4 ,naphtha has from ancient times been worked in ,russia in the ,apsheron peninsula near ,baku1 and is also now worked in ,burmah 7,india71 in ,galicia near the ,carpathians1 and in ,america1 especially in ,pennsylvania and ,canada1 c4 ,naphtha does not consist of one definite hydrocarbon1 but of a mixture of several1 and its density1 external appearance1 and other qualities vary with the amount of the different hydrocarbons of which it is composed4 ,the light kinds of naphtha have a specific gravity about #jh and the heavy kinds up to #jih4 ,the former are very mobile liquids1 and more volatile2 the latter contain less of the volatile hydrocarbons and are less mobile4 ,when the light kinds of naphtha are distilled1 the boiling point taken in the vapours constantly changes1 beginning at #j and going up to above #cej4 ,that which passes over first is a very mobile1 colourless ethereal liquid 7forming gazolene1 ligroin1 benzoline1 c471 from which the hydrocarbons whose boiling points start from #j may be extracted--namely1 the hydrocarbons ,c#d;,h#aj1 ,c#e;,h#ab 7which boils at #cj71 ,c#f;,h#ad 7boils at #fb71 ,c#g;,h#af 7boils about #ij71 c4 ,those fractions of the naphtha distillate which boil above #acj1 and contain hydrocarbons with ,c#i1 ,c#aj1 ,c#aa1 c41 enter into the composition of the oily substance1 universally used for lighting1 called kerosene or photogen or photonaphthalene1 and by other names4 ,the specific gravity of kerosene is from #jgh to #jhd1 and it smells like naphtha4 ,those products of the distillation of naphtha which pass off below #acj and have a specific gravity below #jge1 enter into the composition of light petroleum 7benzoline1 ligroin1 petroleum spirit1 c472 which is used as a solvent for india-rubber1 for removing grease spots1 c4 ,those portions of naphtha 7which can only be distilled without change by means of superheated steam1 otherwise they are largely decomposed7 which boil above #bge and up to #cjj and have a specific gravity higher than #jhe1 form an excellent oil1#ec safe as regards inflammability 7which is very important as diminishing the risks of fire71 and may be used in lamps as an effective substitute for kerosene4#ed ,those portions of naphtha which pass over at a still higher temperature and have a higher specific gravity than #ji1 which are found in abundance 7about #cj p4c47 in the ,baku naphtha1 make excellent lubricating or machine oils4 ,naphtha has many important applications1 and the naphtha industry is now of great commercial importance1 especially as naphtha and its refuse may be used as fuel4#ee ,whether naphtha was formed from organic matter is very doubtful1 as it is found in the most ancient ,silurian strata which correspond with epochs of the earth's existence when there was little organic matter2 it could not penetrate from the higher to the lower 7more ancient7 strata as it floats on water 7and water penetrates through all strata74 ,it therefore tends to rise to the surface of the earth1 and it is always found in highlands parallel to the direction of the mountains4#ef ,much more probably its formation may be attributed to the action of water penetrating through the crevasses formed on the mountain slopes and reaching to the heart of the earth1 to that kernel of heated metallic matter which must be accepted as existing in the interior of the earth4 ,and as meteoric iron often contains carbon 7like cast iron71 so1 accepting the existence of such carburetted iron at unattainable depths in the interior of the earth1 it may be supposed that naphtha was produced by the action of water penetrating through the crevices of the strata during the upheaval of mountain chains1#eg because water with iron carbide ought to give iron oxide and hydrocarbons4#eh ,direct experiment proves that the so-called spiegeleisen 7manganiferous iron1 rich in chemically combined carbon7 when treated with acids gives liquid hydrocarbons#ei which in composition1 appearance1 and properties are completely identical with naphtha4#fj #ea ,the saturated hydrocarbons predominate in ,american petroleum1 especially in its more volatile parts2 in ,baku naphtha the hydrocarbons of the composition ,cn,h#bn form the main part 7,lisenko1 ,markovnikoff1 ,beilstein7 but doubtless 7,mendeleff7 it also contains saturated ones1 ,cn,h#bn#b4 ,the structure of the naphtha hydrocarbons is only known for the lower homologues1 but doubtless the distinction between the hydrocarbons of the ,pennsylvanian and ,baku naphthas1 boiling at the same temperature 7after the requisite refining by repeated fractional distillation1 which can be very conveniently done by means of steam rectification--that is1 by passing the steam through the dense mass71 depends not only on the predominance of saturated hydrocarbons in the former1 and naphthenes1 ,cn,h#bn1 in the latter1 but also on the diversity of composition and structure of the corresponding portions of the distillation4 ,the products of the ,baku naphtha are richer in carbon 7therefore in a suitably constructed lamp they ought to give a brighter light71 they are of greater specific gravity1 and have greater internal friction 7and are therefore more suitable for lubricating machinery7 than the ,american products collected at the same temperature4 #eb ,the formation of naphtha fountains 7which burst forth after the higher clay strata covering the layers of sands impregnated with naphtha have been bored through7 is without doubt caused by the pressure or tension of the combustible hydrocarbon gases which accompany the naphtha1 and are soluble in it under pressure4 ,sometimes these naphtha fountains reach a height of #ajj metres--for instance1 the fountain of #ahhg near ,baku4 ,naphtha fountains generally act periodically and their force diminishes with the lapse of time1 which might be expected1 because the gases which cause the fountains find an outlet1 as the naphtha issuing from the bore-hole carries away the sand which was partially choking it up4 #ec ,this is a so-called intermediate oil 7between kerosene and lubricating oils71 solar oil1 or pyronaphtha4 ,lamps are already being manufactured for burning it but still require improvement4 ,above all1 however1 it requires a more extended market1 and this at present is wanting1 owing to the two following reasons3 7#a7 ,those products of the ,american petroleum which are the most widely spread and almost universally consumed contain but little of this intermediate oil1 and what there is is divided between the kerosene and the lubricating oils2 7#b7 the ,baku naphtha1 which is capable of yielding a great deal 7up to #cj p4c47 of intermediate oil1 is produced in enormous quantities1 about #cjj million poods1 but has no regular markets abroad1 and for the consumption in ,russia 7about #be million poods of kerosene per annum7 and for the limited export 7#fj million poods per annum7 into ,western ,europe 7by the ,trans-,caucasian ,railway7 those volatile and more dangerous parts of the naphtha which enter into the composition of the ,american petroleum are sufficient1 although ,baku naphtha yields about #be p4c4 of such kerosene4 ,for this reason pyronaphtha is not manufactured in sufficient quantities1 and the whole world is consuming the unsafe kerosene4 ,when a pipe line has been laid from ,baku to the ,black ,sea 7in ,america there are many which carry the raw naphtha to the sea-shore1 where it is made into kerosene and other products7 then the whole mass of the ,baku naphtha will furnish safe illuminating oils1 which without doubt will find an immense application4 ,a mixture of the intermediate oil with kerosene or ,baku oil 7specific gravity #jhd to #jhe7 may be considered 7on removing the benzoline7 to be the best illuminating oil1 because it is safe 7flashing point from #dj to #fj71 cheaper 7,baku naphtha gives as much as #fj p4c4 of ,baku oil71 and burns perfectly well in lamps differing but little from those made for burning ,american kerosene 7unsafe1 flashing point #bj to #cj74 #ed ,the substitution of ,baku pyronaphtha1 or intermediate oil1 or ,baku oil 7see ,note #ec71 would not only be a great advantage as regards safety from fire1 but would also be highly economical4 ,a ton 7#fb poods7 of ,american crude petroleum costs at the coast considerably more than #bds4 7#ab roubles71 and yields two-thirds of a ton of kerosene suitable for ordinary lamps4 ,a ton of raw naphtha in ,baku costs less than #ds4 7#a rouble #hj copecks71 and with a pipe line to the shore of the ,black ,sea would not cost more than #h roubles1 or #afs4 ,moreover1 a ton of ,baku naphtha will yield as much as two-thirds of a ton of kerosene1 ,baku oil1 and pyronaphtha suitable for illuminating purposes4 #ee ,naphtha has been applied for heating purposes on a large scale in ,russia1 not only on account of the low cost of naphtha itself and of the residue from the preparation of kerosene1 but also because the products of all the ,baku naphtha do not find an outlet for general consumption4 ,naphtha itself and its various residues form excellent fuel1 burning without smoke and giving a high temperature 7steel and iron may be easily melted in the flame74 ,a hundred poods of good coal 7for instance1 ,don coal7 used as fuel for heating boilers are equivalent to #cf cubic feet 7about #bej poods7 of dry wood1 while only #gj poods of naphtha will be required2 and moreover there is no need for stoking1 as the liquid can be readily and evenly supplied in the required quantity4 ,the economic and other questions relating to ,american and ,baku petroleums have been discussed more in detail in some separate works of mine 7,d4 ,mendeleff73 7#a7 ',the ,naphtha ,industry of ,pennsylvania and the ,caucasus1' #ahgj2 7#b7 ',where to ,build ,naphtha ,works1' #ahhj2 7#c7 ',on the ,naphtha ,question1' #ahhc2 7#d7 ',the ,baku ,naphtha ,question1' #ahhf2 7#e7 the article on the naphtha industry in the account of the ,russian industries printed for the ,chicago ,exhibition4 #ef ,as during the process of the dry distillation of wood1 sea-weed1 and similar vegetable dbris1 and also when fats are decomposed by the action of heat 7in closed vessels71 hydrocarbons similar to those of naphtha are formed1 it was natural that this fact should have been turned to account to explain the formation of the latter4 ,but the hypothesis of the formation of naphtha from vegetable dbris inevitably assumes coal to be the chief element of decomposition1 and naphtha is met with in ,pennsylvania and ,canada1 in the ,silurian and ,devonian strata1 which do not contain coal1 and correspond to an epoch not abounding in organic matter4 ,coal was formed from the vegetable dbris of the ,carboniferous1 ,jurassic1 and other recent strata1 but judging more from its composition and structure1 it has been subjected to the same kind of decomposition as peat2 nor could liquid hydrocarbons have been thus formed to such an extent as we see in naphtha4 ,if we ascribe the derivation of naphtha to the decomposition of fat 7adipose1 animal fat7 we encounter three almost insuperable difficulties3 7#a7 ,animal remains would furnish a great deal of nitrogenous matter1 whilst there is but very little in naphtha2 7#b7 the enormous quantity of naphtha already discovered as compared with the insignificant amount of fat in the animal carcase2 7#c7 the sources of naphtha always running parallel to mountain chains is completely inexplicable4 ,being struck with this last-mentioned circumstance in ,pennsylvania1 and finding that the sources in the ,caucasus surround the whole ,caucasian range 7,baku1 ,tiflis1 ,gouria1 ,kouban1 ,tamman1 ,groznoe1 ,dagestan71 ,i developed in #ahgf the hypothesis of the mineral origin of naphtha expounded further on4 #eg ,during the upheaval of mountain ranges crevasses would be formed at the peaks with openings upwards1 and at the foot of the mountains with openings downwards4 ,these cracks in course of time fill up1 but the younger the mountains the fresher the cracks 7the ,alleghany mountains are1 without doubt1 more ancient than the ,caucasian1 which were formed during the tertiary epoch72 through them water must gain access deep into the recesses of the earth to an extent that could not occur on the level 7on plains74 ,the situation of naphtha at the foot of mountain chains is the principal argument in my hypothesis4 ,another fundamental reason is the consideration of the mean density of the earth4 ,cavendish1 ,airy1 ,cornu1 ,boys1 and many others who have investigated the subject by various methods1 found that1 taking water #a1 the mean density of the earth is nearly #ee4 ,as at the surface water and all rocks 7sand1 clay1 limestone1 granite1 c47 have a density less than #c1 it is evident 7as solid substances are but slightly compressible even under the greatest pressure7 that inside the earth there are substances of a greater density--indeed1 not less than #g or #h4 ,what conclusion1 then1 can be arrived at8 ,anything heavy contained in the bosom of the earth must be distributed not only on its surface1 but throughout the whole solar system1 for everything tends to show that the sun and planets are formed from the same material1 and according to the hypothesis of ,laplace and ,kant it is most probable1 and indeed must necessarily be held1 that the earth and planets are but fragments of the solar atmosphere1 which have had time to cool considerably and become masses semi-liquid inside and solid outside1 forming both planets and satellites4 ,the sun amongst other heavy elements contains a great deal of iron1 as shown by spectrum analysis4 ,there is also much of it in an oxidised condition on the surface of the earth4 ,meteoric stones1 carried as fragmentary planets in the solar system and sometimes falling upon the earth1 consisting of siliceous rocks similar to terrestrial ones1 often contain either dense masses of iron 7for example1 the ,pallosovo iron preserved in the ,st4 ,petersburg ,academy of ,sciences7 or granular masses 7for instance1 the ,okhansk meteorite of #ahhf74 ,it is therefore possible that the interior of the earth contains much iron in a metallic state4 ,this might be anticipated from the hypothesis of ,laplace1 for the iron must have been compressed into a liquid at that period when the other component parts of the earth were still strongly heated1 and oxides of iron could not then have been formed4 ,the iron was covered with slags 7mixtures of silicates like glass fused with rocky matter7 which did not allow it to burn at the expense of the oxygen of the atmosphere or of water1 just at that time when the temperature of the earth was very high4 ,carbon was in the same state2 its oxides were also capable of dissociation 7,deville72 it is also but slightly volatile1 and has an affinity for iron1 and iron carbide is found in meteoric stones 7as well as carbon and even the diamond74 ,thus the supposition of the existence of iron carbides in the interior of the earth was derived by me from many indications1 which are to some extent confirmed by the fact that granular pieces of iron have been found in some basalts 7ancient lava7 as well as in meteoric stones4 ,the occurrence of iron in contact with carbon during the formation of the earth is all the more probable because those elements predominate in nature which have small atomic weights1 and among them the most widely diffused1 the most difficultly fusible1 and therefore the most easily condensed 7,chapter ,,xv47 are carbon and iron4 ,they passed into the liquid state when all compounds were at a temperature of dissociation4 #eh ,the following is the typical equation for this formation3 #c;,fem,cn #dm,h#b,o m,fe#c,o#d 7magnetic oxide7 ,c#en,h#hm 7see ,chapter ,,xvii41 ,note #ch74 #ei ,cloez investigated the hydrocarbons formed when cast-iron is dissolved in hydrochloric acid1 and found ,cn,h#bn and others4 ,i treated crystalline manganiferous cast-iron with the same acid1 and obtained a liquid mixture of hydrocarbons exactly similar to natural naphtha in taste1 smell1 and reaction4 #fj ,probably naphtha was produced during the upheaval of all mountain chains1 but only in some cases were the conditions favourable to its being preserved underground4 ,the water penetrating below formed there a mixture of naphtha and watery vapours1 and this mixture issued through fissures to the cold parts of the earth's crust4 ,the naphtha vapours1 on condensing1 formed naphtha1 which1 if there were no obstacles1 appeared on the surface of land and water4 ,here part of it soaked through formations 7possibly the bituminous slates1 schists1 dolomites1 c41 were thus formed71 another part was carried away on the water1 became oxidised1 evaporated1 and was driven to the shores 7the ,caucasian naphtha probably in this way1 during the existence of the ,aralo-,caspian sea1 was carried as far as the ,sisran banks of the ,volga1 where many strata are impregnated with naphtha and products of its oxidation resembling asphalt and pitch72 a great part of it was burnt in one way or another--that is1 gave carbonic anhydride and water4 ,if the mixture of vapours1 water1 and naphtha formed inside the earth had no free outlet to the surface1 it nevertheless would find its way through fissures to the superior and colder strata1 and there become condensed4 ,some of the formations 7clays7 which do not absorb naphtha were only washed away by the warm water1 and formed mud1 which we also now observe issuing from the earth in the form of mud volcanoes4 ,the neighbourhood of ,baku and the whole of the ,caucasus near the naphtha districts are full of such volcanoes1 which from time to time are in a state of eruption4 ,in old naphtha beds 7such as the ,pennsylvanian7 even these blow-holes are closed1 and the mud volcanoes have had time to be washed away4 ,the naphtha and the gaseous hydrocarbons formed with it under the pressure of the overlying earth and water impregnated the layers of sand1 which are capable of absorbing a great quantity of such liquid1 and if above this there were strata impermeable to naphtha 7dense1 clayey1 damp strata7 the naphtha would accumulate in them4 ,it is thus preserved from remote geological periods up to the present day1 compressed and dissolved under the pressure of the gases which burst out in places forming naphtha fountains4 ,if this be granted1 it may be thought that in the comparatively new 7geologically speaking7 mountain chains1 such as the ,caucasian1 naphtha is even now being formed4 ,such a supposition may explain the remarkable fact that1 in ,pennsylvania1 localities where naphtha had been rapidly worked for five years have become exhausted1 and it becomes necessary to constantly have recourse to sinking new wells in fresh places4 ,thus1 from the year #ahei1 the workings were gradually transferred along a line running parallel to the ,alleghany mountains for a distance of more than #bjj miles1 whilst in ,baku the industry dates from time immemorial 7the ,persians worked near the village of ,ballaghana7 and up to the present time keeps to one and the same place4 ,the amounts of the ,pennsylvanian and ,baku annual outputs are at present equal--namely1 about #bej million poods 7#d million tons74 ,it may be that the ,baku beds1 as being of more recent geological formation1 are not so exhausted by nature as those of ,pennsylvania1 and perhaps in the neighbourhood of ,baku naphtha is still being formed1 which is partially indicated by the continued activity of the mud volcanoes4 ,as many varieties of naphtha contain in solution solid slightly volatile hydrocarbons like paraffin and mineral wax1 the production of ozocerite1 or mountain wax1 is accounted for in conjunction with the formation of naphtha4 ,ozocerite is found in ,galicia1 also in the neighbourhood of ,novorossisk1 in the ,caucasus1 and on the islands of the ,caspian ,sea 7particularly in the ,chileken and ,holy ,islands72 it is met with in large masses1 and is used for the production of paraffin and ceresene1 for the manufacture of candles1 and similar purposes4 ,as the naphtha treasures of the ,caucasus have hardly been exploited 7near ,baku and near ,kouban and ,grosnyi71 and as naphtha finds numerous uses1 the subject presents most interesting features to chemists and geologists1 and is worthy of the close attention of practical men4 ,,chapter ,,ix ,,compounds ,,of ,,carbon ,,with ,,oxygen ,,and ,,nitrogen ,illustration3 ,,fig4 #fa4--;,dumas and ,stas' apparatus for determining the composition of carbonic anhydride4 ,carbon1 graphite1 or a diamond is placed in the tube ,e in the furnace1 and heated in a stream of oxygen displaced from the bottle by water flowing from ,a4 ,the oxygen is purified from carbonic anhydride and water in the tubes ,b1 ,c1 ,d4 ,carbonic anhydride1 together with a certain amount of carbon monoxide1 is formed in ,e4 ,the latter is converted into carbonic acid by passing the products of combustion through a tube ,f1 containing cupric oxide heated in a furnace4 ,the cupric oxide oxidises this ,,co into ,,co#b1 forming metallic copper4 ,the potash bulbs ,h and tubes ,i1 ,j1 ,k retain the carbonic anhydride4 ,thus1 knowing the weight of carbon taken and the weight of the resultant carbonic anhydride 7by weighing ,h1 ,i1 ,j1 ,k before and after the experiment71 the composition of carbonic anhydride and the equivalent of carbon may be determined4 ,carbonic anhydride 7or carbonic acid or carbon dioxide1 ,,co#b7 was the first of all gases distinguished from atmospheric air4 ,paracelsus and ,van ,helmont1 in the sixteenth century1 knew that on heating limestone a particular gas separated1 which is also formed during the alcoholic fermentation of saccharine solutions 7for instance1 in the manufacture of wine72 they knew that it was identical with the gas which is produced by the combustion of charcoal1 and that in some cases it is found in nature4 ,in course of time it was found that this gas is absorbed by alkali1 forming a salt which1 under the action of acid1 again yields this same gas4 ,priestley found that this gas exists in air1 and ,lavoisier determined its formation during respiration1 combustion1 putrefaction1 and during the reduction of the oxides of metals by charcoal2 he determined its composition1 and showed that it only contains oxygen and carbon4 ,berzelius1 ,dumas with ,stas1 and ,roscoe1 determined its composition1 showing that it contains twelve parts of carbon to thirty-two of oxygen4 ,the composition by volume of this gas is determined from the fact that during the combustion of charcoal in oxygen1 the volume remains unchanged2 that is to say1 carbonic anhydride occupies the same volume as the oxygen which it contains--that is1 the atoms of the carbon are1 so to speak1 squeezed in between the atoms of the oxygen4 ,o#b occupies two volumes and is a molecule of ordinary oxygen2 ,,co#b likewise occupies two volumes1 and expresses the composition and molecular weight of the gas4 ,carbonic anhydride exists in nature1 both in a free state and in the most varied compounds4 ,in a free state it is always contained 7,chapter ,v47 in the air1 and in solution is in all kinds of water4 ,it is evolved from volcanoes1 from mountain fissures1 and in some caves4 ,the well-known ,dog grotto1 near ,agnano on the bay of ,bai1 near ,naples1 furnishes the best known example of such an evolution4 ,similar sources of carbonic anhydride are also found in other places4 ,in ,france1 for instance1 there is a well-known poisonous fountain in ,auvergne4 ,it is a round hole1 surrounded with luxurious vegetation and constantly evolving carbonic anhydride4 ,in the woods surrounding the ,lacher ,see near the ,rhine1 in the neighbourhood of extinct volcanoes1 there is a depression constantly filled with this same gas4 ,the insects which fly to this place perish1 animals being unable to breathe this gas4 ,the birds chasing the insects also die1 and this is turned to profit by the local peasantry4 ,many mineral springs carry into the air enormous quantities of this gas4 ,vichy in ,france1 ,sprdel in ,germany1 and ,narzan in ,russia 7in ,kislovodsk near ,piatigorsk7 are known for their carbonated gaseous waters4 ,much of this gas is also evolved in mines1 cellars1 diggings1 and wells4 ,people descending into such places are suffocated4 ,the combustion1 putrefaction1 and fermentation of organic substances give rise to the formation of carbonic anhydride4 ,it is also introduced into the atmosphere during the respiration of animals at all times and during the respiration of plants in darkness and also during their growth4 ,very simple experiments prove the formation of carbonic anhydride under these circumstances2 thus1 for example1 if the air expelled from the lungs be passed through a glass tube into a transparent solution of lime 7or baryta7 in water a white precipitate will soon be formed consisting of an insoluble compound of lime and carbonic anhydride4 ,by allowing the seeds of plants to grow under a bell jar1 or in a closed vessel1 the formation of carbonic anhydride may be similarly confirmed4 ,by confining an animal1 a mouse1 for instance1 under a bell jar1 the quantity of carbonic acid which it evolves may be exactly determined1 and it will he found to be many grams per day for a mouse4 ,such experiments on the respiration of animals have been also made with great exactitude with large animals1 such as men1 bulls1 sheep1 c4 ,by means of enormous hermetically closed bell receivers and the analysis of the gases evolved during respiration it was found that a man expels about #ijj grams 7more than two pounds7 of carbonic anhydride per diem1 and absorbs during this time #gjj grams of oxygen4#a ,it must be remarked that the carbonic anhydride of the air constitutes the fundamental food of plants 7,chapters ,,iii41 ,v41 and ,,viii47 ,carbonic anhydride in a state of combination with a variety of other substances is perhaps even more widely distributed in nature than in a free state4 ,some of these substances are very stable and form a large portion of the earth's crust4 ,for instance1 limestones1 calcium carbonate1 ,ca,c,o#c1 were formed as precipitates in the seas existing previously on the earth2 this is proved by their stratified structure and the number of remains of sea animals which they frequently contain4 ,chalk1 lithographic stone1 limestone1 marls 7a mixture of limestone and clay71 and many other rocks are examples of such sedimentary formations4 ,carbonates with various other bases--such as1 for instance1 magnesia1 ferrous oxide1 zinc oxide1 c4--are often found in nature4 ,the shells of molluscs also have the composition ,ca,c,o#c and many limestones were exclusively formed from the shells of minute organisms4 ,as carbonic anhydride 7together with water7 is produced during the combustion of all organic compounds in a stream of oxygen or by heating them with substances which readily part with their oxygen--for instance1 with copper oxide--this method is employed for estimating the amount of carbon in organic compounds1 more especially as the ,,co#b can be easily collected and the amount of carbon calculated from its weight4 ,for this purpose a hard glass tube1 closed at one end1 is filled with a mixture of the organic substance 7about #jb gram7 and copper oxide4 ,the open end of the tube is fitted with a cork and tube containing calcium chloride for absorbing the water formed by the oxidation of the substance4 ,this tube is hermetically connected 7by a caoutchouc tube7 with potash bulbs or other weighing apparatus 7,chapter ,v47 containing alkali destined to absorb the carbonic anhydride4 ,the increase in weight of this apparatus shows the amounts of carbonic anhydride formed during the combustion of the given substance1 and the quantity of carbon may be determined from this1 because three parts of carbon give eleven parts of carbonic anhydride4 ,illustration3 ,,fig4--#fb4 ,apparatus for the combustion of organic substances by igniting them with oxide of copper4 #a ,the quantity of carbonic acid gas exhaled by a man during the twenty-four hours is not evenly produced2 during the night more oxygen is taken in than during the day 7by night1 in twelve hours1 about #dej grams71 and more carbonic anhydride is separated by day than during night-time and repose2 thus1 of the #ijj grams produced during the twenty-four hours about #cge are given out during the night and #ebe by day4 ,this depends on the formation of carbonic anhydride during the work performed by the man in the day4 ,every movement is the result of some change of matter1 for force cannot be self-created 7in accordance with the law of the conservation of energy74 ,proportionally to the amount of carbon consumed an amount of energy is stored up in the organism and is consumed in the various movements performed by animals4 ,this is proved by the fact that during work a man exhales #ebe grams of carbonic anhydride in twelve hours instead of #cge1 absorbing the same amount of oxygen as before4 ,after a working day a man exhales by night almost the same amount of carbonic anhydride as after a day of rest1 so that during a total twenty-four hours a man exhales about #ijj grams of carbonic anhydride and absorbs about #ihj grams of oxygen4 ,therefore during work the change of matter increases4 ,the carbon expended on the work is obtained from the food2 on this account the food of animals ought certainly to contain carbonaceous substances capable of dissolving under the action of the digestive fluids1 and of passing into the blood1 or1 in other words1 capable of being digested4 ,such food for man and all other animals is formed of vegetable matter1 or of parts of other animals4 ,the latter in every case obtain their carbonaceous matter from plants1 in which it is formed by the separation of the carbon from the carbonic anhydride taken up during the day by the respiration of the plants4 ,the volume of the oxygen exhaled by plants is almost equal to the volume of the carbonic anhydride absorbed2 that is to say1 nearly all the oxygen entering into the plant in the form of carbonic anhydride is liberated in a free state1 whilst the carbon from the carbonic anhydride remains in the plant4 ,at the same time the plant absorbs moisture by its leaves and roots4 ,by a process which is unknown to us1 this absorbed moisture and the carbon obtained from the carbonic anhydride enter into the composition of the plants in the form of so-called carbohydrates1 composing the greater part of the vegetable tissues1 starch and cellulose of the composition ,c#f;,h#aj,o#e being representatives of them4 ,they may be considered like all carbohydrates as compounds of carbon and water1 #f;,c #e;,h#b,o4 ,in this way a circulation of the carbon goes on in nature by means of vegetable and animal organisms1 in which changes the principal factor is the carbonic anhydride of the air4 ,for the preparation of carbonic anhydride in laboratories and often in manufactories1 various kinds of calcium carbonate are used1 being treated with some acid2 it is1 however1 most usual to employ the so-called muriatic acid--that is1 an aqueous solution of hydrochloric acid1 ,h,cl--because1 in the first place1 the substance formed1 calcium chloride4 ,ca,cl#b1 is soluble in water and does not hinder the further action of the acid on the calcium carbonate1 and secondly because1 as we shall see further on1 muriatic acid is a common product of chemical works and one of the cheapest4 ,for calcium carbonate1 either limestone1 chalk1 or marble is used4#b ,ca,c,o#c #b;,h,cl ,ca,cl#b ,h#b,o ,,co#b4 ,the nature of the reaction in this case is the same as in the decomposition of nitre by sulphuric acid2 only in the latter case a hydrate is formed1 and in the former an anhydride of the acid1 because the hydrate1 carbonic acid1 ,h#b;,,co#c1 is unstable and as soon as it separates decomposes into water and its own anhydride4 ,it is evident from the explanation of the cause of the action of sulphuric acid on nitre that not every acid can be employed for obtaining carbonic anhydride2 namely1 those will not set it free which chemically are but slightly energetic1 or those which are insoluble in water1 or are themselves as volatile as carbonic anhydride4#c ,but as many acids are soluble in water and are less volatile than carbonic anhydride1 the latter is evolved by the action of most acids on its salts1 and this reaction takes place at ordinary temperatures4#d #b ,other acids may be used instead of hydrochloric2 for instance1 acetic1 or even sulphuric1 although this latter is not suitable1 because it forms as a product insoluble calcium sulphate 7gypsum7 which surrounds the untouched calcium carbonate1 and thus prevents a further evolution of gas4 ,but if porous limestone--for instance1 chalk--be treated with sulphuric acid diluted with an equal volume of water1 the liquid is absorbed and acting on the mass of the salt1 the evolution of carbonic anhydride continues evenly for a long time4 ,instead of calcium carbonate other carbonates may of course be used2 for instance1 washing-soda1 ,na#b;,,co#c1 which is often chosen when it is required to produce a rapid stream of carbonic anhydride 7for example1 for liquefying it74 ,but natural crystalline magnesium carbonate and similar salts are with difficulty decomposed by hydrochloric and sulphuric acids4 ,when for manufacturing purposes--for instance1 in precipitating lime in sugar-works--a large quantity of carbonic acid gas is required1 it is generally obtained by burning charcoal1 and the products of combustion1 rich in carbonic anhydride1 are pumped into the liquid containing the lime1 and the carbonic anhydride is thus absorbed4 ,another method is also practised1 which consists in using the carbonic anhydride separated during fermentation1 or that evolved from limekilns4 ,during the fermentation of sweet-wort1 grape-juice1 and other similar saccharine solutions1 the glucose ,c#f;,h#ab,o#f changes under the influence of the yeast organism1 forming alcohol 7#b;,c#b;,h#f,o71 and carbonic anhydride 7#b;,,co#b7 which separates in the form of gas2 if the fermentation proceeds in closed bottles sparkling wine is obtained4 ,when carbonic acid gas is prepared for saturating water and other beverages it is necessary to use it in a pure state4 ,whilst in the state in which it is evolved from ordinary limestones by the aid of acids it contains1 besides a certain quantity of acid1 the organic matters of the limestone2 in order to diminish the quantity of these substances the densest kinds of dolomites are used1 which contain less organic matter1 and the gas formed is passed through various washing apparatus1 and then through a solution of potassium permanganate1 which absorbs organic matter and does not take up carbonic anhydride4 #c ,hypochlorous acid1 ,h,cl,o1 and its anhydride1 ,cl#b,o1 do not displace carbonic acid1 and hydrogen sulphide has the same relation to carbonic acid as nitric acid to hydrochloric--an excess of either one displaces the other4 #d ,thus1 in preparing the ordinary effervescing powders1 sodium bicarbonate 7or acid carbonate of soda7 is used1 and mixed with powdered citric or tartaric acid4 ,in a dry state these powders do not evolve carbonic anhydride1 but when mixed with water the evolution takes place briskly1 which is due to the substances passing into solution4 ,the salts of carbonic acid may be recognised from the fact that they evolve carbonic acid with a hissing noise when treated with acids4 ,if vinegar1 which contains acetic acid1 be poured upon limestone1 marble1 malachite 7containing copper carbonate71 c41 carbonic anhydride is evolved with a hissing noise4 ,it is noteworthy that neither hydrochloric acid1 nor even sulphuric acid nor acetic acid1 acts on limestone except in presence of water4 ,we shall refer to this later on4 ,for the preparation of carbonic anhydride in laboratories1 marble is generally used4 ,it is placed in a ,woulfe's bottle and treated with hydrochloric acid in an apparatus similar to the one used for the production of hydrogen4 ,the gas evolved carries away through the tube part of the volatile hydrochloric acid1 and it is therefore necessary to wash the gas by passing it through another ,woulfe's bottle containing water4 ,if it be necessary to obtain dry carbonic anhydride1 it must be passed through chloride of calcium4#e #e ,the direct observations made 7#ahgf7 by ,messrs4 ,bogouski and ,kayander lead to the conclusion that the quantity of carbonic anhydride evolved by the action of acids on marble 7as homogeneous as possible7 is directly proportional to the time of action1 the extent of surface1 and the degree of concentration of the acid1 and inversely proportional to the molecular weight of the acid4 ,if the surface of a piece of ,carrara marble be equal to one decimetre1 the time of action one minute1 and one cubic decimetre or litre contains one gram of hydrochloric acid1 then about #jjb gram of carbonic anhydride will be evolved4 ,if the litre contains n grams of hydrochloric acid1 then by experiment the amount will be n #jjb of carbonic anhydride4 ,therefore1 if the litre contains #cfe 7 ,h,cl7 grams1 about #jgc gram of carbonic anhydride 7about half a litre7 would he evolved per minute4 ,if nitric acid or hydrobromic acid be used instead of hydrochloric1 then1 with a combining proportion of the acid1 the same quantity of carbonic anhydride will be evolved2 thus1 if the litre contains #fc 7 ,,hno#c7 grams of nitric acid1 or #ha 7 ,h,br7 grams of hydrobromic acid1 the quantity of carbonic anhydride evolved will still be #jgc gram4 ,spring1 in #ahij1 made a series of similar determinations4 ,carbonic anhydride may also be prepared by heating many of the salts of carbonic acid2 for instance1 by heating magnesium carbonate1 ,mg,c,o#c 7e4g41 in the form of dolomite71 the separation is easily effected1 particularly in the presence of the vapours of water4 ,the acid salts of carbonic acid 7for instance1 ,na,h,c,o#c1 see further on7 readily and abundantly give carbonic anhydride when heated4 ,carbonic anhydride is colourless1 has a slight smell and a faint acid taste2 its density in a gaseous state is twenty-two times as great as that of hydrogen1 because its molecular weight is forty-four4#f ,it is an example of those gaseous substances which have been long ago transformed into all the three states4 ,in order to obtain liquid carbonic anhydride1 the gas must be submitted to a pressure of thirty-six atmospheres at #j4g ,its absolute boiling point #cb4h ,liquid carbonic anhydride is colourless1 does not mix with water1 but is soluble in alcohol1 ether1 and oils2 at #j its specific gravity is #jhc4h bis ,the boiling point of this liquid lies at -#hj--that is to say1 the pressure of carbonic acid gas at that temperature does not exceed that of the atmosphere4 ,at the ordinary temperature the liquid remains as such for some time under ordinary pressure1 on account of its requiring a considerable amount of heat for its evaporation4 ,if the evaporation takes place rapidly1 especially if the liquid issues in a stream1 such a decrease of temperature occurs that a part of the carbonic anhydride is transformed into a solid snowy mass4 ,water1 mercury1 and many other liquids freeze on coming into contact with snow-like carbonic anhydride4#i ,in this form carbonic anhydride may be preserved for a long time in the open air1 because it requires still more heat to turn it into a gas than when in a liquid state4#i bis #f ,as carbonic anhydride is one and a half times heavier than air1 it diffuses with difficulty1 and therefore does not easily mix with air1 but sinks in it4 ,this may be shown in various ways2 for instance1 the gas may be carefully poured from one vessel into another containing air4 ,if a lighted taper be plunged into the vessel containing carbonic anhydride it is extinguished1 and then1 after pouring the gas into the other cylinder1 it will burn in the former and be extinguished in the latter4 ,if a certain quantity of carbonic anhydride be poured into a vessel containing air1 and soap-bubbles be introduced1 they will only sink as far as the stratum where the atmosphere of carbonic anhydride commences1 as this latter is heavier than the soap-bubbles filled with air4 ,naturally1 after a certain lapse of time1 the carbonic anhydride will be diffused throughout the vessel1 and form a uniform mixture with the air1 just as salt in water4 #g ,this liquefaction was first observed by ,faraday1 who sealed up in a tube a mixture of a carbonate and sulphuric acid4 ,afterwards this method was very considerably improved by ,thilorier and ,natterer1 whose apparatus is given in ,chapter ,,vi4 in describing ,n#b,o4 ,it is1 however1 necessary to remark that the preparation of liquid carbonic anhydride requires good liquefying apparatus1 constant cooling1 and a rapid preparation of large masses of carbonic anhydride4 #h ,carbonic anhydride1 having the same molecular weight as nitrous oxide1 very much resembles it when in a liquid state4 #h bis ,when poured into a tube1 which is then sealed up1 liquefied carbonic anhydride can be easily preserved1 because a thick tube easily supports the pressure 7about #ej atmospheres7 exerted by the liquid at the ordinary temperature4 #i ,when a fine stream of liquid carbonic anhydride is discharged into a closed metallic vessel1 about one-third of its mass solidifies and the remainder evaporates4 ,in employing solid carbonic anhydride for making experiments at low temperatures1 it is best to use it mixed with ether1 otherwise there will be few points of contact4 ,if a stream of air be blown through a mixture of liquid carbonic anhydride and ether1 the evaporation proceeds rapidly1 and great cold is obtained4 ,at present in some special manufactories 7and for making artificial mineral waters7 carbonic anhydride is liquefied on the large scale1 filled into wrought-iron cylinders provided with a valve1 and in this manner it can be transported and preserved safely for a long time4 ,it is used1 for instance1 in breweries4 #i bis ,solid carbonic anhydride1 notwithstanding its very low temperature1 can be safely placed on the hand1 because it continually evolves gas which prevents its coming into actual contact with the skin1 but if a piece be squeezed between the fingers1 it produces a severe frost bite similar to a burn4 ,if the snow-like solid be mixed with ether1 a semi-liquid mass is obtained1 which is employed for artificial refrigeration4 ,this mixture may be used for liquefying many other gases--such as chlorine1 nitrous oxide1 hydrogen sulphide1 and others4 ,the evaporation of such a mixture proceeds with far greater rapidity under the receiver of an air-pump1 and consequently the refrigeration is more intense4 ,by this means many gases may be liquefied which resist other methods--namely1 olefiant gas1 hydrochloric acid gas1 and others4 ,liquid carbonic anhydride in this case congeals in the tube into a glassy transparent mass4 ,pictet availed himself of this method for liquefying many permanent gases 7see ,chapter ,,ii47 ,bleekrode1 by compressing solid ,,co#b in a cylinder by means of a piston1 obtained a semi-transparent stick1 which contained as much as #ac and even #af gram of ,,co#b per cubic centimetre4 ,in this form the ,,co#b slowly evaporated1 and could be kept for a long time4 ,the capacity which carbonic anhydride has of being liquefied stands in connection with its considerable solubility in water1 alcohol1 and other liquids4 ,its solubility in water has been already spoken of in the first chapter4 ,carbonic anhydride is still more soluble in alcohol than in water1 namely at #j one volume of alcohol dissolves #dc volumes of this gas1 and at #bj #bi volumes4 ,aqueous solutions of carbonic anhydride1 under a pressure of several atmospheres1 are now prepared artificially1 because water saturated with this gas promotes digestion and quenches thirst4 ,for this purpose the carbonic anhydride is pumped by means of a force-pump into a closed vessel containing the liquid1 and then bottled off1 taking special means to ensure rapid and air-tight corking4 ,various effervescing drinks and artificially effervescing wines are thus prepared4 ,the presence of carbonic anhydride has an important significance in nature1 because by its means water acquires the property of decomposing and dissolving many substances which are not acted on by pure water2 for instance1 calcium phosphates and carbonates are soluble in water containing carbonic acid4 ,if the water in the interior of the earth is saturated with carbonic acid under pressure1 the quantity of calcium carbonate in solution may reach three grams per litre1 and on issuing at the surface1 as the carbonic anhydride escapes1 the calcium carbonate will be deposited4#aj ,water charged with carbonic anhydride brings about the destruction of many rocky formations by removing the lime1 alkali1 c41 from them4 ,this process has been going on and continues on an enormous scale4 ,rocks contain silica and the oxides of various metals2 amongst others1 the oxides of aluminium1 calcium1 and sodium4 ,water charged with carbonic acid dissolves both the latter1 transforming them into carbonates4 ,the waters of the ocean ought1 as the evolution of the carbonic anhydride proceeds1 to precipitate salts of lime2 these are actually found everywhere on the surface of the ground in those places which previously formed the bed of the ocean4 ,the presence of carbonic anhydride in solution in water is essential to the nourishment and growth of water plants4 #aj ,if such water trickles through crevices and enters a cavern1 the evaporation will be slow1 and therefore in those places from which the water drips growths of calcium carbonate will be formed1 just like the icicles formed on the roof-gutters in winter-time4 ,similar conical and cylindrical stony growths form the so-called stalactites or pendants hanging from above and stalagmites formed on the bottom of caves4 ,sometimes these two kinds meet together1 forming entire columns filling the cave4 ,many of these caves are remarkable for their picturesqueness2 for instance1 the cave of ,antiparos1 in the ,grecian ,archipelago4 ,this same cause also forms spongy masses of calcium carbonate in those places where the springs come to the surface of the earth4 ,it is therefore very evident that a calcareous solution is sometimes capable of penetrating plants and filling the whole of their mass with calcium carbonate4 ,this is one of the forms of petrified plants4 ,calcium phosphate in solution in water containing carbonic acid plays an important part in the nourishment of plants1 because all plants contain both lime and phosphoric acid4 ,although carbonic anhydride is soluble in water1 yet no definite hydrate is formed2#aa nevertheless an idea of the composition of this hydrate may be formed from that of the salts of carbonic acid1 because a hydrate is nothing but a salt in which the metal is replaced by hydrogen4 ,as carbonic anhydride forms salts of the composition ,k#b;,,co#c1 ,na#b;,,co#c1 ,h,na,c,o#c1 c41 therefore carbonic acid ought to have the composition ,h#b;,,co#c--that is1 it ought to contain ,,co#b ,h#b,o4 ,whenever this substance is formed1 it decomposes into its component parts--that is1 into water and carbonic anhydride4 ,the acid properties of carbonic anhydride#aa bis are demonstrated by its being directly absorbed by alkaline solutions and forming salts with them4 ,in distinction from nitric1 ,,hno#c1 and similar monobasic acids which with univalent metals 7exchanging one atom for one atom of hydrogen7 give salts such as those of potassium1 sodium1 and silver containing only one atom of the metal 7,na,n,o#c1 ,ag,n,o#c71 and with bivalent#ab metals 7such as calcium1 barium1 lead7 salts containing two acid groups--for example1 ,ca7,,no#c7b1 ,pb7,,no#c7b--;carbonic acid1 ,h#b;,,co#c1 is bibasic1 that is contains two atoms of hydrogen in the hydrate or two atoms of univalent metals in their salts3 for example1 ,na#b;,,co#c is washing soda1 a normal salt2 ,na,h,c,o#c is the bicarbonate1 an acid salt4 ,therefore1 if ,m be a univalent metal1 its carbonates in general are the normal carbonate ,m#b;,,co#c and the acid carbonate1 ,m,,hco#c2 or if ,m be a bivalent metal 7replacing ,h#b7 its normal carbonate will be ,m,,co#c2 these metals do not usually form acid salts1 as we shall see further on4 ,the bibasic character of carbonic acid is akin to that of sulphuric acid1 ,h#b,,so#d1ac but the latter1 in distinction from the former1 is an example of the energetic or strong acids 7such as nitric or hydrochloric71 whilst in carbonic acid we observe but feeble development of the acid properties2 hence carbonic acid must be considered a weak acid4 ,this conception must1 however1 be taken as only comparative1 as up to this time there is no definitely established rule for measuring the energy#ad of acids4 ,the feeble acid properties of carbonic acid may1 however1 be judged from the joint evidence of many properties4 ,with such energetic alkalis as soda and potash1 carbonic acid forms normal salts1 soluble in water1 but having an alkaline reaction and in many cases themselves acting as alkalis4#ae ,the acid salts of these alkalis1 ,na,h,c,o#c and ,,khco#c1 have a neutral reaction on litmus1 although they1 like acids1 contain hydrogen1 which may be exchanged for metals4 ,the acid salts of such acids--as1 for instance1 of sulphuric acid1 ,na,h,s,o#d--;have a clearly defined acid reaction1 and therefore carbonic acid is unable to neutralise the powerful basic properties of such alkalis as potash or soda4 ,carbonic acid does not even combine at all with feeble bases1 such as alumina1 ,al#b,o#c1 and therefore if a strong solution of sodium carbonate1 ,na#b;,,co#c1 be added to a strong solution of aluminium sulphate1 ,al#b7,,so#d7c1 although according to double saline decompositions aluminium carbonate1 ,al#b7;,,co#c7c1 ought to be formed1 the carbonic acid separates1 for this salt splits up in the presence of water into aluminium hydroxide and carbonic anhydride3 ,al#b7;,,co#c7c #c;,h#b,o ,al#b7,,oh7#f #c;,,co#b4 ,thus feeble bases are unable to retain carbonic acid even at ordinary temperatures4 ,for the same reason1 in the case of bases of medium energy1 although they form carbonates1 the latter are comparatively easily decomposed by heating1 as is shown by the decomposition of copper carbonate1 ,cu,c,o#c 7see ,introduction71 and even of calcium carbonate1 ,ca,c,o#c4 ,only the normal 7not the acid7 salts of such powerful bases as potassium and sodium are capable of standing a red heat without decomposition4 ,the acid salts--for instance1 ,na,h,c,o#c--;decompose even on heating their solutions 7#b,na,h,c,o#c ,na#b;,,co#c ,h#b,o ,,co#b71 evolving carbonic anhydride4 ,the amount of heat given out by the combination of carbonic acid with bases also shows its feeble acid properties1 being considerably less than with energetic acids4 ,thus if a weak solution of forty grams of sodium hydroxide be saturated 7up to the formation of a normal salt7 with sulphuric or nitric acid or another powerful acid1 from thirteen to fifteen thousand calories are given out1 but with carbonic acid only about ten thousand calories4#af ,the majority of carbonates are insoluble in water1 and therefore such solutions as sodium1 potassium1 or ammonium carbonates form in solutions of most other salts1 ,,mx or ,m,x#b1 insoluble precipitates of carbonates1 ,m#b;,,co#c or ,m,,co#c4 ,thus a solution of barium chloride gives with sodium carbonate a precipitate of barium carbonate1 ,ba,c,o#c4 ,for this reason rocks1 especially those of aqueous origin1 very often contain carbonates2 for example1 calcium1 ferrous1 or magnesium carbonates1 c4 #aa ,the crystallohydrate1 ,,co#b1h;,h#b,o of ,wroblewski 7,chapter #a41 ,note #fg71 in the first place1 is only formed under special conditions2 in the second place1 its existence still requires confirmation2 and in the third place1 it does not correspond with that hydrate ,h#b;,,co#c which should occur1 judging from the composition of the salts4 #aa bis ,it is easy to demonstrate the acid properties of carbonic anhydride by taking a long tube1 closed at one end1 and filling it with this gas2 a test-tube is then filled with a solution of an alkali 7for instance1 sodium hydroxide71 which is then poured into the long tube and the open end is corked4 ,the solution is then well shaken in the tube1 and the corked end plunged into water4 ,if the cork be now withdrawn under water1 the water will fill the tube4 ,the vacuum obtained by the absorption of the carbonic anhydride by an alkali is so complete that even an electric discharge will not pass through it4 ,this method is often applied to produce a vacuum4 #ab ,the reasons for distinguishing the uni-1 bi-1 tri-1 and quadrivalent metals will be explained hereafter on passing from the univalent metals 7,na1 ,k1 ,li7 to the bivalent 7,mg4 ,ca1 ,ba71 ,chapter ,,xiv4 #ac ,up to the year #ahdj1 or thereabout1 acids were not distinguished by their basicity4 ,graham1 while studying phosphoric acid1 ,h#c,,po#d1 and ,liebig1 while studying many organic acids1 distinguished mono-1 bi-1 and tribasic acids4 ,gerhardt and ,laurent generalised these relations1 showing that this distinction extends over many reactions 7for instance1 to the faculty of bibasic acids of forming acid salts with alkalis1 ,,kho or ,na,h,o1 or with alcohols1 ,,rho1 c472 but now1 since a definite conception as to atoms and molecules has been arrived at1 the basicity of an acid is determined by the number of hydrogen atoms1 contained in a molecule of the acid1 which can be exchanged for metals4 ,if carbonic acid forms acid salts1 ,na,h,c,o#c1 and normal salts1 ,na#b;,,co#c1 it is evident that the hydrate is ,h#b;,,co#c1 a bibasic acid4 ,otherwise it is at present impossible to account for the composition of these salts4 ,but when ,c #f and ,o #h were taken1 then the formula ,,co#b expressed the composition1 but not the molecular weight1 of carbonic anhydride2 and the composition of the normal salt would be ,na#b;,c#b,o#f or ,na,c,o#c1 therefore carbonic acid might have been considered as a monobasic acid4 ,then the acid salt would have been represented by ,na,c,o#c1;,,hco#c4 ,such questions were the cause of much argument and difference of opinion among chemists about forty years ago4 ,at present there cannot be two opinions on the subject if the law of ,avogadro-,gerhardt and its consequences be strictly adhered to4 ,it may1 however1 be observed here that the monobasic acids ,r7,,oh7 were for a long time considered to be incapable of being decomposed into water and anhydride1 and this property was ascribed to the bibasic acids ,r7,,oh7#b as containing the elements necessary for the separation of the molecule of water1 ,h#b,o4 ,thus ,h#b,,so#d or ,,so#b7,,oh7#b1 ,h#b;,,co#c1 or ,,co7,,oh7#b1 and other bibasic acids decompose into an anhydride1 ,,ro1 and water1 ,h#b,o4 ,but as nitrous1 ,,hno#b1 iodic1 ,,hio#c1 hypochlorous1 ,h,cl,o1 and other monobasic acids easily give their anhydrides ,n#b,o#c1 ,i#b,o#e1 ,cl#b,o1 c41 that method of distinguishing the basicity of acids1 although it fairly well satisfies the requirements of organic chemistry1 cannot be considered correct4 ,it may also be remarked that up to the present time not one of the bibasic acids has been found to have the faculty of being distilled without being decomposed into anhydride and water 7even ,h#b,,so#d1 on being evaporated and distilled1 gives ,,so#c ,h#b,o71 and the decomposition of acids into water and anhydride proceeds particularly easily in dealing with feebly energetic acids1 such as carbonic1 nitrous1 boric1 and hypochlorous4 ,let us add that carbonic acid1 as a hydrate corresponding to marsh gas1 ,c7,,ho7#d ,,co#b #b;,h#b,o1 ought to be tetrabasic4 ,but in general it does not form such salts4 ,basic salts1 however1 such as ,cu,c,o#c;,cu,o1 may be regarded in this sense1 for ,c,cu#b,o#d corresponds with ,,ch#d,o#d1 as ,cu corresponds with ,h#b4 ,amongst the ethereal salts 7alcoholic derivatives7 of carbonic acid corresponding cases are1 however1 observed2 for instance1 ethylic orthocarbonate1 ,c7,c#b;,h#e,o7#d 7obtained by the action of chloropicrin1 ,c7,,no#b7;,cl#c1 on sodium ethoxide1 ,c#b;,h#e,o,na2 boiling point #aeh2 specific gravity1 #jib74 ,the name orthocarbonic acid for ,,ch#d,o#d is taken from orthophosphoric acid1 ,,ph#c,o#d1 which corresponds with ,,ph#c 7see ,chapter on ,phosphorus74 #ad ,long ago endeavours were made to find a measure of affinity of acids and bases1 because some of the acids1 such as sulphuric or nitric1 form comparatively stable salts1 decomposed with difficulty by heat and water1 whilst others1 like carbonic and hypochlorous acids1 do not combine with feeble bases1 and with most of the other bases form salts which are easily decomposed4 ,the same may be said with regard to bases1 among which those of potassium1 ,k#b,o1 sodium1 ,na#b,o1 and barium1 ,ba,o1 may serve as examples of the most powerful1 because they combine with the most feeble acids and form a mass of salts of great stability1 whilst as examples of the feeblest bases alumina1 ,al#b,o#c1 or bismuth oxide1 ,bi#b,o#c1 may be taken1 because they form salts easily decomposed by water and by heat if the acid be volatile4 ,such a division of acids and bases into the feeblest and most powerful is justified by all evidence concerning them1 and is quoted in this work4 ,but the teaching of this subject in certain circles has acquired quite a new tone1 which1 in my opinion1 cannot be accepted without certain reservations and criticisms1 although it comprises many interesting features4 ,the fact is that ,thomsen1 ,ostwald1 and others proposed to express the measure of affinity of acids to bases by figures drawn from data of the measure of displacement of acids in aqueous solutions1 judging 7#a7 from the amount of heat developed by mixing a solution of the salt with a solution of another acid 7the avidity of acids1 according to ,thomsen72 7#b7 from the change of the volumes accompanying such a mutual action of solutions 7,ostwald72 7#c7 from the change of the index of refraction of solutions 7,ostwald71 c4 ,besides this there are many other methods which allow us to form an opinion about the distribution of bases among various acids in aqueous solutions4 ,some of these methods will be described hereafter4 ,it ought1 however1 to be remarked that in making investigations in aqueous solutions the affinity to water is generally left out of sight4 ,if a base ,n1 combining with acids ,x and ,y in presence of them both1 divides in such a way that one-third of it combines with ,x and two-thirds with ,y1 a conclusion is formed that the affinity1 or power of forming salts1 of the acid ,y is twice as great as that of ,x4 ,but the presence of the water is not taken into account4 ,if the acid ,x has an affinity for water and for ,n it will be distributed between them2 and if ,x has a greater affinity for water than ,y1 then less of ,x will combine with ,n than of ,y4 ,if1 in addition to this1 the acid ,x is capable of forming an acid salt ,,nx#b1 and ,y is not1 the conclusion of the relative strength of ,x and ,y will be still more erroneous1 because the ,x set free will form such a salt on the addition of ,y to ,,nx4 ,we shall see in ,chapter ,x4 that when sulphuric and nitric acids in weak aqueous solution act on sodium1 they are distributed exactly in this way3 namely1 one-third of the sodium combines with the sulphuric and two-thirds with the nitric acid2 but1 in my opinion1 this does not show that sulphuric acid1 compared with nitric acid1 possesses but half the degree of affinity for bases like soda1 and only demonstrates the greater affinity of sulphuric acid for water compared with that of nitric acid4 ,in this way the methods of studying the distribution in aqueous solutions probably only shows the difference of the relation of the acid to a base and to water4 ,in view of these considerations1 although the teaching of the distribution of salt-forming elements in aqueous solutions is an object of great and independent interest1 it can hardly serve to determine the measure of affinity between bases and acids4 ,similar considerations ought to be kept in view when determining the energy of acids by means of the electrical conductivity of their weak solutions4 ,this method1 proposed by ,arrhenius 7#ahhd71 and applied on an extensive scale by ,ostwald 7who developed it in great detail in his ,lehrbuch d4 allgemeinen ,chemie1 v4 ii41 #ahhg71 is founded on the fact that the relation of the so-called molecular electrical-conductivity of weak solutions of various acids 7,i7 coincides with the relation in which the same acids stand according to the distribution1 7,,ii7 found by one of the above-mentioned methods1 and with the relation deduced for them from observations upon the velocity of reaction1 7,,iii7 for instance1 according to the rate of the splitting up of an ethereal salt 7into alcohol and acid71 or from the rate of the so-called inversion of sugar--that is1 its transformation into glucose--as is seen by comparing the annexed figures1 in which the energy of hydrochloric acid is taken as equal to #ajj3-- ,i ,,ii ,,iii ,hydrochloric acid1 ,h,cl #ajj #ajj #ajj ,hydrobromic acid1 ,h,br #aja #ih #aje ,nitric acid1 ,,hno#c #ajj #ajj #if ,sulphuric acid1 ,h#b,,so#d #fe #di #gd ,formic acid1 ,,ch#b,o#b #b #d #a ,acetic acid1 ,c#b;,h#d,o#b #a #b #a ,oxalic acid1 ,c#b;,h#b,o#d #bj #bd #ah ,phosphoric acid1 ,,ph#c,o#d #g -- #f ,the coincidence of these figures1 obtained by so many various methods1 presents a most important and instructive relation between phenomena of different kinds1 but in my opinion it does not permit us to assert that the degree of affinity existing between bases and various acids is determined by all these various methods1 because the influence of the water must be taken into consideration4 ,on this account1 until the theory of solution is more thoroughly worked out1 this subject 7which for the present ought to be treated of in special treatises on chemical mechanics7 must be treated with great caution4 ,but now we may hope to decide this question guided by a study of the rate of reaction1 the influence of acids and bases upon indicators1 c41 all of which are treated fully in works on physical and theoretical chemistry4 #ae ,thus1 for instance1 in the washing of fabrics the caustic alkalis1 such as sodium hydroxide1 in weak solutions1 act in removing the fatty matter just in the same way as carbonate solutions2 for instance1 a solution of soda crystals1 ,na#b;,,co#c4 ,soap acts in the same way1 being composed of feeble acids1 either fatty or resinous1 combined with alkali4 ,on this account all such substances are applied in manufacturing processes1 and answer equally well in practice for bleaching and washing fabrics4 ,soda crystals or soap are preferred to caustic alkali1 because an excess of the latter may have a destructive effect on the fabrics4 ,it may be supposed that in aqueous solutions of soap or soda crystals1 part of the base will form caustic alkali2 that is to say1 the water will compete with the weak acids1 and the alkali will be distributed between them and the water4 #af ,although carbonic acid is reckoned among the feeble acids1 yet there are evidently many others still feebler--for instance1 prussic acid1 hypochlorous acid1 many organic acids1 c4 ,bases like alumina1 or such feeble acids as silica1 when in combination with alkalis1 are decomposed in aqueous solutions by carbonic acid1 but on fusion--that is1 without the presence of water--they displace it1 which clearly shows in phenomena of this kind how much depends upon the conditions of reaction and the properties of the substances formed4 ,these relations1 which at first sight appear complex1 may be best understood if we represent that two salts1 ,,mx and ,,ny1 in general always give more or less of two other salts1 ,,my and ,,nx1 and then examine the properties of the derived substances4 ,thus1 in solution1 sodium silicate1 ,na#b,si,o#c1 with carbonic anhydride will to some extent form sodium carbonate and silica1 ,si,o#b2 but the latter1 being colloid1 separates1 and the remaining mass of sodium silicate is again decomposed by carbonic anhydride1 so that finally silica separates and sodium carbonate is formed4 ,in a fused state the case is different2 sodium carbonate will react with silica to form carbonic anhydride and sodium silicate1 but the carbonic anhydride will be separated as a gas1 and therefore in the residue the same reaction will again take place1 and ultimately the carbonic anhydride is entirely eliminated and sodium silicate remains4 ,if1 on the other hand1 nothing is removed from the sphere of the reaction1 distribution takes place4 ,therefore1 although carbonic anhydride is a feeble acid1 still not for this reason1 but only in virtue of its gaseous form1 do all soluble acids displace it in saline solutions 7see ,chapter ,x47 ,carbonic anhydride--which1 like water1 is formed with the development of a large amount of heat--is very stable4 ,only very few substances are capable of depriving it of its oxygen4 ,however1 certain metals1 such as magnesium1 potassium and the like1 on being heated1 burn in it1 depositing carbon and forming oxides4 ,if a mixture of carbonic anhydride and hydrogen be passed through a heated tube1 the formation of water and carbonic oxide will be observed2 ,,co#b ,h#b ,,co ,h#b,o4 ,but only a portion of the carbonic acid gas undergoes this change1 and therefore the result will be a mixture of carbonic anhydride1 carbonic oxide1 hydrogen1 and water1 which does not suffer further change under the action of heat4#ag ,although1 like water1 carbonic anhydride is exceedingly stable1 still on being heated it partially decomposes into carbonic oxide and oxygen4 ,deville showed that such is the case if carbonic anhydride be passed through a long tube containing pieces of porcelain and heated to #a1cjj4 ,if the products of decomposition--namely1 the carbonic oxide and oxygen--be suddenly cooled1 they can be collected separately1 although they partly reunite together4 ,a similar decomposition of carbonic anhydride into carbonic oxide and oxygen takes place on passing a series of electric sparks through it 7for instance1 in the eudiometer74 ,under these conditions an increase of volume occurs1 because two volumes of ,,co#b give two volumes of ,,co and one volume of ,o4 ,the decomposition reaches a certain limit 7less than one-third7 and does not proceed further1 so that the result is a mixture of carbonic anhydride1 carbonic oxide1 and oxygen1 which is not altered in composition by the continued action of the sparks4 ,this is readily understood1 as it is a reversible reaction4 ,if the carbonic anhydride be removed1 then the mixture explodes when a spark is passed and forms carbonic anhydride4#ag bis ,if from an identical mixture the oxygen 7and not the carbonic anhydride7 be removed1 and a series of sparks be again passed1 the decomposition is renewed1 and terminates with the complete dissociation of the carbonic anhydride4 ,phosphorus is used in order to effect the complete absorption of the oxygen4 ,in these examples we see that a definite mixture of changeable substances is capable of arriving at a state of stable equilibrium1 destroyed1 however1 by the removal of one of the substances composing the mixture4 ,this is one of the instances of the influence of mass4 #ag ,hydrogen and carbon are near akin to oxygen as regards affinity1 but it ought to be considered that the affinity of hydrogen is slightly greater than that of carbon1 because during the combustion of hydrocarbons the hydrogen burns first4 ,some idea of this similarity of affinity may be formed by the quantity of heat evolved4 ,gaseous hydrogen1 ,h#b1 on combining with an atom of oxygen1 ,o #af1 develops #fi1jjj heat-units if the water formed be condensed to a liquid state4 ,if the water remains in the form of a gas 7steam7 the latent heat of evaporation must be subtracted1 and then #eh1jjj calories will be developed4 ,carbon1 ,c1 as a solid1 on combining with ,o#b #cb develops about #ig1jjj calories1 forming gaseous ,,co#b4 ,if it were gaseous like hydrogen1 and only contained ,c#b in its molecule1 much more heat would be developed1 and judging by other substances1 whose molecules on passing from the solid to the gaseous state absorb about #aj1jjj to #ae1jjj calories1 it must be held that gaseous carbon on forming gaseous carbonic anhydride would develop not less than #aaj1jjj calories--that is1 approximately twice as much as is developed in the formation of water4 ,and since there is twice as much oxygen in a molecule of carbonic anhydride as in a molecule of water1 the oxygen develops approximately the same quantity of heat on combining with hydrogen as with carbon4 ,that is to say1 that here we find the same close affinity 7see ,chapter ,,ii41 ,note #g7 determined by the quantity of heat as between hydrogen1 zinc1 and iron4 ,for this reason here also1 as in the case of hydrogen and iron1 we ought to expect an equal distribution of oxygen between hydrogen and carbon1 if they are both in excess compared with the amount of oxygen2 but if there be an excess of carbon1 it will decompose water1 whilst an excess of hydrogen will decompose carbonic anhydride4 ,even if these phenomena and similar ones have been explained in isolated cases1 a complete theory of the whole subject is still wanting in the present condition of chemical knowledge4 #ag bis ,the degree or relative magnitude of the dissociation of ,,co#b varies with the temperature and pressure--that is1 it increases with the temperature and as the pressure decreases4 ,deville found that at a pressure of #a atmosphere in the flame of carbonic oxide burning in oxygen1 about #dj per cent4 of the ,,co#b1 is decomposed when the temperature is about #c1jjj1 and at #a1ejj less than #a per cent4 7,krafts72 whilst under a pressure of #aj atmospheres about #cd per cent4 is decomposed at #c1cjj 7,mallard and ,le ,chatelier74 ,it follows therefore that1 under very small pressures1 the dissociation of ,,co#b will be considerable even at comparatively moderate temperatures1 but at the temperature of ordinary furnaces 7about #a1jjj7 even under the small partial pressure of the carbonic acid1 there are only small traces of decomposition which may be neglected in a practical estimation of the combustion of fuels4 ,we may here cite the molecular specific heat of ,,co#b 7i4e4 the amount of heat required to raise #dd units of weight of ,,co#b #a71 according to the determinations and calculations of ,mallard and ,le ,chatelier1 for a constant volume ,cv #fbf #jjjcgt2 for a constant pressure ,cp ,cv #b 7see ,chapter ,,xiv41 ,note #g71 i4e4 the specific heat of ,,co#b increases rapidly with a rise of temperature3 for example1 at #j 7per #a part by weight71 it is1 at a constant pressure #jahh1 at #a1jjj #jbgb1 at #b1jjj1 about #jcef4 ,a perfectly distinct rise of the specific heat 7for example1 at #b1jjj1 #jdji71 is given by a comparison of observations made by the above-mentioned investigators and by ,berthelot and ,vieille 7,kournakoff74 ,the cause of this must be looked for in dissociation4 ,t4 ,m4 ,cheltzoff1 however1 considers upon the basis of his researches upon explosives that it must be admitted that a maximum is reached at a certain temperature 7about #b1ejj71 beyond which the specific heat begins to fall4 ,although carbonic anhydride is decomposed on heating1 yielding oxygen1 it is nevertheless1 like water1 an unchangeable substance at ordinary temperatures4 ,its decomposition1 as effected by plants1 is on this account all the more remarkable2 in this case the whole of the oxygen of the carbonic anhydride is separated in the free state4 ,the mechanism of this change is that the heat and light absorbed by the plants are expended in the decomposition of the carbonic anhydride4 ,this accounts for the enormous influence of temperature and light on the growth of plants4 ,but it is at present not clearly understood how this takes place1 or by what separate intermediate reactions the whole process of decomposition of carbonic anhydride in plants into oxygen and the carbohydrates 7,note #a7 remaining in them1 takes place4 ,it is known that sulphurous anhydride 7in many ways resembling carbonic anhydride7 under the action of light 7and also of heat7 forms sulphur and sulphuric anhydride1 ,,so#c1 and in the presence of water1 sulphuric acid4 ,but no similar decomposition has been obtained directly with carbonic anhydride1 although it forms an exceedingly easily decomposable higher oxide--percarbonic acid2#ah and perhaps that is the reason the oxygen separates4 ,on the other hand1 it is known that plants always form and contain organic acids1 and these must be regarded as derivatives of carbonic acid1 as is seen by all their reactions1 of which we will shortly treat4 ,for this reason it might be thought that the carbonic acid absorbed by the plants first forms 7according to ,baeyer7 formic aldehyde1 ,,ch#b,o1 and from it organic acids1 and that these latter in their final transformation form all the other complex organic substances of the plants4 ,many organic acids are found in plants in considerable quantity2 for instance1 tartaric acid1 ,c#d;,h#f,o#f1 found in grape-juice and in the acid juice of many plants2 malic acid1 ,c#d;,h#f,o#e1 found not only in unripe apples but in still larger quantities in mountain ash berries2 citric acid1 ,c#f;,h#h,o#g1 found in the acid juice of lemons1 in gooseberries1 cranberries1 c42 oxalic acid1 ,c#b;,h#b,o#d1 found in wood-sorrel and many other plants4 ,sometimes these acids exist in a free state in the plants1 and sometimes in the form of salts2 for instance1 tartaric acid is met with in grapes as the salt known as cream of tartar1 but in the impure state called argol1 or tartar1 ,c#d;,h#e,,ko#f4 ,in sorrel we find the so-called salts of sorrel1 or acid potassium oxalate1 ,c#b;,,hko#d4 ,there is a very clear connection between carbonic anhydride and the above-mentioned organic acids--namely1 they all1 under one condition or another1 yield carbonic anhydride1 and can all be formed by means of it from substances destitute of acid properties4 ,the following examples afford the best demonstration of this fact3 if acetic acid1 ,c#b;,h#d,o#b1 the acid of vinegar1 be passed in the form of vapour through a heated tube1 it splits up into carbonic anhydride and marsh gas ,,co#b ,,ch#d4 ,but conversely it can also be obtained from those components into which it decomposes4 ,if one equivalent of hydrogen in marsh gas be replaced 7by indirect means7 by sodium1 and the compound ,,ch#c,na is obtained1 this directly absorbs carbonic anhydride1 forming a salt of acetic acid1 ,,ch#c,na ,,co#b ,c#b;,h#c,na,o#b2 from this acetic acid itself may be easily obtained4 ,thus acetic acid decomposes into marsh gas and carbonic anhydride1 and conversely is obtainable from them4 ,the hydrogen of marsh gas does not1 like that in acids1 show the property of being directly replaced by metals2 i4e4 ,,ch#d does not show any acid character whatever1 but on combining with the elements of carbonic anhydride it acquires the properties of an acid4 ,the investigation of all other organic acids shows similarly that their acid character depends on their containing the elements of carbonic anhydride4 ,for this reason there is no organic acid containing less oxygen in its molecule than there is in carbonic anhydride2 every organic acid contains in its molecule at least two atoms of oxygen4 ,in order to express the relation between carbonic acid1 ,h#b;,,co#c1 and organic acids1 and in order to understand the reason of the acidity of these latter1 it is simplest to turn to that law of substitution which shows 7,chapter ,,vi47 the relation between the hydrogen and oxygen compounds of nitrogen1 and permits us 7,chapter ,,viii47 to regard all hydrocarbons as derived from methane4 ,if we have a given organic compound1 ,a1 which has not the properties of an acid1 but contains hydrogen connected to carbon1 as in hydrocarbons1 then ,,aco#b will be a monobasic organic acid1 ,a#b;,,co#b a bibasic1 ,a#c;,,co#b a tribasic1 and so on--that is1 each molecule of ,,co#b transforms one atom of hydrogen into that state in which it may be replaced by metals1 as in acids4 ,this furnishes a direct proof that in organic acids it is necessary to recognise the group ,,hco#b1 or carboxyl4 ,if the addition of ,,co#b raises the basicity1 the removal of ,,co#b lowers it4 ,thus from the bibasic oxalic acid1 ,c#b;,h#b,o#d1 or phthalic acid1 ,c#h;,h#f,o#d1 by eliminating ,,co#b 7easily effected experimentally7 we obtain the monobasic formic acid1 ,,ch#b,o#b1 or benzoic acid1 ,c#g;,h#f,o#b1 respectively4 ,the nature of carboxyl is directly explained by the law of substitution4 ,judging from what has been stated in ,chapters ,,vi4 and ,,viii4 concerning this law1 it is evident that ,,co#b is ,,ch#d with the exchange of ,h#d for ,o#b1 and that the hydrate of carbonic anhydride1 ,h#b;,,co#c1 is ,,co7,,oh7#b1 that is1 methane1 in which two parts of hydrogen are replaced by two parts of the water radical 7,,oh1 hydroxyl7 and the other two by oxygen4 ,therefore the group ,,co7,,oh71 or carboxyl1 ,,hco#b1 is a part of carbonic acid1 and is equivalent to 7,,oh71 and therefore also to ,h4 ,that is1 it is a univalent residue of carbonic acid capable of replacing one atom of hydrogen4 ,carbonic acid itself is a bibasic acid1 both hydrogen atoms in it being replaceable by metals1 therefore carboxyl1 which contains one of the hydrogen atoms of carbonic acid1 represents a group in which the hydrogen is exchangeable for metals4 ,and therefore if #a1 #b 444 n atoms of non-metallic hydrogen are exchanged #a1 #b 444 n times for carboxyl1 we ought to obtain #a1 #b 444 n-basic acids4 ,organic acids are the products of the carboxyl substitution in hydrocarbons4#ah bis ,if in the saturated hydrocarbons1 ,cn,h#bn #b1 one part of hydrogen is replaced by carboxyl1 the monobasic saturated 7or fatty7 acids1 ,cn,h#bn #a7;,,co#b;,h71 will be obtained1 as1 for instance1 formic acid1 ,,hco#b;,h1 acetic acid1 ,,ch#b;,,co#b;,h1 444 stearic acid1 ,c#ag;,h#ce;,,co#b;,h1 c4 ,the double substitution will give bibasic acids1 ,cn,h#bn7,,co#b;,h77,,co#b;,h72 for instance1 oxalic acid n #j1 malonic acid n #a1 succinic acid n #b1 c4 ,to benzene1 ,c#f;,h#f correspond benzoic acid1 ,c#f;,h#e7;,,co#b;,h71 phthalic acid 7and its isomerides71 ,c#f;,h#d7;,,co#b;,h7#b1 up to mellitic acid1 ,c#f7;,,co#b;,h7#f1 in all of which the basicity is equal to the number of carboxyl groups4 ,as many isomerides exist in hydrocarbons1 it is readily understood not only that such can exist also in organic acids1 but that their number and structure may be foreseen4 ,this complex and most interesting branch of chemistry is treated separately in organic chemistry4 #ah ,percarbonic acid1 ,h#b;,,co#d 7 ,h#b;,,co#c ,o7 is supposed by ,a4 ,bach 7#ahic7 to be formed from carbonic acid in the action of light upon plants1 7in the same manner as1 according to the above scheme1 sulphuric acid from sulphurous7 with the formation of carbon1 which remains in the form of hydrates of carbon3 #c;,h#b;,,co#c #b;,h#b;,,co#d ,,ch#b,o4 ,this substance ,,ch#b,o expresses the composition of formic aldehyde which1 according to ,baeyer1 by polymerisation and further changes1 gives other hydrates of carbon and forms the first product which is formed in plants from ,,co#b4 ,and ,berthelot 7#ahgb7 had already1 at the time of the discovery of persulphuric 7,chapter ,,xx47 and pernitric 7,chapter ,,vi41 ,note #bf7 acids pointed out the formation of the unstable percarbonic anhydride1 ,,co#c4 ,thus1 notwithstanding the hypothetical nature of the above equation1 it may be admitted all the more as it explains the comparative abundance of peroxide of hydrogen 7,schne1 ,chapter ,,iv47 in the air1 and this also at the period of the most energetic growth of plants 7in ,july71 because percarbonic acid should like all peroxides easily give ,h#b,o#b4 ,besides which ,bach 7#ahid7 showed that1 in the first place1 traces of formic aldehyde and oxidising agents 7,,co#c or ,h#b,o#b7 are formed under the simultaneous action of ,,co#b and sunlight upon a solution containing a salt of uranium 7which is oxidised71 and diethylaniline 7which reacts with ,,ch#b,o71 and secondly1 that by subjecting ,ba,o#b1 shaken up in water1 to the action of a stream of ,,co#b in the cold1 extracting 7also in the cold7 with ether1 and then adding an alcoholic solution of ,na,h,o1 crystalline plates of a sodium salt may be obtained1 which with water evolve oxygen and leave sodium carbonate2 they are therefore probably the per-salt4 ,all these facts are of great interest and deserve further verification and elaboration4 #ah bis ,if ,,co#b is the anhydride of a bibasic acid1 and carboxyl corresponds with it1 replacing the hydrogen of hydrocarbons1 and giving them the character of comparatively feeble acids1 then ,,so#c is the anhydride of an energetic bibasic acid1 and sulphoxyl1 ,,so#b7,,oh71 corresponds with it1 being capable of replacing the hydrogen of hydrocarbons1 and forming comparatively energetic sulphur oxyacids 7sulphonic acids72 for instance1 ,c#f;,h#e7;,,cooh71 benzoic acid1 and ,c#f;,h#e7,,so#b,,oh71 benzenesulphonic acid1 are derived from ,c#f;,h#f4 ,as the exchange of ,h for methyl1 ,,ch#c1 is equivalent to the addition of ,,ch#b1 the exchange of carboxyl1 ,,cooh1 is equivalent to the addition of ,,co#b2 so the exchange of ,h for sulphoxyl is equivalent to the addition of ,,so#c4 ,the latter proceeds directly1 for instance3 ,c#f;,h#f ,,so#c ,c#f;,h#e7,,so#b,,oh74 ,as accordingding to the determinations of ,thomsen1 the heat of combustion of the vapours of acids ,,rco#b is known where ,r is a hydrocarbon1 and the heat of combustion of the hydrocarbons ,r themselves1 it may be seen that the formation of acids1 ,,rco#b1 from ,r ,,co#b1 is always accompanied by a small absorption or development of heat4 ,we give the heats of combustion in thousands of calories1 referred to the molecular weights of the substances3-- ,r ,h#b ,,ch#d ,c#b;,h#f ,c#f;,h#f #fhd #bab #cgj #ggg ,,rco#b #fid #bbe #chg #gff ,thus ,h#b1 corresponds with formic acid1 ,,ch#b,o#b2 benzene1 ,c#f;,h#f1 with benzoic acid1 ,c#g;,h#f,o#b4 ,the data for the latter are taken from ,stohmann1 and refer to the solid condition4 ,for formic acid ,stohmann gives the heat of combustion as #ei1jjj calories in a liquid state1 but in a state of vapour1 #fdf thousand units1 which is much less than according to ,thomsen4 ,carbonic ,oxide4--,this gas is formed whenever the combustion of organic substances takes place in the presence of a large excess of incandescent charcoal2 the air first burns the carbon into carbonic anhydride1 but this in penetrating through the red-hot charcoal is transformed into carbonic oxide1 ,,co#b ,c #b;,,co4 ,by this reaction carbonic oxide is prepared by passing carbonic anhydride through charcoal at a red heat4 ,it may be separated from the excess of carbonic anhydride by passing it through a solution of alkali1 which does not absorb carbonic oxide4 ,this reduction of carbonic anhydride explains why carbonic oxide is formed in ordinary clear fires1 where the incoming air passes over a large surface of heated coal4 ,a blue flame is then observed burning above the coal2 this is the burning carbonic oxide4 ,when charcoal is burnt in stacks1 or when a thick layer of coal is burning in a brazier1 and under many similar circumstances1 carbonic oxide is also formed4 ,in metallurgical processes1 for instance when iron is smelted from the ore1 very often the same process of conversion of carbonic anhydride into carbonic oxide occurs1 especially if the combustion of the coal be effected in high1 so-called blast1 furnaces and ovens1 where the air enters at the lower part and is compelled to pass through a thick layer of incandescent coal4 ,in this way1 also1 combustion with flame may be obtained from those kinds of fuel which under ordinary conditions burn without flame3 for instance1 anthracite1 coke1 charcoal4 ,heating by means of a gas-producer--that is1 an apparatus producing combustible carbonic oxide from fuel--is carried on in the same manner4#ai ,in transforming one part of charcoal into carbonic oxide #b1dbj heat units are given out1 and on burning to carbonic anhydride #h1jhj heat units4 ,it is evident that on transforming the charcoal first into carbonic oxide we obtain a gas which in burning is capable of giving out #e1ffj heat units for one part of charcoal4 ,this preparatory transformation of fuel into carbonic oxide1 or producer gas containing a mixture of carbonic oxide 7about #a_/c by volume7 and nitrogen 7#b_/c volume71 in many cases presents most important advantages1 as it is easy to completely burn gaseous fuel without an excess of air1 which would lower the temperature4#bj ,in stoves where solid fuel is burnt it is impossible to effect the complete combustion of the various kinds of fuel without admitting an excess of air4 ,gaseous fuel1 such as carbonic oxide1 is easily completely mixed with air and burnt without excess of it4 ,if1 in addition to this1 the air and gas required for the combustion be previously heated by means of the heat which would otherwise be uselessly carried off in the products of combustion 7smoke7#ba it is easy to reach a high temperature1 so high 7about #a1hjj7 that platinum may be melted4 ,such an arrangement is known as a regenerative furnace4#bb ,by means of this process not only may the high temperatures indispensable in many industries be obtained 7for instance1 glass-working1 steel-melting1 c471 but great advantage also#bc is gained as regards the quantity of fuel1 because the transmission of heat to the object to be heated1 other conditions being equal1 is determined by the difference of temperatures4 #ai ,illustration3 ,,fig4 #fc4--;,gas-producer for the formation of carbon monoxide for heating purposes4 ,in gas-producers all carbonaceous fuels are transformed into inflammable gas4 ,in those which 7on account of their slight density and large amount of water1 or incombustible admixtures which absorb heat7 are not as capable of giving a high temperature in ordinary furnaces--for instance1 fir cones1 peat1 the lower kinds of coal1 c4--the same gas is obtained as with the best kinds of coal1 because the water condenses on cooling1 and the ashes and earthy matter remain in the gas-producer4 ,the construction of a gas-producer is seen from the accompanying drawing4 ,the fuel lies on the fire-bars ,o1 the air enters through them and the ash-hole 7drawn by the draught of the chimney of the stove where the gas burns1 or else forced by a blowing apparatus71 the quantity of air being exactly regulated by means of valves4 ,the gases formed are then led by the tube ,v1 provided with a valve1 into the gas main ,u4 ,the addition of fuel ought to proceed in such a way as to prevent the generated gas escaping2 hence the space ,a is kept filled with the combustible material and covered with a lid4 #bj ,an excess of air lowers the temperature of combustion1 because it becomes heated itself1 as explained in ,chapter ,,iii4 ,in ordinary furnaces the excess of air is three or four times greater than the quantity required for perfect combustion4 ,in the best furnaces 7with fire-bars1 regulated air supply1 and corresponding chimney draught7 it is necessary to introduce twice as much air as is necessary1 otherwise the smoke contains much carbonic oxide4 #ba ,if in manufactories it is necessary1 for instance1 to maintain the temperature in a furnace at #a1jjj1 the flame passes out at this or a higher temperature1 and therefore much fuel is lost in the smoke4 ,for the draught of the chimney a temperature of #ajj to #aej is sufficient1 and therefore the remaining heat ought to be utilised4 ,for this purpose the flues are carried under boilers or other heating apparatus4 ,the preparatory heating of the air is the best means of utilisation when a high temperature is desired 7see ,note #bb74 #bb ,regenerative furnaces were introduced by the ,brothers ,siemens about the year #ahfj in many industries1 and mark a most important progress in the use of fuel1 especially in obtaining high temperatures4 ,the principle is as follows3 ,the products of combustion from the furnace are led into a chamber1 ,i1 and heat up the bricks in it1 and then pass into the outlet flue2 when the bricks are at a red heat the products of combustion are passed 7by altering the valves7 into another adjoining chamber1 ,,ii1 and air requisite for the combustion of the generator gases is passed through ,i4 ,in passing round about the incandescent bricks the air is heated1 and the bricks are cooled--that is1 the heat of the smoke is returned into the furnace4 ,the air is then passed through ,,ii1 and the smoke through ,i4 ,the regenerative burners for illuminating gas are founded on this same principle1 the products of combustion heat the incoming air and gas1 the temperature is higher1 the light brighter1 and an economy of gas is effected4 ,absolute perfection in these appliances has1 of course1 not yet been attained2 further improvement is still possible1 but dissociation imposes a limit because at a certain high temperature combinations do not ensue1 possible temperatures being limited by reverse reactions4 ,here1 as in a number of other cases1 the further investigation of the matter must prove of direct value from a practical point of view4 #bc ,at first sight it appears absurd1 useless1 and paradoxical to lose nearly one-third of the heat which fuel can develop1 by turning it into gas4 ,actually the advantage is enormous1 especially for producing high temperatures1 as is already seen from the fact that fuels rich in oxygen 7for instance1 wood7 when damp are unable1 with any kind of hearth whatever1 to give the temperature required for glass-melting or steel-casting1 whilst in the gas-producer they furnish exactly the same gas as the driest and most carbonaceous fuel4 ,in order to understand the principle which is here involved1 it is sufficient to remember that a large amount of heat1 but having a low temperature1 is in many cases of no use whatever4 ,we are unable here to enter into all the details of the complicated matter of the application of fuel1 and further particulars must be sought for in special technical treatises4 ,the following footnotes1 however1 contain certain fundamental figures for calculations concerning combustion4 ,the transformation of carbonic anhydride1 by means of charcoal1 into carbonic oxide 7,c ,,co#b ,,co ,,co7 is considered a reversible reaction1 because at a high temperature the carbonic oxide splits up into carbon and carbonic anhydride1 as ,sainte-,claire ,deville showed by using the method of the 'cold and hot tube4' ,inside a tube heated in a furnace another thin metallic 7silvered copper7 tube is fitted1 through which a constant stream of cold water flows4 ,the carbonic oxide coming into contact with the heated walls of the exterior tube forms charcoal1 and its minute particles settle in the form of lampblack on the lower side of the cold tube1 and1 since they are cooled1 do not act further on the oxygen or carbonic anhydride formed4#bd ,a series of electric sparks also decomposes carbonic oxide into carbonic anhydride and carbon1 and if the carbonic anhydride be removed by alkali complete decomposition may be obtained 7,deville74#bd bis ,aqueous vapour1 which is so similar to carbonic anhydride in many respects1 acts1 at a high temperature1 on charcoal in an exactly similar way1 ,c ,h#b,o ,h#b ,,co4 ,from #b volumes of carbonic anhydride with charcoal #d volumes of carbonic oxide 7#b molecules7 are obtained1 and precisely the same from #b volumes of water vapour with charcoal #d volumes of a gas consisting of hydrogen and carbonic oxide 7,h#b ,,co7 are formed4 ,this mixture of combustible gases is called water gas4#be ,but aqueous vapour 7and only when strongly superheated1 otherwise it cools the charcoal7 only acts on charcoal to form a large amount of carbonic oxide at a very high temperature 7at which carbonic anhydride dissociates72 it begins to react at about #ejj1 forming carbonic anhydride according to the equation ,c #b;,h#b,o ,,co#b #b;,h#b4 ,besides this1 carbonic oxide on splitting up forms carbonic anhydride1 and therefore water gas always contains a mixture#bf in which hydrogen predominates1 the volume of carbonic oxide being comparatively less1 whilst the amount of carbonic anhydride increases as the temperature of the reaction decreases 7generally it is more than #c per cent47 #bd ,the first product of combustion of charcoal is always carbonic anhydride1 and not carbonic oxide4 ,this is seen from the fact that with a shallow layer of charcoal 7less than a decimetre if the charcoal be closely packed7 carbonic oxide is not formed at all4 ,it is not even produced with a deep layer of charcoal if the temperature is not above #ejj1 and the current of air or oxygen is very slow4 ,with a rapid current of air the charcoal becomes red-hot1 and the temperature rises1 and then carbonic oxide appears 7,lang #ahhh74 ,ernst 7#ahia7 found that below #iie carbonic oxide is always accompanied by ,,co#b1 and that the formation of ,,co#b begins about #djj4 ,naumann and ,pistor determined that the reaction of carbonic anhydride with carbon commences at about #eej1 and that between water and carbon at about #ejj4 ,at the latter temperature carbonic anhydride is formed1 and only with a rise of temperature is carbonic oxide formed 7,lang7 from the action of the carbonic anhydride on the carbon1 and from the reaction ,,co#b ,h#b ,,co ,h#b,o4 ,rathke 7#ahha7 showed that at no temperature whatever is the reaction as expressed by the equation ,,co#b ,c #b;,,co#b1 complete2 a part of the carbonic anhydride remains1 and ,lang determined that at about #a1jjj not less than #c p4c4 of the carbonic anhydride remains untransformed into carbonic oxide1 even after the action has been continued for several hours4 ,the endothermal reactions1 ,c #b;,h#b,o ,,co#b #b;,h#b1 and ,,co ,h#b,o ,,co#b ,h#b1 are just as incomplete4 ,this is made clear if we note that on the one hand the above-mentioned reactions are all reversible1 and therefore bounded by a limit2 and1 on the other hand1 that at about #ejj oxygen begins to combine with hydrogen and carbon1 and also that the lower limits of dissociation of water1 carbonic anhydride1 and carbonic oxide lie near one another between #ejj and #a1bjj4 ,for water and carbonic oxide the lower limit of the commencement of dissociation is unknown1 but judging from the published data 7according to ,le ,chatelier1 #ahhh7 that of carbonic anhydride may be taken as about #a1jej4 ,even at about #bjj half the carbonic anhydride dissociates if the pressure be small1 about #jjja atmosphere4 ,at the atmospheric pressure1 not more than #jje p4c4 of the carbonic anhydride decomposes4 ,the reason of the influence of pressure is here evidently that the splitting up of carbonic anhydride into carbonic oxide and oxygen is accompanied by an increase in volume 7as in the case of the dissociation of nitric peroxide4 ,see ,chapter ,,vi41 ,note #df74 ,as in stoves and lamps1 and also with explosive substances1 the temperature is not higher than #b1jjj to #b1ejj1 it is evident that although the partial pressure of carbonic anhydride is small1 still its dissociation cannot here be considerable1 and probably does not exceed #e p4c4 #bd bis ,besides which ,l4 ,mond 7#ahij7 showed that the powder of freshly reduced metallic nickel 7obtained by heating the oxide to redness in a stream of hydrogen7 is able1 when heated even to #cej1 to completely decompose carbonic oxide into ,,co#b and carbon1 which remains with the nickel and is easily removed from it by heating in a stream of air4 ,here #b;,,co ,,co#b ,c4 ,it should be remarked that heat is evolved in this reaction 7,note #be71 and therefore that the influence of 'contact' may here play a part4 ,indeed1 this reaction must be classed among the most remarkable instances of the influence of contact1 especially as metals analogous to ,ni 7,fe and ,co7 do not effect this reaction 7see ,chapter ,,ii41 ,note #ag74 #be ,a molecular weight of this gas1 or #b volumes ,,co 7#bh grams71 on combustion 7forming ,,co#b7 gives out #fh1jjj heat units 7,thomsen #fg1ifj calories74 ,a molecular weight of hydrogen1 ,h#b 7or #b volumes71 develops on burning into liquid water #fi1jjj heat units 7according to ,thomsen #fh1cjj71 but if it forms aqueous vapour #eh1jjj heat units4 ,charcoal1 resolving itself by combustion into the molecular quantity of ,,co#b 7#b volumes71 develops #ig1jjj heat units4 ,from the data furnished by these exothermal reactions it follows3 7#a7 that the oxidation of charcoal into carbonic oxide develops #bi1jjj heat units2 7#b7 that the reaction ,c ,,co#b #b;,,co absorbs #ci1jjj heat units2 7#c7 ,c ,h#b,o ,h#b ,,co absorbs 7if the water be in a state of vapour7 #bi1jjj calories1 but if the water be liquid #dj1jjj calories 7almost as much as ,c ,,co#b72 7#d7 ,c ,h#b,o ,,co#b #b;,h#b absorbs 7if the water be in a state of vapour7 #ai1jjj heat units2 7#e7 the reaction ,,co ,h#b,o ,,co#b ,h#b develops #aj1jjj heat units if the water be in the state of vapour2 and 7#f7 the decomposition expressed by the equation #b;,,co ,c ,,co#b 7,note #bd bis7 is accompanied by the evolution of #ci1jjj units of heat4 ,hence it follows that #b volumes of ,,co or ,h#b burning into ,,co#b or ,h#b,o develop almost the same amount of heat1 just as also the heat effects corresponding with the equations ,c ,h#b,o ,,co ,h#b ,c ,,co#b ,,co ,,co are nearly equal4 #bf ,water gas1 obtained from steam and charcoal at a white heat1 contains about #ej p4c4 of hydrogen1 about #dj p4c4 of carbonic oxide1 about #e p4c4 of carbonic anhydride1 the remainder being nitrogen from the charcoal and air4 ,compared with producer gas1 which contains much nitrogen1 this is a gas much richer in combustible matter1 and therefore capable of giving high temperatures1 and is for this reason of the greatest utility4 ,if carbonic anhydride could be as readily obtained in as pure a state as water1 then ,,co might be prepared directly from ,,co#b ,c1 and in that case the utilisation of the heat of the carbon would be the same as in water gas1 because ,,co evolves as much heat as ,h#b1 and even more if the temperature of the smoke be over #ajj1 and the water remains in the form of vapour 7,note #be74 ,but producer gas contains a large proportion of nitrogen1 so that its effective temperature is below that given by water gas2 therefore in places where a particularly high temperature is required 7for instance1 for lighting by means of incandescent lime or magnesia1 or for steel melting1 c471 and where the gas can be easily distributed through pipes1 water gas is at present held in high estimation1 but when 7in ordinary furnaces1 re-heating1 glass-melting1 and other furnaces7 a very high temperature is not required1 and there is no need to convey the gas in pipes1 producer gas is generally preferred on account of the simplicity of its preparation1 especially as for water gas such a high temperature is required that the plant soon becomes damaged4 ,there are numerous systems for making water gas1 but the ,american patent of ,t4 ,lowe is generally used4 ,the gas is prepared in a cylindrical generator1 into which hot air is introduced1 in order to raise the coke in it to a white heat4 ,the products of combustion containing carbonic oxide are utilised for superheating steam1 which is then passed over the white hot coke4 ,water gas1 or a mixture of hydrogen and carbonic oxide1 is thus obtained4 ,water gas is sometimes called 'the fuel of the future1' because it is applicable to all purposes1 develops a high temperature1 and is therefore available1 not only for domestic and industrial uses1 but also for gas-motors and for lighting4 ,for the latter purpose platinum1 lime1 magnesia1 zirconia1 and similar substances 7as in the ,drummond light1 ,chapter ,,iii471 are rendered incandescent in the flame1 or else the gas is carburetted--that is1 mixed with the vapours of volatile hydrocarbons 7generally benzene or naphtha1 naphthalene1 or simply naphtha gas71 which communicate to the pale flame of carbonic oxide and hydrogen a great brilliancy1 owing to the high temperature developed by the combustion of the non-luminous gases4 ,as water gas1 possessing these properties1 may be prepared at central works and conveyed in pipes to the consumers1 and as it may be produced from any kind of fuel1 and ought to be much cheaper than ordinary gas1 it may as a matter of fact be expected that in course of time 7when experience shall have determined the cheapest and best way to prepare it7 it will not only supplant ordinary gas1 but will with advantage everywhere replace the ordinary forms of fuel1 which in many respects are inconvenient4 ,at present its consumption spreads principally for lighting purposes1 and for use in gas-engines instead of ordinary illuminating gas4 ,in some cases ,dowson gas is prepared in producers4 ,this is a mixture of water and producer gases obtained by passing steam into an ordinary producer 7,note #ai71 when the temperature of the carbon has become sufficiently high for the reaction ,c ,h#b,o ,,co ,h#b4 ,metals like iron and zinc which at a red heat are capable of decomposing water with the formation of hydrogen1 also decompose carbonic anhydride with the formation of carbonic oxide2 so both the ordinary products of complete combustion1 water and carbonic anhydride1 are very similar in their reactions1 and we shall therefore presently compare hydrogen and carbonic oxide4 ,the metallic oxides of the above-mentioned metals1 when reduced by charcoal1 also give carbonic oxide4 ,priestley obtained it by heating charcoal with zinc oxide4 ,as free carbonic anhydride may be transformed into carbonic oxide1 so1 in precisely the same way1 may that carbonic acid which is in a state of combination2 hence1 if magnesium or barium carbonates 7,mg,c,o#c or ,ba,c,o#c7 be heated to redness with charcoal1 or iron or zinc1 carbonic oxide will be produced--for instance1 it is obtained by heating an intimate mixture of #i parts of chalk and #a part of charcoal in a clay retort4 ,many organic substances#bg on being heated1 or under the action of various agents1 yield carbonic oxide2 amongst these are many organic or carboxylic acids4 ,the simplest are formic and oxalic acids4 ,formic acid1 ,,ch#b,o#b1 on being heated to #bjj1 easily decomposes into carbonic oxide and water1 ,,ch#b,o#b ,,co ,h#b,o4#bg bis ,usually1 however1 carbonic oxide is prepared in laboratories1 not from formic but from oxalic acid1 ,c#b;,h#b,o#d1 the more so as formic acid is itself prepared from oxalic acid4 ,the latter acid is easily obtained by the action of nitric acid on starch1 sugar1 c42 it is also found in nature4 ,oxalic acid is easily decomposed by heat2 its crystals first lose water1 then partly volatilise1 but the greater part is decomposed4 ,the decomposition is of the following nature3 it splits up into water1 carbonic oxide1 and carbonic anhydride1#bh ,c#b;,h#b,o#d ,h#b,o ,,co#b ,,co4 ,this decomposition is generally practically effected by mixing oxalic acid with strong sulphuric acid1 because the latter assists the decomposition by taking up the water4 ,on heating a mixture of oxalic and sulphuric acids a mixture of carbonic oxide and carbonic anhydride is evolved4 ,this mixture is passed through a solution of an alkali in order to absorb the carbonic anhydride1 whilst the carbonic oxide passes on4#bh bis #bg ,the so-called yellow prussiate1 ,k#d;,fe,c#f,n#f1 on being heated with ten parts of strong sulphuric acid forms a considerable quantity of very pure carbonic oxide quite free from carbonic anhydride4 #bg bis ,to perform this reaction1 the formic acid is mixed with glycerine1 because when heated alone it volatilises much below its temperature of decomposition4 ,when heated with sulphuric acid the salts of formic acid yield carbonic oxide4 #bh ,the decomposition of formic and oxalic acids1 with the formation of carbonic oxide1 considering these acids as carboxyl derivatives1 may be explained as follows3--,the first is ,h7,,cooh7 and the second 7,,cooh7#b1 or ,h#b in which one or both halves of the hydrogen are exchanged for carboxyl2 therefore they are equal to ,h#b ,,co#b and ,h#b #b;,,co#b2 but ,h#b reacts with ,,co#b1 as has been stated above1 forming ,,co and ,h#b,o4 ,from this it is also evident that oxalic acid on losing ,,co#b forms formic acid1 and also that the latter may proceed from ,,co ,h#b,o1 as we shall see further on4 #bh bis ,greshoff 7#ahhh7 showed that with a solution of nitrate of silver1 iodoform1 ,,chi#c1 forms ,,co according to the equation ,,chi#c #c;,ag,n,o#c ,h#b,o #c;,ag,i #c;,,hno#c ,,co4 ,the reaction is immediate and is complete4 ,in its physical properties carbonic oxide resembles nitrogen2 this is explained by the equality of their molecular weights4 ,the absence of colour and smell1 the low temperature of the absolute boiling point1 -#adj 7nitrogen1 -#adf71 the property of solidifying at -#bjj 7nitrogen1 -#bjb71 the boiling point of -#aij 7nitrogen1 -#bjc71 and the slight solubility 7,chapter ,i41 ,note #cj71 of carbonic oxide are almost the same as in those of nitrogen4 ,the chemical properties of both gases are1 however1 very different1 and in these carbonic oxide resembles hydrogen4 ,carbonic oxide burns with a blue flame1 giving #b volumes of carbonic anhydride from #b volumes of carbonic oxide1 just as #b volumes of hydrogen give #b volumes of aqueous vapour4 ,it explodes with oxygen1 in the eudiometer1 like hydrogen4#bi ,when breathed it acts as a strong poison1 being absorbed by the blood2#cj this explains the action of charcoal fumes1 the products of the incomplete combustion of charcoal and other carbonaceous fuels4 ,owing to its faculty of combining with oxygen1 carbonic oxide acts as a powerful reducing agent1 taking up the oxygen from many compounds at a red heat1 and being itself transformed into carbonic anhydride4 ,the reducing action of carbonic oxide1 however1 is 7like that of hydrogen1 ,chapter ,,ii47 naturally confined to those oxides which easily part with their oxygen--as1 for instance1 copper oxide--whilst the oxides of magnesium or potassium are not reduced4 ,metallic iron itself is capable of reducing carbonic anhydride to carbonic oxide1 just as it liberates the hydrogen from water4 ,copper1 which does not decompose water1 does not decompose carbonic oxide4 ,if a platinum wire heated to #cjj1 or spongy platinum at the ordinary temperature1 be plunged into a mixture of carbonic oxide and oxygen1 or of hydrogen and oxygen1 the mixture explodes4 ,these reactions are very similar to those peculiar to hydrogen4 ,the following important distinction1 however1 exists between them--namely3 the molecule of hydrogen is composed of ,h#b1 a group of elements divisible into two like parts1 whilst1 as the molecule of carbonic oxide1 ,,co1 contains unlike atoms of carbon and oxygen1 in none of its reactions of combination can it give two molecules of matter containing its elements4 ,this is particularly noticeable in the action of chlorine on hydrogen and on carbonic oxide respectively2 with the former chlorine forms hydrogen chloride1 and with the latter it produces the so-called carbonyl chloride1 ,c,o,cl#b3 that is to say1 the molecule of hydrogen1 ,h#b1 under the action of chlorine divides1 forming two molecules of hydrochloric acid1 whilst the molecule of carbonic oxide enters in its entirety into the molecule of carbonyl chloride4 ,this characterises the so-called diatomic or bivalent reactions of radicles or residues4 ,h is a monatomic residue or radicle1 like ,k1 ,cl1 and others1 whilst carbonic oxide1 ,,co1 is an indivisible 7undecomposable7 bivalent radicle1 equivalent to ,h#b and not to ,h1 and therefore combining with ,x#b and interchangeable with ,h#b4 ,this distinction is evident from the annexed comparison3 ,,hh1 hydrogen4 ,,co1 carbonic oxide4 ,h,cl1 hydrochloric acid4 ,c,o,cl#b1 carbonyl chloride4 ,,hko1 potash4 ,,co7,,ko7#b1 potassium carbonate4 ,,hnh#b1 ammonia4 ,,co7,,nh#b7b1 urea4 ,,hch#c1 methane4 ,,co7,,ch#c7b1 acetone4 ,,hho1 water4 ,,co7,,ho7#b1 carbonic acid4 #bi ,it is remarkable that1 according to the investigations of ,dixon1 perfectly dry carbonic oxide does not explode with oxygen when a spark of low intensity is used1 but an explosion takes place if there is the slightest admixture of moisture4 ,l4 ,meyer1 however1 showed that sparks of an electric discharge of considerable intensity produce an explosion4 ,n4 ,n4 ,beketoff demonstrated that combustion proceeds and spreads slowly unless there be perfect dryness4 ,i think that this may he explained by the fact that water with carbonic oxide gives carbonic anhydride and hydrogen1 but hydrogen with oxygen gives hydrogen peroxide 7,chapter ,,vii471 which with carbonic oxide forms carbonic anhydride and water4 ,the water1 therefore1 is renewed1 and again serves the same purpose4 ,but it may be that here it is necessary to acknowledge a simple contact influence4 ,after ,dixon had shown the influence of traces of moisture upon the reaction ,,co ,o1 many researches were made of a similar nature4 ,the fullest investigation into the influence of moisture upon the course of many chemical reactions was made by ,baker in #ahid4 ,he showed that with perfect dryness1 many chemical transformations 7for example1 the formation of ozone from oxygen1 the decomposition of ,ag,o1 ,k,cl,o#c under the action of heat1 c47 proceeds in exactly the same manner as in the presence of moisture2 but that in many cases traces of moisture have an evident influence4 ,we may mention the following instances3 7#a7 ,dry ,,so#c does not act upon dry ,ca,o or ,cu,o2 7#b7 perfectly dry sal-ammoniac does not give ,,nh#c with dry ,ca,o1 but simply volatilises2 7#c7 dry ,,no and ,o do not react2 7#d7 perfectly dry ,,nh#c and ,h,cl do not combine2 7#e7 perfectly dry sal-ammoniac does not dissociate at #cej 7,chapter ,,vii41 ,note #ae bis72 and 7#f7 perfectly dry chlorine does not act upon metals1 c4 #cj ,carbonic oxide is very rapid in its action1 because it is absorbed by the blood in the same way as oxygen4 ,in addition to this1 the absorption spectrum of the blood changes so that by the help of blood it is easy to detect the slightest traces of carbonic oxide in the air4 ,m4 ,a4 ,kapoustin found that linseed oil and therefore oil paints1 are capable of giving off carbonic oxide while drying 7absorbing oxygen74 ,such monatmic 7univalent7 residues1 ,x1 as ,h1 ,cl1 ,na1 ,,no#b1 ,,nh#d1 ,,ch#c1 ,,co#b;,h 7carboxyl71 ,,oh1 and others1 in accordance with the law of substitution1 combine together1 forming compounds1 ,,xx'2 and with oxygen1 or in general with diatomic 7bivalent7 residues1 ,y--for instance1 ,o1 ,,co1 ,,ch#b1 ,s1 ,ca1 c4 forming compounds ,,xx,y2 but diatomic residues1 ,y1 sometimes capable of existing separately may combine together1 forming ,,yy and with ,x#b or ,,xx1 as we see from the transition of ,,co into ,,co#b and ,c,o,cl#b4 ,this combining power of carbonic oxide appears in many of its reactions4 ,thus it is very easily absorbed by cuprous chloride1 ,cu,cl1 dissolved in fuming hydrochloric acid1 forming a crystalline compound1 ,c,o,cu#b;,cl#b1b;,h#b,o1 decomposable by water2 it combines directly with potassium 7at #ij71 forming 7,,kco7n#ca with platinum dichloride1 ,pt,cl#b1 with chlorine1 ,cl#b1 c4 #ca ,the molecule of metallic potassium 7,scott1 #ahhg71 like that of mercury1 contains only one atom1 and it is probably in virtue of this that the molecules ,,co and ,k combine together4 ,but as in the majority of cases potassium acts as a univalent radicle1 the polymeride ,k#b;,c#b,o#b is formed1 and probably ,k#aj;,c#aj,o#aj1 because products containing ,c#aj are formed by the action of hydrochloric acid4 ,the black mass formed by the combination of carbonic oxide with potassium explodes with great ease1 and oxidises in the air4 ,although ,brodie1 ,lerch1 and ,joannis 7who obtained it in #ahgc in a colourless form by means of ,,nh#c,k1 described in ,chapter ,,vi41 ,note #ad7 have greatly extended our knowledge of this compound1 much still remains unexplained4 ,it probably exists in various polymeric and isomeric forms1 having the composition 7,,kco7n and 7,na,c,o7n4 ,but the most remarkable compounds are 7#a7 the compound of ,,co with metallic nickel1 a colourless volatile liquid1 ,ni7,,co7#d1 obtained by ,l4 ,mond 7described in ,chapter ,,xxii47 and 7#b7 the compounds of carbonic oxide with the alkalis1 for instance with potassium or barium hydroxide1 c4--although it is not directly absorbed by them1 as it has no acid properties4 ,berthelot 7#ahfa7 showed that potash in the presence of water is capable of absorbing carbonic oxide1 but the absorption takes place slowly1 little by little1 and it is only after being heated for many hours that the whole of the carbonic oxide is absorbed by the potash4 ,the salt ,,chko#b is obtained by this absorption2 it corresponds with an acid found in nature--namely1 the simplest organic 7carboxylic7 acid1 formic acid1 ,,ch#b,o#b4 ,it can be extracted from the potassium salt by means of distillation with dilute sulphuric acid1 just as nitric acid is prepared from sodium nitrate4 ,the same acid is found in ants and in nettles 7when the stings of the nettles puncture the skin they break1 and the corrosive formic acid enters into the body72 it is also obtained during the action of oxidising agents on many organic substances2 it is formed from oxalic acid1 and under many conditions splits up into carbonic oxide and water4 ,in the formation of formic acid from carbonic oxide we observe an example of the synthesis of organic compounds1 such as are now very numerous1 and are treated of in detail in works on organic chemistry4 ,formic acid1 ,h7,,cho#b71 carbonic acid1 ,,ho7,,cho#b71 and oxalic acid1 7,,cho#b7b1 are the simple organic or carboxylic acids1 ,r7,,cho#b7 corresponding with ,,hh and ,,hoh4 ,commencing with carbonic oxide1 ,,co1 the formation of carboxylic acids is clearly seen from the fact that ,,co is capable of combining with ,x#b1 that is of forming ,,cox#b4 ,if1 for instance1 one ,x is an aqueous residue1 ,,oh 7hydroxyl71 and the other ,x is hydrogen1 then the simplest organic acid--formic acid1 ,h7,,cooh7--is obtained4 ,as all hydrocarbons 7,chapter ,,viii47 correspond with the simplest1 ,,ch#d1 so all organic acids may be considered to proceed from formic acid4 ,in a similar way it is easy to explain the relation to other compounds of carbon of those compounds which contain nitrogen4 ,by way of an example1 we will take one of the carboxyl acids1 ,r7,,co#b;,h71 where ,r is a hydrocarbon radicle 7residue74 ,such an acid1 like all others1 will give by combination with ,,nh#c an ammoniacal salt1 ,r7,,co#b,,nh#d74 ,this salt contains the elements for the formation of two molecules of water1 and under suitable conditions by the action of bodies capable of taking it up1 water may in fact be separated from ,r7,,co#b,,nh#d71 forming by the loss of one molecule of water1 amides1 ,,rconh#b1 and by the loss of two molecules of water1 nitriles1 ,,rcn1 otherwise known as cyanogen compounds or cyanides4#cb ,if all the carboxyl acids are united not only by many common reactions but also by a mutual conversion into each other 7an instance of which we saw above in the conversion of oxalic acid into formic and carbonic acids7 one would expect the same for all the cyanogen compounds also4 ,the common character of their reactions1 and the reciprocity of their transformation1 were long ago observed by ,gay-,lussac1 who recognised a common group or radicle 7residue7 cyanogen1 ,,cn1 in all of them4 ,the simplest compounds are hydrocyanic or prussic acid1 ,,hcn1 cyanic acid1 ,,ohcn1 and free cyanogen1 7,,cn7#b1 which correspond to the three simplest carboxyl acids3 formic1 ,,hco#b;,h1 carbonic1 ,,ohco#b;,h1 and oxalic1 7,,co#b;,h7#b4 ,cyanogen1 like carboxyl1 is evidently a monatomic residue and acid1 similar to chlorine4 ,as regards the amides ,,rconh#b1 corresponding to the carboxyl acids1 they contain the ammoniacal residue ,,nh#b1 and form a numerous class of organic compounds met with in nature and obtained in many ways1#cc but not distinguished by such characteristic peculiarities as the cyanogen compounds4 #cb ,the connection of the cyanogen compounds with the rest of the hydrocarbons by means of carboxyl was enunciated by me1 about the year #ahfj1 at the first ,annual ,meeting of the ,russian ,naturalists4 #cc ,thus1 for instance1 oxamide1 or the amide of oxalic acid1 7,,cnh#b,o7#b1 is obtained in the form of an insoluble precipitate on adding a solution of ammonia to an alcoholic solution of ethyl oxalate1 7,,co#b;,c#b;,h#e7b1 which is formed by the action of oxalic acid on alcohol3 7,,cho#b7b #b7;,c#b;,h#e7,,oh #b;,,hoh 7,,co#b;,c#b;,h#e7b4 ,as the nearest derivatives of ammonia1 the amides treated with alkalis yield ammonia and form the salt of the acid4 ,the nitriles do not1 however1 give similar reactions so readily4 ,the majority of amides corresponding to acids have a composition ,,rnh#b1 and therefore recombine with water with great ease even when simply boiled with it1 and with still greater facility in presence of acids or alkalis4 ,under the action of alkalis the amides naturally give off ammonia1 through the combination of water with the amide1 when a salt of the acid from which the amide was derived is formed3 ,,rnh#b ,,kho ,,rko ,,nh#c4 ,the same reaction takes place with acids1 only an ammoniacal salt of the acid is of course formed whilst the acid held in the amide is liberated3 ,,rnh#b ,h,cl ,h#b,o ,,rho ,,nh#d;,cl4 ,thus in the majority of cases amides easily pass into ammoniacal salts1 but they differ essentially from them4 ,no ammoniacal salt sublimes or volatilises unchanged1 and generally when heated it gives off water and yields an amide1 whilst many amides volatilise without alteration and frequently are volatile crystalline substances which may be easily sublimed4 ,such1 for instance1 are the amides of benzoic1 formic1 and many other organic acids4 ,the reactions and properties of the amides and nitriles of the organic acids are described in detail in books on organic chemistry2 we will here only touch upon the simplest of them1 and to clearly explain the derivative compounds will first consider the ammoniacal salts and amides of carbonic acid4 ,as carbonic acid is bibasic1 its ammonium salts ought to have the following composition3 acid carbonate of ammonium1 ,h7,,nh#d7;,,co#c1 and normal carbonate1 7,,nh#d7b;,,co#c2 they represent compounds of one or two molecules of ammonia with carbonic acid4 ,the acid salt appears in the form of a non-odoriferous and 7when tested with litmus7 neutral substance1 soluble at the ordinary temperature in six parts of water1 insoluble in alcohol1 and obtainable in a crystalline form either without water of crystallisation or with various proportions of it4 ,if an aqueous solution of ammonia be saturated with an excess of carbonic anhydride1 and then evaporated over sulphuric acid in the bell jar of an air-pump1 crystals of this salt are separated4 ,solutions of all other ammonium carbonates1 when evaporated under the air-pump1 yield crystals of this salt4 ,a solution of this salt1 even at the ordinary temperature1 gives off carbonic anhydride1 as do all the acid salts of carbonic acid 7for instance1 ,na,h,c,o#c71 and at #ch the separation of carbonic anhydride takes place with great rapidity4 ,on losing carbonic anhydride and water1 the acid salt is converted into the normal salt1 #b7,,nh#d7;,,hco#c ,h#b,o ,,co#b 7,,nh#d7b;,,co#c2 the latter1 however1 decomposes in solution1 and can therefore only be obtained in crystals1 7,,nh#d7b;,,co#c1;,h#b,o1 at low temperatures1 and from solutions containing an excess of ammonia as the product of dissociation of this salt3 7,,nh#d7b;,,co#c ,,nh#c 7,,nh#d7;,,hco#c4 ,but the normal salt1#cd according to the general type1 is capable of decomposing with separation of water1 and forming ammonium carbamate1 ,,nh#d,o7,,conh#b7 7,,nh#d7b;,,co#c-;,h#b,o2 this still further complicates the chemical transformations of the carbonates of ammonium4 ,it is in fact evident that1 by changing the ratios of water1 ammonia1 and carbonic acid1 various intermediate salts will be formed containing mixtures or combinations of those mentioned above4 ,thus the ordinary commercial carbonate of ammonia is obtained by heating a mixture of chalk and sulphate of ammonia 7,chapter ,,vi471 or sal-ammoniac1 #b,,nh#d;,cl ,ca,c,o#c ,ca,cl#b 7,,nh#d7b;,,co#c4 ,the normal salt1 however1 through loss of part of the ammonia1 partly forms the acid salt1 and1 partly through loss of water1 forms carbamate1 and most frequently presents the composition ,,nh#d,o7,,conh#b7 #b,,oh7,,co#b,,nh#d7 #d,,nh#c #c;,,co#b #b;,h#b,o4 ,this salt1 in parting under various conditions with ammonia1 carbonic anhydride1 and water1 does not present a constant composition1 and ought rather to be regarded as a mixture of acid salt and amide salt4 ,the latter must be recognised as entering into the composition of the ordinary carbonate of ammonia1 because it contains less water than is required for the normal or acid salt2#ce but on being dissolved in water this salt gives a mixture of acid and normal salts4 #cd ,the acid salt1 7,,nh#d7;,,hco#c1 on losing water ought to form the carbamic acid1 ,,oh7,,cnh#b,o72 but it is not formed1 which is accounted for by the instability of the acid salt itself4 ,carbonic anhydride is given off and ammonia is produced1 which gives ammonium carbamate4 #ce ,in the normal salt1 #b,,nh#c ,,co#b ,h#b,o1 in the acid salt1 ,,nh#c ,,co#b ,h#b,o1 but in the commercial salt only #b;,h#b,o to #c;,,co#b4 ,each of the two ammoniacal salts of carbonic acid has its corresponding amide4 ,that of the acid salt should be acid1 if the water given off takes up the hydrogen of the ammonia1 as it should according to the common type of formation of the amides1 so that ,,ohconh#b1 or carbamic acid1 is formed from ,,ohco#c,,nh#d4 ,this acid is not known in a free state1 but its corresponding ammoniacal salt or ammonium carbamate is known4 ,the latter is easily and immediately formed by mixing #b volumes of dry ammonia with #a volume of dry carbonic anhydride1 #b,,nh#c ,,co#b ,,nh#d,o7,,conh#b72 it is a solid substance1 smells strongly of ammonia1 attracts moisture from the air1 and decomposes completely at #fj4 ,the fact of this decomposition may be proved#cf by the density of its vapour1 which #ac 7,h #a72 this exactly corresponds with the density of a mixture of #b volumes of ammonia and #a volume of carbonic anhydride4 ,it is easily understood that such a combination will take place with any ammonium carbonate under the action of salts which take up the water--for instance1 sodium or potassium carbonate#cg--;as in an anhydrous state ammonia and carbonic anhydride only form one compound1 ,,co#bb,,nh#c4ch ,as the normal ammonium carbonate contains two ammonias1 and as the amides are formed with the separation of water at the expense of the hydrogen of the ammonias1 so this salt has its symmetrical amide1 ,,co7,,nh#b7b4 ,this must be termed carbamide4 ,it is identical with urea1 ,,cn#b;,h#d,o1 which1 contained in the urine 7about #b per cent4 in human urine71 is for the higher animals 7especially the carnivorous7 the ordinary product of excretion#ci and oxidation of the nitrogenous substances found in the organism4 ,if ammonium carbamate be heated to #adj 7in a sealed tube1 ,bazaroff71 or if carbonyl chloride1 ,c,o,cl#b1 be treated with ammonia 7,natanson71 urea will be obtained1 which shows its direct connection with carbonic acid--that is1 the presence of carbonic acid and ammonia in it4 ,from this it will be understood how urea during the putrefaction of urine is converted into ammonium carbonate1 ,,cn#b;,h#d,o ,h#b,o ,,co#b #b,,nh#c4 #cf ,naumann determined the following dissociation tensions of the vapour of ammonium carbamate 7in millimetres of mercury73-- -#aj #j #aj #bj #cj #dj #ej #fj #e #ab #cj #fb #abd #bdh #dgj #ggj ,horstmann and ,isambert studied the tensions corresponding to excess of ,,nh#c or ,,co#b1 and found1 as might have been expected1 that with such excess the mass of the salt formed 7in a solid state7 increases and the decomposition 7transition into vapour7 decreases4 #cg ,calcium chloride enters into double decomposition with ammonium carbamate4 ,acids 7for instance1 sulphuric7 take up ammonia1 and set free carbonic anhydride2 whilst alkalis 7such as potash7 take up carbonic anhydride and set free ammonia1 and therefore1 in this case for removing water only sodium or potassium carbonate can be taken4 ,an aqueous solution of ammonium carbamate does not entirely precipitate a solution of ,ca,cl#b1 probably because calcium carbamate is soluble in water1 and all the 7,,nh#c7b;,,co#b is not converted by dissolving into the normal salt1 7,,nh#d,o7#b;,,co#c4 #ch ,it must be imagined that the reaction takes place at first between equal volumes 7,chapter ,,vii472 but then carbamic acid1 ,,ho7,,cnh#b,o71 is produced1 which1 as an acid1 immediately combines with the ammonia1 forming ,,nh#d,o7,,cnh#b,o74 #ci ,urea is undoubtedly a product of the oxidation of complex nitrogenous matters 7albumin7 of the animal body4 ,it is found in the blood4 ,it is absorbed from the blood by the kidneys4 ,a man excretes about #cj grams of urea per day4 ,as a derivative of carbonic anhydride1 into which it is readily converted1 urea is in a sense a product of oxidation4 ,thus urea1 both by its origin and decomposition1 is an amide of carbonic acid4 ,representing as it does ammonia 7two molecules7 in which hydrogen 7two atoms7 is replaced by the bivalent radicle of carbonic acid1 urea retains the property of ammonia of entering into combination1 with acids 7thus nitric acid forms ,,cn#b;,h#d,o1,,hno#c71 with bases 7for instance1 with mercury oxide71 and with salts 7such as sodium chloride1 ammonium chloride71 but containing an acid residue it has no alkaline properties4 ,it is soluble in water without change1 but at a red heat loses ammonia and forms cyanic acid1 ,,cnho1#ci bis which is a nitrile of carbonic acid--that is to say1 is a cyanogen compound1 corresponding to the acid ammonium carbonate1 ,,oh7,,cnh#d,o#b71 which on parting with #b;,h#b,o ought to form cyanic acid1 ,,cnoh4 ,liquid cyanic acid1 exceedingly unstable at the ordinary temperatures1 gives its stable solid polymer cyanuric acid1 ,o#c;,h#c;,c#c,n#c4 ,both have the same composition1 and they pass one into another at different temperatures4 ,if crystals of cyanuric acid be heated to a temperature1 t1 then the vapour tension1 p1 in millimetres of mercury 7,troost and ,hautefeuille7 will be3 t4 #afj1 #agj1 #bjj1 #bej1 #cjj1 #cej p4 #ef1 #fh1 #acj1 #bbj1 #dcj1 #a1bjj ,the vapour contains cyanic acid1 and1 if it be rapidly cooled1 it condenses into a mobile volatile liquid 7specific gravity at #j #aad74 ,if the liquid cyanic acid be gradually heated1 it passes into a new amorphous polymeride 7cyamelide71 which1 on being heated1 like cyanuric acid1 forms vapours of cyanic acid4 ,if these fumes are heated above #aej they pass directly into cyanuric acid4 ,thus at a temperature of #cej1 the pressure does not rise above #a1bjj mm4 on the addition of vapours of cyanic acid1 because the whole excess is transformed into cyanuric acid4 ,hence1 the above-mentioned figures give the tension of dissociation of cyanuric acid1 or the greatest pressure which the vapours of ,,hocn are able to attain at a given temperature1 whilst at a greater pressure1 or by the introduction of a larger mass of the substance into a given volume1 the whole of the excess is converted into cyanuric acid4 ,the properties of cyanic acid which we have described were principally observed by ,whler1 and clearly show the faculty of polymerisation of cyanogen compounds4 ,this is observed in many other cyanogen derivatives1 and is to be regarded as the consequence of the above-mentioned explanation of their nature4 ,all cyanogen compounds are ammonium salts1 ,r7,,cnh#d,o#b71 deprived of water1 #b;,h#b,o2 therefore the molecules1 ,,rcn1 ought to possess the faculty of combining with two molecules of water or with other molecules in exchange for it 7for instance1 with ,h#b,s1 or ,h,cl1 or #b;,h#b1 c471 and are therefore capable of combining together4 ,the combination of molecules of the same kind to form more complex ones is what is meant by polymerisation4#dj #ci bis ,its polymer1 ,c#c,n#c;,h#c,o#c1 is formed together with it4 ,cyanic acid is a very unstable1 easily changeable liquid1 while cyanuric acid is a crystalline solid which is very stable at the ordinary temperature4 #dj ,just as the aldehydes 7such as ,c#b;,h#d,o7 are alcohols 7like ,c#b;,h#f,o7 which have lost hydrogen and are also capable of entering into combination with many substances1 and of polymerising1 forming slightly volatile polymerides1 which depolymerise on heating4 ,although there are also many similar phenomena 7for instance1 the transformation of yellow into red phosphorus1 the transition of cinnamene into metacinnamene1 c47 of polymerisation1 in no other case are they so clearly and simply expressed as in cyanic acid4 ,the details relating to this must be sought for in treatises on organic and theoretical chemistry4 ,if we touch on certain sides of this question it is principally with the view of showing the phenomenon of polymerisation by typical examples1 for it is of more frequent occurrence than was formerly supposed among compounds of several elements4 ,besidea being a substance very prone to form polymerides1 cyanic acid presents many other features of interest1 expounded in greater detail in organic chemistry4 ,however we may mention here the production of the cyanates by the oxidation of the metallic cyanides4 ,potassium cyanate1 ,,kcno1 is most often obtained in this way4 ,solutions of cyanates by the addition of sulphuric acid yield cyanic acid1 which1 however1 immediately decomposes3 ,,cnho ,h#b,o ,,co#b ,,nh#c4 ,a solution of ammonium cyanate1 ,,cn7,,nh#d7,o1 behaves in the same manner1 but only in the cold4 ,on being heated it completely changes because it is transformed into urea4 ,the composition of both substances is identical1 ,,cn#b;,h#d,o1 but the structure1 or disposition of1 and connection between1 the elements is different3 in the ammonium cyanate one atom of nitrogen exists in the form of cyanogen1 ,,cn--that is1 united with carbon--and the other as ammonium1 ,,nh#d1 but1 as cyanic acid contains the hydroxyl radicle of carbonic acid1 ,,oh7,,cn71 the ammonium in this salt is united with oxygen4 ,the composition of this salt is best expressed by supposing one atom of the hydrogen in water to be replaced by ammonium and the other by cyanogen--i4e4 that its composition is not symmetrical--whilst in urea both the nitrogen atoms are symmetrically and uniformly disposed as regards the radicle ,,co of carbonic acid3 ,,co7,,nh#b7b4 ,for this reason1 urea is much more stable than ammonium cyanate1 and therefore the latter1 on being slightly heated in solution1 is converted into urea4 ,this remarkable isomeric transformation was discovered by ,whler in #ahbh4da ,formamide1 ,,hconh#b1 and hydrocyanic acid1 ,,hcn1 as a nitrile1 correspond with formic acid1 ,,hcooh1 and therefore ammonium formate1 ,,hcoonh#d1 and formamide1 when acted on by heat and by substances which take up water 7phosphoric anhydride7 form hydrocyanic acid1 ,,hcn1 whilst1 under many conditions 7for instance1 on combining with hydrochloric acid in presence of water71 this hydrocyanic acid forms formic acid and ammonia4 ,although containing hydrogen in the presence of two acid-forming elements--namely1 carbon and nitrogen#db--;hydrocyanic acid does not give an acid reaction with litmus 7cyanic acid has very marked acid properties72 but it forms salts1 ,,mcn1 thus presenting the properties of a feeble acid1 and for this reason is called an acid4 ,the small amount of energy which it has is shown by the fact that the cyanides of the alkali metals--for instance1 potassium cyanide 7,,kho ,,hcn ,h#bj ,,kcn7 in solution have a strongly alkaline reaction4#dc ,if ammonia be passed over charcoal at a red heat1 especially in the presence of an alkali1 or if gaseous nitrogen be passed through a mixture of charcoal and an alkali 7especially potash1 ,,kho71 and also if a mixture of nitrogenous organic substances and alkali be heated to a red heat1 in all these cases the alkali metal combines with the carbon and nitrogen1 forming a metallic cyanide1 ,,mcn--for example1 ,,kcn4#dc bis ,potassium cyanide is much used in the arts1 and is obtained1 as above stated1 under many circumstances--as1 for instance1 in iron smelting1 especially with the assistance of wood charcoal1 the ash of which contains much potash4 ,the nitrogen of the air1 the alkali of the ash1 and the charcoal are brought into contact at a high temperature during iron smelting1 and therefore1 under these conditions1 a considerable quantity of potassium cyanide is formed4 ,in practice it is not usual to prepare potassium cyanide directly1 but a peculiar compound of it containing potassium1 iron1 and cyanogen4 ,this compound is potassium ferrocyanide1 and is also known as yellow prussiate of potash4 ,this saline substance 7see ,chapter ,,xxii7 has the composition ,k#d;,fe,c#f,n#f #b;,h#b,o4 ,the name of cyanogen 7,greek3 kuanos7 is derived from the property which this yellow prussiate possesses of forming1 with a solution of a ferric salt1 ,fe,x#c1 the familiar pigment ,prussian blue4 ,the yellow prussiate is manufactured on a large scale1 and is generally used as the source of the other cyanogen compounds4 #da ,it has an important historical interest1 more especially as at that time such an easy preparation of substances occurring in organisms without the aid of organic life was quite unexpected1 for they were supposed to be formed under the influence of the forces acting in organisms1 and without the latter their formation was considered impossible4 ,and in addition to destroying this illusion1 the easy transition of ,,nh#d,,ocn into ,,co7,,nh#b7b is the best example of the passage of one system of equilibrium of atoms into another more stable system4 #db ,if ammonia and methane 7marsh gas7 do not show any acid properties1 that is in all probability due to the presence of a large amount of hydrogen in both2 but in hydrocyanic acid one atom of hydrogen is under the influence of two acid-forming elements4 ,acetylene1 ,c#b;,h#b1 which contains but little hydrogen1 presents acid properties in certain respects1 for its hydrogen is easily replaced by metals4 ,hydronitrous acid1 ,,hn#c1 which contains little hydrogen1 also has the properties of an acid4 #dc ,solutions of cyanides--for instance1 those of potassium or barium--are decomposed by carbonic acid4 ,even the carbonic anhydride of the air acts in a similar way1 and for this reason these solutions do not keep1 because1 in the first place1 free hydrocyanic acid itself decomposes and polymerises1 and1 in the second place1 with alkaline liquids it forms ammonia and formic acid4 ,hydrocyanic acid does not liberate carbonic anhydride from solutions of sodium or potassium carbonates4 ,but a mixture of solutions of potassium carbonate and hydrocyanic acid yields carbonic anhydride on the addition of oxides like zinc oxide1 mercuric oxide1 c4 ,this is due to the great inclination which the cyanides exhibit of forming double salts4 ,for instance1 ,zn,k#b7;,,cn7#d is formed1 which is a soluble double salt4 #dc bis ,the conversion of the atmospheric nitrogen into cyanogen compounds1 although possible1 has not yet been carried out on a large scale1 and one of the problems for future research should be the discovery of a practical and economical means of converting the atmospheric nitrogen into metallic cyanides1 not only because potassium cyanide has found a vast and important use for the extraction of gold from even the poorest ores1 but more especially because the cyanides furnish the means for effecting the synthesis of many complex carbon compounds1 and the nitrogen contained in cyanogen easily passes into other forms of combination such as ammonia1 which is of great importance in agriculture4 ,if four parts of yellow prussiate be mixed with eight parts of water and three parts of sulphuric acid1 and the mixture be heated1 it decomposes1 volatile hydrocyanic acid separating4 ,this was obtained for the first time by ,scheele in #aghb1 but it was only known to him in solution4 ,in #ahji ,ittner prepared anhydrous prussic acid1 and in #ahae ,gay-,lussac finally settled its properties and showed that it contains only hydrogen1 carbon1 and nitrogen1 ,,cnh4 ,if the distillate 7a weak solution of ,,hcn7 be redistilled1 and the first part collected1 the anhydrous acid may be prepared from this stronger solution4 ,in order to do this1 pieces of calcium chloride are added to the concentrated solution1 when the anhydrous acid floats as a separate layer1 because it is not soluble in an aqueous solution of calcium chloride4 ,if this layer be then distilled over a new portion of calcium chloride at the lowest temperature possible1 the prussic acid may be obtained completely free from water4 ,it is1 however1 necessary to use the greatest caution in work of this kind1 because prussic acid1 besides being extremely poisonous1 is exceedingly volatile4#dd #dd ,the mixture of the vapours of water and hydrocyanic acid1 evolved on heating yellow prussiate with sulphuric acid1 may be passed directly through vessels or tubes filled with calcium chloride4 ,these tubes must be cooled1 because1 in the first place1 hydrocyanic acid easily changes on being heated1 and1 in the second place1 the calcium chloride when warm would absorb less water4 ,the mixture of hydrocyanic acid and aqueous vapour on passing over a long layer of calcium chloride gives up water1 and hydrocyanic acid alone remains in the vapour4 ,it ought to be cooled as carefully as possible in order to bring it into a liquid condition4 ,the method which ,gay-,lussac employed for obtaining pure hydrocyanic acid consisted in the action of hydrochloric acid gas on mercuric cyanide4 ,the latter may he obtained in a pure state if a solution of yellow prussiate be boiled with a solution of mercuric nitrate1 filtered1 and crystallised by cooling2 the mercuric cyanide is then obtained in the form of colourless crystals1 ,hg7,,cn7#b4 ,if a strong solution of hydrochloric acid be poured upon these crystals1 and the mixture of vapours evolved1 consisting of aqueous vapour1 hydrochloric acid1 and hydrocyanic acid1 be passed through a tube containing1 first1 marble 7for absorbing the hydrochloric acid71 and then lumps of calcium chloride1 on cooling the hydrocyanic acid will be condensed4 ,in order to obtain the latter in an anhydrous form1 the decomposition of heated mercury cyanide by hydrogen sulphide may be made use of4 ,here the sulphur and cyanogen change places1 and hydrocyanic acid and mercury sulphide are formed3 ,hg7,,cn7#b ,h#b,s #b;,,hcn ,hg,s4 ,anhydrous prussic acid is a very mobile and volatile liquid2 its specific gravity is #jfig at #ah2 at lower temperatures1 especially when mixed with a small quantity of water1 it easily congeals2 it boils at #bf1 and therefore very easily evaporates1 and at ordinary temperatures may be regarded as a gas4 ,an insignificant amount1 when inhaled or brought into contact with the skin1 causes death4 ,it is soluble in all proportions in water1 alcohol1 and ether3 weak aqueous solutions are used in medicine4#de #de ,a weak 7up to #b p4c47 aqueous solution of hydrocyanic acid is obtained by the distillation of certain vegetable substances4 ,the so-called laurel water in particular enjoys considerable notoriety from its containing hydrocyanic acid4 ,it is obtained by the steeping and distillation of laurel leaves4 ,a similar kind of water is formed by the infusion and distillation of bitter almonds4 ,it is well known that bitter almonds are poisonous1 and have a peculiar characteristic taste4 ,this bitter taste is due to the presence of a certain substance called amygdalin1 which can be extracted by alcohol4 ,this amygdalin decomposes in an infusion of bruised almonds1 forming the so-called bitter almond oil1 glucose1 and hydrocyanic acid3 ,c#aj;,h#bg,,no#aa ,h#b,o ,c#g;,h#f,o ,,cnh #b;,c#f;,h#ab,o#f ,amygdalin in ,water ,bitter ,hydrocyanic ,glucose bitter almonds almond acid oil ,if after this the infusion of bitter almonds be distilled with water1 the hydrocyanic acid and the volatile bitter almond oil are carried over with the aqueous vapour4 ,the oil is insoluble in water1 or only sparingly soluble1 while the hydrocyanic acid remains as an aqueous solution4 ,bitter almond water is similar to laurel water1 and is used like the former in medicine1 naturally only in small quantities because any considerable amount has poisonous effects4 ,perfectly pure anhydrous hydrocyanic acid keeps without change1 just like the weak solutions1 but the strong solutions only keep in the presence of other acids4 ,in the presence of many admixtures these solutions easily give a brown polymeric substance1 which is also formed in a solution of potassium cyanide4 ,the salts ,,mcn--for instance1 potassium1 sodium1 ammonium--as well as the salts ,m7,,cn7#b--;for example1 barium1 calcium1 mercury--are soluble in water1 but the cyanides of manganese1 zinc1 lead1 and many others are insoluble in water4 ,they form double salts with potassium cyanide and similar metallic cyanides1 an example of which we will consider in a further description of the yellow prussiate4 ,not only are some of the double salts remarkable for their constancy and comparative stability1 but so also are the soluble salt ,hg,c#b,n#b1 the insoluble silver cyanide ,ag,c,n1 and even potassium cyanide in the absence of water4 ,the last salt1#df when fused1 acts as a reducing agent with its elements ,k and ,c1 and oxidises when fused with lead oxide1 forming potassium cyanate1 ,,kocn1 which establishes the connection between ,,hcn and ,,ohcn--that is1 between the nitriles of formic and carbonic acids--and this connection is the same as that between the acids themselves1 since formic acid1 on oxidation1 yields carbonic acid4 ,free cyanogen1 7,,cn7#b or ,,cncn1 corresponds to hydrocyanic acid in the same manner as free chlorine1 ,cl#b or ,cl,cl1 corresponds to hydrochloric acid4 ,this composition1 judging from what has been already stated1 exactly expresses that of the nitrile of oxalic acid1 and1 as a matter of fact1 oxalate of ammonia and the amide corresponding with it 7oxamide1 ,note #cc71 on being heated with phosphoric anhydride1 which takes up the water1 yield cyanogen1 7,,cn7#b4 ,this substance is also produced by simply heating some of the metallic cyanides4 ,mercuric cyanide is particularly adapted for this purpose1 because it is easily obtained in a pure state and is then very stable4 ,if mercuric cyanide be heated1 it decomposes1 in like manner to mercury oxide1 into metallic mercury and cyanogen3 ,hg,c#b,n#b ,hg ,c#b,n#b4dg ,when cyanogen is formed1 part of it always polymerises into a dark brown insoluble substance called paracyanogen1 capable of forming cyanogen when heated to redness4#dh ,cyanogen is a colourless1 poisonous gas1 with a peculiar smell and easily condensed by cooling into a colourless liquid1 insoluble in water and having a specific gravity of #jhf4 ,it boils at about -#ba1 and therefore cyanogen may be easily condensed into a liquid by a strong freezing mixture4 ,at -#ce liquid cyanogen solidifies4 ,the gas is soluble in water and in alcohol to a considerable extent--namely1 #a volume of water absorbs as much as #d-a_/b volumes1 and alcohol #bc volumes4 ,cyanogen resists the action of a tolerably high temperature without decomposing1 but under the action of the electric spark the carbon is separated1 leaving a volume of nitrogen equal to the volume of the gas taken4 ,as it contains carbon it burns1 and the colour of the flame is reddish-violet1 which is due to the presence of nitrogen1 all compounds of which impart more or less of this reddish-violet hue to the flame4 ,during the combustion of cyanogen1 carbonic anhydride and nitrogen are formed4 ,the same products are obtained in the eudiometer with oxygen or by the action of cyanogen on many oxides at a red heat4 #df ,this salt will be described in ,chapter ,,xiii4 #dg ,for the preparation it is necessary to take completely dry mercuric cyanide1 because when heated in the presence of moisture it gives ammonia1 carbonic anhydride1 and hydrocyanic acid4 ,instead of mercuric cyanide1 a mixture of perfectly dry yellow prussiate and mercuric chloride may be used1 then double decomposition and the formation of mercuric cyanide take place in the retort4 ,silver cyanide also disengages cyanogen1 on being heated4 #dg ,paracyanogen is a brown substance 7having the composition of cyanogen7 which is formed during the preparation of cyanogen by all methods1 and remains as a residue4 ,silver cyanide1 on being slightly heated1 fuses1 and on being further heated evolves a gas2 a considerable quantity of paracyanogen remains in the residue4 ,here it is remarkable that exactly half the cyanogen becomes gaseous1 and the other half is transformed into paracyanogen4 ,metallic silver will be found in the residue with the paracyanogen2 it may be extracted with mercury or nitric acid1 which does not act on paracyanogen4 ,if paracyanogen be heated in a vacuum it decomposes1 forming cyanogen2 but here the pressure p for a given temperature t cannot exceed a certain limit1 so that the phenomenon presents all the external appearance of a physical transformation into vapour2 but1 nevertheless1 it is a complete change in the nature of the substance1 though limited by the pressure of dissociation1 as we saw before in the transformation of cyanuric into hydrocyanic acid1 and as would be expected from the fundamental principles of dissociation4 ,troost and ,hautefeuille 7#ahfh7 found that for paracyanogen1 t #ecj #eha #fjj #fce p #ij #adc #bif #a1jhi mm4 ,however1 even at #eej part of the cyanogen decomposes into carbon and nitrogen4 ,the reverse transition of cyanogen into paracyanogen commences at #cej1 and at #fjj proceeds rapidly4 ,and if the transition of the first kind is likened to evaporation1 then the reverse transition1 or polymerisation1 presents a likeness to the transition of vapours into the solid state4 ,the relation of cyanogen to the metallic cyanides is seen not only in the fact that it is formed from mercuric cyanide1 but also by its forming cyanide of sodium or potassium on being heated with either of those metals1 the sodium or potassium taking fire in the cyanogen4 ,on heating a mixture of hydrogen and cyanogen to #ejj 7,berthelot71#di or under the action of the silent discharge 7,boilleau71 hydrocyanic acid is formed1 so that the reciprocity of the transitions does not leave any doubt in the matter that all the nitriles of the organic acids contain cyanogen1 just as all the organic acids contain carboxyl and in it the elements of carbonic anhydride4 ,besides the amides1#ej the nitriles 7or cyanogen compounds1 ,,rcn71 and nitro-compounds 7containing the radicle of nitric acid1 ,,rno#b71 there are a great number of other substances containing at the same time carbon and nitrogen1 particulars of which must be sought for in special works on organic chemistry4 #di ,cyanogen 7like chlorine7 is absorbed by a solution of sodium hydroxide1 sodium cyanide and cyanate being produced3 ,c#b,n#b #b,na,h,o ,na,c,n ,c,n,na,o ,h#b,o4 ,but the latter salt decomposes relatively easily1 and moreover part of the cyanogen liberated by heat from its compounds undergoes a more complex transformation4 #ej ,if1 in general1 compounds containing the radicle ,,nh#b are called amides1 some of the amines ought to be ranked with them2 namely1 the hydrocarbons ,cn,h#bm1 in which part of the hydrogen is replaced by ,,nh#b2 for instance1 methylamine1 ,,ch#c,,nh#b1 aniline1 ,c#f;,h#e,,nh#b1 c4 ,in general the amines may be represented as ammonia in which part or all of the hydrogen is replaced by hydrocarbon radicles--as1 for example1 trimethylamine1 ,n7,,ch#c7c4 ,they1 like ammonia1 combine with acids and form crystalline salts4 ,analogous substances are sometimes met with in nature1 and bear the general name of alkaloids2 such are1 for instance1 quinine in cinchona bark1 nicotine in tobacco1 c4 ,,chapter ,x ,,sodium ,,chloride--,,berthollet',s ,,laws--,,hydrochloric ,,acid ,in the preceding chapters we have become acquainted with the most important properties of the four elements1 hydrogen1 oxygen1 nitrogen1 and carbon4 ,they are sometimes termed the organogens1 because they enter into the composition of organic substances4 ,their mutual combinations may serve as types for all other chemical compounds--that is1 they present the same atomic relations 7types1 forms1 or grades of combinations7 as those in which the other elements also combine together4 ,hydrogen1 ,,hh1 or1 in general1 ,,hr4 ,water1 ,h#b,o1 ,h#b,r4 ,ammonia1 ,h#c,n1 ,h#c,r4 ,marsh gas1 ,h#d;,c1 ,h#d,r4 ,one1 two1 three1 and four atoms of hydrogen enter into these molecules for one atom of another element4 ,no compounds of one atom of oxygen with three or four atoms of hydrogen are known2 hence the atom of oxygen does not possess certain properties which are found in the atoms of carbon and nitrogen4 ,the faculty of an element to form a compound of definite composition with hydrogen 7or an element analogous to it7 gives the possibility of foretelling the composition of many other of its compounds4 ,thus1 if we know that an element1 ,m1 combines with hydrogen1 forming1 by preference1 a gaseous substance such as ,,hm1 but not forming ,h#b,m1 ,h#c,m1 ,hn,mm1 then we must conclude1 on the basis of the law of substitution1 that this element will give compounds ,m#b,o1 ,m#c,n1 ,,mho1 ,,mh#c;,c1 c4 ,chlorine is an example of this kind4 ,if we know that another element1 ,r1 like oxygen1 gives with hydrogen a molecule ,h#b,r1 then we may expect that it will form compounds similar to hydrogen peroxide1 the metallic oxides1 carbonic anhydride1 or carbonic oxide1 and others4 ,sulphur is an instance of this kind4 ,hence the elements may be classified according to their resemblance to hydrogen1 oxygen1 nitrogen1 and carbon1 and in conformity with this analogy it is possible to foretell1 if not the properties 7for example1 the acidity or basicity71 at any rate the composition1#a of some of their compounds4 ,this forms the substance of the conception of the valency or atomicity of the elements4 ,hydrogen is taken as the representative of the univalent elements1 giving compounds1 ,,rh1 ,r7,,oh71 ,r#b,o1 ,r,cl1 ,r#c,n1 ,r#d;,c1 c4 ,oxygen1 in that form in which it gives water1 is the representative of the bivalent elements1 forming ,,rh#b1 ,,ro1 ,r,cl#b1 ,r,h,cl1 ,r7,,oh7,cl1 ,r7,,oh7#b1 ,r#b;,c1 ,,rcn1 c4 ,nitrogen in ammonia is the representative of the trivalent elements1 giving compounds ,,rh#c1 ,r#b,o#c1 ,r7,,oh7#c1 ,r,cl#c1 ,,rn1 ,,rhc1 c4 ,in carbon are exemplified the properties of the quadrivalent elements1 forming ,,rh#d1 ,,ro#b1 ,,ro7,,oh7#b1 ,r7,,oh7#d1 ,,rhn1 ,r,cl#d1 ,r,h,cl#c1 c4 ,we meet with these forms of combination1 or degrees of union of atoms1 in all other elements1 some being analogous to hydrogen1 others to oxygen1 and others to nitrogen or to carbon4 ,but besides these quantitative analogies or resemblances1 which are foretold by the law of substitution 7,chapter ,,vi471 there exist among the elements qualitative analogies and relations which are not fully seen in the compounds of the elements which have been considered1 but are most distinctly exhibited in the formation of bases1 acids1 and salts of different types and properties4 ,therefore1 for a complete study of the nature of the elements and their compounds it is especially important to become acquainted with the salts1 as substances of a peculiar character1 and with the corresponding acids and bases4 ,common table salt1 or sodium chloride1 ,na,cl1 may in every respect be taken as a type of salts in general1 and we will therefore pass to the consideration of this substance1 and of hydrochloric acid1 and of the base sodium hydroxide1 formed by the non-metal chlorine and the metal sodium1 which correspond with it4 #a ,but it is impossible to foretell all the compounds formed by an element from its atomicity or valency1 because the atomicity of the elements is variable1 and furthermore this variability is not identical for different elements4 ,in ,,co#b1 ,,cox#b1 ,,ch#d1 and the multitude of carbon compounds corresponding with them1 the ,c is quadrivalent1 but in ,,co either the carbon must be taken as bivalent or the atomicity of oxygen be accounted as variable4 ,moreover1 carbon is an example of an element which preserves its atomicity to a greater degree than most of the other elements4 ,nitrogen in ,,nh#c1 ,,nh#b7,,oh71 ,n#b,o#c1 and even in ,,cnh1 must be considered as trivalent1 but in ,,nh#d;,cl1 ,,no#b7,,oh71 and in all their corresponding compounds it is necessarily pentavalent4 ,in ,n#b,o1 if the atomicity of oxygen #b1 nitrogen has an uneven atomicity 7#a1 #c1 #e71 whilst in ,,no it is bivalent4 ,if sulphur be bivalent1 like oxygen1 in many of its compounds 7for example1 ,h#b,s1 ,s,cl#b1 ,,khs1 c471 then it could not be foreseen from this that it would form ,,so#b1 ,,so#c1 ,s,cl#d1 ,s,o,cl#b1 and a series of similar compounds in which its atomicity must be acknowledged as greater than #b4 ,thus ,,so#b1 sulphurous anhydride1 has many points in common with ,,co#b1 and if carbon be quadrivalent then the ,s in ,,so#b is quadrivalent4 ,therefore the principle of atomicity 7valency7 of the elements cannot be considered established as the basis for the study of the elements1 although it gives an easy method of grasping many analogies4 ,i consider the four following as the chief obstacles to acknowledging the atomicity of the elements as a primary conception for the consideration of the properties of the elements3 #a4 ,such univalent elements as ,h1 ,cl1 c41 appear in a free state as molecules ,h#b1 ,cl#b1 c41 and are consequently like the univalent radicles ,,ch#c1 ,,oh1 ,,co#b;,h1 c41 which1 as might be expected1 appear as ,c#b;,h#f1 ,o#b;,h#b1 ,c#b,o#d;,h#b 7ethane1 hydrogen peroxide1 oxalic acid71 whilst on the other hand1 potassium and sodium 7perhaps also iodine at a high temperature7 contain only one atom1 ,k1 ,na1 in the molecule in a free state4 ,hence it follows that free affinities may exist4 ,granting this1 nothing prevents the assumption that free affinities exist in all unsaturated compounds2 for example1 two free affinities in ,,nh#c4 ,if such instances of free affinities be admitted1 then all the possible advantages to be gained by the application of the doctrine of atomicity 7valency7 are lost4 #b4 ,there are instances--for example1 ,na#b;,h--where univalent elements are combined in molecules which are more complex than ,r#b1 and form molecules1 ,r#c1 ,r#d1 c42 this may again be either taken as evidence of the existence of free affinities1 or else necessitates such primary univalent elements as sodium and hydrogen being considered as variable in their atomicity4 #c4 ,the periodic system of the elements1 with which we shall afterwards become acquainted1 shows that there is a law or rule for the variation of the forms of oxygen and hydrogen compounds2 chlorine is univalent with respect to hydrogen1 and septavalent with respect to oxygen2 sulphur is bivalent to hydrogen1 and sexavalent to oxygen2 phosphorus is trivalent to hydrogen and pentavalent in respect to oxygen--the sum is in every case equal to #h4 ,only carbon and its analogues 7for example1 silicon7 are quadrivalent to both hydrogen and oxygen4 ,hence the power of the elements to change their atomicity is an essential part of their nature1 and therefore constant valency cannot he considered as a fundamental property4 #d4 ,crystallo-hydrates 7for instance1 ,na,cl1#b;,h#b,o1 or ,na,br1#b;,h#b,o71 double salts 7such as ,pt,cl#d1b,k,cl1,h#b,si,f#f1 c471 and similar complex compounds 7and1 according to ,chap4 ,i41 solutions also7 demonstrate the capacity not only of the elements themselves1 but also of their saturated and limiting compounds1 of entering into further combination4 ,therefore the admission of a definite limited atomicity of the elements includes in itself an admission of limitation which is not in accordance with the nature of chemical reactions4 ,sodium chloride1 ,na,cl1 the familiar table salt1 occurs1 although in very small quantities1 in all the primary formations of the earth's crust1#b from which it is washed away by the atmospheric waters2 it is contained in small quantities in all waters flowing through these formations1 and is in this manner conveyed to the oceans and seas4 ,the immense mass of salt in the oceans has been accumulated by this process from the remote ages of the earth's creation1 because the water has evaporated from them while the salt has remained in solution4 ,the salt of sea water serves as the source not only for its direct extraction1 but also for the formation of other masses of workable salt1 such as rock salt1 and of saline springs and lakes4#b bis #b ,the primary formations are those which do not bear any distinct traces of having been deposited from water 7have not a stratified formation and contain no remains of animal or vegetable life71 occur under the sedimentary formations of the earth1 and are everywhere uniform in composition and structure1 the latter being generally distinctly crystalline4 ,if it be assumed that the earth was originally in a molten condition1 the first primary formations are those which formed the first solid crust of the earth4 ,but even with this hypothesis of the earth's origin1 it is necessary to admit that the first aqueous deposits must have caused a change in the original crust of the earth1 and therefore under the head of primary formations must be understood the most ancient of the products of decomposition 7mostly by atmospheric1 aqueous1 and organic agency1 c471 from which all the rocks and substances of the earth's surface have arisen4 ,in speaking of the origin of one or another substance1 we can only1 on the basis of facts1 descend to the primary formations1 of which granite1 gneiss1 and trachyte may be taken as examples4 #b bis ,chloride of sodium has been found to occur in the atmosphere in the form of a fine dust2 in the lower strata it is present in larger quantities than in the upper1 so that the rain water falling on mountains contains less ,na,cl than that falling in valleys4 ,mntz 7#ahia7 found that a litre of rain water collected on the summit of the ,pic du ,midi 7#b1hgg metres above the sea level7 contained #jcd milligram of chloride of sodium1 while a litre of rain collected from the valley contained #be-gf milligrams4 ,the extraction of salt from sea water is carried on in several ways4 ,in southern climes1 especially on the shores of the ,atlantic ,ocean and the ,mediterranean and ,black ,seas1 the summer heats are taken advantage of4 ,a convenient low-lying sea shore is chosen1 and a whole series of basins1 communicating with each other1 are constructed along it4 ,the upper of these basins are filled with sea water by pumping1 or else advantage is taken of high tides4 ,these basins are sometimes separated from the sea by natural sand-banks 7limans7 or by artificial means1 and in spring the water already begins to evaporate considerably4 ,as the solution becomes more concentrated1 it is run into the succeeding basins1 and the upper ones are supplied with a fresh quantity of sea water1 or else an arrangement is made enabling the salt water to flow by degrees through the series of basins4 ,it is evident that the beds of the basins should be as far as possible impervious to water1 and for this purpose they are made of beaten clay4 ,the crystals of salt begin to separate out when the concentration attains #bh p4c4 of salt 7which corresponds to #bh of ,baum's hydrometer74 ,they are raked off1 and employed for all those purposes to which table salt is applicable4 ,in the majority of cases only the first half of the sodium chloride which can be separated from the sea water is extracted1 because the second half has a bitter taste from the presence of magnesium salts which separate out together with the sodium salt4 ,but in certain localities--as1 for instance1 in the estuary of the ,rhone1 on the island of ,camarga#c--the evaporation is carried on to the very end1 in order to obtain those magnesium and potassium salts which separate out at the end of the evaporation of sea water4 ,various salts are separated from sea water in its evaporation4 ,from #ajj parts of sea water there separates out1 by natural and artificial evaporation1 about one part of tolerably pure table salt at the very commencement of the operation2 the total amount held in solution being about #b-a_/b p4c4 ,the remaining portion separates out intermixed with the bitter salts of magnesium which1 owing to their solubility and the small amount in which they are present 7less than #a p4c471 only separate out1 in the first crystallisations1 in traces4 ,gypsum1 or calcium sulphate1 ,ca,s,o#d1b;,h#b,o1 because of its sparing solubility1 separates together with or even before the table salt4 ,when about half of the latter has separated1 then a mixture of table salt and magnesium sulphate separates out1 and on still further evaporation the chlorides of potassium and magnesium begin to separate in a state of combination1 forming the double salt ,k,mg,cl#c1f;,h#b,o1 which occurs in nature as carnallite4#d ,after the separation of this salt from sea water1 there remains a mother liquor containing a large amount of magnesium chloride in admixture with various other salts4#e ,the extraction of sea salt is usually carried on for the purpose of procuring table salt1 and therefore directly it begins to separate mixed with a considerable proportion#f of magnesium salts 7when it acquires a bitter taste7 the remaining liquor is run back into the sea4 #c ,the extraction of the potassium salts 7or so-called summer salts7 was carried on at the ,isle of ,camarga about #ahgj1 when ,i had occasion to visit that spot4 ,at the present time the deposits of ,stassfurt provide a much cheaper salt1 owing to the evaporation and separation of the salt being carried on there by natural means and only requiring a treatment and refining1 which is also necessary in addition for the 'summer salt' obtained from sea-water4 #d ,the double salt ,k,cl1,mg,cl#b is a crystallohydrate of ,k,cl and ,mg,cl#b1 and is only formed from solutions containing an excess of magnesium chloride1 because water decomposes this double salt1 extracting the more soluble magnesium chloride from it4 #e ,owing to the fundamental property of salts of interchanging their metals1 it cannot be said that sea water contains this or that salt1 but only that it contains certain amounts of certain metals ,m 7univalent like ,na and ,k1 and bivalent like ,mg and ,ca71 and haloids ,x 7univalent like ,cl1 ,br1 and bivalent like ,,so#d1 ,,co#c71 which are disposed in every possible kind of grouping2 for instance1 ,k as ,k,cl1 ,k,br1 ,k#b,,so#d1 ,mg as ,mg,cl#b1 ,mg,br#b1 ,mg,s,o#d1 and so on for all the other metals4 ,in evaporation different salts separate out consecutively only because they reach saturation4 ,a proof of this may be seen in the fact that a solution of a mixture of sodium chloride and magnesium sulphate 7both of which salts are obtained from sea water1 as was mentioned above71 when evaporated1 deposits crystals of these salts1 but when refrigerated 7if the solution be sufficiently saturated7 the salt ,na#b,,so#d1aj;,h#b,o is first deposited because it is the first to arrive at saturation at low temperatures4 ,consequently this solution contains ,mg,cl#b and ,na#b,,so#d1 besides ,mg,s,o#d and ,na,cl4 ,so it is with sea water4 #f ,the salt extracted from water is piled up in heaps and left exposed to the action of rain water1 which purifies it1 owing to the water becoming saturated with sodium chloride and then no longer dissolving it1 but washing out the impurities4 ,the same process which is employed for artificially obtaining salt in a crystalline form from sea water has been repeatedly accomplished during the geological evolution of the earth on a gigantic scale2 upheavals of the earth have cut off portions of the sea from the remainder 7as the ,dead ,sea was formerly a part of the ,mediterranean1 and the ,sea of ,aral of the ,caspian71 and their water has evaporated and formed 7if the mass of the inflowing fresh water were less than that of the mass evaporated7 deposits of rock salt4 ,it is always accompanied by gypsum1 because the latter is separated from sea water with or before the sodium chloride4 ,for this reason rock salt may always be looked for in those localities where there are deposits of gypsum4 ,but inasmuch as the gypsum remains on the spot where it has been deposited 7as it is a sparingly soluble salt71 whilst the rock salt 7as one which is very soluble7 may be washed away by rain or fresh running water1 it may sometimes happen that although gypsum is still found there may be no salt2 but1 on the other hand1 where there is rock salt there will always be gypsum4 ,as the geological changes of the earth's surface are still proceeding at the present day1 so in the midst of the dry land salt lakes are met with1 which are sometimes scattered over vast districts formerly covered by seas now dried up4 ,such is the origin of many of the salt lakes about the lower portions of the ,volga and in the ,kirghiz steppes1 where at a geological epoch preceding the present the ,aralo-,caspian ,sea extended4 ,such are the ,baskunchaksky 7in the ,government of ,astrakhan1 #aab square kilometres superficial area71 the ,eltonsky 7#adj versts from the left bank of the ,volga1 and #bjj square kilometres in superficial area71 and upward of #gjj other salt lakes lying about the lower portions of the ,volga4 ,in those in which the inflow of fresh water is less than that yearly evaporated1 and in which the concentration of the solution has reached saturation1 the self-deposited salt is found already deposited on their beds1 or is being yearly deposited during the summer months4 ,certain limans1 or sea-side lakes1 of the ,azoff ,sea are essentially of the same character--as1 for instance1 those in the neighbourhood of ,henichesk and ,berdiansk4 ,the saline soils of certain ,central ,asian steppes1 which suffer from a want of atmospheric fresh water1 are of the same origin4 ,their salt originally proceeded from the salt of seas which previously covered these localities1 and has not yet been washed away by fresh water4 ,the main result of the above-described process of nature is the formation of masses of rock salt1 which are1 however1 being gradually washed away by the subsoil waters flowing in their neighbourhood1 and afterwards rising to the surface in certain places as saline springs1 which indicate the presence of masses of deposited rock salt in the depths of the earth4 ,if the subsoil water flows along a stratum of salt for a sufficient length of time it becomes saturated2 but in flowing in its further course along an impervious stratum 7clay7 it becomes diluted by the fresh water leaking through the upper soil1 and therefore the greater the distance of a saline spring from the deposit of rock salt1 the poorer will it be in salt4 ,a perfectly saturated brine1 however1 may be procured from the depths of the earth by means of bore-holes4 ,the deposits of rock salt themselves1 which are sometimes hidden at great depths below the earth's strata1 may be discovered by the guidance of bore-holes and the direction of the strata of the district4 ,deposits of rock salt1 about #ce metres thick and #bj metres below the surface1 were discovered in this manner in the neighbourhood of ,brianstcheffky and ,dekonoffky1 in the ,bakhmut district of the ,government of ,ekaterinoslav4 ,large quantities of most excellent rock salt are now 7since #ahhj7 obtained from these deposits1 whose presence was indicated by the neighbouring salt springs 7near ,slaviansk and ,bakhmut7 and by bore-holes which had been sunk in these localities for procuring strong 7saturated7 brines4 ,but the ,stassfurt deposits of rock salt near ,magdeburg in ,germany are celebrated as being the first discovered in this manner1 and for their many remarkable peculiarities4#g ,the plentiful distribution of saline springs in this and the neighbouring districts suggested the presence of deposits of rock salt in the vicinity4 ,deep bore-holes sunk in this locality did in fact give a richer brine--even quite saturated with salt4 ,on sinking to a still greater depth1 the deposits of salt themselves were at last arrived at4 ,but the first deposit which was met with consisted of a bitter salt unfit for consumption1 and was therefore called refuse salt 7,abraumsalz74 ,on sinking still deeper vast beds of real rock salt were struck4 ,in this instance the presence of these upper strata containing salts of potassium1 magnesium1 and sodium is an excellent proof of the formation of rock salt from sea water4 ,it is very evident that not only a case of evaporation to the end--as far1 for instance1 as the separation of carnallite--but also the preservation of such soluble salts as separate out from sea water after the sodium chloride1 must be a very exceptional phenomenon1 which is not repeated in all deposits of rock salt4 ,the ,stassfurt deposits therefore are of particular interest1 not only from a scientific point of view1 but also because they form a rich source of potassium salts which have many practical uses4#g bis #g ,when the ,german savants pointed out the exact locality of the ,stassfurt salt-beds and their depth below the surface1 on the basis of information collected from various quarters respecting bore-holes and the direction of the strata1 and when the borings1 conducted by the ,government1 struck a salt-bed which was bitter and unfit for use1 there was a great outcry against science1 and the doubtful result even caused the cessation of the further work of deepening the shafts4 ,it required a great effort to persuade the ,government to continue the work4 ,now1 when the pure salt encountered below forms one of the important riches of ,germany1 and when those 'refuse salts' have proved to be most valuable 7as a source of potassium and magnesium71 we should see in the utilisation of the ,stassfurt deposits one of the conquests of science for the common welfare4 #g bis ,in ,western ,europe1 deposits of rock salt have long been known at ,wieliczka1 near ,cracow1 and at ,cardona in ,spain4 ,in ,russia the following deposits are known3 7a7 the vast masses of rock salt 7#c square kilometres area and up to #adj metres thick7 lying directly on the surface of the earth at ,iletzky ,zastchit1 on the left bank of the river ,ural1 in the ,government of ,orenburg2 7b7 the ,chingaksky deposit1 #ij versts from the river ,volga1 in the ,enotaeffsky district of the ,government of ,astrakhan2 7c7 the ,kulepinsky 7and other7 deposits 7whose thickness attains #aej metres71 on the ,araks1 in the ,government of ,erivan in the ,caucasus2 7d7 the ,katchiezmansky deposit in the province of ,kars2 7e7 the ,krasnovodsky deposit in the ,trans-,caspian province2 and 7f7 the ,bardymkulsky salt mines in ,kokhand4 ,a saturated brine1 formed by the continued contact of subsoil water with rock salt1 is extracted by means of bore-holes1 as1 for instance1 in the ,governments of ,perm1 ,kharkoff1 and ,ekaterinoslav4 ,sometimes1 as at ,berchtesgaden 7and at ,hallein7 in ,austria1 spring water is run on to underground beds of rock salt containing much clay4 ,illustration3 ,,fig4 #fd4--;,graduator for the evaporation of the water of saline springs4 ,if a saline spring1 or the salt water pumped from bore-holes1 contains but little salt1 then the first concentration of the natural solution is not carried on by the costly consumption of fuel1 but by the cheaper method of evaporation by means of the wind4 ,for this purpose so-called graduators are constructed3 they consist of long and lofty sheds1 which are sometimes several versts long1 and generally extend in a direction at right angles to that of the usual course of the wind in the district4 ,these sheds are open at the sides1 and are filled with brushwood as shown in fig4 #fd4 ,troughs1 ,a ,b1 ,c ,d1 into which the salt water is pumped1 run along the top4 ,on flowing from these troughs1 through the openings1 a1 the water spreads over the brushwood and distributes itself in a thin layer over it1 so that it presents a very large surface for evaporation1 in consequence of which it rapidly becomes concentrated in warm or windy weather4 ,after trickling over the brushwood1 the solution collects in a reservoir under the graduator1 whence it is usually pumped up by the pumps ,p ,p1 and again run a second and third time through the graduator1 until the solution reaches a degree of concentration at which it becomes profitable to extract the salt by direct heating4 ,generally the evaporation in the graduator is not carried beyond a concentration of #ab to #ae parts of salt in #ajj parts of solution4 ,strong natural solutions of salt1 and also the graduated solutions1 are evaporated in large shallow metallic vessels1 which are either heated by the direct action of the flame from below or from above4 ,these vessels are made of boiler plate1 and are called salt-pans4 ,various means are employed for accelerating the evaporation and for economising fuel1 which are mainly based on an artificial draught to carry off the steam as it is formed1 and on subjecting the saline solution to a preliminary heating by the waste heat of the steam and furnace gases4 ,furthermore1 the first portions of the salt which crystallise out in the salt-pans are invariably contaminated with gypsum1 since the waters of saline springs always contain this substance4 ,it is only the portions of the salt which separate later that are distinguished by their great purity4 ,the salt is ladled out as it is deposited1 left to drain on inclined tables and then dried1 and in this manner the so-called bay salt is obtained4 ,since it has become possible to discover the saline deposits themselves1 the extraction of table salt from the water of saline springs by evaporation1 which previously was in general use1 has begun to be disused1 and is only able to hold its ground in places where fuel is cheap4 ,in order to understand the full importance of the extraction of salt1 it need only be mentioned that on the average #bj lbs4 of table salt are consumed yearly per head of population1 directly in food or for cattle4 ,in those countries where common salt is employed in technical processes1 and especially in ,england1 almost an equal quantity is consumed in the production of substances containing chlorine and sodium1 and especially in the manufacture of washing soda1 c41 and of chlorine compounds 7bleaching powder and hydrochloric acid74 ,the yearly production of salt in ,europe amounts to as much as #g-a_/b million tons4 ,although certain lumps of rock salt and crystals of bay salt sometimes consist of almost pure sodium chloride1 still the ordinary commercial salt contains various impurities1 the most common of which are magnesium salts4 ,if the salt be pure1 its solution gives no precipitate with sodium carbonate1 ,na#b;,,co#c1 showing the absence of magnesium salts1 because magnesium carbonate1 ,mg,c,o#c1 is insoluble in water4 ,rock salt1 which is ground for use1 generally contains also a considerable admixture of clay and other insoluble impurities4#h ,for ordinary use the bulk of the salt obtained can be employed directly without further purification2 but some salts are purified by solution and crystallisation of the solution after standing1 in which case the evaporation is not carried on to dryness1 and the impurities remain in the mother liquor or in the sediment4 ,when perfectly pure salt is required for chemical purposes it is best obtained as follows3 a saturated solution of table salt is prepared1 and hydrochloric acid gas is passed through it2 this precipitates the sodium chloride 7which is not soluble in a strong solution of hydrochloric acid71 while the impurities remain in solution4 ,by repeating the operation and fusing the salt 7when adhering hydrochloric acid is volatilised7 a pure salt is obtained1 which is again crystallised from its solution by evaporation4#i #h ,the fracture of rock salt generally shows the presence of interlayers of impurities which are sometimes very small in weight1 but visible owing to their refraction4 ,in the excellently laid out salt mines of ,briansk ,i counted 7#ahhh71 if my memory does not deceive me1 on an average ten interlayers per metre of thickness1 between which the salt was in general very pure1 and in places quite transparent4 ,if this be the case1 then there would be #cej interlayers for the whole thickness 7about #ce metres7 of the bed4 ,they probably correspond with the yearly deposition of the salt4 ,in this case the deposition would have extended over more than #cjj years4 ,this should be observable at the present day in lakes where the salt is saturated and in course of deposition4 #i ,my own investigations have shown that not only the sulphates1 but also the potassium salts1 are entirely removed by this method4 ,pure sodium chloride1 in well-defined crystals 7slowly deposited at the bottom of the liquid7 or in compact masses 7in which form rock salt is sometimes met with71 is a colourless and transparent substance resembling1 but more brittle and less hard than1 glass4#aj ,common salt always crystallises in the cubic system1 most frequently in cubes1 and more rarely in octahedra4 ,large transparent cubes of common salt1 having edges up to #aj centimetres long1 are sometimes found in masses of rock salt4#aa ,when evaporated in the open the salt often separates out on the surface#ab as cubes1 which grow on to each other in the form of pyramidal square funnels4 ,in still weather1 these clusters are able to support themselves on the surface of the water for a long time1 and sometimes go on increasing to a considerable extent1 but they sink directly the water penetrates inside them4 ,salt fuses to a colourless liquid 7sp4 gr4 #afjb1 according to ,quincke7 at #hea 7,v4 ,meyer72 if pure it solidifies to a non-crystalline mass1 and if impure to an opaque mass whose surface is not smooth4 ,in fusing1 sodium chloride commences to volatilise 7its weight decreases7 and at a white heat it volatilises with great ease and completely2 but at the ordinary temperature it may1 like all ordinary salts1 be considered as non-volatile1 although as yet no exact experiments have been made in this direction4 #aj ,according to the determinations of ,klodt1 the ,briansk rock salt withstands a pressure of #cdj kilograms per square centimetre1 whilst glass withstands #a1gjj kilos4 ,in this respect salt is twice as secure as bricks1 and therefore immense masses may be extracted from underground workings with perfect safety1 without having recourse to brickwork supports1 merely taking advantage of the properties of the salt itself4 #aa ,to obtain well-formed crystals1 a saturated solution is mixed with ferric chloride1 several small crystals of sodium chloride are placed at the bottom1 and the solution is allowed to evaporate slowly in a vessel with a loose-fitting cover4 ,octahedral crystals are obtained by the addition of borax1 urea1 c41 to the solution4 ,very fine crystals are formed in a mass of gelatinous silica4 #ab ,if a solution of sodium chloride be slowly heated from above1 where the evaporation takes place1 then the upper layer will become saturated before the lower and cooler layers1 and therefore crystallisation will begin on the surface1 and the crystals first formed will float1 having also dried from above1 on the surface until they become quite soaked4 ,being heavier than the solution the crystals are partially immersed under it1 and the following crystallisation1 also proceeding on the surface1 will only form crystals along the side of the original crystals4 ,a funnel is formed in this manner4 ,it will be borne on the surface like a boat 7if the liquid be quiescent71 because it will grow more from the upper edges4 ,we can thus understand this at first sight strange funnel form of crystallisation of salt4 ,in explanation why the crystallisation under the above conditions begins at the surface and not at the lower layers1 it must be mentioned that the specific gravity of a crystal of sodium chloride #baf1 and that of a solution saturated at #be contains #bfg p4c4 of salt and has a specific gravity at #be_/d of #abjjd2 at #ae a saturated solution contains #bfe p4c4 of salt and has a sp4 gr4 #abjc at #ae_/d4 ,hence a solution saturated at a higher temperature is specifically lighter1 notwithstanding the greater amount of salt it contains4 ,with many substances surface crystallisation cannot take place because their solubility increases more rapidly with the temperature than their specific gravity decreases4 ,in this case the saturated solution will always be in the lower layers1 where also the crystallisation will take place4 ,besides which it may be added that as a consequence of the properties of water and solutions1 when they are heated from above 7for instance1 by the sun's rays71 the warmer layers being the lightest remain above1 whilst when heated from below they rise to the top4 ,for this reason the water at great depths below the surface is always cold1 which has long been known4 ,these circumstances1 as well as those observed by ,soret 7,chapter ,i41 ,note #ai71 explain the great differences of density and temperature1 and in the amount of salts held in the oceans at different latitudes 7in polar and tropical climes7 and at various depths4 ,a saturated#ac solution of table salt 7containing #bfd p4c47 has at the ordinary temperature a specific gravity of about #ab4 ,the specific gravity of the crystals is #bafg 7#ag74 ,the salt which separates out at the ordinary and higher temperatures contains no water of crystallisation2#ad but if the crystals are formed at a low temperature1 especially from a saturated solution cooled to -#ab1 then they present a prismatic form1 and contain two equivalents of water1 ,na,cl1#b;,h#b,o4 ,at the ordinary temperature these crystals split up into sodium chloride and its solution4#ae ,unsaturated solutions of table salt when cooled below #j give#af crystals of ice1 but when the solution has a composition ,na,cl1#aj;,h#b,o it solidifies completely at a temperature of -#bc4 ,a solution of table salt saturated at its boiling point boils at about #aji1 and contains about #db parts of salt per #ajj parts of water4 #ac ,by combining the results of ,poggiale1 ,mller1 and ,karsten 7they are evidently more accurate than those of ,gay-,lussac and others7 ,i found that a saturated solution at t1 from #j to #ajh1 contains #ceg #jjbdt #jjjjbt#b grams of salt per #ajj grams of water4 ,this formula gives a solubility at #j #ceg grams 7 #bfc p4c471 whilst according to ,karsten it is #cfji1 ,poggiale #cee1 and ,mller #cef grams4 #ad ,perfectly pure fused salt is not hygroscopic1 according to ,karsten1 whilst the crystallised salt1 even when quite pure1 attracts as much as #jf p4c4 of water from moist air1 according to ,stas4 7,in the ,briansk mines1 where the temperature throughout the whole year is about #aj1 it may be observed1 as ,baron ,klodt informed me1 that in the summer during damp weather the walls become moist1 while in winter they are dry74 ,if the salt contain impurities--such as magnesium sulphate1 c4--it is more hygroscopic4 ,if it contain any magnesium chloride1 it partially deliquesces in a damp atmosphere4 ,the crystallised and not perfectly pure salt decrepitates when heated1 owing to its containing water4 ,the pure salt1 and also the transparent rock salt1 or that which has been once fused1 does not decrepitate4 ,fused sodium chloride shows a faint alkaline reaction to litmus1 which has been noticed by many observers1 and is due to the presence of sodium oxide 7probably by the action of the oxygen of the atmosphere74 ,according to ,a4 ,stcherbakoff very sensitive litmus 7washed in alcohol and neutralised with oxalic acid7 shows an alkaline reaction even with the crystallised salt4 ,it may be observed that rock salt sometimes contains cavities filled with a colourless liquid4 ,certain kinds of rock salt emit an odour like that of hydrocarbons4 ,these phenomena have as yet received very little attention4 #ae ,by cooling a solution of table salt saturated at the ordinary temperature to -#ae1 ,i obtained first of all well-formed tabular 7six-sided7 crystals1 which when warmed to the ordinary temperature disintegrated 7with the separation of anhydrous sodium chloride71 and then prismatic needles up to #bj mm4 long were formed from the same solution4 ,i have not yet investigated the reason of the difference in crystalline form4 ,it is known 7,mitscherlich7 that ,na,i1#b;,h#b,o also crystallises either in plates or prisms4 ,sodium bromide also crystallises with #b;,h#b,o at the ordinary temperature4 #af ,notwithstanding the great simplicity 7,chapter ,i41 ,note #di7 of the observations on the formation of ice from solution1 still even for sodium chloride they cannot yet be considered as sufficiently harmonious4 ,according to ,blagden and ,raoult1 the temperature of the formation of ice from a solution containing c grams of salt per #ajj grams of water -#jf;c to c #aj1 according to ,rosetti -#jfdi;c to c #hg1 according to ,de ,coppet 7to c #aj7 -#jee;c-#jjjf;c#b1 according to ,karsten 7to c #aj7-jgfb;c #jjjhd;c#b1 and according to ,guthrie a much lower figure4 ,by taking ,rosetti's figure and applying the rule given in ,chapter ,i41 ,note #di we obtain-- i - #jfdi #ehe_/ahe #bje4 ,pickering 7#ahic7 gives for c #a-jfjc1 for c #b-abbj2 that is 7c up to #bg7 about -7#jfjj #jjje;c7c4 ,the data for strong solutions are not less contradictory4 ,thus with #bj p4c4 of salt1 ice is formed at -#add according to ,karsten1-#ag according to ,guthrie1-#agf according to ,de ,coppet4 ,rdorff states that for strong solutions the temperature of the formation of ice descends in proportion to the contents of the compound1 ,na,cl1#b;,h#b,o 7per #ajj grams of water7 by #jcdb per #a gram of salt1 and ,de ,coppet shows that there is no proportionality1 in a strict sense1 for either a percentage of ,na,cl or of ,na,cl1#b;,h#b,o4 ,of all its physical properties the specific gravity of solutions of sodium chloride is the one which has been the most fully investigated4 ,a comparison of all the existing determinations of the specific gravity of solutions of ,na,cl#ag at #ae 7in vacuo1 taking water at #d as #aj1jjj71 with regard to p 7the percentage amount of the salt in solution71 show that it is expressed by the equation ,s#ae #iiiaf #gaagp #jbadjp#b4 ,for instance1 for a solution #bjj;,h#b,o ,na,cl1 in which case p #af1 ,s#ae #ajajf4 ,it is seen from the formula that the addition of water produces a contraction4#ah ,the specific gravity#ai at certain temperatures and concentrations in vacuo referred to water at #d #aj1jjjbj is here given for #j #ae #cj #ajj p #e #ajcgb #ajcec #ajcjg #iibb #aj #ajgfh #ajgbh #ajffi #ajbgh #ae #aaafd #aaajg #aajdc #ajfeb #bj #aaefh #aaeja #aadbi #aajdc ,it should be remarked that ,baum's hydrometer is graduated by taking a #aj p4c4 solution of sodium chloride as #aj on the scale1 and therefore it gives approximately the percentage amount of the salt in a solution4 ,common salt is somewhat soluble in alcohol1#ba but it is insoluble in ether and in oils4 #ag ,a collection of observations on the specific gravity of solutions of sodium chloride is given in my work cited in ,chapter ,i41 ,note #ej4 ,solutions of common salt have also been frequently investigated as regards rate of diffusion 7,chapter ,i471 but as yet there are no complete data in this respect4 ,it may be mentioned that ,graham and ,de ,vries demonstrated that diffusion in gelatinous masses 7for instance1 gelatin jelly1 or gelatinous silica7 proceeds in the same manner as in water1 which may probably lead to a convenient and accurate method for the investigation of the phenomena of diffusion4 ,n4 ,umoff 7,odessa1 #ahhh7 investigated the diffusion of common salt by means of glass globules of definite density4 ,having poured water into a cylinder over a layer of a solution of sodium chloride1 he observed during a period of several months the position 7height7 of the globules1 which floated up higher and higher as the salt permeated upwards4 ,umoff found that at a constant temperature the distances of the globules 7that is1 the length of a column limited by layers of definite concentration7 remain constant2 that at a given moment of time the concentration1 q1 of different layers situated at a depth z is expressed by the equation ,b-,kz log47,a-q71 where ,a1 ,b1 and ,k are constants2 that at a given moment the rate of diffusion of the different layers is proportional to their depth1 c4 #ah ,if ,s#j be the specific gravity of water1 and ,s the specific gravity of a solution containing p p4c4 of salt1 then by mixing equal weights of water and the solution1 we shall obtain a solution containing #a_/bp of the salt1 and if it be formed without contraction1 then its specific gravity x will be determined by the equation #b_/x #a_/,s#j #a_/,s1 because the volume is equal to the weight divided by the density4 ,in reality1 the specific gravity is always found to be greater than that calculated on the supposition of an absence of contraction4 #ai ,generally the specific gravity is observed by weighing in air and dividing the weight in grams by the volume in cubic centimetres1 the latter being found from the weight of water displaced1 divided by its density at the temperature at which the experiment is carried out4 ,if we call this specific gravity ,s#a1 then as a cubic centimetre of air under the usual conditions weighs about #jjjab gram1 the sp4 gr4 in a vacuum ,s ,s#a #jjjab 7,s#a - #a71 if the density of water #a4 #bj ,if the sp4 gr4 ,s#b be found directly by dividing the weight of a solution by the weight of water at the same temperature and in the same volume1 then the true sp4 gr4 ,s referred to water at #d is found by multiplying ,s#b by the sp4 gr4 of water at the temperature of observation4 #ba ,according to ,schiff #ajj grams of alcohol1 containing p p4c4 by weight of ,c#b;,h#f,o1 dissolves at #ae-- p #aj #bj #dj #fj #hj #bhe #bbf #acb #ei #ab grams ,na,cl4 ,common salt gives very few compounds#bb 7double salts7 and these are very readily decomposed3 it is also decomposed with great difficulty and its dissociation is unknown4#bc ,but it is easily decomposed1 both when fused and in solution1 by the action of a galvanic current4 ,if the dry salt be fused in a crucible and an electric current be passed through it by immersing carbon or platinum electrodes in it 7the positive electrode is made of carbon and the negative of platinum or mercury71 it is decomposed3 the suffocating gas1 chlorine1 is liberated at the positive pole and metallic sodium at the negative pole4 ,both of them act on the excess of water at the moment of their evolution2 the sodium evolves hydrogen and forms caustic soda1 and the chlorine evolves oxygen and forms hydrochloric acid1 and therefore on passing a current through a solution of common salt metallic sodium will not be obtained--but oxygen1 chlorine1 and hydrochloric acid will appear at the positive pole1 and hydrogen and caustic soda at the negative pole4#bc bis ,thus salt1 like other salts1 is decomposed by the action of an electric current into a metal and a haloid 7,chapter ,,iii47 ,naturally1 like all other salts1 it may be formed from the corresponding base and acid with the separation of water4 ,in fact if we mix caustic soda 7base7 with hydrochloric acid 7acid71 table salt is formed1 ,na,h,o ,h,cl ,na,cl ,h#b,o4 #bb ,amongst the double salts formed by sodium chloride that obtained by ,ditte 7#ahgj7 by the evaporation of the solution remaining after heating sodium iodate with hydrochloric acid until chlorine ceases to be liberated1 is a remarkable one4 ,its composition is ,na,i,o#c1,na,cl1#ad;,h#b,o4 ,rammelsberg obtained a similar 7perhaps the same7 salt in well-formed crystals by the direct reaction of both salts4 #bc ,but it gives sodium in the flame of a ,bunsen's burner 7see ,spectrum ,analysis71 doubtless under the reducing action of the elements carbon and hydrogen4 ,in the presence of an excess of hydrochloric acid in the flame 7when the sodium would form sodium chloride71 no sodium is formed in the flame and the salt does not communicate its usual coloration4 #bc bis ,there is no doubt1 however1 but that chloride of sodium is also decomposed in its aqueous solutions with the separation of sodium1 and that it does not simply enter into double decomposition with the water 7,na,cl ,h#b,o ,na,h,o ,h,cl74 ,this is seen from the fact that when a saturated solution of ,na,cl is rapidly decomposed by an electric current1 a large amount of chlorine appears at the anode and a sodium amalgam forms at the mercury cathode1 which acts but slowly upon the strong solution of salt4 ,castner's process for the electrolysis of brine into chlorine and caustic soda is an application of this method which has been already worked in ,england on an industrial scale4 ,with resspect to the double decompositions of sodium chloride it should be observed that they are most varied1 and serve as means of obtaining nearly all the other compounds of sodium and chlorine4 ,the double decompositions of sodium chloride are almost exclusively based on the possibility of the metal sodium being exchanged for hydrogen and other metals4 ,but neither hydrogen nor any other metal can directly displace the sodium from sodium chloride4 ,this would result in the separation of metallic sodium1 which itself displaces hydrogen and the majority of other metals from their compounds1 and is not1 so far as is known1 ever separated by them4 ,the replacement of the sodium in sodium chloride by hydrogen and various metals can only take place by the transference of the sodium into some other combination4 ,if hydrogen or a metal1 ,m1 be combined with an element ,x1 then the double decomposition ,na,cl ,,mx ,na,x ,m,cl takes place4 ,such double decompositions take place under special conditions1 sometimes completely and sometimes only partially1 as we shall endeavour to explain4 ,in order to acquaint ourselves with the double decompositions of sodium chloride1 we will follow the methods actually employed in practice to procure compounds of sodium and of chlorine from common salt4 ,for this purpose we will first describe the treatment of sodium chloride with sulphuric acid for the preparation of hydrochloric acid and sodium sulphate4 ,we will then describe the substances obtained from hydrochloric acid and sodium sulphate4 ,chlorine itself1 and nearly all the compounds of this element1 may be procured from hydrochloric acid1 whilst sodium carbonate1 caustic soda1 metallic sodium itself and all its compounds1 may be obtained from sodium sulphate4 ,even in the animal organism salt undergoes similar changes1 furnishing the sodium1 alkali1 and hydrochloric acid which take part in the processes of animal life4 ,its necessity as a constituent in the food both of human beings and of animals becomes evident when we consider that both hydrochloric acid and salts of sodium are found in the substances which are separated out from the blood into the stomach and intestines4 ,sodium salts are found in the blood and in the bile which is elaborated in the liver and acts on the food in the alimentary canal1 whilst hydrochloric acid is found in the acid juices of the stomach4 ,chlorides of the metals are always found in considerable quantities in the urine1 and if they are excreted they must be replenished in the organism2 and for the replenishment of the loss1 substances containing chlorine compounds must be taken in food4 ,not only do animals consume those small amounts of sodium chloride which are found in drinking water or in plants or other animals1 but experience has shown that many wild animals travel long distances in search of salt springs1 and that domestic animals which in their natural condition do not require salt1 willingly take it1 and that the functions of their organisms become much more regular from their doing so4 ,the action of sulphuric acid on sodium chloride4--,if sulphuric acid be poured over common salt1 then even at the ordinary temperature1 as ,glauber observed1 an odorous gas1 hydrochloric acid1 is evolved4 ,the reaction which takes place consists in the sodium of the salt and the hydrogen of the sulphuric acid changing places4 ,na,cl ,h#b,,so#d ,h,cl ,na,h,s,o#d ,sodium ,sulphuric ,hydrochloric ,acid sodium chloride acid acid sulphate ,at the ordinary temperature this reaction is not complete1 but soon ceases4 ,when the mixture is heated1 the decomposition proceeds until1 if there be sufficient salt present1 all the sulphuric acid taken is converted into acid sodium sulphate4 ,any excess of acid will remain unaltered4 ,if #b molecules of sodium chloride 7#aag parts7 be taken per molecule of sulphuric acid 7#ih parts71 then on heating the mixture to a moderate temperature only one-half 7#ehe7 of the salt will suffer change4 ,complete decomposition1 after which neither hydrogen nor chlorine is left in the residue1 proceeds 7when #aag parts of table salt are taken per #ih parts of sulphuric acid7 at a red heat only4 ,then-- #b,na,cl ,h#b,,so#d #b;,h,cl ,na#b,,so#d ,table ,sulphuric ,hydrochloric ,sodium salt acid acid sulphate ,this double decomposition is the result of the action of the acid salt1 ,na,h,s,o#d1 first formed1 on sodium chloride1 for the acid salt1 since it contains hydrogen1 itself acts like an acid1 ,na,cl ,na,h,s,o#d ,h,cl ,na#b,,so#d4 ,by adding this equation to the first we obtain the second1 which expresses the ultimate reaction4 ,hence in the above reaction1 non-volatile or sparingly volatile table salt and sparingly volatile sulphuric acid are taken1 and as the result of their reaction1 after the hydrogen and sodium have exchanged places1 there is obtained non-volatile sodium sulphate and gaseous hydrochloric acid4 ,the fact of the latter being a gaseous substance forms the main reason for the reaction proceeding to the very end4 ,the mechanism of this kind of double decomposition1 and the cause of the course of the reaction1 are exactly the same as those we saw in the decomposition of nitre 7,chapter ,,vi47 by the action of sulphuric acid4 ,the sulphuric acid in each case displaces the other1 volatile1 acid4 ,not only in these two instances1 but in every instance1 if a volatile acid can be formed by the substitution of the hydrogen of sulphuric acid for a metal1 then this volatile acid will be formed4 ,from this it may be concluded that the volatility of the acid should be considered as the cause of the progress of the reaction2 and indeed if the acid be soluble but not volatile1 or if the reaction take place in an enclosed space where the resulting acid cannot volatilise1 or at the ordinary temperature when it does not pass into the state of elastic vapour--then the decomposition does not proceed to the end1 but only up to a certain limit4 ,in this respect the explanations given at the beginning of this century by the ,french chemist ,berthollet in his work ',essai de ,statique ,chimique' are very important4 ,the doctrine of ,berthollet starts from the supposition that the chemical reaction of substances is determined not only by the degrees of affinity between the different parts1 but also by the relative masses of the reacting substances and by those physical conditions under which the reaction takes place4 ,two substances containing the elements ,,mx and ,,ny1 being brought into contact with each other1 form by double decomposition the compounds ,,my and ,,nx2 but the formation of these two new compounds will not proceed to the end unless one of them is removed from the sphere of action4 ,but it can only be removed if it possesses different physical properties from those of the other substances which are present with it4 ,either it must be a gas while the others are liquid or solid1 or an insoluble solid while the others are liquid or soluble4 ,the relative amounts of the resultant substances1 if nothing separates out from their intermixture1 depend only on the relative quantities of the substances ,,mx and ,,ny1 and upon the degrees of attraction existing between the elements ,m1 ,n1 ,x1 and ,y2 but however great their mass may be1 and however considerable the attractions1 still in any case if nothing separates out from the sphere of action the decomposition will presently cease1 a state of equilibrium will be established1 and instead of two there will remain four substances in the mass3 namely1 a portion of the original bodies ,,mx and ,,ny1 and a certain quantity of the newly formed substances ,,my and ,,nx1 if it be assumed that neither ,,mn or ,,xy nor any other substances are produced1 and this may for the present#bd be admitted in the case of the double decomposition of salts in which ,m and ,x are metals and ,x and ,y haloids4 ,as the ordinary double decomposition here consists merely in the exchange of metals1 the above simplification is applicable4 ,the sum total of existing data concerning the double decomposition of salts leads to the conclusion that from salts ,,mx ,,ny there always arises a certain quantity of ,,nx and ,,my1 as should be the case according to ,berthollet's doctrine4 ,a portion of the historical data concerning this subject will be afterwards mentioned1 but we will at once proceed to point out the observations made by ,spring 7#ahhh7 which show that even in a solid state salts are subject to a similar interchange of metals if in a condition of sufficiently close contact 7it requires time1 a finely divided state1 and intimate mixture74 ,spring took two non-hygroscopic salts1 potassium nitrate1 ,,kno#c1 and well-dried sodium acetate1 ,c#b;,h#c,na,o#b1 and left a mixture of their powders for several months in a desiccator4 ,an interchange of metals took place1 as was seen from the fact that the resultant mass rapidly attracted the moisture of the air1 owing to the formation of sodium nitrate1 ,na,n,o#c1 and potassium acetate1 ,c#b;,h#c,,ko#b1 both of which are highly hygroscopic4#bd bis #bd ,if ,,mx and ,,ny represent the molecules of two salts1 and if there be no third substance present 7such as water in a solution71 the formation of ,,xy would also be possible2 for instance1 cyanogen1 iodine1 c4 are capable of combining with simple haloids1 as well as with the complex groups which in certain salts play the part of haloids4 ,besides which the salts ,,mx and ,,ny or ,,my with ,,nx may form double salts4 ,if the number of molecules be unequal1 or if the valency of the elements or groups contained in them be different1 as in ,na,cl ,h#b,,so#d1 where ,cl is a univalent haloid and ,,so#d is bivalent1 then the matter may be complicated by the formation of other compounds besides ,,my and ,,nx1 and when a solvent participates in the action1 and especially if present in large proportion1 the phenomena must evidently become still more complex2 and this is actually the case in nature4 ,hence while placing before the reader a certain portion of the existing store of knowledge concerning the phenomena of double saline decompositions1 ,i cannot consider the theory of the subject as complete1 and have therefore limited myself to a few data1 the completion of which must be sought in more detailed works on the subject of theoretical chemistry1 without losing sight of what has been said above4 #bd bis ,when the mixture of potassium nitrate and sodium acetate was heated by ,spring to #ajj1 it was completely fused into one mass1 although potassium nitrate fuses at about #cdj and sodium nitrate at about #cbj4 ,when ,berthollet enunciated his doctrine the present views of atoms and molecules had yet to be developed1 and it is now necessary to submit the matter to examination in the light of these conceptions2 we will therefore consider the reaction of salts1 taking ,m and ,n1 ,x and ,y as equivalent to each other--that is1 as capable of replacing each other 'in toto1' as ,na or ,k11 #a_/b;,ca or #a_/b,mg 7bivalent elements7 replace hydrogen4 ,and since1 according to ,berthollet's doctrine1 when m,,mx of one salt comes into contact with n,,ny of another salt1 a certain quantity x,,my and x,,nx is formed1 there remains m-x of the salt ,,mx1 and n-x of the salt ,,ny4 ,if m be greater than n1 then the maximum interchange could lead to x n1 whilst from the salts taken there would be formed n,,my n,,nx 7m-n7,,mx--that is1 a portion of one only of the salts taken would remain unchanged because the reaction could only proceed between n,,mx and n,,ny4 ,if x were actually equal to n1 the mass of the salt ,,mx would not have any influence on the modus operandi of the reaction1 which is equally in accordance with the teaching of ,bergmann1 who supposed double reactions to be independent of the mass and determined by affinity only4 ,if ,m had more affinity for ,x than for ,y1 and ,n more affinity for ,y than for ,x1 then according to ,bergmann there would be no decomposition whatever1 and x would equal #j4 ,if the affinity of ,m for ,y and of ,n for ,x were greater than those in the original grouping1 then the affinity of ,m for ,x and of ,n for ,y would be overcome1 and1 according to ,bergmann's doctrine1 complete interchange would take place--i4e4 x would equal n4 ,according to ,berthollet's teaching1 a distribution of ,m and ,n between ,x and ,y will take place in every case1 not only in proportion to the degrees of affinity1 but also in proportion to the masses1 so that with a small affinity and a large mass the same action can be produced as with a large affinity and a small mass4 ,therefore1 7#a7 x will always be less than n and their ratio x_/n less than unity--that is1 the decomposition will be expressed by the equation1 m,,mx n,,ny 7m-x7,,mx 7n-x7,,ny x,,my x,,nx2 7#b7 by increasing the mass m we increase the decomposition--that is1 we increase x and the ratio x_/7n-x71 until with an infinitely large quantity m the fraction x_/n will equal #a1 and the decomposition will be complete1 however small the affinities uniting ,,my and ,,nx may be2 and 7#c7 if m n1 by taking ,,mx ,,ny or ,,my ,,nx we arrive at one and the same system in either case3 7n-x7,,mx 7n-x7,,ny x,,my x,,nx4 ,these direct consequences of ,berthollet's teaching are verified by experience4 ,thus1 for example1 a mixture of solutions of sodium nitrate and potassium chloride in all cases has entirely the same properties as a mixture of solutions of potassium nitrate and sodium chloride1 of course on condition that the mixed solutions are of identical elementary composition4 ,but this identity of properties might either proceed from one system of salts passing entirely into the other 7,bergmann's hypothesis7 in conformity with the predominating affinities 7for instance1 from ,k,cl ,na,n,o#c there might arise ,,kno#c ,na,cl1 if it be admitted that the affinities of the elements as combined in the latter system are greater than in the former72 or1 on the other hand1 it might be because both systems by the interchange of a portion of their elements give one and the same state of equilibrium1 as according to ,berthollet's teaching4 ,experiment proves the latter hypothesis to be the true one4 ,but before citing the most historically important experiments verifying ,berthollet's doctrine1 we must stop to consider the conception of the mass of the reacting substances4 ,berthollet understood by mass the actual relative quantity of a substance2 but now it is impossible to understand this term otherwise than as the number of molecules1 for they act as chemical units1 and in the special case of double saline decompositions it is better to take it as the number of equivalents4 ,thus in the reaction ,na,cl ,h#b,,so#d the salt is taken in one equivalent and the acid in two4 ,if #b,na,cl ,h#b,,so#d act1 then the number of equivalents are equal1 and so on4 ,the influence of mass on the amount of decomposition x_/n forms the root of ,berthollet's doctrine1 and therefore we will first of all turn our attention to the establishment of this principle in relation to the double decomposition of salts4 ,about #ahdj ,h4 ,rose#be showed that water decomposes metallic sulphides like calcium sulphide1 ,ca,s1 forming hydrogen sulphide1 ,h#b,s1 notwithstanding the fact that the affinity of hydrogen sulphide1 as an acid1 for lime1 ,ca,h#b,o#b1 as a base1 causes them to react on each other1 forming calcium sulphide and water1 ,ca,s #b;,h#b,o4 ,furthermore1 ,rose showed that the greater the amount of water acting on the calcium sulphide1 the more complete is the decomposition4 ,the results of this reaction are evident from the fact that the hydrogen sulphide formed may be expelled from the solution by heating1 and that the resulting lime is sparingly soluble in water4 ,rose clearly saw from this that such feeble agents1 in a chemical sense1 as carbonic anhydride and water1 by acting in a mass and for long periods of time in nature on the durable rocks1 which resist the action of the most powerful acids1 are able to bring about chemical change--to extract1 for example1 from rocks the bases1 lime1 soda1 potash4 ,the influence of the mass of water on antimonious chloride1 bismuth nitrate1 c41 is essentially of the same character4 ,these substances give up to the water a quantity of acid which is greater in proportion as the mass of the water acting on them is greater4#be bis #be ,h4 ,rose is more especially known for his having carefully studied and perfected several methods for the exact chemical analysis of many mineral substances4 ,his predecessor in this branch of research was ,berzelius1 and his successor ,fresenius4 #be bis ,historically the influence of the mass of water was the first well-observed phenomenon in support of ,berthollet's teaching1 and it should not now be forgotten4 ,in double decompositions taking place in dilute solutions where the mass of water is large1 its influence1 notwithstanding the weakness of affinities1 must he great1 according to the very essence of ,berthollet's doctrine4 ,as explaining the action of the mass of water1 the experiments of ,pattison ,muir 7#ahgi7 are very instructive4 ,these experiments demonstrate that the decomposition of bismuth chloride is the more complete the greater the relative quantity of water1 and the less the mass of hydrochloric acid forming one of the products of the reaction4 ,barium sulphate1 ,ba,s,o#d1 which is insoluble in water1 when fused with sodium carbonate1 ,na#b;,,co#c1 gives1 but not completely1 barium carbonate1 ,ba,c,o#c1 7also insoluble71 and sodium sulphate1 ,na#b,,so#d4 ,if a solution of sodium carbonate acts on precipitated barium sulphate1 the same decomposition is also effected 7,dulong1 ,rose71 but it is restricted by a limit and requires time4 ,a mixture of sodium carbonate and sulphate is obtained in the solution and a mixture of barium carbonate and sulphate in the precipitate4 ,if the solution be decanted off and a fresh solution of sodium carbonate be poured over the precipitate1 then a fresh portion of the barium sulphate passes into barium carbonate1 and so by increasing the mass of sodium carbonate it is possible to entirely convert the barium sulphate into barium carbonate4 ,if a definite quantity of sodium sulphate be added to the solution of sodium carbonate1 then the latter will have no action whatever on the barium sulphate1 because then a system in equilibrium determined by the reverse action of the sodium sulphate on the barium carbonate and by the presence of both sodium carbonate and sulphate in the solution1 is at once arrived at4 ,on the other hand1 if the mass of the sodium sulphate in the solution be great1 then the barium carbonate is reconverted into sulphate until a definite state of equilibrium is attained between the two opposite reactions1 producing barium carbonate by the action of the sodium carbonate and barium sulphate by the action of the sodium sulphate4 ,another most important principle of ,berthollet's teaching is the existence of a limit of exchange decomposition1 or the attainment of a state of equilibrium4 ,in this respect the determinations of ,malaguti 7#aheg7 are historically the most important4 ,he took a mixture of solutions of equivalent quantities of two salts1 ,,mx and ,,ny1 and judged the amount of the resulting exchange from the composition of the precipitate produced by the addition of alcohol4 ,when1 for example1 zinc sulphate and sodium chloride 7,zn,s,o#d and #b,na,cl7 were taken1 there were produced by exchange sodium sulphate and zinc chloride4 ,a mixture of zinc sulphate and sodium sulphate was precipitated by an excess of alcohol1 and it appeared from the composition of the precipitate that #gb per cent4 of the salts taken had been decomposed4 ,when1 however1 a mixture of solutions of sodium sulphate and zinc chloride was taken1 the precipitate presented the same composition as before--that is1 about #bh per cent4 of the salts taken had been subjected to decomposition4 ,in a similar experiment with a mixture of sodium chloride and magnesium sulphate1 #b,na,cl ,mg,s,o#d or ,mg,cl#b ,na#b,,so#d1 about half of the metals underwent the decomposition1 which may be expressed by the equation #d,na,cl #b,mg,s,o#d #b,na,cl ,mg,s,o#d ,na#b,,so#d ,mg,cl#b #b,na#b,,so#d #b,mg,cl#b4 ,a no less clear limit expressed itself in another of ,malaguti's researches when he investigated the above-mentioned reversible reactions of the insoluble salts of barium4 ,when1 for example1 barium carbonate and sodium sulphate 7,ba,c,o#c ,na#b,,so#d7 were taken1 then about #gb per cent4 of the salts were decomposed1 that is1 were converted into barium sulphate and sodium carbonate4 ,but when the two latter salts were taken1 then about #ai per cent4 of them passed into barium carbonate and sodium sulphate4 ,probably the end of the reaction was not reached in either case1 because this would require a considerable time and a uniformity of conditions attainable with difficulty4 ,gladstone 7#ahee7 took advantage of the colour of solutions of different ferric salts for determining the measure of exchange between metals4 ,thus a solution of ferric thiocyanate has a most intense red colour1 and by making a comparison between the colour of the resulting solutions and the colour of solutions of known strength it was possible to judge to a certain degree the quantity of the thiocyanate formed4 ,this colorimetric method of determination has an important significance as being the first in which a method was applied for determining the composition of a solution without the removal of any of its component parts4 ,when ,gladstone took equivalent quantities of ferric nitrate and potassium thiocyanate--,fe7,,no#c7c #c,,kcns--only #ac per cent4 of the salts underwent decomposition4 ,on increasing the mass of the latter salt the quantity of ferric thiocyanate formed increased1 but even when more than #cjj equivalents of potassium thiocyanate were taken a portion of the iron still remained as nitrate4 ,it is evident that the affinity acting between ,fe and ,,no#c and between ,k and ,,cns on the one hand1 is greater than the affinity acting between ,fe and ,,cns1 together with the affinity of ,k for ,,no#c1 on the other hand4 ,the investigation of the variation of the fluorescence of quinine sulphate1 as well as the variation of the rotation of the plane of polarisation of nicotine1 gave in the hands of ,gladstone many proofs of the entire applicability of ,berthollet's doctrine1 and in particular demonstrated the influence of mass which forms the chief distinctive feature of the teaching of ,berthollet1 teaching little appreciated in his own time4 ,at the beginning of the year #ahfj1 the doctrine of the limit of reaction and of the influence of mass on the process of chemical transformations received a very important support in the researches of ,berthelot and ,p4 de ,saint-,gilles on the formation of the ethereal salts ,,rx from the alcohols ,,roh and acids ,,hx1 when water is also formed4 ,this conversion is essentially very similar to the formation of salts1 but differs in that it proceeds slowly at the ordinary temperature1 extending over whole years1 and is not complete--that is1 it has a distinct limit determined by a reverse reaction2 thus an ethereal salt ,,rx with water gives an alcohol ,,roh and an acid ,,hx--up to that limit generally corresponding with two-thirds of the alcohol taken1 if the action proceed between molecular quantities of alcohol and acid4 ,thus common alcohol1 ,c#b;,h#e,,oh1 with acetic acid1 ,,hc#b;,h#c,o#b1 gives the following system rapidly when heated1 or slowly at the ordinary temperature1 ,,roh ,,hx #b,,rx #b;,h#b,o1 whether we start from #c,,rho #c;,,hx or from #c,,rx #c;,h#b,o4 ,the process and completion of the reaction in this instance are very easily observed1 because the quantity of free acid is easily determined from the amount of alkali requisite for its saturation1 as neither alcohol nor ethereal salt acts on litmus or other reagent for acids4 ,under the influence of an increased mass of alcohol the reaction proceeds further4 ,if two molecules of alcohol1 ,,rho1 be taken for every one molecule of acetic acid1 ,,hx1 then instead of #ff p4c41 #hc p4c4 of the acid passes into ethereal salt1 and with fifty molecules of ,,rho nearly all the acid is etherised4 ,the researches of ,menschutkin in their details touched on many important aspects of the same subject1 such as the influence of the composition of the alcohol and acid on the limit and rate of exchange--but these1 as well as other details1 must be looked for in special treatises on organic and theoretical chemistry4 ,in any case the study of etherification has supplied chemical mechanics with clear and valuable data1 which directly confirm the two fundamental propositions of ,berthollet2 the influence of mass1 and the limit of reaction--that is1 the equilibrium between opposite reactions4 ,the study of numerous instances of dissociation which we have already touched on1 and shall again meet with on several occasions1 gave the same results4 ,with respect to double saline decompositions1 it is also necessary to mention the researches of ,wiedemann on the decomposing action of a mass of water on the ferric salts1 which could be determined by measuring the magnetism of the solutions1 because the ferric oxide 7soluble colloid7 set free by the water is less magnetic than the ferric salts4 ,a very important epoch in the history of ,berthollet's doctrine was attained when1 in #ahfg1 the ,norwegian chemists1 ,guldberg and ,waage1 expressed it as an algebraical formula4 ,they defined the active mass as the number of molecules contained in a given volume1 and assumed1 as follows from the spirit of ,berthollet's teaching1 that the action between the substances was equal to the product of the masses of the reacting substances4 ,hence if the salts ,,mx and ,,ny be taken in equivalent quantities 7m #a and n #a7 and the salts ,,my and ,,nx are not added to the mixture but proceed from it1 then if k represent the coefficient of the rate of the action of ,,mx on ,,ny and if k represent the same coefficient for the pair ,,my and ,,nx1 then we shall have at the moment when the decomposition equals x a measure of action for the first pair3 k7#a-x77#a-x7 and for the second pair kxx1 and a state of equilibrium or limit will be reached when k7#a-x7#b kx#b1 whence the ratio k_/k x_/7#a-x7#b4 ,therefore in the case of the action of alcohol on an acid1 when x #b_/c1 the magnitude k_/k #d1 that is1 the reaction of the alcohol on the acid is four times as fast as that of the ethereal salt on water4 ,if the ratio k_/k be known1 then the influence of mass may be easily determined from it4 ,thus if instead of one molecule of alcohol two be taken1 then the equation will be k7#b-x77#a-x7 kxx1 whence x #jhe or #he percent41 which is close to the result of experiment4 ,if #cjj molecules of alcohol be taken1 then x proves to be approximately #ajj per cent41 which is also found to be the case by experiment4#bf #bf ,from the above it follows that an excess of acid should influence the reaction like an excess of alcohol4 ,it is in fact shown by experiment that if two molecules of acetic acid be taken to one molecule of alcohol1 #hd p4c4 of alcohol is etherified4 ,if with a large preponderance of acid or of alcohol certain discrepancies are observed1 their cause must be looked for in the incomplete correspondence of the conditions and external influences4 ,but it is impossible to subject the formation of salts to any process directly analogous to that which is so conveniently effected in etherification4 ,many efforts have1 however1 been made to solve the problem of the measure of reaction in this case also4 ,thus1 for example1 ,khichinsky 7#ahff71 ,petrieff 7#ahhe71 and many others investigated the distribution of metals and haloid groups in the case of one metal and several haloids taken in excess1 as acids2 or conversely with an excess of bases1 the distribution of these bases with relation to an acid2 in cases where a portion of the substances forms a precipitate and a portion remains in solution4 ,but such complex cases1 although they in general confirm ,berthollet's teaching 7for instance1 a solution of silver nitrate gives some silver oxide with lead oxide1 and a solution of nitrate of lead precipitates some lead oxide under the action of silver oxide1 as ,petrieff demonstrated71 still1 owing to the complexity of the phenomena 7for instance1 the formation of basic and double salts71 they cannot give simple results4 ,but much more instructive and complete are researches like those made by ,pattison ,muir 7#ahgf71 who took the simple case of the precipitation of calcium carbonate1 ,ca,c,o#c1 from the mixture of solutions of calcium chloride and sodium or potassium carbonate1 and found in this case that not only was the rate of action 7for example1 in the case of ,ca,cl#b ,na#b;,,co#c1 #ge per cent4 of ,ca,c,o#c was precipitated in five minutes1 #he per cent4 in thirty minutes1 and #id per cent4 in two days7 determined by the temperature1 relative mass1 and amount of water 7a large mass of water decreases the rate71 but that the limit of decomposition was also dependent on these influences4 ,however1 even in researches of this kind the conditions of reaction are complicated by the non-uniformity of the media1 inasmuch as a portion of the substance is obtained or remains in the form of a precipitate1 so that the system is heterogeneous4 ,the investigation of double saline decompositions offers many difficulties which cannot be considered as yet entirely overcome4 ,although many efforts have long since been made1 the majority of the researches were carried on in aqueous solutions1 and as water is itself a saline compound and able to combine with salts and enter into double decomposition with them1 such reactions taking place in solutions in reality present very complex cases4#bg ,in this sense the reaction between alcohols and acids is much more simple1 and therefore its significance in confirmation of ,berthollet's doctrine is of particular importance4 ,the only cases which can be compared with these reactions for simplicity are those exchange decompositions investigated by ,g4 ,g4 ,gustavson1 which take place between ,c,cl#d and ,r,brn on the one hand1 and ,c,br#d and ,r,cln on the other4 ,this case is convenient for investigation inasmuch as the ,r,cln and ,r,brn taken 7such as ,b,cl#c1 ,si,cl#d1 ,ti,cl#d1 ,p,o,cl#c1 and ,sn,cl#d7 belong to those substances which are decomposed by water1 whilst ,c,cl#d and ,c,br#d are not decomposed by water2 and therefore1 by heating1 for instance1 a mixture of ,c,cl#d ,si,br#d it is possible to arrive at a conclusion as to the amount of interchange by treating the product with water1 which decomposes the ,si,br#d left unchanged and the ,si,cl#d formed by the exchange1 and therefore by determining the composition of the product acted on by the water it is possible to form a conclusion as to the amount of decomposition4 ,the mixture was always formed with equivalent quantities--for instance1 #d;,b,cl#c #c;,c,br#d4 ,it appeared that there was no exchange whatever on simple intermixture1 but that it proceeded slowly1 when the mixture was heated 7for example1 with the mixture above mentioned at #abc #dhf per cent4 of ,cl was replaced by ,br after #ad days' heating1 and #fhc per cent4 after #bh days1 and #ajab per cent4 when heated at #aej for #fj days74 ,a limit was always reached which corresponded with that of the complemental system2 in the given instance the system #d;,b,br#c #c;,c,cl#d4 ,in this last #hiig per cent4 of bromine in the ,b,br#c was replaced by chlorine2 that is1 there were obtained #hiig molecules of ,b,cl#c and there remained #ajjb molecules of ,b,br#c1 and therefore the same state of equilibrium was reached as that given by the system #d;,b,cl#c #c;,c,br#d4 ,both systems gave one and the same state of equilibrium at the limit1 which is in agreement with ,berthollet's doctrine4#bh #bg ,as an example two methods may be mentioned1 ,thomsen's and ,ostwald's4 ,thomsen 7#ahfi7 applied a thermochemical method to exceedingly dilute solutions without taking the water into further consideration4 ,he took solutions of caustic soda containing #ajj;,h#b,o per ,na,h,o1 and sulphuric acid containing #a_/b;,h#b,,so#d #ajj;,h#b,o4 ,in order that these solutions may be mixed in such quantities that atomic proportions of acid and alkali would act1 for forty grams of caustic soda 7which answers to its equivalent7 there should be employed #di grams of sulphuric acid1 and then #ae1fhi heat units would be evolved4 ,if the normal sodium sulphate so formed be mixed with n equivalents of sulphuric acid1 a certain amount of heat is absorbed1 namely a quantity equal to 7n4#afej7_/7n #jh7 heat units4 ,an equivalent of caustic soda1 in combining with an equivalent of nitric acid1 evolves #ac1fag units of heat1 and the augmentation of the amount of nitric acid entails an absorption of heat for each equivalent equal to -#bg units2 so also in combining with hydrochloric acids #ac1gdj heat units are absorbed1 and for each equivalent of hydrochloric acid beyond this amount there are absorbed -#cb heat units4 ,thomsen mixed each one of three neutral salts1 sodium sulphate1 sodium chloride and sodium nitrate1 with an acid which is not contained in it2 for instance1 he mixed a solution of sodium sulphate with a solution of nitric acid and determined the number of heat units then absorbed4 ,an absorption of heat ensued because a normal salt was taken in the first instance1 and the mixture of all the above normal salts with acid produces an absorption of heat4 ,the amount of heat absorbed enabled him to obtain an insight into the process taking place in this mixture1 for sulphuric acid added to sodium sulphate absorbs a considerable quantity of heat1 whilst hydrochloric and nitric acids absorb a very small amount of heat in this case4 ,by mixing an equivalent of sodium sulphate with various numbers of equivalents of nitric acid1 ,thomsen observed that the amount of heat absorbed increased more and more as the amount of nitric acid was increased2 thus when ,,hno#c was taken per #a_/b,na#b,,so#d1 #a1geb heat units were absorbed per equivalent of soda contained in the sodium sulphate4 ,when twice as much nitric acid was taken1 #b1jbf heat units1 and when three times as much1 #b1jej heat units were absorbed4 ,had the double decomposition been complete in the case where one equivalent of nitric acid was taken per equivalent of ,na#b,,so#d then according to calculation from similar data there should have been absorbed -#b1ihi units of heat1 while in reality only -#a1geb units were absorbed4 ,hence ,thomsen concluded that a displacement of only about two-thirds of the sulphuric acid had taken place--that is1 the ratio k 3 k for the reaction #a_/b,na#b,,so#d ,,hno#c and ,na,n,o#c #a_/b;,h#b,,so#d is equal1 as for ethereal salts1 to #d4 ,by taking this figure and admitting the above supposition1 ,thomsen found that for all mixtures of soda with nitric acid1 and of sodium nitrate with sulphuric acid1 the amounts of heat followed ,guldberg and ,waage's law2 that is1 the limit of decomposition reached was greater the greater the mass of acid added4 ,the relation of hydrochloric to sulphuric acid gave the same results4 ,therefore the researches of ,thomsen fully confirm the hypotheses of ,guldberg and ,waage and the doctrine of ,berthollet4 ,thomson concludes his investigation with the words3 7a7 ',when equivalent quantities of ,na,h,o1 ,,hno#c 7or ,h,cl7 and #a_/b;,h#b,,so#d react on one another in an aqueous solution1 then two-thirds of the soda combines with the nitric and one-third with the sulphuric acid2 7b7 this subdivision repeats itself1 whether the soda be taken combined with nitric or with sulphuric acid2 7c7 and therefore nitric acid has double the tendency to combine with the base that sulphuric acid has1 and hence in an aqueous solution it is a stronger acid than the latter4' ',it is therefore necessary1' ,thomsen afterwards remarks1 'to have an expression indicating the tendency of an acid for the saturation of bases4 ,this idea cannot be expressed by the word affinity1 because by this term is most often understood that force which it is necessary to overcome in order to decompose a substance into its component parts4 ,this force should therefore be measured by the amount of work or heat employed for the decomposition of the substance4 ,the above-mentioned phenomenon is of an entirely different nature1' and ,thomsen introduces the term avidity1 by which he designates the tendency of acids for neutralisation4 ',therefore the avidity of nitric acid with respect to soda is twice as great as the avidity of sulphuric acid4 ,an exactly similar result is obtained with hydrochloric acid1 so that its avidity with respect to soda is also double the avidity of sulphuric acid4 ,experiments conducted with other acids showed that not one of the acids investigated had so great an avidity as nitric acid2 some had a greater avidity than sulphuric acid1 others less1 and in some instances the avidity #j4' ,the reader will naturally see clearly that the path chosen by ,thomsen deserves to be worked out1 for his results concern important questions of chemistry1 but great faith cannot be placed in the deductions he has already arrived at1 because great complexity of relations is to be seen in the very method of his investigation4 ,it is especially important to turn attention to the fact that all the reactions investigated are reactions of double decomposition4 ,in them ,a and ,b do not combine with ,c and distribute themselves according to their affinity or avidity for combination1 but reversible reactions are induced4 ,,mx and ,,ny give ,,my and ,,nx1 and conversely2 therefore the affinity or avidity for combination is not here directly determined1 but only the difference or relation of the affinities or avidities4 ,the affinity of nitric acid not only for the water of constitution1 but also for that serving for solution1 is much less than that of sulphuric acid4 ,this is seen from thermal data4 ,the reaction ,n#b,o#e ,h#b,o gives #c1fjj heat units1 and the solution of the resultant hydrate1 #b,,nho#c1 in a large excess of water evolves #ad1ihf heat units4 ,the formation of ,,so#c ,h#b,o evolves #ba1cjh heat units1 and the solution of ,h#b,,so#d in an excess of water #ag1hfj--that is1 sulphuric acid gives more heat in both cases4 ,the interchange between ,na#b,,so#d and #b;,,hno#c is not only accomplished at the expense of the production of ,na,n,o#c1 but also at the expense of the formation of ,h#b,,so#d1 hence the affinity of sulphuric acid for water plays its part in the phenomena of displacement4 ,therefore in determinations like those made by ,thomsen the water does not form a medium which is present without participating in the process2 it also takes part in the reaction4 7,compare ,chapter ,,ix41 ,note #ad47 ,whilst retaining essentially the methods of ,thomsen1 ,ostwald 7#ahgf7 determined the variation of the sp4 gr4 7and afterwards of volume71 proceeding in the same dilute solutions1 on the saturation of acids by bases1 and in the decomposition of the salts of one acid by the other1 and arrived at conclusions of just the same nature as ,thomsen's4 ,ostwald's method will be clearly understood from an example4 ,a solution of caustic soda containing an almost molecular 7#dj grams7 weight per litre had a specific gravity of #ajdjea4 ,the specific gravities of solutions of equal volume and equivalent composition of sulphuric and nitric acids were #ajbigj and #ajcjhd respectively4 ,on mixing the solutions of ,na,h,o and ,h#b,,so#d there was formed a solution of ,na#b,,so#d of sp4 gr4 #ajbiei2 hence there ensued a decrease of specific gravity which we will term ,q1 equal to #ajdjea #ajbigj-b7ajbiei7 #jjaajc4 ,so also the specific gravity after mixture of the solutions of ,na,h,o and ,,hno#c was #ajbfcc1 and therefore ,q #jjahfi4 ,when one volume of the solution of nitric acid was added to two volumes of the solution of sodium sulphate1 a solution of sp4 gr4 #ajbgha was obtained1 and therefore the resultant decrease of sp4 gr4 ,q#a #b7ajbiei7 #ajcjhd-c7ajbgha7 #jjjfei4 ,had there been no chemical reaction between the salts1 then according to ,ostwald's reasoning the specific gravity of the solutions would not have changed1 and if the nitric acid had entirely displaced the sulphuric acid ,q#b would be #jjahfi-jjaajc #jjjgff4 ,it is evident that a portion of the sulphuric acid was displaced by the nitric acid4 ,but the measure of displacement is not equal to the ratio between ,q#a and ,q#b1 because a decrease of sp4 gr4 also occurs on mixing the solution of sodium sulphate with sulphuric acid1 whilst the mixing of the solutions of sodium nitrate and nitric acid only produces a slight variation of sp4 gr4 which falls within the limits of experimental error4 ,ostwald deduces from similar data the same conclusions as ,thomsen1 and thus reconfirms the formula deduced by ,guldberg and ,waage1 and the teaching of ,berthollet4 ,the participation of water is seen still more clearly in the methods adopted by ,ostwald than in those of ,thomsen1 because in the saturation of solutions of acids by alkalis 7which ,kremers1 ,reinhold1 and others had previously studied7 there is observed1 not a contraction1 as might have been expected from the quantity of heat which is then evolved1 but an expansion1 of volume 7a decrease of specific gravity1 if we calculate as ,ostwald did in his first investigations74 ,thus by mixing #a1hhj grams of a solution of sulphuric acid of the composition ,,so#c #ajj;,h#b,o1 occupying a volume of #a1hae c4c41 with a corresponding quantity of a solution #b7,na,h,o #e;,h#b,o71 whose volume #a1gic c4c41 we obtain not #c1fjh but #c1fcc c4c41 an expansion of #be c4c4 per gram molecule of the resulting salt1 ,na#b,,so#d4 ,it is the same in other cases4 ,nitric and hydrochloric acids give a still greater expansion than sulphuric acid1 and potassium hydroxide than sodium hydroxide1 whilst a solution of ammonia gives a contraction4 ,the relation to water must be considered as the cause of these phenomena4 ,when sodium hydroxide and sulphuric acid dissolve in water they develop heat and give a vigorous contraction2 the water is separated from such solutions with great difficulty4 ,after mutual saturation they form the salt ,na#b,,so#d1 which retains the water but feebly and evolves but little heat with it1 i4e41 in other words1 has little affinity for water4 ,in the saturation of sulphuric acid by soda the water is1 so to say1 displaced from a stable combination and passes into an unstable combination2 hence an expansion 7decrease of sp4 gr47 takes place4 ,it is not the reaction of the acid on the alkali1 but the reaction of water1 that produces the phenomenon by which ,ostwald desires to measure the degree of salt formation4 ,the water1 which escaped attention1 itself has affinity1 and influences those phenomena which are being investigated4 ,furthermore1 in the given instance its influence is very great because its mass is large4 ,when it is not present1 or only present in small quantities1 the attraction of the base to the acid leads to contraction1 and not expansion4 ,na#b,o has a sp4 gr4 #bh1 hence its molecular volume #bb2 the sp4 gr4 of ,,so#c is #ai and volume #da1 hence the sum of their volumes is #fc2 for ,na#b,,so#d the sp4 gr4 is #bfe and volume #ecf1 consequently there is a contraction of #aj c4c4 per gram-molecule of salt4 ,the volume of ,h#b,,so#d #ecc1 that of #b,na,h,o #cgd2 there is produced #b;,h#b,o1 volume #cf1 ,na#b,,so#d1 volume #ecf4 ,there react #ijg c4c41 and on saturation there result #hif c4c42 consequently contraction again ensues1 although less1 and although this reaction is one of substitution and not of combination4 ,consequently the phenomena studied by ,ostwald depend but little on the measure of the reaction of the salts1 and more on the relations of the dissolved substances to water4 ,in substitutions1 for instance #b,na,n,o#c ,h#b,,so#d #b;,,hno#c ,na#b,,so#d1 the volumes vary but slightly3 in the above example they are #b7chh7 #ecc and #b7dab7 #ecf2 hence #aca volumes act1 and #acf volumes are produced4 ,it may be concluded1 therefore1 on the basis of what has been said1 that on taking water into consideration the phenomena studied by ,thomsen and ,ostwald are much more complex than they at first appear1 and that this method can scarcely lead to a correct interpretation as to the distribution of acids between bases4 ,we may add that ,p4 ,d4 ,chroustcheff 7#ahij7 introduced a new method for this class of research1 by investigating the electro-conductivity of solutions and their mixtures1 and obtained remarkable results 7for example1 that hydrochloric acid almost entirely displaces formic acid and only #b_/c of sulphuric acid71 but details of these methods must be looked for in text-books of theoretical chemistry4 #bh ,g4 ,g4 ,gustavson's researches1 which were conducted in the laboratory of the ,st4 ,petersburg ,university in #ahga-gb1 are among the first in which the measure of the affinity of the elements for the halogens is recognised with perfect clearness in the limit of substitution and in the rate of reaction4 ,the researches conducted by ,a4 ,l4 ,potilitzin 7of which mention will be made in ,chapter ,,xi41 ,note #ff7 in the same laboratory touch on another aspect of the same problem which has not yet made much progress1 notwithstanding its importance and the fact that the theoretical side of the subject 7thanks especially to ,guldberg and ,van't ,hoff7 has since been rapidly pushed forward4 ,if the researches of ,gustavson took account of the influence of mass1 and were more fully supplied with data concerning velocities and temperatures1 they would be very important1 because of the great significance which the case considered has for the understanding of double saline decompositions in the absence of water4 ,furthermore ,gustavson showed that the greater the atomic weight of the element 7,b1 ,si1 ,ti1 ,as1 ,sn7 combined with chlorine the greater the amount of chlorine replaced by bromine by the action of ,c,br#d1 and consequently the less the amount of bromine replaced by chlorine by the action of ,c,cl#d on bromine compounds4 ,for instance1 for chlorine compounds the percentage of substitution 7at the limit7 is-- ,b,cl#c ,si,cl#d ,ti,cl#d ,as,cl#c ,sn,cl#d #aja #abe #dcf #gah #gge ,it should he observed1 however1 that ,thorpe1 on the basis of his experiments1 denies the universality of this conclusion4 ,i may mention one conclusion which it appears to me may be drawn from the above-cited figures of ,gustavson1 if they are subsequently verified even within narrow limits4 ,if ,c,br#d be heated with ,r,cl#d1 then an exchange of the bromine for chlorine takes place4 ,but what would be the result if it were mixed with ,c,cl#d8 ,judging by the magnitude of the atomic weights1 ,b #aa1 ,c #ab1 ,si #bh1 about #aa p4c4 of the chlorine would be replaced by bromine4 ,but to what does this point8 ,i think that this shows the existence of a motion of the atoms in the molecule4 ,the mixture of ,c,cl#d and ,c,br#d does not remain in a condition of static equilibrium2 not only are the molecules contained in it in a state of motion1 but also the atoms in the molecules1 and the above figures show the measure of their translation under these conditions4 ,the bromine in the ,c,br#d is1 within the limit1 substituted by the chlorine of the ,c,cl#d in a quantity of about #aa out of #ajj3 that is1 a portion of the atoms of bromine previously to this moment in combination with one atom of carbon pass over to the other atom of carbon1 and the chlorine passes over from this second atom of carbon to replace it4 ,therefore1 also1 in the homogeneous mass ,c,cl#d all the atoms of ,cl do not remain constantly combined with the same atoms of carbon1 and there is on exchange of atoms between different molecules in a homogeneous medium also4 ,this hypothesis may in my opinion explain certain phenomena of dissociation1 but though mentioning it ,i do not consider it worth while to dwell upon it4 ,i will only observe that a similar hypothesis suggested itself to me in my researches on solutions1 and that ,pfaundler enunciated an essentially similar hypothesis1 and in recent times a like view is beginning to find favour with respect to the electrolysis of saline solutions4 ,thus we now find ample confirmation from various quarters for the following rules of ,berthollet1 applying them to double saline decompositions3 #a4 ,from two salts ,,mx and ,,ny containing different haloids and metals there result from their reaction two others1 ,,my and ,,nx1 but such a substitution will not proceed to the end unless one product passes from the sphere of action4 #b4 ,this reaction is limited by the existence of an equilibrium between ,,mx1 ,,ny1 ,,my1 and ,,nx1 because a reverse reaction is quite as possible as the direct reaction4 #c4 ,this limit is determined both by the measure of the active affinities and by the relative masses of the substances as measured by the number of the reacting molecules4 #d4 ,other conditions being constant1 the chemical action is proportional to the product of the chemical masses in action4#bi #bi ,berthollet's doctrine is hardly at all affected in principle by showing that there are cases in which there is no decomposition between salts1 because the affinity may be so small that even a large mass would still give no observable displacements4 ,the fundamental condition for the application of ,berthollet's doctrine1 as well as ,deville's doctrine of dissociation1 lies in the reversibility of reactions4 ,there are practically irreversible reactions 7for instance1 ,c,cl#d #b;,h#b,o ,,co#b #d;,h,cl71 just as there are non-volatile substances4 ,but while accepting the doctrine of reversible reactions and retaining the theory of the evaporation of liquids1 it is possible to admit the existence of non-volatile substances1 and in just the same way of reactions1 without any visible conformity to ,berthollet's doctrine4 ,this doctrine evidently comes nearer than the opposite doctrine of ,bergmann to solving the complex problems of chemical mechanics for the successful solution of which at the present time the most valuable help is to be expected from the working out of data concerning dissociation1 the influence of mass1 and the equilibrium and velocity of reactions4 ,but it is evident that from this point of view we must not regard a solvent as a non-participant space1 but must take into consideration the chemical reactions accompanying solution1 or else bring about reactions without solution4 ,thus if the salts ,,mx and ,,ny after reaction partly formed salts ,,my and ,,nx1 then a state of equilibrium is reached and the reaction ceases2 but if one of the resultant compounds1 in virtue of its physical properties1 passes from the sphere of action of the remaining substances1 then the reaction will continue4 ,this exit from the sphere of action depends on the physical properties of the substance and on the conditions under which the reaction takes place4 ,thus1 for instance1 the salt ,,nx may1 in the case of reaction between solutions1 separate as a precipitate1 an insoluble substance1 while the other three substances remain in solution1 or it may pass into vapour1 and in this manner also pass away from the sphere of action of the remaining substances4 ,let us now suppose that it passes away in some form or other from the sphere of action of the remaining substances--for instance1 that it is transformed into a precipitate or vapour--then a fresh reaction will set in and a re-formation of the salt ,,nx4 ,if this be removed1 then1 although the quantity of the elements ,n and ,x in the mass will be diminished1 still1 according to ,berthollet's law1 a certain amount of ,,nx should be again formed4 ,when this substance is again formed1 then1 owing to its physical properties1 it will again pass away2 hence the reaction1 in consequence of the physical properties of the resultant substances1 is able to proceed to completion notwithstanding the possible weakness of the attraction existing between the elements entering into the composition of the resultant substance ,,nx4 ,naturally1 if the resultant substance is formed of elements having a considerable degree of affinity1 then the complete decomposition is considerably facilitated4 ,such a representation of the modus operandi of chemical transformations is applicable with great clearness to a number of reactions studied in chemistry1 and1 what is especially important1 the application of this aspect of ,berthollet's teaching does not in any way require the determination of the measure of affinity acting between the substances present4 ,for instance1 the action of ammonia on solutions of salts2 the displacement1 by its means1 of basic hydrates insoluble in water2 the separation of volatile nitric acid by the aid of non-volatile sulphuric acid1 as well as the decomposition of common salt by means of sulphuric acid1 when gaseous hydrochloric acid is formed--may be taken as examples of reactions which proceed to the end1 inasmuch as one of the resultant substances is entirely removed from the sphere of action1 but they in no way indicate the measure of affinity4#cj #cj ,common salt not only enters into double decomposition with acids but also with every salt4 ,however1 as clearly follows from ,berthollet's doctrine1 this form of decomposition will only in a few cases render it possible for new metallic chlorides to be obtained1 because the decomposition will not be carried on to the end unless the metallic chloride formed separates from the mass of the active substances4 ,thus1 for example1 if a solution of common salt be mixed with a solution of magnesium sulphate1 double decomposition ensues1 but not completely1 because all the substances remain in the solution4 ,in this case the decomposition must result in the formation of sodium sulphate and magnesium chloride1 substances which are soluble in water2 nothing is disengaged1 and therefore the decomposition #b,na,cl ,mg,s,o#d ,mg,cl#b ,na#b,,so#d cannot proceed to the end4 ,however1 the sodium sulphate formed in this manner may be separated by freezing the mixture4 ,the complete separation of the sodium sulphate will naturally not take place1 owing to a portion of the salt remaining in the solution4 ,nevertheless1 this kind of decomposition is made use of for the preparation of sodium sulphate from the residues left after the evaporation of sea-water1 which contain a mixture of magnesium sulphate and common salt4 ,such a mixture is found at ,stassfurt in a natural form4 ,it might be said that this form of double decomposition is only accomplished with a change of temperature2 but this would not be true1 as may be concluded from other analogous cases4 ,thus1 for instance1 a solution of copper sulphate is of a blue colour1 while a solution of copper chloride is green4 ,if we mix the two salts together the green tint is distinctly visible1 so that by this means the presence of the copper chloride in the solution of copper sulphate is clearly seen4 ,if now we add a solution of common salt to a solution of copper sulphate1 a green coloration is obtained1 which indicates the formation of copper chloride4 ,in this instance it is not separated1 but it is immediately formed on the addition of common salt1 as it should be according to ,berthollet's doctrine4 ,the complete formation of a metallic chloride from common salt can only occur1 judging from the above1 when it separates from the sphere of action4 ,the salts of silver are instances in point1 because the silver chloride is insoluble in water2 and therefore if we add a solution of sodium chloride to a solution of a silver salt1 silver chloride and the sodium salt of that acid which was in the silver salt are formed4 ,as a proof that double decompositions like the above are actually accomplished in the sense of ,berthollet's doctrine1 the fact may be cited that common salt may be entirely decomposed by nitric acid1 and nitre may be completely decomposed by hydrochloric acid1 just as they are decomposed by sulphuric acid2 but this only takes place when1 in the first instance1 an excess of nitric acid is taken1 and in the second instance1 an excess of hydrochloric acid1 for a given quantity of the sodium salt1 and when the resultant acid passes off4 ,if sodium chloride be put into a porcelain evaporating basin1 nitric acid added to it1 and the mixture heated1 then both hydrochloric and nitric acids are expelled by the heat4 ,thus the nitric acid partially acts on the sodium chloride1 but on heating1 as both acids are volatile1 they are both converted into vapour2 and therefore the residue will contain a mixture of a certain quantity of the sodium chloride taken and of the sodium nitrate formed4 ,if a fresh quantity of nitric acid be then added1 reaction will again set in1 a certain portion of hydrochloric acid is again evolved1 and on heating is expelled together with nitric acid4 ,if this be repeated several times1 it is possible to expel all the hydrochloric acid1 and to obtain sodium nitrate only in the residue4 ,if1 on the contrary1 we take sodium nitrate and add hydrochloric acid to it in an aqueous solution1 a certain quantity of the hydrochloric acid displaces a portion of the nitric acid1 and on heating the excess of hydrochloric acid passes away with the nitric acid formed4 ,on repeating this process1 it is possible to displace the nitric acid with an excess of hydrochloric acid1 just as it was possible to displace the hydrochloric acid by an excess of nitric acid4 ,the influence of the mass of the substance in action and the influence of volatility are here very distinctly seen4 ,hence it may be affirmed that sulphuric acid does not displace hydrochloric acid because of an especially high degree of affinity1 but that this reaction is only carried on to the end because the sulphuric acid is not volatile1 whilst the hydrochloric acid which is formed is volatile4 ,the preparation of hydrochloric acid in the laboratory and on a large scale is based upon these data4 ,in the first instance1 an excess of sulphuric acid is employed in order that the reaction may proceed easily at a low temperature1 whilst on a large scale1 when it is necessary to economise every material1 equivalent quantities are taken in order to obtain the normal salt ,na#b,,so#d and not the acid salt1 which would require twice as much acid4 ,the hydrochloric acid evolved is a gas which is very soluble in water4 ,it is most frequently used in practice in this state of solution under the name of muriatic acid4#ca #ca ,the apparatus shown in fig4 #df 7,chapter ,,vi41 ,note #ab7 is generally employed for the preparation of small quantities of hydrochloric acid4 ,common salt is placed in the retort2 the salt is generally previously fused1 as it otherwise froths and boils over in the apparatus4 ,when the apparatus is placed in order sulphuric acid mixed with water is poured down the thistle funnel into the retort4 ,strong sulphuric acid 7about half as much again as the weight of the salt7 is usually taken1 and it is diluted with a small quantity of water 7half7 if it be desired to retard the action1 as in using strong sulphuric acid the action immediately begins with great vigour4 ,the mixture1 at first without the aid of heat and then at a moderate temperature 7in a water-bath71 evolves hydrochloric acid4 ,commercial hydrochloric acid contains many impurities2 it is usually purified by distillation1 the middle portions being collected4 ,it is purified from arsenic by adding ,fe,cl#b1 distilling1 and rejecting the first third of the distillate4 ,if free hydrochloric acid gas be required1 it is passed through a vessel containing strong sulphuric acid to dry it1 and is collected over a mercury hath4 ,phosphoric anhydride absorbs hydrogen chloride 7,bailey and ,fowler1 #ahhh2 #b,p#b,o#c #c;,h,cl ,p,o,cl#c #c;,,hpo#c7 at the ordinary temperature1 and therefore the gas cannot he dried by this substance4 ,illustration3 ,,fig4 #fe4--,section of a salt-cake furnace4 ,b1 pan in which the sodium chloride and sulphuric acid are first mixed and heated4 ,c1 muffle for the ultimate decomposition4 ,in chemical works the decomposition of sodium chloride by means of sulphuric acid is carried on on a very large scale1 chiefly with a view to the preparation of normal sodium sulphate1 the hydrochloric acid being a bye-product4#ca bis ,the furnace employed is termed a salt cake furnace4 ,it is represented in fig4 #fe1 and consists of the following two parts3 the pan ,b and the roaster ,c1 or enclosed space built up of large bricks a and enveloped on all sides by the smoke and flames from the fire grate1 ,f4 ,the ultimate decomposition of the salt by the sulphuric acid is accomplished in the roaster4 ,but the first decomposition of sodium chloride by sulphuric acid does not require so high a temperature as the ultimate decomposition1 and is therefore carried on in the front and cooler portion1 ,b1 whose bottom is heated by gas flues4 ,when the reaction in this portion ceases and the evolution of hydrochloric acid stops1 then the mass1 which contains about half of the sodium chloride still undecomposed1 and the sulphuric acid in the form of acid sodium sulphate1 is removed from ,b and thrown into the roaster ,c1 where the action is completed4 ,normal sodium sulphate1 which we shall afterwards describe1 remains in the roaster4 ,it is employed both directly in the manufacture of glass1 and in the preparation of other sodium compounds--for instance1 in the preparation of soda ash1 as will afterwards be described4 ,for the present we will only turn our attention to the hydrochloric acid evolved in ,b and ,c4 #ca bis ,in chemical works where sulphuric acid of #fj ,baum 7#bb p4c4 of water7 is employed1 #aag parts of sodium chloride are taken to about #abe parts of sulphuric acid4 ,the hydrochloric acid gas evolved is subjected to condensation by dissolving it in water4#cb ,if the apparatus in which the decomposition is accomplished were hermetically closed1 and only presented one outlet1 then the escape of the hydrochloric acid would only proceed through the escape pipe intended for this purpose4 ,but as it is impossible to construct a perfectly hermetically closed furnace of this kind1 it is necessary to increase the draught by artificial means1 or to oblige the hydrochloric acid gas to pass through those arrangements in which it is to be condensed4 ,this is done by connecting the ends of the tubes through which the hydrochloric acid gas escapes from the furnace with high chimneys1 where a strong draught is set up from the combustion of the fuel4 ,this causes a current of hydrochloric acid gas to pass through the absorbing apparatus in a definite direction4 ,here it encounters a current of water flowing in the opposite direction1 by which it is absorbed4 ,it is not customary to cause the acid to pass through the water1 but only to bring it into contact with the surface of the water4 ,the absorption apparatus consists of large earthenware vessels having four orifices1 two above and two lateral ones in the wide central portion of each vessel4 ,the upper orifices serve for connecting the vessels together1 and the hydrochloric acid gas escaping from the furnace passes through these tubes4 ,the water for absorbing the acid enters at the upper1 and flows out from the lower1 vessel1 passing through the lateral orifices in the vessels4 ,the water flows from the chimney towards the furnace and it is therefore evident that the outflowing water will be the most saturated with acid1 of which it actually contains about #bj per cent4 ,the absorption in these vessels is not complete4 ,the ultimate absorption of the hydrochloric acid is carried on in the so-called coke towers1 which usually consist of two adjacent chimneys4 ,a lattice-work of bricks is laid on the bottom of these towers1 on which coke is piled up to the top of the tower4 ,water1 distributing itself over the coke1 trickles down to the bottom of the tower1 and in so doing absorbs the hydrochloric acid gas rising upwards4 #cb ,as in works which treat common salt in order to obtain sodium sulphate1 the hydrochloric acid is sometimes held to be of no value1 it might be allowed to escape with the waste furnace gases into the atmosphere1 which would greatly injure the air of the neighbourhood and destroy all vegetation4 ,in all countries1 therefore1 there are laws forbidding the factories to proceed in this manner1 and requiring the absorption of the hydrochloric acid by water at the works themselves1 and not permitting the solution to be run into rivers and streams1 whose waters it would spoil4 ,it may be remarked that the absorption of hydrochloric acid presents no particular difficulties 7the absorption of sulphurous acid is much more difficult7 because hydrochloric acid has a great affinity for water and gives a hydrate which boils above #ajj4 ,hence1 even steam and hot water1 as well as weaker solutions1 can be used for absorbing the acid4 ,however1 ,warder 7#ahhh7 showed that weak solutions of composition ,h#b,o n,h,cl when boiled 7the residue will be almost ,h,cl1#h;,h#b,o7 evolve 7not water but7 a solution of the composition ,h#b,o #dden#d;,h,cl2 for example1 on distilling ,h,cl1#aj;,h#b,o1 ,h,cl1#bc;,h#b,o is first obtained in the distillate4 ,as the strength of the residue becomes greater1 so also does that of the distillate1 and therefore in order to completely absorb hydrochloric acid it is necessary in the end to have recourse to water4 ,as in ,russia the manufacture of sodium sulphate from sodium chloride has not yet been sufficiently developed1 and as hydrochloric acid is required for many technical purposes 7for instance1 for the preparation of zinc chloride1 which is employed for soaking railway sleepers71 therefore salt is often treated mainly for the manufacture of hydrochloric acid4 ,it will be readily understood that hydrochloric acid may be obtained from all other metallic chlorides4#cc ,it is frequently formed in other reactions1 many of which we shall meet with in the further course of this work4 ,it is1 for instance1 formed by the action of water on sulphur chloride1 phosphorus chloride1 antimony chloride1 c4 #cc ,thus the metallic chlorides1 which are decomposed to a greater or less degree by water1 correspond with feeble bases4 ,such are1 for example1 ,mg,cl#b1 ,al,cl#c1 ,sb,cl#c1 ,bi,cl#c4 ,the decomposition of magnesium chloride 7and also carnallite7 by sulphuric acid proceeds at the ordinary temperature2 water decomposes ,mg,cl#b to the extent of #ej p4c4 when aided by heat1 and may be employed as a convenient method for the production of hydrochloric acid4 ,hydrochloric acid is also produced by the ignition of certain metallic chlorides in a stream of hydrogen1 especially of those metals which are easily reduced and difficultly oxidised--for instance1 silver chloride4 ,lead chloride1 when heated to redness in a current of steam1 gives hydrochloric acid and lead oxide4 ,the multitude of the cases of formation of hydrochloric acid are understood from the fact that it is a substance which is comparatively very stable1 resembling water in this respect1 and even most probably more stable than water1 because1 at a high temperature and even under the action of light1 chlorine decomposes water1 with the formation of hydrochloric acid4 ,the combination of chlorine and hydrogen also proceeds by their direct action1 as we shall afterwards describe4 ,hydrochloric acid is a colourless gas having a pungent suffocating odour and an acid taste4 ,this gas fumes in air and attracts moisture1 because it forms vapour containing a compound of hydrochloric acid and water4 ,hydrochloric acid is liquefied by cold1 and under a pressure of #dj atmospheres1 into a colourless liquid of sp4 gr4 #jijh at #j1cd boiling point -#ce and absolute boiling point #eb4 ,we have already seen 7,chapter ,i47 that hydrochloric acid combines very energetically with water1 and in so doing evolves a considerable amount of heat4 ,the solution saturated in the cold attains a density #abc4 ,on heating such a solution containing about #de parts of acid per #ajj parts1 the hydrochloric acid gas is expelled with only a slight admixture of aqueous vapour4 ,but it is impossible to entirely separate the whole of the hydrochloric acid from the water by this means1 as could be done in the case of an ammoniacal solution4 ,the temperature required for the evolution of the gas rises and reaches #aaj-aaa1 and after this remains constant--that is1 a solution having a constant boiling point is obtained 7as with ,,hno#c71 which1 however1 does not 7,roscoe and ,dittmar7 present a constant composition under different pressures1 because the hydrate is decomposed in distillation1 as is seen from the determinations of its vapour density 7,bineau74 ,judging from the facts 7#a7 that with decrease of the pressure under which the distillation proceeds the solution of constant boiling point approaches to a composition of #be p4c4 of hydrochloric acid1#ce 7#b7 that by passing a stream of dry air through a solution of hydrochloric acid there is obtained in the residue a solution which also approaches to #be p4c4 of acid1 and more nearly as the temperature falls1#cf 7#c7 that many of the properties of solutions of hydrochloric acid vary distinctly according as they contain more or less than #be p4c4 of hydrochloric acid 7for instance1 antimonious sulphide gives hydrogen sulphide with a stronger acid1 but is not acted on by a weaker solution1 also a stronger solution fumes in the air1 c471 and 7#d7 that the composition ,h,cl1#f;,h#b,o corresponds with #bebf p4c4 ,h,cl--judging from all these data1 and also from the loss of tension which occurs in the combination of hydrochloric acid with water1 it may be said that they form a definite hydrate of the composition ,h,cl1#f;,h#b,o4 ,besides this hydrate there exists also a crystallo-hydrate1 ,h,cl1#b;,h#b,o1#cg which is formed by the absorption of hydrochloric acid by a saturated solution at a temperature of -#bc4 ,it crystallises and melts at -#ah4ch #cd ,according to ,ansdell 7#ahhj7 the sp4 gr4 of liquid hydrochloric acid at #j #jijh1 at #aafg #jhed1 at #bbg #jhjh1 at #cc #jgdh4 ,hence it is seen that the expansion of this liquid is greater than that of gases 7,chapter ,,ii41 ,note #cd74 #ce ,according to ,roscoe and ,dittmar at a pressure of three atmospheres the solution of constant boiling point contains #ah p4c4 of hydrogen chloride1 and at a pressure of one-tenth atmosphere #bc p4c4 ,the percentage is intermediate at medium pressures4 #cf ,at #j #be p4c41 at #ajj #bjg p4c42 ,roscoe and ,dittmar4 #cg ,this crystallo-hydrate 7obtained by ,pierre and ,puchot1 and investigated by ,roozeboom7 is analogous to ,na,cl1#b;,h#b,o4 ,the crystals ,h,cl1#b;,h#b,o at -#bb have a specific gravity #adf2 the vapour tension 7under dissociation7 of the solution having a composition ,h,cl1#b;,h#b,o at -#bd #gfj1 at -#ai #a1jaj1 at -#ah #a1jeg1 at -#ag #a1aab mm4 of mercury4 ,in a solid state the crystallo-hydrate at -#agg has the same tension1 whilst at lower temperatures it is much less3 at -#bd about #aej1 at -#ai about #ehj mm4 ,a mixture of fuming hydrochloric acid with snow reduces the temperature to -#ch4 ,if another equivalent of water be added to the hydrate ,h,cl1#b;,h#b,o at -#ah1 the temperature of solidification falls to -#be1 and the hydrate ,h,cl1#c;,h#b,o is formed 7,pickering1 #ahic74 #ch ,according to ,roscoe at #j one hundred grams of water at a pressure p 7in millimetres of mercury7 dissolves-- p #ajj #bjj #cjj #ejj #gjj #a1jjj ,grams ,h,cl #feg #gjg #gch #ghb #hag #hef ,at a pressure of #gfj millimetres and temperature t1 one hundred grams of water dissolves t #j #h #af #bd #dj #fj ,grams ,h,cl #hbe #ghc #gdb #gjj #fcc #efa ,roozeboom 7#ahhf7 showed that at t solutions containing c grams of hydrogen chloride per #ajj grams of water may 7with the variation of the pressure p7 be formed together with the crystallo-hydrate ,h,cl1#b;,h#b,o3 t -#bhh -#ba -#ai -#ah -#agg c #hdb #hfh #ibf #ihd #ajad p -- #ccd #ehj #ijj #a1jgc mm4 ,the last combination answers to the melted crystallo-hydrate ,h,cl1#b;,h#b,o1 which splits up at temperatures above -#agg1 and at a constant atmospheric pressure when there are no crystals-- t -#bd -#ba -#ah -#aj #j c #ajab #ihc #ieg #hih #hdb ,from these data it is seen that the hydrate ,h,cl1#b;,h#b,o can exist in a liquid state1 which is not the case for the hydrates of carbonic and sulphurous anhydrides1 chlorine1 c4 ,according to ,marignac1 the specific heat c of a solution ,h,cl m,h#b,o 7at about #cj1 taking the specific heat of water #a7 is given by the expression-- ,c7#cfe m#ah7 #ahm - #bhci #adj_/m - #bfh_/m#b if m be not less than #fbe4 ,for example1 for ,h,cl #be;,h#b,o1 ,c #jhgg4 ,according to ,thomsen's data1 the amount of heat ,q1 expressed in thousands of calories1 evolved in the solution of #cfe grams of gaseous hydrochloric acid in m,h#b,o or #ahm grams of water is equal to-- m #b #d #aj #ej #djj ,q #aad #adc #afb #aga #agc ,in these quantities the latent heat of liquefaction is included1 which must be taken as #e-i thousand calories per molecular quantity of hydrogen chloride4 ,the researches of ,scheffer 7#ahhh7 on the rate of diffusion 7in water7 of solutions of hydrochloric acid show that the coefficient of diffusion k decreases with the amount of water n1 if the composition of the solution is ,h,cl1n,h#b,o at #j3-- n #e #fi #ih #ad #bga #abie k #bca #bjh #ahf #afg #aeb #aci ,it also appears that strong solutions diffuse more rapidly into dilute solutions than into water4 ,the mean specific gravities at #ae1 taking water at its maximum density 7#d7 as #aj1jjj1 for solutions containing p per cent4 of hydrogen chloride are-- p ,s p ,s #e #aj1bdb #be #aa1bff #aj #aj1dij #cj #aa1ebb #ae #aj1gdd #ce #aa1ggc #bj #aa1jja #dj #aa1iig ,the formula ,s #i1iiaf #didcp #jjegap#b1 up to p #bebf1 which answers to the hydrate ,h,cl1#f;,h#b,o mentioned above1 gives the specific gravity4 ,above this percentage ,s #i1ghea #feajp-#jbdjp#b4 ,the rise of specific gravity with an increase of percentage 7or the differential ds_/dp7 reaches a maximum at about #be p4c4#ci ,the intermediate solution1 ,h,cl1#f;,h#b,o1 is further distinguished by the fact that the variation of the specific gravity with the variation of temperature is a constant quantity1 so that the specific gravity of this solution is equal to #aa1cebg7a-jjjjddgt71 where #jjjjddg is the coefficient of expansion of the solution4#dj ,in the case of more dilute solutions1 as with water1 the specific gravity per #a 7or the differential ds_/dt7 rises with a rise of temperature4#da p #j #e #aj #ae #bj ,s#j - ,s#ae #gb #bc #ch #eb #fd ,s#ae - ,s#cj #cda #db #ej #ei #fg ,whilst for solutions which contain a greater proportion of hydrogen chloride than ,h,cl1#f;,h#b,o1 these coefficients decrease with a rise of temperature2 for instance1 for #cj p4c4 of hydrogen chloride ,s#j-,s#ae #hh and ,s#ae-,s#cj #hg 7according to ,marignac's data74 ,in the case of ,h,cl1#f;,h#b,o these differences are constant1 and equal #gf4 #ci ,if it be admitted that the maximum of the differential corresponds with ,h,cl1#f;,h#b,o1 then it might be thought that the specific gravity is expressed by a parabola of the third order2 but such an admission does not give expressions in accordance with fact4 ,this is all more fully considered in my work mentioned in ,chapter ,i41 ,note #ai4 #dj ,as in water1 the coefficient of expansion 7or the quantity k in the expression ,st ,s#j-k,s#jt1 or ,vt #a_/7a-kt77 attains a magnitude #jjjjddg at about #dh1 it might be thought that at #dh all solutions of hydrochloric acid would have the same coefficient of expansion1 but in reality this is not the case4 ,at low and at the ordinary temperatures the coefficient of expansion of aqueous solutions is greater than that of water1 and increases with the amount of substance dissolved4 #da ,the figures cited above may serve for the direct determination of that variation of the specific gravity of solutions of hydrochloric acid with the temperature4 ,thus1 knowing that at #ae the specific gravity of a #aj p4c4 solution of hydrochloric acid #aj1dib1 we find that at t it #aj1ecj-t7#bac #jjbgt74 ,whence also may be found the coefficient of expansion 7,note #dj74 ,thus the formation of two definite hydrates1 ,h,cl1#b;,h#b,o and ,h,cl1#f;,h#b,o1 between hydrochloric acid and water may be accepted upon the basis of many facts4 ,but both of them1 if they occur in a liquid state1 dissociate with great facility into hydrogen chloride and water1 and are completely decomposed when distilled4 ,all solutions of hydrochloric acid present the properties of an energetic acid4 ,they not only transform blue vegetable colouring matter into red1 and disengage carbonic acid gas from carbonates1 c41 but they also entirely saturate bases1 even such energetic ones as potash1 lime1 c4 ,in a dry state1 however1 hydrochloric acid does not alter vegetable dyes1 and does not effect many double decompositions which easily take place in the presence of water4 ,this is explained by the fact that the gaso-elastic state of the hydrochloric acid prevents its entering into reaction4 ,however1 incandescent iron1 zinc1 sodium1 c41 act on gaseous hydrochloric acid1 displacing the hydrogen and leaving half a volume of hydrogen for each volume of hydrochloric acid gas2 this reaction may serve for determining the composition of hydrochloric acid4 ,combined with water hydrochloric acid acts as an acid much resembling nitric acid#db in its energy and in many of its reactions2 however1 the latter contains oxygen1 which is disengaged with great ease1 and so very frequently acts as an oxidiser1 which hydrochloric acid is not capable of doing4 ,the majority of metals 7even those which do not displace the ,h from ,h#b,,so#d1 but which1 like copper1 decompose it to the limit of ,,so#b7 displace the hydrogen from hydrochloric acid4 ,thus hydrogen is disengaged by the action of zinc1 and even of copper and tin4#db bis ,only a few metals withstand its action2 for example1 gold and platinum4 ,lead in compact masses is only acted on feebly1 because the lead chloride formed is insoluble and prevents the further action of the acid on the metal4 ,the same is to be remarked with respect to the feeble action of hydrochloric acid on mercury and silver1 because the compounds of these metals1 ,ag,cl and ,hg,cl1 are insoluble in water4 ,metallic chlorides are not only formed by the action of hydrochloric acid on the metals1 but also by many other methods2 for instance1 by the action of hydrochloric acid on the carbonates1 oxides1 and hydroxides1 and also by the action of chlorine on metals and certain of their compounds4 ,metallic chlorides have a composition ,m,cl2 for example1 ,na,cl1 ,k,cl1 ,ag,cl1 ,hg,cl1 if the metal replaces hydrogen equivalent for equivalent1 or1 as it is said1 if it be monatomic or univalent4 ,in the case of bivalent metals1 they have a composition ,m,cl#b2 for example1 ,ca,cl#b1 ,cu,cl#b1 ,pb,cl#b1 ,hg,cl#b1 ,fe,cl#b1 ,mn,cl#b4 ,the composition of the haloid salts of other metals presents a further variation2 for example1 ,al,cl#c1 ,pt,cl#d1 c4 ,many metals1 for instance ,fe1 give several degrees of combination with chlorine 7,fe,cl#b1 ,fe,cl#c7 as with hydrogen4 ,in their composition the metallic chlorides differ from the corresponding oxides1 in that the ,o is replaced by ,cl#b1 as should follow from the law of substitution1 because oxygen gives ,,oh#b1 and is consequently bivalent1 whilst chlorine forms ,h,cl1 and is therefore univalent4 ,so1 for instance1 ferrous oxide1 ,fe,o1 corresponds with ferrous chloride1 ,fe,cl#b1 and the oxide ,fe#b,o#c with ferric chloride1 which is also seen from the origin of these compounds1 for ,fe,cl#b is obtained by the action of hydrochloric acid on ferrous oxide or carbonate and ,fe,cl#c by its action on ferric oxide4 ,in a word1 all the typical properties of acids are shown by hydrochloric acid1 and all the typical properties of salts in the metallic chlorides derived from it4 ,acids and salts composed like ,h,cl and ,mn,cl#bm without any oxygen bear the name of haloid salts2 for instance1 ,h,cl is a haloid acid1 ,na,cl a haloid salt1 chlorine a halogen4 ,the capacity of hydrochloric acid to give1 by its action on bases1 ,,mo1 a metallic chloride1 ,m,cl#b1 and water1 is limited at high temperatures by the reverse reaction ,m,cl#b ,h#b,o ,,mo #b;,h,cl1 and the more pronounced are the basic properties of ,,mo the feebler is the reverse action1 while for feebler bases such as ,al#b,o#c1 ,mg,o1 c41 this reverse reaction proceeds with ease4 ,metallic chlorides corresponding with the peroxides either do not exist1 or are easily decomposed with the disengagement of chlorine4 ,thus there is no compound ,ba,cl#d corresponding with the peroxide ,ba,o#b4 ,metallic chlorides having the general aspect of salts1 like their representative sodium chloride1 are1 as a rule1 easily fusible1 more so than the oxides 7for instance1 ,ca,o is infusible at a furnace heat1 whilst ,ca,cl#b is easily fused7 and many other salts4 ,under the action of heat many chlorides are more stable than the oxides1 some can even be converted into vapour2 thus corrosive sublimate1 ,hg,cl#b1 is particularly volatile1 whilst the oxide ,hg,o decomposes at a red heat4 ,silver chloride1 ,ag,cl1 is fusible and is decomposed with difficulty1 whilst ,ag#b,o is easily decomposed4 ,the majority of the metallic chlorides are soluble in water1 but silver chloride1 cuprous chloride1 mercurous chloride1 and lead chloride are sparingly soluble in water1 and are therefore easily obtained as precipitates when a solution of the salts of these metals is mixed with a solution of any chloride or even with hydrochloric acid4 ,the metal contained in a haloid salt may often be replaced by another metal1 or even by hydrogen1 just as is the case with a metal in an oxide4 ,thus copper displaces mercury from a solution of mercuric chloride1 ,hg,cl#b ,cu ,cu,cl#b ,hg1 and hydrogen at a red heat displaces silver from silver chloride1 #b;,ag,cl ,h#b ,ag#b #b;,h,cl4 ,these1 and a whole series of similar reactions1 form the typical methods of double saline decompositions4 ,the measure of decomposition and the conditions under which reactions of double saline decompositions proceed in one or in the other direction are determined by the properties of the compounds which take part in the reaction1 and of those capable of formation at the temperature1 c41 as was shown in the preceding portions of this chapter1 and as will be frequently found hereafter4 #db ,thus1 for instance1 with feeble bases they evolve in dilute solutions 7,chapter ,,iii41 ,note #ec7 almost equal amounts of heat2 their relation to sulphuric acid is quite identical4 ,they both form fuming solutions as well as hydrates2 they both form solutions of constant boiling point4 #db bis ,pybalkin 7#ahia7 found that copper begins to disengage hydrogen at #ajj1 and that chloride of copper begins to give up its chlorine to hydrogen gas at #bcj2 for silver these temperatures are #aag and #bfj--that is1 there is less difference between them4 ,if hydrochloric acid enters into double decomposition with basic oxides and their hydrates1 this is only due to its acid properties2 and for the same reason it rarely enters into double decomposition with acids and acid anhydrides4 ,sometimes1 however1 it combines with the latter1 as1 for instance1 with the anhydride of sulphuric acid1 forming the compound ,,so#c;,h,cl2 and in other cases it acts on acids1 giving up its hydrogen to their oxygen and forming chlorine1 as will be seen in the following chapter4 ,hydrochloric acid1 as may already be concluded from the composition of its molecule1 belongs to the monobasic acids1 and does not1 therefore1 give true acid salts 7like ,h,na,s,o#d or ,h,na,c,o#c72 nevertheless many metallic chlorides1 formed from powerful bases1 are capable of combining with hydrochloric acid1 just as they combine with water1 or with ammonia1 or as they give double salts4 ,compounds have long been known of hydrochloric acid with auric1 platinic1 and antimonious chlorides1 and other similar metallic chlorides corresponding with very feeble bases4 ,but ,berthelot1 ,engel1 and others have shown that the capacity of ,h,cl for combining with ,mn,clm is much more frequently encountered than was previously supposed4 ,thus1 for instance1 dry hydrochloric acid when passed into a solution of zinc chloride 7containing an excess of the salt7 gives in the cold 7#j7 a compound ,h,cl1,zn,cl#b1b;,h#b,o1 and at the ordinary temperature ,h,cl1#b,zn,cl#b1b;,h#b,o1 just as it is able at low temperatures to form the crystallo-hydrate ,zn,cl#b1c;,h#b,o 7,engel1 #ahhf74 ,similar compounds are obtained with ,cd,cl#b1;,cu,cl#b1 ,hg,cl#b1;,fe#b;,cl#f1 c4 7,berthelot1 ,ditte1 ,cheltzoff1 ,lachinoff1 and others74 ,these compounds with hydrochloric acid are generally more soluble in water than the metallic chlorides themselves1 so that whilst hydrochloric acid decreases the solubility of ,mn,clm1 corresponding with energetic bases 7for instance1 sodium or barium chlorides71 it increases the solubility of the metallic chlorides corresponding with feeble bases 7cadmium chloride1 ferric chloride1 c47 ,silver chloride1 which is insoluble in water1 is soluble in hydrochloric acid4 ,hydrochloric acid also combines with certain unsaturated hydrocarbons 7for instance1 with turpentine1 ,c#aj;,h#af1b;,h,cl7 and their derivatives4 ,sal-ammoniac1 or ammonia hydrochloride1 ,,nh#d;,cl ,,nh#c1;,h,cl1 also belongs to this class of compounds4#dc ,if hydrogen chloride gas be mixed with ammonia gas a solid compound consisting of equal volumes of each is immediately formed4 ,the same compound is obtained on mixing solutions of the two gases4 ,it is also produced by the action of hydrochloric acid on ammonium carbonate4 ,sal-ammoniac is usually prepared1 in practice1 by the last method4#dd ,the specific gravity of sal-ammoniac is #aee4 ,we have already seen 7,chapter ,,vi47 that sal-ammoniac1 like all other ammonium salts1 easily decomposes2 for instance1 by volatilisation with alkalis1 and even partially when its solution is boiled4 ,the other properties and reactions of sal-ammoniac1 especially in solution1 fully recall those already mentioned in speaking of sodium chloride4 ,thus1 for instance1 with silver nitrate it gives a precipitate of silver chloride2 with sulphuric acid it gives hydrochloric acid and ammonium sulphate1 and it forms double salts with certain metallic chlorides and other salts4#de #dc ,when an unsaturated hydrocarbon1 or1 in general1 an unsaturated compound1 assimilates to itself the molecules ,cl#b1 ,h,cl1 ,,so#c1 ,h#b,,so#d1 c41 the cause of the reaction is most simple4 ,as nitrogen1 besides the type ,,nx#c to which ,,nh#c1 belongs1 gives compounds of the type ,,nx#e--;for example1 ,,no#b7,,oh7--the formation of the salts of ammonium should be understood in this way4 ,,nh#c gives ,,nh#d;,cl because ,,nx#c is capable of giving ,,nx#e4 ,but as saturated compounds--for instance1 ,,so#c1;,h#b,o1 ,na,cl1 c4--are also capable of combination even between themselves1 it is impossible to deny the capacity of ,h,cl also for combination4 ,,so#c combines with ,h#b,o1 and also with ,h,cl and the unsaturated hydrocarbons4 ,it is impossible to recognise the distinction formerly sought to be established between atomic and molecular compounds1 and regarding1 for instance1 ,p,cl#c as an atomic compound and ,p,cl#e as a molecular one1 only because it easily splits up into molecules ,p,cl#c and ,cl#b4 #dd ,sal-ammoniac is prepared from ammonium carbonate1 obtained in the dry distillation of nitrogenous substances 7,chapter ,,vi471 by saturating the resultant solution with hydrochloric acid4 ,a solution of sal-ammoniac is thus produced1 which is evaporated1 and in the residue a mass is obtained containing a mixture of various other1 especially tarry1 products of dry distillation4 ,the sal-ammoniac is generally purified by sublimation4 ,for this purpose iron vessels covered with hemispherical metallic covers are employed1 or else simply clay crucibles covered by other crucibles4 ,the upper portion1 or head1 of the apparatus of this kind will have a lower temperature than the lower portion1 which is under the direct action of the flame4 ,the sal-ammoniac volatilises when heated1 and settles on the cooler portion of the apparatus4 ,it is thus freed from many impurities1 and is obtained as a crystalline crust1 generally several centimetres thick1 in which form it is commonly sold4 ,the solubility of sal-ammoniac rises rapidly with the temperature3 at #j1 #ajj parts of water dissolve about #bh parts of ,,nh#d;,cl1 at #ej about #ej parts1 and at the ordinary temperature about #ce parts4 ,this is sometimes taken advantage of for separating ,,nh#d;,cl from solutions of other salts4 #de ,the solubility of sal-ammoniac in #ajj parts of water 7according to ,alluard7 is-- #j #aj #bj #cj #dj #fj #hj #ajj #aaj #bhdj #cbdh #cgbh #dagb #df #ee #fd #gc #gg ,a saturated solution boils at #aaeh4 ,the specific gravity at #ae_/d of solutions of sal-ammoniac 7water #d #aj1jjj7 #i1iiaf-cabfp-#jjhep#b1 where p is the amount by weight of ammonium chloride in #ajj parts of solution4 ,with the majority of salts the differential ds_/dp increases1 but here it decreases with the increase of p4 ,for 7unlike the sodium and potassium salts7 a solution of the alkali plus a solution of acid occupy a greater volume than that of the resultant ammonium salt4 ,in the solution of solid ammonium chloride a contraction1 and not expansion1 generally takes place4 ,it may further be remarked that solutions of sal-ammoniac have an acid reaction even when prepared from the salt remaining after prolonged washing of the sublimed salt with water 7,a4 ,stcherbakoff74 ,,chapter ,,xi ,,the ,,halogens3 ,,chlorine1 ,,bromine1 ,,iodine1 ,,and ,,fluorine ,although hydrochloric acid1 like water1 is one of the most stable substances1 it is nevertheless decomposed not only by the action of a galvanic current1#a but also by a high temperature4 ,sainte-,claire ,deville showed that decomposition already occurs at #a1cjj1 because a cold tube 7as with ,,co1 ,chapter ,,ix47 covered with an amalgam of silver absorbs chlorine from hydrochloric acid in a red-hot tube1 and the escaping gas contains hydrogen4 ,v4 ,meyer and ,langer 7#ahhe7 observed the decomposition of hydrochloric acid at #a1fij in a platinum vessel2 the decomposition in this instance was proved not only from the fact that hydrogen diffused through the platinum 7p4 #adb71 owing to which the volume was diminished1 but also from chlorine being obtained in the residue 7the hydrogen chloride was mixed with nitrogen71 which liberated iodine from potassium iodide4#b ,the usual method for the preparation of chlorine consists in the abstraction of the hydrogen by oxidising agents4#b bis #a ,the decomposition of fused sodium chloride by an electric current has been proposed in ,america and ,russia 7,n4 ,n4 ,beketoff7 as a means for the preparation of chlorine and sodium4 ,a strong solution of hydrochloric acid is decomposed into equal volumes of chlorine and hydrogen by the action of an electric current4 ,if sodium chloride and lead be melted in a crucible1 the former being connected with the cathode and a carbon anode immersed in the lead1 then the lead dissolves sodium and chlorine is disengaged as gas4 ,this electrolytic method has not yet been practised on a large scale1 probably because gaseous chlorine has not many applications1 and because of the difficulty there is in dealing with it4 #b ,to obtain so high a temperature 7at which the best kinds of porcelain soften7 ,langer and ,meyer employed the dense graphitoidal carbon from gas retorts1 and a powerful blast4 ,they determined the temperature by the alteration of the volume of nitrogen in the platinum vessel1 for this gas does not permeate through platinum1 and is unaltered by heat4 #b bis ,the acid properties of hydrochloric acid were known when ,lavoisier pointed out the formation of acids by the combination of water with the oxides of the non-metals1 and therefore there was reason for thinking that hydrochloric acid was formed by the combination of water with the oxide of some element4 ,hence when ,scheele obtained chlorine by the action of hydrochloric acid on manganese peroxide he considered it as the acid contained in common salt4 ,when it became known that chlorine gives hydrochloric acid with hydrogen1 ,lavoisier and ,berthollet supposed it to be a compound with oxygen of an anhydride contained in hydrochloric acid4 ,they supposed that hydrochloric acid contained water and the oxide of a particular radicle1 and that chlorine was a higher degree of oxidation of this radicle muvias 7from the ,latin neme of hydrochloric acid1 acidum muriaticum74 ,it was only in #ahaa that ,gay-,lussac and ,thnard in ,france and ,davy in ,england arrived at the conclusion that the substance obtained by ,scheele does not contain oxygen1 nor under any conditions give water with hydrogen1 and that there is no water in hydrochloric acid gas1 and therefore concluded that chlorine is an elementary substance4 ,they named it 'chlorine' from the ,greek word ,greek3 chlros1 signifying a green colour1 because of the peculiar colour by which this gas is characterised4 ,an aqueous solution of hydrochloric acid is generally employed for t he evolution of chlorine4 ,the hydrogen has to be abstracted from the hydrochloric acid4 ,this is accomplished by nearly all oxidising substances1 and especially by those which are able to evolve oxygen at a red heat 7besides bases1 such as mercury and silver oxides1 which are able to give salts with hydrogen chloride72 for example1 manganese peroxide1 potassium chlorate1 chromic acid1 c4 ,the decomposition essentially consists in the oxygen of the oxidising substance displacing the chlorine from #b;,h,cl1 forming water1 ,h#b,o1 and setting the chlorine free1 #b;,h,cl ,o 7disengaged by the oxidising substances7 ,h#b,o ,cl#b4 ,even nitric acid partially produces a like reaction2 but as we shall afterwards see its action is more complicated1 and it is therefore not suitable for the preparation of pure chlorine4#c ,but other oxidising substances which do not give any other volatile products with hydrochloric acid may be employed for the preparation of chlorine4 ,among these may be mentioned3 potassium chlorate1 acid potassium chromate1 sodium manganate1 manganese peroxide1 c4 ,manganese peroxide is commonly employed in the laboratory1 and on a large scale1 for the preparation of chlorine4 ,the chemical process in this case may be represented as follows3 an exchange takes place between #d;,h,cl and ,mn,o#b1 in which the manganese takes the place of the four atoms of hydrogen1 or the chlorine and oxygen exchange places--that is1 ,mn,cl#d and #b;,h#b,o are produced4 ,the chlorine compound1 ,mn,cl#d1 obtained is very unstable2 it splits up into chlorine1 which as a gas passes from the sphere of action1 and a lower compound containing less chlorine than the substance first formed1 which remains in the apparatus in which the mixture is heated1 ,mn,cl#d ,mn,cl#b ,cl#b4c bis ,the action of hydrochloric acid requires a temperature of about #ajj4 ,in the laboratory the preparation of chlorine is carried on in flasks1 heated over a water-bath1 by acting on manganese peroxide with hydrochloric acid or a mixture of common salt and sulphuric acid#d and washing the gas with water to remove hydrochloric acid4#e ,chlorine cannot be collected over mercury1 because it combines with it as with many other metals1 and it is soluble in water2 however1 it is but slightly soluble in hot water or brine4 ,owing to its great weight1 chlorine may be directly collected in a dry vessel by carrying the gas-conducting tube down to the bottom of the vessel4 ,the chlorine will lie in a heavy layer at the bottom of the vessel1 displace the air1 and the extent to which it fills the vessel may be followed by its colour4#f #c ,however1 nitric acid has been proposed as a means for obtaining chlorine1 but by methods which have the drawback of being very complicated #c bis ,this representation of the process of the reaction is most natural4 ,however1 this decomposition is generally represented as if chlorine gave only one degree of combination with manganese1 ,mn,cl#b1 and therefore directly reacts in the following manner--,mn,o#b #d;,h,cl ,mn,cl#b #b;,h#b,o ,cl#b1 in which case it is supposed that manganese peroxide1 ,mn,o#b1 breaks up1 as it were1 into manganous oxide1 ,mn,o and oxygen1 both of which react with hydrochloric acid1 the manganous oxide acting upon ,h,cl as a base1 giving ,mn,cl#b and at the same time #b;,h,cl ,o ,h#b,o ,cl#b4 ,in reality1 a mixture of oxygen and hydrochloric acid does give chlorine at a red heat1 and this reaction may also take place at the moment of its evolution in this case4 ,all the oxides of manganese 7,mn#b,o#c1 ,mn,o#b1 ,mn,o#c1 ,mn#b,o#g71 with the exception of manganous oxide1 ,mn,o1 disengage chlorine from hydrochloric acid1 because manganous chloride1 ,mn,cl#b1 is the only compound of chlorine and manganese which exists as a stable compound1 all the higher chlorides of manganese being unstable and evolving chlorine4 ,hence we here take note of two separate changes3 7#a7 an exchange between oxygen and chlorine1 and 7#b7 the instability of the higher chlorine compounds4 ,as 7according to the law of substitution7 in the substitution of oxygen by chlorine1 ,cl#b takes the place of ,o1 the chlorine compounds will contain more atoms than the corresponding oxygen compounds4 ,it is not surprising1 therefore1 that certain of the chlorine compounds corresponding with oxygen compounds do not exist1 or if they are formed are very unstable4 ,and furthermore1 an atom of chlorine is heavier than an atom of oxygen1 and therefore a given element would have to retain a large mass of chlorine if in the higher oxides the oxygen were replaced by chlorine4 ,for this reason equivalent compounds of chlorine do not exist for all oxygen compounds4 ,many of the former are immediately decomposed1 when formed1 with the evolution of chlorine4 ,from this it is evident that there should exist such chlorine compounds as would evolve chlorine as peroxides evolve oxygen1 and indeed a large number of such compounds are known4 ,amongst them may be mentioned antimony pentachloride1 ,sb,cl#e1 which splits up into chlorine and antimony trichloride when heated4 ,cupric chloride1 corresponding with copper oxide1 and having a composition ,cu,cl#b1 similar to ,cu,o1 when heated parts with half its chlorine1 just as barium peroxide evolves half its oxygen4 ,this method may even be taken advantage of for the preparation of chlorine and cuprous chloride1 ,cu,cl4 ,the latter attracts oxygen from the atmosphere1 and in so doing is converted from a colourless substance into a green compound whose composition is ,cu#b;,cl#b,o4 ,with hydrochloric acid this substance gives cupric chloride 7,cu#b;,cl#b,o #b;,h,cl ,h#b,o #b;,cu,cl#b71 which has only to be dried and heated in order again to obtain chlorine4 ,thus1 in solution1 and at the ordinary temperature1 the compound ,cu,cl#b is stable1 but when heated it splits up4 ,on this property is founded ,deacon's process for the preparation of chlorine from hydrochloric acid with the aid of air and copper salts1 by passing a mixture of air and hydrochloric acid at about #ddj over bricks saturated with a solution of a copper salt 7a mixture of solutions of ,cu,s,o#d and ,na#b,,so#d74 ,cu,cl#b is then formed by the double decomposition of the salt of copper and the hydrochloric acid2 the ,cu,cl#b liberates chlorine1 and the ,cu,cl forms ,cu#b;,cl#b,o with the oxygen of the air1 which again gives ,cu,cl#b with #b;,h,cl1 and so on4 ,magnesium chloride1 which is obtained from sea-water1 carnallite1 c41 may serve not only as a means for the preparation of hydrochloric acid1 but also of chlorine1 because its basic salt 7magnesium oxychloride7 when heated in the air gives magnesium oxide and chlorine 7,weldon-,pechiney's process1 #ahhh74 ,chlorine is now prepared on a large scale by this method4 ,several new methods based upon this reaction have been proposed for procuring chlorine from the bye-products of other chemical processes4 ,thus1 ,lyte and ,tattars 7#ahia7 obtained up to #fg p4c4 of chlorine from ,ca,cl#b in this manner4 ,a solution of ,ca,cl#b1 containing a certain amount of common salt1 is evaporated and oxide of magnesium added to it4 ,when the solution attains a density of #abdde 7at #ae71 it is treated with carbonic acid1 which precipitates carbonate of calcium1 while chloride of magnesium remains in solution4 ,after adding ammonium chloride1 the solution is evaporated to dryness and the double chloride of magnesium and ammonium formed is ignited1 which drives off the chloride of ammonium4 ,the chloride of magnesium which remains behind is used in the ,weldon-,pechiney process4 ,the ,de ,wilde-,reychler 7#ahib7 process for the manufacture of chlorine consists in passing alternate currents of hot air and hydrochloric acid gas through a cylinder containing a mixture of the chlorides of magnesium and manganese4 ,a certain amount of sulphate of magnesium which does not participate in any way in the reaction1 is added to the mixture to prevent its fusing4 ,the reactions may be expressed by the following equations3 7#a7 #c,mg,cl#b #c,mn,cl#b #h,o ,mg#c,mn#c,o#h #ab;,cl2 7#b7 ,mg#c,mn#c,o#h #af;,h,cl #c,mg,cl#b #c,mn,cl#b #h;,h#b,o #d;,cl4 ,as nitric acid is able to take up the hydrogen from hydrochloric acid1 a heated mixture of these acids is also employed for the preparation of chlorine4 ,the resultant mixture of chlorine and lower oxides of nitrogen is mixed with air and steam which regenerates the ,,hno#c1 while the chlorine remains as a gas together with nitrogen1 in which form it is quite capable of bleaching1 forming chloride of lime1 c4 ,besides these1 ,solvay and ,mond's methods of preparing chlorine must be mentioned4 ,the first is based upon the reaction ,ca,cl#b ,si,o#b ,o7air7 ,ca,o,si,o#b ,cl#b1 the second on the action of the oxygen of the air 7heated7 upon ,mg,cl#b 7and certain similar chlorides7 ,mg,cl#b ,o ,mg,o ,cl#b ,the remaining ,mg,o is treated with sal-ammoniac to re-form ,mg,cl#b 7,mg,o #b,,nh#d;,cl ,mg,cl#b ,h#b,o #b,,nh#c7 and the resultant ,,nh#c again converted into sal-ammoniac1 so that hydrochloric acid is the only substance consumed4 ,the latter processes have not yet found much application4 #d ,the following proportions are accordingly taken by weight3 #e parts of powdered manganese peroxide1 #aa parts of salt 7best fused1 to prevent its frothing71 and #ad parts of sulphuric acid previously mixed with an equal volume of water4 ,the mixture is heated in a salt bath1 so as to obtain a temperature above #ajj4 ,the corks in the apparatus must be soaked in paraffin 7otherwise they are corroded by the chlorine71 and black india-rubber tubing smeared with vaseline must be used1 and not vulcanised rubber 7which contains sulphur1 and becomes brittle under the action of the chlorine74 ,the reaction which proceeds may be expressed thus3 ,mn,o#b #b,na,cl #b;,h#b,,so#d ,mn,s,o#d ,na#b,,so#d #b;,h#b,o ,cl#b4 ,the method of preparation of ,cl#b from manganese peroxide and hydrochloric acid was discovered by ,scheele1 and from sodium chloride by ,berthollet4 #e ,the reaction of hydrochloric acid upon bleaching powder gives chlorine without the aid of heat1 ,ca,cl#b,o#b #d;,h,cl ,ca,cl#b #b;,h#b,o #b;,cl#b and is therefore also used for the preparation of chlorine4 ,this reaction is very violent if all the acid be added at once2 it should be poured in drop by drop 7,merm1 ,kmmerer74 ,c4 ,winkler proposed to mix bleaching powder with one quarter of burnt and powdered gypsum1 and having damped the mixture with water1 to press and cut it up into cubes and dry at the ordinary temperature4 ,these cubes can be used for the preparation of chlorine in the same apparatus as that used for the evolution of hydrogen and carbonic anhydride--the disengagement of the chlorine proceeds uniformly4 ,a mixture of potassium dichromate and hydrochloric acid evolves chlorine perfectly free from oxygen 7,v4 ,meyer and ,langer74 #f ,illustration3 ,,fig4 #ff4--;,clay retort for the preparation of chlorine on a large scale4 ,chlorine is manufactured on a large scale from manganese peroxide and hydrochloric acid4 ,it is most conveniently prepared in the apparatus shown in fig4 #ff1 which consists of a three-necked earthenware vessel whose central orifice is the largest4 ,a clay or lead funnel1 furnished with a number of orifices1 is placed in the central wide neck of the vessel4 ,roughly-ground lumps of natural manganese peroxide are placed in the funnel1 which is then closed by the cover ,n1 and luted with clay4 ,one orifice is closed by a clay stopper1 and is used for the introduction of the hydrochloric acid and withdrawal of the residues4 ,the chlorine disengaged passes along a leaden gas-conducting tube placed in the other orifice4 ,a row of these vessels is surrounded by a water-bath to ensure their being uniformly heated4 ,manganese chloride is found in the residue4 ,in ,weldon's process lime is added to the acid solution of manganese chloride4 ,a double decomposition takes place1 resulting in the formation of manganous hydroxide and calcium chloride4 ,when the insoluble manganous hydroxide has settled1 a further excess of milk of lime is added 7to make a mixture #b,mn7,,oh7#b ,ca,o x,ca,cl#b1 which is found to be the best proportion1 judging from experiment71 and then air is forced through the mixture4 ,the hydroxide is thus converted from a colourless to a brown substance1 containing peroxide1 ,mn,o#b1 and oxide of manganese1 ,mn#b,o#c4 ,this is due to the manganous oxide absorbing oxygen from the air4 ,under the action of hydrochloric acid this mixture evolves chlorine1 because of all the compounds of chlorine and manganese the chloride ,mn,cl#b is the only one which is stable 7see ,note #c74 ,thus one and the same mass of manganese may be repeatedly used for the preparation of chlorine4 ,the same result is attained in other ways4 ,if manganous oxide be subjected to the action of oxides of nitrogen and air 7,coleman's process71 then manganese nitrate is formed1 which at a red heat gives oxides of nitrogen 7which are again used in the process7 and manganese peroxide1 which is thus renewed for the fresh evolution of chlorine4 ,chlorine is a gas of a yellowish green colour1 and has a very suffocating and characteristic odour4 ,on lowering the temperature to -#ej or increasing the pressure to six atmospheres 7at #j7 chlorine condenses#g into a liquid which has a yellowish-green colour1 a density of #ac1 and boils at -#cd4 ,the density and atomic weight of chlorine is #cee times greater than that of hydrogen1 hence the molecule contains ,cl#bh4 ,at #j one volume of water dissolves about #a-a_/b volume of chlorine1 at #aj about #c volumes1 at #ej again #a-a_/b volume4#i ,such a solution of chlorine is termed 'chlorine water2' and is employed in a diluted form in medicine and as a laboratory reagent4 ,it is prepared by passing chlorine through a series of ,woulfe's bottles or into an inverted retort filled with water4 ,under the action of light1 chlorine water gives oxygen and hydrochloric acid4 ,at #j a saturated solution of chlorine yields a crystallo-hydrate1 ,cl#b1h;,h#b,o1 which easily splits up into chlorine and water when heated1 so that if it be sealed up in a tube and heated to #ce1 two layers of liquid are formed--a lower stratum of chlorine containing a small quantity of water1 and an upper stratum of water containing a small quantity of chlorine4#aj #g ,davy and ,faraday liquefied chlorine in #ahbc by heating the crystallo-hydrate ,cl#bh;,h#b,o in a bent tube 7as with ,,nh#c71 surrounded by warm water1 while the other end of the tube was immersed in a freezing mixture4 ,meselan condensed chlorine in freshly-burnt charcoal 7placed in a glass tube71 which when cold absorbs an equal weight of chlorine4 ,the tube was then fused up1 the bent end cooled1 and the charcoal heated1 by which means the chlorine was expelled from the charcoal1 and the pressure increased4 #h ,judging from ,ludwig's observations 7#ahfh71 and from the fact that the coefficient of expansion of gases increases with their molecular weight 7,chapter ,,ii41 ,note #bf1 for hydrogen #jcfg1 carbonic anhydride #jcgc1 hydrogen bromide #jchf71 it might be expected that the expansion of chlorine would be greater than that of air or of the gases composing it4 ,v4 ,meyer and ,langer 7#ahhe7 having remarked that at #a1djj the density of chlorine 7taking its expansion as equal to that of nitrogen7 #bi1 consider that the molecules of chlorine split up and partially give molecules ,cl1 but it might be maintained that the decrease in density observed only depends on the increase of the coefficient of expansion4 #i ,investigations on the solubility of chlorine in water 7the solutions evolve all their chlorine on boiling and passing air through them7 show many different peculiarities4 ,first ,gay-,lussac1 and subsequently ,pelouze1 determined that the solubility increases between #j and #h-aj 7from #a-a_/b to #b vols4 of chlorine per #ajj vols4 of water at #j up to #c to #b-c_/d at #aj74 ,in the following note we shall see that this is not due to the breaking-up of the hydrate at about #h to #aj1 but to its formation below #i4 ,roscoe observed an increase in the solubility of chlorine in the presence of hydrogen--even in the dark4 ,berthelot determined an increase of solubility with the progress of time4 ,schnbein and others suppose that chlorine acts on water1 forming hypochlorous and hypochloric acids1 7,h,cl,o ,h,cl74 ,the equilibrium between chlorine and steam as gases and between water1 liquid chlorine1 ice1 and the solid crystallo-hydrate of chlorine is evidently very complex4 ,gibbs1 ,guldberg 7#ahgj7 and others gave a theory for similar states of equilibrium1 which was afterwards developed by ,roozeboom 7#ahhg71 but it would be inopportune here to enter into its details4 ,it will be sufficient in the first place to mention that there is now no doubt 7according to the theory of heat1 and the direct observations of ,ramsay and ,young7 that the vapour tensions at one and the same temperature are different for the liquid and solid states of substances2 secondly1 to call attention to the following note2 and1 thirdly1 to state that1 in the presence of the crystallo-hydrate1 water between ,o#bd and #bhg 7when the hydrate and a solution may occur simultaneously7 dissolves a different amount of chlorine than it does in the absence of the crystallo-hydrate4 #aj ,according to ,faraday's data the hydrate of chlorine contains ,cl#b1aj;,h#b,o1 but ,roozeboom 7#ahhe7 showed that it is poorer in water and ,cl#b1h;,h#b,o4 ,at first small1 almost colourless1 crystals are obtained1 but they gradually form 7if the temperature be below their critical point #bhg1 above which they do not exist7 large yellow crystals1 like those of potassium chromate4 ,the specific gravity is #abc4 ,the hydrate is formed if there be more chlorine in a solution than it is able to dissolve under the dissociation pressure corresponding with a given temperature4 ,in the presence of the hydrate the percentage amount of chlorine at #j #je1 at #i #ji1 and at #bj #ahb4 ,at temperatures below #i the solubility 7determined by ,gay-,lussac and ,pelouze1 see ,note #i7 is dependent on the formation of the hydrate2 whilst at higher temperatures under the ordinary pressure the hydrate cannot be formed1 and the solubility of chlorine falls1 as it does for all gases 7,chapter ,i474 ,if the crystallo-hydrate is not formed1 then below #i the solubility follows the same rule 7#f #ajg p4c4 ,cl1 #i #jie p4c474 ,according to ,roozeboom1 the chlorine evolved by the hydrate presents the following tensions of dissociation3 at #j #bdi mm41 at #d #cih1 at #h #fbj1 at #aj #gig1 at #ad #a1djj mm4 ,in this case a portion of the crystallo-hydrate remains solid4 ,at #if the tension of dissociation is equal to the atmospheric pressure4 ,at a higher pressure the crystallo-hydrate may form at temperatures above #i up to #bhg1 when the vapour tension of the hydrate equals the tension of the chlorine4 ,it is evident that the equilibrium which is established is on the one hand a case of a complex heterogeneous system1 and on the other hand a case of the solution of solid and gaseous substances in water4 ,the crystallo-hydrate or chlorine water must be kept in the dark1 or the access of light be prevented by coloured glass1 otherwise oxygen is evolved and hydrochloric acid formed4 ,chlorine explodes with hydrogen1 if a mixture of equal volumes be exposed to the direct action of the sun's rays#aa or brought into contact with spongy platinum1 or a strongly heated substance1 or when subjected to the action of an electric spark4 ,the explosion in this case takes place for exactly the same reasons--i4e4 the evolution of heat and expansion of the resultant product--as in the case of detonating gas 7,chapter ,,iii47 ,diffused light acts in the same way1 but slowly1 whilst direct sunlight causes an explosion4#ab ,the hydrochloric acid gas produced by the reaction of chlorine on hydrogen occupies 7at the original temperature and pressure7 a volume equal to the sum of the original volumes2 that is1 a reaction of substitution here takes place3 ,h#b ,cl#b ,h,cl ,h,cl4 ,in this reaction twenty-two thousand heat units are evolved for one part by weight #a gram of hydrogen4#ac #aa ,the chemical action of light on a mixture of chlorine and hydrogen was discovered by ,gay-,lussac and ,thnard 7#ahji74 ,it has been investigated by many savants1 and especially by ,draper1 ,bunsen1 and ,roscoe4 ,electric or magnesium light1 or the light emitted by the combustion of carbon bisulphide in nitric oxide1 and actinic light in general1 acts in the same manner as sunlight1 in proportion to its intensity4 ,at temperatures below -#ab light no longer brings about reaction1 or at all events does not give an explosion4 ,it was long supposed that chlorine that had been subjected to the action of light was afterwards able to act on hydrogen in the dark1 but it was shown that this only takes place with moist chlorine1 and depends on the formation of oxides of chlorine4 ,the presence of foreign gases1 and even of excess of chlorine or of hydrogen1 very much enfeebles the explosion1 and therefore the experiment is conducted with a detonating mixture prepared by the action of an electric current on a strong solution 7sp4 gr4 #aae7 of hydrochloric acid1 in which case the water is not decomposed--that is1 no oxygen becomes mixed with the chlorine4 #ab ,the quantity of chlorine and hydrogen which combine is proportional to the intensity of the light--not of all the rays1 but only those so-termed chemical 7actinic7 rays which produce chemical action4 ,hence a mixture of chlorine and hydrogen1 when exposed to the action of light in vessels of known capacity and surface1 may be employed as an actinometer--that is1 as a means for estimating the intensity of the chemical rays1 the influence of the heat rays being previously destroyed1 which may be done by passing the rays through water4 ,investigations of this kind 7photo-chemical7 showed that chemical action is chiefly limited to the violet end of the spectrum1 and that even the invisible ultra-violet rays produce this action4 ,a colourless gas flame contains no chemically active rays2 the flame coloured green by a salt of copper evinces more chemical action than the colourless flame1 but the flame brightly coloured yellow by salts of sodium has no more chemical action than that of the colourless flame4 ,as the chemical action of light becomes evident in plants1 photography1 the bleaching of tissues1 and the fading of colours in the sunlight1 and as a means for studying the phenomenon is given in the reaction of chlorine on hydrogen1 this subject has been the most fully investigated in photo-chemistry4 ,the researches of ,bunsen and ,roscoe in the fifties and sixties are the most complete in this respect4 ,their actinometer contains hydrogen and chlorine1 and is surrounded by a solution of chlorine in water4 ,the hydrochloric acid is absorbed as it forms1 and therefore the variation in volume indicates the progress of the combination4 ,as was to be expected1 the action of light proved to be proportional to the time of exposure and intensity of the light1 so that it was possible to conduct detailed photometrical investigations respecting the time of day and season of the year1 various sources of light1 its absorption1 c4 ,this subject is considered in detail in special works1 and we only stop to mention one circumstance1 that a small quantity of a foreign gas decreases the action of light2 for example1 #a_/ccj of hydrogen by #ch p4c41 #a_/bjj of oxygen by #aj p4c41 #a_/ajj of chlorine by #fj p4c41 c4 ,according to the researches of ,klimenko and ,pekatoros 7#ahhi71 the photo-chemical alteration of chlorine water is retarded by the presence of traces of metallic chlorides1 and this influence varies with different metals4 ,as much heat is evolved in the reaction of chlorine on hydrogen1 and as this reaction1 being exothermal1 may proceed by itself1 the action of light is essentially the same as that of heat--that is1 it brings the chlorine and hydrogen into the condition necessary for the reaction--it1 as we may say1 disturbs the original equilibrium2 this is the work done by the luminous energy4 ,it seems to me that the action of light on the mixed gases should be understood in this sense1 as ,pringsheim 7#ahgg7 pointed out4 #ac ,in the formation of steam 7from one part by weight #a gram of hydrogen7 #bi1jjj heat units are evolved4 ,the following are the quantities of heat 7thousands of units7 evolved in the formation of various other corresponding compounds of oxygen and of chlorine 7from ,thomsen's1 and1 for ,na#b,o1 ,beketoff's results73 #b,na,cl1 #aie2 ,ca,cl#b1 #agj2 ,hg,cl#b1 #fc2 #b;,ag,cl1 #ei4 ,na#b,o1 #ajj2 ,ca,o1 #aca2 ,hg,o1 #db2 ,ag#b,o1 #f4 #b;,as,cl#c1 #adc2 #b,p,cl#e1 #baj2 ,c,cl#d1 #ba2 #b;,h,cl1 #dd 7gas74 ,as#b,o#c1 #aee2 ,p#b,o#e1 #cgj2 ,,co#b1 #ig2 ,h#b,o1 #eh 7gas74 ,with the first four elements the formation of the chlorine compound gives the most heat1 and with the four following the formation of the oxygen compound evolves the greater amount of heat4 ,the first four chlorides are true salts formed from ,h,cl and the oxide1 whilst the remainder have other properties1 as is seen from the fact that they are not formed from hydrochloric acid and the oxide1 but give hydrochloric acid with water4 ,these relations show that the affinity of chlorine for hydrogen is very great and analogous to the affinity between hydrogen and oxygen4 ,thus#ad on the one hand by passing a mixture of steam and chlorine through a red-hot tube1 or by exposing water and chlorine to the sunlight1 oxygen is disengaged1 whilst on the other hand1 as we saw above1 oxygen in many cases displaces chlorine from its compound with hydrogen1 and therefore the reaction ,h#b,o ,cl#b #b;,h,cl ,o belongs to the number of reversible reactions1 and hydrogen will distribute itself between oxygen and chlorine4 ,this determines the relation of ,cl to substances containing hydrogen and its reactions in the presence of water1 to which we shall turn our attention after having pointed out the relation of chlorine to other elements4 #ad ,this has been already pointed out in ,chapter ,,iii41 ,note #e4 ,many metals when brought into contact with chlorine immediately combine with it1 and form those metallic chlorides which correspond with hydrogen chloride and with the oxide of the metal taken4 ,this combination may proceed rapidly with the evolution of heat and light2 that is1 metals are able to burn in chlorine4 ,thus1 for example1 sodium#ae burns in chlorine1 synthesising common salt4 ,metals in the form of powders burn without the aid of heat1 and become highly incandescent in the process2 for instance1 antimony1 which is a metal easily converted into a powder4#af ,even such metals as gold and platinum1#ag which do not combine directly with oxygen and give very unstable compounds with it1 unite directly with chlorine to form metallic chlorides4 ,either chlorine water or aqua regia may be employed for this purpose instead of gaseous chlorine4 ,these dissolve gold and platinum1 converting them into metallic chlorides4 ,aqua regia is a mixture of #a part of nitric acid with #b to #c parts of hydrochloric acid4 ,this mixture converts into soluble chlorides not only those metals which are acted on by hydrochloric and nitric acids1 but also gold and platinum1 which are insoluble in either acid separately4 ,this action of aqua regia depends on the fact that nitric acid in acting on hydrochloric acid evolves chlorine4 ,if the chlorine evolved be transferred to a metal1 then a fresh quantity is formed from the remaining acids and also combines with the metal4#ah ,thus the aqua regia acts by virtue of the chlorine which it contains and disengages4 #ae ,sodium remains unaltered in perfectly dry chlorine at the ordinary temperature1 and even when slightly warmed2 but the combination is exceedingly violent at a red heat4 #af ,an instructive experiment on combustion in chlorine may be conducted as follows3 leaves of ,dutch metal 7used instead of gold for gilding7 are placed in a glass globe1 and a gas-conducting tube furnished with a glass cock is placed in the cork closing it1 and the air is pumped out of the globe4 ,the gas-conducting tube is then connected with a vessel containing chlorine1 and the cock opened2 the chlorine rushes in1 and the metallic leaves are consumed4 #ag ,the behaviour of platinum to chlorine at a high temperature 7#a1djj7 is very remarkable1 because platinous chloride1 ,pt,cl#b1 is then formed1 whilst this substance decomposes at a much lower temperature into chlorine and platinum4 ,hence1 when chlorine comes into contact with platinum at such high temperatures1 it forms fumes of platinous chloride1 and they on cooling decompose1 with the liberation of platinum1 so that the phenomenon appears to be dependent on the volatility of platinum4 ,deville proved the formation of platinous chloride by inserting a cold tube inside a red-hot one 7as in the experiment on carbonic oxide74 ,however1 ,v4 ,meyer was able to observe the density of chlorine in a platinum vessel at #a1fij1 at which temperature chlorine does not exert this action on platinum1 or at least only to an insignificant degree4 #ah ,when left exposed to the air aqua regia disengages chlorine1 and afterwards it no longer acts on gold4 ,gay-,lussac1 in explaining the action of aqua regia1 showed that when heated it evolves1 besides chlorine1 the vapours of two chloranhydrides--that of nitric acid1 ,,no#b;,cl 7nitric acid1 ,,no#b,,oh1 in which ,,ho is replaced by chlorine2 see ,chapter on ,phosphorus71 and that of nitrous acid1 ,n,o,cl--but these do not act on gold4 ,the formation of aqua regia may therefore be expressed by #d,,nho#c #h;,h,cl #b,,no#b;,cl #b,n,o,cl #f;,h#b,o #b;,cl#b4 ,the formation of the chlorides ,,no#b;,cl and ,n,o,cl is explained by the fact that the nitric acid is deoxidised1 gives the oxides ,,no and ,,no#b1 and they directly combine with chlorine to form the above anhydrides4 ,the majority of non-metals also react directly on chlorine2 hot sulphur and phosphorus burn in it and combine with it at the ordinary temperature4 ,only nitrogen1 carbon1 and oxygen do not combine directly with it4 ,the chlorine compounds formed by the non-metals--for instance1 phosphorus trichloride1 ,p,cl#c1 and sulphurous chloride1 c41 do not have the properties of salts1 and1 as we shall afterwards see more fully1 correspond to acid anhydrides and acids2 for example1 ,p,cl#c--to phosphorous acid1 ,p7,,oh7#c3 ,na,cl ,fe,cl#b ,sn,cl#d ,p,cl#c ,h,cl ,na7,,ho7 ,fe7,,ho7#b ,sn7,,ho7#d ,p7,,ho7#c ,h7,,ho7 ,as the above-mentioned relation in composition--i4e4 substitution of ,cl by the aqueous residue--exists between many chlorine compounds and their corresponding hydrates1 and as furthermore some 7acid7 hydrates are obtained from chlorine compounds by the action of water1 for instance1 ,p,cl#c #c;,h#bj ,p7,,ho7#c #c;,h,cl ,phosphorus ,water ,phosphorous ,hydrochloric trichloride acid acid whilst other chlorine compounds are formed from hydroxides and hydrochloric acid1 with the liberation of water1 for example1 ,na,h,o ,h,cl ,na,cl ,h#b,o we endeavour to express this intimate connection between the hydrates and chlorine compounds by calling the latter chloranhydrides4 ,in general terms1 if the hydrate be basic1 then1 ,m7,,ho7 ,h,cl ,m,cl ,h#b,o hydrate hydrochloric acid chloranhydride water and if the hydrate ,,roh be acid1 then1 ,r,cl ,h#b,o ,r7,,ho7 ,h,cl ,chloranhydride water hydrate hydrochloric acid ,the chloranhydrides ,m,cl corresponding to the bases are evidently metallic chlorides or salts corresponding to ,h,cl4 ,in this manner a distinct equivalency is marked between the compounds of chlorine and the so-called hydroxyl radicle 7,,ho71 which is also expressed in the analogy existing between chlorine1 ,cl#b1 and hydrogen peroxide1 7,,ho7#b4 ,as regards the chloranhydrides corresponding to acids and non-metals1 they bear but little resemblance to metallic salts4 ,they are nearly all volatile1 and have a powerful suffocating smell which irritates the eyes and respiratory organs4 ,they react on water like many anhydrides of the acids1 with the evolution of heat and liberation of hydrochloric acid1 forming acid hydrates4 ,for this reason they cannot usually be obtained from hydrates--that is1 acids--by the action of hydrochloric acid1 as in that case water would be formed together with them1 and water decomposes them1 converting them into hydrates4 ,there are many intermediate chlorine compounds between true saline metallic chlorides like sodium chloride and true acid chloranhydrides1 just as there are all kinds of transitions between bases and acids4 ,acid chloranhydrides are not only obtained from chlorine and non-metals1 but also from many lower oxides1 by the aid of chlorine4 ,thus1 for example1 ,,co1 ,,no1 ,,no#b1 ,,so#b1 and other lower oxides which are capable of combining with oxygen may also combine with a corresponding quantity of chlorine4 ,thus ,c,o,cl#b1 ,n,o,cl1 ,,no#b;,cl1 ,,so#b;,cl#b1 c41 are obtained4 ,they correspond with the hydrates ,,co7,,oh7#b1 ,,no7,,oh71 ,,no#b7,,oh71 ,,so#b7,,oh7#b1 c41 and to the anhydrides ,,co#b1 ,n#b,o#c1 ,n#b,o#e1 ,,so#c1 c4 ,here we should notice two aspects of the matter3 7#a7 chlorine combines with that with which oxygen is able to combine1 because it is in many respects equally if not more energetic than oxygen and replaces it in the proportion ,cl#b 3 ,o2 7#b7 that highest limit of possible combination which is proper to a given element or grouping of elements is very easily and often attained by combination with chlorine4 ,if phosphorus gives ,p,cl#c and ,p,cl#e1 it is evident that ,p,cl#e is the higher form of combination compared with ,p,cl#c4 ,to the form ,p,cl#e1 or in general ,,px#e1 correspond ,,ph#d;,i1 ,,po7,,oh7#c1 ,p,o,cl#c1 c4 ,if chlorine does not always directly give compounds of the highest possible forms for a given element1 then generally the lower forms combine with it in order to reach or approach the limit4 ,this is particularly clear in hydrocarbons1 where we see the limit ,cn,h#bn#b very distinctly4 ,the unsaturated hydrocarbons are sometimes able to combine with chlorine with the greatest ease and thus reach the limit4 ,thus ethylene1 ,c#b;,h#d1 combines with ,cl#b1 forming the so-called ,dutch liquid or ethylene chloride1 ,c#b;,h#d;,cl#b1 because it then reaches the limit ,cn,x#bn#b4 ,in this and all similar cases the combined chlorine is able by reactions of substitution to give a hydroxide and a whole series of other derivatives4 ,thus a hydroxide called glycol1 ,c#b;,h#d7,,oh7#b1 is obtained from ,c#b;,h#d;,cl#b4 ,chlorine in the presence of water very often acts directly as an oxidising agent4 ,a substance ,a combines with chlorine and gives1 for example1 ,a,cl#b1 and this in turn a hydroxide1 ,a7,,oh7#b1 which on losing water forms ,,ao4 ,here the chlorine has oxidised the substance ,a4 ,this frequently happens in the simultaneous action of water and chlorine3 ,a ,h#b,o ,cl#b #b;,h,cl ,,ao4 ,examples of this oxidising action of chlorine may frequently be observed both in practical chemistry and technical processes4 ,thus1 for instance1 chlorine in the presence of water oxidises sulphur and metallic sulphides4 ,in this case the sulphur is converted into sulphuric acid1 and the chlorine into hydrochloric acid1 or a metallic chloride if a metallic sulphide be taken4 ,a mixture of carbonic oxide and chlorine passed into water gives carbonic anhydride and hydrochloric acid4 ,sulphurous anhydride is oxidised by chlorine in the presence of water into sulphuric acid1 just as it is by the action of nitric acid3 ,,so#b #b;,h#b,o ,cl#b ,h#b,,so#d #b;,h,cl4 ,the oxidising action of chlorine in the presence of water is taken advantage of in practice for the rapid bleaching of tissues and fibres4 ,the colouring matter of the fibres is altered by oxidation and converted into a colourless substance1 but the chlorine afterwards acts on the tissue itself4 ,bleaching by means of chlorine therefore requires a certain amount of technical skill in order that the chlorine should not act on the fibres themselves1 but that its action should be limited to the colouring matter only4 ,the fibre for making writing paper1 for instance1 is bleached in this manner4 ,the bleaching property of chlorine was discovered by ,berthollet1 and forms an important acquisition to the arts1 because it has in the majority of cases replaced that which before was the universal method of bleaching--namely1 exposure to the sun of the fabrics damped with water1 which is still employed for linens1 c4 ,time and great trouble1 and therefore money also1 have been considerably saved by this change4#ai #ai ,ozone and peroxide of hydrogen also bleach tissues4 ,as the action of peroxide of hydrogen is easily controlled by taking a weak solution1 and as it has hardly any action upon the tissues themselves1 it is replacing chlorine more and more as a bleaching agent4 ,the oxidising property of chlorine is apparent in destroying the majority of organic tissues1 and proves fatal to organisms4 ,this action of chlorine is taken advantage of in quarantine stations4 ,but the simple fumigation by chlorine must be carried on with great care in dwelling places1 because chlorine disengaged into the atmosphere renders it harmful to the health4 ,the power of chlorine for combination is intimately connected with its capacity for substitution1 because1 according to the law of substitution1 if chlorine combines with hydrogen1 then it also replaces hydrogen1 and furthermore the combination and substitution are accomplished in the same quantities4 ,therefore the atom of chlorine which combines with the atom of hydrogen is also able to replace the atom of hydrogen4 ,we mention this property of chlorine not only because it illustrates the application of the law of substitution in clear and historically important examples1 but more especially because reactions of this kind explain those indirect methods of the formation of many substances which we have often mentioned and to which recourse is had in many cases in chemistry4 ,thus chlorine does not act on carbon1#bj oxygen1 or nitrogen1 but nevertheless its compounds with these elements may be obtained by the indirect method of the substitution of hydrogen by chlorine4 #bj ,a certain propensity of carbon to attract chlorine is evidenced in the immense absorption of chlorine by charcoal 7,note #g71 but1 so far as is at present known 7if ,i am not mistaken1 no one has tried the aid of light71 no combination takes place between the chlorine and carbon4 ,as chlorine easily combines with hydrogen1 and does not act on carbon1 it decomposes hydrocarbons 7and many of their derivatives7 at a high temperature1 depriving them of their hydrogen and liberating the carbon1 as1 for example1 is clearly seen when a lighted candle is placed in a vessel containing chlorine4 ,the flame becomes smaller1 but continues to burn for a certain time1 a large amount of soot is obtained1 and hydrochloric acid is formed4 ,in this case the gaseous and incandescent substances of the flame are decomposed by the chlorine1 the hydrogen combines with it1 and the carbon is disengaged as soot4#ba ,this action of chlorine on hydrocarbons1 c41 proceeds otherwise at lower temperatures1 as we will now consider4 #ba ,the same reaction takes place under the action of oxygen1 with the difference that it burns the carbon1 which chlorine is not able to do4 ,if chlorine and oxygen compete together at a high temperature1 the oxygen will unite with the carbon1 and the chlorine with the hydrogen4 ,a very important epoch in the history of chemistry was inaugurated by the discovery of ,dumas and ,laurent that chlorine is able to displace and replace hydrogen4 ,this discovery is important from the fact that chlorine proved to be an element which combines with great ease simultaneously with both the hydrogen and the element with which the hydrogen was combined4 ,this clearly proved that there is no opposite polarity between elements forming stable compounds4 ,chlorine does not combine with hydrogen because it has opposite properties1 as ,dumas and ,laurent stated previously1 accounting hydrogen to be electro-positive and chlorine electro-negative2 this is not the reason of their combining together1 for the same chlorine which combines with hydrogen is also able to replace it without altering many of the properties of the resultant substance4 ,this substitution of hydrogen by chlorine is termed metalepsis4 ,the mechanism of this substitution is very constant4 ,if we take a hydrogen compound1 preferably a hydrocarbon1 and if chlorine acts directly on it1 then there is produced on the one hand hydrochloric acid and on the other hand a compound containing chlorine in the place of the hydrogen--so that the chlorine divides itself into two equal portions1 one portion is evolved as hydrochloric acid1 and the other portion takes the place of the hydrogen thus liberated4 ,hence this metalepsis is always accompanied by the formation of hydrochloric acid4#bb ,the scheme of the process is as follows3 ,cn,hm,x ,cl#b ,cn,hm-#a;,cl,x ,h,cl ,hydrocarbon ,free ,product of ,hydrochloric chlorine metalepsis acid ,or1 in general terms-- ,,rh ,cl#b ,r,cl ,h,cl4 #bb ,this division of chlorine into two portions may at the same time be taken as a clear confirmation of the conception of molecules4 ,according to ,avogadro-,gerhardt's law1 the molecule of chlorine 7p4 #caj7 contains two atoms of this substance2 one atom replaces hydrogen1 and the other combines with it4 ,the conditions under which metalepsis takes place are also very constant4 ,in the dark chlorine does not usually act on hydrogen compounds1 but the action commences under the influence of light4 ,the direct action of the sun's rays is particularly propitious to metalepsis4 ,it is also remarkable that the presence of traces of certain substances1#bc especially of iodine1 aluminium chloride1 antimony chloride1 c41 promotes the action4 ,a trace of iodine added to the substance subjected to metalepsis often produces the same effect as sunlight4#bd #bc ,such carriers or media for the transference of chlorine and the halogens in general were long known to exist in iodine and antimonious chloride1 and have been most fully studied by ,gustavson and ,friedel1 of the ,petroffsky ,academy--the former with respect to aluminium bromide1 and the latter with respect to aluminium chloride4 ,gustavson showed that if a trace of metallic aluminium be dissolved in bromine 7it floats on bromine1 and when combination takes place much heat and light are evolved71 the latter becomes endowed with the property of entering into metalepsis1 which it is not able to do of its own accord4 ,when pure1 for instance1 it acts very slowly on benzene1 ,c#f;,h#f1 but in the presence of a trace of aluminium bromide the reaction proceeds violently and easily1 so that each drop of the hydrocarbon gives a mass of hydrobromic acid1 and of the product of metalepsis4 ,gustavson showed that the modus operandi of this instructive reaction is based on the property of aluminium bromide to enter into combination with hydrocarbons and their derivatives4 ,the details of this and all researches concerning the metalepsis of the hydrocarbons must be looked for in works on organic chemistry4 #bd ,as small admixtures of iodine1 aluminium bromide1 c41 aid the metalepsis of large quantities of a substance1 just as nitric oxide aids the reaction of sulphurous anhydride on oxygen and water1 so the principle is essentially the same in both cases4 ,effects of this kind 7which should also be explained by a chemical reaction proceeding at the surfaces7 only differ from true contact phenomena in that the latter are produced by solid bodies and are accomplished at their surfaces1 whilst in the former all is in solution4 ,probably the action of iodine is founded on the formation of iodine chloride1 which reacts more easily than chlorine4 ,if marsh gas be mixed with chlorine and the mixture ignited1 then the hydrogen is entirely taken up from the marsh gas and hydrochloric acid and carbon formed1 but there is no metalepsis4#be ,but if a mixture of equal volumes of chlorine and marsh gas be exposed to the action of diffused light1 then the greenish yellow mixture gradually becomes colourless1 and hydrochloric acid and the first product of metalepsis--namely1 methyl chloride--are formed3 ,,ch#d ,cl#b ,,ch#c;,cl ,h,cl ,marsh gas ,chlorine ,methyl chloride ,hydrochloric acid #be ,metalepsis belongs to the number of delicate reactions--if it may be so expressed--as compared with the energetic reaction of combustion4 ,many cases of substitution are of this kind4 ,reactions of metalepsis are accompanied by an evolution of heat1 but in a less quantity than that evolved in the formation of the resulting quantity of the halogen acids4 ,thus the reaction ,c#b;,h#f ,cl#b ,c#b;,h#e;,cl ,h,cl1 according to the data given by ,thomsen1 evolves about #bj1jjj heat units1 whilst the formation of hydrochloric acid evolves #bb1jjj units4 ,the volume of the mixture remains unaltered4 ,the methyl chloride which is formed is a gas4 ,if it be separated from the hydrochloric acid 7it is soluble in acetic acid1 in which hydrochloric acid is but sparingly soluble7 and be again mixed with chlorine1 then it may be subjected to a further metalepsical substitution--the second atom of hydrogen may be substituted by chlorine1 and a liquid substance1 ,,ch#b;,cl#b1 called methylene chloride1 will be obtained4 ,in the same manner the substitution may be carried on still further1 and ,c,h,cl#c1 or chloroform1 and lastly carbon tetrachloride1 ,c,cl#d1 will be produced4 ,of these substances the best known is chloroform1 owing to its being formed from many organic substances 7by the action of bleaching powder7 and to its being used in medicine as an ansthetic2 chloroform boils at #fb and carbon tetrachloride at #gh4 ,they are both colourless odoriferous liquids1 heavier than water4 ,the progressive substitution of hydrogen by chlorine is thus evident1 and it can be clearly seen that the double decompositions are accomplished between molecular quantities of the substance--that is1 between equal volumes in a gaseous state4 ,carbon tetrachloride1 which is obtained by the metalepsis of marsh gas1 cannot be obtained directly from chlorine and carbon1 but it may be obtained from certain compounds of carbon--for instance1 from carbon bisulphide--if its vapour mixed with chlorine be passed through a red-hot tube4 ,both the sulphur and carbon then combine with the chlorine4 ,it is evident that by ultimate metalepsis a corresponding carbon chloride may be obtained from any hydrocarbon--indeed1 the number of chlorides of carbon ,cn,cl#bm already known is very large4 ,as a rule1 the fundamental chemical characters of hydrocarbons are not changed by metalepsis2 that is1 if a neutral substance be taken1 then the product of metalepsis is also a neutral substance1 or if an acid be taken the product of metalepsis also has acid properties4 ,even the crystalline form not unfrequently remains unaltered after metalepsis4 ,the metalepsis of acetic acid1 ,,ch#c;,,cooh1 is historically the most important4 ,it contains three of the atoms of the hydrogen of marsh gas1 the fourth being replaced by carboxyl1 and therefore by the action of chlorine it gives three products of metalepsis 7according to the amount of the chlorine and conditions under which the reaction takes place71 mono-1 di-1 and tri-chloracetic acids--,,ch#b;,cl,,cooh1 ,c,h,cl#b;,,cooh1 and ,c,cl#c;,,cooh2 they are all1 like acetic acid1 monobasic4 ,the resulting products of metalepsis1 in containing an element which so easily acts on metals as chlorine1 possess the possibility of attaining a further complexity of molecules of which the original hydrocarbon is often in no way capable4 ,thus on treating with an alkali 7or first with a salt and then with an alkali1 or with a basic oxide and water1 c47 the chlorine forms a salt with its metal1 and the hydroxyl radicle takes the place of the chlorine--for example1 ,,ch#c,,oh is obtained from ,,ch#c;,cl4 ,by the action of metallic derivatives of hydrocarbons--for example1 ,,ch#c,na--the chlorine also gives a salt1 and the hydrocarbon radicle--for instance1 ,,ch#c--takes the place of the chlorine4 ,in this1 or in a similar manner1 ,,ch#c;,,ch#c1 or ,c#b;,h#f is obtained from ,,ch#c;,cl and ,c#f;,h#e;,,ch#c from ,c#f;,h#f4 ,the products of metalepsis also often react on ammonia1 forming hydrochloric acid 7and thence ,,nh#d;,cl7 and an amide2 that is1 the product of metalepsis1 with the ammonia radicle ,,nh#b1 c4 in the place of chlorine4 ,thus by means of metalepsical substitution methods were found in chemistry for an artificial and general means of the formation of complex carbon compounds from more simple compounds which are often totally incapable of direct reaction4 ,besides which1 this key opened the doors of that secret edifice of complex organic compounds into which man had up to then feared to enter1 supposing the hydrocarbon elements to be united only under the influence of those mystic forces acting in organisms4#bf #bf ,with the predominance of the representation of compound radicles 7this doctrine dates from ,lavoisier and ,gay-,lussac7 in organic chemistry1 it was a very important moment in its history when it became possible to gain an insight into the structure of the radicles themselves4 ,it was clear1 for instance1 that ethyl1 ,c#b;,h#e1 or the radicle of common alcohol1 ,c#b;,h#e,,oh1 passes1 without changing1 into a number of ethyl derivatives1 but its relation to the still simpler hydrocarbons was not clear1 and occupied the attention of science in the 'forties' and 'fifties4' ,having obtained ethyl hydride1 ,c#b;,h#e;,h ,c#b;,h#f1 it was looked on as containing the same ethyl1 just as methyl hydride1 ,,ch#d ,,ch#c;,h1 was considered as existing in methane4 ,having obtained free methyl1 ,,ch#c;,,ch#c ,c#b;,h#f1 from it1 it was considered as a derivative of methyl alcohol1 ,,ch#c,,oh1 and as only isomeric with ethyl hydride4 ,by means of the products of metalepsis it was proved that this is not a case of isomerism but of strict identity1 and it therefore became clear that ethyl is methylated methyl1 ,c#b;,h#e ,,ch#b;,,ch#c4 ,in its time a still greater impetus was given by the study of the reactions of monochloracetic acid1 ,,ch#b;,cl,,cooh1 or ,,co7,,ch#b;,cl77,,oh74 ,it appeared that metalepsical chlorine1 like the chlorine of chloranhydrides--for instance1 of methyl chloride1 ,,ch#c;,cl1 or ethyl chloride1 ,c#b;,h#e;,cl--is capable of substitution2 for example1 glycollic acid1 ,,ch#b7,,oh77,,co#b;,h71 or ,,co7,,ch#b,,oh77,,oh71 was obtained from it1 and it appeared that the ,,oh in the group ,,ch#b7,,oh7 reacted like that in alcohols1 and it became clear1 therefore1 that it was necessary to examine the radicles themselves by analysing them from the point of view of the bonds connecting the constituent atoms4 ,whence arose the present doctrine of the structure of the carbon compounds4 7,see ,chapter ,,viii41 ,note #db47 ,it is not only hydrocarbons which are subject to metalepsis4 ,certain other hydrogen compounds1 under the action of chlorine1 also give corresponding chlorine derivatives in exactly the same manner2 for instance1 ammonia1 caustic potash1 caustic lime1 and a whole series of alkaline substances4#bg ,in fact1 just as the hydrogen in marsh gas can be replaced by chlorine and form methyl chloride1 so the hydrogen in caustic potash1 ,,kho1 ammonia1 ,,nh#c1 and calcium hydroxide1 ,ca,h#b,o#b or ,ca7,,oh7#b1 may be replaced by chlorine and give potassium hypochlorite1 ,k,cl,o1 calcium hypochlorite1 ,ca,cl#b,o#b1 and the so-called chloride of nitrogen1 ,n,cl#c4 ,for not only is the correlation in composition the same as in the substitution in marsh gas1 but the whole mechanism of the reaction is the same4 ,here also two atoms of chlorine act3 one takes the place of the hydrogen whilst the other is evolved as hydrochloric acid1 only in the former case the hydrochloric acid evolved remained free1 and in the latter1 in presence of alkaline substances1 it reacts on them4 ,thus1 in the action of chlorine on caustic potash1 the hydrochloric acid formed acts on another quantity of caustic potash and gives potassium chloride and water1 and therefore not only ,,kho ,cl#b ,h,cl ,k,cl,o1 but also ,,kho ,h,cl ,h#b,o ,k,cl1 and the result of both simultaneous phases will be #b,,kho ,cl#b ,h#b,o ,k,cl ,k,cl,o4 ,we will here discuss certain special cases4 #bg ,by including many instances of the action of chlorine under metalepsis we not only explain the indirect formation of ,c,cl#d1 ,n,cl#c1 and ,cl#b,o by one method1 but we also arrive at the fact that the reactions of the metalepsis of the hydrocarbons lose that exclusiveness which was often ascribed to them4 ,also by subjecting the chemical representations to the law of substitution we may foretell metalepsis as a particular case of a general law4 ,the action of chlorine on ammonia may either result in the entire breaking up of the ammonia1 with the evolution of gaseous nitrogen1 or in a product of metalepsis 7as with ,,ch#d74 ,with an excess of chlorine and the aid of heat the ammonia is decomposed1 with the disengagement of free nitrogen4#bh ,this reaction evidently results in the formation of sal-ammoniac1 #h,,nh#c #c;,cl#b #f,,nh#d;,cl ,n#b4 ,but if the ammonium salt be in excess1 then the reaction takes the direction of the replacement of the hydrogen in the ammonia by chlorine4 ,the principal result is that ,,nh#c #c;,cl#b forms ,n,cl#c #c;,h,cl4#bi ,the resulting product of metalepsis1 or chloride of nitrogen1 ,n,cl#c1 discovered by ,dulong1 is a liquid having the property of decomposing with excessive ease not only when heated1 but even under the action of mechanical influences1 as by a blow or by contact with certain solid substances4 ,the explosion which accompanies the decomposition is due to the fact that the liquid chloride of nitrogen gives gaseous products1 nitrogen and chlorine4#bi bis #bh ,this may be taken advantage of in the preparation of nitrogen4 ,if a large excess of chlorine water be poured into a beaker1 and a small quantity of a solution of ammonia be added1 then1 after shaking1 nitrogen is evolved4 ,if chlorine act on a dilute solution of ammonia1 the volume of nitrogen does not correspond with the volume of the chlorine taken1 because ammonium hypochlorite is formed4 ,if ammonia gas be passed through a fine orifice into a vessel containing chlorine1 the reaction of the formation of nitrogen is accompanied by the emission of light and the appearance of a cloud of sal-ammoniac4 ,in all these instances an excess of chlorine must be present4 #bi ,the hydrochloric acid formed combines with ammonia1 and therefore the final result is #d,,nh#c #c;,cl#b ,n,cl#c #c,,nh#d;,cl4 ,for this reason1 more ammonia must enter into the reaction1 but the metalepsical reaction in reality only takes place with an excess of ammonia or its salt4 ,if bubbles of chlorine be passed through a fine tube into a vessel containing ammonia gas1 each bubble gives rise to an explosion4 ,if1 however1 chlorine be passed into a solution of ammonia1 the reaction at first brings about the formation of nitrogen1 because chloride of nitrogen acts on ammonia like chlorine4 ,but when sal-ammoniac has begun to form1 then the reaction directs itself towards the formation of chloride of nitrogen4 ,the first action of chlorine on a solution of sal-ammoniac always causes the formation of chloride of nitrogen1 which then reacts on ammonia thus3 ,n,cl#c #d,,nh#c ,n#b #c,,nh#d;,cl4 ,therefore1 so long as the liquid is alkaline from the presence of ammonia the chief product will be nitrogen4 ,the reaction ,,nh#d;,cl #c;,cl#b ,n,cl#c #d;,h,cl is reversible2 with a dilute solution it proceeds in the above-described direction 7perhaps owing to the affinity of the hydrochloric acid for the excess of water71 but with a strong solution of hydrochloric acid it takes the opposite direction 7probably by virtue of the affinity of hydrochloric acid for ammonia74 ,therefore there must exist a very interesting case of equilibrium between ammonia1 hydrochloric acid1 chlorine1 water1 and chloride of nitrogen which has not yet been investigated4 ,the reaction ,n,cl#c #d;,h,cl ,,nh#d;,cl #c;,cl#b enabled ,deville and ,hautefeuille to determine the composition of chloride of nitrogen4 ,when slowly decomposed by water1 chloride of nitrogen gives1 like a chloranhydride1 nitrous acid or its anhydride1 #b,n,cl#c #c;,h#b,o ,n#b,o#c #f;,h,cl4 ,from these observations it is evident that chloride of nitrogen presents great chemical interest1 which is strengthened by its analogy with trichloride of phosphorus4 ,the researches of ,f4 ,f4 ,selivanoff 7#ahia-id7 prove that ,n,cl#c may be regarded as an ammonium derivative of hypochlorous acid4 ,chloride of nitrogen is decomposed by dilute sulphuric acid in the following manner3 ,n,cl#c #c;,h#b,o ,h#b,,so#d ,,nh#d;,,hso#d #c;,h,cl,o4 ,this reaction is reversible and is only complete when some substance1 combining with ,h,cl,o 7for instance1 succinimide7 or decomposing it1 is added to the liquid4 ,this is easily understood from the fact that hypochlorous acid itself1 ,h,cl,o1 may1 according to the view held in this book1 be regarded as the product of the metalepsis of water1 and consequently bears the same relation to ,n,cl#c as ,h#b,o does to ,,nh#c1 or as ,,rho to ,,rnh#b1 ,r#b,,nh1 and ,r#c,n--that is to say1 ,n,cl#c corresponds as an ammonium derivative to ,cl,o,h and ,cl#b in exactly the same manner as ,,nr#c corresponds to ,,roh and ,r#b4 ,the connection of ,n,cl#c and other similar explosive chloro-nitrogen compounds 7called chloryl compounds by ,selivanoff2 for example1 the ,c#b;,h#e,n,cl#b of ,wurtz is chloryl ethylamine71 such as ,n,r,cl#b 7as ,,nc#b;,h#e;,cl#b71 and ,,nr#b;,cl 7for instance1 ,n7,,ch#c;,,co7,h,cl1 chlorylacetamide1 and ,n7,c#b;,h#e7b;,cl1 chloryl diethylamine7 with ,h,cl,o is evident from the fact that under certain circumstances these compounds give hypochlorous acid1 with water1 for instance1 ,,nr#b;,cl ,h#b,o ,,nr#b;,h ,h,cl,o1 and frequently act 7like ,n,cl#c and ,h,cl,o1 or ,cl#b7 in an oxidising and chloridising manner4 ,we may take chloryl succinimide1 ,c#b;,h#d7;,,co7#b,n,cl for example4 ,it was obtained by ,bender by the action of ,h,cl,o upon succinimide1 ,c#b;,h#d7;,,co7#b,,nh1 and is decomposed by water with the re-formation of amide and ,h,cl,o 7the reaction is reversible74 ,selivanoff obtained1 investigated1 and classified many of the compounds ,,nr#b;,cl and ,n,r,cl#b1 where ,r is a residue of organic acids or alcohols1 and showed their distinction from the chloranhydrides1 and thus supplemented the history of chloride of nitrogen1 which is the simplest of the amides containing chlorine1 ,,nr#c1 where ,r is fully substituted by chlorine4 #bi bis ,in preparing ,n,cl#c every precaution must he used to guard against an explosion1 and care should he taken that the ,n,cl#c remains under a layer of water4 ,whenever an ammoniacal substance comes into contact with chlorine great care must be taken1 because it may be a case of the formation of such products and a very dangerous explosion may ensue4 ,the liquid product of the metalepsis of ammonia may be most safely prepared in the form of small drops by the action of a galvanic current on a slightly warm solution of sal-ammoniac2 chlorine is then evolved at the positive pole1 and this chlorine acting on the ammonia gradually forms the product of metalepsis which floats on the surface of the liquid 7being carried up by the gas71 and if a layer of turpentine be poured on to it these small drops1 on coming into contact with the turpentine1 give feeble explosions1 which are in no way dangerous owing to the small mass of the substance formed4 ,drops of chloride of nitrogen may with great caution be collected for investigation in the following manner4 ,the neck of a funnel is immersed in a basin containing mercury1 and first a saturated solution of common salt is poured into the funnel1 and above it a solution of sal-ammoniac in #i parts of water4 ,chlorine is then slowly passed through the solutions1 when drops of chloride of nitrogen fall into the salt water4 ,chloride of nitrogen is a yellow oily liquid of sp4 gr4 #afe1 which boils at #ga1 and breaks up into ,n ,cl#c at #ig4 ,the contact of phosphorus1 turpentine1 india-rubber1 c4 causes an explosion1 which is sometimes so violent that a small drop will pierce through a thick board4 ,the great ease with which chloride of nitrogen decomposes is dependent upon the fact that it is formed with an absorption of heat1 which it evolves when decomposed1 to the amount of about #ch1jjj heat units for ,n,cl#c1 as ,deville and ,hautefeuille determined4 ,chlorine1 when absorbed by a solution of caustic soda 7and also of other alkalis7 at the ordinary temperature1 causes the replacement of the hydrogen in the caustic soda by the chlorine1 with the formation of sodium chloride by the hydrochloric acid1 so that the reaction may be represented in two phases1 as described above4 ,in this manner1 sodium hypochlorite1 ,na,cl,o1 and sodium chloride are simultaneously formed3 #b,na,h,o ,cl#b ,na,cl ,na,cl,o ,h#b,o4 ,the resultant solution contains ,na,cl,o and is termed 'eau de ,javelle4' ,an exactly similar reaction takes place when chlorine is passed over dry hydrate of lime at the ordinary temperature3 #b;,ca7,,ho7#b #b;,cl#b ,ca,cl#b,o#b ,ca,cl#b #b;,h#b,o4 ,a mixture of the product of metalepsis with calcium chloride is obtained4 ,this mixture is employed in practice on a large scale1 and is termed 'bleaching powder1' owing to its acting1 especially when mixed with acids1 as a bleaching agent on tissues1 so that it resembles chlorine in this respect4 ,it is however preferable to chlorine1 because the destructive action of the chlorine can be moderated in this case1 and because it is much more convenient to deal with a solid substance than with gaseous chlorine4 ,bleaching powder is also called chloride of lime1 because it is obtained from chlorine and hydrate of lime1 and contains#cj both these substances4 ,it may be prepared in the laboratory by passing a current of chlorine through a cold mixture of water and lime 7milk of lime74 ,the mixture must be kept cold1 as otherwise #c;,ca7,cl,o7#b passes into #b;,ca,cl#b ,ca7,cl,o#c7b4 ,in the manufacture of bleaching powder in large quantities at chemical works1 the purest possible slaked lime is taken and laid in a thin layer in large flat chambers1 ,m 7whose walls are made of ,yorkshire flags or tarred wood1 on which chlorine has no action71 and into which chlorine gas is introduced by lead tubes4 ,the distribution of the plant is shown in the annexed drawing 7fig4 #fg74 ,illustration3 ,,fig4 #fg4--;,apparatus for the manufacture of bleaching powder 7on a small scale7 by the action of chlorine1 which is generated in the vessels ,c1 on lime1 which is charged into ,m4 #cj ,quicklime1 ,ca,o 7or calcium carbonate1 ,ca,c,o#c71 does not absorb chlorine when cold1 but at a red heat1 in a current of chlorine1 it forms calcium chloride1 with the evolution of oxygen4 7,this was confirmed in #ahic by ,wells1 at ,oxford47 ,this reaction corresponds with the decomposing action of chlorine on methane1 ammonia1 and water4 ,slaked lime 7calcium hydroxide1 ,ca,h#b,o#b7 also1 when dry1 does not absorb chlorine at #ajj4 ,the absorption proceeds at the ordinary temperature 7below #dj74 ,the dry mass thus obtained contains not less than three equivalents of calcium hydroxide to four equivalents of chlorine1 so that its composition is ,ca7,,ho7#be;,cl#d4 ,in all probability a simple absorption of chlorine by the lime at first takes place in this case1 as may be seen from the fact that even carbonic anhydride1 when acting on the dry mass obtained as above1 disengages all the chlorine from it1 leaving only calcium carbonate4 ,but if the bleaching powder be obtained by a wet method1 or if it be dissolved in water 7in which it is very soluble71 and carbonic anhydride be passed into it1 then chlorine is no longer disengaged1 but chlorine oxide1 ,cl#b,o1 and only half of the chlorine is converted into this oxide1 while the other half remains in the liquid as calcium chloride4 ,from this it may be inferred that calcium chloride is formed by the action of water on bleaching powder1 and this is proved to be the case by the fact that small quantities of water extract a considerable amount of calcium chloride from bleaching powder4 ,if a large quantity of water act on bleaching powder an excess of calcium hydroxide remains1 a portion of which is not subjected to change4 ,the action of the water may be expressed by the following formul3 ,from the dry mass ,ca#c7;,,ho7#f;,cl#d there is formed lime1 ,ca7,,ho7#b1 calcium chloride1 ,ca,cl#b1 and a saline substance1 ,ca7,cl,o7#b4 ,ca#c;,h#f,o#f;,cl#d ,ca,h#b,o#b ,ca,cl#b,o#b ,ca,cl#b #b;,h#b,o4 ,the resulting substances are not equally soluble2 water first extracts the calcium chloride1 which is the most soluble1 then the compound ,ca7,cl,o7#b and ultimately calcium hydroxide is left4 ,a mixture of calcium chloride and hypochlorite passes into solution4 ,on evaporation there remains ,ca#b,o#b;,cl#dc;,h#b,o4 ,the dry bleaching powder does not absorb more chlorine1 but the solution is able to absorb it in considerable quantity4 ,if the liquid be boiled1 a considerable amount of chlorine monoxide is evolved4 ,after this calcium chloride alone remains in solution1 and the decomposition may be expressed as follows3 ,ca,cl#b ,ca,cl#b,o#b #b;,cl#b #b;,ca,cl#b #b;,cl#b,o4 ,chlorine monoxide may be prepared in this manner4 ,it is sometimes said that bleaching powder contains a substance1 ,ca7,,oh7#b;,cl#b1 that is calcium peroxide1 ,ca,o#b1 in which one atom of oxygen is replaced by 7,,oh7#b1 and the other by ,cl#b2 but1 judging from what has been said above1 this can only be the case in the dry state1 and not in solutions4 ,on being kept for some time1 bleaching powder sometimes decomposes1 with the evolution of oxygen 7because ,ca,cl#b,o#b ,ca,cl#b ,o#b1 see p4 #afc72 the same takes place when it is heated4 ,the products of the metalepsis of alkaline hydrates1 ,na,cl,o and ,ca7,cl,o7#b1 which are present in solutions of ',javelle salt' and bleaching powder 7they are not obtained free from metallic chlorides71 must be counted as salts1 because their metals are capable of substitution4 ,but the hydrate ,h,cl,o corresponding with these salts1 or hypochlorous acid1 is not obtained in a free or pure state1 for two reasons3 in the first place1 because this hydrate1 as a very feeble acid1 splits up 7like ,h#b;,,co#c or ,,hno#c7 into water and the anhydride1 or chlorine monoxide1 ,cl#b,o #b;,h,cl,o-,h#b,o2 and1 in the second place1 because1 in a number of instances1 it evolves oxygen with great facility1 forming hydrochloric acid3 ,h,cl,o ,h,cl ,o4 ,both hypochlorous acid and chlorine monoxide may be regarded as products of the metalepsis of water1 because ,,hoh corresponds with ,cl,o,h and ,cl,o,cl4 ,hence in many instances bleaching salts 7a mixture of hypochlorites and chlorides7 break up1 with the evolution of 7#a7 chlorine1 under the action of an excess of a powerful acid capable of evolving hydrochloric acid from sodium or calcium chlorides1 and this takes place most simply under the action of hydrochloric acid itself1 because 7p4 #dfb7 ,na,cl ,na,cl,o #c;,h,cl #b,na,cl ,h,cl ,cl#b ,h#b,o2 7#b7 oxygen1 as we saw in ,chapter ,,iii4--,the bleaching properties and1 in general1 oxidising action of bleaching salts is based on this evolution of oxygen 7or chlorine72 oxygen is also disengaged on heating the dry salts--for instance1 ,na,cl ,na,cl,o #b,na,cl ,o2 7#c7 and1 lastly1 chlorine monoxide1 which contains both chlorine and oxygen4 ,thus1 if a little sulphuric1 nitric1 or similar acid 7not enough to liberate hydrochloric acid from the ,ca,cl#b7 be added to a solution of a bleaching salt 7which has an alkaline reaction1 owing either to an excess of alkali or to the feeble acid properties of ,h,cl,o71 then the hypochlorous acid set free gives water and chlorine monoxide4 ,if carbonic anhydride 7or boracic or a similar very feeble acid7 act on the solution of a bleaching salt1 then hydrochloric acid is not evolved from the sodium or calcium chlorides1 but the hypochlorous acid is displaced and gives chlorine monoxide1#ca because hypochlorous acid is one of the most feeble acids4 ,another method for the preparation of chlorine monoxide is based on these feeble acid properties of hypochlorous acid4 ,zinc oxide and mercury oxide1 under the action of chlorine in the presence of water1 do not give a salt of hypochlorous acid1 but form a chloride and hypochlorous acid1 which fact shows the incapacity of this acid to combine with the bases mentioned4 ,therefore1 if such oxides as those of zinc or mercury be shaken up in water1 and chlorine be passed through the turbid liquid1#cb a reaction occurs which may be expressed in the following manner3 #b;,hg,o #b;,cl#b ,hg#b,o,cl#b ,cl#b,o4 ,in this case1 a compound of mercury oxide with mercury chloride1 or the so-called mercury oxychloride1 is obtained3 ,hg#b,o,cl#b ,hg,o ,hg,cl#b4 ,this is insoluble in water1 and is not affected by hypochlorous anhydride1 so that the solution will contain hypochlorous acid only1 but the greater part of it splits up into the anhydride and water4#cb bis #ca ,for this reason it is necessary that in the preparation of bleaching powder the chlorine should be free from hydrochloric acid1 and even the lime from calcium chloride4 ,an excess of chlorine1 in acting on a solution of bleaching powder1 may also give chlorine monoxide1 because calcium carbonate also gives chlorine monoxide under the action of chlorine4 ,this reaction may be brought about by treating freshly precipitated calcium carbonate with a stream of chlorine in water3 #b;,cl#b ,ca,c,o#c ,,co#b ,ca,cl#b ,cl#b,o4 ,from this we may conclude that1 although carbonic anhydride displaces hypochlorous anhydride1 it may be itself displaced by an excess of the latter4 #cb ,dry red mercury oxide acts on chlorine1 forming dry hypochlorous anhydride 7chlorine monoxide7 7,balard72 when mixed with water1 red mercury oxide acts feebly on chlorine1 and when freshly precipitated it evolves oxygen and chlorine4 ,an oxide of mercury which easily and abundantly evolves chlorine monoxide under the action of chlorine in the presence of water may be prepared as follows3 the oxide of mercury1 precipitated from a mercuric salt by an alkali1 is heated to #cjj and cooled 7,pelouze74 ,if a salt1 ,m,cl,o1 be added to a solution of mercuric salt1 ,hg,x#b1 mercuric oxide is liberated1 because the hypochlorite is decomposed4 #cb bis ,a solution of hypochlorous anhydride is also obtained by the action of chlorine on many salts2 for example1 in the action of chlorine on a solution of sodium sulphate the following reaction takes place3 ,na#b,,so#d ,h#b,o ,cl#b ,na,cl ,h,cl,o ,na,h,s,o#d4 ,here the hypochlorous acid is formed1 together with ,h,cl1 at the expense of chlorine and water1 for ,cl#b ,h#b,o ,h,cl ,h,cl,o4 ,if the crystallo-hydrate of chlorine be mixed with mercury oxide1 the hydrochloric acid formed in the reaction gives mercury chloride1 and hypochlorous acid remains in solution4 ,a dilute solution of hypochlorous acid or chlorine monoxide may be concentrated by distillation1 and if a substance which takes up water 7without destroying the acid7--for instance1 calcium nitrate--be added to the stronger solution1 then the anhydride of hypochlorous acid--i4e4 chlorine monoxide--is disengaged4 ,chlorine monoxide1 which corresponds to bleaching and hypochlorous salts1 containing as it does the two elements oxygen and chlorine1 forms a characteristic example of a compound of elements which1 in the majority of cases1 act chemically in an analogous manner4 ,chlorine monoxide1 as prepared from an aqueous solution by the abstraction of water or by the action of dry chlorine on cold mercury oxide1 is1 at the ordinary temperature1 a gas or vapour which condenses into a red liquid boiling at #bj and giving a vapour whose density 7#dc referred to hydrogen7 shows that #b vols4 of chlorine and #a vol4 of oxygen give #b vols4 of chlorine monoxide4 ,in an anhydrous form the gas or liquid easily explodes1 splitting up into chlorine and oxygen4 ,this explosiveness is determined by the fact that heat is evolved in the decomposition to the amount of about #ae1jjj heat units for ,cl#b,o4#cc ,the explosion may even take place spontaneously1 and also in the presence of many oxidisable substances 7for instance1 sulphur1 organic compounds1 c471 but the solution1 although unstable and showing a strong oxidising tendency1 does not explode4#cd ,it is evident that the presence of hypochlorous acid1 ,h,cl,o1 may be assumed in an aqueous solution of ,cl#b,o1 since ,cl#b,o ,h#b,o #b;,h,cl,o4 #cc ,all explosive substances are of this kind--ozone1 hydrogen peroxide1 chloride of nitrogen1 nitro-compounds1 c4 ,hence they cannot be formed directly from the elements or their simplest compounds1 but1 on the contrary1 decompose into them4 ,in a liquid state chlorine monoxide explodes even on contact with powdery substances1 or when rapidly agitated--for instance1 if a file be rasped over the vessel in which it is contained4 #cd ,a solution of chlorine monoxide1 or hypochlorous acid1 does not explode1 owing to the presence of the mass of water4 ,in dissolving1 chlorine monoxide evolves about #i1jjj heat units1 so that its store of heat becomes less4 ,the capacity of hypochlorous acid 7studied by ,carius and others7 for entering into combination with the unsaturated hydrocarbons is very often taken advantage of in organic chemistry4 ,thus its solution absorbs ethylene1 forming the chlorhydrin ,c#b;,h#d;,cl,o,h4 ,the oxidising action of hypochlorous acid and its salts is not only applied to bleaching but also to many reactions of oxidation4 ,thus it converts the lower oxides of manganese into the peroxide4 ,hypochlorous acid1 its salts1 and chlorine monoxide serve as a transition between hydrochloric acid1 chlorides1 and chlorine1 and a whole series of compounds containing the same elements combined with a still greater quantity of oxygen4 ,the higher oxides of chlorine1 as their origin indicates1 are closely connected with hypochlorous acid and its salts3 ,cl#b1 ,na,cl1 ,h,cl1 hydrochloric acid4 ,cl#b,o1 ,na,cl,o1 ,h,cl,o1 hypochlorous acid4 ,cl#b,o#c1 ,na,cl,o#b1 ,h,cl,o#b1 chlorous acid4#ce ,cl#b,o#e1 ,na,cl,o#c1 ,h,cl,o#c1 chloric acid4 ,cl#b,o#g1 ,na,cl,o#d1 ,h,cl,o#d1 perchloric acid4 ,when heated1 solutions of hypochlorites undergo a remarkable change4 ,themselves so unstable1 they1 without any further addition1 yield two fresh salts which are both much more stable2 one contains more oxygen than ,m,cl,o1 the other contains none at all4 #c,m,cl,o ,m,cl,o#c #b,m,cl hypochlorite chlorate chloride #ce ,chlorous acid1 ,h,cl,o#b 7according to the data given by ,millon1 ,brandau1 and others7 in many respects resembles hypochlorous acid1 ,h,cl,o1 whilst they both differ from chloric and perchloric acids in their degree of stability1 which is expressed1 for instance1 in their bleaching properties2 the two higher acids do not bleach1 but both the lower ones do so 7oxidise at the ordinary temperature74 ,on the other hand1 chlorous acid is analogous to nitrous acid1 ,,hno#b4 ,the anhydride of chlorous acid1 ,cl#b,o#c1 is not known in a pure state1 but it probably occurs in admixture with chlorine dioxide1 ,cl,o#b1 which is obtained by the action of nitric and sulphuric acids on a mixture of potassium chlorate with such reducing substances as nitric oxide1 arsenious oxide1 sugar1 c4 ,all that is at present known is that pure chlorine dioxide ,cl,o#b 7see ,notes #ci-dc7 is gradually converted into a mixture of hypochlorous and chlorous acids under the action of water 7and alkalis72 that is1 it acts like nitric peroxide1 ,,no#b 7giving ,,hno#c and ,,hno#b71 or as a mixed anhydride1 #b;,cl,o#b ,h#b,o ,h,cl,o#c ,h,cl,o#b4 ,the silver salt1 ,ag,cl,o#b1 is sparingly soluble in water4 ,the investigations of ,garzarolli-,thurnlackh and others seem to show that the anhydride ,cl#b,o#c does not exist in a free state4 ,part of the salt--namely1 two-thirds of it--parts with its oxygen in order to oxidise the remaining third4#cf ,from an intermediate substance1 ,,rx1 two extremes1 ,r and ,,rx#c are formed1 just as nitrous anhydride splits up into nitric oxide and nitric anhydride 7or nitric acid74 ,the resulting salt1 ,m,cl,o#c1 corresponds with chloric acid and potassium chlorate1 ,k,cl,o#c4 ,it is evident that a similar salt may be obtained directly by the action of chlorine on an alkali if its solution be heated1 because ,r,cl,o will be first formed1 and then ,r,cl,o#c2 for example1 #f,,kho #c;,cl#b ,k,cl,o#c #e,k,cl #c;,h#b,o4 ,chlorates are so prepared2 for instance1 potassium chlorate1 which is easily separated from potassium chloride1 being sparingly soluble in cold water4#cg #cf ,hydrochloric acid1 which is an example of compounds of this kind1 is a saturated substance which does not combine directly with oxygen1 but in which1 nevertheless1 a considerable quantity of oxygen may be inserted between the elements forming it4 ,the same may be observed in a number of other cases4 ,thus oxygen may be added or inserted between the elements1 sometimes in considerable quantities1 in the saturated hydrocarbons2 for instance1 in ,c#c;,h#h1 three atoms of oxygen produce an alcohol1 glycerin or glycerol1 ,c#c;,h#e7,,oh7#c4 ,we shall meet with similar examples hereafter4 ,this is generally explained by regarding oxygen as a bivalent element--that is1 as capable of combining with two different elements1 such as chlorine1 hydrogen1 c4 ,on the basis of this view1 it may be inserted between each pair of combined elements2 the oxygen will then be combined with one of the elements by one of its affinities and with the other element by its other affinity4 ,this view does not1 however1 express the entire truth of the matter1 even when applied to the compounds of chlorine4 ,hypochlorous acid1 ,h,o,cl--that is1 hydrochloric acid in which one atom of oxygen is inserted--is1 as we have already seen1 a substance of small stability2 it might therefore be expected that on the addition of a fresh quantity of oxygen1 a still less stable substance would be obtained1 because1 according to the above view1 the chlorine and hydrogen1 which form such a stable compound together1 are then still further removed from each other4 ,but it appears that chloric and perchloric acid1 ,h,cl,o#c and ,h,cl,o#d1 are much more stable substances4 ,furthermore1 the addition of oxygen has also its limit1 it can only be added to a certain extent4 ,if the above representation were true and not merely hypothetical1 there would be no limit to the combination of oxygen1 and the more it entered into one continuous chain the more unstable would be the resultant compound4 ,but not more than four atoms of oxygen can be added to hydrogen sulphide1 nor to hydrochloric acid1 nor to hydrogen phosphide4 ,this peculiarity must lie in the properties of oxygen itself2 four atoms of oxygen seem to have the power of forming a kind of radicle which retains two or several atoms of various other substances--for example1 chlorine and hydrogen1 hydrogen and sulphur1 sodium and manganese1 phosphorus and metals1 c41 forming comparatively stable compounds1 ,na,cl,o#d1 ,na#b,,so#d1 ,na,mn,o#d1 ,na#c,,po#d1 c4 ,see ,chapter ,x4 ,note #a and ,chapter ,,xv4 #cg ,if chlorine be passed through a cold solution of potash1 a bleaching compound1 potassium chloride and hypochlorite1 ,k,cl ,k,cl,o1 is formed1 but if it be passed through a hot solution potassium chlorate is formed4 ,as this is sparingly soluble in water1 it chokes the gas-conducting tube1 which should therefore be widened out at the end4 ,potassium chlorate is usually obtained on a large scale from calcium chlorate1 which is prepared by passing chlorine 7as long as it is absorbed7 into water containing lime1 the mixture being kept warm4 ,a mixture of calcium chlorate and chloride is thus formed in the solution4 ,potassium chloride is then added to the warm solution1 and on cooling a precipitate of potassium chlorate is formed as a substance which is sparingly soluble in cold water1 especially in the presence of other salts4 ,the double decomposition taking place is ,ca7,cl,o#c7b #b,k,cl ,ca,cl#b #b,k,cl,o#c4 ,on a small scale in the laboratory potassium chlorate is best prepared from a strong solution of bleaching powder by passing chlorine through it and then adding potassium chloride4 ,k,cl,o#c is always formed by the action of an electric current on a solution of ,k,cl1 especially at #hj 7,hussermann and ,naschold1 #ahid71 so that this method is now used on a large scale4 ,potassium chlorate crystallises easily in large colourless tabular crystals4 ,its solubility in #ajj parts of water at #j #c parts1 #bj #h parts1 #dj #ad parts1 #fj #be parts1 #hj #dj parts4 ,for comparison we will cite the following figures showing the solubility of potassium chloride and perchlorate in #ajj parts of water3 potassium chloride at ,o #bh parts1 #bj #ce parts1 #dj #dj parts1 #ajj #eg parts2 potassium perchlorate at #j about #a part1 #bj about #a-c_/d part1 #ajj about #ah parts4 ,when heated1 potassium chlorate melts 7the melting point has been given as from #cce-cgf2 according to the latest determination by ,carnelley1 #cei7 and decomposes with the evolution of oxygen1 potassium perchlorate being at first formed1 as will afterwards be described 7see ,note #dg74 ,a mixture of potassium chlorate and nitric and hydrochloric acids effects oxidation and chlorination in solutions4 ,it deflagrates when thrown upon incandescent carbon1 and when mixed with sulphur 7#a_/c by weight7 it ignites it on being struck1 in which case an explosion takes place4 ,the same occurs with many metallic sulphides and organic substances4 ,such mixtures are also ignited by a drop of sulphuric acid4 ,all these effects are due to the large amount of oxygen contained in potassium chlorate1 and to the ease with which it is evolved4 ,a mixture of two parts of potassium chlorate1 one part of sugar1 and one part of yellow prussiate of potash acts like gunpowder1 but burns too rapidly1 and therefore bursts the guns1 and it also has a very strong oxidising action on their metal4 ,the sodium salt1 ,na,cl,o#c1 is much more soluble than the potassium salt1 and it is therefore more difficult to free it from sodium chloride1 c4 ,the barium salt is also more soluble than the potassium salt2 ,o #bd parts1 #bj #cg parts1 #hj #ih parts of salt per #ajj of water4 ,if dilute sulphuric acid be added to a solution of potassium chlorate1 chloric acid is liberated1 but it cannot be separated by distillation1 as it is decomposed in the process4 ,to obtain the free acid1 sulphuric acid must be added to a solution of barium chlorate4#ch ,the sulphuric acid gives a precipitate of barium sulphate1 and free chloric acid remains in solution4 ,the solution may be evaporated under the receiver of an air-pump4 ,this solution is colourless1 has no smell1 and acts as a powerful acid 7it neutralises sodium hydroxide1 decomposes sodium carbonate1 gives hydrogen with zinc1 c472 when heated above #dj1 however1 it decomposes1 forming chlorine1 oxygen1 and perchloric acid3 #d;,h,cl,o#c #b;,h,cl,o#d ,h#b,o ,cl#b ,o#c4 ,in a concentrated condition the acid acts as an exceedingly energetic oxidiser1 so that organic substances brought into contact with it burst into flame4 ,iodine1 sulphurous acid1 and similar oxidisable substances form higher oxidation products and reduce the chloric acid to hydrochloric acid4 ,hydrochloric acid gas gives chlorine with chloric acid 7and consequently with ,k,cl,o#c also7 acting in the same manner as it acts on the lower acids3 ,h,cl,o#c #e;,h,cl #c;,h#b,o #c;,cl#b4 #ch ,barium chlorate1 ,ba7,cl,o#c7b1;,h#b,o1 is prepared in the following way3 impure chloric acid is first prepared and saturated with baryta1 and the barium salt purified by crystallisation4 ,the impure free chloric acid is obtained by converting the potassium in potassium chlorate into an insoluble salt4 ,this is done by adding tartaric or hydrofluosilicic acid to a solution of potassium chlorate1 because potassium tartrate and potassium silicofluoride are very sparingly soluble in water4 ,chloric acid is easily soluble in water4 ,by cautiously acting on potassium chlorate with sulphuric acid1 the dioxide 7chloric peroxide71 ,cl,o#b1ci is obtained 7,davy1 ,millon74 ,this gas is easily liquefied in a freezing mixture1 and boils at #aj4 ,the vapour density 7about #ce if ,h #a7 shows that the molecule of this substance is ,cl,o#b4dj ,in a gaseous or liquid state it very easily explodes 7for instance1 at #fj1 or by contact with organic compounds or finely divided substances1 c471 forming ,cl and ,o#b1 and in many instances#da therefore it acts as an oxidising agent1 although 7like nitric peroxide7 it may itself be further oxidised4#db ,in dissolving in water or alkalis chloric peroxide gives chlorous and hypochlorous acids--#b;,cl,o#b #b,,kho ,k,cl,o#c ,k,cl,o#b ,h#b,o--and therefore1 like nitric peroxide1 the dioxide may be regarded as an intermediate oxide between the 7unknown7 anhydrides of chlorous and chloric acids3 #d;,cl,o#b ,cl#b,o#c ,cl#b,o#c4dc #ci ,to prepare ,cl,o#b #ajj grams of sulphuric acid are cooled in a mixture of ice and salt1 and #ae grams of powdered potassium chlorate are gradually added to the acid1 which is then carefully distilled at #bj to #dj1 the vapour given off being condensed in a freezing mixture4 ,potassium perchlorate is then formed3 #c,k,cl,o#c #b;,h#b,,so#d #b,,khso#d ,k,cl,o#d #b;,cl,o#b ,h#b,o4 ,the reaction may result in an explosion4 ,calvert and ,davies obtained chloric peroxide without the least danger by heating a mixture of oxalic acid and potassium chlorate in a test tube in a water-bath4 ,in this case #b,k,cl,o#c #c;,c#b;,h#b,o#d1b;,h#b,o #b;,c#b;,,hko#d #b;,,co#b #b;,cl,o#b #h;,h#b,o4 ,the reaction is still further facilitated by the addition of a small quantity of sulphuric acid4 ,if a solution of ,h,cl acts upon ,k,cl,o#c at the ordinary temperature1 a mixture of ,cl#b and ,cl,o#b is formed1 but if the temperature be raised to #hj the greater part of the ,cl,o#b decomposes1 and when passed through a hot solution of ,mn,cl#b it oxidises it4 ,gooch and ,kreider proposed 7#ahid7 to employ this method for preparing small quantities of chlorine in the laboratory4 #dj ,by analogy with nitric peroxide it might be expected that at low temperatures a doubling of the molecule into ,cl#b,o#d would take place1 as the reactions of ,cl,o#b point to its being a mixed anhydride of ,h,cl,o#b and ,h,cl,o#c4 #da ,owing to the formation of this chlorine dioxide1 a mixture of potassium chlorate and sugar is ignited by a drop of sulphuric acid4 ,this property was formerly made use of for making matches1 and is now sometimes employed for setting fire to explosive charges by means of an arrangement in which the acid is caused to fall on the mixture at the moment required4 ,an interesting experiment on the combustion of phosphorus under water may be conducted with chlorine dioxide4 ,pieces of phosphorus and of potassium chlorate are placed under water1 and sulphuric acid is poured on to them 7through a long funnel72 the phosphorus then burns at the expense of the chlorine dioxide4 #db ,potassium permanganate oxidises chlorine dioxide into chloric acid 7,frst74 #dc ,the euchlorine obtained by ,davy by gently heating potassium chlorate with hydrochloric acid is 7,pebal7 a mixture of chlorine dioxide and free chlorine4 ,the liquid and gaseous chlorine oxide 7,note #ce71 which ,millon considered to be ,cl#b,o#c1 probably contains a mixture of ,cl,o#b 7vapour density #ce71 ,cl#b,o#c 7whose vapour density should be #ei71 and chlorine 7vapour density #cee71 since its vapour density was determined to be about #dj4 ,as the salts of chloric acid1 ,h,cl,o#c1 are produced by the splitting up of the salts of hypochlorous acid1 so in the same way the salts of perchloric acid1 ,h,cl,o#d1 are produced from the salts of chloric acid1 ,h,cl,o#c4 ,but this is the highest form of the oxidation of ,h,cl4 ,perchloric acid1 ,h,cl,o#d1 is the most stable of all the acids of chlorine4 ,when fused potassium chlorate begins to swell up and solidify1 after having parted with one-third of its oxygen1 potassium chloride and potassium perchlorate have been formed according to the equation #b,k,cl,o#c ,k,cl,o#d ,k,cl ,o#b4 ,the formation of this salt is easily observed in the preparation of oxygen from potassium chlorate1 owing to the fact that the potassium perchlorate fuses with greater difficulty than the chlorate1 and therefore appears in the molten salt as solid grains 7see ,chapter ,,iii4 ,note #ab74 ,under the action of certain acids--for instance1 sulphuric and nitric--potassium chlorate also gives potassium perchlorate4 ,this latter may be easily purified1 because it is but sparingly soluble in water1 although all the other salts of perchloric acid are very soluble and even deliquesce in the air4 ,the perchlorates1 although they contain more oxygen than the chlorates1 are decomposed with greater difficulty1 and even when thrown on ignited charcoal give a much feebler deflagration than the chlorates4 ,sulphuric acid 7at a temperature not below #ajj7 evolves volatile and to a certain extent stable perchloric acid from potassium perchlorate4 ,neither sulphuric nor any other acid will further decompose perchloric acid as it decomposes chloric acid4 ,of all the acids of chlorine1 perchloric acid alone can be distilled4#dd ,the pure hydrate ,h,cl,o#dde is a colourless and exceedingly caustic substance which fumes in the air and has a specific gravity #agh at #ae 7sometimes1 after being kept for some time1 it decomposes with a violent explosion74 ,it explodes violently when brought into contact with charcoal1 paper1 wood1 and other organic substances4 ,if a small quantity of water be added to this hydrate1 and it be cooled1 a crystallo-hydrate1 ,cl,h,o#d1;,h#b,o1 separates out4 ,this is much more stable1 but the liquid hydrate ,h,cl,o#d1b;,h#b,o is still more so4 ,the acid dissolves in water in all proportions1 and its solutions are distinguished for their stability4#df ,when ignited both the acid and its salts are decomposed1 with the evolution of oxygen4#dg #dd ,if a solution of chloric acid1 ,h,cl,o#c1 be first concentrated over sulphuric acid under the receiver of an air-pump and afterwards distilled1 chlorine and oxygen are evolved and perchloric acid is formed3 #d;,h,cl,o#c #b;,h,cl,o#d ,cl#b #c,o ,h#b,o4 ,roscoe accordingly decomposed directly a solution of potassium chlorate by hydrofluosilicic acid1 decanted it from the precipitate of potassium silicofluoride1 ,k#b,si,f#f1 concentrated the solution of chloric acid1 and then distilled it1 perchloric acid being then obtained 7see following footnote74 ,that chloric acid is capable of passing into perchloric acid is also seen from the fact that potassium permanganate is decolorised1 although slowly1 by the action of a solution of chloric acid4 ,on decomposing a solution of potassium chlorate by the action of an electric current1 potassium perchlorate is obtained at the positive electrode 7where the oxygen is evolved74 ,perchloric acid is also formed by the action of an electric current on solutions of chlorine and chlorine monoxide4 ,perchloric acid was obtained by ,count ,stadion and afterwards by ,serullas1 and was studied by ,roscoe and others4 #de ,perchloric acid1 which is obtained in a free state by the action of sulphuric acid on its salts1 may be separated from a solution very easily by distillation1 being volatile1 although it is partially decomposed by distillation4 ,the solution obtained after distillation may be concentrated by evaporation in open vessels4 ,in the distillation the solution reaches a temperature of #bjj1 and then a very constant liquid hydrate of the composition ,h,cl,o#d1b;,h#b,o is obtained in the distillate4 ,if this hydrate be mixed with sulphuric acid1 it begins to decompose at #ajj1 but nevertheless a portion of the acid passes over into the receiver without decomposing1 forming a crystalline hydrate ,h,cl,o#d1;,h#b,o which melts at #ej4 ,on carefully heating this hydrate it breaks up into perchloric acid1 which distills over below #ajj1 and into the liquid hydrate ,h,cl,o#d1b;,h#b,o4 ,the acid ,h,cl,o#d may also be obtained by adding one-fourth part of strong sulphuric acid to potassium chlorate1 carefully distilling and subjecting the crystals of the hydrate ,h,cl,o#d1;,h#b,o obtained in the distillate to a fresh distillation4 ,perchloric acid1 ,h,cl,o#d1 itself does not distil1 and is decomposed on distillation until the more stable hydrate ,h,cl,o#d1 ,h#b,o is formed2 this decomposes into ,h,cl,o#d and ,h,cl,o#d1b;,h#b,o1 which latter hydrate distils without decomposition4 ,this forms an excellent example of the influence of water on stability1 and of the property of chlorine of giving compounds of the type ,cl,x#g1 of which all the above hydrates1 ,cl,o#c7,,oh71 ,cl,o#b7,,oh7#c1 and ,cl,o7,,oh7#e1 are members4 ,probably further research will lead to the discovery of a hydrate ,cl7,,oh7#g4 #df ,according to ,roscoe the specific gravity of perchloric acid #aghb and of the hydrate ,h,cl,o#d1;,h#b,o in a liquid state 7#ej7 #ahaa2 hence a considerable contraction takes place in the combination of ,h,cl,o#d with ,h#b,o4 #dg ,the decomposition of salts analogous to potassium chlorate has been more fully studied in recent years by ,potilitzin and ,p4 ,frankland4 ,professor ,potilitzin1 by decomposing1 for example1 lithium chlorate ,li,cl,o#c1 found 7from the quantity of lithium chloride and oxygen7 that at first the decomposition of the fused salt 7#cfh7 takes place according to the equation1 #c,li,cl,o#c #b,li,cl ,li,cl,o#d #e,o1 and that towards the end the remaining salt is decomposed thus3 #e,li,cl,o#c #d,li,cl ,li,cl,o#d #aj,o4 ,the phenomena observed by ,potilitzin obliged him to admit that lithium perchlorate is capable of decomposing simultaneously with lithium chlorate1 with the formation of the latter salt and oxygen2 and this was confirmed by direct experiment1 which showed that lithium chlorate is always formed in the decomposition of the perchlorate4 ,potilitzin drew particular attention to the fact that the decomposition of potassium chlorate and of salts analogous to it1 although exothermal 7,chapter ,,iii41 ,note #ab71 not only does not proceed spontaneously1 but requires time and a rise of temperature in order to attain completion1 which again shows that chemical equilibria are not determined by the heat effects of reactions only4 ,p4 ,frankland and ,j4 ,dingwall 7#ahhg7 showed that at #ddh 7in the vapour of sulphur7 a mixture of potassium chlorate and powdered glass is decomposed almost in accordance with the equation #b,k,cl,o#c ,k,cl,o#d ,k,cl ,o#b1 whilst the salt by itself evolves about half as much oxygen1 in accordance with the equation1 #h,k,cl,o#c #e,k,cl,o#d #c,k,cl #b,o#b4 ,the decomposition of potassium perchlorate in admixture with manganese peroxide proceeds to completion1 ,k,cl,o#d ,k,cl #b,o#b4 ,but in decomposing by itself the salt at first gives potassium chlorate1 approximately according to the equation #g,k,cl,o#d #b,k,cl,o#c #e,k,cl #aa,o#b4 ,thus there is now no doubt that when potassium chlorate is heated1 the perchlorate is formed1 and that this salt1 in decomposing with evolution of oxygen1 again gives the former salt4 ,in the decomposition of barium hypochlorite1 #ej per cent4 of the whole amount passes into chlorate1 in the decomposition of strontium hypochlorite 7,potilitzin1 #ahij7 #abe per cent41 and of calcium hypochlorite about #be per cent4 ,besides which ,potilitzin showed that the decomposition of the hypochlorites and also of the chlorates is always accompanied by the formation of a certain quantity of the oxides and by the evolution of chlorine1 the chlorine being displaced by the oxygen disengaged4 ,spring and ,prost 7#ahhi7 represent the evolution of oxygen from ,k,cl,o#c as due to the salt first splitting up into base and anhydride1 thus 7#a7 #b,m,cl,o#c ,m#b,o ,cl#b,o#e2 7#b7 ,cl#b,o#e ,cl#b ,o#c2 and 7#c7 ,m#b,o ,cl #b,m,cl ,o4 ,i may further remark that the decomposition of potassium chlorate as a reaction evolving heat easily lends itself for this very reason to the contact action of manganese peroxide and other similar admixtures2 for such very feeble influences as those of contact may become evident either in those cases 7for instance1 detonating gas1 hydrogen peroxide1 c471 when the reaction is accompanied by the evolution of heat1 or when 7for instance1 ,h#b ,i#b1 c47 little heat is absorbed or evolved4 ,in these cases it is evident that the existing equilibrium is not very stable1 and that a small alteration in the conditions at the surfaces of contact may suffice to upset it4 ,in order to conceive the modus operandi of contact phenomena1 it is enough to imagine1 for instance1 that at the surface of contact the movement of the atoms in the molecules changes from a circular to an elliptical path4 ,momentary and transitory compounds may he formed1 but their formation cannot affect the explanation of the phenomena4 ,on comparing chlorine as an element not only with nitrogen and carbon but with all the other non-metallic elements 7chlorine has so little analogy with the metals that a comparison with them would be superfluous71 we find in it the following fundamental properties of the halogens or salt-producers4 ,with metals chlorine gives salts 7such as sodium chloride1 c472 with hydrogen a very energetic and monobasic acid ,h,cl1 and the same quantity of chlorine is able by metalepsis to replace the hydrogen2 with oxygen it forms unstable oxides of an acid character4 ,these properties of chlorine are possessed by three other elements1 bromine1 iodine1 and fluorine4 ,they are members of one natural family4 ,each representative has its peculiarities1 its individual properties and points of distinction1 in combination and in the free state--otherwise they would not be independent elements2 but the repetition in all of them of the same chief characteristics of the family enables one more quickly to grasp all their various properties and to classify the elements themselves4 ,in order to have a guiding thread in forming comparisons between the elements1 attention must however be turned not only to their points of resemblance but also to those of their properties and characters in which they differ most from each other4 ,and the atomic weights of the elements must be considered as their most elementary property1 since this is a quantity which is most firmly established1 and must be taken account of in all the reactions of the element4 ,the halogens have the following atomic weights-- ,f #ai1 ,cl #cee1 ,br #hj1 ,i #abg4 ,all the properties1 physical and chemical1 of the elements and their corresponding compounds must evidently be in a certain dependence on this fundamental point1 if the grouping in one family be natural4#dg bis ,and we find in reality that1 for instance1 the properties of bromine1 whose atomic weight is almost the mean between those of iodine and chlorine1 occupy a mean position between those of these two elements4 ,the second measurable property of the elements is their equivalence or their capacity for forming compounds of definite forms4 ,thus carbon or nitrogen in this respect differs widely from the halogens4 ,although the form ,cl,o#b corresponds with ,,no#b and ,,co#b1 yet the last is the highest oxide of carbon1 whilst that of nitrogen is ,n#b,o#e1 and for chlorine1 if there were an anhydride of perchloric acid1 its composition would be ,cl#b,o#g1 which is quite different from that of carbon4 ,in respect to the forms of their compounds the halogens1 like all elements of one family or group1 are perfectly analogous to each other1 as is seen from their hydrogen compounds3 ,,hf1 ,h,cl1 ,h,br1 ,,hi4 #dg bis ,see1 for example the melting point of ,na,cl1 ,na,br1 ,na,i in ,chapter ,,ii4 ,note #bg4 ,according to ,f4 ,freyer and ,v4 ,meyer 7#ahib71 the following are the boiling points of some of the corresponding compounds of chlorine and bromine3 ,b,cl#e #ag ,b,br#c #ij ,si,cl#c #ei ,si,br#d #aec ,p,cl#c #gf ,p,br#c #age ,sb,cl#c #bbc ,sb,br#c #bge ,bi,cl#c #ddg ,bi,br#c #dec ,sn,cl#d #fjf ,sn,br#d #fai ,zn,cl#b #gcj ,zn,br#b #fej ,thus for all the more volatile compounds the replacement of chlorine by bromine raises the boiling point1 but in the ease of ,zn,x#b it lowers it 7,chapter ,,xv4 ,note #ai74 ,their oxygen compounds exhibit a similar analogy4 ,only fluorine does not give any oxygen compounds4 ,the iodine and bromine compounds corresponding with ,h,cl,o#c and ,h,cl,o#d are ,h,br,o#c and ,h,br,o#d1 ,,hio#c and ,,hio#d4 ,on comparing the properties of these acids we can even predict that fluorine will not form any oxygen compound4 ,for iodine is easily oxidised--for instance1 by nitric acid--whilst chlorine is not directly oxidised4 ,the oxygen acids of iodine are comparatively more stable than those of chlorine2 and1 generally speaking1 the affinity of iodine for oxygen is much greater than that of chlorine4 ,here also bromine occupies an intermediate position4 ,in fluorine we may therefore expect a still smaller affinity for oxygen than in chlorine--and up to now it has not been combined with oxygen4 ,if any oxygen compounds of fluorine should be obtained1 they will naturally be exceedingly unstable4 ,the relation of these elements to hydrogen is the reverse of the above4 ,fluorine has so great an affinity for hydrogen that it decomposes water at the ordinary temperature2 whilst iodine has so little affinity for hydrogen that hydriodic acid1 ,,hi1 is formed with difficulty1 is easily decomposed1 and acts as a reducing agent in a number of cases4 ,from the form of their compounds the halogens are univalent elements with respect to hydrogen and septivalent with respect to oxygen1 ,n being trivalent to hydrogen 7it gives ,,nh#c7 and quinqui-valent to oxygen 7it gives ,n#b,o#e71 and ,c being quadrivalent to both ,h and ,o as it forms ,,ch#d and ,,co#b4 ,and as not only their oxygen compounds1 but also their hydrogen compounds1 have acid properties1 the halogens are elements of an exclusively acid character4 ,such metals as sodium1 potassium1 barium only give basic oxides4 ,in the case of nitrogen1 although it forms acid oxides1 still in ammonia we find that capacity to give an alkali with hydrogen which indicates a less distinctly acid character than in the halogens4 ,in no other elements is the acid-giving property so strongly developed as in the halogens4 ,in describing certain peculiarities characterising the halogens1 we shall at every step encounter a confirmation of the above-mentioned general relations4 ,as fluorine decomposes water with the evolution of oxygen1 ,f#b ,h#b,o #b;,,hf ,o1 for a long time all efforts to obtain it in free state by means of methods similar to those for the preparation of chlorine proved fruitless4#dh ,thus by the action of hydrofluoric acid on manganese peroxide1 or by decomposing a solution of hydrofluoric acid by an electric current1 either oxygen or a mixture of oxygen and fluorine were obtained instead of fluorine4 ,probably a certain quantity of fluorine#dh bis was set free by the action of oxygen or an electric current on incandescent and fused calcium fluoride1 but at a high temperature fluorine acts even on platinum1 and therefore it was not obtained4 ,when chlorine acted on silver fluoride1 ,ag,f1 in a vessel of natural fluor spar1 ,ca,f#b1 fluorine was also liberated2 but it was mixed with chlorine1 and it was impossible to study the properties of the resultant gas4 ,brauner 7#ahha7 also obtained fluorine by igniting cerium fluoride1 #b;,ce,f#d #b;,ce,f#c ,f#b2 but this1 like all preceding efforts1 only showed fluorine to be a gas which decomposes water1 and is capable of acting in a number of instances like chlorine1 but gave no possibility of testing its properties4 ,it was evident that it was necessary to avoid as far as possible the presence of water and a rise of temperature2 this ,moissan succeeded in doing in #ahhf4 ,he decomposed anhydrous hydrofluoric acid1 liquefied at a temperature of -#bc and contained in a ,u-shaped tube 7to which a small quantity of potassium fluoride had been added to make it a better conductor71 by the action of a powerful electric current 7twenty ,bunsen's elements in series74 ,hydrogen was then evolved at the negative pole1 and fluorine appeared at the positive pole 7of iridium platinum7 as a pale green gas which decomposed water with the formation of ozone and hydrofluoric acid1 and combined directly with silicon 7forming silicon fluoride1 ,si,f#d71 boron 7forming ,,bf#c71 sulphur1 c4 ,its density 7,h #a7 is #ah1 so that its molecule is ,f#b4 ,but the action of fluorine on metals at the ordinary temperature is comparatively feeble1 because the metallic fluoride formed coats the remaining mass of the metals2 it is1 however1 completely absorbed by iron4 ,hydrocarbons 7such as naphtha71 alcohol1 c41 immediately absorb fluorine1 with the formation of hydrofluoric acid4 ,fluorine when mixed with hydrogen can easily be made to explode violently1 forming hydrofluoric acid4#di #dh ,even before free fluorine was obtained 7#ahhf7 it was evident from experience gained in the efforts made to obtain it1 and from analogy1 that it would decompose water 7see first ,russian edition of the ,principles of ,chemistry74 #dh bis ,it is most likely that in this experiment of ,fremy's1 which corresponds with the action of oxygen on calcium chloride1 fluorine was set free1 but that a converse reaction also proceeded1 ,ca,o ,f#b ,ca,f#b ,o--that is1 the calcium distributed itself between the oxygen and fluorine4 ,mn,f#d1 which is capable of splitting up into ,mn,f#b and ,f#b1 is without doubt formed by the action of a strong solution of hydrofluoric acid on manganese peroxide1 but under the action of water the fluorine gives hydrofluoric acid1 and probably this is aided by the affinity of the manganese fluoride and hydrofluoric acid4 ,in all the attempts made 7by ,davy1 ,knox1 ,louget1 ,fremy1 ,gore1 and others7 to decompose fluorides 7those of lead1 silver1 calcium1 and others7 by chlorine1 there were doubtless also cases of distribution1 a portion of the metal combined with chlorine and a portion of the fluorine was evolved2 but it is improbable that any decisive results were obtained4 ,fremy probably obtained fluorine1 but not in a pure state4 #di ,according to ,moissan1 fluorine is disengaged by the action of an electric current on fused hydrogen potassium fluoride1 ,,khf#b4 ,the present state of chemical knowledge is such that the knowledge of the properties of an element is much more general than the knowledge of the free element itself4 ,it is useful and satisfactory to learn that even fluorine in the free state has not succeeded in eluding experiment and research1 that the efforts to isolate it have been crowned with success1 but the sum total of chemical data concerning fluorine as an element gains but little by this achievement4 ,the gain will1 however1 be augmented if it be now possible to subject fluorine to a comparative study in relation to oxygen and chlorine4 ,there is particular interest in the phenomena of the distribution of fluorine and oxygen1 or fluorine and chlorine1 competing under different conditions and relations4 ,we may add that ,moissan 7#ahib7 found that free fluorine decomposes ,h#b,s1 ,h,cl1 ,h,br1 ,,cs#b1 and ,,cnh with a flash2 it does not act upon ,o#b1 ,n#b1 ,,co1 and ,,co#b2 ,mg1 ,al1 ,ag1 and ,ni1 when heated1 burn in it1 as also do ,s1 ,se1 ,p 7forms ,,pf#e72 it reacts upon ,h#b even in the dark1 with the evolution of #cffjj units of heat4 ,at a temperature of -#ie1 ,f#b still retains its gaseous state4 ,soot and carbon in general 7but not the diamond7 when heated in gaseous fluorine form fluoride of carbon1 ,,cf#d 7,moissan1 #ahij72 this compound is also formed at #cjj by the double decomposition of ,c,cl#d and ,ag,f2 it is a gas which liquefies at #aj under a pressure of #e atmospheres4 ,with an alcoholic solution of ,,kho1 ,,cf#d gives ,k#b;,,co#c1 according to the equation ,,cf#d #f,,kho ,k#b;,,co#c #d,,kf #c;,h#b,o4 ,,cf#d is not soluble in water1 but it is easily soluble in ,c,cl#d and alcohol4 ,in #ahid ,brauner obtained fluorine directly by igniting the easily formed#di bis double lead salt ,,hf1#c,,kf1,pb,f#d1 which first1 at #bcj1 decomposes with the evolution of ,,hf1 and then splits up forming #c,,kf1,pb,f#b and fluorine ,f#b1 which is recognised by the fact that it liberates iodine from ,,ki and easily combines with silicon1 forming ,si,f#d4 ,this method gives chemically pure fluorine1 and is based upon the breaking up of the higher compound--tetrafluoride of lead1 ,pb,f#d1 corresponding to ,pb,o#b1 into free fluorine1 ,f#b1 and the lower more stable form--bifluoride of lead1 ,pb,f#b1 which corresponds to ,pb,o2 that is1 this method resembles the ordinary method of obtaining chlorine by means of ,mn,o#b1 as ,mn,cl#d here breaks up into ,mn,cl#b and chlorine1 just as ,pb,f#d splits up into ,pb,f#b and fluorine4 #di bis ,t4 ,nikolukin 7#ahhe7 and subsequently ,friedrich and ,classen obtained ,pb,cl#d and a double ammonium salt of tetrachloride of lead 7starting from the binoxide71 ,pb,cl#db,,nh#d;,cl2 ,hutchinson and ,pallard obtained a similar salt of acetic acid 7#ahic7 corresponding to ,pb,x#d by treating red lead with strong acetic acid2 the composition of this salt is ,pb7,c#b;,h#c,o#b7d2 it melts 7and decomposes7 at about #age4 ,brauner 7#ahid7 obtained a salt corresponding to tetrafluoride of lead1 ,pb,f#d1 and the acid corresponding to it1 ,h#d,pb,f#h4 ,for example1 by treating potassium plumbate 7,chapter ,,xviii4 ,note #ee7 with strong ,,hf1 and also the above-mentioned tetra-acetate with a solution of ,,khf#b1 ,brauner obtained crystalline ,,hk#c,pb,f#h--;i4e4 the salt from which he obtained fluorine4 ,among the compounds of fluorine1 calcium fluoride1 ,ca,f#b1 is somewhat widely distributed in nature as fluor spar1#ej whilst cryolite1 or aluminium sodium fluoride1 ,na#c;,al,f#f1 is found more rarely 7in large masses in ,greenland74 ,cryolite1 like fluor spar1 is also insoluble in water1 and gives hydrofluoric acid with sulphuric acid4 ,small quantities of fluorine have also in a number of cases been found in the bodies of animals1 in the blood1 urine1 and bones4 ,if fluorides occur in the bodies of animals1 they must have been introduced in food1 and must occur in plants and in water4 ,and as a matter of fact river1 and especially sea1 water always contains a certain1 although small1 quantity of fluorine compounds4 #ej ,it is called spar because it very frequently occurs as crystals of a clearly laminar structure1 and is therefore easily split up into pieces bounded by planes4 ,it is called fluor spar because when used as a flux it renders ores fusible1 owing to its reacting with silica1 ,si,o#b #b;,ca,f#b #b;,ca,o ,si,f#d2 the silicon fluoride escapes as a gas and the lime combines with a further quantity of silica1 and gives a vitreous slag4 ,fluor spar occurs in mineral veins and rocks1 sometimes in considerable quantities4 ,it always crystallises in the cubic system1 sometimes in very large semi-transparent cubic crystals1 which are colourless or of different colours4 ,it is insoluble in water4 ,it melts under the action of heat1 and crystallises on cooling4 ,the specific gravity is #ca4 ,when steam is passed over incandescent fluor spar1 lime and hydrofluoric acid are formed3 ,ca,f#b ,h#b,o ,ca,o #b;,,hf4 ,a double decomposition is also easily produced by fusing fluor spar with sodium or potassium hydroxides1 or potash1 or even with their carbonates2 the fluorine then passes over to the potassium or sodium1 and the oxygen to the calcium4 ,in solutions--for example1 ,ca7,,no#c7b #b,,kf ,ca,f#b 7precipitate7 #b,,kno#c 7in solution7--the formation of calcium fluoride takes place1 owing to its very sparing solubility4 #bf1jjj parts of water dissolve one part of fluor spar4 ,hydrifluoric acid1 ,,hf1 cannot be obtained from fluor spar in glass retorts1 because glass is acted on by and destroys the acid4 ,it is prepared in lead vessels1 and when it is required pure1 in platinum vessels1 because lead also acts on hydrofluoric acid1 although only very feebly on the surface1 and when once a coating of fluoride and sulphate of lead is formed no further action takes place4 ,powdered fluor spar and sulphuric acid evolve hydrofluoric acid 7which fumes in the air7 even at the ordinary temperature1 ,ca,f#b ,h#b,,so#d ,ca,s,o#d #b;,,hf4 ,at #acj fluor spar is completely decomposed by sulphuric acid4 ,the acid is then evolved as vapour1 which may be condensed by a freezing mixture into an anhydrous acid4 ,the condensation is aided by pouring water into the receiver of the condenser1 as the acid is easily soluble in cold water4 ,in the liquid anhydrous form hydrofluoric acid boils at #ai1 and its specific gravity at #abh #jihdi4ea ,it dissolves in water with the evolution of a considerable amount of heat1 and gives a solution of constant boiling point which distils over at #abj2 showing that the acid is able to combine with water4 ,the specific gravity of the compound is #aae1 and its composition ,,hf1#b;,h#b,o4#eb ,with an excess of water a dilute solution distils over first4 ,the aqueous solution and the acid itself must be kept in platinum vessels1 but the dilute acid may be conveniently preserved in vessels made of various organic materials1 such as gutta-percha1 or even in glass vessels having an interior coating of paraffin4 ,hydrofluoric acid does not act on hydrocarbons and many other substances1 but it acts in a highly corrosive manner on metals1 glass1 porcelain1 and the majority of rock substances4#ec ,it also attacks the skin1 and is distinguished by its poisonous properties1 so that in working with the acid a strong draught must be kept up1 to prevent the possibility of the fumes being inhaled4 ,the non-metals do not act on hydrofluoric acid1 but all metals--with the exception of mercury1 silver1 gold1 and platinum1 and1 to a certain degree1 lead--decompose it with the evolution of hydrogen4 ,with bases it gives directly metallic fluorides1 and behaves in many respects like hydrochloric acid4 ,there are1 however1 several distinct individual differences1 which are furthermore much greater than those between hydrochloric1 hydrobromic1 and hydriodic acids4 ,thus the silver compounds of the latter are insoluble in water1 whilst silver fluoride is soluble4 ,calcium fluoride1 on the contrary1 is insoluble in water1 whilst calcium chloride1 bromide1 and iodide are not only soluble1 but attract water with great energy4 ,neither hydrochloric1 hydrobromic1 nor hydriodic acid acts on sand and glass1 whilst hydrofluoric acid corrodes them1 forming gaseous silicon fluoride4 ,the other halogen acids only form normal salts1 ,k,cl1 ,na,cl1 with ,na or ,k1 whilst hydrofluoric acid gives acid salts1 for instance ,,hkf#b 7and by dissolving ,,kf in liquid ,,hf1 ,,khf#bb;,,hf is obtained74 ,this latter property is in close connection with the fact that at the ordinary temperature the vapour density of hydrofluoric acid is nearly #bj1 which corresponds with a formula ,h#b;,f#b1 as ,mallet 7#ahha7 showed2 but a depolymerisation occurs with a rise of temperature1 and the density approaches #aj1 which answers to the formula ,,hf4#ed #ea ,according to ,gore4 ,fremy obtained anhydrous hydrofluoric acid by decomposing lead fluoride at a red heat1 by hydrogen1 or by beating the double salt ,,hkf#b1 which easily crystallises 7in cubes7 from a solution of hydrofluoric acid1 half of which has been saturated with potassium hydroxide4 ,its vapour density corresponds to the formula ,,hf4 #eb ,this composition corresponds to the crystallo-hydrate ,h,cl1#b;,h#b,o4 ,all the properties of hydrofluoric acid recall those of hydrochloric acid1 and therefore the comparative ease with which hydrofluoric acid is liquefied 7it boils at #ai1 hydrochloric acid at -#ce7 must be explained by a polymerisation taking place at low temperatures1 as will be afterwards explained1 ,h#b;,f#b being formed1 and therefore in a liquid state it differs from hydrochloric acid1 in which a phenomenon of a similar kind has not yet been observed4 #ec ,the corrosive action of hydrofluoric acid on glass and similar siliceous compounds is based upon the fact that it acts on silica1 ,si,o#b1 as we shall consider more fully in describing that compound1 forming gaseous silicon fluoride1 ,si,o#b #d;,,hf ,si,f#d #b;,h#b,o4 ,silica1 on the other hand1 forms the binding 7acid7 element of glass and of the mass of mineral substances forming the salts of silica4 ,when it is removed the cohesion is destroyed4 ,this is made use of in the arts1 and in the laboratory1 for etching designs and scales1 c41 on glass4 ,in engraving on glass the surface is covered with a varnish composed of four parts of wax and one part of turpentine4 ,this varnish is not acted on by hydrofluoric acid1 and it is soft enough to allow of designs being drawn upon it whose lines lay bare the glass4 ,the drawing is made with a steel point1 and the glass is afterwards laid in a lead trough in which a mixture of fluor spar and sulphuric acid is placed4 ,the sulphuric acid must be used in considerable excess1 as otherwise transparent lines are obtained 7owing to the formation of hydrofluosilicic acid74 ,after being exposed for some time1 the varnish is removed 7melted7 and the design drawn by the steel point is found reproduced in dull lines4 ,the drawing may be also made by the direct application of a mixture of a silicofluoride and sulphuric acid1 which forms hydrofluoric acid4 #ed ,mallet 7#ahha7 determined the density at #cj and #ajj1 previous to which ,gore 7#ahfi7 had determined the vapour density at #ajj1 whilst ,thorpe and ,hambly 7#ahhh7 made fourteen determinations between #bf and #hh1 and showed that within this limit of temperature the density gradually diminishes1 just like the vapour of acetic acid1 nitrogen dioxide1 and others4 ,the tendency of ,,hf to polymerise into ,h#b;,f#b is probably connected with the property of many fluorides of forming acid salts--for example1 ,,khf#b and ,h#b,si,f#f4 ,we saw above that ,h,cl has the same property 7forming1 for instance1 ,h#b,pt,cl#f1 c41 p4 #deg71 and hence this property of hydrofluoric acid does not stand isolated from the properties of the other halogens4 ,the analogy between chlorine and the other two halogens1 bromine and iodine1 is much more perfect4 ,not only have their hydrates or halogen acids much in common1 but they themselves resemble chlorine in many respects1#ee and even the properties of the corresponding metallic compounds of bromine and iodine are very much alike4 ,thus1 the chlorides1 bromides1 and iodides of sodium and potassium crystallise in the cubic system1 and are soluble in water2 the chlorides of calcium1 aluminium1 magnesium1 and barium are just as soluble in water as the bromides and iodides of these metals4 ,the iodides and bromides of silver and lead are sparingly soluble in water1 like the chlorides of these metals4 ,the oxygen compounds of bromine and iodine also present a very strong analogy to the corresponding compounds of chlorine4 ,a hypobromous acid is known corresponding with hypochlorous acid4 ,the salts of this acid have the same bleaching property as the salts of hypochlorous acid4 ,iodine was discovered in #ahaa by ,courtois in kelp1 and was shortly afterwards investigated by ,clement1 ,gay-,lussac1 and ,davy4 ,bromine was discovered in #ahbf by ,balard in the mother liquor of sea water4 #ee ,for instance1 the experiment with ,dutch metal foil 7,note #af7 may be made with bromine just as well as with chlorine4 ,a very instructive experiment on the direct combination of the halogens with metals maybe made by throwing a small piece 7a shaving7 of aluminium into a vessel containing liquid bromine2 the aluminium1 being lighter1 floats on the bromine1 and after a certain time reaction sets in accompanied by the evolution of heat1 light1 and fumes of bromine4 ,the incandescent piece of metal moves rapidly over the surface of the bromine in which the resultant aluminium bromide dissolves4 ,for the sake of comparison we will proceed to cite several thermochemical data 7,thomsen7 for analogous actions of 7#a7 chlorine1 7#b7 bromine1 and 7#c7 iodine1 with respect to metals2 the halogen being expressed by the symbol ,x1 and the plus sign connecting the reacting substances4 ,all the figures are given in thousands of calories1 and refer to molecular quantities in grams and to the ordinary temperature3-- #a #b #c ,k#b ,x#b #baa #aia #afj ,na#b ,x#b #aie #agb #ach ,ag#b ,x#b #ei #de #bh ,hg#b ,x#b #hc #fh #dh ,hg ,x#b #fc #ea #cd ,ca ,x#b #agj #ada -- ,ba ,x#b #aie #agj -- ,zn ,x#b #ig #gf #di ,pb ,x#b #hc #fd #dj ,al ,x#b #afa #abj #gj ,we may remark that the latent heat of vaporisation of the molecular weight ,br#b is about #gb1 and of iodine #fj thousand heat units1 whilst the latent heat of fusion of ,br#b is about #jc1 and of ,i#b about #cj thousand heat units4 ,from this it is evident that the difference between the amounts of heat evolved does not depend on the difference in physical state4 ,for instance1 the vapour of iodine in combining with ,zn to form ,zn,i#b would give #dh #h #c1 or about sixty thousand heat units1 or #a-a_/b times less than ,zn ,cl#b4 ,bromine and iodine1 like chlorine1 occur in sea water in combination with metals4 ,however1 the amount of bromides1 and especially of iodides1 in sea water is so small that their presence can only be discovered by means of sensitive reactions4#ef ,in the extraction of salt from sea water the bromides remain in the mother liquor4 ,iodine and bromine also occur combined with silver1 in admixture with silver chloride1 as a rare ore which is mainly found in ,america4 ,certain mineral waters 7those of ,kreuznach and ,staro-rosssk7 contain metallic bromides and iodides1 always in admixture with an excess of sodium chloride4 ,those upper strata of the ,stassfurt rock salt 7,chapter ,x47 which are a source of potassium salts also contain metallic bromides1#eg which collect in the mother liquors left after the crystallisation of the potassium salts2 and this now forms the chief source 7together with certain ,american springs7 of the bromine in common use4 ,bromine may be easily liberated from a mixture of bromides and chlorides1 owing to the fact that chlorine displaces bromine from its compounds with sodium1 magnesium1 calcium1 c4 ,a colourless solution of bromides and chlorides turns an orange colour after the passage of chlorine1 owing to the disengagement of bromine4#eh ,bromine may be extracted on a large scale by a similar method1 but it is simpler to add a small quantity of manganese peroxide and sulphuric acid to the mother liquid direct4 ,this sets free a portion of the chlorine1 and this chlorine liberates the bromine4 #ef ,one litre of sea-water contains about #bj grams of chlorine1 and about #jjg gram of bromine4 ,the ,dead ,sea contains about ten times as much bromine4 #eg ,but there is no iodine in ,stassfurt carnallite4 #eh ,the chlorine must not1 however1 be in large excess1 as otherwise the bromine would contain chlorine4 ,commercial bromine not unfrequently contains chlorine1 as bromine chloride2 this is more soluble in water than bromine1 from which it may thus be freed4 ,to obtain pure bromine the commercial bromine is washed with water1 dried by sulphuric acid1 and distilled1 the portion coming over at #eh being collected2 the greater part is then converted into potassium bromide and dissolved1 and the remainder is added to the solution in order to separate iodine1 which is removed by shaking with carbon bisulphide4 ,by heating the potassium bromide thus obtained with manganese peroxide and sulphuric acid1 bromine is obtained quite free from iodine1 which1 however1 is not present in certain kinds of commercial bromine 7the ,stassfurt1 for instance74 ,by treatment with potash1 the bromine is then converted into a mixture of potassium bromide and bromate1 and the mixture 7which is in the proportion given in the equation7 is distilled with sulphuric acid1 bromine being then evolved3 #e,k,br ,k,br,o#c #f;,h#b,,so#d #f,,khso#d #c;,h#b,o #c;,br#b4 ,after dissolving the bromine in a strong solution of calcium bromide and precipitating with an excess of water1 it loses all the chlorine it contained1 because chlorine forms calcium chloride with ,ca,br#b4 ,bromine is a dark brown liquid1 giving brown fumes1 and having a poisonous suffocating smell1 whence its name 7from the ,greek ,greek3 brmos1 signifying evil smelling74 ,the vapour density of bromine shows that its molecule is ,br#b4 ,in the cold bromine freezes into brown-grey scales like iodine4 ,the melting point of pure bromine is -#gje4ei ,the density of liquid bromide at #j is #cahg1 and at #ae about #cj4 ,the boiling point of bromine is about #ehg4 ,bromine1 like chlorine1 is soluble in water2 #a part of bromine at #e requires #bg parts of water1 and at #ae #bi parts of water4 ,the aqueous solution of bromine is of an orange colour1 and when cooled to -#b yields crystals containing #aj molecules of water to #a molecule of bromine4#fj ,alcohol dissolves a greater quantity of bromine1 and ether a still greater amount4 ,but after a certain time products of the action of the bromine on these organic substances are formed in the solutions4 ,aqueous solutions of the bromides also absorb a large amount of bromine4 #ei ,there has long existed a difference of opinion as to the melting point of pure bromine4 ,by some investigators 7,regnault1 ,pierre7 it was given as between -#g and -#h1 and by others 7,balard1 ,liebig1 ,quincke1 ,baumhauer7 as between -#bj and -#be4 ,there is now no doubt1 thanks more especially to the researches of ,ramsay and ,young 7#ahhe71 that pure bromine melts at about -#g4 ,this figure is not only established by direct experiment 7,van der ,plaats confirmed it71 but also by means of the determination of the vapour tensions4 ,for solid bromine the vapour tension p in mm4 at t was found to be-- p #bj #be #cj #ce #dj #de mm4 t -#aff -#ad -#ab -#aj -#he -#g ,for liquid bromine-- p #ej #ajj #bjj #djj #fjj #gfj mm4 t -#ej #hb #bcd #djd #eai #ehg ,these curves intersect at -#gje4 ,besides which1 in comparing the vapour tension of many liquids 7for example1 those given in ,chapter ,,ii41 ,note #bg71 ,ramsay and ,young observed that the ratio of the absolute temperatures 7t #bgc7 corresponding with equal tension varies for every pair of substances in rectilinear proportion in dependence upon t1 and1 therefore1 for the above pressure p1 ,ramsay and ,young determined the ratio of t #bgc for water and bromine1 and found that the straight lines expressing these ratios for liquid and solid bromine intersect also at #gje2 thus1 for example1 for solid bromine-- p #bj #be #cj #ce #dj #de #bgc t #befd #bei #bfa #bfc #bfdf #bff #bgc t #biec #bii #cjba #cjdh #cjgb #cjic c #aaeb #aaed #aaeg #aaei #aafa #aafc where t indicates the temperature of water corresponding with a vapour tension p1 and where c is the ratio of #bgc t to #bgc t4 ,the magnitude of c is evidently expressed with great accuracy by the straight line c #aagjc #jjjaat4 ,in exactly the same way we find the ratio for liquid bromine and water to be c#a #aaehe #jjjjegt4 ,the intersection of these straight lines in fact corresponds with -#gjf1 which again confirms the melting point given above for bromine4 ,in this manner it is possible with the existing store of data to accurately establish and verify the melting point of substances4 ,ramsay and ,young established the thermal constants of iodine by exactly the same method4 #fj ,the observations made by ,paterno and ,nasini 7by ,raoult's method1 ,chapter ,i4 ,note #di7 on the temperature of the formation of ice 7-#aaae1 with #acia gram of bromine in #ajj grams of water7 in an aqueous solution of bromine1 showed that bromine is contained in solutions as the molecule ,br#b4 ,similar experiments conducted on iodine 7,kloboukoff #ahhi and ,beckmann #ahij7 show that in solution the molecule is ,i#b4 ,b4 ,roozeboom investigated the hydrate of bromine as completely as the hydrate of chlorine 7,notes #i1 #aj74 ,the temperature of the complete decomposition of the hydrate is #fb2 the density of ,br#b1aj;,h#b,o #adi4 ,with respect to iodine1 it is almost exclusively extracted from the mother liquors after the crystallisation of natural sodium nitrate 7,chili saltpetre7 and from the ashes of the sea-weed cast upon the shores of ,france1 ,great ,britain1 and ,spain1 sometimes in considerable quantities1 by the high tides4 ,the majority of these sea-weeds are of the genera ,fucus1 ,laminaria1 c4 ,the fused ashes of these sea-weeds are called 'kelp' in ,scotland and 'varech' in ,normandy4 ,a somewhat considerable quantity of iodine is contained in these sea-weeds4 ,after being burnt 7or subjected to dry distillation7 an ash is left which chiefly contains salts of potassium1 sodium1 and calcium4 ,the metals occur in the sea-weed as salts of organic acids4 ,on being burnt these organic salts are decomposed1 forming carbonates of potassium and sodium4 ,hence1 sodium carbonate is found in the ash of sea plants4 ,the ash is dissolved in hot water1 and on evaporation sodium carbonate and other salts separate1 but a portion of the substances remains in solution4 ,these mother liquors left after the separation of the sodium carbonate contain chlorine1 bromine1 and iodine in combination with metals1 the chlorine and iodine being in excess of the bromine4 #ac1jjj kilos of kelp give about #a1jjj kilos of sodium carbonate and #ae kilos of iodine4 ,the liberation of the iodine from the mother liquor is effected with comparative ease1 because chlorine disengages iodine from potassium iodide and its other combinations with the metals4 ,not only chlorine1 but also sulphuric acid1 liberates iodine from sodium iodide4 ,sulphuric acid1 in acting on an iodide1 sets hydriodic acid free1 but the latter easily decomposes1 especially in the presence of substances capable of evolving oxygen1 such as chromic acid1 nitrous acid1 and even ferric salts4#fa ,owing to its sparing solubility in water1 the iodine liberated separates as a precipitate4 ,to obtain pure iodine it is sufficient to distil it1 and neglect the first and last portions of the distillate1 the middle portion only being collected4 ,iodine passes directly from a state of vapour into a crystalline form1 and settles on the cool portions of the apparatus in tabular crystals1 having a black grey colour and metallic lustre4#fb #fa ,in general1 #b;,,hi ,o ,i#b ,h#b,o1 if the oxygen proceed from a substance from which it is easily evolved4 ,for this reason compounds corresponding with the higher stages of oxidation or chlorination frequently give a lower stage when treated with hydriodic acid4 ,ferric oxide1 ,fe#b,o#c1 is a higher oxide1 and ferrous oxide1 ,fe,o1 a lower oxide2 the former corresponds with ,fe,x#c1 and the latter with ,fe,x#b1 and this passage from the higher to the lower takes place under the action of hydriodic acid4 ,thus hydrogen peroxide and ozone 7,chapter ,,iv47 are able to liberate iodine from hydriodic acid4 ,compounds of copper oxide1 ,cu,o or ,cu,x#b1 give compounds of the suboxide ,cu#b,o1 or ,cu,x4 ,even sulphuric acid1 which corresponds to the higher stage ,,so#c1 is able to act thus1 forming the lower oxide ,,so#b4 ,the liberation of iodine from hydriodic acid proceeds with still greater ease under the action of substances capable of disengaging oxygen4 ,in practice1 many methods are employed for liberating iodine from acid liquids containing1 for example1 sulphuric acid and hydriodic acid4 ,the higher oxides of nitrogen are most commonly used2 they then pass into nitric oxide4 ,iodine may even be disengaged from hydriodic acid by the action of iodic acid1 c4 ,but there is a limit in these reactions of the oxidation of hydriodic acid because1 under certain conditions1 especially in dilute solutions1 the iodine set free is itself able to act as an oxidising agent--that is1 it exhibits the character of chlorine1 and of the halogens in general1 to which we shall again have occasion to refer4 ,in ,chili1 where a large quantity of iodine is extracted in the manufacture of ,chili nitre1 which contains ,na,i,o#c1 it is mixed with the acid and normal sulphites of sodium in solution2 the iodine is then precipitated according to the equation #b,na,i,o#c #c,na#b,,so#c #b,na,h,s,o#c #e,na#b,,so#d ,i#b ,h#b,o4 ,the iodine thus obtained is purified by sublimation4 #fb ,for the final purification of iodine1 ,stas dissolved it in a strong solution of potassium iodide1 and precipitated it by the addition of water 7see ,note #eh74 ,the specific gravity of the crystals of iodine is #die4 ,it melts at #aad and boils at #ahd4 ,its vapour is formed at a much lower temperature1 and is of a violet colour1 whence iodine receives its name 7,greek3 ioeids1 violet74 ,the smell of iodine recalls the characteristic smell of hypochlorous acid2 it has a sharp sour taste4 ,it destroys the skin and organs of the body1 and is therefore frequently employed for cauterising and as an irritant for the skin4 ,in small quantities it turns the skin brown1 but the coloration disappears after a certain time1 partly owing to the volatility of the iodine4 ,water dissolves only #a_/ejjj part of iodine4 ,a brown solution is thus obtained1 which bleaches1 but much more feebly than bromine and chlorine4 ,water which contains salts1 and especially iodides1 in solution dissolves iodine in considerable quantities1 and the resultant solution is of a dark brown colour4 ,pure alcohol dissolves a small amount of iodine1 and in so doing acquires a brown colour1 but the solubility of iodine is considerably increased by the presence of a small quantity of an iodine compound--for instance1 ethyl iodide--in the alcohol4#fc ,ether dissolves a larger amount of iodine than alcohol2 but iodine is particularly soluble in liquid hydrocarbons1 in carbon bisulphide1 and in chloroform4 ,a small quantity of iodine dissolved in carbon bisulphide tints it rose-colour1 but in a somewhat larger amount it gives a violet colour4 ,chloroform 7quite free from alcohol7 is also tinted rose colour by a small amount of iodine4 ,this gives an easy means for detecting the presence of free iodine in small quantities4 ,the blue coloration which free iodine gives with starch may also1 as has already been frequently mentioned 7see ,chapter ,,iv471 serve for the detection of iodine4 #fc ,the solubility of iodine in solutions containing iodides1 and compounds of iodine in general1 may serve1 on the one hand1 as an indication that solution is due to a similarity between the solvent and dissolved substance1 and1 on the other hand1 as an indirect proof of that view as to solutions which was cited in ,chapter ,i41 because in many instances unstable highly iodised compounds1 resembling crystallo-hydrates1 have been obtained from such solutions4 ,thus iodide of tetramethylammonium1 ,n7,,ch#c7d;,i1 combines with ,i#b1 and ,i#d4 ,even a solution of iodine in a saturated solution of potassium iodide presents indications of the formation of a definite compound ,,ki#c4 ,thus1 an alcoholic solution of ,,ki#c does not give up iodine to carbon bisulphide1 although this solvent takes up iodine from an alcoholic solution of iodine itself 7,girault1 ,jrgensen1 and others74 ,the instability of these compounds resembles the instability of many crystallo-hydrates1 for instance of ,h,cl1#b;,h#b,o4 ,if we compare the four elements1 fluorine1 chlorine1 bromine1 and iodine1 we see in them an example of analogous substances which arrange themselves by their physical properties in the same order as they stand in respect to their atomic and molecular weights4 ,if the weight of the molecule be large1 the substance has a higher specific gravity1 a higher melting and boiling point1 and a whole series of properties depending on this difference in its fundamental properties4 ,chlorine in a free state boils at about -#ce1 bromine boils at #fj1 and iodine only above #ahj4 ,according to ,avogadro-,gerhardt's law1 the vapour densities of these elements in a gaseous state are proportional to their atomic weights1 and here1 at all events approximately1 the densities in a liquid 7or solid7 state are also almost in the ratio of their atomic weights4 ,dividing the atomic weight of chlorine 7#cee7 by its specific gravity in a liquid state 7#ac71 we obtain a volume #bg1 for bromine 7#hj_/ca7 #bf1 and for iodine also 7#abg_/di7 #bf4fd #fd ,the equality of the atomic volumes of the halogens themselves is all the more remarkable because in all the halogen compounds the volume augments with the substitution of fluorine by chlorine1 bromine1 and iodine4 ,thus1 for example1 the volume of sodium fluoride 7obtained by dividing the weight expressed by its formula by its specific gravity7 is about #ae1 of sodium chloride #bg1 of sodium bromide #cb1 and of sodium iodide #da4 ,the volume of silicon chloroform1 ,si,h,cl#c1 is #hb1 and those of the corresponding bromine and iodine compounds are #ajh and #abb respectively4 ,the same difference also exists in solutions2 for example1 ,na,cl #bjj;,h#b,o has a sp4 gr4 7at #ae_/d7 of #ajajf1 consequently the volume of the solution #c1fehe_/ajajf #c1fbj1 hence the volume of sodium chloride in solution #c1fbj-c1fjc 7this is the volume of #bjj ,h#b,o7 #ag1 and in similar solutions1 ,na,br #bf and ,na,i #ce4 ,the metallic bromides and iodides are in the majority of cases1 in most respects analogous to the corresponding chlorides1#fe but chlorine displaces the bromine and iodine from them1 and bromine liberates iodine from iodides1 which is taken advantage of in the preparation of these halogens4 ,however1 the researches of ,potilitzin showed that a reverse displacement of chlorine by bromine may occur both in solutions and in ignited metallic chlorides in an atmosphere of bromine vapour--that is1 a distribution of the metal 7according to ,berthollet's doctrine7 takes place between the halogens1 although however the larger portion1 still unites with the chlorine1 which shows its greater affinity for metals as compared with that of bromine and iodine4#ff ,the latter1 however1 sometimes behave with respect to metallic oxides in exactly the same manner as chlorine4 ,gay-,lussac1 by igniting potassium carbonate in iodine vapour1 obtained 7as with chlorine7 an evolution of oxygen and carbonic anhydride1 ,k#b;,,co#c ,i#b #b,,ki ,,co#b ,o1 only the reactions between the halogens and oxygen are more easily reversible with bromine and iodine than with chlorine4 ,thus1 at a red heat oxygen displaces iodine from barium iodide4 ,aluminium iodide burns in a current of oxygen 7,deville and ,troost71 and a similar1 although not so clearly marked1 relation exists for aluminium chloride1 and shows that the halogens have a distinctly smaller affinity for those metals which only form feeble bases4 ,this is still more the case with the non-metals1 which form acids and evolve much more heat with oxygen than with the halogens 7,note #ac74 ,but in all these instances the affinity 7and amount of heat evolved7 of iodine and bromine is less than that of chlorine1 probably because the atomic weights are greater4 ,the smaller store of energy in iodine and bromine is seen still more clearly in the relation of the halogens to hydrogen4 ,in a gaseous state they all enter1 with more or less ease1 into direct combination with gaseous hydrogen--for example1 in the presence of spongy platinum1 forming halogen acids1 ,,hx--but the latter are far from being equally stable2 hydrogen chloride is the most stable1 hydrogen iodide the least so1 and hydrogen bromide occupies an intermediate position4 ,a very high temperature is required to decompose hydrogen chloride even partially1 whilst hydrogen iodide is decomposed by light even at the ordinary temperature and very easily by a red heat4 ,hence the reaction ,i#b ,h#b ,,hi ,,hi is very easily reversible1 and consequently has a limit1 and hydrogen iodide easily dissociates4#fg ,judging by the direct measurement of the heat evolved 7#bb1jjj heat units7 in the formation of ,h,cl1 the conversion of #b;,h,cl into ,h#b ,cl#b requires the expenditure of #dd1jjj heat units4 ,the decomposition of #b;,h,br into ,h#b ,br#b only requires1 if the bromine be obtained in a gaseous state1 a consumption of about #bd1jjj units1 whilst in the decomposition of #b;,,hi into ,h#b ,i#b as vapour about #c1jjj heat units are evolved2#fh these facts1 without doubt1 stand in causal connection with the great stability of hydrogen chloride1 the easy decomposability of hydrogen iodide1 and the intermediate properties of hydrogen bromide4 ,from this it would be expected that chlorine is capable of decomposing water with the evolution of oxygen1 whilst iodine has not the energy to produce this disengagement1#fi although it is able to liberate the oxygen from the oxides of potassium and sodium1 the affinity of these metals for the halogens being very considerable4 ,for this reason oxygen1 especially in compounds from which it can be evolved readily 7for instance1 ,cl,h,o1 ,cr,o#c1 c471 easily decomposes hydrogen iodide4 ,a mixture of hydrogen iodide and oxygen burns in the presence of an ignited substance1 forming water and iodine4 ,drops of nitric acid in an atmosphere of hydrogen iodide cause the disengagement of violet fumes of iodine and brown fumes of nitric peroxide4 ,in the presence of alkalis and an excess of water1 however1 iodine is able to effect oxidation like chlorine--that is1 it decomposes water2 the action is here aided by the affinity of hydrogen iodide for the alkali and water1 just as sulphuric acid helps zinc to decompose water4 ,but the relative instability of hydriodic acid is best seen in comparing the acids in a gaseous state4 ,if the halogen acids be dissolved in water1 they evolve so much heat that they approach much nearer to each other in properties4 ,this is seen from thermochemical data1 for in the formation of ,,hx in solution 7in a large excess of water7 from the gaseous elements there is evolved for ,h,cl #ci1jjj1 for ,h,br #cb1jjj1 and for ,,hi #ah1jjj heat units4#gj ,but it is especially evident from the fact that solutions of hydrogen bromide and iodide in water have many points in common with solutions of hydrogen chloride1 both in their capacity to form hydrates and fuming solutions of constant boiling point1 and in their capacity to form haloid salts1 c4 by reacting on bases4 #fe ,but the density 7and also molecular volume1 ,note #fd7 of a bromine compound is always greater than that of a chlorine compound1 whilst that of an iodine compound is still greater4 ,the order is the same in many other respects4 ,for example1 an iodine compound has a higher boiling point than a bromine compound1 c4 #ff ,a4 ,l4 ,potilitzin showed that in heating various metallic chlorides in a closed tube1 with an equivalent quantity of bromine1 a distribution of the metal between the halogens always occurs1 and that the amounts of chlorine replaced by the bromine in the ultimate product are proportional to the atomic weights of the metals taken and inversely proportional to their equivalence4 ,thus1 if ,na,cl ,br be taken1 then out of #ajj parts of chlorine1 #eed are replaced by the bromine1 whilst with ,ag,cl ,br #bgbh parts are replaced4 ,these figures are in the ratio #a 3 #di1 and the atomic weights ,na 3 ,ag #a 3 #dg4 ,in general terms1 if a chloride ,m,cln be taken1 it gives with n,br a percentage substitution #d,m_/n#b where ,m is the atomic weight of the metal4 ,this law was deduced from observations on the chlorides of ,li1 ,k1 ,na1 ,ag 7n #a71 ,ca1 ,sr1 ,ba1 ,co1 ,ni1 ,hg1 ,pb 7n #b71 ,bi 7n #c71 ,sn 7n #d71 and ,fe#b 7n #f74 ,in these determinations of ,potilitzin we see not only a brilliant confirmation of ,berthollet's doctrine1 but also the first effort to directly determine the affinities of elements by means of displacement4 ,the chief object of these researches consisted in proving whether a displacement occurs in those cases where heat is absorbed1 and in this instance it should be absorbed1 because the formation of all metallic bromides is attended with the evolution of less heat than that of the chlorides1 as is seen by the figures given in ,note #ee4 ,if the mass of the bromine be increased1 then the amount of chlorine displaced also increases4 ,for example1 if masses of bromine of #a and #d equivalents act on a molecule of sodium chloride1 then the percentages of the chlorine displaced will be #fjh p4c4 and #abdf p4c42 in the action of #a1 #d1 #be1 and #ajj molecules of bromine on a molecule of barium chloride1 there will be displaced #gh1 #agf1 #cej1 and #dej p4c4 of chlorine4 ,if an equivalent quantity of hydrochloric acid act on metallic bromides in closed tubes1 and in the absence of water at a temperature of #cjj1 then the percentages of the substitution of the bromine by the chlorine in the double decomposition taking place between univalent metals are inversely proportional to their atomic weights4 ,for example1 ,na,br ,h,cl gives at the limit #ba p4c4 of displacement1 ,k,cl #ab p4c4 and ,ag,cl #d-a_/d p4c4 ,essentially the same action takes place in an aqueous solution1 although the phenomenon is complicated by the participation of the water4 ,the reactions proceed spontaneously in one or the other direction at the ordinary temperature but at different rates4 ,in the action of a dilute solution 7#a equivalent per #e litres7 of sodium chloride on silver bromide at the ordinary temperature the amount of bromine replaced in six and a half days is #bjg p4c41 and with potassium chloride #ae p4c4 ,with an excess of the chloride the magnitude of the substitution increases4 ,these conversions also proceed with the absorption of heat4 ,the reverse reactions evolving heat proceed incomparably more rapidly1 but also to a certain limit2 for example1 in the reaction ,ag,cl ,r,br the following percentages of silver bromide are formed in different times3 hours #b #c #bb #if #abj ,k #gihb #hgd #hhbb -- #idba ,na #hcfc #ijgd #iagj #iedi -- ,that is1 the conversions which are accompanied by an evolution of heat proceed with very much greater rapidity than the reverse conversions4 #fg ,the dissociation of hydriodic acid has been studied in detail by ,hautefeuille and ,lemoine1 from whose researches we extract the following information4 ,the decomposition of hydriodic acid is decided1 but proceeds slowly at #ahj2 the rate and limit of decomposition increase with a rise of temperature4 ,the reverse action--that is1 ,i#b ,h#b #b;,,hi--proceeds not only under the influence of spongy platinum 7,corenwinder71 which also accelerates the decomposition of hydriodic acid1 but also by itself1 although slowly4 ,the limit of the reverse reaction remains the same with or without spongy platinum4 ,an increase of pressure has a very powerful accelerative effect on the rate of formation of hydriodic acid1 and therefore spongy platinum by condensing gases has the same effect as increase of pressure4 ,at the atmospheric pressure the decomposition of hydriodic acid reaches the limit at #bej in several months1 and at #ddj in several hours4 ,the limit at #bej is about #ah p4c4 of decomposition--that is1 out of #ajj parts of hydrogen previously combined in hydriodic acid1 about #ah p4c4 may be disengaged at this temperature 7this hydrogen may be easily measured1 and the measure of dissociation determined71 but not more2 the limit at #ddj is about #bf p4c4 ,if the pressure under which #b;,,hi passes into ,h#b ,i#b be #d-a_/b atmospheres1 then the limit is #bd p4c42 under a pressure of #a_/e atmosphere the limit is #bi p4c4 ,the small influence of pressure on the dissociation of hydriodic acid 7compared with ,n#b,o#d1 ,chapter ,,vi4 ,note #df7 is due to the fact that the reaction #b;,,hi ,i#b ,h#b is not accompanied by a change of volume4 ,in order to show the influence of time1 we will cite the following figures referring to #cej3 7#a7 ,reaction ,h#b ,i#b2 after #c hours1 #hh p4c4 of hydrogen remained free2 #h hours1 #fi p4c42 #cd hours1 #dh p4c42 #gf hours1 #bi p4c42 and #cbg hours1 #ahe p4c4 7#b7 ,the reverse decomposition of #b;,,hi2 after #i hours1 #c p4c4 of hydrogen was set free1 and after #bej hours #ahf p4c4--that is1 the limit was reached4 ,the addition of extraneous hydrogen diminishes the limit of the reaction of decomposition1 or increases the formation of hydriodic acid from iodine and hydrogen1 as would be expected from ,berthollet's doctrine 7,chapter ,x474 ,thus at #ddj #bf p4c4 of hydriodic acid is decomposed if there be no admixture of hydrogen1 while if ,h#b be added1 then at the limit only half as large a mass of ,,hi is decomposed4 ,therefore1 if an infinite mass of hydrogen be added there will be no decomposition of the hydriodic acid4 ,light aids the decomposition of hydriodic acid very powerfully4 ,at the ordinary temperature #hj p4c4 is decomposed under the influence of light1 whilst under the influence of heat alone this limit corresponds with a very high temperature4 ,the distinct action of light1 spongy platinum1 and of impurities in glass 7especially of sodium sulphate1 which decomposes hydriodic acid71 not only render the investigations difficult1 but also show that in reactions like #b;,,hi ,i#b ,h#b1 which are accompanied by slight heat effects1 all foreign and feeble influences may strongly affect the progress of the action 7,note #dg74 #fh ,the thermal determinations of ,thomsen 7at #ah7 gave in thousands of calories1 ,cl ,h #bb1 ,h,cl ,aq 7that is1 on dissolving ,h,cl in a large amount of water7 #agc1 and therefore ,h ,cl ,aq #cic4 ,in taking molecules1 all these figures must be doubled4 ,br ,h #hd2 ,h,br ,aq #aii2 ,h ,br ,aq #bhc4 ,according to ,berthelot #gb are required for the vaporisation of ,br#b1 hence ,br#b ,h#b #afh #gb #bd1 if ,br#b be taken as vapour for comparison with ,cl#b4 ,h ,i -#fj1 ,,hi ,aq #aib2 ,h ,i ,aq #acb1 and1 according to ,berthelot1 the heat of fusion of ,i#b #cj1 and of vaporisation #fj thousand heat units1 and therefore ,i#b ,h#b -#b7fj7 #c #f -#cj1 if the iodine be taken as vapour4 ,berthelot1 on the basis of his determinations1 gives1 however1 #jh thousand heat units4 ,similar contradictory results are often met with in thermochemistry owing to the imperfection of the existing methods1 and particularly the necessity of depending on indirect methods for obtaining the fundamental figures4 ,thus ,thomsen decomposed a dilute solution of potassium iodide by gaseous chlorine2 the reaction gave #bfb1 whence1 having first determined the heat effects of the reactions ,,kho ,h,cl1 ,,kho ,,hi and ,cl ,h in aqueous solutions1 it was possible to find ,h ,i ,aq2 then1 knowing ,,hi ,aq1 to find ,i ,h4 ,it is evident that unavoidable errors may accumulate4 #fi ,one can believe1 however1 on the basis of ,berthollet's doctrine1 and the observations of ,potilitzin 7,note #ff71 that a certain slow decomposition of water by iodine takes place4 ,on this view the observations of ,dossios and ,weith on the fact that the solubility of iodine in water increases after the lapse of several months will be comprehensible4 ,hydriodic acid is then formed1 and it increases the solubility4 ,if the iodine be extracted from such a solution by carbon bisulphide1 then1 as the authors showed1 after the action of nitrous anhydride iodine may be again detected in the solution by means of starch4 ,it can easily be understood that a number of similar reactions1 requiring much time and taking place in small quantities1 have up to now eluded the attention of investigators1 who even still doubt the universal application of ,berthollet's doctrine1 or only see the thermochemical side of reactions1 or else neglect to pay attention to the element of time and the influence of mass4 #gj ,on the basis of the data in ,note #fh4 ,in consequence of what has been said above1 it follows that hydrobromic and hydriodic acids1 being substances which are but slightly stable1 cannot be evolved in a gaseous state under many of those conditions under which hydrochloric acid is formed4 ,thus if sulphuric acid in solution acts on sodium iodide1 all the same phenomena take place as with sodium chloride 7a portion of the sodium iodide gives hydriodic acid1 and all remains in solution71 but if sodium iodide be mixed with strong sulphuric acid1 then the oxygen of the latter decomposes the hydriodic acid set free1 with liberation of iodine1 ,h#b,,so#d #b;,,hi #b;,h#b,o ,,so#b ,i#b4 ,this reaction takes place in the reverse direction in the presence of a large quantity of water 7#b1jjj parts of water per #a part of ,,so#b71 in which case not only the affinity of hydriodic acid for water is brought to light but also the action of water in directing chemical reactions in which it participates4#ga ,therefore1 with a halogen salt1 it is easy to obtain gaseous hydrochloric acid by the action of sulphuric acid1 but neither hydrobromic nor hydriodic acid can be so obtained in the free state 7as gases74#gb ,other methods have to be resorted to for their preparation1 and recourse must not be had to compounds of oxygen1 which are so easily able to destroy these acids4 ,therefore hydrogen sulphide1 phosphorus1 c41 which themselves easily take up oxygen1 are introduced as means for the conversion of bromine and iodine into hydrobromic and hydriodic acids in the presence of water4 ,for example1 in the action of phosphorus the essence of the matter is that the oxygen of the water goes to the phosphorus1 and the union of the remaining elements leads to the formation of hydrobromic or hydriodic acid2 but the matter is complicated by the reversibility of the reaction1 the affinity for water1 and other circumstances which are understood by following ,berthollet's doctrine4 ,chlorine 7and bromine also7 directly decomposes hydrogen sulphide1 forming hydrochloric acid and liberating sulphur1 both in a gaseous form and in solutions1 whilst iodine only decomposes hydrogen sulphide in weak solutions1 when its affinity for hydrogen is aided by the affinity of hydrogen iodide for water4 ,in a gaseous state iodine does not act on hydrogen sulphide1#gc whilst sulphur is able to decompose gaseous hydriodic acid1 forming hydrogen sulphide and a compound of sulphur and iodine which with water forms hydriodic acid4#gd #ga ,a number of similar cases confirm what has been said in ,chapter ,x4 #gb ,this is prevented by the reducibility of sulphuric acid4 ,if volatile acids be taken they pass over1 together with the hydrobromic and hydriodic acids1 when distilled2 whilst many non-volatile acids which are not reduced by hydrobromic and hydriodic acids only act feebly 7like phosphoric acid71 or do not act at all 7like boric acid74 #gc ,this is in agreement with the thermochemical data1 because if all the substances be taken in the gaseous state 7for sulphur the heat of fusion is #jc1 and the heat of vaporisation #bc7 we have ,h#b ,s #dg2 ,h#b ,cl#b #dd2 ,h#b ,br#b #bd1 and ,h#b ,i#b -#c thousand heat units2 hence the formation of ,h#b,s gives less heat than that of ,h,cl and ,h,br1 but more than that of ,,hi4 ,in dilute solutions ,h#b ,s ,aq #ic1 and consequently less than the formation of all the halogen acids1 as ,h#b,s evolves but little heat with water1 and therefore in dilute solutions chlorine1 bromine1 and iodine decompose hydrogen sulphide4 #gd ,here there are three elements1 hydrogen1 sulphur1 and iodine1 each pair of which is able to form a compound1 ,,hi1 ,h#b,s1 and ,,si1 besides which the latter may unite in various proportions4 ,the complexity of chemical mechanics is seen in such examples as these4 ,it is evident that only the study of the simplest cases can give the key to the more complex problems1 and on the other hand it is evident from the examples cited in the last pages that1 without penetrating into the conditions of chemical equilibria1 it would be impossible to explain chemical phenomena4 ,by following the footsteps of ,berthollet the possibility of unravelling the problems will be reached2 but work in this direction has only been begun during the last ten years1 and much remains to be done in collecting experimental material1 for which occasions present themselves at every step4 ,in speaking of the halogens ,i wished to turn the reader's attention to problems of this kind4 ,if hydrogen sulphide be passed through water containing iodine1 the reaction ,h#b,s ,i#b #b;,,hi ,s proceeds so long as the solution is dilute1 but when the mass of free ,,hi increases the reaction stops1 because the iodine then passes into solution4 ,a solution having a composition approximating to #b;,,hi #d;,i#b #i;,h#b,o 7according to ,bineau7 does not react with ,h#b,s1 notwithstanding the quantity of free iodine4 ,therefore only weak solutions of hydriodic acid can be obtained by passing hydrogen sulphide into water with iodine4#gd bis #gd bis ,the same essentially takes place when sulphurous anhydride1 in a dilute solution1 gives hydriodic acid and sulphuric acid with iodine4 ,on concentration a reverse reaction takes place4 ,the equilibrated systems and the part played by water are everywhere distinctly seen4 ,to obtain#ge gaseous hydrobromic and hydriodic acids it is most convenient to take advantage of the reactions between phosphorus1 the halogens1 and water1 the latter being present in small quantity 7otherwise the halogen acids formed are dissolved by it72 the halogen is gradually added to the phosphorus moistened with water4 ,thus if red phosphorus be placed in a flask and moistened with water1 and bromine be added drop by drop 7from a tap funnel71 hydrobromic acid is abundantly and uniformly disengaged4#gf ,hydrogen iodide is prepared by adding #a part of common 7yellow7 dry phosphorus to #aj parts of dry iodine in a glass flask4 ,on shaking the flask1 union proceeds quietly between them 7light and heat being evolved71 and when the mass of iodide of phosphorus which is formed has cooled1 water is added drop by drop 7from a tap funnel7 and hydrogen iodide is evolved directly without the aid of heat4 ,these methods of preparation will be at once understood when it is remembered 7p4 #dfh7 that phosphorus chloride gives hydrogen chloride with water4 ,it is exactly the same here--the oxygen of the water passes over to the phosphorus1 and the hydrogen to the iodine1 thus1 ,,pi#c #c;,h#b,o ,,ph#c,o#c #c;,,hi4#gg #ge ,methods of formation and preparation are nothing more than particular cases of chemical reaction4 ,if the knowledge of chemical mechanics were more exact and complete than it now is it would be possible to foretell all cases of preparation with every detail 7of the quantity of water1 temperature1 pressure1 mass1 c47 ,the study of practical methods of preparation is therefore one of the paths for the study of chemical mechanics4 ,the reaction of iodine on phosphorus and water is a case like that mentioned in ,note #gd1 and the matter is here further complicated by the possibility of the formation of the compound ,,ph#c with ,,hi1 as well as the production of ,,pi#b1 ,,pi#c1 and the affinity of hydriodic acid and the acids of phosphorus for water4 ,the theoretical interest of equilibria in all their complexity is naturally very great1 but it falls into the background in presence of the primary interest of discovering practical methods for the isolation of substances1 and the means of employing them for the requirements of man4 ,it is only after the satisfaction of these requirements that interests of the other order arise1 which in their turn must exert an influence on the former4 ,for these reasons1 whilst considering it opportune to point out the theoretical interest of chemical equilibria1 the chief attention of the reader is directed in this work to questions of practical importance4 #gf ,hydrobromic acid is also obtained by the action of bromine on paraffin heated to #ahj4 ,gustavson proposed to prepare it by the action of bromine 7best added in drops together with traces of aluminium bromide7 on anthracene 7a solid hydrocarbon from coal tar74 ,balard prepared it by passing bromine vapour over moist pieces of common phosphorus4 ,the liquid tribromide of phosphorus1 directly obtained from phosphorus and bromine1 also gives hydrobromic acid when treated with water4 ,bromide of potassium or sodium1 when treated with sulphuric acid in the presence of phosphorus1 also gives hydrobromic acid1 but hydriodic acid is decomposed by this method4 ,in order to free hydrobromic acid from bromine vapour it is passed over moist phosphorus and dried either by phosphoric anhydride or calcium bromide 7calcium chloride cannot be used1 as hydrochloric acid would be formed74 ,neither hydrobromic nor hydriodic acids can be collected over mercury1 on which they act1 but they may be directly collected in a dry vessel by leading the gas-conducting tube to the bottom of the vessel1 both gases being much heavier than air4 ,merz and ,holtzmann 7#ahhi7 proposed to prepare ,h,br directly from bromine and hydrogen4 ,for this purpose pure dry hydrogen is passed through a flask containing boiling bromine4 ,the mixture of gas and vapour then passes through a tube provided with one or two bulbs1 which is heated moderately in the middle4 ,hydrobromic acid is formed with a series of flashes at the part heated4 ,the resultant ,h,br1 together with traces of bromine1 passes into a ,woulfe's bottle into which hydrogen is also introduced1 and the mixture is then carried through another heated tube1 after which it is passed through water which dissolves the hydrobromic acid4 ,according to the method proposed by ,newth 7#ahib7 a mixture of bromine and hydrogen is led through a tube containing a platinum spiral1 which is heated to redness after the air has been displaced from the tube4 ,if the vessel containing the bromine be kept at #fj1 the hydrogen takes up almost the theoretical amount of bromine required for the formation of ,h,br4 ,although the flame which appears in the neighbourhood of the platinum spiral does not penetrate into the vessel containing the bromine1 still1 for safety1 a tube filled with cotton wool may be interposed4 ,hydroiodic acid is obtained in the same manner as hydrobromic4 ,the iodine is heated in a small flask1 and its vapour is carried over by hydrogen into a strongly heated tube1 ,the gas passing from the tube is found to contain a considerable amount of ,,hi1 together with some free iodine4 ,at a low red heat about #ag p4c4 of the iodine vapour enters into combination2 at a higher temperature1 #gh p4c4 to #gi p4c42 and at a strong heat about #hb p4c4 #gg ,but generally more phosphorus is taken than is required for the formation of ,,pi#c1 because otherwise a portion of the iodine distils over4 ,if less than one-tenth part of iodine be taken1 much phosphonium iodide1 ,,ph#d;,i1 is formed4 ,this proportion was established by ,gay-,lussac and ,kolbe4 ,hydriodic acid is also prepared in many other ways4 ,bannoff dissolves two parts of iodine in one part of a previously prepared strong 7sp4 gr4 #afg7 solution of hydriodic acid1 and pours it on to red phosphorus in a retort4 ,personne takes a mixture of fifteen parts of water1 ten of iodine1 and one of red phosphorus1 which1 when heated1 disengages hydriodic acid mixed with iodine vapour2 the latter is removed by passing it over moist phosphorus 7,note #gf74 ,it must be remembered however that reverse reaction 7,oppenheim7 may take place between the hydriodic acid and phosphorus1 in which the compounds ,,ph#d;,i and ,,pi#b are formed4 ,it should be observed that the reaction between phosphorus1 iodine and water must be carried out in the above proportions and with caution1 as they may react with explosion4 ,with red phosphorus the reaction proceeds quietly1 but nevertheless requires care4 ,l4 ,meyer showed that with an excess of iodine the reaction proceeds without the formation of bye-products 7,,ph#d;,i71 according to the equation ,p #e;,i #d;,h#b,o ,,ph#c,o#d #e;,,hi4 ,for this purpose #ajj grams of iodine and #aj grams of water are placed in a retort1 and a paste of #e grams of red phosphorus and #aj grams of water is added little by little 7at first with great care74 ,the hydriodic acid may be obtained free from iodine by directing the neck of the retort upwards and causing the gas to pass through a shallow layer of water 7respecting the formation of ,,hi1 see also ,note #ge74 ,in a gaseous form hydrobromic and hydriodic acids are closely analogous to hydrochloric acid2 they are liquefied by pressure and cold1 they fume in the air1 form solutions and hydrates1 of constant boiling point1 and react on metals1 oxides and salts1 c4#gh ,only the relatively easy decomposability of hydrobromic acid and especially of hydriodic acid1 clearly distinguish these acids from hydrochloric acid4 ,for this reason1 hydriodic acid acts in a number of cases as a deoxidiser or reducer1 and frequently even serves as a means for the transference of hydrogen4 ,thus ,berthelot1 ,baeyer1 ,wreden1 and others1 by heating unsaturated hydrocarbons in a solution of hydriodic acid1 obtained their compounds with hydrogen nearer to the limit ,cn,h#bn #b or even the saturated compounds4 ,for example1 benzene1 ,c#f;,h#f1 when heated in a closed tube with a strong solution of hydriodic acid1 gives hexylene1 ,c#f;,h#ab4 ,the easy decomposability of hydriodic acid accounts for the fact that iodine does not act by metalepsis on hydrocarbons1 for the hydrogen iodide liberated with the product of metalepsis1 ,,ri1 formed1 gives iodine and the hydrogen compound1 ,,rh1 back again4 ,and therefore1 to obtain the products of iodine substitution1 either iodic acid1 ,,hio#c 7,kekul71 or mercury oxide1 ,hg,o 7,weselsky71 is added1 as they immediately react on the hydrogen iodide1 thus3 ,,hio#c #e;,,hi #c;,h#b,o #c;,i#b1 or1 ,hg,o #b;,,hi ,hg,i#b ,h#b,o4 ,from these considerations it will be readily understood that iodine acts like chlorine 7or bromine7 on ammonia and sodium hydroxide1 for in these cases the hydriodic acid produced forms ,,nh#d;,i and ,na,i4 ,with tincture of iodine or even the solid element1 a solution of ammonia immediately forms a highly-explosive solid black product of metalepsis1 ,,nhi#b1 generally known as iodide of nitrogen1 although it still contains hydrogen 7this was proved beyond doubt by ,szuhay #ahic71 which may be replaced by silver 7with the formation of ,n,ag,i#b73 #c,,nh#c #b;,i#b #b,,nh#d;,i ,,nhi#b4 ,however1 the composition of the last product is variable1 and with an excess of water ,,ni#c seems to be formed4 ,iodide of nitrogen is just as explosive as nitrogen chloride4#gh bis ,in the action of iodine on sodium hydroxide no bleaching compound is formed 7whilst bromine gives one71 but a direct reaction is always accomplished with the formation of an iodate1 #f,na,h,o #c;,i#b #e,na,i #c;,h#b,o ,na,i,o#c 7,gay-,lussac74 ,solutions of other alkalis1 and even a mixture of water and oxide of mercury1 act in the same manner4#gi ,this direct formation of iodic acid1 ,,hio#c ,,io#b7,,oh71 shows the propensity of iodine to give compounds of the type ,,ix#e4 ,indeed1 this capacity of iodine to form compounds of a high type emphasises itself in many ways4 ,but it is most important to turn attention to the fact that iodic acid is easily and directly formed by the action of oxidising substances on iodine4 ,thus1 for instance1 strong nitric acid directly converts iodine into iodic acid1 whilst it has no oxidising action on chlorine4#gi bis ,this shows a greater affinity in iodine for oxygen than in chlorine1 and this conclusion is confirmed by the fact that iodine displaces chlorine from its oxygen acids1#hj and that in the presence of water chlorine oxidises iodine4#ha ,even ozone or a silent discharge passed through a mixture of oxygen and iodine vapour is able to directly oxidise iodine#hb into iodic acid4 ,it is disengaged from solutions as a hydrate1 ,,hio#c1 which loses water at #agj1 and gives an anhydride1 ,i#b,o#e4 ,both these substances are crystalline 7sp4 gr4 ,i#b,o#e #ejcg1 ,,hio#c #dhfi at #j71 colourless and soluble in water2#hc both decompose at a red heat into iodine and oxygen1 are in many cases powerfully oxidising--for instance1 they oxidise sulphurous anhydride1 hydrogen sulphide1 carbonic oxide1 c4--form chloride of iodine and water with hydrochloric acid1 and with bases form salts1 not only normal ,,mio#c1 but also acid2 for example1 ,,kio#c;,,hio#c1 ,,kio#cb;,,hio#c4hc bis ,with hydriodic acid iodic acid immediately reacts1 disengaging iodine1 ,,hio#c #e;,,hi #c;,h#b,o #c;,i#b4 #gh ,the specific gravities of their solutions as deduced by me on the basis of ,topse and ,berthelot's determinations for #ae_/d are as follows3-- #aj #bj #cj #dj #ej #fj p4c4 ,h,br #ajga #aaef #abeh #acgd #aeje #afej ,,hi #ajge #aafd #abfg #acii #aefg #agfi ,hydrobromic acid forms two hydrates1 ,h,br1#b;,h#b,o and ,h,br1,h#b,o1 which have been studied by ,roozeboom with as much completeness as the hydrate of hydrochloric acid 7,chapter ,x4 ,note #cg74 ,with metallic silver1 solutions of hydriodic acid give hydrogen with great ease1 forming silver iodide4 ,mercury1 lead1 and other metals act in a similar manner4 #gh bis ,iodide of nitrogen1 ,,nhi#b is obtained as a brown pulverulent precipitate on adding a solution of iodine 7in alcohol1 for instance7 to a solution of ammonia4 ,if it be collected on a filter-paper1 it does not decompose so long as the precipitate is moist2 but when dry it explodes violently1 so that it can only be experimented upon in small quantities4 ,usually the filter-paper is torn into bits while moist1 and the pieces laid upon a brick2 on drying an explosion proceeds not only from friction or a blow1 but even spontaneously4 ,the more dilute the solution of ammonia1 the greater is the amount of iodine required for the formation of the precipitate of ,,nhi#b4 ,a low temperature facilitates its formation4 ,,nhi#b dissolves in ammonia water1 and when heated the solution forms ,,hio#c and iodine4 ,with ,,ki1 iodide of nitrogen gives iodine1 ,,nh#c and ,,kho4 ,these reactions 7,selivanoff7 are explained by the formation of ,,hio from ,,nhi#b #b;,h#b,o ,,nh#c #b;,,hio--and then ,,ki ,,hio ,i#b ,,kho4 ,selivanoff 7see ,note #bi7 usually observed a temporary formation of hypoiodous acid1 ,,hio1 in the reaction of ammonia upon iodine1 so that here the formation of ,,nhi#b is preceded by that of ,,hio--i4e4 first ,i#b ,h#b,o ,,hio ,,hi1 and then not only the ,,hi combines with ,,nh#c1 but also #b;,,hio ,,nh#c ,,nhi#b #b;,h#b,o4 ,with dilute sulphuric acid iodide of nitrogen 7like ,n,cl#c7 forms hypoiodous acid1 but it immediately passes into iodic acid1 as is expressed by the equation #e;,,hio #b;,i#b ,,hio#c #b;,h#b,o 7first #c;,,hio ,,hio#c #b;,,hi1 and then ,,hi ,,hio ,i#b ,h#b,o74 ,moreover1 ,selivanoff found that iodide of nitrogen1 ,,nhi#b1 dissolves in an excess of ammonia water1 and that with potassium iodide the solution gives the reaction for hypoiodous acid 7the evolution of iodine in an alkaline solution74 ,this shows that ,,hio participates in the formation and decomposition of ,,nhi#b1 and therefore the condition of the iodine 7its metaleptic position7 in them is analogous1 and differs from the condition of the halogens in the haloid-anhydrides 7for instance1 ,,no#b;,cl74 ,the latter are tolerably stable1 while 7the haloid being designated by ,x7 ,,nhx#b1 ,,nx#c1 ,,xoh1 ,,rxo 7see ,chapter ,,xiii4 ,note #dc71 c41 are unstable1 easily decomposed with the evolution of heat1 and1 under the action of water1 the haloid is easily replaced by hydrogen 7,selivanoff71 as would be expected in true products of metalepsis4 #gi ,hypoiodous acid1 ,,hio1 is not known1 but organic compounds1 ,,rio1 of this type are known4 ,to illustrate the peculiarities of their properties we will mention one of these compounds1 namely1 iodosobenzol1 ,c#f;,h#e;,,io4 ,this substance was obtained by ,willgerodt 7#ahib71 and also by ,v4 ,meyer1 ,wachter1 and ,askenasy1 by the action of caustic alkalis upon phenoldiiodochloride1 ,c#f;,h#e;,i,cl#b 7according to the equation1 ,c#f;,h#e;,i,cl#b #b,,moh ,c#f;,h#e;,,io #b,m,cl ,h#b,o74 ,iodosobenzol is an amorphous yellow substance1 whose melting point could not be determined because it explodes at #baj1 decomposing with the evolution of iodine vapour4 ,this substance dissolves in hot water and alcohol1 but is not soluble in the majority of other neutral organic solvents4 ,if acids do not oxidise ,c#f;,h#e;,,io1 they give saline compounds in which iodosobenzol appears as a basic oxide of a diatomic metal1 ,c#f;,h#e;,i4 ,thus1 for instance1 when an acetic acid solution of iodosobenzol is treated with a solution of nitric acid1 it gives large monoclinic crystals of a nitric acid salt having the composition ,c#f;,h#e;,i7,,no#c7b 7like ,ca7,,no#c7b74 ,in appearing as the analogue of basic oxides1 iodosobenzol displaces iodine from potassium iodide 7in a solution acidulated with acetic or hydrochloric acid7--i4e4 it acts with its oxygen like ,h,cl,o4 ,the action of peroxide of hydrogen1 chromic acid1 and other similar oxidising agents gives iodoxybenzol1 ,c#f;,h#e;,,io#b1 which is a neutral substance--i4e4 incapable of giving salts with acids 7compare ,chapter ,,xiii4 ,note #dc74 #gi bis ,the oxidation of iodine by strong nitric acid was discovered by ,connell2 ,millon showed that it is effected1 although more slowly1 by the action of the hydrates of nitric acid up to ,,hno#c1;,h#b,o1 but that the solution ,,hno#c1b;,h#b,o1 and weaker solutions1 do not oxidise1 but simply dissolve1 iodine4 ,the participation of water in reactions is seen in this instance4 ,it is also seen1 for example1 in the fact that dry ammonia combines directly with iodine--for instance1 at #j forming the compound ,i#b1d,,nh#c--whilst iodide of nitrogen is only formed in presence of water4 #hj ,bromine also displaces chlorine--for instance1 from chloric acid1 directly forming bromic acid4 ,if a solution of potassium chlorate be taken 7#ge parts per #djj parts of water71 and iodine be added to it 7#hj parts71 and then a small quantity of nitric acid1 chlorine is disengaged on boiling1 and potassium iodate is formed in the solution4 ,in this instance the nitric acid first evolves a certain portion of the chloric acid1 and the latter1 with the iodine1 evolves chlorine4 ,the iodic acid thus formed acts on a further quantity of the potassium chlorate1 sets a portion of the chloric acid free1 and in this manner the action is kept up4 ,potilitzin 7#ahhg7 remarked1 however1 that not only do bromine and iodine displace the chlorine from chloric acid and potassium chlorate1 but also chlorine displaces bromine from sodium bromate1 and1 furthermore1 the reaction does not proceed as a direct substitution of the halogens1 but is accompanied by the formation of free acids2 for example1 #e,na,cl,o#c #c;,br#b #c;,h#b,o #e,na,br #e;,h,cl,o#c ,h,br,o#c4 #ha ,if iodine be stirred up in water1 and chlorine passed through the mixture1 the iodine is dissolved2 the liquid becomes colourless1 and contains1 according to the relative amounts of water and chlorine1 either ,i,h,cl#b1 or ,i,cl#c1 or ,,hio#c4 ,if there be a small amount of water1 then the iodic acid may separate out directly as crystals1 but a complete conversion 7,bornemann7 only occurs when not less than ten parts of water are taken to one part of iodine--,i,cl #c;,h#b,o #b;,cl#b ,,iho#c #e;,h,cl4 #hb ,schnbein and ,ogier proved this4 ,ogier found that at #de ozone immediately oxidises iodine vapour1 forming first of all the oxide ,i#b,o#c1 which is decomposed by water or on heating into iodic anhydride and iodine4 ,iodic acid is formed at the positive pole when a solution of hydriodic acid is decomposed by a galvanic current 7,riche74 ,it is also formed in the combustion of hydrogen mixed with a small quantity of hydriodic acid 7,salet74 #hc ,kmmerer showed that a solution of sp4 gr4 #babg at #ad1 containing #b;,,hio#c1i;,h#b,o1 solidified completely in the cold4 ,on comparing solutions ,,hi m,h#b,o with ,,hio#c m,h#b,o1 we find that the specific gravity increases but the volume decreases1 whilst in the passage of solutions ,h,cl m,h#b,o to ,h,cl,o#c m,h#b,o both the specific gravity and the volume increase1 which is also observed in certain other cases 7for example1 ,h#c,,po#c and ,h#c,,po#d74 #hc bis ,ditte 7#ahij7 obtained many iodates of great variety4 ,a neutral salt1 #b7,li,i,o#c7;,h#b,o1 is obtained by saturating a solution of lithia with iodic acid4 ,there is an analogous ammonium salt1 #b7,,nh#d;,,io#c7;,h#b,o4 ,he also obtained hydrates of a more complex composition1 such as #f7,,nh#d;,,io#c7;,h#b,o and #f7,,nh#d;,,io#c7b;,h#b,o4 ,salts of the alkaline earths1 ,ba7,,io#c7b;,h#b,o and ,sr7,,io#c7b;,h#b,o1 may be obtained by a reaction of double decomposition from the normal salts of the type #b7,me,i,o#c7;,h#b,o4 ,when evaporated at #gj to #hj with nitric acid these salts lose water4 ,a mixture of solutions of nitrate of zinc and an alkaline iodate precipitates ,zn7,,io#c7bb;,h#b,o4 ,an anhydrous salt is thrown out if nitric acid be added to the solutions4 ,analogous salts of cadmium1 silver1 and copper give compounds of the type #b,me,,io#cd,,nh#c and ,me7,,io#c7bd,,nh#c1 with gaseous ammonia 7,me and ,me being elements of the first 7,ag7 and second 7,cd1 ,zn1 ,cu7 groups74 ,with an aqueous solution of ammonia the above salts give substances of a different composition1 such as ,zn7,,io#c7b7,,nh#d7b,o1 ,cd7,,io#c7b7,,nh#d7b,o4 ,copper gives ,cu7,,io#c7bd7,,nh#d7b,o and ,cu7,,io#c7b7,,nh#d7b,o4 ,these salts may be regarded as compounds of ,i#b,o#e1 and ,me,o and 7,,nh#d7b,o2 for example1 ,zn7,,io#c7b7,,nh#d7b,o may be regarded as ,zn,o7,,nh#d7b,,oi#b,o#e1 or1 as derived from the hydrate1 ,i#b,o#eb;,h#b,o #b7;,,hio#c7;,h#b,o4 ,as with chlorine1 so with iodine1 a periodic acid1 ,,hio#d1 is formed4 ,this acid is produced in the form of its salts1 by the action of chlorine on alkaline solutions of iodates1 and also by the action of iodine on chloric acid4#hd ,it crystallises from solutions as a hydrate containing #b;,h#b,o 7corresponding with ,h,cl,o#d1b;,h#b,o71 but as it forms salts containing up to #e atoms of metals1 this water must be counted as water of constitution4 ,therefore ,,io7,,oh7#e ,,hio#d1b;,h#b,o corresponds with the highest form of halogen compounds1 ,,ix#g4he ,in decomposing 7at #bjj7 or acting as an oxidiser1 periodic acid first gives iodic acid1 but it may also be ultimately decomposed4 #hd ,if sodium iodate be mixed with a solution of sodium hydroxide1 heated1 and chlorine passed through the solution1 a sparingly soluble salt separates out1 which corresponds with periodic acid1 and has the composition ,na#d;,i#b,o#i1c;,h#b,o4 #f,na,h,o #b,na,i,o#c #d;,cl #d,na,cl ,na#d;,i#b,o#i #c;,h#b,o4 ,this compound is sparingly soluble in water1 but dissolves easily in a very dilute solution of nitric acid4 ,if silver nitrate be added to this solution a precipitate is formed which contains the corresponding compound of silver1 ,ag#d;,i#b,o#i1c;,h#b,o4 ,if this sparingly soluble silver compound be dissolved in hot nitric acid1 orange crystals of a salt having the composition ,ag,i,o#d separate on evaporation4 ,this salt is formed from the preceding by the nitric acid taking up silver oxide--,ag#d;,i#b,o#i #b;,,hno#c #b;,ag,n,o#c #b;,ag,i,o#d ,h#b,o4 ,the silver salt is decomposed by water1 with the re-formation of the preceding salt1 whilst iodic acid remains in solution-- #d;,ag,i,o#d ,h#b,o ,ag#d;,i#b,o#i #b;,,hio#d4 ,the structure of the first of these salts1 ,na#d;,i#b,o#i1c;,h#b,o1 presents itself in a simpler form if the water of crystallisation is regarded as an integral portion of the salt2 the formula is then divided in two1 and takes the form of ,,io7,,oh7#c7,o,na7#b--that is1 it answers to the type ,,iox#e1 or ,,ix#g1 like ,ag,i,o#d which is ,,io#c7,o,ag74 ,the composition of all the salts of periodic acids are expressed by this type ,,ix#g4 ,kimmins 7#ahhi7 refers all the salts of periodic acid to four types--the meta-salts of ,,hio#d 7salts of ,ag1 ,cu1 ,pb71 the meso-salts of ,h#c;,,io#e 7,pb,h1 ,ag#b;,h1 ,cd,h71 the para-salts of ,h#e;,,io#f 7,na#b;,h#c1 ,na#c;,h#b71 and the di-salts of ,h#d;,i#b,o#i 7,k#d1 ,ag#d1 ,ni#b74 ,the three first are direct compounds of the type ,,ix#g1 namely1 ,,io#c7,,oh71 ,,io#b7,,oh7#c1 and ,,io7,,oh7#e1 and the last are types of diperiodic salts1 which correspond with the type of the meso-salts1 as pyrophosphoric salts correspond with orthophosphoric salts--i4e4 #b;,h#c;,,io#e-;,h#b,o ,h#d;,i#b,o#i4 #he ,periodic acid1 discovered by ,magnus and ,ammermller1 and whose salts were afterwards studied by ,langlois1 ,rammelsberg1 and many others1 presents an example of hydrates in which it is evident that there is not that distinction between the water of hydration and of crystallisation which was at first considered to be so clear4 ,in ,h,cl,o1#b;,h#b,o the water1 #b;,h#b,o1 is not displaced by bases1 and must be regarded as water of crystallisation1 whilst in ,,hio#d1b;,h#b,o it must be regarded as water of hydration4 ,we shall afterwards see that the system of the elements obliges us to consider the halogens as substances giving a highest saline type1 ,,gx#g1 where ,g signifies a halogen1 and ,x oxygen 7,o ,x#b71 ,,oh1 and other like elements4 ,the hydrate ,,io7,,oh7#e corresponding with many of the salts of periodic acid 7for example1 the salts of barium1 strontium1 mercury7 does not exhaust all the possible forms4 ,it is evident that various other pyro-1 meta-1 c41 forms are possible by the loss of water1 as will be more fully explained in speaking of phosphoric acid1 and as was pointed out in the preceding note4 ,compounds formed between chlorine and iodine must be classed among the most interesting halogen bodies4#hf ,these elements combine together directly with evolution of heat1 and form iodine monochloride1 ,i,cl1 or iodine trichloride1 ,i,cl#c4hg ,as water reacts on these substances1 forming iodic acid and iodine1 they have to be prepared from dry iodine and chlorine4#hh ,both substances are formed in a number of reactions2 for example1 by the action of aqua regia on iodine1 of chlorine on hydriodic acid1 of hydrochloric acid on periodic acid1 of iodine on potassium chlorate 7with the aid of heat1 c47 ,trapp obtained iodine monochloride1 in beautiful red crystals1 by passing a rapid current of chlorine into molten iodine4 ,the monochloride then distils over and solidifies1 melting at #bg4 ,by passing chlorine over the crystals of the monochloride1 it is easy to obtain iodine trichloride in orange crystals1 which melt at #cd and volatilise at #dg1 but in so doing decompose 7into ,cl#b and ,cl,i74 ,the chemical properties of these chlorides entirely resemble those of chlorine and iodine1 as would be expected1 because1 in this instance1 a combination of similar substances has taken place as in the formation of solutions or alloys4 ,thus1 for instance1 the unsaturated hydrocarbons 7for example1 ,c#b;,h#d71 which are capable of directly combining with chlorine and iodine1 also directly combine with iodine monochloride4 #hf ,with respect to hydrogen1 oxygen1 chlorine1 and other elements1 bromine occupies an intermediate position between chlorine and iodine1 and therefore there is no particular need for considering at length the compounds of bromine4 ,this is the great advantage of a natural grouping of the elements4 #hg ,they were both obtained by ,gay-,lussac and many others4 ,recent data respecting iodine monochloride1 ,i,cl1 entirely confirm the numerous observations of ,trapp 7#ahed71 and even confirm his statement as to the existence of two isomeric 7liquid and crystalline7 forms 7,stortenbeker74 ,with a small excess of iodine1 iodine monochloride remains liquid1 but in the presence of traces of iodine trichloride it easily crystallises4 ,tanatar 7#ahic7 showed that of the two modifications of ,i,cl1 one is stable1 and melts at #bg2 while the other1 which easily passes into the first1 and is formed in the absence of ,i,cl#c1 melts at #ad4 ,schtzenberger amplified the data concerning the action of water on the chlorides 7,note #hh71 and ,christomanos gave the fullest data regarding the trichloride4 ,after being kept for some time1 the liquid monochloride of iodine yields red deliquescent octahedra1 having the composition ,i,cl#d1 which are therefore formed from the monochloride with the liberation of free iodine1 which dissolves in the remaining quantity of the monochloride4 ,this substance1 however1 judging by certain observations1 is impure iodine trichloride4 ,if #a part of iodine be stirred up in #bj parts of water1 and chlorine be passed through the liquid1 then all the iodine is dissolved1 and a colourless liquid is ultimately obtained which contains a certain proportion of chlorine1 because this compound gives a metallic chloride and iodate with alkalis without evolving any free iodine3 ,i,cl#e #f,,kho #e,k,cl ,,kio#c #c;,h#b,o4 ,the existence of a pentachloride ,i,cl#e is1 however1 denied1 because this substance has not been obtained in a free state4 ,stortenbeker 7#ahhh7 investigated the equilibrium of the system containing the molecules ,i#b1 ,i,cl1 ,i,cl#c1 and ,cl#b1 in the same way that ,roozeboom 7,chapter ,x4 ,note #ch7 examined the equilibrium of the molecules ,h,cl1 ,h,cl1#b;,h#b,o1 and ,h#b,o4 ,he found that iodine monochloride appears in two states1 one 7the ordinary7 is stable and melts at #bgb1 whilst the other is obtained by rapid cooling1 and melts at #aci1 and easily passes into the first form4 ,iodine trichloride melts at #aja only in a closed tube under a pressure of #af atmospheres4 #hh ,by the action of water on iodine monochloride and trichloride a compound ,i,h,cl#b is obtained1 which does not seem to be altered by water4 ,besides this compound1 iodine and iodic acid are always formed1 #aj;,i,cl #c;,h#b,o ,,hio#c #e;,i,h,cl#b #b;,i#b2 and in this respect iodine trichloride may be regarded as a mixture1 ,i,cl ,i,cl#e #b;,i,cl#c1 but ,i,cl#e #c;,h#b,o ,,iho#c #e;,h,cl2 hence iodic acid1 iodine1 the compound ,i,h,cl#b1 and hydrochloric acid are also formed by the action of water4 ,,chapterr ,,xii ,,sodium ,the neutral salt1 sodium sulphate1 ,na#b,,so#d1 obtained when a mixture of sulphuric acid and common salt is strongly heated 7,chapter ,x471#a forms a colourless saline mass consisting of fine crystals1 soluble in water4 ,it is the product of many other double decompositions1 sometimes carried out on a large scale2 for example1 when ammonium sulphate is heated with common salt1 in which case the sal-ammoniac is volatilised1 c4 ,a similar decomposition also takes place when1 for instance1 a mixture of lead sulphate and common salt is heated2 this mixture easily fuses1 and if the temperature be further raised heavy vapours of lead chloride appear4 ,when the disengagement of these vapours ceases1 the remaining mass1 on being treated with water1 yields a solution of sodium sulphate mixed with a solution of undecomposed common salt4 ,a considerable quantity1 however1 of the lead sulphate remains unchanged during this reaction1 ,pb,s,o#d #b,na,cl ,pb,cl#b ,na#b,,so#d1 the vapours will contain lead chloride1 and the residue will contain the mixture of the three remaining salts4 ,the cause and nature of the reaction are just the same as were pointed out when considering the action of sulphuric acid upon ,na,cl4 ,here too it may be shown that the double decomposition is determined by the removal of ,pb,cl#b from the sphere of the action of the remaining substances4 ,this is seen from the fact that sodium sulphate1 on being dissolved in water and mixed with a solution of any lead salt 7and even with a solution of lead chloride1 although this latter is but sparingly soluble in water71 immediately gives a white precipitate of lead sulphate4 ,in this case the lead takes up the elements of sulphuric acid from the sodium sulphate in the solutions4 ,on heating1 the reverse phenomenon is observed4 ,the reaction in the solution depends upon the insolubility of the lead sulphate1 and the decomposition which takes place on heating is due to the volatility of the lead chloride4 ,silver sulphate1 ,ag#b,,so#d1 in solution with common salt1 gives silver chloride1 because the latter is insoluble in water1 ,ag#b,,so#d #b,na,cl ,na#b,,so#d #b;,ag,cl4 ,sodium carbonate1 mixed in solution with the sulphates of iron1 copper1 manganese1 magnesium1 c41 gives in solution sodium sulphate1 and in the precipitate a carbonate of the corresponding metal1 because these salts of carbonic acid are insoluble in water2 for instance1 ,mg,s,o#d ,na#b;,,co#c ,na#b,,so#d ,mg,c,o#c4 ,in precisely the same way sodium hydroxide acts on solutions of the majority of the salts of sulphuric acid containing metals1 the hydroxides of which are insoluble in water--for instance1 ,cu,s,o#d #b,na,h,o ,cu7,,ho7#b ,na#b,,so#d4 ,sulphate of magnesium1 ,mg,s,o#d1 on being mixed in solution with common salt1 forms1 although not completely1 chloride of magnesium1 and sodium sulphate4 ,on cooling the mixture of such 7concentrated7 solutions sodium sulphate is deposited1 as was shown in ,chapter ,x4 ,this is made use of for preparing it on the large scale in works where sea-water is treated4 ,in this case1 on cooling1 the reaction #b,na,cl ,mg,s,o#d ,mg,cl#b ,na#b,,so#d takes place4 #a ,whilst describing in some detail the properties of sodium chloride1 hydrochloric acid1 and sodium sulphate1 ,i wish to impart1 by separate examples1 an idea of the properties of saline substances1 but the dimensions of this work and its purpose and aim do not permit of entering into particulars concerning every salt1 acid1 or other substance4 ,the fundamental object of this work--an account of the characteristics of the elements and an acquaintance with the forces acting between atoms--has nothing to gain from the multiplication of the number of as yet ungeneralised properties and relations4 ,thus where sulphates and salts of sodium are in contact1 it may be expected that sodium sulphate will be formed and separated if the conditions are favourable2 for this reason it is not surprising that sodium sulphate is often found in the native state4 ,some of the springs and salt lakes in the steppes beyond the ,volga1 and in the ,caucasus1 contain a considerable quantity of sodium sulphate1 and yield it by simple evaporation of the solutions4 ,beds of this salt are also met with2 thus at a depth of only #e feet1 about #ch versts to the east of ,tiflis1 at the foot of the range of the ',wolf's mane' 7,voltchia griva7 mountains1 a deep stratum of very pure ,glauber's salt1 ,na#b,,so#d1aj;,h#b,o1 has been found4#b ,a layer two metres thick of the same salt lies at the bottom of several lakes 7an area of about #aj square kilometres7 in the ,kouban district near ,batalpaschinsk1 and here its working has been commenced 7#ahhg74 ,in ,spain1 near ,arangoulz and in many parts of the ,western ,states of ,north ,america1 mineral sodium sulphate has likewise been found1 and is already being worked4 #b ,anhydrous 7ignited7 sodium sulphate1 ,na#b,,so#d1 is known in trade as 'sulphate' or salt-cake1 in mineralogy thenardite4 ,crystalline decahydrated salt is termed in mineralogy mirabilite1 and in trade ,glauber's salt4 ,on fusing it1 the monohydrate ,na#b,,so#d;,h#b,o is obtained1 together with a supersaturated solution4 ,the methods of obtaining salts by means of double decomposition from others already prepared are so general1 that in describing a given salt there is no necessity to enumerate the cases hitherto observed of its being formed through various double decompositions4#c ,the possibility of this occurrence ought to be foreseen according to ,berthollet's doctrine from the properties of the salt in question4 ,on this account it is important to know the properties of salts2 all the more so because up to the present time those very properties 7solubility1 formation of crystallo-hydrates1 volatility1 c47 which may be made use of for separating them from other salts have not been generalised4#d ,these properties as yet remain subjects for investigation1 and are rarely to be foreseen4 ,the crystallo-hydrate of the normal sodium sulphate1 ,na#b,,so#d1aj;,h#b,o1 very easily parts with water1 and may be obtained in an anhydrous state if it be carefully heated until the weight remains constant2 but if heated further1 it partly loses the elements of sulphuric anhydride4 ,the normal salt fuses at #hdc 7red heat71 and volatilises to a slight extent when very strongly heated1 in which case it naturally decomposes with the evolution of ,,so#c4 ,at #j #ajj parts of water dissolve #e parts of the anhydrous salt1 at #aj #i parts1 at #bj #aid1 at #cj #dj1 and at #cd #ee parts1 the same being the case in the presence of an excess of crystals of ,na#b,,so#d1aj;,h#b,o4#e ,at #cd the latter fuses1 and the solubility decreases at higher temperatures4#f ,a concentrated solution at #cd has a composition nearly approaching to ,na#b,,so#d #ad;,h#b,o1 and the decahydrated salt contains #ghi of the anhydrous salt combined with #ajj parts of water4 ,from the above figures it is seen that the decahydrated salt cannot fuse without decomposing1#g like hydrate of chlorine1 ,cl#b1h;,h#b,o 7,chapter ,,xi41 ,note #aj74 ,not only the fused decahydrated salt1 but also the concentrated solution at #cd 7not all at once1 but gradually71 yields the monohydrated salt1 ,na#b,,so#d1;,h#b,o4 ,the heptahydrated salt1 ,na#b,,so#d1g;,h#b,o1 also splits up1 even at low temperatures1 with the formation of this monohydrated salt1 and therefore from #ce the solubility can be given only for the latter4 ,for #ajj parts of water this is as follows3 at #dj #dhh1 at #ej #dfg1 at #hj #dcg1 at #ajj #dbe parts of the anhydrous salt4 ,if the decahydrated salt be fused1 and the solution allowed to cool in the presence of the monohydrated salt1 then at #cj #ejd parts of anhydrous salt are retained in the solution1 and at #bj #ebh parts4 ,hence1 with respect to the anhydrous and monohydrated salts1 the solubility is identical1 and falls with increasing temperature1 whilst with respect to decahydrated salt1 the solubility rises with increasing temperature4 ,so that if in contact with a solution of sodium sulphate there are only crystals of that heptahydrated salt 7,chapter ,i41 ,note #ed71 ,na#b,,so#d1g;,h#b,o1 which is formed from saturated solutions1 then saturation sets in when the solution has the following composition per #ajj parts of salt3 at #j #aif1 at #aj #cje1 at #bj #ddg1 and at #be #ebi parts of anhydrous salt4 ,above #bg the heptahydrated salt1 like the decahydrated salt at #cd1 splits up into the monohydrated salt and a saturated solution4 ,thus sodium sulphate has three curves of solubility3 one for ,na#b,,so#d1g;,h#b,o 7from #j to #bf71 one for ,na#b,,so#d1aj;,h#b,o 7from #j to #cd71 and one for ,na#b,,so#d1;,h#b,o 7a descending curve beginning at #bf71 because there are three of these crystallo-hydrates1 and the solubility of a substance only depends upon the particular condition of that portion of it which has separated from the solution or is present in excess4#h #c ,the salts may be obtained not only by methods of substitution of various kinds1 but also by many other combinations4 ,thus sodium sulphate may be formed from sodium oxide and sulphuric anhydride1 by oxidising sodium sulphide1 ,na#b,s1 or sodium sulphite1 ,na#b,,so#c1 c4 ,when sodium chloride is heated in a mixture of the vapours of water1 air1 and sulphurous anhydride1 sodium sulphate is formed4 ,according to this method 7patented by ,hargreaves and ,robinson71 sodium sulphate1 ,na#b,,so#d1 is obtained from ,na,cl without the preliminary manufacture of ,h#b,,so#d4 ,lumps of ,na,cl pressed into bricks are loosely packed into a cylinder and subjected1 at a red heat1 to the action of steam1 air and ,,so#b4 ,under these conditions1 ,h,cl1 sulphate1 and a certain amount of unaltered ,na,cl are obtained4 ,this mixture is converted into soda by ,gossage's process 7see ,note #ae7 and may have some practical value4 #d ,many observations have been made1 but little general information has been obtained from particular cases4 ,in addition to which1 the properties of a given salt are changed by the presence of other salts4 ,this takes place not only in virtue of mutual decomposition or formation of double salts capable of separate existence1 but is determined by the influence which some salts exert on others1 or by forces similar to those which act during solution4 ,here nothing has been generalised to that extent which would render it possible to predict without previous investigation1 if there be no close analogy to help us4 ,let us state one of these numerous cases3 #ajj parts of water at #bj dissolve #cd parts of potassium nitrate but on the addition of sodium nitrate the solubility of potassium nitrate increases to #dh parts in #aj of water 7,carnelley and ,thomson74 ,in general1 in all cases of which there are accurate observations it appears that the presence of foreign salts changes the properties of any given salt4 #e ,the information concerning solubility 7,chapter ,i47 is given according to the determinations of ,gay-,lussac1 ,lovell1 and ,mulder4 #f ,in ,chapter ,i41 ,note #bd1 we have already seen that with many other sulphates the solubility also decreases after a certain temperature is passed4 ,gypsum1 ,ca,s,o#d1b;,h#b,o1 lime1 and many other compounds present such a phenomenon4 ,an observation of ,tilden's 7#ahhd7 is most instructive2 he showed that on raising the temperature 7in closed vessels7 above #adj the solubility of sodium sulphate again begins to increase4 ,at #ajj #ajj parts of water dissolve about #dc parts of anhydrous salt1 at #adj #db parts1 at #afj #dc parts1 at #ahj #dd parts1 at #bcj #df parts4 ,according to tard 7#ahib7 the solubility of #cj parts of ,na#b,,so#d in #ajj of solution 7or #dc per #ajj of water7 corresponds to #hj1 and above #bdj the solubility again falls1 and very rapidly1 so that at #cbj the solution contains #ab per #ajj of solution 7about #ad per #ajj of water7 and a further rise of temperature is followed by a further deposition of the salt4 ,it is evident that the phenomenon of saturation1 determined by the presence of an excess of the dissolved substance1 is very complex1 and therefore that for the theory of solutions considered as liquid indefinite chemical compounds1 many useful statements can hardly be given4 #g ,already referred to in ,chapter ,i41 ,note #ef4 ,the example of sodium sulphate is historically very important for the theory of solutions4 ,notwithstanding the number of investigations which have been made1 it is still insufficiently studied1 especially from the point of the vapour tension of solutions and crystallo-hydrates1 so that those processes cannot be applied to it which ,guldberg1 ,roozeboom1 ,van't ,hoff1 and others applied to solutions and crystallo-hydrates4 ,it would also be most important to investigate the influence of pressure on the various phenomena corresponding with the combinations of water and sodium sulphate1 because when crystals are separated--for instance1 of the decahydrated salt--an increase of volume takes place1 as can be seen from the following data3--the sp4 gr4 of the anhydrous salt is #bff1 that of the decahydrated salt #adf1 but the sp4 gr4 of solutions at #ae_/d #i1iib #ijbp #jcep#b where p represents the percentage of anhydrous salt in the solution1 and the sp4 gr4 of water at #d #aj1jjj4 ,hence for solutions containing #bj p4c4 of anhydrous salt the sp4 gr4 #aaicf2 therefore the volume of #ajj grams of this solution #hch c4c41 and the volume of anhydrous salt contained in it is equal to #bj_/bff1 or #ge c4c41 and the volume of water #hja c4c4 ,therefore1 the solution1 on decomposing into anhydrous salt and water1 increases in volume 7from #hch to #hgf72 but in the same way #hch c4c4 of #bj p4c4 solution are formed from 7#ded_/adf 7 #caa c4c4 of the decahydrated salt1 and #edf c4c4 of water--that is to say1 that during the formation of a solution from #heg c4c41 #hch c4c4 are formed4 #h ,from this example it is evident the solution remains unaltered until from the contact of a solid it becomes either saturated or supersaturated1 crystallisation being determined by the attraction to a solid1 as the phenomenon of supersaturation clearly demonstrates4 ,this partially explains certain apparently contradictory determinations of solubility4 ,the best investigated example of such complex relations is cited in ,chapter ,,xiv41 ,note #ej 7for ,ca,cl#b74 ,thus solutions of sodium sulphate may give crystallo-hydrates of three kinds on cooling the saturated solution3 the unstable heptahydrated salt is obtained at temperatures below #bf1 the decahydrated salt forms under ordinary conditions at temperatures below #cd1 and the monohydrated salt at temperatures above #cd4 ,both the latter crystallo-hydrates present a stable state of equilibrium1 and the heptahydrated salt decomposes into them1 probably according to the equation #c,na#b,,so#d1g;,h#bj #b,na#b,,so#d1aj;,h#b,o ,na#b,,so#d1;,h#b,o4 ,the ordinary decahydrated salt is called ,glauber's salt4 ,all forms of these crystallo-hydrates lose their water entirely1 and give the anhydrous salt when dried over sulphuric acid4#i #i ,according to ,pickering's experiments 7#ahhf71 the molecular weight in grams 7that is1 #adb grams7 of anhydrous sodium sulphate1 on being dissolved in a large mass of water1 at #j absorbs 7hence the-sign7-#a1ajj heat units1 at #aj-gjj1 at #ae-bge1 at #bj gives out #be1 at #be #cjj calories4 ,for the decahydrated salt1 ,na#b,,so#d1aj;,h#b,o1 #e-d1bbe1 #aj-d1jjj1 #ae-c1egj1 #bj-c1afj1 #be-b1gge4 ,hence 7just as in ,chapter ,i41 ,note #ef7 the heat of the combination ,na#b,,so#d1aj;,h#b,o at #e #c1abe1 #aj #c1bej1 #bj #c1bjj1 and #be #c1jej4 ,it is evident that the decahydrated salt dissolving in water gives a decrease of temperature4 ,solutions in hydrochloric acid give a still greater decrease1 because they contain the water of crystallisation in a solid state--that is1 like ice--and this on melting absorbs heat4 ,a mixture of #ae parts of ,na#b,,so#d1aj;,h#b,o and #ab parts of strong hydrochloric acid produces sufficient cold to freeze water4 ,during the treatment with hydrochloric acid a certain quantity of sodium chloride is formed4 ,sodium sulphate1 ,na#b,,so#d1 only enters into a few reactions of combination with other salts1 and chiefly with salts of the same acid1 forming double sulphates4 ,thus1 for example1 if a solution of sodium sulphate be mixed with a solution of aluminium1 magnesium1 or ferrous sulphate1 it gives crystals of a double salt when evaporated4 ,sulphuric acid itself forms a compound with sodium sulphate1 which is exactly like these double salts4 ,it is formed with great ease when sodium sulphate is dissolved in sulphuric acid and the solution evaporated4 ,on evaporation1 crystals of the acid salt separate1 ,na#b,,so#d ,h#b,,so#d #b,na,h,s,o#d4 ,this separates from hot solutions1 whilst the crystallo-hydrate1 ,na,h,s,o#d1;,h#b,o1#aj separates from cold solutions4 ,the crystals when exposed to damp air decompose into ,h#b,,so#d1 which deliquesces1 and ,na#b,,so#d 7,graham1 ,rose72 alcohol also extracts sulphuric acid from the acid salt4 ,this shows the feeble force which holds the sulphuric acid to the sodium sulphate4#aa ,both acid sodium sulphate and all mixtures of the normal salt and sulphuric acid lose water when heated1 and are converted into sodium pyrosulphate1 ,na#b,s#b,o#g1 at a low red heat4#aa bis ,this anhydrous salt1 at a bright red heat1 parts with the elements of sulphuric anhydride1 the normal sodium sulphate remaining behind--,na#b,s#b,o#g ,na#b,,so#d ,,so#c4 ,from this it is seen that the normal salt is able to combine with water1 with other sulphates1 and with sulphuric anhydride or acid1 c4 #aj ,the very large and well-formed crystals of this salt resemble the hydrate ,h#b,,so#d1;,h#b,o1 or ,,so7,,oh7#d4 ,in general the replacement of hydrogen by sodium modifies many of the properties of acids less than its replacement by other metals4 ,this most probably depends on the volumes being nearly equal4 #aa ,in solution 7,berthelot7 the acid salt in all probability decomposes most in the greatest mass of water4 ,the specific gravity 7according to the determinations of ,marignac7 of solutions at #ae_/d #i1iib #ggibp #jbcap#b 7see ,note #g74 ,from these figures1 and from the specific gravities of sulphuric acid1 it is evident that on mixing solutions of this acid and sodium sulphate expansion will always take place2 for instance1 ,h#b,,so#d #be;,h#b,o with ,na#b,,so#d #be;,h#b,o increases from #dhc volumes to #dhf4 ,in addition to which1 in weak solutions heat is absorbed1 as shown in ,chapter ,x41 ,note #bg4 ,nevertheless1 even more acid salts may be formed and obtained in a crystalline form4 ,for instance1 on cooling a solution of #a part of sodium sulphate in #g parts of sulphuric acid1 crystals of the composition ,na,h,s,o#d1;,h#b,,so#d are separated 7,schultz1 #ahfh74 ,this compound fuses at about #ajj2 the ordinary acid salt1 ,na,h,s,o#d1 at #adi4 #aa bis ,on decreasing the pressure1 sodium hydrogen sulphate1 ,na,h,s,o#d1 dissociates much more easily than at the ordinary pressure2 it loses water and forms the pyrosulphate1 ,na#b,s#b,o#g2 this reaction is utilised in chemical works4 ,sodium sulphate may by double decomposition be converted into a sodium salt of any other acid1 by means of heat and taking advantage of the volatility1 or by means of solution and taking advantage of the different degree of solubility of the different salts4 ,thus1 for instance1 owing to the insolubility of barium sulphate1 sodium hydroxide or caustic soda may be prepared from sodium sulphate1 if barium hydroxide be added to its solution1 ,na#b,,so#d ,ba7,,ho7#b ,ba,s,o#d #b,na,h,o4 ,and by taking any salt of barium1 ,ba,x#b1 the corresponding salt of sodium may be obtained1 ,na#b,,so#d ,ba,x#b ,ba,s,o#d #b,na,x4 ,barium sulphate thus formed1 being a very sparingly-soluble salt1 is obtained as a precipitate1 whilst the sodium hydroxide1 or salt1 ,na,x1 is obtained in solution1 because all salts of sodium are soluble4 ,berthollet's doctrine permits all such cases to be foreseen4 ,the reactions of decomposition of sodium sulphate are above all noticeable by the separation of oxygen4 ,sodium sulphate by itself is very stable1 and it is only at a temperature sufficient to melt iron that it is possible to separate the elements ,,so#c from it1 and then only partially4 ,however1 the oxygen may be separated from sodium sulphate1 as from all other sulphates1 by means of many substances which are able to combine with oxygen1 such as charcoal and sulphur1 but hydrogen is not able to produce this action4 ,if sodium sulphate be heated with charcoal1 then carbonic oxide and anhydride are evolved1 and there is produced1 according to the circumstances1 either the lower oxygen compound1 sodium sulphite1 ,na#b,,so#c 7for instance1 in the formation of glass72 or else the decomposition proceeds further1 and sodium sulphide1 ,na#b,s1 is formed1 according to the equation ,na#b,,so#d #b;,c #b;,,co#b ,na#b,s4 ,on the basis of this reaction the greater part of the sulphate of sodium prepared at chemical works is converted into soda ash--that is1 sodium carbonate1 ,na#b;,,co#c1 which is used for many purposes4 ,in the form of carbonates1 the metallic oxides behave in many cases just as they do in the state of oxides or hydroxides1 owing to the feeble acid properties of carbonic acid4 ,however1 the majority of the salts of carbonic acid are insoluble1 whilst sodium carbonate is one of the few soluble salts of this acid1 and therefore reacts with facility4 ,hence sodium carbonate is employed for many purposes1 in which its alkaline properties come into play4 ,thus1 even under the action of feeble organic acids it immediately parts with its carbonic acid1 and gives a sodium salt of the acid taken4 ,its solutions exhibit an alkaline reaction on litmus4 ,it aids the passage of certain organic substances 7tar1 acids7 into solution1 and is therefore used1 like caustic alkalis and soap 7which latter also acts by virtue of the alkali it contains71 for the removal of certain organic substances1 especially in bleaching cotton and similar fabrics4 ,besides which a considerable quantity of sodium carbonate is used for the preparation of sodium hydroxide or caustic soda1 which has also a very wide application4 ,in large chemical works where sodium carbonate is manufactured from ,na#b,,so#d1 it is usual first to manufacture sulphuric acid1 and then by its aid to convert common salt into sodium sulphate1 and lastly to convert the sodium sulphate thus obtained into carbonate and caustic soda4 ,hence these works prepare both alkaline substances 7soda ash and caustic soda7 and acid substances 7sulphuric and hydrochloric acids71 the two classes of chemical products which are distinguished for the greatest energy of their reactions and are therefore most frequently applied to technical purposes4 ,factories manufacturing soda are generally called alkali works4 ,the process of the conversion of sodium sulphate into sodium carbonate consists in strongly heating a mixture of the sulphate with charcoal and calcium carbonate4 ,the following reactions then take place3 the sodium sulphate is first deoxidised by the charcoal1 forming sodium sulphide and carbonic anhydride1 ,na#b,,so#d #b;,c ,na#b,s #b;,,co#b4 ,the sodium sulphide thus formed then enters into double decomposition with the calcium carbonate taken1 and gives calcium sulphide and sodium carbonate1 ,na#b,s ,ca,c,o#c ,na#b;,,co#c ,ca,s4 ,illustration3 ,,fig4 #fh4--,reverberatory furnace for the manufacture of sodium carbonate4 ,f1 grate4 ,a1 bridge4 ,m1 hearth for the ultimate calcination of the mixture of sodium sulphate1 coal1 and calcium carbonate1 which is charged from above into the part of the furnace furthest removed from the fire ,f4 ,p1 ,p1 doors for stirring and bringing the mass towards the grate ,f by means of stirrers ,r4 ,at the end of the operation the semifused mass is charged into trucks ,c4 ,besides which1 under the action of the heat1 a portion of the excess of calcium carbonate is decomposed into lime and carbonic anhydride1 ,ca,c,o#c ,ca,o ,,co#b1 and the carbonic anhydride with the excess of charcoal forms carbon monoxide1 which towards the end of the operation shows itself by the appearance of a blue flame4 ,thus from a mass containing sodium sulphate we obtain a mass which includes sodium carbonate1 calcium sulphide1 and calcium oxide1 but none of the sodium sulphide which was formed on first heating the mixture4 ,the entire process1 which proceeds at a high temperature1 may be expressed by a combination of the three above-mentioned formul1 if it be considered that the product contains one equivalent of calcium oxide to two equivalents of calcium sulphide4#ab ,the sum of the reactions may then be expressed thus3 #b,na#b,,so#d #c;,ca,c,o#c #i;,c #b,na#b;,,co#c ,ca,o1#b;,ca,s #aj;,,co4 ,indeed1 the quantities in which the substances are mixed together at chemical works approaches to the proportion required by this equation4 ,the entire process of decomposition is carried on in reverberatory furnaces1 into which a mixture of #a1jjj parts of sodium sulphate1 #a1jdj parts of calcium carbonate 7as a somewhat porous limestone71 and #ejj parts of small coal is introduced from above4 ,this mixture is first heated in the portion of the furnace which is furthest removed from the fire-grate2 it is then brought to the portion nearest to the fire-grate1 when it is stirred during heating4 ,the partially fused mass obtained at the end of the process is cooled1 and then subjected to methodical lixiviation#ac to extract the sodium carbonate1 the mixture of calcium oxide and sulphide forming the so-called 'soda waste' or 'alkali waste4'#ad #ab ,calcium sulphide1 ,ca,s1 like many metallic sulphides which are soluble in water1 is decomposed by it 7,chapter ,x471 ,ca,s ,h#b,o ,ca,o ,h#b,s1 because hydrogen sulphide is a very feeble acid4 ,if calcium sulphide be acted on by a large mass of water1 lime may be precipitated1 and a state of equilibrium will be reached1 when the system ,ca,o #b;,ca,s remains unchanged4 ,lime1 being a product of the action of water on ,ca,s1 limits this action4 ,therefore1 if in black ash the lime were not in excess1 a part of the sulphide would be in solution 7actually there is but very little74 ,in this manner in the manufacture of sodium carbonate the conditions of equilibrium which enter into double decompositions have been made use of 7see above71 and the aim is to form directly the unchangeable product ,ca,o1#b;,ca,s4 ,this was first regarded as a special insoluble compound1 but there is no evidence of its independent existence4 #ac ,illustration3 ,,fig4 #fi4--;,apparatus for the methodical lixiviation of black ash1 c4 ,water flows into the tanks from the pipes r1 r1 and the saturated liquid is drawn off from c1 c4 ,methodical lixiviation is the extraction1 by means of water1 of a soluble substance from the mass containing it4 ,it is carried on so as not to obtain weak aqueous solutions1 and in such a way that the residue shall not contain any of the soluble substance4 ,this problem is practically of great importance in many industries4 ,it is required to extract from the mass all that is soluble in water4 ,this is easily effected if water be first poured on the mass1 the strong solution thus obtained decanted1 then water again poured on1 time being allowed for it to act1 then again decanted1 and so on until fresh water does not take up anything4 ,but then finally such weak solutions are obtained that it would be very disadvantageous to evaporate them4 ,this is avoided by pouring the fresh hot water destined for the lixiviation1 not onto the fresh mass1 but upon a mass which has already been subjected to a first lixiviation by weak solutions4 ,in this way the fresh water gives a weak solution4 ,the strong solution which goes to the evaporating pan flows from those parts of the apparatus which contain the fresh1 as yet unlixiviated1 mass1 and thus in the latter parts the weak alkali formed in the other parts of the apparatus becomes saturated as far as possible with the soluble substance4 ,generally several intercommunicating vessels are constructed 7standing at the same level7 into which in turn the fresh mass is charged which is intended for lixiviation2 the water is poured in1 the alkali drawn off1 and the lixiviated residue removed4 ,the illustration represents such an apparatus1 consisting of four communicating vessels4 ,the water poured into one of them flows through the two nearest and issues from the third4 ,the fresh mass being placed in one of these boxes or vessels1 the stream of water passing through the apparatus is directed in such a manner as to finally issue from this vessel containing the fresh unlixiviated mass4 ,the fresh water is added to the vessel containing the material which has been almost completely exhausted4 ,passing through this vessel it is conveyed by the pipe 7syphon passing from the bottom of the first box to the top of the second7 communicating with the second2 it finally passes 7also through a syphon pipe7 into the box 7the third7 containing the fresh material4 ,the water will extract all that is soluble in the first vessel1 leaving only an insoluble residue4 ,this vessel is then ready to be emptied1 and refilled with fresh material4 ,the levels of the liquids in the various vessels will naturally be different1 in consequence of the various strengths of the solutions which they contain4 ,it must not1 however1 be thought that sodium carbonate alone passes into the solution2 there is also a good deal of caustic soda with it1 formed by the action of lime on the carbonate of sodium1 and there are also certain sodium sulphur compounds with which we shall partly become acquainted hereafter4 ,the sodium carbonate1 therefore1 is not obtained in a very pure state4 ,the solution is concentrated by evaporation4 ,this is conducted by means of the waste heat from the soda furnaces1 together with that of the gases given off4 ,the process in the soda furnaces can only be carried on at a high temperature1 and therefore the smoke and gases issuing from them are necessarily very hot4 ,if the heat they contain was not made use of there would be a great waste of fuel2 consequently in immediate proximity to these furnaces there is generally a series of pans or evaporating boilers1 under which the gases pass1 and into which the alkali solution is poured4 ,on evaporating the solution1 first of all the undecomposed sodium sulphate separates1 then the sodium carbonate or soda crystals4 ,these crystals as they separate are raked out and placed on planks1 where the liquid drains away from them4 ,caustic soda remains in the residue1 and also any sodium chloride which was not decomposed in the foregoing process4 ,part of the sodium carbonate is recrystallised in order to purify it more thoroughly4 ,in order to do this a saturated solution is left to crystallise at a temperature below #cj in a current of air1 in order to promote the separation of the water vapour4 ,the large transparent crystals 7efflorescent in air7 of ,na#b;,,co#c1aj;,h#b,o are then formed which have already been spoken of 7,chapter ,i474 #ad ,the whole of the sulphur used in the production of the sulphuric acid employed in decomposing the common salt is contained in this residue4 ,this is the great burden and expense of the soda works which use ,leblanc's method4 ,as an instructive example from a chemical point of view1 it is worth while mentioning here two of the various methods of recovering the sulphur from the soda waste4 ,chance's process is treated in ,chapter ,,xx41 ,note #f4 ,kynaston 7#ahhe7 treats the soda waste with a solution 7sp4 gr4 l#ba7 of magnesium chloride1 which disengages sulphuretted hydrogen3 ,ca,s ,mg,cl#b #b;,h#b,o ,ca,cl#b ,mg7,,oh7#b ,h#b,s4 ,sulphurous anhydride is passed through the residue in order to form the insoluble calcium sulphite3 ,ca,cl#b ,mg7,,oh7#b ,,so#b ,ca,s,o#c ,mg,cl#b ,h#b,o4 ,the solution of magnesium chloride obtained is again used1 and the washed calcium sulphite is brought into contact at a low temperature with hydrochloric acid 7a weak aqueous solution7 and hydrogen sulphide1 the whole of the sulphur then separating3 ,ca,s,o#c #b;,h#b,s #b;,h,cl ,ca,cl#b #c;,h#b,o #c,s4 ,but most efforts have been directed towards avoiding the formation of soda waste4 ,the above-mentioned process for making soda was discovered in the year #ahjh by the ,french doctor ,leblanc1 and is known as the ,leblanc process4 ,the particulars of the discovery are somewhat remarkable4 ,sodium carbonate1 having a considerable application in industry1 was for a long time prepared exclusively from the ash of marine plants 7,chapter ,,xi41 page #dig74 ,even up to the present time this process is carried on in ,normandy4 ,in ,france1 where for a long time the manufacture of large quantities of soap 7so-called ,marseilles soap7 and various fabrics required a large amount of soda1 the quantity prepared at the coast was insufficient to meet the demand4 ,for this reason during the wars at the beginning of the century1 when the import of foreign goods into ,france was interdicted1 the want of sodium carbonate was felt4 ,the ,french ,academy offered a prize for the discovery of a profitable method of preparing it from common salt4 ,leblanc then proposed the above-mentioned process1 which is remarkable for its great simplicity4#ae #ae ,among the drawbacks of the ,leblanc process are the accumulation of 'soda waste' 7,note #ad7 owing to the impossibility at the comparatively low price of sulphur 7especially in the form of pyrites7 of finding employment for the sulphur and sulphur compounds for which this waste is sometimes treated1 and also the insufficient purity of the sodium carbonate for many purposes4 ,the advantages of the ,leblanc process1 besides its simplicity and cheapness1 are that almost the whole of the acids obtained as bye-products have a commercial value2 for chlorine and bleaching powder are produced from the large amount of hydrochloric acid which appears as a bye-product2 caustic soda also is very easily made1 and the demand for it increases every year4 ,in those places where salt1 pyrites1 charcoal1 and limestone 7the materials required for alkali works7 are found side by side--as1 for instance1 in the ,ural or ,don districts--conditions are favourable to the development of the manufacture of sodium carbonate on an enormous scale2 and where1 as in the ,caucasus1 sodium sulphate occurs naturally1 the conditions are still more favourable4 ,a large amount1 however1 of the latter salt1 even from soda works1 is used in making glass4 ,the most important soda works1 as regards the quantity of products obtained from them1 are the ,english works4 ,as an example of the other numerous and varied methods of manufacturing soda from sodium chloride1 the following may be mentioned3 ,sodium chloride is decomposed by oxide of lead1 ,pb,o1 forming lead chloride and sodium oxide1 which1 with carbonic anhydride1 yields sodium carbonate 7,scheele's process74 ,in ,cornu's method sodium chloride is treated with lime1 and then exposed to the air1 when it yields a small quantity of sodium carbonate4 ,in ,e4 ,kopp's process sodium sulphate 7#abe parts7 is mixed with oxide of iron 7#hj parts7 and charcoal 7#ee parts71 and the mixture is heated in reverberatory furnaces4 ,here a compound1 ,na#f;,fe#d,s#c1 is formed1 which is insoluble in water absorbs oxygen and carbonic anhydride1 and then forms sodium carbonate and ferrous sulphide2 this when roasted gives sulphurous anhydride1 the indispensable material for the manufacture of sulphuric acid1 and ferric oxide which is again used in the process4 ,in ,grant's method sodium sulphate is transformed into sodium sulphide1 and the latter is decomposed by a stream of carbonic anhydride and steam1 when hydrogen sulphide is disengaged and sodium carbonate formed4 ,gossage prepares ,na#b,s from ,na#b,,so#d 7by heating it with carbon71 dissolves it in water and subjects the solution to the action of an excess of ,,co#b in coke towers1 thus obtaining ,h#b,s 7a gas which gives ,,so#b under perfect combustion1 or sulphur when incompletely burnt1 ,chapter ,,xx41 ,note #f7 and bicarbonate of sodium2 ,na#b,s #b;,,co#b #b;,h#b,o ,h#b,s #b;,h,na,c,o#c4 ,the latter gives soda and ,,co#b when ignited4 ,this process quite eliminates the formation of soda-waste 7see ,note #c7 and should in my opinion be suitable for the treatment of native ,na#b,,so#d1 like that which is found in the ,caucasus1 all the more since ,h#b,s gives sulphur as a bye-product4 ,repeated efforts have been made in recent times to obtain soda 7and chlorine1 see ,chapter ,,ii41 ,note #a7 from strong solutions of salt 7,chapter ,x41 ,note #bc bis7 by the action of an electric current1 but until now these methods have not been worked out sufficiently for practical use1 probably partly owing to the complicated apparatus needed1 and the fact that the chlorine given off at the anode corrodes the electrodes and vessels and has but a limited industrial application4 ,we may mention that according to ,hempel 7#ahij7 soda in crystals is deposited when an electric current and a stream of carbonic acid gas are passed through a saturated solution of ,na,cl4 ,sodium carbonate may likewise be obtained from cryolite 7,chapter ,,xvii41 ,note #bc7 the method of treating this will be mentioned under ,aluminium4 ,of all other industrial processes for manufacturing sodium carbonate1 the ammonia process is the most worthy of mention4#af ,in this the vapours of ammonia1 and then an excess of carbonic anhydride1 are directly introduced into a concentrated solution of sodium chloride in order to form the acid ammonium carbonate1 ,,nh#d;,,hco#c4 ,then1 by means of the double saline decomposition of this salt1 sodium chloride is decomposed1 and in virtue of its slight solubility acid sodium carbonate1 ,na,h,c,o#c1 is precipitated and ammonium chloride1 ,,nh#d;,cl1 is obtained in solution 7with a portion of the sodium chloride and acid sodium carbonate74 ,the reaction proceeds in the solution owing to the sparing solubility of the ,na,h,c,o#c according to the equation ,na,cl ,,nh#d;,,hco#c ,,nh#d;,cl ,na,h,c,o#c4 ,the ammonia is recovered from the solution by heating with lime or magnesia1#af bis and the precipitated acid sodium carbonate is converted into the normal salt by heating4 ,it is thus obtained in a very pure state4#ag #af ,this process 7,chapter ,,xvii47 was first pointed out by ,turck1 worked out by ,schloesing1 and finally applied industrially by ,solvay4 ,the first 7#ahhc7 large soda factories erected in ,russia for working this process are on the banks of the ,kama at ,berezniak1 near ,ousolia1 and belong to ,lubimoff4 ,but ,russia1 which still imports from abroad a large quantity of bleaching powder and exports a large amount of manganese ore1 most of all requires works carrying on the ,leblanc process4 ,in #ahij a factory of this kind was erected by ,p4 ,k4 ,oushkoff1 on the ,kama1 near ,elagoubi4 #af bis ,mond 7see ,chapter ,,xi41 ,note #c bis7 separates the ,,nh#d;,cl from the residual solutions by cooling 7,chapter ,x41 ,note #dd72 ignites the sal-ammoniac and passes the vapour over ,mg,o1 and so re-obtains the ,,nh#c1 and forms ,mg,cl#b3 the former goes back for the manufacture of soda1 while the latter is employed either for making ,h,cl or ,cl#b4 #ag ,commercial soda ash 7calcined1 anhydrous7 is rarely pure2 the crystallised soda is generally purer4 ,in order to purify it further1 it is best to boil a concentrated solution of soda ash until two-thirds of the liquid remain1 collect the soda which settles1 wash with cold water1 and then shake up with a strong solution of ammonia1 pour off the residue1 and heat4 ,the impurities will then remain in the mother liquors1 c4 ,some numerical data may be given for sodium carbonate4 ,the specific gravity of the anhydrous salt is #bdh1 that of the decahydrated salt #adf4 ,two varieties are known of the heptahydrated salt 7,lwel1 ,marignac1 ,rammelsberg71 which are formed together by allowing a saturated solution to cool under a layer of alcohol2 the one is less stable 7like the corresponding sulphate7 and at #j has a solubility of #cb parts 7of anhydrous salt7 in #ajj water2 the other is more stable1 and its solubility #bj parts 7of anhydrous salt7 per #ajj of water4 ,the solubility of the decahydrated salt in #ajj water at #j1 #gj2 at #bj1 #bag2 at #cj1 #cgb parts 7of anhydrous salt74 ,at #hj the solubility is only #dfa1 at #ij #deg1 at #ajj1 #ded parts 7of anhydrous salt74 ,that is1 it falls as the temperature rises1 like ,na#b,,so#d4 ,the specific gravity 7,note #g7 of the solutions of sodium carbonate1 according to the data of ,gerlach and ,kohlrausch1 at #ae_/d is expressed by the formula1 s #i1iib #ajdep #jafep#b4 ,weak solutions occupy a volume not only less than the sum of the volumes of the anhydrous salt and the water1 but even less than the water contained in them4 ,for instance1 #a1jjj grams of a #a p4c4 solution occupy 7at #ae7 a volume of #iijd c4c4 7sp4 gr4 #ajjig71 but contain #iij grams of water1 occupying at #ae a volume of #iijh c4c4 ,a similar case1 which is comparatively rare occurs also with sodium hydroxide1 in those dilute solutions for which the factor ,a is greater than #ajj if the sp4 gr4 of water at #d #ajj1jjj1 and if the sp4 gr4 of the solution be expressed by the formula ,s ,s#j ,ap ,bp#b1 where ,s#j is the specific gravity of the water4 ,for #e p4c4 the sp4 gr4 #ae_/d #ajebj2 for #aj p4c4 #aajeg2 for #ae p4c4 #aafjc4 ,the changes in the sp4 gr4 with the temperature are here almost the same as with solutions of sodium chloride with an equal value of p4 ,sodium carbonate1 like sodium sulphate1 loses all its water on being heated1 and when anhydrous fuses at a bright-red heat 7#ajih74 ,a small quantity of sodium carbonate placed in the loop of a platinum wire volatilises in the heat of a gas flame1 and therefore in the furnaces of glass works part of the soda is always transformed into the condition of vapour4 ,sodium carbonate resembles sodium sulphate in its relation to water4#ah ,here also the greatest solubility is at the temperature of #cg2 both salts1 on crystallising at the ordinary temperature1 combine with ten molecules of water1 and such crystals of soda1 like crystals of ,glauber's salt1 fuse at #cd4 ,sodium carbonate also forms a supersaturated solution1 and1 according to the conditions1 gives various combinations with water of crystallisation 7mentioned on page #ajh71 c4 #ah ,the resemblance is so great that1 notwithstanding the difference in the molecular composition of ,na#b,,so#d and ,na#b;,,co#c1 they ought to be classed under the type 7,na,o7#b,r1 where ,r ,,so#b or ,,co4 ,many other sodium salts also contain #aj mol4 ,h#b,o4 ,at a red heat superheated steam liberates carbonic anhydride from sodium carbonate and forms caustic soda1 ,na#b;,,co#c ,h#b,o #b,na,h,o ,,co#b4 ,here the carbonic anhydride is replaced by water2 this depends on the feebly acid character of carbonic anhydride4 ,by direct heating1 sodium carbonate is only slightly decomposed into sodium oxide and carbonic anhydride2 thus1 when sodium carbonate is fused1 about #a per cent4 of carbonic anhydride is disengaged4#ai ,the carbonates of many other metals--for instance1 of calcium1 copper1 magnesium1 iron1 c4--on being heated lose all their carbonic anhydride4 ,this shows the considerable basic energy which sodium possesses4 ,with the soluble salts of most metals1 sodium carbonate gives precipitates either of insoluble carbonates of the metals1 or else of the hydroxides 7in this latter case carbonic anhydride is disengaged72 for instance1 with barium salts it precipitates an insoluble barium carbonate 7,ba,cl#b ,na#b;,,co#c #b,na,cl ,ba,c,o#c7 and with the aluminium salts it precipitates aluminium hydroxide1 carbonic anhydride being disengaged3 #c,na#b;,,co#c ,al#b7,,so#d7c #c;,h#b,o #c,na#b,,so#d #b;,al7,,oh7#c #c;,,co#b4 ,sodium carbonate1 like all the salts of carbonic acid1 evolves carbonic anhydride on treatment with all acids which are to any extent energetic4 ,but if an acid diluted with water be gradually added to a solution of sodium carbonate1 at first such an evolution does not take place1 because the excess of the carbonic anhydride forms acid sodium carbonate 7sodium bicarbonate71 ,na,h,c,o#c4bj ,the acid sodium carbonate is an unstable salt4 ,not only when heated alone1 but even on being slightly heated in solution1 and also at the ordinary temperature in damp air1 it loses carbonic anhydride and forms the normal salt4 ,and at the same time it is easy to obtain it in a pure crystalline form1 if a strong solution of sodium carbonate be cooled and a stream of carbonic anhydride gas passed through it4 ,the acid salt is less soluble in water than the normal1#ba and therefore a strong solution of the latter gives crystals of the acid salt if carbonic anhydride be passed through it4 ,the acid salt may be yet more conveniently formed from effloresced crystals of sodium carbonate1 which1 on being considerably heated1 very easily absorb carbonic anhydride4#bb ,the acid salt crystallises well1 but not1 however1 in such large crystals as the normal salt2 it has a brackish and not an alkaline taste like that of the normal salt2 its reaction is feebly alkaline1 nearly neutral4 ,at #gj its solution begins to lose carbonic anhydride1 and on boiling the evolution becomes very abundant4 ,from the preceding remarks it is clear that in most reactions this salt1 especially when heated1 acts similarly to the normal salt1 but has1 naturally1 some distinction from it4 ,thus1 for example1 if a solution of sodium carbonate be added to a normal magnesium salt1 a turbidity 7precipitate7 is formed of magnesium carbonate4 ,mg,c,o#c4 ,no such precipitate is formed by the acid salt1 because magnesium carbonate is soluble in the presence of an excess of carbonic anhydride4 #ai ,according to the observations of ,pickering4 ,according to ,rose1 when solutions of sodium carbonate are boiled a certain amount of carbonic anhydride is disengaged4 #bj ,the composition of this salt1 however1 may be also represented as a combination of carbonic acid1 ,h#b;,,co#c1 with the normal salt1 ,na#b;,,co#c1 just as the latter also combines with water4 ,such a combination is all the more likely because 7#a7 there exists another salt1 ,na#b;,,co#c1b,na,h,c,o#c1b;,h#b,o 7sodium sesquicarbonate71 obtained by cooling a boiling solution of sodium bicarbonate1 or by mixing this salt with the normal salt2 but the formula of this salt cannot be derived from that of normal carbonic acid1 as the formula of the bicarbonate can4 ,at the same time the sesqui-salt has all the properties of a definite compound2 it crystallises in transparent crystals1 has a constant composition1 its solubility 7at #j in #ajj of water1 #abf of anhydrous salt7 differs from the solubility of the normal and acid salts2 it is found in nature1 and is known by the names of trona and urao4 ,the observations of ,watts and ,richards showed 7#ahhf7 that on pouring a strong solution of the acid salt into a solution of the normal salt saturated by heating1 crystals of the salt ,na,h,c,o#c1,na#b;,,co#c1b;,h#b,o may be easily obtained1 as long as the temperature is above #ce4 ,the natural urao 7,boussingault7 has1 according to ,laurent1 the same composition4 ,this salt is very stable in air1 and may be used for purifying sodium carbonate on the large scale4 ,such compounds have been little studied from a theoretical point of view1 although particularly interesting1 since in all probability they correspond with ortho-carbonic acid1 ,c7,,oh7#d1 and at the same time correspond with double salts like astrakhanite 7,chapter ,,xiv41 ,note #be74 7#b7 ,water of crystallisation does not enter into the composition of the crystals of the acid salt1 so that on its formation 7occurring only at low temperatures1 as in the formation of crystalline compounds with water7 the water of crystallisation of the normal salt separates and the water is1 as it were1 replaced by the elements of carbonic acid4 ,if anhydrous sodium carbonate be mixed with the amount of water requisite for the formation of ,na#b;,,co#c1;,h#b,o1 this salt will1 when powdered1 absorb ,,co#b as easily at the ordinary temperature as it does water4 #ba #ajj parts of water at #j dissolve #g parts of the acid salt1 which corresponds with #dc parts of the anhydrous normal salt1 but at #j #ajj parts of water dissolve #g parts of the latter4 ,the solubility of the bi-or acid salt varies with considerable regularity2 #ajj parts of water dissolves at #ae #i parts of the salt1 at #cj #aa parts4 ,the ammonium1 and more especially the calcium1 salt1 is much more soluble in water4 ,the ammonia process 7see p4 #ebd7 is founded upon this4 ,ammonium bicarbonate 7acid carbonate7 at #j has a solubility of #ab parts in #ajj water1 at #cj of #bg parts4 ,the solubility therefore increases very rapidly with the temperature4 ,and its saturated solution is more stable than a solution of sodium bicarbonate4 ,in fact1 saturated solutions of these salts have a gaseous tension like that of a mixture of carbonic anhydride and water--namely1 at #ae and at #ej1 for the sodium salt #abj and #gej millimetres1 for the ammonium salt #abj and #efc millimetres4 ,these data are of great importance in understanding the phenomena connected with the ammonia process4 ,they indicate that with an increased pressure the formation of the sodium salt ought to increase if there be an excess of ammonium salt4 #bb ,crystalline sodium carbonate 7broken into lumps7 also absorbs carbonic anhydride1 but the water contained in the crystals is then disengaged3 ,na#b;,,co#c1aj;,h#b,o ,,co#b ,na#b;,,co#c1;,h#b;,,co#c #i;,h#b,o1 and dissolves part of the carbonate2 therefore part of the sodium carbonate passes into solution together with all the impurities4 ,when it is required to avoid the formation of this solution1 a mixture of ignited and crystalline sodium carbonate is taken4 ,sodium bicarbonate is prepared chiefly for medicinal use1 and is then often termed carbonate of soda1 also1 for instance1 in the so-called soda powders1 for preparing certain artificial mineral waters1 for the manufacture of digestive lozenges like those made at ,essentuki1 ,vichy1 c4 ,sodium carbonate is used for the preparation of caustic soda#bc--that is1 the hydrate of sodium oxide1 or the alkali which corresponds to sodium4 ,for this purpose the action of lime on a solution of sodium carbonate is generally made use of4 ,the process is as follows3 a weak1 generally #aj per cent41 solution of sodium carbonate is taken1#bd and boiled in a cast-iron1 wrought-iron1 or silver boiler 7sodium hydroxide does not act on these metals71 and lime is added1 little by little1 during the boiling4 ,this latter is soluble in water1 although but very slightly4 ,the clear solution becomes turbid on the addition of the lime because a precipitate is formed2 this precipitate consists of calcium carbonate1 almost insoluble in water1 whilst caustic soda is formed and remains in solution4 ,the decomposition is effected according to the equation3 ,na#b;,,co#c ,ca7,,ho7#b ,ca,c,o#c #b,na,h,o4 ,on cooling the solution the calcium carbonate easily settles as a precipitate1 and the clear solution or alkali above it contains the easily soluble sodium hydroxide formed in the reaction4#be ,after the necessary quantity of lime has been added1 the solution is allowed to stand1 and is then decanted off and evaporated in cast or wrought iron boilers1 or in silver pans if a perfectly pure product is required4#bf ,the evaporation cannot be conducted in china1 glass1 or similar vessels1 because caustic soda attacks these materials1 although but slightly4 ,the solution does not crystallise on evaporation1 because the solubility of caustic soda when hot is very great1 but crystals containing water of crystallisation may be obtained by cooling4 ,if the evaporation of the alkali be conducted until the specific gravity reaches #ach1 and the liquid is then cooled to #j1 transparent crystals appear containing #b,na,h,o1#g;,h#b,o2 they fuse at #f4bg ,if the evaporation be conducted so long as water is disengaged1 which requires a considerable amount of heat1 then1 on cooling1 the hydroxide1 ,na,h,o1 solidifies in a semi-transparent crystalline mass1#bh which eagerly absorbs moisture and carbonic anhydride from the air4#bi ,its specific gravity is #bac2cj it is easily soluble in water1 with disengagement of a considerable quantity of heat4#ca ,a saturated solution at the ordinary temperature has a specific gravity of about #ae1 contains about #de per cent4 of sodium hydroxide1 and boils at #acj2 at #ee water dissolves an equal weight of it4#cb ,caustic soda is not only soluble in water but in alcohol1 and even in ether4 ,dilute solutions of sodium hydroxide produce a soapy feeling on the skin because the active base of soap consists of caustic soda4#cc ,strong solutions have a corroding action4 #bc ,in chemistry1 sodium oxide is termed 'soda1' which word must be carefully distinguished from the word sodium1 meaning the metal4 #bd ,with a small quantity of water1 the reaction either does not take place1 or even proceeds in the reverse way--that is1 sodium and potassium hydroxides remove carbonic anhydride from calcium carbonate 7,liebig1 ,watson1 ,mitscherlich1 and others74 ,the influence of the mass of water is evident4 ,according to ,gerberts1 however1 strong solutions of sodium carbonate are decomposed by lime1 which is very interesting if confirmed by further investigation4 #be ,as long as any undecomposed sodium carbonate remains in solution1 excess of acid added to the solution disengages carbonic anhydride1 and the solution after dilution gives a white precipitate with a barium salt soluble in acids1 showing the presence of a carbonate in solution 7if there be sulphate present1 it also forms a white precipitate1 but this is insoluble in acids74 ,for the decomposition of sodium carbonate1 milk of lime--that is1 slaked slime suspended in water--is employed4 ,formerly pure sodium hydroxide was prepared 7according to ,berthollet's process7 by dissolving the impure substance in alcohol 7sodium carbonate and sulphate are not soluble71 but now that metallic sodium has become cheap and is purified by distillation1 pure caustic soda is prepared by acting on a small quantity of water with sodium4 ,perfectly pure sodium hydroxide may also be obtained by allowing strong solutions to crystallise 7in the cold7 7,note #bg74 ,in alkali works where the ,leblanc process is used1 caustic soda is prepared directly from the alkali remaining in the mother liquors after the separation of the sodium carbonate by evaporation 7,note #ad74 ,if excess of lime and charcoal have been used1 much sodium hydroxide maybe obtained4 ,after the removal as much as possible of the sodium carbonate1 a red liquid 7from iron oxide7 is left1 containing sodium hydroxide mixed with compounds of sulphur and of cyanogen 7see ,chapter ,,ix47 and also containing iron4 ,this red alkali is evaporated and air is blown through it1 which oxidises the impurities 7for this purpose sometimes sodium nitrate is added1 or bleaching powder1 c47 and leaves fused caustic soda4 ,the fused mass is allowed to settle in order to separate the ferruginous precipitate1 and poured into iron drums1 where the sodium hydroxide solidifies4 ,such caustic soda contains about #aj p4c4 of water in excess and some saline impurities1 but when properly manufactured is almost free from carbonate and from iron4 ,the greater part of the caustic soda1 which forms so important an article of commerce1 is manufactured in this manner4 #bf ,lwig gave a method of preparing sodium hydroxide from sodium carbonate by heating it to a dull red heat with an excess of ferric oxide4 ,carbonic anhydride is given off1 and warm water extracts the caustic soda from the remaining mass4 ,this reaction1 as experiment shows1 proceeds very easily1 and is an example of contact action similar to that of ferric oxide on the decomposition of potassium chlorate4 ,the reason of this may be that a small quantity of the sodium carbonate enters into double decomposition with the ferric oxide1 and the ferric carbonate produced is decomposed into carbonic anhydride and ferric oxide1 the action of which is renewed4 ,similar explanations expressing the reason for a reaction really adds but little to that elementary conception of contact which1 according to my opinion1 consists in the change of motion of the atoms in the molecules under the influence of the substance in contact4 ,in order to represent this clearly it is sufficient1 for instance1 to imagine that in the sodium carbonate the elements ,,co#b move in a circle round the elements ,na#b,o1 but at the points of contact with ,fe#b,o#c the motion becomes elliptic with a long axis1 and at some distance from ,na#b,o the elements of ,,co#b are parted1 not having the faculty of attaching themselves to ,fe#b,o#c4 #bg ,by allowing strong solutions of sodium hydroxide to crystallise in the cold1 impurities--such as1 for instance1 sodium sulphate--may be separated from them4 ,the fused crystallo-hydrate #b,na,h,o1#g;,h#b,o forms a solution having a specific gravity of #adje 7,hermes74 ,the crystals on dissolving in water produce cold1 while ,na,h,o produces heat4 ,besides which ,pickering obtained hydrates with #a1 #b1 #d1 #e1 and #g ,h#b,o4 #bh ,in solid caustic soda there is generally an excess of water beyond that required by the formula ,na,h,o4 ,the caustic soda used in laboratories is generally cast in sticks1 which are broken into pieces4 ,it must be preserved in carefully closed vessels1 because it absorbs water and carbonic anhydride from the air4 #bi ,by the way it changes in air it is easy to distinguish caustic soda from caustic potash1 which in general resembles it4 ,both alkalis absorb water and carbonic anhydride from the air1 but caustic potash forms a deliquescent mass of potassium carbonate1 whilst caustic soda forms a dry powder of efflorescent salt4 #cj ,as the molecular weight of ,na,h,o #dj1 the volume of its molecule #dj_/bac #ahe1 which very nearly approaches the volume of a molecule of water4 ,the same rule applies to the compounds of sodium in general--for instance1 its salts have a molecular volume approaching the volume of the acids from which they are derived4 #ca ,the molecular quantity of sodium hydroxide 7#dj grams71 on being dissolved in a large mass 7#bjj gram molecules7 of water1 develops1 according to ,berthelot #i1ghj1 and according to ,thomsen #i1idj1 heat-units1 but at #ajj about #ac1jjj 7,berthelot74 ,solutions of ,na,h,o n,h#b,o1 on being mixed with water1 evolve heat if they contain less than #f;,h#b,o1 but if more they absorb beat4 #cb ,the specific gravity of solutions of sodium hydroxide at #ae_/d is given in the short table below3-- ,na,h,o1 p4c4 #e #aj #ae #bj #cj #dj ,sp4 gr4 #ajeg #aaac #aafi #abbd #acca #adcf #a1jjj grams of a #e p4c4 solution occupies a volume of #idf c4c42 that is1 less than the water serving to make the solution 7see ,note #ah74 #cc ,sodium hydroxide and some other alkalis are capable of hydrolysing--saponifying1 as it is termed--the compounds of acids with alcohols4 ,if ,,rho 7or ,r7,,ho7n7 represent the composition of an alcohol--that is1 of the hydroxide of a hydrocarbon radicle--and ,,qho an acid1 then the compound of the acid with the alcohol or ethereal salt of the given acid will have the composition ,,rqo4 ,ethereal salts1 therefore1 present a likeness to metallic salts1 just as alcohols resemble basic hydroxides4 ,sodium hydroxide acts on ethereal salts in the same way that it acts on the majority of metallic salts--namely1 it liberates alcohol1 and forms the sodium salt of that acid which was in the ethereal salt4 ,the reaction takes place in the following way3-- ,,rqo ,na,h,o ,na,q,o ,,rho ,ethereal ,caustic ,sodium ,alcohol salt soda salt ,such a decomposition is termed saponification2 similar reactions were known very long ago for the ethereal salts corresponding with glycerin1 ,c#c;,h#e7,,oh7#c 7,chapter ,,ix471 found in animals and plants1 and composing what are called fats or oils4 ,caustic soda1 acting on fat and oil1 forms glycerin1 and sodium salts of those acids which were in union with the glycerin in the fat1 as ,chevreul showed at the beginning of this century4 ,the sodium salts of the fatty acids are commonly known as soaps4 ,that is to say1 soap is made from fat and caustic soda1 glycerin being separated and a sodium salt or soap formed4 ,as glycerin is usually found in union with certain acids1 so also are the sodium salts of the same acids found in soap4 ,the greater part of the acids found in conjunction with glycerin in fats are the solid palmitic and stearic acids1 ,c#af;,h#cb,o#b and ,c#ah;,h#ch,o#b1 and the liquid oleic acid1 ,c#ah;,h#cd,o#b4 ,in preparing soap the fatty substances are mixed with a solution of caustic soda until an emulsion is formed2 the proper quantity of caustic soda is then added in order to produce saponification on heating1 the soap being separated from the solution either by means of an excess of caustic soda or else by common salt1 which displaces the soap from the aqueous solution 7salt water does not dissolve soap1 neither does it form a lather74 ,water acting on soap partly decomposes it 7because the acids of the soap are feeble71 and the alkali set free acts during the application of soap4 ,hence it may be replaced by a very feeble alkali4 ,strong solutions of alkali corrode the skin and tissues4 ,they are not formed from soap1 because the reaction is reversible1 and the alkali is only set free by the excess of water4 ,thus we see how the teaching of ,berthollet renders it possible to understand many phenomena which occur in every-day experience 7see ,chapter ,,ix41 ,note #ae74 ,the chemical reactions of sodium hydroxide serve as a type for those of a whole class of alkalis--that is1 of soluble basic hydroxides1 ,,moh4 ,the solution of sodium hydroxide is a very caustic liquid--that is to say1 it acts in a destructive way on most substances1 for instance on most organic tissues--hence caustic soda1 like all soluble alkalis1 is a poisonous substance2 acids1 for example hydrochloric1 serve as antidotes4 ,the action of caustic soda on bones1 fat1 starch1 and similar vegetable and animal substances explains its action on organisms4 ,thus bones1 when plunged into a weak solution of caustic soda1 fall to powder1#cd and evolve a smell of ammonia1 owing to the caustic soda changing the gelatinous organic substance of the bones 7which contains carbon1 hydrogen1 nitrogen1 oxygen1 and sulphur1 like albumin71 dissolving it and in part destroying it1 whence ammonia is disengaged4 ,fats1 tallow1 and oils become saponified by a solution of caustic soda--that is to say1 they form with it soaps soluble in water1 or sodium salts of the organic acids contained in the fats4#ce ,the most characteristic reactions of sodium hydroxide are determined by the fact that it saturates all acids1 forming salts with them1 which are almost all soluble in water1 and in this respect caustic soda is as characteristic amongst the bases as nitric acid is among the acids4 ,it is impossible to detect sodium by means of the formation of precipitates of insoluble sodium salts1 as may be done with other metals1 many of whose salts are but slightly soluble4 ,the powerful alkaline properties of caustic soda determine its capacity for combining with even the feeblest acids1 its property of disengaging ammonia from ammonium salts1 its faculty of forming precipitates from solutions of salts whose bases are insoluble in water1 c4 ,if a solution of the salt of almost any metal be mixed with caustic soda1 then a soluble sodium salt will be formed1 and an insoluble hydroxide of the metal will be separated--for instance1 copper nitrate yields copper hydroxide1 ,cu7,,no#c7b #b,na,h,o ,cu7,,ho7#b #b,na,n,o#c4 ,even many basic oxides precipitated by caustic soda are capable of combining with it and forming soluble compounds1 and therefore caustic soda in the presence of salts of such metals first forms a precipitate of hydroxide1 and then1 employed in excess1 dissolves this precipitate4 ,this phenomenon occurs1 for example1 when caustic soda is added to the salts of aluminium4 ,this shows the property of such an alkali as caustic soda of combining not only with acids1 but also with feeble basic oxides4 ,for this reason caustic soda acts on most elements which are capable of forming acids or oxides similar to them2 thus the metal aluminium gives hydrogen with caustic soda in consequence of the formation of alumina1 which combines with the caustic soda--that is1 in this case1 the caustic alkali acts on the metal just as sulphuric acid does on ,fe or ,zn4 ,if caustic soda acts in this manner on a metalloid capable of combining with the hydrogen evolved 7aluminium does not give a compound with hydrogen71 then it forms such a hydrogen compound4 ,thus1 for instance1 phosphorus acts in this way on caustic soda1 yielding hydrogen phosphide4 ,when the hydrogen compound disengaged is capable of combining with the alkali1 then1 naturally1 a salt of the corresponding acid is formed4 ,for example1 chlorine and sulphur act in this way on caustic soda4 ,chlorine1 with the hydrogen of the caustic soda1 forms hydrochloric acid1 and the latter forms common salt with the sodium hydroxide1 whilst the other atom in the molecule of chlorine1 ,cl#b1 takes the place of the hydrogen1 and forms the hypochlorite1 ,na,cl,o4 ,in the same way1 by the action of sodium hydroxide on sulphur1 hydrogen sulphide is formed1 which acts on the soda forming sodium sulphide1 in addition to which sodium thiosulphate is formed 7see ,chapter ,,xx47 ,by virtue of such reactions1 sodium hydroxide acts on many metals and non-metals4 ,such action is often accelerated by the presence of the oxygen of the air1 as by this means the formation of acids and oxides rich in oxygen is facilitated4 ,thus many metals and their lower oxides1 in the presence of an alkali1 absorb oxygen and form acids4 ,even manganese peroxide1 when mixed with caustic soda1 is capable of absorbing the oxygen of the air1 and forming sodium manganate4 ,organic acids when heated with caustic soda give up to it the elements of carbonic anhydride1 forming sodium carbonate1 and separating that hydrocarbon group which exists1 in combination with carbonic anhydride1 in the organic acid4 #cd ,on this is founded the process of ,henkoff and ,engelhardt for treating bones4 ,the bones are mixed with ashes1 lime1 and water2 it is true that in this case more potassium hydroxide than sodium hydroxide is formed1 but their action is almost identical4 #ce ,as explained in ,note #cc4 ,thus sodium hydroxide1 like the soluble alkalis in general1 ranks amongst the most active substances in the chemical sense of the term1 and but few substances are capable of resisting it4 ,even siliceous rocks1 as we shall see further on1 are transformed by it1 forming when fused with it vitreous slags4 ,sodium hydroxide 7like ammonium and potassium hydroxides71 as a typical example of the basic hydrates1 in distinction from many other basic oxides1 easily forms acid salts with acids 7for instance1 ,na,h,s,o#d1 ,na,h,c,o#c71 and does not form any basic salts at all2 whilst many less energetic bases1 such as the oxides of copper and lead1 easily form basic salts1 but acid salts only with difficulty4 ,this capability of forming acid salts1 particularly with polybasic acids1 may be explained by the energetic basic properties of sodium hydroxide1 contrasted with the small development of these properties in the bases which easily form basic salts4 ,an energetic base is capable of retaining a considerable quantity of acid1 which a slightly energetic base would not have the power of doing4 ,also1 as will be shown in the subsequent chapters1 sodium belongs to the univalent metals1 being exchangeable for hydrogen atom for atom--that is1 amongst metals sodium may1 like chlorine amongst the non-metals1 serve as the representative of the univalent properties4 ,most of the elements which are not capable of forming acid salts are bivalent4 ,whence it may be understood that in a bibasic acid--for instance1 carbonic1 ,h#b;,,co#c1 or sulphuric1 ,h#b,,so#d--the hydrogen may be exchanged1 atom for atom1 for sodium1 and yield an acid salt by means of the first substitution1 and a normal salt by means of the second--for instance1 ,na,h,s,o#d1 and ,na#b,,so#d1 whilst such bivalent metals as calcium and barium do not form acid salts because one of their atoms at once takes the place of both hydrogen atoms1 forming1 for example1 ,ca,c,o#c and ,ca,s,o#d4ce bis #ce bis ,it might be expected1 from what has been mentioned above1 that bivalent metals would easily form acid salts with acids containing more than two atoms of hydrogen--for instance1 with tribasic acids1 such as phosphoric acid1 ,h#c,,po#d--;and actually such salts do exist2 but all such relations are complicated by the fact that the character of the base very often changes and becomes weakened with the increase of valency and the change of atomic weight2 the feebler bases 7like silver oxide71 although corresponding with univalent metals1 do not form acid salts1 while the feeblest bases 7,cu,o1 ,pb,o1 c47 easily form basic salts1 and notwithstanding their valency do not form acid salts which are in any degree stable--that is1 which are undecomposable by water4 ,basic and acid salts ought to be regarded rather as compounds similar to crystallo-hydrates1 because such acids as sulphuric form with sodium not only an acid and a normal salt1 as might be expected from the valency of sodium1 but also salts containing a greater quantity of acid4 ,in sodium sesquicarbonate we saw an example of such compounds4 ,taking all this into consideration1 we must say that the property of more or less easily forming acid salts depends more upon the energy of the base than upon its valency1 and the best statement is that the capacity of a base for forming acid and basic salts is characteristic1 just as the faculty of forming compounds with hydrogen is characteristic of elements4 ,we have seen the transformation of common salt into sodium sulphate1 of this latter into sodium carbonate1 and of sodium carbonate into caustic soda4 ,lavoisier still regarded sodium hydroxide as an element1 because he was unacquainted with its decomposition with the formation of metallic sodium1 which separates the hydrogen from water1 reforming caustic soda4 ,the preparation of metallic sodium was one of the greatest discoveries in chemistry1 not only because through it the conception of elements became broader and more correct1 but especially because in sodium1 chemical properties were observed which were but feebly shown in the other metals more familiarly known4 ,this discovery was made in #ahjg by the ,english chemist ,davy by means of the galvanic current4 ,by connecting with the positive pole 7of copper or carbon7 a piece of caustic soda 7moistened in order to obtain electrical conductivity71 and boring a hole in it filled with mercury connected with the negative pole of a strong ,volta's pile1 ,davy observed that on passing the current a peculiar metal dissolved in the mercury1 less volatile than mercury1 and capable of decomposing water1 again forming caustic soda4 ,in this way 7by analysis and synthesis7 ,davy demonstrated the compound nature of alkalis4 ,on being decomposed by the galvanic current1 caustic soda disengages hydrogen and sodium at the negative pole and oxygen at the positive pole4 ,davy showed that the metal formed volatilises at a red heat1 and this is its most important physical property in relation to its extraction1 all later methods being founded on it4 ,besides this ,davy observed that sodium easily oxidises1 its vapour taking fire in air1 and the latter circumstance was for a long time an obstacle to the easy preparation of this metal4 ,the properties of sodium were subsequently more thoroughly investigated by ,gay-,lussac and ,thnard1 who observed that metallic iron at a high temperature was capable of reducing caustic soda to sodium4#cf ,brunner latterly discovered that not only iron1 but also charcoal1 has this property1 although hydrogen has not4#cg ,but still the methods of extracting sodium were very troublesome1 and consequently it was a great rarity4 ,the principal obstacle to its production was that an endeavour was made to condense the easily-oxidising vapours of sodium in vacuo in complicated apparatus4 ,for this reason1 when ,donny and ,maresca1 having thoroughly studied the matter1 constructed a specially simple condenser1 the production of sodium was much facilitated4 ,furthermore1 in practice the most important epoch in the history of the production of sodium is comprised in the investigation of ,sainte-,claire ,deville1 who avoided the complex methods in vogue up to that time1 and furnished those simple means by which the production of sodium is now rendered feasible in chemical works4 #cf ,deville supposes that such a decomposition of sodium hydroxide by metallic iron depends solely on the dissociation of the alkali at a white heat into sodium1 hydrogen1 and oxygen4 ,here the part played by the iron is only that it retains the oxygen formed1 otherwise the decomposed elements would again reunite upon cooling1 as in other cases of dissociation4 ,if it be supposed that the temperature at the commencement of the dissociation of the iron oxides is higher than that of sodium oxide1 then the decomposition may be explained by ,deville's hypothesis4 ,deville demonstrates his views by the following experiment3--,an iron bottle1 filled with iron borings1 was heated in such a way that the upper part became red hot1 the lower part remaining cooler2 sodium hydroxide was introduced into the upper part4 ,the decomposition was then effected--that is1 sodium vapours were produced 7this experiment was really performed with potassium hydroxide74 ,on opening the bottle it was found that the iron in the upper part was not oxidised1 but only that in the lower part4 ,this may be explained by the decomposition of the alkali into sodium1 hydrogen1 and oxygen taking place in the upper part1 whilst the iron in the lower part absorbed the oxygen set free4 ,if the whole bottle be subjected to the same moderate heat as the lower extremity1 no metallic vapours are formed4 ,in that case1 according to the hypothesis1 the temperature is insufficient for the dissociation of the sodium hydroxide4 #cg ,it has been previously remarked 7,chapter ,,ii4 ,note #i7 that ,beketoff showed the displacement of sodium by hydrogen1 not from sodium hydroxide but from the oxide ,na#b,o2 then1 however1 only one half is displaced1 with the formation of ,na,h,o4 ,for the production of sodium according to ,deville's method1 a mixture of anhydrous sodium carbonate 7#g parts71 charcoal 7two parts71 and lime or chalk 7#g parts7 is heated4 ,this latter ingredient is only added in order that the sodium carbonate1 on fusing1 shall not separate from the charcoal4#ch ,the chalk on being heated loses carbonic anhydride1 leaving infusible lime1 which is permeated by the sodium carbonate and forms a thick mass1 in which the charcoal is intimately mixed with the sodium carbonate4 ,when the charcoal is heated with the sodium carbonate1 at a white heat1 carbonic oxide and vapours of sodium are disengaged1 according to the equation3 ,na#b;,,co#c #b;,c ,na#b #c;,,co #ch ,since the close of the eighties in ,england1 where the preparation of sodium is at present carried out on a large commercial scale 7from #ahfj to #ahgj it was only manufactured in a few works in ,france71 it has been the practice to add to ,deville's mixture iron1 or iron oxide which with the charcoal gives metallic and carburetted iron1 which still further facilitates the decomposition4 ,at present a kilogram of sodium may be purchased for about the same sum 7#b_/-7 as a gram cost thirty years ago4 ,castner1 in ,england1 greatly improved the manufacture of sodium in large quantities1 and so cheapened it as a reducing agent in the preparation of metallic aluminium4 ,he heated a mixture of #dd parts of ,na,h,o1 and #g parts of carbide of iron in large iron retorts at #a1jjj and obtained about #f-a_/b parts of metallic sodium4 ,the reaction proceeds more easily than with carbon or iron alone1 and the decomposition of the ,na,h,o proceeds according to the equation3 #c,na,h,o ,c ,na#b;,,co#c #c;,h ,na4 ,subsequently1 in #ahia1 aluminium was prepared by electrolysis 7see ,chapter ,,xvii471 and metallic sodium found two new uses2 7#a7 for the manufacture of peroxide of sodium 7see later on7 which is used in bleaching works1 and 7#b7 in the manufacture of potassium and sodium cyanide from yellow prussiate 7,chapter ,,xiii41 ,note #ab74 ,illustration3 ,,fig4 #gj4--,manufacture of sodium by ,deville's process4 ,a ,c1 iron tube containing a mixture of soda1 charcoal1 and chalk4 ,b1 condenser4 ,on cooling the vapours and gases disengaged1 the vapours condense into molten metal 7in this form sodium does not easily oxidise1 whilst in vapour it burns7 and the carbonic oxide remains as gas4 ,illustration3 ,,fig4 #ga4--;,donny and ,maresca's sodium condenser1 consisting of two cast-iron plates screwed together4 ,in sodium works an iron tube1 about a metre long and a decimeter in diameter1 is made out of boiler plate4 ,the pipe is luted into a furnace having a strong draught1 capable of giving a high temperature1 and the tube is charged with the mixture required for the preparation of sodium4 ,one end of the tube is closed with a cast-iron stopper ,a with clay luting1 and the other with the cast-iron stopper ,c provided with an aperture4 ,on heating1 first of all the moisture contained in the various substances is given off1 then carbonic anhydride and the products of the dry distillation of the charcoal1 then the latter begins to act on the sodium carbonate1 and carbonic oxide and vapours of sodium appear4 ,it is easy to observe the appearance of the latter1 because on issuing from the aperture in the stopper ,c they take fire spontaneously and burn with a very bright yellow flame4 ,a pipe is then introduced into the aperture ,c1 compelling the vapours and gases formed to pass through the condenser ,b4 ,this condenser consists of two square cast-iron trays1 ,a and ,a1 fig4 #ga1 with wide edges firmly screwed together4 ,between these two trays there is a space in which the condensation of the vapours of sodium is effected1 the thin metallic walls of the condenser being cooled by the air but remaining hot enough to preserve the sodium in a liquid state1 so that it does not choke the apparatus1 but continually flows from it4 ,the vapours of sodium1 condensing in the cooler1 flow in the shape of liquid metal into a vessel containing some non-volatile naphtha or hydrocarbon4 ,this is used in order to prevent the sodium oxidising as it issues from the condenser at a somewhat high temperature4 ,in order to obtain sodium of a pure quality it is necessary to distil it once more1 which may even be done in porcelain retorts1 but the distillation must be conducted in a stream of some gas on which sodium does not act1 for instance in a stream of nitrogen2 carbonic anhydride is not applicable1 because sodium partially decomposes it1 absorbing oxygen from it4 ,although the above described methods of preparing sodium by chemical means have proved very convenient in practice1 still it is now 7since #ahic7 found profitable in ,england to obtain it 7to the amount of several tons a week7 by ,davy's classical method1 i4e4 by the action of an electric current at a moderately high temperature1 because the means for producing an electric current 7by motors and dynamos7 now render this quite feasible4 ,this may be regarded as a sign that in process of time many other technical methods for producing various substances by decomposition may be profitably carried on by electrolysis4 ,pure sodium is a lustrous metal1 white as silver1 soft as wax2 it becomes brittle in the cold4 ,in ordinary moist air it quickly tarnishes and becomes covered with a film of hydroxide1 ,na,h,o1 formed at the expense of the water in the air4 ,in perfectly dry air sodium retains its lustre for an indefinite time4 ,its density at the ordinary temperature is equal to #jih1 so that it is lighter than water2 it fuses very easily at a temperature of #ie1 and distils at a bright red heat 7#gdb according to ,perman1 #ahhi74 ,scott 7#ahhg7 determined the density of sodium vapour and found it to be nearly #ab 7if ,h #a74 ,this shows that its molecule contains one atom 7like mercury and cadmium7 ,na4#ch bis ,it forms alloys with most metals1 combining with them1 heat being sometimes evolved and sometimes absorbed4 ,thus1 if sodium 7having a clean surface7 be thrown into mercury1 especially when heated1 there is a flash1 and such a considerable amount of heat is evolved that part of the mercury is transformed into vapour4#ci ,compounds or solutions of sodium in mercury1 or amalgams of sodium1 even when containing #b parts of sodium to #ajj parts of mercury1 are solids4 ,only those amalgams which are the very poorest in sodium are liquid4 ,such alloys of sodium with mercury are often used instead of sodium in chemical investigations1 because in combination with mercury sodium is not easily acted on by air1 and is heavier than water1 and therefore more convenient to handle1 whilst at the same time it retains the principal properties of sodium1#dj for instance it decomposes water1 forming ,na,h,o4 #ch bis ,this is also shown by the fall in the temperature of solidification of tin produced by the addition of sodium 7and also ,al and ,zn74 ,heycock and ,neville 7#ahhi74 #ci ,by dissolving sodium amalgams in water and acids1 and deducting the heat of solution of the sodium1 ,berthelot found that for each atom of the sodium in amalgams containing a larger amount of mercury than ,na,hg#e1 the amount of heat evolved increases1 after which the heat of formation falls1 and the heat evolved decreases4 ,in the formation of ,na,hg#e about #ah1ejj calories are evolved2 when ,na,hg#c is formed1 about #ad1jjj2 and for ,na,hg about #aj1jjj calories4 ,kraft regarded the definite crystalline amalgam as having the composition of ,na,hg#f1 but at the present time1 in accordance with ,grimaldi's results1 it is thought to be ,na,hg#e4 ,a similar amalgam is very easily obtained if a #c p4c4 amalgam be left several days in a solution of sodium hydroxide until a crystalline mass is formed1 from which the mercury may be removed by strongly pressing in chamois leather4 ,this amalgam with a solution of potassium hydroxide forms a potassium amalgam1 ,k,hg#aj4 ,it may be mentioned here that the latent heat of fusion 7of atomic quantities7 of ,hg #cfj 7,personne71 ,na #gcj 7,joannis71 and ,k #faj calories 7,joannis74 #dj ,alloys are so similar to solutions 7exhibiting such complete parallelism in properties7 that they are included in the same class of so-called indefinite compounds4 ,but in alloys1 as substances passing from the liquid to the solid state1 it is easier to discover the formation of definite chemical compounds4 ,besides the alloys of ,na with ,hg1 those with tin 7,bailey #ahib found ,na#b,sn71 lead 7,na,pb71 bismuth 7,na#c;,bi71 c4 7,joannis #ahib and others7 have been investigated4 ,it is easy to form an alloy of mercury and sodium having a crystalline structure1 and a definite atomic composition1 ,na,hg#e4 ,the alloy of sodium with hydrogen or sodium hydride1 ,na#b;,h1 which has the external appearance of a metal1#da is a most instructive example of the characteristics of alloys4 ,at the ordinary temperature sodium does not absorb hydrogen1 but from #cjj to #dba the absorption takes place at the ordinary pressure 7and at an increased pressure even at higher temperatures71 as shown by ,troost and ,hautefeuille 7#ahgd74 ,one volume of sodium absorbs as much as #bch volumes of hydrogen4 ,the metal increases in volume1 and when once formed the alloy can be preserved for some time without change at the ordinary temperature4 ,the appearance of sodium hydride resembles that of sodium itself2 it is as soft as this latter1 when heated it becomes brittle1 and decomposes above #cjj1 evolving hydrogen4 ,in this decomposition all the phenomena of dissociation are very clearly shown--that is1 the hydrogen gas evolved has a definite tension#db corresponding with each definite temperature4 ,this confirms the fact that the formation of substances capable of dissociation can only be accomplished within the dissociation limits4 ,sodium hydride melts more easily than sodium itself1 and then does not undergo decomposition if it is in an atmosphere of hydrogen4 ,it oxidises easily in air1 but not so easily as potassium hydride4 ,the chemical reactions of sodium are retained in its hydride1 and1 if we may so express it1 they are even increased by the addition of hydrogen4 ,at all events1 in the properties of sodium hydride#dc we see other properties than in such hydrogen compounds as ,h,cl1 ,h#b,o1 ,h#c,n1 ,h#d;,c1 or even in the gaseous metallic hydrides ,as,h#c1 ,te,h#b4 ,platinum1 palladium1 nickel1 and iron1 in absorbing hydrogen form compounds in which hydrogen is in a similar state4 ,in them1 as in sodium hydride1 the hydrogen is compressed1 absorbed1 occluded 7,chapter ,,ii47#dc bis #da ,potassium forms a similar compound1 but lithium1 under the same circumstances1 does not4 #db ,the tension of dissociation of hydrogen p1 in millimetres of mercury1 is3-- t #ccj #cej #djj #dcj for ,na#b;,h p #bh #eg #ddg #iaj for ,k#b;,h #de #gb #edh #aajj #dc ,in general1 during the formation of alloys the volumes change very slightly1 and therefore from the volume of ,na#b;,h some idea may be formed of the volume of hydrogen in a solid or liquid state4 ,even ,archimedes concluded that there was gold in an alloy of copper and gold by reason of its volume and density4 ,from the fact that the density of ,na#b;,h is equal to #jiei1 it may be seen that the volume of #dg grams 7the gram molecule7 of this compound #dij c4c4 ,the volume of #df grams of sodium contained in the ,na#b;,h 7the density under the same conditions being #jig7 is equal to #dgd c4c4 ,therefore the volume of #a gram of hydrogen in ,na#b;,h is equal to #af c4c41 and consequently the density of metallic hydrogen1 or the weight of #a c4c41 approaches #jf gram4 ,this density is also proper to the hydrogen alloyed with potassium and palladium4 ,judging from the scanty information which is at present available1 liquid hydrogen near its absolute boiling point 7,chapter ,,ii47 has a much lower density4 #dc bis ,we may remark that at low temperatures ,na absorbs ,,nh#c and forms 7,,nh#c,na7#b 7see ,chapter ,,vi41 ,note #ad72 this substance absorbs ,,co and gives 7,na,c,o7n 7,chapter ,,ix41 ,note #ca71 although by itself ,na does not combine directly with ,,co 7but ,k does74 ,the most important chemical property of sodium is its power of easily decomposing water and evolving hydrogen from the majority of the hydrogen compounds1 and especially from all acids1 and hydrates in which hydroxyl must be recognised4 ,this depends on its power of combining with the elements which are in combination with the hydrogen4 ,we already know that sodium disengages hydrogen1 not only from water1 hydrochloric acid1#dd and all other acids1 but also from ammonia1#dd bis with the formation of sodamide ,,nh#b,na1 although it does not displace hydrogen from the hydrocarbons4#de ,sodium burns both in chlorine and in oxygen1 evolving much heat4 ,these properties are closely connected with its power of taking up oxygen1 chlorine1 and similar elements from most of their compounds4 ,just as it removes the oxygen from the oxides of nitrogen and from carbonic anhydride1 so also does it decompose the majority of oxides at definite temperatures4 ,here the action is essentially the same as in the decomposition of water4 ,thus1 for instance1 when acting on magnesium chloride the sodium displaces the magnesium1 and when acting on aluminium chloride it displaces metallic aluminium4 ,sulphur1 phosphorus1 arsenic and a whole series of other elements1 also combine with sodium4#df #dd ,h4 ,a4 ,schmidt remarked that perfectly dry hydrogen chloride is decomposed with great difficulty by sodium1 although the decomposition proceeds easily with potassium and with sodium in moist hydrogen chloride4 ,wanklyn also remarked that sodium burns with great difficulty in dry chlorine4 ,probably these facts are related to other phenomena observed by ,dixon1 who found that perfectly dry carbonic oxide does not explode with oxygen on passing an electric spark4 #dd bis ,sodamide1 ,,nh#b,na1 7,chapter ,,iv41 ,note #ad71 discovered by ,gay-,lussac and ,thnard1 has formed the object of repeated research1 but has been most fully investigated by ,a4 ,w4 ,titherley 7#ahid74 ,until recently the following was all that was known about this compound3-- ,by heating sodium in dry ammonia1 ,gay-,lussac and ,thnard obtained an olive-green1 easily-fusible mass1 sodamide1 ,,nh#b,na1 hydrogen being separated4 ,this substance with water forms sodium hydroxide and ammonia2 with carbonic oxide1 ,,co1 it forms sodium cyanide1 ,na,c,n1 and water1 ,h#b,o2 and with dry hydrogen chloride it forms sodium and ammonium chlorides4 ,these and other reactions of sodamide show that the metal in it preserves its energetic properties in reaction1 and that this compound of sodium is more stable than the corresponding chlorine amide4 ,when heated1 sodamide1 ,,nh#b,na1 only partially decomposes1 with evolution of hydrogen1 the principal part of it giving ammonia and sodium nitride1 ,na#c,n1 according to the equation #c,,nh#b,na #b,,nh#c ,n,na#c4 ,the latter is an almost black powdery mass1 decomposed by water into ammonia and sodium hydroxide4 ,titherley's researches added the following data3-- ,iron or silver vessels should be used in preparing this body1 because glass and porcelain are corroded at #cjj-djj1 at which temperature ammonia gas acts upon sodium and forms the amide with the evolution of hydrogen4 ,the reaction proceeds slowly1 but is complete if there be an excess of ,,nh#c4 ,pure ,,nh#b,na is colourless 7its colouration is due to various impurities71 semi-transparent1 shows traces of crystallisation1 has a conchoidal fracture1 and melts at #ade4 ,judging from the increase in weight of the sodium and the quantity of hydrogen which is disengaged1 the composition of the amide is exactly ,,nh#b,na4 ,it partially volatilises 7sublimes7 in vacuo at #bjj1 and breaks up into #b,na ,n#b #b;,h#b at #ejj4 ,the same amide is formed when oxide of sodium is heated in ,,nh#c3 ,na#b,o #b,,nh#c #b,na,h#b,n ,h#b,o4 ,na,h,o is also formed to some extent by the resultant ,h#b,o4 ,potassium and lithium form similar amides4 ,with water1 alcohol1 and acids1 ,,nh#b,na gives ,,nh#c and ,na,h,o1 which react further4 ,anhydrous ,ca,o absorbs ,,nh#b,na when heated without decomposing it4 ,when sodamide is heated with ,si,o#b1 ,,nh#c is disengaged1 and silicon nitride formed4 ,it acts still more readily upon boric anhydride when heated with it3 #b,,nh#b,na ,b#b,o#c #b;,,bn #b,na,h,o ,h#b,o4 ,when slightly heated1 ,,nh#b,na ,n,o,cl ,na,cl ,n#b ,h#b,o 7,n,h,na#b and ,n,na#c are apparently not formed at a higher temperature74 ,the halogen organic compounds react with the aid of heat1 but with so much energy that the reaction frequently leads to the ultimate destruction of the organic groups and production of carbon4 #de ,as sodium does not displace hydrogen from the hydrocarbons1 it may be preserved in liquid hydrocarbons4 ,naphtha is generally used for this purpose1 as it consists of a mixture of various liquid hydrocarbons4 ,however1 in naphtha sodium usually becomes coated with a crust composed of matter produced by the action of the sodium on certain of the substances contained in the mixture composing naphtha4 ,in order that sodium may retain its lustre in naphtha1 secondary octyl alcohol is added4 7,this alcohol is obtained by distilling castor oil with caustic potash47 ,sodium keeps well in a mixture of pure benzene and paraffin4 #df ,if sodium does not directly displace the hydrogen in hydrocarbons1 still by indirect means compounds may be obtained which contain sodium and hydrocarbon groups4 ,some of these compounds have been produced1 although not in a pure state4 ,thus1 for instance1 zinc ethyl1 ,zn7,c#b;,h#e7b1 when treated with sodium1 loses zinc and forms sodium ethyl1 ,c#b;,h#e,na1 but this decomposition is not complete1 and the compound formed cannot be separated by distillation from the remaining zinc ethyl4 ,in this compound the energy of the sodium is clearly manifest1 for it reacts with substances containing haloids1 oxygen1 c41 and directly absorbs carbonic anhydride1 forming a salt of a carboxylic acid 7propionic74 ,with oxygen sodium unites in three degrees of combination1 forming a suboxide ,na#d,o1#df bis an oxide1 ,na#b,o1 and a peroxide1 ,na,o4 ,they are thus termed because ,na#b,o is a stable basic oxide 7with water it forms a basic hydroxide71 whilst ,na#d,o and ,na,o do not form corresponding saline hydrates and salts4 ,the suboxide is a grey inflammable substance which easily decomposes water1 disengaging hydrogen2 it is formed by the slow oxidation of sodium at the ordinary temperature4 ,the peroxide is a greenish yellow substance1 fusing at a bright red heat2 it is produced by burning sodium in an excess of oxygen1 and it yields oxygen when treated with water3 ,suboxide3 ,na#d,o #c;,h#b,o #d,na,h,o ,h#bdg ,oxide3 ,na#b,o ,h#b,o #b,na,h,o#dh ,peroxide3 ,na#b,o#b ,h#b,o #b,na,h,o ,o#di ,all three oxides form sodium hydroxide with water1 but only the oxide ,na#b,o is directly transformed into a hydrate4 ,the other oxides liberate either hydrogen or oxygen2 they also present a similar distinction with reference to many other agents4 ,thus carbonic anhydride combines directly with the oxide ,na#b,o1 which when heated in the gas burns1 forming sodium carbonate1 whilst the peroxide yields oxygen in addition4 ,when treated with acids1 sodium and all its oxides only form the salts corresponding with sodium oxide--that is1 of the formula or type ,na,x4 ,thus the oxide of sodium1 ,na#b,o1 is the only salt-forming oxide of this metal1 as water is in the case of hydrogen4 ,although the peroxide ,h#b,o#b is derived from hydrogen1 and ,na#b,o#b from sodium1 yet there are no corresponding salts known1 and if they are formed they are probably as unstable as hydrogen peroxide4 ,although carbon forms carbonic oxide1 ,,co1 still it has only one salt-forming oxide--carbonic anhydride1 ,,co#b4 ,nitrogen and chlorine both give several salt-forming oxides and types of salts4 ,but of the oxides of nitrogen1 ,,no and ,,no#b do not form salts1 as do ,n#b,o#c1 ,n#b,o#d1 and ,n#b,o#e1 although ,n#b,o#d does not form special salts1 and ,n#b,o#e corresponds with the highest form of the saline compounds of nitrogen4 ,such distinctions between the elements1 according to their power of giving one or several saline forms1 is a radical property of no less importance than the basic or acid properties of their oxides4 ,sodium as a typical metal does not form any acid oxides1 whilst chlorine1 as a typical non-metal1 does not form bases with oxygen4 ,therefore sodium as an element may be thus characterised3 it forms one very stable salt-forming oxide1 ,na#b,o1 having powerful basic properties1 and its salts are of the general formula1 ,na,x1 therefore in its compounds it is1 like hydrogen1 a basic and univalent element4 #df bis ,it is even doubtful whether the suboxide exists 7see ,note #dg74 #dg ,a compound1 ,na#b;,cl1 which corresponds with the suboxide1 is apparently formed when a galvanic current is passed through fused common salt2 the sodium liberated dissolves in the common salt1 and does not separate from the compound either on cooling or on treatment with mercury4 ,it is therefore supposed to be ,na#b;,cl2 the more so as the mass obtained gives hydrogen when treated with water3 ,na#b;,cl ,h#b,o ,h ,na,h,o ,na,cl1 that is1 it acts like suboxide of sodium4 ,if ,na#b;,cl really exists as a salt1 then the corresponding base ,na#d,o1 according to the rule with other bases of the composition ,m#d,o1 ought to be called a quaternary oxide4 ,according to certain evidence1 a suboxide is formed when thin sheets or fine drops of sodium slowly oxidise in moist air4 #dh ,according to observations easily made1 sodium when fused in air oxidises but does not burn1 the combustion only commencing with the formation of vapour--that is1 when considerably heated4 ,davy and ,karsten obtained the oxides of potassium1 ,k#b,o1 and of sodium1 ,na#b,o1 by heating the metals with their hydroxides1 whence ,na,h,o ,na ,na#b,o ,h1 but ,n4 ,n4 ,beketoff failed to obtain oxides by this means4 ,he prepared them by directly igniting the metals in dry air1 and afterwards heating with the metal in order to destroy any peroxide4 ,the oxide produced1 ,na#b,o1 when heated in an atmosphere of hydrogen1 gave a mixture of sodium and its hydroxide3 ,na#b,o ,h ,na,h,o ,na 7see ,chapter ,,ii41 ,note #i74 ,if both the observations mentioned are accurate1 then the reaction is reversible4 ,sodium oxide ought to be formed during the decomposition of sodium carbonate by oxide of iron 7see ,note #bf71 and during the decomposition of sodium nitrite4 ,according to ,karsten1 its specific gravity is #bh1 according to ,beketoff #bc4 ,the difficulty in obtaining it is owing to an excess of sodium forming the suboxide1 and an excess of oxygen the peroxide4 ,the grey colour peculiar to the suboxide and oxide perhaps shows that they contain metallic sodium4 ,in addition to this1 in the presence of water it may contain sodium hydride and ,na,h,o4 #di ,of the oxides of sodium1 that easiest to form is the peroxide1 ,na,o or ,na#b,o#b2 this is obtained when sodium is burnt in an excess of oxygen4 ,if ,na,n,o#c be melted1 it gives ,na#b,o#b with metallic ,na4 ,in a fused state the peroxide is reddish yellow1 but it becomes almost colourless when cold4 ,when heated with iodine vapour1 it loses oxygen3 ,na#b,o#b ,i#b ,na#b,,oi#b ,o4 ,the compound ,na#b,,oi#b is akin to the compound ,cu#b,o,cl#b obtained by oxidising ,cu,cl4 ,this reaction is one of the few in which iodine directly displaces oxygen4 ,the substance ,na#b,,oi#b is soluble in water1 and when acidified gives free iodine and a sodium salt4 ,carbonic oxide is absorbed by heated sodium peroxide with formation of sodium carbonate3 ,na#b;,,co#c ,na#b,o#b ,,co1 whilst carbonic anhydride liberates oxygen from it4 ,with nitrous oxide it reacts thus3 ,na#b,o#b #b,n#b,o #b,na,n,o#b ,n#b2 with nitric oxide it combines directly1 forming sodium nitrite1 ,na,o ,,no ,na,n,o#b4 ,sodium peroxide1 when treated with water1 does not give hydrogen peroxide1 because the latter in the presence of the alkali formed 7,na#b,o#b #b;,h#b,o #b,na,h,o ,h#b,o#b7 decomposes into water and oxygen4 ,in the presence of dilute sulphuric acid it forms ,h#b,o#b 7,na#b,o#b ,h#b,,so#d ,na#b,,so#d ,h#b,o#b74 ,peroxide of sodium is now prepared on a large scale 7by the action of air upon ,na at #cjj7 for bleaching wool1 silk c4 7when it acts in virtue of the ,h#b,o#b formed74 ,the oxidising properties of ,na#b,o#b under the action of heat are seen1 for instance1 in the fact that when heated with ,i it forms sodium iodate2 with ,pb,o1 ,na#b,pb,o#c2 with pyrites1 sulphates1 c4 ,when peroxide of sodium comes into contact with water1 it evolves much heat1 forming ,h#b,o#b1 and decomposing with the disengagement of oxygen2 but1 as a rule1 there is no explosion4 ,but if ,na#b,o#b be placed in contact with organic matter1 such as sawdust1 cotton1 c41 it gives a violent explosion when heated1 ignited1 or acted on by water4 ,peroxide of sodium forms an excellent oxidising agent for the preparation of the higher product of oxidation of ,mn1 ,cr1 ,w1 c41 and also for oxidising the metallic sulphides4 ,it should therefore find many applications in chemical analysis4 ,to prepare ,na#b,o#b on a large scale1 ,castner melts ,na in an aluminium vessel1 and at #cjj passes first air deprived of a portion of its oxygen 7having been already once used71 and then ordinary dry air over it4 ,on comparing sodium and its analogues1 which will be described later with other metallic elements1 it will be seen that these properties1 together with the relative lightness of the metal itself and its compounds1 and the magnitude of its atomic weight comprise the most essential properties of this element1 clearly distinguishing it from others1 and enabling us easily to recognise its analogues4 ,,chapter ,,xiii ,,potassium1 ,,rubidium1 ,c,,sium1 ,,and ,,lithium4 ,,spectrum ,,analysis ,just as the series of halogens1 fluorine1 bromine and iodine correspond with the chlorine contained in common salt1 so also there exists a corresponding series of elements3 lithium1 ,li #g1 potassium1 ,k #ci1 rubidium1 ,rb #he1 and csium1 ,cs #acc1 which are analogous to the sodium in common salt4 ,these elements bear as great a resemblance to sodium1 ,na #bc1 as fluorine1 ,f #ai1 bromine1 ,br #hj1 and iodine1 ,i #abg1 do to chlorine1 ,cl #cee4 ,indeed1 in a free state1 these elements1 like sodium1 are soft metals which rapidly oxidise in moist air and decompose water at the ordinary temperature1 forming soluble hydroxides having clearly-defined basic properties and the composition ,,rho1 like that of caustic soda4 ,the resemblance between these metals is sometimes seen with striking clearness1 especially in compounds such as salts4#a ,the corresponding salts of nitric1 sulphuric1 carbonic1 and nearly all acids with these metals have many points in common4 ,the metals which resemble sodium so much in their reactions are termed the metals of the alkalis4 #a ,tutton's researches 7#ahid7 upon the analogy of the crystalline forms of ,k#b,,so#d1 ,rb#b,,so#d and ,cs#b,,so#d may be taken as a typical example of the comparison of analogous compounds4 ,we cite the following data from these excellent researches3 the sp4 gr4 at #bj_/d of ,k#b,,so#d is #bffcc of ,rb#b,,so#d1 #cfaac1 and of ,cs#b,,so#d1 #dbdcd4 ,the coefficient of cubical expansion 7the mean between #bj and #fj7 for the ,k salt is #jjjec1 for the ,rb salt #jjjeb1 for the ,cs salt #jjjea4 ,the linear expansion 7the maximum for the vertical axis7 along the axis of crystallisation is the same for all three salts1 within the limits of experimental error4 ,the replacement of potassium by rubidium causes the distance between the centres of the molecules in the direction of the three axes of crystallisation to increase equally1 and less than with the replacement of rubidium by csium4 ,the index of refraction for all rays and for every crystalline path 7direction7 is greater for the rubidium salt than for the potassium salt1 and less than for the csium salt1 and the differences are nearly in the ratio #b 3 #e4 ,the lengths of the rhombic crystalline axes for ,k#b,,so#d are in the ratio #jegbg 3 #a 3 #jgdah1 for ,rb#b,,so#d1 #jegbc 3 #a 3 #jgdhe1 and for ,cs#b,,so#d1 #jegab 3 #a 3 #jgeba4 ,the development of the basic and brachy-pinacoids gradually increases in passing from ,k to ,rb and ,cs4 ,the optical properties also follow the same order both at the ordinary and at a higher temperature4 ,tutton draws the general conclusion that the crystallographic properties of the isomorphic rhombic sulphates ,r#b,,so#d are a function of the atomic weight of the metals contained in them 7see ,chapter ,,xv47 ,such researches as these should do much towards hastening the establishment of a true molecular mechanics of physico-chemical phenomena4 ,among the metals of the alkalis1 the most widely distributed in nature1 after sodium1 is potassium4 ,like sodium1 it does not appear either in a free state or as oxide or hydroxide1 but in the form of salts1 which present much in common with the salts of sodium in the manner of their occurrence4 ,the compounds of potassium and sodium in the earth's crust occur as mineral compounds of silica4 ,with silica1 ,si,o#b1 potassium oxide1 like sodium oxide1 forms saline mineral substances resembling glass4 ,if other oxides1 such as lime1 ,ca,o1 and alumina1 ,al#b,o#c1 combine with these compounds1 glass is formed1 a vitreous stony mass1 distinguished by its great stability1 and its very slight variation under the action of water4 ,it is such complex silicious compounds as these which contain potash 7potassium oxide71 ,k#b,o1 or soda 7sodium oxide71 ,na#b,o1 and sometimes both together1 silica1 ,si,o#b1 lime1 ,ca,o1 alumina1 ,al#b,o#c1 and other oxides1 that form the chief mass of rocks1 out of which1 judging by the direction of the strata1 the chief mass of the accessible crust 7envelope7 of the earth is made up4 ,the primary rocks1 like granite1 porphyry1 c41#a bis are formed of such crystalline silicious rocks as these4 ,the oxides entering into the composition of these rocks do not form a homogeneous amorphous mass like glass1 but are distributed in a series of peculiar1 and in the majority of cases crystalline1 compounds1 into which the primary rocks may be divided4 ,thus a felspar 7orthoclase7 in granite contains from #h to #ae per cent4 of potassium1 whilst another variety 7plagioclase7 which also occurs in granite contains #ab to #f per cent4 of potassium1 and #f to #ab per cent4 of sodium4 ,the mica in granite contains #c to #aj per cent4 of potassium4 ,as already mentioned1 and further explained in ,chapter ,,xvii41 the friable1 crumbling1 and stratified formations which in our times cover a large part of the earth's surface have been formed from these primary rocks by the action of the atmosphere and of water containing carbonic acid4 ,it is evident that in the chemical alteration of the primary rocks by the action of water1 the compounds of potassium1 as well as the compounds of sodium1 must have been dissolved by the water 7as they are soluble in water71 and that therefore the compounds of potassium must be accumulated together with those of sodium in sea water4 ,and indeed compounds of potassium are always found in sea water1 as we have already pointed out 7,chapters ,i4 and ,x474 ,this forms one of the sources from which they are extracted4 ,after the evaporation of sea water1 there remains a mother liquor1 which contains potassium chloride and a large proportion of magnesium chloride4 ,on cooling this solution crystals separate out which contain chlorides of magnesium and potassium4 ,a double salt of this kind1 called carnallite1 ,k,mg,cl#c1f;,h#b,o1 occurs at ,stassfurt4 ,this carnallite#b is now employed as a material for the extraction of potassium chloride1 and of all the compounds of this element4#c ,besides which1 potassium chloride itself is sometimes found at ,stassfurt as sylvine4#c bis ,by a method of double saline decomposition1 the chloride of potassium may be converted into all the other potassium salts1#d some of which are of practical use4 ,the potassium salts have1 however1 their greatest importance as an indispensable component of the food of plants4#e #a bis ,the origin of the primary rocks has been mentioned in ,chapter ,x41 ,note #b4 #b ,carnallite belongs to the number of double salts which are directly decomposed by water1 and it only crystallises from solutions which contain an excess of magnesium chloride4 ,it may be prepared artificially by mixing strong solutions of potassium and magnesium chlorides1 when colourless crystals of sp4 gr4 #afj separate1 whilst the ,stassfurt salt is usually of a reddish tint1 owing to traces of iron4 ,at the ordinary temperature sixty-five parts of carnallite are soluble in one hundred parts of water in the presence of an excess of the salt4 ,it deliquesces in the air1 forming a solution of magnesium chloride and leaving potassium chloride4 ,the quantity of carnallite produced at ,stassfurt is now as much as #ajj1jjj tons a year4 #c ,the method of separating sodium chloride from potassium chloride has been described in ,chapter ,i4 ,on evaporation of a mixture of the saturated solutions1 sodium chloride separates2 and then1 on cooling1 potassium chloride separates1 owing to the difference of rate of variation of their solubilities with the temperature4 ,the following are the most trustworthy figures for the solubility of potassium chloride in one hundred parts of water 7for sodium chloride1 see ,chapter ,x41 ,note #ac73-- #aj #bj #dj #fj #ajj #cb #ce #dj #df #eg ,when mixed with solutions of other salts the solubility of potassium chloride naturally varies1 but not to any great extent4 #c bis ,the specific gravity of the solid salt is #aii--that is1 less than that of sodium chloride4 ,all the salts of sodium are specifically heavier than the corresponding salts of potassium1 as are also their solutions for equal percentage compositions4 ,if the specific gravity of water at #d #aj1jjj1 then at #ae the specific gravity of a solution of p p4c4 potassium chloride #i1iib #fcbip #jbbfp#b1 and therefore for #aj p4c4 #ajfdg1 #bj p4c4 #aacdh1 c4 ,potassium chloride combines with iodine trichloride to form a compound ,k,cl ,i,cl#c ,k,i,cl#d1 which has a yellow colour1 is fusible1 loses iodine trichloride at a red heat1 and gives potassium iodate and hydrochloric acid with water4 ,it is not only formed by direct combination1 but also by many other methods2 for instance1 by passing chlorine into a solution of potassium iodide so long as the gas is absorbed1 ,,ki #b;,cl#b ,k,cl1,i,cl#c4 ,potassium iodide1 when treated with potassium chlorate and strong hydrochloric acid1 also gives this compound2 another method for its formation is given by the equation ,k,cl,o#c ,i #f;,h,cl ,k,cl1,i,cl#c #c;,cl #c;,h#b,o4 ,this is a kind of salt corresponding with ,,kio#b 7unknown7 in which the oxygen is replaced by chlorine4 ,if valency be taken as the starting-point in the study of chemical compounds1 and the elements considered as having a constant atomicity 7number of bonds7--that is1 if ,k1 ,cl1 and ,i be taken as univalent elements--then it is impossible to explain the formation of such a compound because1 according to this view1 univalent elements are only able to form dual compounds with each other2 such as1 ,k,cl1 ,cl,i1 ,,ki1 c41 whilst here they are grouped together in the molecule ,k,i,cl#d4 ,wells1 ,wheeler1 and ,penfield 7#ahib7 obtained a large number of such poly-haloid salts4 ,they may all be divided into two large classes3 the tri-haloid and the penta-haloid salts4 ,they have been obtained not only for ,k but also for ,rb and ,cs1 and partially also for ,na and ,li4 ,the general method of their formation consists in dissolving the ordinary halogen salt of the metal in water1 and treating it with the requisite amount of free halogen4 ,the poly-haloid salt separates out after evaporating the solution at a more or less low temperature4 ,in this manner1 among the tri-haloid salts1 may be obtained3 ,,ki#c1 ,k,br#b;,i1 ,k,cl#b;,i1 and the corresponding salts of rubidium and csium1 for instance1 ,cs,i#c1 ,cs,br,i#b1 ,cs,br#b;,i1 ,cs,cl,br,i1,cs,cl#b;,i1 ,cs,br#c1 ,cs,cl,br#b1 ,cs,cl#b;,br1 and in general ,,mx#c where ,x is a halogen4 ,the colour of the crystals varies according to the halogen1 thus ,cs,i#c is black1 ,cr,br#c yellowish red1 ,cr,br,i#b reddish brown1 ,cs,br#b;,i red1 ,cs,cl#b;,br yellow4 ,the csium salts are the most stable1 and those of potassium least so1 as also those which contain ,br and ,i separately or together2 for csium no compounds containing ,cl and ,i were obtained4 ,the penta-haloid salts form a smaller class2 among these salts potassium forms ,k,cl#d;,i1 rubidium ,rb,cl#d;,i1 csium ,cs,i#e1 ,cs,br1 ,cs,cl#d;,i1 lithium ,li,cl#d;,i 7with #d;,h#b,o7 and sodium ,na,cl#d;,i 7with #b;,h#b,o74 ,the most stable are those salts containing the metal with the greatest atomic weight--csium 7see ,chapter ,,xi41 ,note #fc74 #d ,it is possible to extract the compounds of potassium directly from the primary rocks which are so widely distributed over the earth's surface and so abundant in some localities4 ,from a chemical point of view this problem presents no difficulty2 for instance1 by fusing powdered orthoclase with lime and fluor spar 7,ward's method7 and then extracting the alkali with water 7on fusion the silica gives an insoluble compound with lime71 or by treating the orthoclase with hydrofluoric acid 7in which case silicon fluoride is evolved as a gas7 it is possible to transfer the alkali of the orthoclase to an aqueous solution1 and to separate it in this manner from the other insoluble oxides4 ,however1 as yet there is no profit in1 nor necessity for1 recourse to this treatment1 as carnallite and potash form abundant materials for the extraction of potassium compounds by cheaper methods4 ,furthermore1 the salts of potassium are now in the majority of chemical reactions replaced by salts of sodium1 especially since the preparation of sodium carbonate has been facilitated by the ,leblanc process4 ,the replacement of potassium compounds by sodium compounds not only has the advantage that the salts of sodium are in general cheaper than those of potassium1 but also that a smaller quantity of a sodium salt is needed for a given reaction than of a potassium salt1 because the combining weight of sodium 7#bc7 is less than that of potassium 7#ci74 #e ,it has been shown by direct experiment on the cultivation of plants in artificial soils and in solutions that under conditions 7physical1 chemical1 and physiological7 otherwise identical plants are able to thrive and become fully developed in the entire absence of sodium salts1 but that their development is impossible without potassium salts4 ,the primary rocks contain an almost equal proportion of potassium and sodium4 ,but in sea water the compounds of the latter metal predominate4 ,it may be asked1 what became of the compounds of potassium in the disintegration of the primary rocks1 if so small a quantity went to the sea water8 ,they remained with the other products of the decomposition of the primary rocks4 ,when granite or any other similar rock formation is disintegrated1 there are formed1 besides the soluble substances1 also insoluble substances--sand and finely-divided clay1 containing water1 alumina1 and silica4 ,this clay is carried away by the water1 and is then deposited in strata4 ,it1 and especially its admixture with vegetable remains1 retain compounds of potassium in a greater quantity than those of sodium4 ,this has been proved with absolute certainty to be the case1 and is due to the absorptive power of the soil4 ,if a dilute solution of a potassium compound be filtered through common mould used for growing plants1 containing clay and the remains of vegetable decomposition1 this mould will be found to have retained a somewhat considerable percentage of the potassium compounds4 ,if a salt of potassium be taken1 then during the filtration an equivalent quantity of a salt of calcium--which is also found1 as a rule1 in soils--is set free4 ,such a process of filtration through finely divided earthy substances proceeds in nature1 and the compounds of potassium are everywhere retained by the friable earth in considerable quantity4 ,this explains the presence of so small an amount of potassium salts in the water of rivers1 lakes1 streams1 and oceans1 where the lime and soda have accumulated4 ,the compounds of potassium retained by the friable mass of the earth are absorbed as an aqueous solution by the roots of plants4 ,plants1 as everyone knows1 when burnt leave an ash1 and this ash1 besides various other substances1 without exception contains compounds of potassium4 ,many land plants contain a very small amount of sodium compounds1#f whilst potassium and its compounds occur in all kinds of vegetable ash4 ,among the generally cultivated plants1 grass1 potatoes1 the turnip1 and buckwheat are particularly rich in potassium compounds4 ,the ash of plants1 and especially of herbaceous plants1 buckwheat straw1 sunflower and potato leaves are used in practice for the extraction of potassium compounds4 ,there is no doubt that potassium occurs in the plants themselves in the form of complex compounds1 and often as salts of organic acids4 ,in certain cases such salts of potassium are even extracted from the juice of plants4 ,thus1 sorrel and oxalis1 for example1 contain in their juices the acid oxalate of potassium1 ,c#b;,,hko#d1 which is employed for removing ink stains4 ,grape juice contains the so-called cream of tartar1 which is the acid tartrate of potassium1 ,c#d;,h#e,,ko#f4g ,this salt also separates as a sediment from wine4 ,when the plants1 containing one or more of the salts of potassium1 are burnt1 the carbonaceous matter is oxidised1 and in consequence the potassium is obtained in the ash as carbonate1 ,k#b;,,co#c1 which is generally known as potashes4 ,hence potashes occur ready prepared in the ash of plants1 and therefore the ash of land plants is employed as a source for the extraction of potassium compounds4 ,potassium carbonate is extracted by lixiviating the ash with water4#h ,potassium carbonate may also be obtained from the chloride by a method similar to that by which sodium carbonate is prepared from sodium chloride4#h bis ,there is no difficulty in obtaining any salt of potassium--for example1 the sulphate1#i bromide1 and iodide#aj--;by the action of the corresponding acid on ,k,cl and especially on the carbonate1 whilst the hydroxide1 caustic potash1 ,,kho1 which is in many respects analogous to caustic soda1 is easily obtained by means of lime in exactly the same manner in which sodium hydroxide is prepared from sodium carbonate4#aa ,therefore1 in order to complete our knowledge of the alkali metals1 we will only describe two salts of potassium which are of practical importance1 and whose analogues have not been described in the preceding chapter1 potassium cyanide and potassium nitrate4 #f ,if herbaceous plants contain much sodium salts1 it is evident that these salts mainly come from the sodium compounds in the water absorbed by the plants4 #g ,as plants always contain mineral substances and cannot thrive in a medium which does not contain them1 more especially in one which is free from the salts of the four basic oxides1 ,k#b,o1 ,ca,o1 ,mg,o1 and ,fe#b,o#c1 and of the four acid oxides1 ,,co#b1 ,n#b,o#e1 ,p#b,o#e1 and ,,so#c1 and as the amount of ash-forming substances in plants is small1 the question inevitably arises as to what part these play in the development of plants4 ,with the existing chemical data only one answer is possible to this question1 and it is still only a hypothesis4 ,this answer was particularly clearly expressed by ,professor ,gustavson of the ,petroffsky ,agricultural ,academy4 ,starting from the fact 7,chapter ,,xi41 ,note #ee7 that a small quantity of aluminium renders possible or facilitates the reaction of bromine on hydrocarbons at the ordinary temperature1 it is easy to arrive at the conclusion1 which is very probable and in accordance with many data respecting the reactions of organic compounds1 that the addition of mineral substances to organic compounds lowers the temperature of reaction and in general facilitates chemical reactions in plants1 and thus aids the conversion of the most simple nourishing substances into the complex component parts of the plant organism4 ,the province of chemical reactions proceeding in organic substances in the presence of a small quantity of mineral substances has as yet been but little investigated1 although there are already several disconnected data concerning reactions of this kind1 and although a great deal is known with regard to such reactions among inorganic compounds4 ,the essence of the matter may be expressed thus--two substances1 ,a and ,b1 do not react on each other of their own accord1 but the addition of a small quantity of a third particularly active substance1 ,c1 produces the reaction of ,a on ,b1 because ,a combines with ,c1 forming ,,ac1 and ,b reacts on this new compound1 which has a different store of chemical energy1 forming the compound ,,ab or its products1 and setting ,c free again or retaining it4 ,it may here be remarked that all the mineral substances necessary for plants 7those enumerated at the beginning of the note7 are the highest saline compounds of their elements1 that they enter into the plants as salts1 that the lower forms of oxidation of the same elements 7for instance1 sulphites and phosphites7 are harmful to plants 7poisonous71 and that strong solutions of the salts assimilated by plants 7their osmotic pressure being great and contracting the cells1 as ,de ,vries showed1 7see ,chapter ,i41 ,note #ai77 not only do not enter into the plants but kill them 7poison them74 #h ,besides which1 it will be understood from the preceding paragraph that the salts of potassium may become exhausted from the soil by long cultivation1 and that there may therefore be cases when the direct fertilisation by salts of potassium may be profitable4 ,but manure and animal excrements1 ashes1 and1 in general1 nearly all refuse which may serve for fertilising the soil1 contain a considerable quantity of potassium salts1 and therefore1 as regards the natural salts of potassium 7,stassfurt71 and especially potassium sulphate1 if they often improve the crops1 it is in all probability due to their action on the properties of the soil4 ,the agriculturist cannot therefore be advised to add potassium salts1 without making special experiments showing the advantage of such a fertiliser on a given kind of soil and plant4 ,the animal body also contains potassium compounds1 which is natural1 since animals consume plants4 ,for example1 milk1 and especially human milk1 contains a somewhat considerable quantity of potassium compounds4 ,cow's milk1 however1 does not contain much potassium salt4 ,sodium compounds generally predominate in the bodies of animals4 ,the excrement of animals1 and especially of herbivorous animals1 on the contrary1 often contains a large proportion of potassium salts4 ,thus sheep's dung is rich in them1 and in washing sheep's wool salts of potassium pass into the water4 ,the ash of tree stems1 as the already dormant portion of the plant 7,chapter ,,viii41 ,note #a71 contains little potash4 ,for the extraction of potash1 which was formerly carried on extensively in the east of ,russia 7before the discovery of the ,stassfurt salt71 the ash of grasses1 and the green portions of potatoes1 buckwheat1 c41 are taken and treated with water 7lixiviated71 the solution is evaporated1 and the residue ignited in order to destroy the organic matter present in the extract4 ,the residue thus obtained is composed of raw potash4 ,it is refined by a second dissolution in a small quantity of water1 for the potash itself is very soluble in water1 whilst the impurities are sparingly soluble4 ,the solution thus obtained is again evaporated1 and the residue ignited1 and this potash is then called refined potash1 or pearlash4 ,this method of treatment cannot give chemically pure potassium carbonate4 ,a certain amount of impurities remain4 ,to obtain chemically pure potassium carbonate1 some other salt of potassium is generally taken and purified by crystallisation4 ,potassium carbonate crystallises with difficulty1 and it cannot therefore be purified by this means1 whilst other salts1 such as the tartrate1 acid carbonate1 sulphate1 or nitrate1 c41 crystallise easily and may thus be directly purified4 ,the tartrate is most frequently employed1 since it is prepared in large quantities 7as a sediment from wine7 for medicinal use under the name of cream of tartar4 ,when ignited without access of air1 it leaves a mixture of charcoal and potassium carbonate4 ,the charcoal so obtained being in a finely-divided condition1 the mixture 7called 'black flux'71 is sometimes used for reducing metals from their oxides with the aid of heat4 ,a certain quantity of nitre is added to burn the charcoal formed by heating the cream of tartar4 ,potassium carbonate thus prepared is further purified by converting it into the acid salt1 by passing a current of carbonic anhydride through a strong solution4 ,,khco#c is then formed1 which is less soluble than the normal salt 7as is also the case with the corresponding sodium salts71 and therefore crystals of the acid salt separate from the solution on cooling4 ,when ignited1 they part with their water and carbonic anhydride1 and pure potassium carbonate remains behind4 ,the physical properties of potassium carbonate distinguish it sufficiently from sodium carbonate2 it is obtained from solutions as a powdery white mass1 having an alkaline taste and reaction1 and1 as a rule1 shows only traces of crystallisation4 ,it also attracts the moisture of the air with great energy4 ,the crystals do not contain water1 but absorb it from the air1 deliquescing into a saturated solution4 ,it melts at a red heat 7#ajde71 and at a still higher temperature is even converted into vapour1 as has been observed at glass works where it is employed4 ,it is very soluble4 ,at the ordinary temperature1 water dissolves an equal weight of the salt4 ,crystals containing two equivalents of water separate from such a saturated solution when strongly cooled 7,morel obtained ,k#b;,,co#cc;,h#b,o in well-formed crystals at #aj74 ,there is no necessity to describe its reactions1 because they are all analogous to those of sodium carbonate4 ,when manufactured sodium carbonate was but little known1 the consumption of potassium carbonate was very considerable1 and even now washing soda is frequently replaced for household purposes by 'lye'--i4e4 an aqueous solution obtained from ashes4 ,it contains potassium carbonate1 which acts like the sodium salt in washing tissues1 linen1 c4 ,a mixture of potassium and sodium carbonates fuses with much greater ease than the separate salts1 and a mixture of their solutions gives well-crystallised salts--for instance 7,marguerite's salt71 ,k#b;,,co#c1f;,h#b,o1#b,na#b;,,co#c1f;,h #b,o4 ,crystallisation also occurs in other multiple proportions of ,k and ,na 7in the above case #a 3 #b1 but #a 3 #a and #a 3 #c are known71 and always with #f mol4 ,h#b,o4 ,this is evidently a combination by similarity1 as in alloys1 solutions1 c4 #h bis ,about #be1jjj tons of potash annually are now prepared from ,k,cl by this method at ,stassfurt4 #i ,potassium sulphate1 ,k#b,,so#d1 crystallises from its solutions in an anhydrous condition1 in which respect it differs from the corresponding sodium salt1 just as potassium carbonate differs from sodium carbonate4 ,in general1 it must be observed that the majority of sodium salts combine more easily with water of crystallisation than the potassium salts4 ,the solubility of potassium sulphate does not show the same peculiarities as that of sodium sulphate1 because it does not combine with water of crystallisation2 at the ordinary temperature #ajj parts of water dissolve about #aj parts of the salt1 at #j #hc parts1 and at #ajj about #bf parts4 ,the acid sulphate1 ,,khso#d1 obtained easily by heating crystals of the normal salt with sulphuric acid1 is frequently employed in chemical practice4 ,on heating the mixture of acid and salt1 fumes of sulphuric acid are at first given off2 when they cease to be evolved1 the acid salt is contained in the residue4 ,at a higher temperature 7of above #fjj7 the acid salt parts with all the acid contained in it1 the normal salt being re-formed4 ,the definite composition of this acid salt1 and the ease with which it decomposes1 render it exceedingly valuable for certain chemical transformations accomplished by means of sulphuric acid at a high temperature1 because it is possible to take1 in the form of this salt1 a strictly definite quantity of sulphuric acid1 and to cause it to act on a given substance at a high temperature1 which it is often necessary to do1 more especially in chemical analysis4 ,in this case1 the acid salt acts in exactly the same manner as sulphuric acid itself1 but the latter is inefficient at temperatures above #djj1 because it all evaporates1 while at that temperature the acid salt still remains in a fused state1 and acts with the elements of sulphuric acid on the substance taken4 ,hence by its means the boiling-point of sulphuric acid is raised4 ,thus the acid potassium sulphate is employed1 where for conversion of certain oxides1 such as those of iron1 aluminium1 and chromium1 into salts1 a high temperature is required4 ,weber1 by heating potassium sulphate with an excess of sulphuric acid at #ajj1 observed the formation of a lower stratum1 which was found to contain a definite compound containing eight equivalents of ,,so#c per equivalent of ,k#b,o4 ,the salts of rubidium1 csium1 and thallium give a similar result1 but those of sodium and lithium do not4 7,see ,note #a47 #aj ,the bromide and iodide of potassium are used1 like the corresponding sodium compounds1 in medicine and photography4 ,potassium iodide is easily obtained in a pure state by saturating a solution of hydriodic acid with caustic potash4 ,in practice1 however1 this method is rarely had recourse to1 other more simple processes being employed although they do not give so pure a product4 ,they aim at the direct formation of hydriodic acid in the liquid in the presence of potassium hydroxide or carbonate4 ,thus iodine is thrown into a solution of pure potash1 and hydrogen sulphide passed through the mixture1 the iodine being thus converted into hydriodic acid4 ,or a solution is prepared from phosphorus1 iodine1 and water1 containing hydriodic and phosphoric acid2 lime is then added to this solution1 when calcium iodide is obtained in solution1 and calcium phosphate as a precipitate4 ,the solution of calcium iodide gives1 with potassium carbonate1 insoluble calcium carbonate and a solution of potassium iodide4 ,if iodine is added to a slightly-heated solution of caustic potash 7free from carbonate--that is1 freshly prepared71 so long as the solution is not coloured from the presence of an excess of iodine1 there is formed 7as in the action of chlorine on a solution of caustic potash7 a mixture of potassium iodide and iodate4 ,on evaporating the solution thus obtained and igniting the residue1 the iodate is destroyed and converted into iodide1 the oxygen being disengaged1 and potassium iodide only is left behind4 ,on dissolving the residue in water and then evaporating1 cubical crystals of the anhydrous salt are obtained1 which are soluble in water and alcohol1 and on fusion give an alkaline reaction1 owing to the fact that when ignited a portion of the salt decomposes1 forming potassium oxide4 ,the neutral salt may be obtained by adding hydriodic acid to this alkaline salt until it gives an acid reaction4 ,it is best to add some finely-divided charcoal to the mixture of iodate and iodide before igniting it1 as this facilitates the evolution of the oxygen from the iodate4 ,the iodate may also be converted into iodide by the action of certain reducing agents1 such as zinc amalgam1 which when boiled with a solution containing an iodate converts it into iodide4 ,potassium iodide may also be prepared by mixing a solution of ferrous iodide 7it is best if the solution contain an excess of iodine7 and potassium carbonate1 in which case ferrous carbonate ,fe,c,o#c1 is precipitated 7with an excess of iodine the precipitate is granular1 and contains a compound of the suboxide and oxide of iron71 while potassium iodide remains in solution4 ,ferrous iodide1 ,fe,i#b1 is obtained by the direct action of iodine on iron in water4 ,potassium iodide considerably lowers the temperature 7by #bd71 when it dissolves in water1 #ajj parts of the salt dissolve in #gce parts of water at #abe1 in #gj parts at #ah1 whilst the saturated solution which boils at #abj contains #ajj parts of salt per #de parts of water4 ,solutions of potassium iodide dissolve a considerable amount of iodine2 strong solutions even dissolving as much or more iodine than they contain as potassium iodide 7see ,note #c bis and ,chapter ,,xi41 ,note #fd74 #aa ,caustic potash is not only formed by the action of lime on dilute solutions of potassium carbonate 7as sodium hydroxide is prepared from sodium carbonate71 but by igniting potassium nitrate with finely-divided copper 7see ,note #ae71 and also by mixing solutions of potassium sulphate 7or even of alum1 ,k,al,s#b,o#h7 and barium hydroxide1 ,ba,h#b,o#b4 ,it is sometimes purified by dissolving it in alcohol 7the impurities1 for example1 potassium sulphate and carbonate1 are not dissolved7 and then evaporating the alcohol4 ,the specific gravity of potassium hydroxide is #bjd1 but that of its solutions 7see ,chapter ,,xii41 ,note #ah7 at #ae ,s #i1iib #ijdp #jbhp#b 7here p#b is 1 and for sodium hydroxide it is-74 ,strong solutions1 when cooled1 yield a crystallo-hydrate1 ,,kho1#d;,h#b,o1 which dissolves in water1 producing cold 7like #b,na,h,o1#g;,h#b,o71 whilst potassium hydroxide in solution develops a considerable amount of heat4 ,potassium cyanide1 which presents in its chemical relations a certain analogy with the halogen salts of potassium1 is not only formed according to the equation1 ,,kho ,,hcn ,h#b,o ,,kcn1 but also whenever a nitrogenous carbon compound--for instance1 animal matter--is heated in the presence of metallic potassium1 or of a compound of potassium1 and even when a mixture of potash and carbon is heated in a stream of nitrogen4 ,potassium cyanide is obtained from yellow prussiate1 which has been already mentioned in ,chapter ,,ix41 and whose preparation on a large scale will be described in ,chapter ,,xxii4 ,if the yellow prussiate be ground to a powder and dried1 so that it loses its water of crystallisation1 it then melts at a red heat1 and decomposes into carbide of iron1 nitrogen1 and potassium cyanide1 ,fe,k#d;,c#f,n#f #d,,kcn ,fe,c#b ,n#b4 ,after the decomposition it is found that the yellow salt has been converted into a white mass of potassium cyanide4 ,the carbide of iron formed collects at the bottom of the vessel4 ,if the mass thus obtained be treated with water1 the potassium cyanide is partially decomposed by the water1 but if it be treated with alcohol1 then the cyanide is dissolved1 and on cooling separates in a crystalline form4#ab ,a solution of potassium cyanide has a powerfully alkaline reaction1 a smell like that of bitter almonds1 peculiar to prussic acid1 and acts as a most powerful poison4 ,although exceedingly stable in a fused state1 potassium cyanide easily changes when in solution4 ,prussic acid is so very feebly energetic that even water decomposes potassium cyanide4 ,a solution of the salt1 even without access of air1 easily turns brown and decomposes1 and when heated evolves ammonia and forms potassium formate2 this is easily comprehensible from the representation of the cyanogen compounds which was developed in ,chapter ,,ix41 ,,kcn #b;,h#b,o ,,chko#b ,,nh#c4 ,furthermore1 as carbonic anhydride acts on potassium cyanide with evolution of prussic acid1 and as potassium cyanate1 which is also unstable1 is formed by the action of air1 it will be easily seen that solutions of potassium cyanide are very unstable4 ,potassium cyanide1 containing as it does carbon and potassium1 is a substance which can act in a very vigorously reducing manner1 especially when fused2 it is therefore used as a powerful reducing agent at a red heat4#ac ,the property of potassium cyanide of giving double salts with other cyanides is very clearly shown by the fact that many metals dissolve in a solution of potassium cyanide1 with the evolution of hydrogen4 ,for example1 iron1 copper1 and zinc act in this manner4 ,thus-- #d,,kcn #b;,h#b,o ,zn ,k#b,zn,c#d,n#d #b,,kho ,h#b #ab ,when the yellow prussiate is heated to redness1 all the cyanogen which was in combination with the iron is decomposed into nitrogen1 which is evolved as gas1 and carbon1 which combines with the iron4 ,in order to avoid this1 potassium carbonate is added to the yellow prussiate while it is being fused4 ,a mixture of #h parts of anhydrous yellow prussiate and #c parts of pure potassium carbonate is generally taken4 ,double decomposition then takes place1 resulting in the formation of ferrous carbonate and potassium cyanide4 ,but by this method1 as by the first1 a pure salt is not obtained1 because a portion of the potassium cyanide is oxidised at the expense of the iron carbonate and forms potassium cyanate1 ,fe,c,o#c ,,kcn ,,co#b ,fe ,,kcno2 and the potassium cyanide very easily forms oxide1 which acts on the sides of the vessel in which the mixture is heated 7to avoid this iron vessels should be used74 ,by adding one part of charcoal powder to the mixture of #h parts of anhydrous yellow prussiate and #c parts of potassium carbonate a mass is obtained which is free from cyanate1 because the carbon absorbs the oxygen1 but in that case it is impossible to obtain a colourless potassium cyanide by simple fusion1 although this may be easily done by dissolving it in alcohol4 ,cyanide of potassium may also be obtained from potassium thiocyanate1 which is formed from ammonium thiocyanate obtained by the action of ammonia upon bisulphide of carbon 7see works upon ,organic ,chemistry74 ,potassium cyanide is now prepared in large quantities from yellow prussiate for gilding and silvering4 ,when fused in large quantities the action of the oxygen of the air is limited1 and with great care the operation may be successfully conducted1 and therefore1 on a large scale1 very pure salt is sometimes obtained4 ,when slowly cooled1 the fused salt separates in cubical crystals like potassium chloride4 ,pure ,,kcn is obtained by passing ,,cnh gas into an alcoholic solution of ,,kho4 ,the large amount of potassium cyanide which is now required for the extraction of gold from its ores1 is being replaced by a mixture 7,rossler and ,gasslaker1 #ahib7 of ,,kcn and ,na,c,n1 prepared by heating powdered and dried yellow prussiate with metallic sodium3 ,k#d;,fe7,,cn7#f #b,na #d,,kcn #b,na,c,n ,fe4 ,this method offers two advantages over the above methods3 7#a7 the whole of the cyanide is obtained1 and does not decompose with the formation of ,n#b2 and 7#b7 no cyanates are formed1 as is the case when carbonate of potash is heated with the prussiate4 #ac ,a considerable quantity of potassium cyanide is used in the arts1 more particularly for the preparation of metallic solutions which are decomposed by the action of a galvanic current2 thus it is very frequently employed in electro-silvering and gilding4 ,an alkaline solution is prepared1 which is moderately stable owing to the fact that potassium cyanide in the form of certain double salts--that is1 combined with other cyanides--is far more stable than when alone 7yellow prussiate1 which contains potassium cyanide in combination with ferrous cyanide1 is an example of this74 ,gold and silver are soluble in potassium cyanide in the presence of air1 in which case the hydrogen1 which would otherwise be evolved in the reaction1 combines with the oxygen of the air1 forming water 7,eissler1 ,mac,laurin1 #ahic71 for example1 #d;,au #d,,kcn ,o ,h#b,o #b;,au,k,c#b,n#b #b,,kho1 which is taken advantage of for extracting gold from its ores 7,chapter ,,xxiv474#ac bis ,platinum1 mercury1 and tin are not dissolved in a solution of potassium cyanide1 even with access of air4 #ac bis ,a dilute solution of ,,kcn is taken1 not containing more than #a per cent4 ,,kcn4 ,mac,laurin explains this by the fact that strong solutions dissolve gold less rapidly1 owing to their dissolving less air1 whose oxygen is necessary for the reaction4 ,potassium nitrate1 or common nitre or saltpetre1 ,,kno#c1 is chiefly used as a component part of gunpowder1 in which it cannot be replaced by the sodium salt1 because the latter is deliquescent4 ,it is necessary that the nitre in gunpowder should be perfectly pure1 as even small traces of sodium1 magnesium1 and calcium salts1 especially chlorides1 render the nitre and the gunpowder capable of attracting moisture4 ,nitre may easily be obtained pure1 owing to its great disposition to form crystals both large and small1 which aids its separation from other salts4 ,the considerable differences between the solubility of nitre at different temperatures aids this crystallisation4 ,a solution of nitre saturated at its boiling point 7#aaf7 contains #cce parts of nitre to #ajj parts of water1 whilst at the ordinary temperature--for instance1 #bj--the solution is only able to retain #cb parts of the salt4 ,therefore1 in the preparation and refining of nitre1 its solution1 saturated at the boiling point1 is cooled1 and nearly all the nitre is obtained in the form of crystals4 ,if the solution be quietly and slowly cooled in large quantities then large crystals are formed1 but if it be rapidly cooled and agitated then small crystals are obtained4 ,in this manner1 if not all1 at all events the majority1 of the impurities present in small quantities remain in the mother liquor4 ,if an unsaturated solution of nitre be rapidly cooled1 so as to prevent the formation of large crystals 7in whose crevices the mother liquor1 together with the impurities1 would remain71 the very minute crystals of nitre known as saltpetre flour are obtained4 ,common nitre occurs in nature1 but only in small quantities in admixture with other nitrates1 and especially with sodium1 magnesium1 and calcium nitrates4 ,such a mixture of salts of nitric acid is formed in nature in fertile earth1 and in those localities where1 as in the soil1 nitrogenous organic remains are decomposed in the presence of alkalis or alkaline bases with free access of air4 ,this method of the formation of nitrates requires moisture1 besides the free access of air1 and takes place principally during warm weather4#ad ,in warm countries1 and in temperate climates during the summer months1 fertile soils produce a small quantity of nitre4 ,in this respect ,india is especially known as affording a considerable supply of nitre extracted from the soil4 ,the nitre-bearing soil after the rainy season sometimes becomes covered during the summer with crystals of nitre1 formed by the evaporation of the water in which it was previously dissolved4 ,this soil is collected1 subjected to repeated lixiviations1 and treated for nitre as will be presently described4 ,in temperate climates nitrates are obtained from the lime rubbish of demolished buildings which have stood for many years1 and especially from those portions which have been in contact with the ground4 ,the conditions there are very favourable for the formation of nitre1 because the lime used as a cement in buildings contains the base necessary for the formation of nitrates1 while the excrement1 urine1 and animal refuse are sources of nitrogen4 ,by the methodical lixiviation of this kind of rubbish a solution of nitrogenous salts is formed similar to that obtained by the lixiviation of fertile soil4 ,a similar solution is also obtained by the lixiviation of the so-called nitre plantations4 ,they are composed of manure interlaid with brushwood1 and strewn over with ashes1 lime1 and other alkaline rubbish4 ,these nitre plantations are set up in those localities where the manure is not required for the fertilisation of the soil1 as1 for example1 in the south-eastern 'black earth' ,governments of ,russia4 ,the same process of oxidation of nitrogenous matter freely exposed to air and moisture during the warm season in the presence of alkalis takes place in nitre plantations as in fertile soil and in the walls of buildings4 ,from all these sources there is obtained a solution containing various salts of nitric acid mixed with soluble organic matter4 ,the simplest method of treating this impure solution of nitre is to add a solution of potassium carbonate1 or to simply treat it with ashes containing this substance4 ,the potassium carbonate enters into double decomposition with the calcium and magnesium salts1 forming insoluble carbonates of these bases and leaving the nitre in solution4 ,thus1 for instance1 ,k#b;,,co#c ,ca7,,no#c7b #b,,kno#c ,ca,c,o#c4 ,both calcium and magnesium carbonates are insoluble1 and therefore after treatment with potassium carbonate the solution no longer contains salts of these metals but only the salts of sodium and potassium together with organic matter4 ,the latter partially separates on heating in an insoluble form1 and is entirely destroyed by heating the nitre to a low red heat4 ,the nitre thus obtained is easily purified by repeated crystallisation4 ,the greater part of the nitre used for making gunpowder is now obtained from the sodium salt ,chili saltpetre or cubic nitre1 which occurs in nature1 as already mentioned4 ,the conversion of this salt into common nitre is also carried on by means of a double decomposition4 ,this is done either by adding potassium carbonate 7when1 on mixing the strong and hot solutions1 sodium carbonate is directly obtained as a precipitate71 or1 as is now most frequent1 potassium chloride4 ,when a mixture of strong solutions of potassium chloride and sodium nitrate is evaporated1 sodium chloride first separates1 because this salt1 which is formed by the double decomposition ,k,cl ,na,n,o#c ,,kno#c ,na,cl1 is almost equally soluble in hot and cold water2 on cooling1 therefore1 a large amount of potassium nitrate separates from the saturated solution1 while the sodium chloride remains dissolved4 ,the nitre is ultimately purified by recrystallisation and by washing with a saturated solution of nitre1 which cannot dissolve a further quantity of nitre but only the impurities4 #ad ,besides which ,schloesing and ,mntz1 by employing similar methods to ,pasteur1 showed that the formation of nitre in the decomposition of nitrogenous substances is accomplished by the aid of peculiar micro-organisms 7ferments71 without which the simultaneous action of the other necessary conditions 7alkalis1 moisture1 a temperature of #cg1 air1 and nitrogenous substances7 cannot give nitre4 ,nitre is a colourless salt having a peculiar cool taste4 ,it crystallises easily in long striated six-sided rhombic prisms terminating in rhombic pyramids4 ,its crystals 7sp4 gr4 #aic7 do not contain water1 but their cavities generally contain a certain quantity of the solution from which they have crystallised4 ,for this reason in refining nitre1 the production of large crystals is prevented1 saltpetre flour being prepared4 ,at a low red heat 7#cci7 nitre melts to a colourless liquid4#ad bis ,potassium nitrate at the ordinary temperature and in a solid form is inactive and stable1 but at a high temperature it acts as a powerful oxidising agent1 giving up a considerable amount of oxygen to substances mixed with it4#ae ,when thrown on to incandescent charcoal it brings about its rapid combustion1 and a mechanical mixture of powdered charcoal and nitre ignites when brought into contact with a red-hot substance1 and continues to burn by itself4 ,in this action1 nitrogen is evolved1 and the oxygen oxidises the charcoal1 in consequence of which potassium carbonate and carbonic anhydride are formed3 #d,,kno#c #e;,c #b,k#b;,,co#c #c;,,co#b #b,n#b4 ,this phenomenon depends on the fact that oxygen in combining with carbon evolves more heat than it does in combining with nitrogen4 ,hence1 when once the combustion has been started at the expense of the nitre1 it is able to go on without requiring the aid of external heat4 ,a similar oxidation or combustion at the expense of the contained oxygen takes place when nitre is heated with different combustible substances4 ,if a mixture of sulphur and nitre be thrown upon a red-hot surface1 the sulphur burns1 forming potassium sulphate and sulphurous anhydride4 ,in this case1 also1 the nitrogen of the nitre is evolved as gas3 #b,,kno#c #b,s ,k#b,,so#d ,n#b ,,so#b4 ,a similar phenomenon occurs when nitre is heated with many metals4 ,the oxidation of those metals which are able to form acid oxides with an excess of oxygen is especially remarkable4 ,in this case they remain in combination with potassium oxide as potassium salts4 ,manganese1 antimony1 arsenic1 iron1 chromium1 c4 are instances of this kind4 ,these elements1 like carbon and sulphur1 displace free nitrogen4 ,the lower oxides of these metals when fused with nitre pass into the higher oxides4 ,organic substances are also oxidised when heated with nitre--that is1 they burn at the expense of the nitre4 ,it will be readily understood from this that nitre is frequently used in practical chemistry and the arts as an oxidising agent at high temperatures4 ,its application in gunpowder is based on this property2 gunpowder consists of a mechanical mixture of finely-ground sulphur1 nitre1 and charcoal4 ,the relative proportion of these substances varies according to the destination of the powder and to the kind of charcoal employed 7a friable1 incompletely-burnt charcoal1 containing therefore hydrogen and oxygen1 is employed74 ,gases are formed in its combustion1 chiefly nitrogen and carbonic anhydride1 which create a considerable pressure if their escape be in any way impeded4 ,this action of gunpowder may be expressed by the equation3 #b,,kno#c #c;,c ,s ,k#b,s #c;,,co#b ,n#b4 #ad bis ,before fusing1 the crystals of potassium nitrate change their form1 and take the same form as sodium nitrate--that is1 they change into rhombohedra4 ,nitre crystallises from hot solutions1 and in general under the influence of a rise of temperature1 in a different form from that given at the ordinary or lower temperatures4 ,fused nitre solidifies to a radiated crystalline mass2 but it does not exhibit this structure if metallic chlorides be present1 so that this method may be taken advantage of to determine the degree of purity of nitre4 ,carnelley and ,thomson 7#ahhh7 determined the fusing point of mixtures of potassium and sodium nitrates4 ,the first salt fuses at #cci and the second at #caf1 and if p be the percentage amount of potassium nitrate1 then the results obtained were-- p #aj #bj #cj #dj #ej #fj #gj #hj #ij #bih #bhc #bfh #bdb #bca #bca #bdb #bhd #cjf which confirms ,shaffgotsch's observation 7#aheg7 that the lowest fusing point 7about #bca7 is given by mixing molecular quantities 7p #edc7 of the salts--that is1 in the formation of the alloy1 ,,kno#c1,na,n,o#c4 ,a somewhat similar result was discovered by the same observers for the solubility of mixtures of these salts at #bj in #ajj parts of water4 ,thus1 if p be the weight of potassium nitrate mixed with #ajj-p parts by weight of sodium nitrate taken for solution1 and c be the quantity of the mixed salts which dissolves in #ajj1 the solubility of sodium nitrate being #he1 and of potassium nitrate #cd1 parts in #ajj parts of water1 then-- p #aj #bj #cj #dj #ej #fj #gj #hj #ij c #aaj #acf #acf #ach #ajf #ha #gc #ed #da ,the maximum solubility proved not to correspond with the most fusible mixture1 but to one much richer in sodium nitrate4 ,both these phenomena show that in homogeneous liquid mixtures the chemical forces that act between substances are the same as those that determine the molecular weights of substances1 even when the mixture consists of such analogous substances as potassium and sodium nitrates1 between which there is no direct chemical interchange4 ,it is instructive to note also that the maximum solubility does not correspond with the minimum fusing point1 which naturally depends on the fact that in solution a third substance1 namely water1 plays a part1 although an attraction between the salts1 like that which exists between sodium and potassium carbonates 7,note #h71 also partially acts4 #ae ,fused nitre1 with a further rise of temperature1 disengages oxygen and then nitrogen4 ,the nitrite ,,kno#b is first formed and then potassium oxide4 ,the admixture of certain metals--for example1 of finely-divided copper--aids the last decomposition4 ,the oxygen in this case naturally passes over to the metal4 ,it is found by this equation that gunpowder should contain thirty-six parts of charcoal 7#acc p4c471 and thirty-two parts 7#aai p4c47 of sulphur1 to #bjb parts 7#gdh p4c47 of nitre1 which is very near to its actual composition4#af #af ,in ,china1 where the manufacture of gunpowder has long been carried on1 #geg parts of nitre1 #add of charcoal1 and #ii of sulphur are used4 ,ordinary powder for sporting purposes contains #hj parts of nitre1 #ab of charcoal1 and #h of sulphur1 whilst the gunpowder used in heavy ordnance contains #ge of nitre1 #ae of charcoal1 and #aj of sulphur4 ,gunpowder explodes when heated to #cjj1 when struck1 or by contact with a spark4 ,a compact or finely-divided mass of gunpowder burns slowly and has but little disruptive action1 because it burns gradually4 ,to act properly the gunpowder must have a definite rate of combustion1 so that the pressure should increase during the passage of the projectile along the barrel of the fire-arm4 ,this is done by making the powder in large granules or in the shape of six-sided prisms with holes through them 7prismatic powder74 ,the products of combustion are of two kinds3 7#a7 gases which produce the pressure and are the cause of the dynamical action of gunpowder1 and 7#b7 a solid residue1 usually of a black colour owing to its containing unburnt particles of charcoal4 ,besides charcoal1 the residue generally contains potassium sulphide1 ,k#b,s1 and a whole series of other salts--for instance1 carbonate and sulphate4 ,it is apparent from this that the combustion of gunpowder is not so simple as it appears to be from the above formula1 and hence the weight of the residue is also greater than indicated by that formula4 ,according to the formula1 #bgj parts of gunpowder give #aaj parts of residue--that is1 #ajj parts of powder give #cgd parts of residue1 ,k#b,s1 whilst in reality the weight of the residue varies from #dj p4c4 to #gj p4c4 7generally #eb p4c474 ,this difference depends on the fact that so much oxygen 7of the nitre7 remains in the residue1 and it is evident that if the residue varies the composition of the gases evolved by the powder will vary also1 and therefore the entire process will be different in different cases4 ,the difference in the composition of the gases and residue depends1 as the researches of ,gay-,lussac1 ,shishkoff and ,bunsen1 ,nobel and ,abel1 ,federoff1 ,debus1 c41 show1 on the conditions under which the combustion of the powder proceeds4 ,when gunpowder burns in an open space1 the gaseous products which are formed do not remain in contact with the residue1 and then a considerable portion of the charcoal entering into the composition of the powder remains unburnt1 because the charcoal burns after the sulphur at the expense of the oxygen of the nitre4 ,in this extreme case the commencement of the combustion of the gunpowder may be expressed by the equation1 #b,,kno#c #c;,c ,s #b;,c ,k#b,,so#d ,,co#b ,n#b4 ,the residue in a blank cartridge often consists of a mixture of ,c1 ,k#b,,so#d1 ,k#b;,,co#c1 and ,k#b,s#b,o#c4 ,if the combustion of the gunpowder be impeded--if it take place in a cartridge in the barrel of a gun--the quantity of potassium sulphate will first be diminished1 then the amount of sulphite1 whilst the amount of carbonic anhydride in the gases and the amount of potassium sulphide in the residue will increase4 ,the quantity of charcoal entering into the action will then be also increased1 and hence the amount in the residue will decrease4 ,under these circumstances the weight of the residue will be less--for example1 #d,k#b;,,co#c #d,s ,k#b,,so#d #c,k#b,s #d;,,co#b4 ,besides which1 carbonic oxide has been found in the gases1 and potassium bisulphide1 ,k#b,s#b1 in the residue of gunpowder4 ,the amount of potassium sulphide1 ,k#b,s1 increases with the completeness of the combustion1 and is formed in the residue at the expense of the potassium sulphite4 ,in recent times the knowledge of the action of gunpowder and other explosives has made much progress1 and has developed into a vast province of artillery science1 which1 guided by the discoveries of chemistry1 has worked out a 'smokeless powder' which burns without leaving a residue1 and does not therefore give any 'powder smoke' 7to hinder the rapidity of firing and aiming71 and at the same time disengages a greater volume of gas and consequently gives 7under proper conditions of combustion7 the possibility of communicating to the charge a greater initial velocity1 and therefore greater distance1 force1 and accuracy of aim4 ,such 'smokeless powder' is prepared either from the varieties of nitrocellulose 7,chapter ,,vi41 ,note #cg7 or from a mixture of them with nitro-glycerine 7ibid74 ,in burning they give1 besides steam and nitrogen1 generally a large amount of oxide of carbon 7this is a very serious drawback in all the present forms of smokeless powder1 because carbonic oxide is poisonous71 and also ,,co#b1 ,h#b1 c4 ,metallic potassium was obtained like sodium2 first by the action of a galvanic current1 then by reduction of the hydroxide by means of metallic iron1 and lastly1 by the action of charcoal on the carbonate at a high temperature4 ,the behaviour of metallic potassium differs1 however1 from that of sodium1 because it easily combines with carbonic oxide1 forming an explosive and inflammable mass4#ag #ag ,the substances obtained in this case are mentioned in ,chapter ,,ix41 ,note #ca4 ,potassium is quite as volatile as sodium1 if not more so4 ,at the ordinary temperature potassium is even softer than sodium2 its freshly-cut surfaces present a whiter colour than sodium1 but1 like the latter1 and with even greater ease1 it oxidises in moist air4 ,it is brittle at low temperatures1 but is quite soft at #be1 and melts at #eh4 ,at a low red heat 7#ffg1 ,perkin7 it distils without change1 forming a green vapour1 whose density1#ah according to ,a4 ,scott 7#ahhg71 is equal to #ai 7if that of hydrogen #a74 ,this shows that the molecule of potassium 7like that of sodium1 mercury1 and zinc7 contains but one atom4 ,this is also the case with many other metals1 judging by recent researches4#ai ,the specific gravity of potassium at #ae is #jhg1 and is therefore less than that of sodium1 as is also the case with all its compounds4#bj ,potassium decomposes water with great ease at the ordinary temperature1 evolving #de1jjj heat units per atomic weight in grams4 ,the heat evolved is sufficient to inflame the hydrogen1 the flame being coloured violet from the presence of particles of potassium4#ba #ah ,a4 ,scott 7#ahhg7 determined the vapour densities of many of the alkali elements and their compounds in a platinum vessel heated in a furnace and previously filled with nitrogen4 ,but these1 the first data concerning a subject of great importance1 have not yet been sufficiently fully described1 nor have they received as much attention as could be desired4 ,taking the density of hydrogen as unity1 ,scott found the vapour densities of the following substances to be-- ,na #abge 7#aae74 ,,ki #ib 7#hd74 ,k #ai 7#aie74 ,rb,cl #gj 7#fj74 ,cs,cl #hie 7#hdb74 ,cs,i #acc 7#acj74 ,fe,cl#c #fh4 ,ag,cl #hj 7#gag74 ,in brackets are given the densities corresponding with the formul1 according to ,avogadro-,gerhardt's law4 ,this figure is not given for ,fe,cl#c1 because in all probability under these conditions 7the temperature at which it was determined7 a portion of the ,fe,cl#c was decomposed4 ,if it was not decomposed1 then a density #ha would correspond with the formula ,fe,cl#c1 and if the decomposition were ,fe#b;,cl#f #b;,fe,cl#b ,cl#b1 then the density should be #ed4 ,with regard to the silver chloride1 there is reason to think that the platinum decomposed this salt4 ,the majority of ,scott's results so closely correspond with the formul that a better concord cannot be expected in such determinations4 ,v4 ,meyer 7#ahhg7 gives #ic as the density of ,,ki4 #ai ,the molecules of non-metals are more complex--for instance1 ,h#b1 ,o#c1 ,cl#b1 c4 ,but arsenic1 whose superficial appearance recalls that of metals1 but whose chemical properties approach more nearly to the non-metals1 has a complex molecule containing ,as#d4 #bj ,as the atomic weight of potassium is greater than that of sodium1 the volumes of the molecules1 or the quotients of the molecular weight by the specific gravity1 for potassium compounds are greater than those of sodium compounds1 because both the denominator and numerator of the fraction increase4 ,we cite for comparison the volumes of the corresponding compounds-- ,na #bd ,na,h,o #ah ,na,cl #bh ,na,n,o#c #cg ,na#b,,so#d #ed ,k #de ,,kho #bg ,k,cl #ci ,,kno#c #dh ,k#b,,so#d #ff #ba ,the same precautions must be taken in decomposing water by potassium as have to be observed with sodium 7,chapter ,,ii41 ,note #h74 ,it must be observed that potassium decomposes carbonic anhydride and carbonic oxide when heated1 the carbon being liberated and the oxygen taken up by the metal1 whilst on the other hand charcoal takes up oxygen from potassium1 as is seen from the preparation of potassium by heating potash with charcoal1 hence the reaction ,k#b,o ,c ,k#b ,,co is reversible and the relation is the same in this case as between hydrogen and zinc4 ,with regard to the relation of potassium to hydrogen and oxygen1 it is closely analogous to sodium in this respect4 ,thus1 with hydrogen it forms potassium hydride1 ,k#b;,h 7between #bjj and #daa71 and with oxygen it gives a suboxide ,k#d,o1 oxide ,k#b,o1 and peroxide1 only more oxygen enters into the composition of the latter than in sodium peroxide2 potassium peroxide contains ,,ko#b1 but it is probable that in the combustion of potassium an oxide ,,ko is also formed4 ,potassium1 like sodium1 is soluble in mercury4#bb ,in a word1 the relation between sodium and potassium is as close as that between chlorine and bromine1 or1 better still1 between fluorine and chlorine1 as the atomic weight of sodium1 #bc1 is as much greater than that of fluorine1 #ai1 as that of potassium1 #ci1 is greater than that of chlorine1 #cee4 #bb ,potassium forms alloys with sodium in all proportions4 ,the alloys containing #a and #c equivalents of potassium to one equivalent of sodium are liquids1 like mercury at the ordinary temperature4 ,joannis1 by determining the amount of heat developed by these alloys in decomposing water1 found the evolution for ,na#b,k1 ,na,k1 ,na,k#b and ,na,k#c to be #dde1 #dda1 #dch and #ddd thousand heat units respectively 7for ,na #dbf and for ,k #ded74 ,the formation of the alloy ,na,k#b is therefore accompanied by the development of heat1 whilst the other alloys may be regarded as solutions of potassium or sodium in this alloy4 ,in any case a fall of the temperature of fusion is evident in this instance as in the alloys of nitre 7,note #ad74 ,the liquid alloy ,na,k#b is now used for filling thermometers employed for temperatures above #cfj1 when mercury boils4 ,the resemblance between potassium and sodium is so great that their compounds can only be easily distinguished in the form of certain of their salts4 ,for instance1 the acid potassium tartrate1 ,c#d;,h#e,,ko#f 7cream of tartar71 is distinguished by its sparing solubility in water and in alcohol1 and in a solution of tartaric acid1 whilst the corresponding sodium salt is easily soluble4 ,therefore1 if a solution of tartaric acid be added in considerable excess to the solutions of the majority of potassium salts1 a precipitate of the sparingly-soluble acid salt is formed1 which does not occur with salts of sodium4 ,the chlorides ,k,cl and ,na,cl in solutions easily give double salts ,k#b,pt,cl#f and ,na#b,pt,cl#f1 with platinic chloride1 ,pt,cl#d1 and the solubility of these salts is very different1 especially in a mixture of alcohol and ether4 ,the sodium salt is easily soluble1 whilst the potassium salt is insoluble or almost so1 and therefore the reaction with platinic chloride is that most often used for the separation of potassium from sodium1 as is more fully described in works on analytical chemistry4 ,it is possible to discover the least traces of these metals in admixture together1 by means of their property of imparting different colours to a flame4 ,the presence of a salt of sodium in a flame is recognised by a brilliant yellow coloration1 and a pure potassium salt colours a colourless flame violet4 ,however1 in the presence of a sodium salt the pale violet coloration given by a potassium salt is quite undistinguishable1 and it is at first sight impossible in this case to discover the potassium salt in the presence of that of sodium4 ,but by decomposing the light given by a flame coloured by these metals or a mixture of them1 by means of a prism1 they are both easily distinguishable1 because the yellow light emitted by the sodium salt depends on a group of light rays having a definite index of refraction which corresponds with the yellow portion of the solar spectrum1 having the index of refraction of the ,fraunhofer line 7strictly speaking1 group of lines7 ,d1 whilst the salts of potassium give a light from which these rays are entirely absent1 but which contain rays of a red and violet colour4 ,therefore1 if a potassium salt occur in a flame1 on decomposing the light 7after passing it through a narrow slit7 by means of a prism1 there will be seen red and violet bands of light situated at a considerable distance from each other2 whilst if a sodium salt be present a yellow line will also appear4 ,if both metals simultaneously occur in a flame and emit light1 the spectrum lines corresponding to the potassium and the sodium will appear simultaneously4 ,illustration3 ,,fig4 #gb4--,spectroscope4 ,the prism and table are covered with an opaque cover4 ,the spectrum obtained from the flame coloured by a substance introduced on the wire is viewed through ,b4 ,a light is placed before the scale ,d in order to illuminate the image of the scale reflected through ,b by the side of the prism4 ,for convenience in carrying on this kind of testing1 spectroscopes 7fig4 #gb7 are constructed1#bc consisting of a refracting prism and three tubes placed in the plane of the refracting angle of the prism4 ,one of the tubes1 ,c1 has a vertical slit at the end1 giving access to the light to be tested1 which then passes into the tube 7collimator71 containing a lens which gives the rays a parallel direction4 ,the rays of light having passed through the slit1 and having become parallel1 are refracted and dispersed in the prism1 and the spectrum formed is observed through the eye-piece of the other telescope ,b4 ,the third tube ,d contains a horizontal transparent scale 7at the outer end7 which is divided into equal divisions4 ,the light from a source such as a gas burner or candle placed before this tube1 passes through the scale1 and is reflected on that face of the prism which stands before the telescope ,b1 so that the image of the scale is seen through this telescope simultaneously with the spectrum given by the rays passing through the slit of the tube ,c4 ,in this manner the image of the scale and the spectrum given by the source of light under investigation are seen simultaneously4 ,if the sun's rays be directed through the slit of the tube ,c1 then the observer looking through the eye-piece of ,b will see the solar spectrum1 and 7if the aperture of the slit be narrow and the apparatus correctly adjusted7 the dark ,fraunhofer lines in it4#bd ,small-sized spectroscopes are usually so adjusted that 7looking through ,b7 the violet portion of the spectrum is seen to the right and the red portion to the left1 and the ,fraunhofer line ,d 7in the bright yellow portion of the spectrum7 is situated on the #ejth division of the scale4#be ,if the light emitted by an incandescent solid--for example1 the ,drummond light--be passed through the spectroscope1 then all the colours of the solar spectrum are seen1 but not the ,fraunhofer lines4 ,to observe the result given by a flame coloured by various salts a ,bunsen gas burner 7or the pale flame of hydrogen gas issuing from a platinum orifice7 giving so pale a flame that its spectrum will be practically invisible is placed before the slit4 ,if any compound of sodium be placed in the flame of the gas burner 7for which purpose a platinum wire on whose end sodium chloride is fused is fixed to the stand71 then the flame is coloured yellow1 and on looking through the spectroscope the observer will see a bright yellow line falling upon the #ejth division of the scale1 which is seen together with the spectrum in the telescope4 ,no yellow lines of other refractive index1 nor any rays of any other colour1 will be seen1 and1 therefore1 the spectrum corresponding with sodium compounds consists of yellow rays of that index of refraction which belong to the ,fraunhofer 7black7 line ,d of the solar spectrum4 ,if a potassium salt be introduced into the flame instead of a sodium salt1 then two bands will be seen which are much feebler than the bright sodium band--namely1 one red line near the ,fraunhofer line ,a and another violet line4 ,besides which1 a pale1 almost continuous1 spectrum will be observed in the central portions of the scale4 ,if a mixture of sodium and potassium salts be now introduced into the flame1 three lines will be seen simultaneously--namely1 the red and pale violet lines of potassium and the yellow line of sodium4 ,in this manner it is possible1 by the aid of the spectroscope1 to determine the relation between the spectra of metals and known portions of the solar spectrum4 ,the continuity of the latter is interrupted by dark lines 7that is1 by an absence of light of a definite index of refraction71 termed the ,fraunhofer lines of the solar spectrum4 ,it has been shown by careful observations 7by ,fraunhofer1 ,brewster1 ,foucault1 ngstrom1 ,kirchhoff1 ,cornu1 ,lockyer1 ,dewar1 and others7 that there exists an exact agreement between the spectra of certain metals and certain of the ,fraunhofer lines4 ,thus the bright yellow sodium line exactly corresponds with the dark ,fraunhofer line ,d of the solar spectrum4 ,a similar agreement is observed in the case of many other metals4 ,this is not an approximate or chance correlation4 ,in fact1 if a spectroscope having a large number of refracting prisms and a high magnifying power be used1 it is seen that the dark line ,d of the solar spectrum consists of an entire system of closely adjacent but definitely situated fine and wide 7sharp1 distinct7 dark lines1#bf and an exactly similar group of bright lines is obtained when the yellow sodium line is examined through the same apparatus1 so that each bright sodium line exactly corresponds with a dark line in the solar spectrum4#bf bis ,this conformity of the bright lines formed by sodium with the dark lines of the solar spectrum cannot be accidental4 ,this conclusion is further confirmed by the fact that the bright lines of other metals correspond with dark lines of the solar spectrum4 ,thus1 for example1 a series of sparks passing between the iron electrodes of a ,ruhmkorff coil gives #dej very distinct lines characterising this metal4 ,all these #dej bright lines1 constituting the whole spectrum corresponding with iron1 are repeated1 as ,kirchhoff showed1 in the solar spectrum as dark ,fraunhofer lines which occur in exactly the same situations as the bright lines in the iron spectrum1 just as the sodium lines correspond with the band ,d in the solar spectrum4 ,many observers have in this manner studied the solar spectrum and the spectra of different metals simultaneously1 and discovered in the former lines which correspond not only with sodium and iron1 but also with many other metals4#bg ,the spectra of such elements as hydrogen1 oxygen1 nitrogen1 and other gases may be observed in the so-called ,geissler's tubes--that is1 in glass tubes containing rarefied gases1 through which the discharge of a ,ruhmkorff's coil is passed4 ,thus hydrogen gives a spectrum composed of three lines--a red line corresponding with the ,fraunhofer line ,c1 a green line corresponding with the line ,f1 and a violet line corresponding with one of the lines between ,g and ,h4 ,of these rays the red is the brightest1 and therefore the general colour of luminous hydrogen 7with an electric discharge through a ,geissler tube7 is reddish4 #bc ,for accurate measurements and comparative researches more complicated spectroscopes are required which give a greater dispersion1 and are furnished for this purpose with several prisms--for example1 in ,browning's spectroscope the light passes through six prisms1 and then1 having undergone an internal total reflection1 passes through the upper portion of the same six prisms1 and again by an internal total reflection passes into the ocular tube4 ,with such a powerful dispersion the relative position of the spectral lines may be determined with accuracy4 ,for the absolute and exact determination of the wave lengths it is particularly important that the spectroscope should be furnished with diffraction gratings4 ,the construction of spectroscopes destined for special purposes 7for example1 for investigating the light of stars1 or for determining the absorption spectra in microscopic preparations1 c47 is exceedingly varied4 ,details of the subject must be looked for in works on physics and on spectrum analysis4 ,among the latter the best known for their completeness and merit are those of ,roscoe1 ,kayser1 ,vogel1 and ,lecoq de ,boisbaudran4 #bd ,the arrangement of all the parts of the apparatus so as to give the clearest possible vision and accuracy of observation must evidently precede every kind of spectroscopic determination4 ,details concerning the practical use of the spectroscope must be looked for in special works on the subject4 ,in this treatise the reader is supposed to have a certain knowledge of the physical data respecting the refraction of light1 and its dispersion and diffraction1 and the theory of light1 which allows of the determination of the length of the waves of light in absolute measure on the basis of observations with diffraction gratings1 the distance between whose divisions may be easily measured in fractions of a millimetre2 by such means it is possible to determine the wave-length of any given ray of light4 #be ,in order to give an idea of the size of the scale1 we may observe that the ordinary spectrum extends from the zero of the scale 7where the red portion is situated7 to the #agjth division 7where the end of the visible violet portion of the spectrum is situated71 and that the ,fraunhofer line ,a 7the extreme prominent line in the red7 corresponds with the #agth division of the scale2 the ,fraunhofer line ,f 7at the beginning of the blue1 near the green colour7 is situated on the #ijth division1 and the line ,g1 which is clearly seen in the beginning of the violet portion of the spectrum1 corresponds with the #abgth division of the scale4 #bf ,the two most distinct lines of ,d1 or of sodium1 have wave-lengths of #ehie and #ehhi millionths of a millimeter1 besides which fainter and fainter lines are seen whose wave-lengths in millionths of a millimeter are #ehhg and #ehha1 #fafj and #faed1 #eaee and #eaeb1 #dihc and #dihb1 c41 according to ,liveing and ,dewar4 #bf bis ,in the ordinary spectroscopes which are usually employed in chemical research1 one yellow band1 which does not split up into thinner lines1 is seen instead of the system of sodium lines1 owing to the small dispersive power of the prism and the width of the slit of the object tube4 #bg ,the most accurate investigations made in this respect are carried on with spectra obtained by diffraction1 because in this case the position of the dark and bright lines does not depend on the index of refraction of the material of the prism1 nor on the dispersive power of the apparatus4 ,the best--that is1 the most general and accurate--method of expressing the results of such determinations consists in determining the lengths of the waves corresponding to the rays of a definite index of refraction4 7,sometimes instead of this the fraction of #a divided by the square of the wave-length is given47 ,we will express this wave-length in millionth parts of a millimetre 7the ten-millionth parts are already doubtful1 and fall within the limits of error74 ,in order to illustrate the relation between the wave-lengths and the positions of the lines of the spectrum1 we will cite the wave-lengths corresponding with the chief ,fraunhofer lines and colours of the spectrum4 ,fraunhofer line ,a ,b ,c ,d ,e b ,wave-length #gfaj #fhge #feff #ehie-ehhi #ebgc #eahg ----------------- --------- ---- --- ,colour red orange yellow green ,fraunhofer line ,f ,g ,h ,wave-length #dhfe #dcaj #cigb ---------- ---- ,colour blue violet ,in the following table are given the wave-lengths of the light rays 7the longest and most distinct1 see later7 for certain elements1 those in black type being the most clearly defined and distinct lines1 which are easily obtained either in the flame of a ,bunsen's burner1 or in ,geissler's tubes1 or in general1 by an electric discharge4 ,these lines refer to the elements 7the lines of compounds are different1 as will be afterwards explained1 but many compounds are decomposed by the flame or by an electric discharge71 and moreover to the elements in an incandescent and rarefied gaseous state1 for the spectra sometimes vary considerably with a variation of temperature and pressure4 ,it may be mentioned that the red colour corresponds with lines having a wave-length of from #ghj 7with a greater wave-length the lines are hardly visible1 and are ultra red7 to #fej1 the orange from #fej to #eij1 the yellow from #eij to #ebj1 the green from #ebj to #dij1 the blue from #dij to #dbj1 and the violet from #dbj to #chj millionth parts of a millimetre4 ,beyond #chj the lines are scarcely visible1 and belong to the ultra-violet4 ,for fluorine ,moissan found as many as #ac bright lines from #gdd to #fbc4 ,in the table 7p4 #efe7 which is arranged in conformity with the image of the spectrum as it is seen 7the red lines on the left-hand and the violet on the right-hand side71 the figures in black type correspond with lines which are so bright and distinctly visible that they may easily be made use of1 both in determining the relation between the divisions of the scale and the wave-lengths1 and in determining the admixture of a given element with another4 ,brackets join those lines between which several other lines are clearly visible if the dispersive power of the spectroscope permits distinguishing the neighbouring lines4 ,in the ordinary laboratory spectroscopes with one prism1 even with all possible precision of arrangement and with a brilliancy of light permitting the observations being made with a very narrow aperture1 the lines whose wave-lengths only differ by #b-c millionths of a millimetre1 are blurred together2 and with a wide aperture a series of lines differing by even as much as #bj millionths of a millimetre appear as one wide line4 ,with a faint light 7that is1 with a small quantity of light entering into the spectroscope7 only the most brilliant lines are clearly visible4 ,the length of the lines does not always correspond with their brilliancy4 ,according to ,lockyer this length is determined by placing the carbon electrodes 7between which the incandescent vapours of the metals are formed71 not horizontally to the slit 7as they are generally placed1 to give more light71 but vertically to it4 ,then certain lines appear long and others short4 ,as a rule 7,lockyer1 ,dewar1 ,cornu71 the longest lines are those with which it is easiest to obtain reversed spectra 7see later74 ,consequently1 these lines are the most characteristic4 ,only the longest and most brilliant are given in our table1 which is composed on the basis of a collection of the data at our disposal for bright spectra of the incandescent and rarefied vapours of the elements4 ,as the spectra change with great variations of temperature and vapour density 7the faint lines become brilliant whilst the bright lines sometimes disappear71 which is particularly clear from ,ciamician's researches on the halogens1 until the method of observation and the theory of the subject are enlarged1 particular theoretical importance should not be given to the wave-lengths showing the maximum brilliancy1 which only possess a practical significance in the common methods of spectroscopic observations4 ,in general the spectra of metals are simpler than those of the halogens1 and the latter are variable2 at an increased pressure all spectral lines become broader4 ---------------------------------------- ---------------- ,n#b ,o#b ,cl#b ,br#b ,i#b ,pb ,sn ,tl ---------------------------------------- ---------------- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- #ffb -- -- -- -- -- -- -- #fcb -- -- #fcf -- -- #fde -- #fbj #fae -- -- #fba -- -- -- #ehe -- -- -- #fac #fjeg #ehj -- #egd -- #edf -- #egi #efjg #eef -- #edd #edc #eci #edd #efj #eedg -- #edi #ece #ecc #ebh #ebc #ede #ecg -- #ece #ebg #eaf #eai #eag #ejf -- -- -- #eaf #die #did #dgi -- -- -- #dhi -- #did #dhj #dgj -- -- -- -- #deg #dgj -- #dfb -- -- #deb -- #ddb #dfe -- #ded #dde -- -- -- #dcf #ddg #dcf #dcg -- -- -- -- #dbf #dcb #dca -- -- -- -- -- -- -- -- -- #dba -- -- -- -- -- -- -- -- -- -- -- #dji -- -- -- -- #djf -- -- -- -- -- -- -- -- -- -- ---------------------------------------- ---------------- ---------------------------------------- ---------------- ,in ,ga ,al ,ba ,sr ,ca ,mg ,zn ,cd ,hg ---------------------------------------- ---------------- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- #fdf -- -- -- -- -- -- #fbd #fdig #fda #fdd -- #fcf #fdch -- #fai -- #fbc #fad #fjf -- -- -- -- #fae -- -- -- -- -- #fab -- -- -- #egi -- -- #egb #eece -- -- -- -- -- #egg -- -- #egj #edi #edh #eei -- -- #ecgg #edf #ebe -- -- -- #ebd -- #eah -- #eccf -- -- -- -- -- -- -- #eaf #dib #ejhe -- -- -- -- #dicc -- -- -- #dha -- -- -- -- -- -- -- -- #dga #dgb #dgii -- #dea -- #dff #dee #dfj -- -- #dfh #dfgg -- -- -- -- -- -- #dde #ddh -- -- -- -- -- -- -- -- #ddb -- -- -- #dcf -- -- -- -- #dcj -- -- -- -- -- -- #dag -- -- #dba #dbc -- -- -- -- -- -- -- #dac -- -- -- -- -- -- #daj #djc #cif -- #djh #cig #chd -- -- #djd -- -- #cid -- -- #cic #chc - - -- -- ---------------------------------------- ---------------- ---------------------------------------- ---------------- ,mn ,fe ,cu ,ag ,cs ,rb ,k ,na ,li ---------------------------------------- ---------------- -- -- -- -- -- #ghj #ggj -- -- -- -- -- -- -- -- #gff -- -- -- -- -- -- -- -- -- -- #fgjf -- #fdj -- -- -- -- -- -- -- #fjb -- -- -- #fbb #fbif -- -- #faj #fja #efa #egh -- #fjj -- #ehc #ehie -- #eea #edd #egj #edfd -- -- #egh #ehhi -- #ecd #ecg #ebb -- -- -- -- -- -- -- #ecb #eae #ebjh -- -- #ece -- -- -- #eba #eaa -- -- -- #ecb -- -- #dhb #dif -- -- -- -- -- -- #dig #dga #dhi -- -- -- -- -- -- -- -- -- -- -- #dei -- -- -- #dfjc -- #dda -- -- #def -- -- -- -- -- #dcj -- -- -- -- -- -- -- -- #dbg -- -- -- -- -- -- -- #dbd -- -- #dba -- #dbj -- -- -- #djc #djg -- -- -- -- -- -- -- -- #djd -- -- -- -- #djd -- -- -- -- -- -- -- -- -- -- -- ---------------------------------------- ---------------- ------- ,h#b ------- -- -- -- #fefb -- -- -- -- -- -- #dhfa -- -- -- #dcd -- -- -- -- -- ------- ,illustration3 ,,fig4 #gc4--;,absorption spectrum 7,lecoq de ,boisbaudran7 of salts of didymium in concentrated and dilute solutions4 ,the correlation of the ,fraunhofer lines with the spectra of metals depends on the phenomenon of the so-called reversal of the spectrum4 ,this phenomenon consists in this1 that instead of the bright spectrum corresponding with a metal1 under certain circumstances a similar dark spectrum in the form of ,fraunhofer lines may be obtained1 as will be explained directly4 ,in order to clearly understand the phenomenon of reversed spectra1 it must be known that when light passes through certain transparent substances these substances retain rays of a certain refrangibility4 ,the colour of solutions is a proof of this4 ,light which has passed through a yellow solution of a uranium salt contains no violet rays1 and after having passed through a red solution of a permanganate1 does not contain many rays in the yellow1 blue1 and green portions of the spectrum4 ,solutions of copper salts absorb nearly all red rays4 ,sometimes colourless solutions also absorb rays of certain definite refractive indexes1 and give absorption spectra4 ,thus solutions of salts of didymium absorb rays of a certain refrangibility1 and therefore an impression of black lines is received1#bh as shown in fig4 #gc4 ,many vapours 7iodine7 and gases 7nitric peroxide7 give similar spectra4 ,light which has passed through a deep layer of aqueous vapour1 oxygen1 or nitrogen also gives an absorption spectrum4 ,for this reason the peculiar 7winter7 dark lines discovered by ,brewster are observed in sunlight1 especially in the evening and morning1 when the sun's rays pass through the atmosphere 7containing these substances7 by a longer path than at mid-day4 ,it is evident that the ,fraunhofer lines may be ascribed to the absorption of certain rays of light in its passage from the luminous mass of the sun to the earth4 ,the remarkable progress made in all spectroscopic research dates from the investigations made by ,kirchhoff 7#ahei7 on the relation between absorption spectra and the spectra of luminous incandescent gases4 ,it had already been observed long before 7by ,fraunhofer1 ,foucault1 ngstrom7 that the bright spectrum of the sodium flame gives two bright lines which are in exactly the same position as two black lines known as ,d in the solar spectrum1 which evidently belong to an absorption spectrum4 ,when ,kirchhoff caused diffused sunlight to fall upon the slit of a spectroscope1 and placed a sodium flame before it1 a perfect superposition was observed--the bright sodium lines completely covered the black lines ,d of the solar spectrum4 ,when further the continuous spectrum of a ,drummond light showed the black line ,d on placing a sodium flame between it and the slit of the spectroscope--that is1 when the ,fraunhofer line of the solar spectrum was artificially produced--then there was no doubt that its appearance in the solar spectrum was due to the light passing somewhere through incandescent vapours of sodium4 ,hence a new theory of reversed spectra#bi arose--that is1 of the relation between the waves of light emitted and absorbed by a substance under given conditions of temperature2 this is expressed by ,kirchhoff's law1 discovered by a careful analysis of the phenomena4 ,this law may be formulated in an elementary way as follows3 ,at a given temperature the relation between the intensity of the light emitted 7of a definite wave-length7 and the absorptive capacity with respect to the same colour 7of the same wave-length7 is a constant quantity4#cj ,as a black dull surface emits and also absorbs a considerable quantity of heat rays whilst a polished metallic surface both absorbs and emits but few1 so a flame coloured by sodium emits a considerable quantity of yellow rays of a definite refrangibility1 and has the property of absorbing a considerable quantity of the rays of the same refractive index4 ,in general1 the medium which emits definite rays also absorbs them4 #bh ,the method of observing absorption spectra consists in taking a continuous spectrum of white light 7one which does not show either dark lines or particularly bright luminous bands--for instance1 the light of a candle1 lamp1 or other source74 ,the collimator 7that is1 the tube with the slit7 is directed towards this light1 and then all the colours of the spectrum are visible in the ocular tube4 ,a transparent absorptive medium--for instance1 a solution or tube containing a gas--is then placed between the source of light and the apparatus 7or anywhere inside the apparatus itself in the path of the rays74 ,in this case either the entire spectrum is uniformly fainter1 or absorption bands appear on the bright field of the continuous spectrum in definite positions along it4 ,these bands have different lengths and positions1 and distinctness and intensity of absorption1 according to the properties of the absorptive medium4 ,like the luminous spectra given by incandescent gases and vapours1 the absorption spectra of a number of substances have already been studied1 and some with great precision--as1 for example1 the spectrum of the brown vapours of nitrogen dioxide by ,hasselberg 7at ,pulkowa71 the spectra of colouring matters 7,eder and others71 especially of those applied to orthochromatic photography1 the spectra of blood1 chlorophyll 7the green constituent of leaves71 and other similar substances1 all the more carefully as by the aid of their spectra the presence of these substances may be discovered in small quantities 7even in microscopical quantities1 by the aid of special appliances on the microscope71 and the changes they undergo investigated4 ,illustration3 ,,fig4 #gd4--;,absorption spectra of nitrogen dioxide and iodine4 ,the absorption spectra1 obtained at the ordinary temperature and proper to substances in all physical states1 offer a most extensive but as yet little studied field1 both for the general theory of spectroscopy1 and for gaining an insight into the structure of substances4 ,the investigation of colouring matters has already shown that in certain cases a definite change of composition and structure entails not only a definite change of the colours but also a displacement of the absorption bands by a definite number of wave-lengths4 #bi ,a number of methods have been invented to demonstrate the reversibility of spectra2 among these methods we will cite two which are very easily carried out4 ,in ,bunsen's method sodium chloride is put into an apparatus for evolving hydrogen 7the spray of the salt is then carried off by the hydrogen and colours the flame with the yellow sodium colour71 and the hydrogen is ignited in two burners--in one large one with a wide flame giving a bright yellow sodium light1 and in another with a small fine orifice whose flame is pale3 this flame will throw a dark patch on the large bright flame4 ,in ,ladoffsky's method the front tube 7p4 #efa7 is unscrewed from a spectroscope directed towards the light of a lamp 7a continuous spectrum71 and the flame of a spirit lamp coloured by a small quantity of ,na,cl is placed between the tube and the prism2 a black band corresponding to sodium will then be seen on looking through the ocular tube4 ,this experiment is always successful if only there be the requisite relation between the strength of light of the two lamps4 #cj ,the absorptive capacity is the relation between the intensity of the light 7of a given wave-length7 falling upon and retained by a substance4 ,bunsen and ,roscoe showed by direct experiment that this ratio is a constant quantity for every substance4 ,if ,a stand for this ratio for a given substance at a given temperature--for instance1 for a flame coloured by sodium--and ,e be the intensity of the light of the same wave-length emitted at the same temperature by the same substance1 then ,kirchhoff's law1 the explanation and deduction of which must be looked for in text-books of physics1 states that the fraction ,a_/,e is a constant quantity depending on the nature of a substance 7as ,a depends on it7 and determined by the temperature and wave-length4 ,illustration3 ,,fig4 #ge4--;,bright spectra of copper compounds4 ,thus the bright spectral rays characteristic of a given metal may be reversed--that is1 converted into dark lines--by passing light which gives a continuous spectrum through a space containing the heated vapours of the given metal4 ,a similar phenomenon to that thus artificially produced is observed in sunlight1 which shows dark lines characteristic of known metals--that is1 the ,fraunhofer lines form an absorption spectrum or depend on a reversed spectrum2 it being presupposed that the sun itself1 like all known sources of artificial light1 gives a continuous spectrum without ,fraunhofer lines4#ca ,we must imagine that the sun1 owing to the high temperature which is proper to it1 emits a brilliant light which gives a continuous spectrum1 and that this light1 before reaching our eyes1 passes through a space full of the vapours of different metals and their compounds4 ,as the earth's atmosphere#cb contains very little1 or no1 metallic vapours1 and as they cannot be supposed to exist in the celestial space1#cb bis the only place in which the existence of such vapours can be admitted is in the atmosphere surrounding the sun itself4 ,as the cause of the sun's luminosity must be looked for in its high temperature1 the existence of an atmosphere containing metallic vapours is readily understood1 because at that high temperature such metals as sodium1 and even iron1 are separated from their compounds and converted into vapour4 ,the sun must be imagined as surrounded by an atmosphere of incandescent vaporous and gaseous matter1#cc including those elements whose reversed spectra correspond with the ,fraunhofer lines--namely1 sodium1 iron1 hydrogen1 lithium1 calcium1 magnesium1 c4 ,thus in spectrum analysis we find a means of determining the composition of the inaccessible heavenly luminaries1 and much has been done in this respect since ,kirchhoff's theory was formulated4 ,by observations on the spectra of many heavenly bodies1 changes have been discovered going on in them1#cd and many of the elements known to us have been found with certainty in them4#ce ,from this it must be concluded that the same elements which exist on the earth occur throughout the whole universe1 and that at that degree of heat which is proper to the sun those simple substances which we accept as the elements in chemistry are still undecomposed and remain unchanged4 ,a high temperature forms one of those conditions under which compounds most easily decompose2 and if sodium or a similar element were a compound1 in all probability it would be decomposed into component parts at the high temperature of the sun4 ,this may indeed be concluded from the fact that in ordinary spectroscopic experiments the spectra obtained often belong to the metals and not to the compounds taken2 this depends on the decomposition of these compounds in the heat of the flame4 ,if common salt be introduced into the flame of a gas-burner1 a portion of it is decomposed1 first forming1 in all probability1 with water1 hydrochloric acid and sodium hydroxide1 and the latter then becoming partially decomposed by the hydrocarbons1 giving metallic sodium1 whose incandescent vapour emits light of a definite refrangibility4 ,this conclusion is arrived at from the following experiment3--,if hydrochloric acid gas be introduced into a flame coloured by sodium it is observed that the sodium spectrum disappears1 owing to the fact that metallic sodium cannot remain in the flame in the presence of an excess of hydrochloric acid4 ,the same thing takes place on the addition of sal-ammoniac1 which in the heat of the flame gives hydrochloric acid4 ,if a porcelain tube containing sodium chloride 7or sodium hydroxide or carbonate71 and closed at both ends by glass plates1 be so powerfully heated that the salt volatilises1 then the sodium spectrum is not observable2 but if the salt be replaced by sodium1 then either the bright line or the absorption spectra is obtained1 according to whether the light emitted by the incandescent vapour be observed1 or light passing through the tube4 ,thus the above spectrum is not given by sodium chloride or other sodium compound1 but is proper to the metal sodium itself4 ,this is also the case with other analogous metals4 ,the chlorides and other halogen compounds of barium1 calcium1 copper1 c41 give independent spectra which differ from those of the metals4 ,if barium chloride be introduced into a flame1 it gives a mixed spectrum belonging to metallic barium and barium chloride4 ,if besides barium chloride1 hydrochloric acid or sal-ammoniac be introduced into the flame1 then the spectrum of the metal disappears1 and that of the chloride remains1 which differs distinctly from the spectrum of barium fluoride1 barium bromide1 or barium iodide4 ,a certain common resemblance and certain common lines are observed in the spectra of two different compounds of one and the same element obtained in the above-described manner1 and also in the spectrum of the metal1 but they all have their peculiarities4 ,the independent spectra of the compounds of copper are easily observed 7fig4 #ge74 ,thus certain compounds which exist in a state of vapour1 and are luminous at a high temperature1 give their independent spectra4 ,in the majority of cases the spectra of compounds are composed of indistinct luminous lines and complete bright bands1 whilst metallic elements generally give a few clearly-defined spectral lines4#cf ,there is no reason for supposing that the spectrum of a compound is equal to the sum of the spectra of its elements--that is1 every compound which is not decomposed by heat has its own proper spectrum4 ,this is best proved by absorption spectra1 which are essentially only reversed spectra observed at low temperatures4 ,if every salt of sodium1 lithium1 and potassium gives one and the same spectrum1 this must be ascribed to the presence in the flame of the free metals liberated by the decomposition of their salts4 ,therefore the phenomena of the spectrum are determined by molecules1 and not by atoms--that is1 the molecules of the metal sodium1 and not its atoms1 produce those particular vibrations which determine the spectrum of a sodium salt4 ,where there is no free metallic sodium there is no sodium spectrum4 #ca ,heated metals begin to emit light 7only visible in the dark7 at about #dbj 7varying with the metal74 ,on further heating1 solids first emit red1 then yellow1 and lastly white light4 ,compressed or heavy gases 7see ,chapter ,,iii41 ,note #dd71 when strongly heated1 also emit white light4 ,heated liquids 7for example1 molten steel or platinum7 also give a white compound light4 ,this is readily understood4 ,in a dense mass of matter the collisions of the molecules and atoms are so frequent that waves of only a few definite lengths cannot appear2 the reverse is possible in rarefied gases or vapours4 #cb ,brewster1 as is mentioned above1 first distinguished the atmospheric1 cosmical ,fraunhofer lines from the solar lines4 ,janssen showed that the spectrum of the atmosphere contains lines which depend on the absorption produced by aqueous vapour4 ,egoreff1 ,olszewski1 ,janssen1 and ,liveing and ,dewar showed by a series of experiments that the oxygen of the atmosphere gives rise to certain lines of the solar spectrum1 especially the line ,a4 ,liveing and ,dewar took a layer of #afe c4m4 of oxygen compressed under a pressure of #he atmospheres1 and determined its absorption spectrum1 and found that1 besides the ,fraunhofer lines ,a and ,b1 it contained the following groups3 #fcj-fbb1 #eha-efh1 #ece1 #dhj-dge4 ,the same lines were found for liquid oxygen4 #cb bis ,if the material of the whole heavenly space formed the absorbent medium1 the spectra of the stars would be the same as the solar spectrum2 but ,huyghens1 ,lockyer1 and others showed not only that this is the case for only a few stars1 but that the majority of stars give spectra of a different character with dark and bright lines and bands4 #cc ,eruptions1 like our volcanic eruptions1 but on an incomparably larger scale1 are of frequent occurrence on the sun4 ,they are seen as protuberances visible during a total eclipse of the sun1 in the form of vaporous masses on the edge of the solar disc and emitting a faint light4 ,these protuberances of the sun are now observed at all times by means of the spectroscope 7,lockyer's method71 because they contain luminous vapours 7giving bright lines7 of hydrogen and other elements4 #cd ,the great interest and vastness of astro-physical observations concerning the sun1 comets1 stars1 nebul1 c41 render this new province of natural science very important1 and necessitate referring the reader to special works on the subject4 ,the most important astro-physical data since the time of ,kellner are those referring to the displacement of the lines of the spectrum4 ,just as a musical note changes its pitch with the approach or withdrawal of the resonant object or the ear1 so the pitch of the luminous note or wave-length of the light varies if the luminous 7or absorbent7 vapour and the earth from which we observe it approach or recede from each other2 this expresses itself in a visible displacement of the spectral lines4 ,the solar eruptions even give broken lines in the spectrum1 because the rapidly moving eruptive masses of vapour and gases either travel in the direction of the eye or fall back towards the sun4 ,as the earth travels with the solar system among the stars1 so it is possible to determine the direction and velocity with which the sun travels in space by the displacement of the spectral lines and light of the stars4 ,the changes proceeding on the sun in its mass1 which must be pronounced as vaporous1 and in its atmosphere1 are now studied by means of the spectroscope4 ,for this purpose1 many special astro-physical observatories now exist where these investigations are carried on4 ,we may remark that if the observer or luminous object moves with a velocity v1 the ray1 whose wave-length is ,greek3 l1 has an apparent wave-length ,greek3 l7nv7_/n1 where n is the velocity of light4 ,thus ,tolon1 ,huyghens1 and others proved that the star ,aldebaran approaches the solar system with a velocity of #cj kilometres per second1 while ,arcturus is receding with a velocity of #de kilometres4 ,the majority of stars give a distinct hydrogen spectrum1 besides which nebul also give the spectrum of nitrogen4 ,lockyer classes the stars from their spectra1 according to their period of formation1 showing that some stars are in a period of increasing temperature 7of formation or aggregation71 whilst others are in a period of cooling4 ,altogether1 in the astro-physical investigation of the spectra of heavenly bodies we find one of the most interesting subjects of recent science4 #ce ,spectrum analysis has proved the indubitable existence in the sun and stars of a number of elements known in chemistry4 ,huyghens1 ,secchi1 ,lockyer1 and others have furnished a large amount of material upon this subject4 ,a compilation of existing information on it has been given by ,prof4 ,s4 ,a4 ,kleiber1 in the ,journal of the ,russian ,physico-chemical ,society for #ahhe 7vol4 xviii4 p4 #adf74 ,besides which1 a peculiar element called helium has been discovered1 which is characterised by a line 7whose wave-length is #ehge1 situated near ,d71 which is seen very brightly in the projections 7protuberances7 and spots of the sun1 but which does not belong to any known element1 and is not reproducible as a reversed1 dark line4 ,this may be a right conclusion--that is to say1 it is possible that an element may be discovered to which the spectrum of helium corresponds--but it may be that the helium line belongs to one of the known elements1 because spectra vary in the brilliancy and position of their lines with changes of temperature and pressure4 ,thus1 for instance1 ,lockyer could only see the line #dbc1 at the very end of the calcium spectrum1 at comparatively low temperatures1 whilst the lines #cig and #cic appear at a higher temperature1 and at a still higher temperature the line #dbc becomes quite invisible4 #cf ,spectroscopic observations are still further complicated by the fact that one and the same substance gives different spectra at different temperatures4 ,this is especially the case with gases whose spectra are obtained by an electric discharge in tubes4 ,plcker1 ,wllner1 ,schuster1 and others showed that at low temperatures and pressures the spectra of iodine1 sulphur1 nitrogen1 oxygen1 c4 are quite different from the spectra of the same elements at high temperatures and pressures4 ,this may either depend on the fact that the elements change their molecular structure with a change of temperature1 just as ozone is converted into oxygen 7for instance1 from ,n#b molecules are obtained containing only one atom of nitrogen71 or else it may be because at low temperature certain rays have a greater relative intensity than those which appear at higher temperatures4 ,if we suppose that the molecules of a gas are in continual motion1 with a velocity dependent on the temperature1 then it must be admitted that they often strike against each other and rebound1 and thus communicate peculiar motions to each other and the supposed ether1 which express themselves in luminiferous phenomena4 ,a rise of the temperature or an increase in the density of a gas must have an influence on the collision of its molecules and luminiferous motions thus produced1 and this may be the cause of the difference of the spectra under these circumstances4 ,it has been shown by direct experiment that gases compressed by pressure1 when the collision of the molecules must be frequent and varied1 exhibit a more complex spectrum on the passage of an electric spark than rarefied gases1 and that even a continuous spectrum appears4 ,in order to show the variability of the spectrum according to the circumstances under which it proceeds1 it may be mentioned that potassium sulphate fused on a platinum wire gives1 on the passage of a series of sparks1 a distinct system of lines1 #ehc-egh1 whilst when a series of sparks is passed through a solution of this salt this system of lines is faint1 and when ,roscoe and ,schuster observed the absorption spectrum of the vapour of metallic potassium 7which is green7 they remarked a number of lines of the same intensity as the above system in the red1 orange1 and yellow portions4 ,illustration3 ,,fig4 #gf4--,method of showing the spectrum of substances in solution4 ,the spectra of solutions are best observed by means of ,lecoq de ,boisbaudran's arrangement1 shown in fig4 #gf4 ,a bent capillary tube1 ,d ,f1 inside which a platinum wire1 ,a a 7from #jc to #je mm4 in diameter7 is fused1 is immersed in a narrow cylinder1 ,c 7in which it is firmly held by a cork74 ,the projecting end1 a1 of the wire is covered by a fine capillary tube1 d1 which extends #a-b mm4 beyond the wire4 ,another straight capillary tube1 ,e1 with a platinum wire1 ,b b1 about #a mm4 in diameter 7a finer wire soon becomes hot71 is held 7by a cork or in a stand7 above the end of the tube1 ,d4 ,if the wire ,a be now connected with the positive1 and the wire ,b with the negative terminal of a ,ruhmkorff's coil 7if the wires be connected in the opposite order1 the spectrum of air is obtained71 a series of sparks rapidly following each other appear between a and b1 and their light may be examined by placing the apparatus in front of the slit of a spectroscope4 ,the variations to which a spectrum is liable may easily be observed by increasing the distance between the wires1 altering the direction of the current or strength of the solution1 c4 ,spectrum analysis has not only endowed science with a knowledge of the composition of distant heavenly bodies 7of the sun1 stars1 nebul1 comets1 c471 but has also given a new method for studying the matter of the earth's surface4 ,with its help ,bunsen discovered two new elements belonging to the group of the alkali metals1 and thallium1 indium1 and gallium were afterwards discovered by the same means4 ,the spectroscope is employed in the study of rare metals 7which in solution often give distinct absorption spectra71 of dyes1 and of many organic substances1 c4#cg ,with respect to the metals which are analogous to sodium1 they all give similar very volatile salts and such very characteristic spectra that the least traces of them#ch are discovered with great ease by means of the spectroscope4 ,for instance1 lithium gives a very brilliant red coloration to a flame and a very bright red spectral line 7wave-length1 #fgj millionths mm471 which indicates the presence of this metal in admixture with compounds of other alkali metals4 #cg ,the importance of the spectroscope for the purpose of chemical research was already shown by ,gladstone in #ahef1 but it did not become an accessory to the laboratory until after the discoveries of ,kirchhoff and ,bunsen4 ,it may be hoped that in time spectroscopic researches will meet certain wants of the theoretical 7philosophical7 side of chemistry1 but as yet all that has been done in this respect can only be regarded as attempts which have not yet led to any trustworthy conclusions4 ,thus many investigators1 by collating the wave-lengths of all the light vibrations excited by a given element1 endeavour to find the law governing their mutual relations2 others 7especially ,hartley and ,ciamician71 by comparing the spectra of analogous elements 7for instance1 chlorine1 bromine1 and iodine71 have succeeded in noticing definite features of resemblance in them1 whilst others 7,grnwald7 search for relations between the spectra of compounds and their component elements1 c42 but--owing to the multiplicity of the spectral lines proper to many elements1 and 7especially in the ultra-red and ultra-violet ends of the spectrum7 the existence of lines which are undistinguishable owing to their faintness1 and also owing to the comparative novelty of spectroscopic research--this subject cannot be considered as in any way perfected4 ,nevertheless1 in certain instances there is evidently some relationship between the wave-lengths of all the spectral lines formed by a given element4 ,thus1 in the hydrogen spectrum the wave-length #cfdedb m#b_/7m#b-d71 if m varies as a series of whole numbers from #c to #ae 7,walmer1 ,hagebach1 and others74 ,for example1 when m #c1 the wave-length of one of the brightest lines of the hydrogen spectrum is obtained 7#fefb71 when m #g1 one of the visible violet lines 7#cifh71 and when m is greater than #i1 the ultra-violet lines of the hydrogen spectrum4 #ch ,in order to show the degree of sensitiveness of spectroscopic reactions the following observation of ,dr4 ,bence ,jones may be cited3 ,if a solution of #c grains of a lithium salt be injected under the skin of a guinea-pig1 after the lapse of four minutes1 lithium can be discovered in the bile and liquids of the eye1 and1 after ten minutes1 in all parts of the animal4 ,illustration3 ,,fig4 #gg4--,preparation of lithium by the action of a galvanic current on fused lithium chloride4 ,lithium1 ,li1 is1 like potassium and sodium1 somewhat widely spread in siliceous rocks1 but only occurs in small quantities and as mere traces in considerable masses of potassium and sodium salts4 ,only a very few rather rare minerals contain more than traces of it1#ci for example1 spodumene and lithia mica4 ,many compounds of lithium are in all respects closely analogous to the corresponding compounds of sodium and potassium2 but the carbonate is sparingly soluble in cold water1 which fact is taken advantage of for separating lithium from potassium and sodium4 ,this salt1 ,li#b;,,co#c1 is easily converted into the other compounds of lithium4 ,thus1 for instance1 the lithium hydroxide1 ,li,h,o1 is obtained in exactly the same way as caustic soda1 by the action of lime on the carbonate1 and it is soluble in water and crystallises 7from its solution in alcohol7 as ,li,h,o1,h#b,o4 ,metallic lithium is obtained by the action of a galvanic current on fused lithium chloride2 for this purpose a cast-iron crucible1 furnished with a stout cover1 is filled with lithium chloride1 heated until the latter fuses1 and a strong galvanic current is then passed through the molten mass4 ,the positive pole 7fig4 #gg7 consists of a dense carbon rod ,c 7surrounded by a porcelain tube ,p fixed in an iron tube ,,bb71 and the negative pole of an iron wire1 on which the metal is deposited after the current has passed through the molten mass for a certain length of time4 ,chlorine is evolved at the positive pole4 ,when a somewhat considerable quantity of the metal has accumulated on the wire it is withdrawn1 the metal is collected from it1 and the experiment is then carried on as before4#ci bis ,lithium is the lightest of all metals1 its specific gravity is #jei1 owing to which it floats even on naphtha2 it melts at #ahj1 but does not volatilise at a red heat4 ,its appearance recalls that of sodium1 and1 like it1 it has a yellow tint4 ,at #bjj it burns in air with a very bright flame1 forming lithium oxide4 ,in decomposing water it does not ignite the hydrogen4 ,the characteristic test for lithium compounds is the red coloration which they impart to a colourless flame4#dj #ci ,thus spodumene contains up to #f p4c4 of lithium oxide1 and petolite1 and lepidolite or lithia mica1 about #c p4c4 of lithium oxide4 ,this mica is met with in certain granites in a somewhat considerable quantity1 and is therefore most frequently employed for the preparation of lithium compounds4 ,the treatment of lepidolite is carried on on a large scale1 because certain salts of lithium are employed in medicine as a remedy for certain diseases 7stone1 gouty affections71 as they have the power of dissolving the insoluble uric acid which is then deposited4 ,lepidolite1 which is unacted on by acids in its natural state1 decomposes under the action of strong hydrochloric acid after it has been fused4 ,after being subjected to the action of the hydrochloric acid for several hours all the silica is obtained in an insoluble form1 whilst the metallic oxides pass into solution as chlorides4 ,this solution is mixed with nitric acid to convert the ferrous salts into ferric1 and sodium carbonate is then added until the liquid becomes neutral1 by which means a precipitate is formed of the oxides of iron1 alumina1 magnesia1 c41 as insoluble oxides and carbonates4 ,the solution 7with an excess of water7 then contains the chlorides of the alkaline metals ,k,cl1 ,na,cl1 ,li,cl1 which do not give a precipitate with sodium carbonate in a dilute solution4 ,it is then evaporated1 and a strong solution of sodium carbonate added4 ,this precipitates lithium carbonate1 which1 although soluble in water1 is much less so than sodium carbonate1 and therefore the latter precipitates lithium from strong solutions as carbonate1 #b,li,cl ,na#b;,,co#c #b,na,cl ,li#b;,,co#c4 ,lithium carbonate1 which resembles sodium carbonate in many respects1 is a substance which is very slightly soluble in cold water and is only moderately soluble in boiling water4 ,in this respect lithium forms a transition between the metals of the alkalis and other metals1 especially those of the alkaline earths 7magnesium1 barium71 whose carbonates are only sparingly soluble4 ,oxide of lithium1 ,li#b,o1 may be obtained by heating lithium carbonate with charcoal4 ,lithium oxide in dissolving gives 7per gram-molecule7 #bf1jjj heat units2 but the combination of ,li#b with ,o evolves #adj1jjj calories--that is1 more than ,na#b,o 7#ajj1jjj calories7 and ,k#b,o 7#ig1jjj calories71 as shown by ,beketoff 7#ahhg74 ,oeuvrard 7#ahib7 heated lithium to redness in nitrogen1 and observed the absorption of ,n and formation of ,li#c,n1 like ,na#c,n 7see ,chapter ,,xii4 ,note #ej74 ,li,cl1 ,li,br1 and ,li,i form crystallo-hydrates with ,h#b,o1 #b;,h#b,o1 and #c;,h#b,o4 ,as a rule1 ,li,br1#b;,h#b,o crystallises out1 but ,bogorodsky 7#ahid7 showed that a solution containing ,li,br #cg;,h#b,o1 cooled to -#fb1 separates out crystals ,li,br1#c;,h#b,o1 which decompose at #d with the separation of ,h#b,o4 ,li,f is but slightly soluble 7in #hjj parts7 in water 7and still less so in a solution of ,,nh#d;,f74 #ci bis ,guntz 7#ahic7 recommends adding ,k,cl to the ,li,cl in preparing ,li by this method1 and to act with a current of #aj ampres at #bj volts1 and not to heat above #dej1 so as to avoid the formation of ,li#b;,cl4 #dj ,in determining the presence of lithium in a given compound1 it is best to treat the material under investigation with acid 7in the case of mineral silicon compounds hydrofluoric acid must be taken71 and to treat the residue with sulphuric acid1 evaporate to dryness1 and extract with alcohol1 which dissolves a certain amount of the lithium sulphate4 ,it is easy to discover lithium in such an alcoholic solution by means of the coloration imparted to the flame on burning it1 and in case of doubt by investigating its light in a spectroscope1 because lithium gives a red line1 which is very characteristic and is found as a dark line in the solar spectrum4 ,lithium was first discovered in #ahag in petolite by ,arfvedson4 ,bunsen in #ahfj tried to determine by means of the spectroscope whether any other as yet unknown metals might not occur in different natural products together with lithium1 potassium1 and sodium1 and he soon discovered two new alkali metals showing independent spectra4 ,they are named after the characteristic coloration which they impart to the flame4 ,one which gives a red and violet band is named rubidium1 from rubidius 7dark red71 and the other is called csium1 because it colours a pale flame sky blue1 which depends on its containing bright blue rays1 which appear in the spectrum of csium as two blue bands 7table on p4 #efe74 ,both metals accompany sodium1 potassium1 and lithium1 but in small quantities2 rubidium occurs in larger quantity than csium4 ,the amount of the oxides of csium and rubidium in lepidolite does not generally exceed one-half per cent4 ,rubidium has also been found in the ashes of many plants1 while the ,stassfurt carnallite 7the mother-liquor obtained after having been treated for ,k,cl7 forms an abundant source for rubidium and also partly for csium4 ,rubidium also occurs1 although in very small quantities1 in the majority of mineral waters4 ,in a very few cases csium is not accompanied by rubidium2 thus1 in a certain granite on the ,isle of ,elba1 csium has been discovered1 but not rubidium4 ,this granite contains a very rare mineral called pollux1 which contains as much as #cd per cent4 of csium oxide4 ,guided by the spectroscope1 and aided by the fact that the double salts of platinic chloride and rubidium and csium chlorides are still less soluble in water than the corresponding potassium salt1 ,k#b,pt,cl#f1da ,bunsen succeeded in separating both metals from each other and from potassium1 and demonstrated the great resemblance they bear to each other4 ,the isolated metals1#db rubidium and csium1 have respectively the specific gravities #aeb and #bcff1 and melting points #ci and #bg as ,n4 ,n4 ,beketoff showed 7#ahid71 he having obtained csium by heating ,cs,al,o#b with ,mg7#db bis74 #da ,the salts of the majority of metals are precipitated as carbonates on the addition of ammonium carbonate--for instance1 the salts of calcium1 iron1 c4 ,the alkalis whose carbonates are soluble are not1 however1 precipitated in this case4 ,on evaporating the resultant solution and igniting the residue 7to remove the ammonium salts71 we obtain salts of the alkali metals4 ,they may he separated by adding hydrochloric acid together with a solution of platinic chloride4 ,the chlorides of lithium and sodium give easily soluble double salts with platinic chloride1 whilst the chlorides of potassium1 rubidium1 and csium form double salts which are sparingly soluble4 ,a hundred parts of water at #j dissolve #jgd part of the potassium platinochloride2 the corresponding rubidium platinochloride is only dissolved to the amount of #jacd part1 and the csium salt1 #jjbd part2 at #ajj #eac parts of potassium platinochloride1 ,k#b,pt,cl#f1 are dissolved1 #jfcd part of rubidium platinochloride1 and #jagg part of csium platinochloride4 ,from this it is clear how the salts of rubidium and csium may be isolated4 ,the separation of csium from rubidium by this method is very tedious4 ,it can be better effected by taking advantage of the difference of the solubility of their carbonates in alcohol2 csium carbonate1 ,cs#b;,,co#c1 is soluble in alcohol1 whilst the corresponding salts of rubidium and potassium are almost insoluble4 ,setterberg separated these metals as alums1 but the best method1 that given by ,scharples1 is founded on the fact that from a mixture of the chlorides of potassium1 sodium1 csium1 and rubidium in the presence of hydrochloric acid1 stannic chloride precipitates a double salt of csium1 which is very slightly soluble4 ,the salts of ,rb and ,cs are closely analogous to those of potassium4 #db ,bunsen obtained rubidium by distilling a mixture of the tartrate with soot1 and ,beketoff 7#ahhh7 by heating the hydroxide with aluminium1 #b,rb,h,o ,al ,rb,al,o#b ,h#b ,rb4 ,by the action of #he grams of rubidium on water1 #id1jjj heat units are evolved4 ,setterberg obtained csium 7#ahhb7 by the electrolysis of a fused mixture of cyanide of csium and of barium4 ,winkler 7#ahij7 showed that metallic magnesium reduces the hydrates and carbonates of ,rb and ,cs like the other alkaline metals4 ,n4 ,n4 ,beketoff obtained them with aluminium 7see following note74 #db bis ,beketoff 7#ahhh7 showed that metallic aluminium reduces the hydrates of the alkaline metals at a red heat 7they should be perfectly dry7 with the formation of aluminates 7,chapter ,,xvii471 ,r,al,o#b--;for example1 #b,,kho ,al ,k,al,o#b ,k ,h#b4 ,it is evident that in this case only half of the alkaline metal is obtained free4 ,on the other hand1 ,k4 ,winkler 7#ahhi7 showed that magnesium powder is also able to reduce the alkaline metals from their hydrates and carbonates4 ,n4 ,n4 ,beketoff and ,tscherbacheff 7#ahid7 prepared csium upon this principle by heating its aluminate ,cs,al,o#b with magnesium powder4 ,in this case aluminate of magnesium is formed1 and the whole of the csium is obtained as metal3 #b;,cs,al,o#b ,mg ,mg,o,al#b,o#e #b;,cs4 ,a certain excess of alumina was taken 7in order to obtain a less hygroscopic mass of aluminate71 and magnesium powder 7in order to decompose the last traces of water72 the ,cs,al,o#b was prepared by the precipitation of csium alums by caustic baryta1 and evaporating the resultant solution4 ,we may add that ,n4 ,n4 ,beketoff 7#ahhg7 prepared oxide of potassium1 ,k#b,o1 by heating the peroxide1 ,,ko1 in the vapour of potassium 7disengaged from its alloy with silver71 and showed that in dissolving in an excess of water it evolves 7for the above-given molecular weight7 #fg1djj calories 7while #b,,kho in dissolving in water evolves #bd1ibj cal42 so that ,k#b,o ,h#b,o gives #db1dhj cal471 whence 7knowing that ,k#b ,o,h#b,o in an excess of water evolves #afd1ejj7 it follows that ,k#b ,o evolves #ig1ajj cal4 ,this quantity is somewhat less than that 7#ajj1bfj cal47 which corresponds to sodium1 and the energy of the action of potassium upon water is explained by the fact that ,k#b,o evolves more heat than ,na#b,o in combining with water 7see ,chapter ,,ii4 ,note #i74 ,just as hydrogen displaces half the ,na from ,na#b,o forming ,na,h,o1 so also ,n4 ,n4 ,beketoff found from experiment and thermochemical reasonings that hydrogen displaces half the potassium from ,k#b,o forming ,,kho and evolving #g1aij calories4 ,oxide of lithium1 ,li#b,o1 which is easily formed by igniting ,li#b;,,co#c with carbon 7when ,li#b,o #b;,,co is formed71 disengages #bf1jjj cals4 with an excess of water1 while the reaction ,li#b ,o gives #aad1jjj cals4 and the reaction ,li#b ,h#b,o gives only #ac1jjj cals41 and metallic lithium cannot be liberated from oxide of lithium with hydrogen 7nor with carbon74 ,thus in the series ,li1 ,na1 ,k1 the formation of ,r#b,o gives most heat with ,li and least with ,k1 while the formation of ,r,cl evolves most heat with ,k 7#aje1jjj cals47 and least of all with ,li 7#ic1ejj cals474 ,rubidium1 in forming ,rb#b,o1 gives #id1jjj cals4 7,beketoff74 ,csium1 in acting upon an excess of water1 evolves #ea1ejj cals41 and the reaction ,cs#b ,o evolves about #ajj1jjj cals4--i4e4 more than ,k and ,rb1 and almost as much as ,na--and oxide of csium reacts with hydrogen 7according to the equation ,cs#b,o ,h ,cs,h,o ,cs7 more easily than any of the oxides of the alkali metals1 and this reaction takes place at the ordinary temperature 7the hydrogen is absorbed71 as ,beketoff showed 7#ahic74 ,he also obtained a mixed oxide1 ,ag,cs,o1 which was easily formed in the presence of silver1 and absorbed hydrogen with the formation of ,cs,h,o4 ,judging by the properties of the free metals1 and of their corresponding and even very complex compounds1 lithium1 sodium1 potassium1 rubidium1 and csium present an indubitable chemical resemblance4 ,the fact that the metals easily decompose water1 and that their hydroxides ,,rho and carbonates ,r#b;,,co#c are soluble in water1 whilst the hydroxides and carbonates of nearly all other metals are insoluble1 shows that these metals form a natural group of alkali metals4 ,the halogens and the alkali metals form1 by their character1 the two extremes of the elements4 ,many of the other elements are metals approaching the alkali metals1 both in their capacity of forming salts and in not forming acid compounds1 but are not so energetic as the alkali metals1 that is1 they form less energetic bases4 ,such are the common metals1 silver1 iron1 copper1 c4 ,some other elements1 in the character of their compounds1 approach the halogens1 and1 like them1 combine with hydrogen1 but these compounds do not show the energetic property of the halogen acids2 in a free state they easily combine with metals1 but they do not then form such saline compounds as the halogens do--in a word1 the halogen properties are less sharply defined in them than in the halogens themselves4 ,sulphur1 phosphorus1 arsenic1 c4 belong to this order of elements4 ,the clearest distinction of the properties of the halogens and alkali metals is expressed in the fact that the former give acids and do not form bases1 whilst the latter1 on the contrary1 only give bases4 ,the first are true acid elements1 the latter clearly-defined basic or metallic elements4 ,on combining together1 the halogens form1 in a chemical sense1 unstable compounds1 and the alkali metals alloys in which the character of the metals remains unaltered1 just as in the compound ,i,cl the character of the halogens remains undisguised2 thus both classes of elements on combining with members of their own class form non-characteristic compounds1 which have the properties of their components4 ,on the other hand1 the halogens on combining with the alkali metals form compounds which are1 in all respects1 stable1 and in which the original characters of the halogens and alkali metals have entirely disappeared4 ,the formation of such compounds is accompanied by evolution of a large amount of heat1 and by an entire change of both the physical and chemical properties of the substances originally taken4 ,the alloy of sodium and potassium1 although liquid at the ordinary temperature1 is perfectly metallic1 like both its components4 ,the compound of sodium and chlorine has neither the appearance nor the properties of the original elements2 sodium chloride melts at a higher temperature1 and is more difficultly volatile1 than either sodium or chlorine4 ,with all these qualitative differences there is1 however1 an important quantitative resemblance between the halogens and the alkali metals4 ,this resemblance is clearly expressed by stating that both orders of elements belong to those which are univalent with respect to hydrogen4 ,it is thus correct to say that both the above-named orders of elements replace hydrogen atom for atom4 ,chlorine is able to take the place of hydrogen by metalepsis1 and the alkali metals take the place of hydrogen in water and acids4 ,as it is possible to consecutively replace every equivalent of hydrogen in a hydrocarbon by chlorine1 so it is possible in an acid containing several equivalents of hydrogen to replace the hydrogen consecutively equivalent after equivalent by an alkali metal2 hence an atom of these elements is analogous to an atom of hydrogen1 which is taken1 in all cases1 as the unit for the comparison of the other elements4 ,in ammonia1 and in water1 chlorine and sodium are able to bring about a direct replacement4 ,according to the law of substitution1 the formation of sodium chloride1 ,na,cl1 at once shows the equivalence of the atoms of the alkali metals and the halogens4 ,the halogens and hydrogen and the alkali metals combine with such elements as oxygen1 and it is easily proved that in such compounds one atom of oxygen is able to retain two atoms of the halogens1 of hydrogen1 and of the alkali metals4 ,for this purpose it is enough to compare the compounds ,,kho1 ,k#b,o1 ,h,cl,o1 and ,cl#b,o1 with water4 ,it must not be forgotten1 however1 that the halogens give1 with oxygen1 besides compounds of the type ,r#b,o1 higher acid grades of oxidation1 which the alkali metals and hydrogen are not capable of forming4 ,we shall soon see that these relations are also subject to a special law1 showing a gradual transition of the properties of the elements from the alkali metals to the halogens4#dc #dc ,we may here observe that the halogens1 and especially iodine1 may play the part of metals 7hence iodine is more easily replaced by metals than the other halogens1 and it approaches nearer to the metals in its physical properties than the other halogens74 ,schtzenberger obtained a compound ,c#b;,h#c,o7,o,cl71 which he called chlorine acetate1 by acting on acetic anhydride1 7,c#b;,h#c,o7#b,o1 with chlorine monoxide1 ,cl#b,o4 ,with iodine this compound gives off chlorine and forms iodine acetate1 ,c#b;,h#c,o7,,oi71 which also is formed by the action of iodine chloride on sodium acetate1 ,c#b;,h#c,o7,o,na74 ,these compounds are evidently nothing else than mixed anhydrides of hypochlorous and hypoiodous acids1 or the products of the substitution of hydrogen in ,,rho by a halogen 7see ,chapter ,,xi41 ,notes #bi and #gh bis74 ,such compounds are very unstable1 decompose with an explosion when heated1 and are changed by the action of water and of many other reagents1 which is in accordance with the fact that they contain very closely allied elements1 as does ,cl#b,o itself1 or ,i,cl or ,k,na4 ,by the action of chlorine monoxide on a mixture of iodine and acetic anhydride1 ,schtzenberger also obtained the compound ,i7,c#b;,h#c,o#b7c1 which is analogous to ,i,cl#c1 because the group ,c#b;,h#c,o#b is1 like ,cl1 a halogen1 forming salts with the metals4 ,similar properties are found in iodosobenzene 7,chapter ,,xi41 ,note #gi74 ,the atomic weights of the alkali metals1 lithium #g1 sodium #bc1 potassium #ci1 rubidium #he1 and csium #acc1 show that here1 as in the class of halogens1 the elements may be arranged according to their atomic weights in order to compare the properties of the analogous compounds of the members of this group4 ,thus1 for example1 the platinochlorides of lithium and sodium are soluble in water2 those of potassium1 rubidium1 and csium sparingly soluble1 and the greater the atomic weight of the metal the less soluble is the salt4 ,in other cases the reverse is observed--the greater the atomic weight the more soluble are the corresponding salts4 ,the variation of properties with the variation in atomic weights even shows itself in the metals themselves2 thus lithium volatilises with difficulty1 whilst sodium is obtained by distillation1 potassium volatilises more easily than sodium1 and rubidium and csium as we have seen1 are still more volatile4 ,,chapter ,,xiv ,,the ,,valency ,,and ,,specific ,,heat ,,of ,,the ,,metals4 ,,magnesium4 ,,calcium1 ,,strontium1 ,,barium1 ,,and ,,beryllium ,it is easy by investigating the composition of corresponding compounds1 to establish the equivalent weights of the metals compared with hydrogen--that is1 the quantity which replaces one part by weight of hydrogen4 ,if a metal decomposes acids directly1 with the evolution of hydrogen1 the equivalent weight of the metal may be determined by taking a definite weight of it and measuring the volume of hydrogen evolved by its action on an excess of acid2 it is then easy to calculate the weight of the hydrogen from its volume4#a ,the same result may be arrived at by determining the composition of the normal salts of the metal2 for instance1 by finding the weight of metal which combines with #cee parts of chlorine or #hj parts of bromine4#b ,the equivalent of a metal may be also ascertained by simultaneously 7i4e4 in one circuit7 decomposing an acid and a fused salt of a given metal by an electric current and determining the relation between the amounts of hydrogen and metal separated1 because1 according to ,faraday's law1 electrolytes 7conductors of the second order7 are always decomposed in equivalent quantities4#b bis ,the equivalent of a metal may even be found by simply determining the relation between its weight and that of its salt-giving oxide1 as by this we know the quantity of the metal which combines with #h parts by weight of oxygen1 and this will be the equivalent1 because #h parts of oxygen combine with #a part by weight of hydrogen4 ,one method is verified by another1 and all the processes for the accurate determination of equivalents require the greatest care to avoid the absorption of moisture1 further oxidation1 volatility1 and other accidental influences which affect exact weighings4 ,the description of the methods necessary for the attainment of exact results belongs to the province of analytical chemistry4 #a ,under favourable circumstances 7by taking all the requisite precautions71 the weight of the equivalent may be accurately determined by this method4 ,thus ,reynolds and ,ramsay 7#ahhg7 determined the equivalent of zinc to be #cbg by this method 7from the average of #bi experiments71 whilst by other methods it has been fixed 7by different observers7 between #cbee and #ccie4 ,the differences in their equivalents may be demonstrated by taking equal weights of different metals1 and collecting the hydrogen evolved by them 7under the action of an acid or alkali74 #b ,the most accurate determinations of this kind were carried on by ,stas1 and will be described in ,chapter ,,xxiv4 #b bis ,the amount of electricity in one coulomb according to the present nomenclature of electrical units 7see ,works on ,physics and ,electro-technology7 disengages #jjjjjajcf gram of hydrogen1 #jjjaab gram of silver1 #jjjjcbfc gram of copper from the salts of the oxide1 and #jjjjfebf gram from the salts of the suboxide1 c4 ,these amounts stand in the same ratio as the equivalents1 i4e4 as the quantities replaced by one part by weight of hydrogen4 ,the intimate bond which is becoming more and more marked existing between the electrolytic and purely chemical relations of substances 7especially in solutions7 and the application of electrolysis to the preparation of numerous substances on a large scale1 together with the employment of electricity for obtaining high temperatures1 c41 makes me regret that the plan and dimensions of this book1 and the impossibility of giving a concise and objective exposition of the necessary electrical facts1 prevent my entering upon this province of knowledge1 although ,i consider it my duty to recommend its study to all those who desire to take part in the further development of our science4 ,there is only one side of the subject respecting the direct correlation between thermochemical data and electro-motive force1 which ,i think right to mention here1 as it justifies the general conception1 enunciated by ,faraday1 that the galvanic current is an aspect of the transference of chemical motion or reaction along the conductors4 ,from experiments conducted by ,favre1 ,thomsen1 ,garni1 ,berthelot1 ,cheltzoff1 and others1 upon the amount of heat evolved in a closed circuit1 it follows that the electro-motive force of the current or its capacity to do a certain work1 ,e1 is proportional to the whole amount of heat1 ,q1 disengaged by the reaction forming the source of the current4 ,if ,e be expressed in volts1 and ,q in thousands of units of heat referred to equivalent weights1 then ,e #jjdcf,q4 ,for example in a ,daniells battery ,e #aji both by experiment and theory1 because in it there takes place the decomposition of ,cu,s,o#d into ,cu ,o together with the formation of ,zn ,o and ,zn,o ,,so#c;,aq1 and these reactions correspond to ,q #bejf thousand units of heat4 ,so also in all other primary batteries 7e4g4 ,bunsen's1 ,poggendorff's1 c47 and secondary ones 7for instance1 those acting according to the reaction ,pb ,h#b,,so#d ,pb,o#b1 as ,cheltzoff showed7 ,e #jjdcf,q4 ,for univalent metals1 like those of the alkalis1 the weight of the equivalent is equal to the weight of the atom4 ,for bivalent metals the atomic weight is equal to the weight of two equivalents1 for n-valent metals it is equal to the weight of n equivalents4 ,thus aluminium1 ,al #bg1 is trivalent1 that is1 its equivalent #i2 magnesium1 ,mg #bd1 is bivalent1 and its equivalent #ab4 ,therefore1 if potassium or sodium1 or in general a univalent metal1 ,m1 give compounds ,m#b,o1 ,,mho1 ,m,cl1 ,,mno#c1 ,m#b,,so#d1 c41 and in general ,,mx1 then for bivalent metals like magnesium or calcium the corresponding compounds will be ,mg,o1 ,mg7,,ho7#b1 ,mg,cl#b1 ,mg7,,no#c7b1 ,mg,s,o#d1 c41 or in general ,,mx#b4 ,by what are we to be guided in ascribing to some metals univalency and to others bi-1 ter-1 quadri-1 444 n-valency8 ,what obliges us to make this difference8 ,why are not all metals given the same valency--for instance1 why is not magnesium considered as univalent8 ,if this be done1 taking ,mg #ab 7and not #bd as now71 not only is a simplicity of expression of the composition of all the compounds of magnesium attained1 but we also gain the advantage that their composition will be the same as those of the corresponding compounds of sodium and potassium4 ,these combinations were so expressed formerly--why has this since been changed8 ,these questions could only be answered after the establishment of the idea of multiples of the atomic weights as the minimum quantities of certain elements combining with others to form compounds--in a word1 since the time of the establishment of ,avogadro-,gerhardt's law 7,chapter ,,vii474 ,by taking such an element as arsenic1 which has many volatile compounds1 it is easy to determine the density of these compounds1 and therefore to establish their molecular weights1 and hence to find the indubitable atomic weight1 exactly as for oxygen1 nitrogen1 chlorine1 carbon1 c4 ,it appears that ,as #ge1 and its compounds correspond1 like the compounds of nitrogen1 with the forms ,as,x#c1 and ,as,x#e2 for example1 ,as,h#c1 ,as,cl#c1 ,as,f#e1 ,as#b,o#e1 c4 ,it is evident that we are here dealing with a metal 7or rather element7 of two valencies1 which moreover is never univalent1 but tri- or quinqui-valent4 ,this example alone is sufficient for the recognition of the existence of polyvalent atoms among the metals4 ,and as antimony and bismuth are closely analogous to arsenic in all their compounds1 7just as potassium is analogous to rubidium and csium72 so1 although very few volatile compounds of bismuth are known1 it was necessary to ascribe to them formul corresponding with those ascribed to arsenic4 ,as we shall see in describing them1 there are also many analogous metals among the bivalent elements1 some of which also give volatile compounds4 ,for example1 zinc1 which is itself volatile1 gives several volatile compounds 7for instance1 zinc ethyl1 ,zn,c#d;,h#aj1 which boils at #aah1 vapour density #fac71 and in the molecules of all these compounds there is never less than #fe parts of zinc1 which is equivalent to ,h#b1 because #fe parts of zinc displace #b parts by weight of hydrogen2 so that zinc is just such an example of the bivalent metals as oxygen1 whose equivalent #h 7because ,h#b is replaced by ,o #af71 is a representative of the bivalent elements1 or as arsenic is of the tri- and quinqui-valent elements4 ,and1 as we shall afterwards see1 magnesium is in many respects closely analogous to zinc1 which fact obliges us to regard magnesium as a bivalent metal4 ,such metals as mercury and copper1 which are able to give not one but two bases1 are of particular importance for distinguishing univalent and bivalent metals4 ,thus copper gives the suboxide ,cu#b,o and the oxide ,cu,o--that is1 the compounds ,cu,x corresponding with the suboxide are analogous 7in the quantitative relations1 by their composition7 to ,na,x or ,ag,x1 and the compounds of the oxide ,cu,x#b1 to ,mg,x#b1 ,zn,x#b1 and in general to the bivalent metals4 ,it is clear that in such examples we must make a distinction between atomic weights and equivalents4 ,in this manner the valency1 that is1 the number of equivalents entering into the atom of the metals may in many cases be established by means of comparatively few volatile metallic compounds1 with the aid of a search into their analogies 7concerning which see ,chapter ,,xv474 ,the law of specific heats discovered by ,dulong and ,petit has frequently been applied to the same purpose#c in the history of chemistry1 especially since the development given to this law by the researches of ,regnault1 and since ,cannizzaro 7#ahfj7 showed the agreement between the deductions of this law and the consequences arising from ,avogadro-,gerhardt's law4 #c ,the chief means by which we determine the valency of the elements1 or what multiple of the equivalent should be ascribed to the atom1 are3 7#a7 ,the law of ,avogadro-,gerhardt4 ,this method is the most general and trustworthy1 and has already been applied to a great number of elements4 7#b7 ,the different grades of oxidation and their isomorphism or analogy in general2 for example1 ,fe #ef because the suboxide 7ferrous oxide7 is isomorphous with magnesium oxide1 c41 and the oxide 7ferric oxide7 contains half as much oxygen again as the suboxide4 ,berzelius1 ,marignac1 and others took advantage of this method for determining the composition of the compounds of many elements4 7#c7 ,the specific heat1 according to ,dulong and ,petit's law4 ,regnault1 and more especially ,cannizzaro1 used this method to distinguish univalent from bivalent metals4 7#d7 ,the periodic law 7see ,chapter ,,xv47 has served as a means for the determination of the atomic weights of cerium1 uranium1 yttrium1 c41 and more especially of gallium1 scandium1 and germanium4 ,the correction of the results of one method by those of others is generally had recourse to1 and is quite necessary1 because1 phenomena of dissociation1 polymerisation1 c41 may complicate the individual determinations by each method4 ,it will be well to observe that a number of other methods1 especially from the province of those physical properties which are clearly dependent on the magnitude of the atom 7or equivalent7 or of the molecule1 may lead to the same result4 ,i may point out1 for instance1 that even the specific gravity of solutions of the metallic chlorides may serve for this purpose4 ,thus1 if beryllium he taken as trivalent--that is1 if the composition of its chloride be taken as ,be,cl#c 7or a polymeride of it71 then the specific gravity of solutions of beryllium chloride will not fit into the series of the other metallic chlorides4 ,but by ascribing to it an atomic weight ,be #g1 or taking ,be as bivalent1 and the composition of its chloride as ,be,cl#b1 we arrive at the general rule given in ,chapter ,,vii41 ,note #bh4 ,thus ,w4 ,g4 ,burdakoff determined in my laboratory that the specific gravity at #ae_/d of the solution ,be,cl#b #bjj;,h#b,o #ajach--that is1 greater than the corresponding solution ,k,cl #bjj;,h#b,o 7 #ajaba71 and less than the solution ,mg,cl#b #bjj;,h#b,o 7 #ajbjc71 as would follow from the magnitude of the molecular weight ,be,cl#b #hj1 since ,k,cl #gde and ,mg,cl#b #ie4 ,dulong and ,petit1 having determined the specific heat of a number of solid elementary substances1 observed that as the atomic weights of the elements increase1 their specific heats decrease1 and that the product of the specific heat ,q into the atomic weight ,a is an almost constant quantity4 ,this means that to bring different elements into a known thermal state an equal amount of work is required if atomic quantities of the elements are taken2 that is1 the amounts of heat expended in heating equal quantities by weight of the elements are far from equal1 but are in inverse proportion to the atomic weights4 ,for thermal changes the atom is a unit2 all atoms1 notwithstanding the difference of weight and nature1 are equal4 ,this is the simplest expression of the fact discovered by ,dulong and ,petit4 ,the specific heat measures that quantity of heat which is required to raise the temperature of one unit of weight of a substance by one degree4 ,if the magnitude of the specific heat of elements be multiplied by the atomic weight1 then we obtain the atomic heat--that is1 the amount of heat required to raise the temperature of the atomic weight of an element by one degree4 ,it is these products which for the majority of the elements prove to be approximately1 if not quite1 identical4 ,a complete identity cannot be expected1 because the specific heat of one and the same substance varies with the temperature1 with its passage from one state into another1 and frequently with even a simple mechanical change of density 7for instance by hammering71 not to speak of allotropic changes1 c4 ,we will cite several figures#d proving the truth of the conclusions arrived at by ,dulong and ,petit with respect to solid elementary bodies4 ,li ,na ,mg ,p ,a #g #bc #bd #ca ,q #jidjh #jbicd #jbde #jbjb ,,aq #fei #fge #ehh #fbf ,fe ,cu ,zn ,br ,a #ef #fc #fe #hj ,q #jaab #jjic #jjic #jjhdc ,,aq #fbg #ehf #fjd #fgd ,pd ,ag ,sn ,i ,a #ajf #ajh #aah #abg ,q #jjeib #jjef #jjee #jeda ,,aq #fbh #fje #fdi #fhg ,pt ,au ,hg ,pb ,a #aif #aih #bjj #bjf ,q #jjcbe #jjcbd #jjccc #jjcae ,,aq #fcg #fda #fff #fdi #d ,the specific heats here given refer to different limits of temperature1 but in the majority of cases between #j and #ajj2 only in the case of bromine the specific heat is taken 7for the solid state7 at a temperature below -#g1 according to ,regnault's determination4 ,the variation of the specific heat with a change of temperature is a very complex phenomenon1 the consideration of which ,i think would here be out of place4 ,i will only cite a few figures as an example4 ,according to ,bystrom1 the specific heat of iron at #j #jaaaf1 at #ajj #jaaad1 at #bjj #jaahh1 at #cjj #jabfg1 and at #a1djj #jdjca4 ,between these last limits of temperature a change takes place in iron 7a spontaneous heating1 recalescence71 as we shall see in ,chapter ,,xxii4 ,for quartz ,si,o#b ,pionchon gives ,q #jagcg #cidt#aj-f-bgt#baj-i up to #djj1 for metallic aluminium 7,richards1 #ahib7 at #j #jbbb1 at #bj #jbbd1 at #ajj #jbcb2 consequently1 as a rule1 the specific heat varies slightly with the temperature4 ,still more remarkable are ,h4 ,e4 ,weber's observations on the great variation of the specific heat of charcoal1 the diamond and boron3 #j #ajj #bjj #fjj #ijj ,wood charcoal #jae #jbc #jbi #jdd #jdf ,diamond #jaj #jai #jbb #jdd #jde ,boron #jbb #jbi #jce -- -- ,these determinations1 which have been verified by ,dewar1 ,le ,chatelier 7,chapter ,,viii41 ,note #ac71 ,moissan1 and ,gauthier1 the latter finding for boron ,,aq #f at #djj1 are of especial importance as confirming the universality of ,dulong and ,petit's law1 because the elements mentioned above form exceptions to the general rule when the mean specific heat is taken for temperatures between #j and #ajj4 ,thus in the case of the diamond the product of ,a ,q at #j #ab1 and for boron #bd4 ,but if we take the specific heat towards which there is evidently a tendency with a rise of temperature1 we obtain a product approaching to #f as with other elements4 ,thus with the diamond and charcoal1 it is evident that the specific heat tends towards #jdg1 which multiplied by #ab gives #ef1 the same as for magnesium and aluminium4 ,i may here direct the reader's attention to the fact that for solid elements having a small atomic weight1 the specific heat varies considerably if we take the average figures for temperatures #j to #ajj3 ,li #g ,be #i ,b #aa ,c #ab ,q #jid #jdb #jbd #jbj ,,aq #ff #ch #bf #bd ,it is therefore clear that the specific heat of beryllium determined at a low temperature cannot serve for establishing its atomicity4 ,on the other hand1 the low atomic heat of charcoal1 graphite1 and the diamond1 boron1 c41 may perhaps depend on the complexity of the molecules of these elements4 ,the necessity for acknowledging a great complexity of the molecules of carbon was explained in ,chapter ,,viii4 ,in the case of sulphur the molecule contains at least ,s#f and its atomic heat #cb #jafc #ebb1 which is distinctly below the normal4 ,if a large number of atoms of carbon are contained in the molecule of charcoal1 this would to a certain extent account for its comparatively small atomic heat4 ,with respect to the specific heat of compounds1 it will not be out of place to mention here the conclusion arrived at by ,kopp1 that the molecular heat 7that is1 the product of ,,mq7 may be looked on as the sum of the atomic heats of its component elements2 but as this rule is not a general one1 and can only be applied to give an approximate estimate of the specific heats of substances1 ,i do not think it necessary to go into the details of the conclusions described in ,liebig's ',annalen ,supplement-,band1' #ahfd1 which includes a number of determinations made by ,kopp4 ,it is seen from this that the product of the specific heat of the element into the atomic weight is an almost constant quantity1 which is nearly #f4 ,hence it is possible to determine the valency by the specific heats of the metals4 ,thus1 for instance1 the specific heats of lithium1 sodium1 and potassium convince us of the fact that their atomic weights are indeed those which we chose1 because by multiplying the specific heats found by experiment by the corresponding atomic weights we obtain the following figures3 ,li1 #fei1 ,na1 #fge and ,k1 #fdg4 ,of the alkaline earth metals the specific heats have been determined3 of magnesium #jbde 7,regnault and ,kopp71 of calcium #jagj 7,bunsen71 and of barium #jje 7,mendeleff74 ,if the same composition be ascribed to the compounds of magnesium as to the corresponding compounds of potassium1 then the equivalent of magnesium will be equal to #ab4 ,on multiplying this atomic weight by the specific heat of magnesium1 we obtain a figure #bid1 which is half that which is given by the other solid elements and therefore the atomic weight of magnesium must be taken as equal to #bd and not to #ab4 ,then the atomic heat of magnesium #bd #jbde #ei2 for calcium1 giving its compounds a composition ,ca,x#b--;for example ,ca,cl#b1 ,ca,s,o#d1 ,ca,o 7,ca #dj7--we obtain an atomic heat #dj #jag #fh1 and for barium it is equal to #acg #jje #fh2 that is1 they must be counted as bivalent1 or that their atom replaces ,h#b1 ,na#b1 or ,k#b4 ,this conclusion may be confirmed by a method of analogy1 as we shall afterwards see4 ,the application of the principle of specific heats to the determination of the magnitudes of the atomic weights of those metals1 the magnitude of whose atomic weights could not be determined by ,avogadro-,gerhardt's law1 was made about #ahfj by the ,italian professor ,cannizzaro4 ,exactly the same conclusions respecting the bivalence of magnesium and its analogues are obtained by comparing the specific heats of their compounds1 especially of the halogen compounds as the most simple1 with the specific heats of the corresponding alkali compounds4 ,thus1 for instance1 the specific heats of magnesium and calcium chlorides1 ,mg,cl#b and ,ca,cl#b1 are #jaid and #jafd1 and of sodium and potassium chlorides1 ,na,cl and ,k,cl1 #jbad and #jagb1 and therefore their molecular heats 7or the products ,,qm1 where ,m is the weight of the molecule7 are #ahd and #ahb1 #abe and #abh1 and hence the atomic heats 7or the quotient of ,,qm by the number of atoms7 are all nearly #f1 as with the elements4 ,whilst if1 instead of the actual atomic weights ,mg #bd and ,ca #dj1 their equivalents #ab and #bj be taken1 then the atomic heats of the chlorides of magnesium and calcium would be about #df1 whilst those of potassium and sodium chlorides are about #fc4e ,we must remark1 however1 that as the specific heat or the amount of heat required to raise the temperature of a unit of weight one degree#f is a complex quantity--including not only the increase of the energy of a substance with its rise in temperature1 but also the external work of expansion#g and the internal work accomplished in the molecules causing them to decompose according to the rise of temperature#h--therefore it is impossible to expect in the magnitude of the specific heat the great simplicity of relation to composition which we see1 for instance1 in the density of gaseous substances4 ,hence1 although the specific heat is one of the important means for determining the atomicity of the elements1 still the mainstay for a true judgment of atomicity is only given by ,avogadro-,gerhardt's law1 i4e4 this other method can only be accessory or preliminary1 and when possible recourse should be had to the determination of the vapour density4 #e ,it must be remarked that in the case of oxygen 7and also hydrogen and carbon7 compounds the quotient of ,,mq_/n1 where n is the number of atoms in the molecule1 is always less than #f for solids2 for example1 for ,mg,o #ej1 ,cu,o #ea1 ,mn,o#b #df1 ice 7,q #jejd7 #c1 ,si,o#b #ce1 c4 ,at present it is impossible to say whether this depends on the smaller specific heat of the atom of oxygen in its solid compounds 7,kopp1 ,note #d7 or on some other cause2 but1 nevertheless1 taking into account this decrease depending on the presence of oxygen1 a reflection of the atomicity of the elements may to a certain extent be seen in the specific heat of the oxides4 ,thus for alumina1 ,al#b,o#c 7,q #jbag71 ,,mq #bbc1 and therefore the quotient ,,mq_/n #de1 which is nearly that given by magnesium oxide1 ,mg,o4 ,but if we ascribe the same composition to alumina1 as to magnesia--that is1 if aluminium were counted as divalent--we should obtain the figure #cg1 which is much less4 ,in general1 in compounds of identical atomic composition and of analogous chemical properties the molecular heats ,,mq are nearly equal1 as many investigators have long remarked4 ,for example1 ,zn,s #aag and ,hg,s #aah2 ,mg,s,o#d #bgj and ,zn,s,o#d #bhj1 c4 #f ,if ,w be the amount of heat contained in a mass m of a substance at a temperature t1 and d,w the amount expended in heating it from t to t dt1 then the specific heat ,q d,w7m dt74 ,the specific heat not only varies with the composition and complexity of the molecules of a substance1 but also with the temperature1 pressure1 and physical state of a substance4 ,even for gases the variation of ,q with t is to be observed4 ,thus it is seen from the experiments of ,regnault and ,wiedemann that the specific heat of carbonic anhydride at #j #jai1 at #ajj #jbb1 and at #bjj #jbd4 ,but the variation of the specific heat of permanent gases with the temperature is1 as far as we know1 very inconsiderable4 ,according to ,mallard and ,le ,chatelier it is #jjjjf_/,m per #a1 where ,m is the molecular weight 7for instance1 for ,o#b1 ,m #cb74 ,therefore the specific heat of those permanent gases which contain two atoms in the molecule 7,h#b1 ,o#b1 ,n#b1 ,,co1 and ,,no7 may be1 as is shown by experiment1 taken as not varying with the temperature4 ,the constancy of the specific heat of perfect gases forms one of the fundamental propositions of the whole theory of heat and on it depends the determination of temperatures by means of gas-thermometers containing hydrogen1 nitrogen1 or air4 ,le ,chatelier 7#ahhg71 on the basis of existing determinations1 concludes that the molecular heat--that is1 the product ,,mq--of all gases varies in proportion to the temperature1 and tends to become equal 7 #fh7 at the temperature of absolute zero 7that is1 at -#bgc72 and therefore ,,mq #fh a7#bgc t71 where a is a constant quantity which increases with the complexity of the gaseous molecule and ,q is the specific heat of the gas under a constant pressure4 ,for permanent gases a almost #j1 and therefore ,,mq #fh--that is1 the atomic heat 7if the molecule contains two atoms7 #cd1 as it is in fact 7,chapter ,,ix41 ,note #ag bis74 ,as regards liquids 7as well as the vapours formed by them71 the specific heat always rises with the temperature4 ,thus for benzene it equals #jch #jjjadt4 ,r4 ,schiff 7#ahhg7 showed that the variation of the specific heat of many organic liquids is proportional to the change of temperature 7as in the case of gases1 according to ,le ,chatelier71 and reduced these variations into dependence with their composition and absolute boiling point4 ,it is very probable that the theory of liquids will make use of these simple relations which recall the simplicity of the variation of the specific gravity 7,chapter ,,ii41 ,note #cd71 cohesion1 and other properties of liquids with the temperature4 ,they are all expressed by the linear function of the temperature1 a bt1 with the same degree of proximity as the property of gases is expressed by the equation pv ,rt4 ,as regards the relation between the specific heats of liquids 7or of solids7 and of their vapours1 the specific heat of the vapour 7and also of the solid7 is always less than that of the liquid4 ,for example1 benzene vapour #jbb1 liquid #jch2 chloroform vapour #jac1 liquid #jbc2 steam #jdge1 liquid water #aj4 ,but the complexity of the relations existing in specific heat is seen from the fact that the specific heat of ice #jejb is less than that of liquid water4 ,according to ,regnault1 in the case of bromine the specific heat of the vapour #jjee at 7#aej71 of the liquid #jajg 7at #cj71 and of solid bromine #jjhd 7at -#ae74 ,the specific heat of solid benzoic acid 7according to experiment and calculation1 ,hess1 #ahhh7 between #j and #ajj is #jca1 and of liquid benzoic acid #jej4 ,one of the problems of the present day is the explanation of those complex relations which exist between the composition and such properties as specific heat1 latent heat1 expansion by heat1 compression1 internal friction1 cohesion1 and so forth4 ,they can only be connected by a complete theory of liquids1 which may now soon be expected1 more especially as many sides of the subject have already been partially explained4 #g ,according to the above reasons the quantity of heat1 ,q1 required to raise the temperature of one part by weight of a substance by one degree may be expressed by the sum ,q ,k ,b ,d1 where ,k is the heat actually expended in heating the substance1 or what is termed the absolute specific heat1 ,b the amount of heat expended in the internal work accomplished with the rise of temperature1 and ,d the amount of heat expended in external work4 ,in the case of gases the last quantity may be easily determined1 knowing their coefficient of expansion1 which is approximately #jjjcfh4 ,by applying to this case the same argument given at the end of ,note #aa1 ,chapter ,i41 we find that one cubic metre of a gas heated #a produces an external work of #ajccc #jjjcfh1 or #chjb kilogrammetres1 on which #chjb_/dbd or #jjhig heat units are expended4 ,this is the heat expended for the external work produced by one cubic metre of a gas1 but the specific heat refers to units of weight1 and therefore it is necessary in order to know ,d to reduce the above quantity to a unit of weight4 ,one cubic metre of hydrogen at #j and #gfj mm4 pressure weighs #jjhif kilo1 a gas of molecular weight ,m has a density ,m_/#b1 consequently a cubic metre weighs 7at #j and #gfj mm47 #jjddh,m kilo1 and therefore #a kilogram of the gas occupies a volume #a_/jjddh,m cubic metres1 and hence the external work ,d in the heating of #a kilo of the given gas through #a #jjhif_/jjddh,m1 or ,d #b_/,m4 ,taking the magnitude of the internal work ,b for gases as negligible if permanent gases are taken1 and therefore supposing ,b #j1 we find the specific heat of gases at a constant pressure ,q ,k #b ,m1 where ,k is the specific heat at a constant volume1 or the true specific heat1 and ,m the molecular weight4 ,hence ,k ,q-#b_/,m4 ,the magnitude of the specific heat ,q is given by direct experiment4 ,according to ,regnault's experiments1 for oxygen it #jbage1 for hydrogen #cdje1 for nitrogen #jbdch2 the molecular weights of these gases are #cb1 #b1 and #bh1 and therefore for oxygen ,k #jbage-jjfbe #jaeej1 for hydrogen ,k #cdjej-ajjj #bdjej1 and for nitrogen ,k #jbdch-jjgad #jagbd4 ,these true specific heats of elements are in inverse proportion to their atomic weights--that is1 their product by the atomic weight is a constant quantity4 ,in fact1 for oxygen this product #jaee #af #bdh1 for hydrogen #bdj1 for nitrogen #jggbd #ad #bdad1 and therefore if ,a stand for the atomic weight we obtain the expression ,k ,a a constant1 which may be taken as #bde4 ,this is the true expression of ,dulong and ,petit's law1 because ,k is the true specific heat and ,a the weight of the atom4 ,it should be remarked1 moreover1 that the product of the observed specific heat ,q into ,a is also a constant quantity 7for oxygen #cdh1 for hydrogen #cdj71 because the external work ,d is also inversely proportional to the atomic weight4 ,in the case of gases we distinguish the specific heat at a constant pressure c 7we designated this quantity above by ,q71 and at a constant volume c4 ,it is evident that the relation between the two specific heats1 k1 judging from the above1 is the ratio of ,q to ,k1 or equal to the ratio of #bden #b to #bden4 ,when n #a this ratio k #ah2 when n #b1 k #ad1 when n #c1 k #ac1 and with an exceedingly large number n1 of atoms in the molecule1 k #a4 ,that is1 the ratio between the specific heats decreases from #ah to #aj as the number of atoms1 n1 contained in the molecule increases4 ,this deduction is verified to a certain extent by direct experiment4 ,for such gases as hydrogen1 oxygen1 nitrogen1 carbonic oxide1 air1 and others in which n #b1 the magnitude of k is determined by methods described in works on physics 7for example1 by the change of temperature with an alteration of pressure1 by the velocity of sound1 c47 and is found in reality to be nearly #ad1 and for such gases as carbonic anhydride1 nitric dioxide1 and others it is nearly #ac4 ,kundt and ,warburg 7#ahge71 by means of the approximate method mentioned in ,note #bi1 ,chapter ,,vii41 determined k for mercury vapour when n #a1 and found it to be #afg--that is1 a larger quantity than for air1 as would be expected from the above4 ,it may be admitted that the true atomic heat of gases #bdc1 only under the condition that they are distant from a liquid state1 and do not undergo a chemical change when heated--that is1 when no internal work is produced in them 7,b #j74 ,therefore this work may to a certain extent be judged by the observed specific heat4 ,thus1 for instance1 for chlorine 7,q #jab1 ,regnault2 k #acc1 according to ,straker and ,martin1 and therefore ,k #jji1 ,,mk #fd71 the atomic heat 7#cb7 is much greater than for other gases containing two atoms in a molecule1 and it must be assumed1 therefore1 that when it is heated some great internal work is accomplished4 ,in order to generalise the facts concerning the specific heat of gases and solids1 it appears to me possible to accept the following general proposition3 the atomic heat 7that is1 ,,aq or ,,qm_/n1 where ,m is the molecular weight and n the number of molecules7 is smaller 7in solids it attains its highest value #fh and in gases #cd71 the more complex the molecule 7i4e4 the greater the number 7n7 of atoms forming it7 and so much smaller1 up to a certain point 7in similar physical states7 the smaller the mean atomic weight ,m_/n4 #h ,as an example1 it will be sufficient to refer to the specific heat of nitrogen tetroxide1 ,n#b,o#d1 which1 when heated1 gradually passes into ,,no#b--that is1 chemical work of decomposition proceeds1 which consumes heat4 ,speaking generally1 specific heat is a complex quantity1 in which it is clear that thermal data 7for instance1 the heat of reaction7 alone cannot give an idea either of chemical or of physical changes individually1 but always depend on an association of the one and the other4 ,if a substance be heated from t#j to t#a it cannot but suffer a chemical change 7that is1 the state of the atoms in the molecules changes more or less in one way or another7 if dissociation sets in at a temperature t#a4 ,even in the case of the elements whose molecules contain only one atom1 a true chemical change is possible with a rise of temperature1 because more heat is evolved in chemical reactions than that quantity which participates in purely physical changes4 ,one gram of hydrogen 7specific heat #cd at a constant pressure7 cooled to the temperature of absolute zero will evolve altogether about one thousand units of heat1 #h grams of oxygen half this amount1 whilst in combining together they evolve in the formation of #i grams of water more than thirty times as much heat4 ,hence the store of chemical energy 7that is1 of the motion of the atoms1 vortex1 or other7 is much greater than the physical store proper to the molecules1 but it is the change accomplished by the former that is the cause of chemical transformations4 ,here we evidently touch on those limits of existing knowledge beyond which the teaching of science does not yet allow us to pass4 ,many new scientific discoveries have still to be made before this is possible4 ,among the bivalent metals the first place1 with respect to their distribution in nature1 is occupied by magnesium and calcium1 just as sodium and potassium stand first amongst the univalent metals4 ,the relation which exists between the atomic weights of these four metals confirms the above comparison4 ,in fact1 the combining weight of magnesium is equal to #bd1 and of calcium #dj2 whilst the combining weights of sodium and potassium are #bc and #ci--that is1 the latter are one unit less than the former4#i ,they all belong to the number of light metals1 as they have but a small specific gravity1 in which respect they differ from the ordinary1 generally known heavy1 or ore1 metals 7for instance1 iron1 copper1 silver1 and lead71 which are distinguished by a much greater specific gravity4 ,there is no doubt that their low specific gravity has a significance1 not only as a simple point of distinction1 but also as a property which determines the fundamental properties of these metals4 ,indeed1 all the light metals have a series of points of resemblance with the metals of the alkalis2 thus both magnesium and calcium1 like the metals of the alkalis1 decompose water 7without the addition of acids71 although not so easily as the latter metals4 ,the process of the decomposition is essentially one and the same2 for example1 ,ca #b;,h#b,o ,ca,h#b,o#b ,h#b--that is1 hydrogen is liberated and a hydroxide of the metal formed4 ,these hydroxides are bases which neutralise nearly all acids4 ,however1 the hydroxides ,,rh#b,o#b of calcium and magnesium are in no respect so energetic as the hydroxides of the true metals of the alkalis2 thus when heated they lose water1 are not so soluble1 develop less heat with acids1 and form various salts1 which are less stable and more easily decomposed by heat than the corresponding salts of sodium and potassium4 ,thus calcium and magnesium carbonates easily part with carbonic anhydride when ignited2 the nitrates are also very easily decomposed by heat1 calcium and magnesium oxides1 ,ca,o and ,mg,o1 being left behind4 ,the chlorides of magnesium and calcium1 when heated with water1 evolve hydrogen chloride1 forming the corresponding hydroxides1 and when ignited the oxides themselves4 ,all these points are evidence of a weakening of the alkaline properties4 #i ,as if ,na,h ,mg and ,,kh ,ca1 which is in accordance with their valency4 ,,kh includes two monovalent elements1 and is a bivalent group like ,ca4 ,these metals have been termed the metals of the alkaline earths1 because they1 like the alkali metals1 form energetic bases4 ,they are called alkaline earths because they are met with in nature in a state of combination1 forming the insoluble mass of the earth1 and because as oxides1 ,,ro1 they themselves have an earthy appearance4 ,not a few salts of these metals are known which are insoluble in water1 whilst the corresponding salts of the alkali metals are generally soluble--for example1 the carbonates1 phosphates1 borates1 and other salts of the alkaline earth metals are nearly insoluble4 ,this enables us to separate the metals of the alkaline earths from the metals of the alkalis4 ,for this purpose a solution of ammonium carbonate is added to a mixed solution of salts of both kinds of metals1 when by a double decomposition the insoluble carbonates of the metals of the alkaline earths are formed and fall as a precipitate1 whilst the metals of the alkalis remain in solution3 ,,rx#b ,na#b;,,co#c ,,rco#c #b,na,x4 ,we may here remark that the oxides of the metals of the alkaline earths are frequently called by special names3 ,mg,o is called magnesia or bitter earth2 ,ca,o1 lime2 ,sr,o1 strontia2 and ,ba,o1 baryta4 ,in the primary rocks the oxides of calcium and magnesium are combined with silica1 sometimes in variable quantities1 so that in some cases the lime predominates and in other cases the magnesium4 ,the two oxides1 being analogous to each other1 replace each other in equivalent quantities4 ,the various forms of augite1 hornblende1 or amphibole1 and of similar minerals1 which enter into the composition of nearly all rocks1 contain lime and magnesia and silica4 ,the majority of the primary rocks also contain alumina1 potash1 and soda4 ,these rocks1 under the action of water 7containing carbonic acid7 and air1 give up lime and magnesia to the water1 and therefore they are contained in all kinds of water1 and especially in sea-water4 ,the carbonates ,ca,c,o#c and ,mg,c,o#c1 frequently met with in nature1 are soluble in an excess of water saturated with carbonic anhydride1#aj and therefore many natural waters contain these salts1 and are able to yield them when evaporated4 ,however1 one kilogram of water saturated with carbonic anhydride does not dissolve more than three grams of calcium carbonate4 ,by gradually expelling the carbonic anhydride from such water1 an insoluble precipitate of calcium carbonate separates out4 ,it may confidently be stated that the formation of the very widely distributed strata of calcium and magnesium carbonates was of this nature1 because these strata are of a sedimentary character--that is1 such as would be exhibited by a gradually accumulating deposit on the bottom of the sea1 and1 moreover1 frequently containing the remains of marine plants1 and animals1 shells1 c4 ,it is very probable that the presence of these organisms in the sea has played the chief part in the precipitation of the carbonates from the sea water1 because the plants absorb ,,co#b1 and many of the organisms ,ca,c,o#c1 and after death give deposits of carbonate of lime2 for instance1 chalk1 which is almost entirely composed of the minute remains of the calcareous shields of such organisms4 ,these deposits of calcium and magnesium carbonates are the most important sources of these metals4 ,lime generally predominates1 because it is present in rocks and running water in greater quantity than magnesia1 and in this case these sedimentary rocks are termed limestone4 ,some common flagstones used for paving1 c41 and chalk may be taken as examples of this kind of formation4 ,those limestones in which a considerable portion of the calcium is replaced by magnesium are termed dolomites4 ,the dolomites are distinguished by their hardness1 and by their not parting with the whole of their carbonic anhydride so easily as the limestones under the action of acids4 ,dolomites#aa sometimes contain an equal number of molecules of calcium carbonate and magnesium carbonate1 and they also sometimes appear in a crystalline form1 which is easily intelligible1 because calcium carbonate itself is exceedingly common in this form in nature1 and is then known as calc spar1 whilst natural crystalline magnesium carbonate is termed magnesite4 ,the formation of the crystalline varieties of the insoluble carbonates is explained by the possibility of a slow deposition from solutions containing carbonic acid4 ,besides which 7,chapter ,x47 calcium and magnesium sulphates are obtained from sea water1 and therefore they are met with both as deposits and in springs4 ,it must be observed that magnesium is held in considerable quantities in sea water1 because the sulphate and chloride of magnesium are very soluble in water1 whilst calcium sulphate is but little soluble1 and is used in the formation of shells2 and therefore if the occurrence of considerable deposits of magnesium sulphate cannot be expected in nature1 still1 on the other hand1 one would expect 7and they do actually occur7 large masses of calcium sulphate or gypsum1 ,ca,s,o#d1b;,h#b,o4 ,gypsum sometimes forms strata of immense size1 which extend over many hectometres--for example1 in ,russia on the ,volga1 and in the ,donetz and ,baltic provinces4 #aj ,sodium carbonate and other carbonates of the alkalis give acid salts which are less soluble than the normal2 here1 on the contrary1 with an excess of carbonic anhydride1 a salt is formed which is more soluble than the normal1 but this acid salt is more unstable than sodium hydrogen carbonate1 ,na,h,c,o#c4 #aa ,the formation of dolomite may be explained1 if only we imagine that a solution of a magnesium salt acts on calcium carbonate4 ,magnesium carbonate may be formed by double decomposition1 and it must be supposed that this process ceases at a certain limit 7,chapter ,,xii471 when we shall obtain a mixture of the carbonates of calcium and magnesium4 ,haitinger heated a mixture of calcium carbonate1 ,ca,c,o#c1 with a solution of an equivalent quantity of magnesium sulphate1 ,mg,s,o#d1 in a closed tube at #bjj1 and then a portion of the magnesia actually passed into the state of magnesium carbonate1 ,mg,c,o#c1 and a portion of the lime was converted into gypsum1 ,ca,s,o#d4 ,lubavin 7#ahib7 showed that ,mg,c,o#c is more soluble than ,ca,c,o#c in salt water1 which is of some significance in explaining the composition of sea water4 ,lime and magnesia also1 but in much smaller quantities 7only to the amount of several fractions of a per cent4 and rarely more71 enter into the composition of every fertile soil1 and without these bases the soil is unable to support vegetation4 ,lime is particularly important in this respect1 and its presence in a larger quantity generally improves the harvest1 although purely calcareous soils are as a rule infertile4 ,for this reason the soil is fertilised both with lime#ab itself and with marl--that is1 with clay mixed with a certain quantity of calcium carbonate1 strata of which are found nearly everywhere4 #ab ,the undoubted action of lime in increasing the fertility of soils--if not in every case1 at all events1 with ordinary soils which have long been under corn--is based not so much on the need of plants for the lime itself as on those chemical and physical changes which it produces in the soil1 as a particularly powerful base which aids the alteration of the mineral and organic elements of the soil4 ,from the soil the lime and magnesia 7in a smaller quantity7 pass into the substance of plants1 where they occur as salts4 ,certain of these salts separate in the interior of plants in a crystalline form--for example1 calcium oxalate4 ,the lime occurring in plants serves as the source for the formation of the various calcareous secretions which are so common in animals of all classes4 ,the bones of the highest animal orders1 the shells of mollusca1 the covering of the sea-urchin1 and similar solid secretions of sea animals1 contain calcium salts2 namely1 the shells mainly calcium carbonate1 and the bones mainly calcium phosphate4 ,certain limestones are almost entirely formed of such deposits4 ,odessa is situated on a limestone of this kind1 composed of shells4 ,thus magnesium and calcium occur throughout the entire realm of nature1 but calcium predominates4 ,as lime and magnesia form bases which are in many respects analogous1 they were not distinguished from each other for a long time4 ,magnesia was obtained for the first time in the seventeenth century from ,italy1 and used as a medicine2 and it was only in the last century that ,black1 ,bergmann1 and others distinguished magnesia from lime4 ,metallic magnesium 7and calcium also7 is not obtained by heating magnesium oxide or the carbonate with charcoal1 as the alkali metals are obtained1#ac but is liberated by the action of a galvanic current on fused magnesium chloride 7best mixed with potassium chloride72 ,davy and ,bussy obtained metallic magnesium by acting on magnesium chloride with the vapours of potassium4 ,at the present time 7,deville's process7 magnesium is prepared in rather considerable quantities by a similar process1 only the potassium is replaced by sodium4 ,anhydrous magnesium chloride1 together with sodium chloride and calcium fluoride1 is fused in a close crucible4 ,the latter substances only serve to facilitate the formation of a fusible mass before and after the reaction1 which is indispensable in order to prevent the access and action of air4 ,one part of finely divided sodium to five parts of magnesium chloride is thrown into the strongly heated molten mass1 and after stirring the reaction proceeds very quickly1 and magnesium separates1 ,mg,cl#b ,na#b ,mg #b,na,cl4 ,in working on a large scale1 the powdery metallic magnesium is then subjected to distillation at a white heat4 ,the distillation of the magnesium is necessary1 because the undistilled metal is not homogeneous#ad and burns unevenly3 the metal is prepared for the purpose of illumination4 ,magnesium is a white metal1 like silver2 it is not soft like the alkali metals1 but is1 on the contrary1 hard like the majority of the ordinary metals4 ,this follows from the fact that it melts at a somewhat high temperature--namely1 about #ejj--;and boils at about #ajjj4 ,it is malleable and ductile1 like the generality of metals1 so that it can be drawn into wires and rolled into ribbon2 it is most frequently used for lighting purposes in the latter form4 ,unlike the alkali metals1 magnesium does not decompose the atmospheric moisture at the ordinary temperature1 so that it is almost unacted on by air2 it is not even acted on by water at the ordinary temperature1 so that it may be washed to free it from sodium chloride4 ,magnesium only decomposes water with the evolution of hydrogen at the boiling point of water1#ae and more rapidly at still higher temperatures4 ,this is explained by the fact that in decomposing water magnesium forms an insoluble hydroxide1 ,mg,h#b,o#b1 which covers the metal and hinders the further action of the water4 ,magnesium easily displaces hydrogen from acids1 forming magnesium salts4 ,when ignited it burns1 not only in oxygen but in air 7and even in carbonic anhydride71 forming a white powder of magnesium oxide1 or magnesia2 in burning it emits a white and exceedingly brilliant light4 ,the strength of this light naturally depends on the fact that magnesium 7#bd parts by weight7 in burning evolves about #adj thousand heat units1 and that the product of combustion1 ,mg,o1 is infusible by heat2 so that the vapour of the burning magnesium contains an ignited powder of non-volatile and infusible magnesia1 and consequently presents all the conditions for the production of a brilliant light4 ,the light emitted by burning magnesium contains many rays which act chemically1 and are situated in the violet and ultra-violet parts of the spectrum4 ,for this reason burning magnesium may be employed for producing photographic images4#af #ac ,sodium and potassium only decompose magnesium oxide at a white heat and very feebly1 probably for two reasons4 ,in the first place1 because the reaction ,mg ,o develops more heat 7about #adj thousand calories7 than ,k#b ,o or ,na#b ,o 7about #ajj thousand calories72 and1 in the second place1 because magnesia is not fusible at the heat of a furnace and cannot act on the charcoal1 sodium1 or potassium--that is1 it does not pass into that mobile state which is necessary for reaction4 ,the first reason alone is not sufficient to explain the absence of the reaction between charcoal and magnesia1 because iron and charcoal in combining with oxygen evolve less heat than sodium or potassium1 yet1 nevertheless1 they can displace them4 ,with respect to magnesium chloride1 it acts on sodium and potassium1 not only because their combination with chlorine evolves more heat than the combination of chlorine and magnesium 7,mg ,cl#b gives #aej and ,na#b ,cl#b about #aie thousand calories71 but also because a fusion1 both of the magnesium chloride and of the double salt1 takes place under the action of heat4 ,it is probable1 however1 that a reverse reaction will take place4 ,a reverse reaction might probably be expected1 and ,winkler 7#ahij7 showed that ,mg reduces the oxides of the alkali metals 7,chapter ,,xiii41 ,note #db74 #ad ,commercial magnesium generally contains a certain amount of magnesium nitride 7,deville and ,caron71 ,mg#c,n#b--that is1 a product of substitution of ammonia which is directly formed 7as is easily shown by experiment7 when magnesium is heated in nitrogen4 ,it is a yellowish green powder1 which gives ammonia and magnesia with water1 and cyanogen when heated with carbonic anhydride4 ,pashkoffsky 7#ahic7 showed that ,mg#c,n#b is easily formed and is the sole product when ,mg is heated to redness in a current of ,,nh#c4 ,perfectly pure magnesium may be obtained by the action of a galvanic current4 #ae ,hydrogen peroxide 7,weltzien7 dissolves magnesium4 ,the reaction has not been investigated4 #af ,a special form of apparatus is used for burning magnesium4 ,it is a clockwork arrangement in which a cylinder rotates1 round which a ribbon or wire of magnesium is wound4 ,the wire is subjected to a uniform unwinding and burning as the cylinder rotates1 and in this manner the combustion may continue uniform for a certain time4 ,the same is attained in special lamps1 by causing a mixture of sand and finely divided magnesium to fall from a funnel-shaped reservoir on to the flame4 ,in photography it is best to blow finely divided magnesium into a colourless 7spirit or gas7 flame1 and for instantaneous photography to light a cartridge of a mixture of magnesium and chlorate of potassium by means of a spark from a ,ruhmkorff's coil 7,d4 ,mendeleff1 #ahhi74 ,owing to its great affinity for oxygen1 magnesium reduces many metals 7zinc1 iron1 bismuth1 antimony1 cadmium1 tin1 lead1 copper1 silver1 and others7 from solutions of their salts at the ordinary temperature1#ag and at a red heat finely divided magnesium takes up the oxygen from silica1 alumina1 boric anhydride1 c42 so that silicon and similar elements may be obtained by directly heating a mixture of powdered silica and magnesium in an infusible glass tube4#ah #ag ,according to the observations of ,maack1 ,comaille1 ,bttger1 and others4 ,the reduction by heat mentioned further on was pointed out by ,geuther1 ,phipson1 ,parkinson and ,gattermann4 #ah ,this action of metallic magnesium in all probability depends1 although only partially 7see ,note #ac71 on its volatility1 and on the fact that1 in combining with a given quantity of oxygen1 it evolves more heat than aluminium1 silicon1 potassium1 and other elements4 ,the affinity of magnesium for the halogens is much more feeble than for oxygen1#ai as is at once evident from the fact that a solution of iodine acts feebly on magnesium2 still magnesium burns in the vapours of iodine1 bromine1 and chlorine4 ,the character of magnesium is also seen in the fact that all its salts1 especially in the presence of water1 are decomposable at a comparatively moderate temperature1 the elements of the acid being evolved1 and the magnesium oxide1 which is non-volatile and unchangeable by heat1 being left4 ,this naturally refers to those acids which are themselves volatilised by heat4 ,even magnesium sulphate is completely decomposed at the temperature at which iron melts1 oxide of magnesium remaining behind4 ,this decomposition of magnesium salts by heat proceeds much more easily than that of calcium salts4 ,for example1 magnesium carbonate is totally decomposed at #agj1 magnesium oxide being left behind4 ,this magnesia1 or magnesium oxide1 is met with both in an anhydrous and hydrated state in nature 7the anhydrous magnesia as the mineral periclase1 ,mg,o1 and the hydrated magnesia as brucite1 ,mg,h#b,o#b74 ,magnesia is a well-known medicine 7calcined magnesia--magnesia usta74 ,it is a white1 extremely fine1 and very voluminous powder1 of specific gravity #cd2 it is infusible by heat1 and only shrinks or shrivels in an oxyhydrogen flame4 ,after long contact the anhydrous magnesia combines with water1 although very slowly1 forming the hydroxide ,mg7,,ho7#b1 which1 however1 parts with its water with great ease when heated even below a red heat1 and again yields anhydrous magnesia4 ,this hydroxide is obtained directly as a gelatinous amorphous substance when a soluble alkali is mixed with a solution of any magnesium salt1 ,mg,cl#b #b,,kho ,mg7,,ho7#b #b,k,cl4 ,this decomposition is complete1 and nearly all the magnesium passes into the precipitate2 and this clearly shows the almost perfect insolubility of magnesia in water4 ,water dissolves a scarcely perceptible quantity of magnesium hydroxide--namely1 one part is dissolved by #ee1jjj parts of water4 ,such a solution1 however1 has an alkaline reaction1 and gives1 with a salt of phosphoric acid1 a precipitate of magnesium phosphate1 which is still more insoluble4 ,magnesia is not only dissolved by acids1 forming salts1 but it also displaces certain other bases--for example1 ammonia from ammonium salts when boiled2 and the hydroxide also absorbs carbonic anhydride from the air4 ,the magnesium salts1 like those of calcium1 potassium1 and sodium1 are colourless if they are formed from colourless acids4 ,those which are soluble have a bitter taste1 whence magnesia has been termed bitter-earth4 ,in comparison with the alkalis magnesia is a feeble base1 inasmuch as it forms somewhat unstable salts1 easily gives basic salts1 forms acid salts with difficulty1 and is able to give double salts with the salts of the alkalis1 which facts are characteristic of feeble bases1 as we shall see in becoming acquainted with the different metals4 #ai ,davy1 on heating magnesia in chlorine1 concluded that there was a complete substitution1 because the volume of the oxygen was half the volume of the chlorine2 it is probable1 however1 that owing to the formation of chlorine oxide 7,chapter ,,xi41 ,note #cj7 the decomposition is not complete and is limited by a reverse reaction4 ,the power of magnesium salts to form double and basic salts is very frequently shown in reactions1 and is specially marked as regards ammonium salts4 ,if saturated solutions of magnesium and ammonium sulphates are mixed together1 a crystalline double salt ,mg7,,nh#d7b7,,so#d7b1f;,h#b,o1#bj is immediately precipitated4 ,a strong solution of ordinary ammonium carbonate dissolves magnesium oxide or carbonate1 and precipitates crystals of a double salt1 ,mg7,,nh#d7b7;,,co#c7b1d;,h#b,o1 from which water extracts the ammonium carbonate4 ,with an excess of an ammonium salt the double salt passes into solution1#ba and therefore if a solution contain a magnesium salt and an excess of an ammonium salt--for instance1 sal-ammoniac--then sodium carbonate will no longer precipitate magnesium carbonate4 ,a mixture of solutions of magnesium and ammonium chlorides1 on evaporation or refrigeration1 gives a double salt1 ,mg7,,nh#d7;,cl#c1f;,h#b,o4#bb ,the salts of potassium1 like those of ammonium1 are able to enter into combination with the magnesium salts4#bc ,for instance1 the double salt1 ,mg,k,cl#c1f;,h#b,o1 which is known as carnallite1#bd and occurs in the salt mines of ,stassfurt1 may be formed by freezing a saturated solution of potassium chloride with an excess of magnesium chloride4 ,a saturated solution of magnesium sulphate dissolves potassium sulphate1 and solid magnesium sulphate is soluble in a saturated solution of potassium sulphate4 ,a double salt1 ,k#b,mg7,,so#d7b1f;,h#b,o1 which closely resembles the above-mentioned ammonium salt1 crystallises from these solutions4#be ,the nearest analogues of magnesium are able to give exactly similar double salts1 both in crystalline form 7monoclinic system7 and composition2 they1 like this salt 7see ,chapter ,,xv471 are easily able 7at #adj7 to part with all their water of crystallisation1 and correspond with the salts of sulphuric acid1 whose type may be taken as magnesium sulphate1 ,mg,s,o#d4bf ,it occurs at ,stassfurt as kieserite1 ,mg,s,o#d1;,h#b,o1 and generally separates from solutions as a heptahydrated salt1 ,mg,s,o#d1g;,h#b,o1 and from supersaturated solutions as a hexahydrated salt1 ,mg,s,o#d1f;,h#b,o2 at temperatures below #j it crystallises out as a dodecahydrated salt1 ,mg,s,o#d1ab;,h#b,o1 and a solution of the composition ,mg,s,o#d1b;,h#b,o solidifies completely at -#e4bg ,thus between water and magnesium sulphate there may exist several definite and more or less stable degrees of equilibrium2 the double salt ,mg,s,o#d,k#b,,so#d1f;,h#b,o may be regarded as one of these equilibrated systems1 the more so since it contains #f;,h#b,o1 whilst ,mg,s,o#d forms its most stable system with #g;,h#b,o1 and the double salt may be considered as this crystallo-hydrate in which one molecule of water is replaced by the molecule ,k#b,,so#d4bh #bj ,even a solution of ammonium chloride gives this salt with magnesium sulphate4 ,its sp4 gr4 is #agb2 #ajj parts of water at #j dissolve #i1 at #bj #agi parts of the anhydrous salt4 ,at about #acj it loses all its water4 #ba ,this is an example of equilibrium and of the influence of mass2 the double salt is decomposed by water1 but if instead of water we take a solution of that soluble part which is formed in the decomposition of the double salt1 then the latter dissolves as a whole4 #bb ,if an excess of ammonia be added to a solution of magnesium chloride1 only half the magnesium is thrown down in the precipitate1 #b,mg,cl#b #b,,nh#d4,,oh ,mg7,,oh7#b ,mg4,,nh#d;,cl#c ,,nh#d;,cl4 ,a solution of ammonium chloride reacts with magnesia1 evolving ammonia and forming a solution of the same salt1 ,mg,o #c,,nh#d;,cl ,mg,n,h#d;,cl#c ,h#b,o #b,,nh#c4 ,among the double salts of ammonium and magnesium1 the phosphate1 ,mg,n,h#d,,po#d1f;,h#b,o1 is almost insoluble in water 7#jjg gram is soluble in a litre71 even in the presence of ammonia4 ,magnesia is very frequently precipitated as this salt from solutions in which it is held by ammonium salts4 ,as lime is not retained in solution by the presence of ammonium salts1 but is precipitated nevertheless by sodium carbonate1 c41 it is very easy to separate calcium from magnesium by taking advantage of these properties4 #bc ,in order to see the nature and cause of formation of double salts1 it is sufficient 7although this does not embrace the whole essence of the matter7 to consider that one of the metals of such salts 7for instance1 potassium7 easily gives acid salts1 and the other 7in this instance1 magnesium7 basic salts2 the properties of distinctly basic elements predominate in the former1 whilst in the latter these properties are enfeebled1 and the salts formed by them bear the character of acids--for example1 the salts of aluminium or magnesium act in many cases like acids4 ,by their mutual combination these two opposite properties of the salts are both satisfied4 #bd ,carnallite has been mentioned in ,chapter ,x4 7,note #d7 and in ,chapter ,,xiii4 ,these deposits also contain much kainite1 ,k,mg,cl7,,so#d71c;,h#b,o 7sp4 gr4 #bac2 #ajj parts of water dissolve #gif parts at #ah74 ,this double salt contains two metals and two haloids4 ,feit 7#ahhi7 also obtained a bromide corresponding to carnallite4 #be ,the component parts of certain double salts diffuse at different rates1 and as the diffused solution contains a different proportion of the component salts than the solution taken of the double salt1 it shows that such salts are decomposed by water4 ,according to ,rdorff1 the double salts1 like carnallite1 ,mg,k#b7,,so#d7b1f;,h#b,o1 and the alums1 all belong to this order 7#ahhh74 ,but such salts as tartar emetic1 the double oxalates1 and double cyanides are not separated by diffusion1 which in all probability depends both on the relative rate of the diffusion of the component salts and on the degree of affinity acting between them4 ,those complex states of equilibrium which exist between water1 the individual salts ,,mx and ,,ny1 and the double salt ,,mnxy1 have been already partially analysed 7as will be shown hereafter7 in that case when the system is heterogeneous 7that is1 when something separates out in a solid state from the liquid solution71 but in the case of equilibria in a homogeneous liquid medium 7in a solution7 the phenomenon is not so clear1 because it concerns that very theory of solution which cannot yet be considered as established 7,chapter ,i41 ,note #i1 and others74 ,as regards the heterogeneous decomposition of double salts1 it has long been known that such salts as carnallite and ,k#b,mg7,,so#d7b give up the more soluble salt if an insufficient quantity of water for their complete solution be taken4 ,the complete saturation of #ajj parts of water requires at #j #ada1 at #bj #be1 and at #fj #ejb parts of the latter double salt 7anhydrous71 while #ajj parts of water dissolve #bg parts of magnesium sulphate at #j1 #cf parts at #bj1 and #ee parts at #fj1 of the anhydrous salt taken4 ,of all the states of equilibrium exhibited by double salts the most fully investigated as yet is the system containing water1 sodium sulphate1 magnesium sulphate1 and their double salt1 ,na#b,mg7,,so#d7b1 which crystallises with #d and #f mol4 ,,oh#b4 ,the first crystallo-hydrate1 ,mg,na#b7,,so#d7b1d;,h#b,o1 occurs at ,stassfurt1 and as a sedimentary deposit in many of the salt lakes near ,astrakhan1 and is therefore called astrakhanite4 ,the specific gravity of the monoclinic prisms of this salt is #bbb4 ,if this salt1 in a finely divided state1 be mixed with the necessary quantity of water 7according to the equation ,mg,na#b7,,so#d7b1d;,h#b,o #ac;,h#b,o ,na#b,,so#d1aj;,h#b,o ,mg,s,o#d1g;,h#b,o71 the mixture solidifies like plaster of ,paris into a homogeneous mass if the temperature be below #bb 7,van't ,hoff und ,van ,deventer1 #ahhf2 ,bakhuis ,roozeboom1 #ahhg72 but if the temperature be above this transition-point the water and double salt do not react on each other3 that is1 they do not solidify or give a mixture of sodium and magnesium sulphates4 ,if a mixture 7in equivalent quantities7 of solutions of these salts be evaporated1 and crystals of astrakhanite and of the individual salts capable of proceeding from it be added to the concentrated solution to avoid the possibility of a supersaturated solution1 then at temperatures above #bb astrakhanite is exclusively formed 7this is the method of its production71 but at lower temperatures the individual salts are alone produced4 ,if equivalent amounts of ,glauber's salt and magnesium sulphate be mixed together in a solid state1 there is no change at temperatures below #bb1 but at higher temperatures astrakhanite and water are formed4 ,the volume occupied by ,na#b,,so#d1aj;,h#b,o in grams #cbb_/adf #bbje cubic centimetres1 and by ,mg,s,o#d1g;,h#b,o #bdf_/afh #adfd c4c42 hence their mixture in equivalent quantities occupies a volume of #cffi c4c4 ,the volume of astrakhanite #ccd_/bbb #aeje c4c41 and the volume of #ac;,h#b,o #bcd c4c41 hence their sum #chje c4c41 and therefore it is easy to follow the formation of the astrakhanite in a suitable apparatus 7a kind of thermometer containing oil and a powdered mixture of sodium and magnesium sulphates71 and to see by the variation in volume that below #bb it remains unchanged1 and at higher temperatures proceeds the more quickly the higher the temperature4 ,at the transition temperature the solubility of astrakhanite and of the mixture of the component salts is one and the same1 whilst at higher temperatures a solution which is saturated for a mixture of the individual salts would be supersaturated for astrakhanite1 and at lower temperatures the solution of astrakhanite will be supersaturated for the component salts1 as has been shown with especial detail by ,karsten1 ,deacon1 and others4 ,roozeboom showed that there are two limits to the composition of the solutions which can exist for a double salt2 these limits are respectively obtained by dissolving a mixture of the double salt with each of its component simple salts4 ,van't ,hoff demonstrated1 besides this1 that the tendency towards the formation of double salts has a distinct influence on the progress of double decomposition1 for at temperatures above #ca the mixture #b,mg,s,o#d1g;,h#b,o #b,na,cl passes into ,mg,na#b7,,so#d7b1d;,h#b,o ,mg,cl#b1f;,h#b,o #d;,h#b,o1 whilst below #ca there is not this double decomposition1 but it proceeds in the opposite direction1 as may be demonstrated by the above-described methods4 ,van der ,heyd obtained a potassium astrakhanite1 ,k#b,,so#d,mg,s,o#d1d;,h#b,o1 from solutions of the component salts at #ajj4 ,from these experiments on double salts we see that there is as close a dependence between the temperature and the formation of substances as there is between the temperature and a change of state4 ,it is a case of ,deville's principles of dissociation1 extended in the direction of the passage of a solid into a liquid4 ,on the other hand1 we see here how essential a rle water plays in the formation of compounds1 and how the affinity for water of crystallisation is essentially analogous to the affinity between salts1 and hence also to the affinity of acids for bases1 because the formation of double salts does not differ in any essential point 7except the degree of affinity--that is1 from a quantitative aspect7 from the formation of salts themselves4 ,when sodium hydroxide with nitric acid gives sodium nitrate and water the phenomenon is essentially the same as in the formation of astrakhanite from the salts ,na#b,,so#d1aj;,h#b,o and ,mg,s,o#d1g;,h#b,o4 ,water is disengaged in both cases1 and hence the volumes are altered4 #bf ,this salt1 and especially its crystallo-hydrate with #g;,h#b,o1 is generally known as ,epsom salts4 ,it has long been used as a purgative4 ,it is easily obtained from magnesia and sulphuric acid1 and it separates on the evaporation of sea water and of many saline springs4 ,when carbonic anhydride is obtained by the action of sulphuric acid on magnesite1 magnesium sulphate remains in solution4 ,when dolomite--that is1 a mixture of magnesium and calcium carbonates--is subjected to the action of a solution of hydrochloric acid until about half of the salt remains1 the calcium carbonate is mostly dissolved and magnesium carbonate is left1 which by treatment with sulphuric acid gives a solution of magnesium sulphate4 #bg ,the anhydrous salt1 ,mg,s,o#d 7sp4 gr4 #bfa71 attracts moisture 7#g mol4 ,h#b,o7 from moist air2 when heated in steam or hydrogen chloride it gives sulphuric acid1 and when heated with carbon it is decomposed according to the equation #b,mg,s,o#d ,c #b,,so#b ,,co#b #b,mg,o4 ,the monohydrated salt 7kieserite71 ,mg,s,o#d1;,h#b,o 7sp4 gr4 #bef71 dissolves in water with difficulty2 it is formed by heating the other crystallo-hydrates to #ace4 ,the hexahydrated salt is dimorphous4 ,if a solution1 saturated at the boiling-point1 be prepared1 and cooled without access of crystals of the heptahydrated salt1 then ,mg,s,o#d1f;,h#b,o crystallises out in monoclinic prisms 7,loewel1 ,marignac71 which are quite as unstable as the salt1 ,na#b,,so#d1g;,h#b,o2 but if prismatic crystals of the cubic system of the copper-nickel salts of the composition ,,mso#d1f;,h#b,o be added1 then crystals of ,mg,s,o#d1f;,h#b,o are deposited on them as prisms of the cubic system 7,lecoq de ,boisbaudran74 ,the common crystallo-hydrate1 ,mg,s,o#d1g;,h#b,o1 ,epsom salts1 belongs to the rhombic system1 and is obtained by crystallisation below #cj4 ,its specific gravity is #afi4 ,in a vacuum1 or at #ajj1 it loses #e;,h#b,o1 at #acb #f;,h#b,o1 and at #baj all the #g;,h#b,o 7,graham74 ,if crystals of ferrous or cobaltic sulphate be placed in a saturated solution1 hexagonal crystals of the heptahydrated salt are formed 7,lecoq de ,boisbaudran72 they present an unstable state of equilibrium1 and soon become cloudy1 probably owing to their transformation into the more stable common form4 ,fritzsche1 by cooling saturated solutions below #j1 obtained a mixture of crystals of ice and of a dodecahydrated salt1 which easily split up at temperatures above #j4 ,guthrie showed that dilute solutions of magnesium sulphate1 when refrigerated1 separate ice until the solution attains a composition ,mg,s,o#d1bd;,h#b,o1 which will completely freeze into a crystallo-hydrate at -#ec4 ,according to ,coppet and ,rdorff1 the temperature of the formation of ice falls by #jjgc for every part by weight of the heptahydrated salt per #ajj of water4 ,this figure gives 7,chapter ,i41 ,note #di7 i #a for both the heptahydrated and the anhydrous salt1 from which it is evident that it is impossible to judge the state of combination in which a dissolved substance occurs by the temperature of the formation of ice4 ,the solubility of the different crystallo-hydrates of magnesium sulphate1 according to ,loewel1 also varies1 like those of sodium sulphate or carbonate 7see ,chapter ,,xii41 ,notes #g and #ah74 ,at #j #ajj parts of water dissolves #djge ,mg,s,o#d in the presence of the hexahydrated salt1 #cdfg ,mg,s,o#d in the presence of the hexagonal heptahydrated salt1 and only #bf parts of ,mg,s,o#d in the presence of the ordinary heptahydrated salt--that is1 solutions giving the remaining crystallo-hydrates will be supersaturated for the ordinary heptahydrated salt4 ,all this shows how many diverse aspects of more or less stable equilibria may exist between water and a substance dissolved in it2 this has already been enlarged on in ,chapter ,i4 ,carefully purified magnesium sulphate in its aqueous solution gives1 according to ,stcherbakoff1 an alkaline reaction with litmus1 and an acid reaction with phenolphthalein4 ,the specific gravity of solutions of certain salts of magnesium and calcium reduced to #ae_/d 7see my work cited1 ,chapter ,i41 ,note #ai71 are1 if water at #d #aj1jjj1 ,mg,s,o#d3 s #i1iib #iihip #jeecp#b ,mg,cl#b3 s #i1iib #hacap #jcgbp#b ,ca,cl#b3 s #i1iib #hjbdp #jdgfp#b #bh ,graham even distinguished the last equivalent of the water of crystallisation of the heptahydrated salt as that which is replaced by other salts1 pointing out that double salts like ,mg,k#b7,,so#d7b1f;,h#b,o lose all their water at #ace1 whilst ,mg,s,o#d1g;,h#b,o only parts with #f;,h#b,o4 ,the power of forming basic salts is a very remarkable peculiarity of magnesia and other feeble bases1 and especially of those corresponding with polyvalent metals4 ,the very powerful bases corresponding with univalent metals--like potassium and sodium--do not form basic salts1 and1 indeed1 are more prone to give acid salts1 whilst magnesium easily and frequently forms basic salts1 especially with feeble acids1 although there are some oxides--as1 for example1 copper and lead oxides--which still more frequently give basic salts4 ,if a cold solution of magnesium sulphate be mixed with a solution of sodium carbonate there is formed a gelatinous precipitate of a basic salt1 ,mg7,,ho7#b1d,mg,c,o#c1i;,h#b,o2 but all the magnesia is not precipitated in this case1 as a portion of it remains in solution as an acid double salt4 ,if sodium carbonate be added to a boiling solution of magnesium sulphate a precipitate of a still more basic salt is formed1 #d,mg,s,o#d #d,na#b;,,co#c #d;,h#b,o #d,na#b,,so#d ,,co#b ,mg7,,oh7#b1c,mg,c,o#c1c;,h#b,o4 ,this basic salt forms the ordinary drug magnesia 7magnesia alba71 in the form of light porous lumps4 ,other basic salts are formed under certain modifications of temperature and conditions of decomposition4 ,but the normal salt1 ,mg,c,o#c1 which occurs in nature as magnesite in the form of rhombohedra of specific gravity #cjef1 cannot be obtained by such a method of precipitation4 ,in fact1 the formation of the different basic salts shows the power of water to decompose the normal salt4 ,it is possible1 however1 to obtain this salt both in an anhydrous and hydrated state4 ,a solution of magnesium carbonate in water containing carbonic acid is taken for this purpose4 ,the reason for this is easily understood--carbonic anhydride is one of the products of the decomposition of magnesium carbonate in the presence of water4 ,if this solution be left to evaporate spontaneously the normal salt separates in a hydrated form1 but in the evaporation of a heated solution1 through which a stream of carbonic anhydride is passed1 the anhydrous salt is formed as a crystalline mass1 which remains unaltered in the air1 like the natural mineral4#bi ,the decomposing influence of water on the salts of magnesium1 which is directly dependent on the feeble basic properties of magnesia1#cj is most clearly seen in magnesium chloride1 ,mg,cl#b4 ,this salt is contained#ca in the last mother-liquors of the evaporation of sea-water4 ,on cooling a sufficiently concentrated solution1 the crystallo-hydrate1 ,mg,cl#b1f;,h#b,o1 separates2#cb but if it be further heated 7above #ajf7 to remove the water1 then hydrochloric acid passes off together with the latter1 so that there ultimately remains magnesia with a small quantity of magnesium chloride4#cc ,from what has been said it is evident that anhydrous magnesium chloride cannot be obtained by simple evaporation4 ,but if sal-ammoniac or sodium chloride be added to a solution of magnesium chloride1 then the evolution of hydrochloric acid does not take place1 and after complete evaporation the residue is perfectly soluble in water4 ,this renders it possible to obtain anhydrous magnesium chloride from its aqueous solution4 ,indeed the mixture with sal-ammoniac 7in excess7 may be dried 7the residue consists of an anhydrous double salt1 ,mg,cl#b1b,,nh#d;,cl7 and then ignited 7#dfj71 when the sal-ammoniac is converted into vapour and a fused mass of anhydrous magnesium chloride remains behind4 ,the anhydrous chloride evolves a very considerable amount of heat on the addition of water1 which shows the great affinity the salt has for water4#cd ,anhydrous magnesium chloride is not only obtained by the above method1 but is also formed by the direct combination of chlorine and magnesium1 and by the action of chlorine on magnesium oxide1 oxygen being evolved2 this proceeds still more easily by heating magnesia with charcoal in a stream of chlorine1 when the charcoal serves to take up the oxygen4 ,this latter method is also employed for the preparation of chlorides which are formed in an anhydrous condition with still greater difficulty than magnesium chloride4 ,anhydrous magnesium chloride forms a colourless1 transparent mass1 composed of flexible crystalline plates of a pearly lustre4 ,it fuses at a low red heat 7#gjh7 into a colourless liquid1 remains unchanged in a dry state1 but under the action of moisture is partially decomposed even at the ordinary temperature1 with formation of hydrochloric acid4 ,when heated in the presence of oxygen 7air7 it gives chlorine and the basic salt1 which is formed with even greater facility under the action of heat in the presence of steam1 when ,h,cl is formed1 according to the equation #b,mg,cl#b ,h#b,o ,mg,o,mg,cl#b #b;,h,cl4#cd bis #bi ,the crystalline form of the anhydrous salt obtained in this manner is not the same as that of the natural salt4 ,the former gives rhombohedra1 like those in which calcium carbonate appears as calc spar1 whilst the natural salt appears as rhombic prisms1 like those sometimes presented by the same carbonate as aragonite1 which will shortly be described4 #cj ,magnesium sulphate enters into certain reactions which are proper to sulphuric acid itself4 ,thus1 for instance1 if a carefully prepared mixture of equivalent quantities of hydrated magnesium sulphate and sodium chloride be heated to redness1 the evolution of hydrochloric acid is observed just as in the action of sulphuric acid on common salt1 ,mg,s,o#d #b,na,cl ,h#b,o ,na#b,,so#d ,mg,o #b;,h,cl4 ,magnesium sulphate acts in a similar manner on nitrates1 with the evolution of nitric acid4 ,a mixture of it with common salt and manganese peroxide gives chlorine4 ,sulphuric acid is sometimes replaced by magnesium sulphate in galvanic batteries--for example1 in the well-known ,meidinger battery4 ,in the above-mentioned reactions we see a striking example of the similarity of the reactions of acids and salts1 especially of salts which contain such feeble bases as magnesia4 #ca ,as sea-water contains many salts1 ,m,cl and ,mg,x#b1 it follows1 according to ,berthollet's teaching1 that ,mg,cl#b is also present4 #cb ,as the crystallo-hydrates of the salts of sodium often contain #aj;,h#b,o1 so many of the salts of magnesium contain #f;,h#b,o4 #cc ,this decomposition is most simply defined as the result of the two reverse reactions1 ,mg,cl#b ,h#b,o ,mg,o #b;,h,cl and ,mg,o #b;,h,cl ,mg,cl#b ,h#b,o1 or as a distribution between ,o and ,cl#b on the one hand and ,h#b and ,mg on the other4 7,with ,o1 ,mg,cl#b gives chlorine1 see ,chapter ,x41 ,note #cc1 and ,chapter ,,ii41 ,note #c bis and others1 where the reactions and applications of ,mg,cl#b are given47 ,it is then clear that1 according to ,berthollet's doctrine1 the mass of the hydrochloric acid converts the magnesium oxide into chloride1 and the mass of the water converts the magnesium chloride into oxide4 ,the crystallo-hydrate1 ,mg,cl#b1f;,h#b,o1 forms the limit of the reversibility4 ,but an intermediate state of equilibrium may exist in the form of basic salts4 ,on mixing ignited magnesia with a solution of magnesium chloride of specific gravity about #ab1 a solid mass is obtained which is scarcely decomposed by water at the ordinary temperature 7see ,chapter ,,xvi41 ,note #d74 ,a similar means is employed for cementing sawdust into a solid mass1 called cylolite1 used for flooring1 c4 ,we may remark that ,mg,br#b crystallises not only with #f;,h#b,o 7temperature of fusion #aeb71 but also with #aj;,h#b,o 7temperature of fusion #ab1 formed at -#ah74 7,panfiloff1 #ahid74 #cd ,according to ,thomsen1 the combination of ,mg,cl#b with #f;,h#b,o evolves #cc1jjj calories1 and its solution in an excess of water #cf1jjj4 #cd bis ,hence ,mg,cl#b may be employed for the preparation of chlorine and hydrochloric acid 7,chapters ,x4 and ,,xi474 ,in general magnesium chloride1 which is obtained in large quantities from sea water and ,stassfurt carnallite1 may find numerous practical uses4 ,calcium 7or the metal of lime7 and its compounds in many respects present a great resemblance to magnesium compounds1 but are also clearly distinguished from them by many properties4#ce ,in general1 calcium stands to magnesium in the same relation as potassium occupies in respect to sodium4 ,davy obtained metallic calcium1 like potassium1 as an amalgam by the action of a galvanic current2 but neither charcoal nor iron decomposes calcium oxide1 and even sodium decomposes calcium chloride#cf with difficulty4 ,but a galvanic current easily decomposes calcium chloride1 and metallic sodium somewhat easily decomposes calcium iodide when heated4 ,as in the case of hydrogen1 potassium1 and magnesium1 the affinity of iodine for calcium is feebler than that of chlorine 7and oxygen71 and therefore it is not surprising that calcium iodide may be subjected to that decomposition1 which the chloride and oxide undergo with difficulty4#cg ,metallic calcium is of a yellow colour1 and has a considerable lustre1 which it preserves in dry air4 ,its specific gravity is #aeh4 ,calcium is distinguished by its great ductility2 it melts at a red heat and then burns in the air with a very brilliant flame2 the brilliancy is due to the formation of finely divided infusible calcium oxide4 ,judging from the fact that calcium in burning gives a very large flame1 it is probable that this metal is volatile4 ,calcium decomposes water at the ordinary temperature1 and is oxidised in moist air1 but not so rapidly as sodium4 ,in burning1 it gives its oxide or lime1 ,ca,o1 a substance which is familiar to every one1 and of which we have already frequently had occasion to speak4 ,this oxide is not met with in nature in a free state1 because it is an energetic base which everywhere encounters acid substances forming salts with them4 ,it is generally combined with silica1 or occurs as calcium carbonate or sulphate4 ,the carbonate and nitrate are decomposed1 at a red heat1 with the formation of lime4 ,as a rule1 the carbonate1 which is so frequently met with in nature1 serves as the source of the calcium oxide1 both commercial and pure4 ,when heated1 calcium carbonate dissociates3 ,ca,c,o#c ,ca,o ,,co#b4 ,in practice the decomposition is conducted at a bright red heat1 in the presence of steam1 or a current of a foreign gas1 in heaps or in special kilns4#ch #ce ,there are many other methods of separating calcium from magnesium besides that mentioned above 7,note #bb74 ,among them it will be sufficient to mention the behaviour of these bases towards a solution of sugar2 hydrated lime is exceedingly soluble in an aqueous solution of sugar1 whilst magnesia is but little soluble4 ,all the lime may be extracted from dolomite by burning it1 slaking the mixture of oxides thus obtained1 and adding a #aj p4c4 solution of sugar4 ,carbonic anhydride precipitates calcium carbonate from this solution4 ,the addition of sugar 7molasses7 to the lime used for building purposes powerfully increases the binding power of the mortar1 as ,i have myself found4 ,i have been told that in the ,east 7,india1 ,japan7 the addition of sugar to cement has long been practised4 #cf ,moreover ,caron obtained an alloy of calcium and zinc by fusing calcium chloride with zinc and sodium4 ,the zinc distilled from this alloy at a white heat1 leaving calcium behind 7,note #ej74 #cg ,calcium iodide may be prepared by saturating lime with hydriodic acid4 ,it is a very soluble salt 7at #bj one part of the salt requires #jdi part and at #dc #jce part of water for solution71 is deliquescent in the air1 and resembles calcium chloride in many respects4 ,it changes but little when evaporated1 and like calcium chloride fuses when heated1 and therefore all the water may be driven off by heat4 ,if anhydrous calcium iodide be heated with an equivalent quantity of sodium in a closely covered iron crucible1 sodium iodide and metallic calcium are formed 7,lis-,bodart74 ,dumas advises carrying on this reaction in a closed space under pressure4 #ch ,kilns which act either intermittently or continuously are built for this purpose4 ,those of the first kind are filled with alternate layers of fuel and limestone2 the fuel is lighted1 and the heat developed by its combustion serves for decomposing the limestone4 ,when the process is completed the kiln is allowed to cool somewhat1 the lime raked out1 and the same process repeated4 ,in the continuously acting furnaces1 constructed like that shown in fig4 #gh1 the kiln itself only contains limestone1 and there are lateral hearths for burning the fuel1 whose flame passes through the limestone and serves for its decomposition4 ,such furnaces are able to work continuously1 because the unburnt limestone may be charged from above and the burnt lime raked out from below4 ,it is not every limestone that is suitable for the preparation of lime1 because many contain impurities1 principally clay1 dolomite1 and sand4 ,such limestones when burnt either fuse partially or give an impure lime1 called poor lime in distinction from that obtained from purer limestone1 which is called rich lime4 ,the latter kind is characterised by its disintegrating into a fine powder when treated with water1 and is suitable for the majority of uses to which lime is applied1 and for which the poor lime is sometimes quite unfit4 ,however1 certain kinds of poor lime 7as we shall see in ,chapter ,,xviii41 ,note #be7 are used in the preparation of hydraulic cements1 which solidify into a hard mass under water4 ,in order to obtain perfectly pure lime it is necessary to take the purest possible materials4 ,in the laboratory1 marble or shells are used for this purpose as a pure form of calcium carbonate4 ,they are first burnt in a furnace1 then put in a crucible and moistened with a small quantity of water1 and finally strongly ignited1 by which means a pure lime is obtained4 ,pure lime may be more rapidly prepared by taking calcium nitrate1 ,ca,n#b,o#f1 which is easily obtained by dissolving limestone in nitric acid4 ,the solution obtained is boiled with a small quantity of lime in order to precipitate the foreign oxides which are insoluble in water4 ,the oxides of iron1 aluminium1 c41 are precipitated by this means4 ,the salt is then crystallised and ignited3 ,ca,n#b,o#f ,ca,o #b,,no#b ,o4 ,in the decomposition of calcium carbonate the lime preserves the form of the lumps subjected to ignition2 this is one of the signs distinguishing quicklime when it is freshly burnt and unaltered by air4 ,it attracts moisture from the air and then disintegrates to a powder2 if left long exposed in the air1 it also attracts carbonic anhydride and increases in volume2 it does not entirely pass into carbonate1 but forms a compound of the latter with caustic lime4 ,illustration3 ,,fig4 #gh4--;,continually-acting kiln for burning lime4 ,the lime is charged from above and calcined by four lateral grates1 ,r1 ,m4 ,d1 fire-bars4 ,b1 space for withdrawing the burnt lime4 ,k1 stoke-house4 ,m4 fire grate4 ,q1 ,r1 under-grate4 ,calcium oxide--that is1 quicklime--is a substance 7sp4 gr4 #cae7 which is unaffected by heat1#ci and may therefore serve as a fire-resisting material1 and was employed by ,deville for the construction of furnaces in which platinum was melted1 and silver volatilised by the action of the heat evolved by the combustion of detonating gas4 ,the hydrated lime1 slaked lime1 or calcium hydroxide1 ,ca,h#b,o#b 7specific gravity #bjg7 is a most common alkaline substance1 employed largely in building for making mortars or cements1 in which case its binding property is mainly due to the absorption of carbonic anhydride4#dj ,lime1 like other alkalis1 acts on many animal and vegetable substances1 and for this reason has many practical uses--for example1 for removing fats1 and in agriculture for accelerating the decomposition of organic substances in the so-called composts or accumulations of vegetable and animal remains used for fertilising land4 ,calcium hydroxide easily loses its water at a moderate heat 7#ecj71 but it does not part with water at #ajj4 ,when mixed with water1 lime forms a pasty mass known as slaked lime and in a more dilute form as milk of lime1 because when shaken up in water it remains suspended in it for a long time and presents the appearance of a milky liquid4 ,but1 besides this1 lime is directly soluble in water1 not to any considerable extent1 but still in such a quantity that lime water is precipitated by carbonic anhydride1 and has clearly distinguishable alkaline properties4 ,one part of lime requires at the ordinary temperature about #hjj parts of water for solution4 ,at #ajj it requires about #aejj parts of water1 and therefore lime-water becomes cloudy when boiled4 ,if lime-water be evaporated in a vacuum1 calcium hydroxide separates in six-sided crystals4#da ,if lime-water be mixed with hydrogen peroxide minute crystals of calcium peroxide1 ,ca,o#b1h;,h#b,o1 separate2 this compound is very unstable and1 like barium peroxide1 is decomposed by heat4 ,lime1 as a powerful base1 combines with all acids1 and in this respect presents a transition from the true alkalis to magnesia4 ,many of the salts of calcium 7the carbonate1 phosphate1 borate1 and oxalate7 are insoluble in water2 besides which the sulphate is only sparingly soluble4 ,as a more energetic base than magnesia1 lime forms salts1 ,ca,x#b1 which are distinguished by their stability in comparison with the salts ,mg,x#b2 neither does lime so easily form basic and double salts as magnesia4 #ci ,lime1 when raised to a white heat in the vapour of potassium1 gives calcium1 and in chlorine it gives off oxygen4 ,sulphur1 phosphorus1 c41 when heated with lime1 are absorbed by it4 #dj ,the greater quantity of lime is used in making mortar for binding bricks or stones together1 in the form of lime or cement1 or the so-called slaked lime4 ,for this purpose the lime is mixed with water and sand1 which serves to separate the particles of lime from each other4 ,if only lime paste were put between two bricks they would not hold firmly together1 because after the water had evaporated the lime would occupy a smaller space than before1 and therefore cracks and powder would form in its mass1 so that it would not at all produce that complete cementation of the bricks which it is desired to attain4 ,pieces of stone--that is1 sand--mixed with the lime hinder this process of disintegration1 because the lime binds together the individual grains of sand mixed with it1 and forms one concrete mass1 in consequence of a process which proceeds after the desiccation or removal of the water4 ,the process of the solidification of lime1 taken as slaked lime1 consists first in the direct evaporation of the water and crystallisation of the hydrate1 so that the lime binds the stones and sand mixed with it1 just as glue binds two pieces of wood4 ,but this preliminary binding action of lime is feeble 7as is seen by direct experiment7 unless there be further alteration of the lime leading to the formation of carbonates1 silicates1 and other salts of calcium which are distinguished by their great cohesiveness4 ,with the progress of time the cement is partially subjected to the action of the carbonic anhydride in the air1 owing to which calcium carbonate is formed1 but not more than half the lime is thus converted into carbonate4 ,besides which1 the lime partially acts on the silica of the bricks1 and it is owing to these new combinations simultaneously forming in the cement that it gradually becomes stronger and stronger4 ,hence the binding action of the lime becomes stronger with the lapse of time4 ,this is the reason 7and not1 as is sometimes said1 because the ancients knew how to build stronger than we do7 why buildings which have stood for centuries possess a very strongly binding cement4 ,hydraulic cements will be described later 7,chapter ,,xviii41 ,note #be74 #da ,professor ,glinka measured the transparent bright crystals of calcium hydroxide which are formed in common hydraulic 7,portland7 cement4 ,anhydrous lime does not absorb dry carbonic anhydride at the ordinary temperature4 ,this was already known by ,scheele1 and ,prof4 ,schuliachenko showed that there is no absorption even at #cfj4 ,it only proceeds at a red heat1#db and then only leads to the formation of a mixture of calcium oxide and carbonate 7,rose74 ,but if the lime be slaked or dissolved1 the absorption of carbonic anhydride proceeds rapidly and completely4 ,these phenomena are connected with the dissociation of calcium carbonate1 studied by ,debray 7#ahfg7 under the influence of the conceptions of dissociation introduced into science by ,henri ,saint-,claire ,deville4 ,just as there is no vapour tension for non-volatile substances1 so there is no dissociation tension of carbonic anhydride for calcium carbonate at the ordinary temperature4 ,just as every volatile substance has a maximum possible vapour tension for every temperature1 so also calcium carbonate has its corresponding dissociation tension2 this at #ggj 7the boiling point of cadmium7 is about #he mm4 7of the mercury column71 and at #icj 7the boiling point of ,zn7 it is about #ebj mm4 ,as1 if the tension be greater1 there will be no evaporation1 so also there will he no decomposition4 ,debray took crystals of calc spar1 and could not observe the least change in them at the boiling point of zinc 7#icj7 in an atmosphere of carbonic anhydride taken at the atmospheric pressure 7#gfj mm471 whilst on the other hand calcium carbonate may be completely decomposed at a much lower temperature if the tension of the carbonic anhydride be kept below the dissociation tension1 which may be done either by directly pumping away the gas with an air-pump1 or by mixing it with some other gas--that is1 by diminishing the partial pressure of the carbonic anhydride1#dc just as an object may be dried at the ordinary temperature by removing the aqueous vapour or by carrying it off in a stream of another gas4 ,thus it is possible to obtain calcium carbonate from lime and carbonic anhydride at a certain temperature above that at which dissociation begins1 and conversely to decompose calcium carbonate at the same temperature into lime and carbonic anhydride4#dd ,at the ordinary temperature the reaction of the first order 7combination7 cannot proceed because the second 7decomposition1 dissociation7 cannot take place1 and thus all the most important phenomena with respect to the behaviour of lime towards carbonic anhydride are explained by starting from one common basis4#de #db ,the act of heating brings the substance into that state of internal motion which is required for reaction4 ,it should be considered that by the act of heating not only is the bond between the parts1 or cohesion of the molecules1 altered 7generally diminished71 not only is the motion or store of energy of the whole molecule increased1 but also that in all probability the motion of the atoms themselves in molecules undergoes a change4 ,the same kind of change is accomplished by the act of solution1 or of combination in general1 judging from the fact that a dissolved or combined substance--for instance1 lime with water--reacts on carbonic anhydride as it does under the action of heat4 ,for the comprehension of chemical phenomena it is exceedingly useful to recognise clearly this parallelism4 ,rose's observation on the formation 7by the slow diffusion of solutions of calcium chloride and sodium carbonate7 of aragonite from dilute1 and of calc spar from strong1 solutions is easily understood from this point of view4 ,as aragonite is always formed from hot solutions1 it appears that dilution with water acts like heat4 ,the following experiment of ,khlmann is particularly instructive in this sense4 ,anhydrous 7perfectly dry7 barium oxide does not react with monohydrated sulphuric acid1 ,h#b,,so#d 7containing neither free water nor anhydride1 ,,so#c74 ,but if either an incandescent object or a moist substance is brought into contact with the mixture a violent reaction immediately begins 7it is essentially the same as combustion71 and the whole mass reacts4 ,the influence of solution on the process of reaction is instructively illustrated by the following experiment4 ,lime1 or barium oxide1 is placed in a flask or retort having an upper orifice and connected with a tube immersed in mercury4 ,a funnel furnished with a stopcock and filled with water is fixed into the upper orifice of the retort1 which is then filled with dry carbonic anhydride4 ,there is no absorption4 ,when a constant temperature is arrived at1 the unslaked oxide is made to absorb all the carbonic anhydride by carefully admitting water4 ,a vacuum is formed1 as is seen by the mercury rising in the neck of the retort4 ,with water the absorption goes on to the end1 whilst under the action of heat there remains the dissociating tension of the carbonic anhydride4 ,furthermore1 we here see that1 with a certain resemblance1 there is also a distinction1 depending on the fact that at low temperatures calcium carbonate does not dissociate2 this determines the complete absorption of the carbonic anhydride in the aqueous solution4 #dc ,experience has shown that by moistening partially-burnt lime with water and reheating it1 it is easy to drive off the last traces of carbonic anhydride from it1 and that1 in general1 by blowing air or steam through the lime1 and even by using moist fuel1 it is possible to accelerate the decomposition of the calcium carbonate4 ,the partial pressure is decreased by these means4 #dd ,before the introduction of ,deville's theory of dissociation1 the modus operandi of decompositions like that under consideration was understood in the sense that decomposition starts at a certain temperature1 and that it is accelerated by a rise of temperature1 but it was not considered possible that combination could proceed at the same temperature as that at which decomposition goes on4 ,berthollet and ,deville introduced the conception of equilibrium into chemical science1 and elucidated the question of reversible reactions4 ,naturally the subject is still far from being clear--the questions of the rate and completeness of reaction1 of contact1 c41 still intrude themselves--but an important step has been made in chemical mechanics1 and we have started on a new path which promises further progress1 towards which much has been done not only by ,deville himself1 but more especially by the ,french chemists ,debray1 ,troost1 ,lemoine1 ,hautefeuille1 ,le ,chatelier1 and others4 ,among other things those investigators have shown the close resemblance between the phenomena of evaporation and dissociation1 and pointed out that the amount of heat absorbed by a dissociating substance may be calculated according to the law of the variation of dissociation-pressure1 in exactly the same manner as it is possible to calculate the latent heat of the evaporation of water1 knowing the variation of the tension with the temperature1 on the basis of the second law of the mechanical theory of heat4 ,details of this subject must be looked for in special works on physical chemistry4 ,one and the same conception of the mechanical theory of heat is applicable to dissociation and evaporation4 #de ,but the question as to the formation of a basic calcium carbonate with a rise of temperature still remains undecided4 ,the presence of water complicates all the relations between lime and carbonic anhydride1 all the more as the existence of an attraction between calcium carbonate and water is seen from its being able to give a crystallo-hydrate1 ,ca,c,o#c1e;,h#b,o 7,pelouze71 which crystallises in rhombic prisms of sp4 gr4 about #agg and loses its water at #bj4 ,these crystals are obtained when a solution of lime in sugar and water is left long exposed to the air and slowly attracts carbonic anhydride from it1 and also by the evaporation of such a solution at a temperature of about #c4 ,on the other band1 it is probable that an acid salt1 ,ca,h#b7;,,co#c7b1 is formed in an aqueous solution1 not only because water containing carbonic acid dissolves calcium carbonate1 but more especially in view of the researches of ,schloesing 7#ahgb71 which showed that at #af a litre of water in an atmosphere of carbonic anhydride 7pressure #jihd atmosphere7 dissolves #ajhf gram of calcium carbonate and #aggh gram of carbonic anhydride1 which corresponds with the formation of calcium hydrogen carbonate1 and the solution of carbonic anhydride in the remaining water4 ,caro showed that a litre of water is able to dissolve as much as #c grams of calcium carbonate if the pressure be increased to #d and more atmospheres4 ,the calcium carbonate is precipitated when the carbonic anhydride passes off in the air or in a current of another gas2 this also takes place in many natural springs4 ,tufa1 stalactites1 and other like formations from waters containing calcium carbonate and carbonic acid in solution are formed in this manner4 ,the solubility of calcium carbonate itself at the ordinary temperature does not exceed #ac milligrams per litre of water4 ,calcium carbonate1 ,ca,c,o#c1 is sometimes met with in nature in a crystalline form1 and it forms an example of the phenomenon termed dimorphism--that is1 it appears in two crystalline forms4 ,when it exhibits combinations of forms belonging to the hexagonal system 7six-sided prisms1 rhombohedra1 c47 it is called calc spar4 ,calc spar has a specific gravity of #bg1 and is further characterised by a distinct cleavage along the planes of the fundamental rhombohedron having an angle of #aje4 ,perfectly transparent ,iceland spar presents a clear example of double refraction 7for which reason it is frequently employed in physical apparatus74 ,the other form of calcium carbonate occurs in crystals belonging to the rhombic system1 and it is then called aragonite2 its specific gravity is #cj4 ,if calcium carbonate be artificially produced by slow crystallisation at the ordinary temperature1 it appears in the rhombohedral form1 but if the crystallisation be aided by heat it then appears as aragonite4 ,it may therefore be supposed that calc spar presents the form corresponding with a low temperature1 and aragonite with a higher temperature during crystallisation4#df #df ,dimorphous bodies differ from true isomers and polymers in that they do not differ in their chemical reactions1 which are determined by a difference in the distribution 7motion7 of the atoms in the molecules1 and therefore dimorphism is usually ascribed to a difference in the distribution of similar molecules1 building up a crystal4 ,although such a hypothesis is quite admissible in the spirit of the atomic and molecular theory1 yet1 as in such a redistribution of the molecules a perfect conservation of the distribution of the atoms in them cannot be imagined1 and in every effort of chemical reaction there must take place a certain motion among the atoms2 so in my opinion there is no firm basis for distinguishing dimorphism from the general conception of isomerism1 under which the cases of those organic bodies which are dextro and lvo rotatory 7with respect to polarised light7 have recently been brought with such brilliant success4 ,when calcium carbonate separates out from solutions1 it has at first a gelatinous appearance1 which leads to the supposition that this salt appears in a colloidal state4 ,it only crystallises with the progress of time4 ,the colloidal state of calcium carbonate is particularly clear from the following observations made by ,prof4 ,famintzin1 who showed that when it separates from solutions it is obtained under certain conditions in the form of grains having the peculiar paste-like structure proper to starch1 which fact has not only an independent interest1 but presents an example of a mineral substance being obtained in a form until then only known in the organic substances elaborated in plants4 ,this shows that the forms 7cells1 vessels1 c47 in which vegetable and animal substances occur in organisms do not present in themselves anything peculiar to organisms1 but are only the result of those particular conditions in which these substances are formed4 ,traube and afterwards ,monnier and ,vogt 7#ahhb7 obtained formations which1 under the microscope1 were in every respect identical in appearance with vegetable cells1 by means of a similar slow formation of precipitates 7by reacting on sulphates of different metals with sodium silicate or carbonate74 ,calcium sulphate in combination with two equivalents of water1 ,ca,s,o#d1b;,h#b,o1 is very widely distributed in nature1 and is known as gypsum4 ,gypsum loses one and a half and two equivalents of water at a moderate temperature1#dg and anhydrous or burnt gypsum is then obtained1 which is also known as plaster of ,paris1 and is employed in large quantities for modelling4#dh ,this use depends on the fact that burnt and finely-divided and sifted gypsum forms a paste when mixed with water2 after a certain time this paste becomes slightly heated and solidifies1 owing to the fact that the anhydrous calcium sulphate1 ,ca,s,o#d1 again combines with water4 ,when the plaster of ,paris and water are first made into a paste they form a mechanical mixture1 but when the mass solidifies1 then a compound of the calcium sulphate with two molecules of water is produced2 and this may be regarded as derived from ,s7,,oh7#f by the substitution of two atoms of hydrogen by one atom of bivalent calcium4 ,natural gypsum sometimes appears as perfectly colourless1 or variegated1 marble-like1 masses1 and sometimes in perfectly colourless crystals1 selenite1 of specific gravity #bcc4 ,the semi-transparent gypsum1 or alabaster1 is often carved into small statues4 ,besides which an anhydrous calcium sulphate1 ,ca,s,o#d1 called anhydrite 7specific gravity #big71 occurs in nature4 ,it sometimes occurs along with gypsum4 ,it is no longer capable of combining directly with water1 and differs in this respect from the anhydrous salt obtained by gently igniting gypsum4 ,if gypsum be very strongly heated it shrinks and loses its power of combining with water4#dh bis ,one part of calcium sulphate requires at #j #ebe parts of water for solution1 at #ch #dff parts1 and at #ajj #ega parts of water4 ,the maximum solubility of gypsum is at about #cf1 which is nearly the same temperature as that at which sodium sulphate is most soluble4#di #dg ,according to ,le ,chatelier 7#ahhh71 #a-a_/b;,h#b,o is lost at #abj--that is1 ,h#b,o1#b;,ca,s,o#d is formed1 but at #aid all the water is expelled4 ,according to ,shenstone and ,cundall 7#ahhh7 gypsum begins to lose water at #gj in dry air4 ,the semi-hydrated compound ,h#b,o1#b;,ca,s,o#d is also formed when gypsum is heated with water in a closed vessel at #aej 7,hoppe-,seyler74 #dh ,for stucco-work it is usual to add lime and sand1 as the mass is then harder and does not solidify so quickly4 ,for imitating marble1 glue is added to the plaster1 and the mass is polished when thoroughly dry4 ,re-burnt gypsum cannot be used over again1 as that which has once solidified is1 like the natural anhydride1 not able to recombine with water4 ,it is evident that the structure of the molecules in the crystallised mass1 or in general in any dense mass1 exerts an influence on the chemical action1 which is more particularly evident in metals in their different forms 7powder1 crystalline1 rolled1 c47 #dh bis ,according to ,mac,coleb1 gypsum dehydrated at #bjj has a specific gravity #begg1 and heated to its point of fusion1 #bfed4 ,potilitzin 7#ahid7 also admits the two above-named modifications of anhydrous gypsum1 which1 moreover1 always contain the semi-hydrated hydrate 7,note #dg71 and he explains by their relation to water the phenomena observed in the solidification of a mixture of burnt gypsum and water4 #di ,as ,marignac showed1 gypsum1 especially when desiccated at #abj1 easily gives supersaturated solutions with respect to ,ca,s,o#d1b;,h#b,o1 which contain as much as #a part of ,ca,s,o#d to #aaj parts of water4 ,boiling dilute hydrochloric acid dissolves gypsum1 forming calcium chloride4 ,the behaviour of gypsum towards the alkaline carbonates has been described in ,chapter ,x4 ,alcohol precipitates gypsum from its aqueous solutions1 because1 like the sulphates in general1 it is sparingly soluble in alcohol4 ,gypsum1 like all the sulphates1 when heated with charcoal1 gives up its oxygen1 forming the sulphide1 ,ca,s4 ,calcium ulphate1 like magnesium sulphate1 is capable of forming double salts1 but with difficulty1 and they are chemically less stable4 ,they contain1 as is always the case with double salts1 less water of crystallisation than the component salts4 ,rose1 ,struv1 and others obtained the salt ,ca,k#b7,,so#d7b1;,h#b,o2 a mixture of gypsum with an equivalent amount of potassium sulphate and water solidifies into a homogeneous mass4 ,fritzsche obtained the corresponding sodium salt in a hydrated and anhydrous state1 by heating a mixture of gypsum with a saturated solution of sodium sulphate4 ,the anhydrous salt occurs in nature as glauberite4 ,fritzsche also obtained gaylussite1 ,na#b;,ca7,,co#c7b1e;,h#b,o1 by pouring a saturated solution of sodium carbonate on to freshly-precipitated calcium carbonate4 ,calcium also forms basic salts1 but only a few4 ,veeren 7#ahib7 obtained ,ca7,,no#c7b;,ca7,,oh7#b1b-a_/b;,h#b,o by leaving powdered caustic lime in a saturated solution of ,ca7,,no#c7b until it solidified4 ,this salt is decomposed by water4 ,as lime is a more energetic base than magnesia1 so calcium chloride1 ,ca,cl#b1 is not so easily decomposed by water1 and its solutions only disengage a small quantity of hydrochloric acid when evaporated1 and when the evaporation is conducted in a stream of hydrochloric acid it easily gives an anhydrous salt which fuses at #gai2 otherwise an aqueous solution yields a crystallo-hydrate1 ,ca,cl#b1f;,h#b,o1 which melts at #cj4ej #ej ,calcium chloride has a specific gravity #bbj1 or1 when fused1 #bab1 and the sp4 gr4 of the crystallised salt ,ca,cl#b1f;,h#b,o is #afi4 ,if the volume of the crystals at #j #a1 then at #bi it is #ajbj1 and the volume of the fused mass at the same temperature is #aaah 7,kopp7 7specific gravity of solutions1 see ,note #bg74 ,the solution containing #ej p4c4 ,ca,cl#b boils at #acj1 #gj p4c4 at #aeh4 ,superheated steam decomposes calcium chloride with more difficulty than magnesium chloride and with greater ease than barium chloride 7,kuhnheim74 ,sodium does not decompose fused calcium chloride even on prolonged heating 7,lis-,bodart71 but an alloy of sodium with zinc1 lead1 and bismuth decomposes it1 forming an alloy of calcium with one of the above-named metals 7,caron74 ,the zinc alloy may be obtained with as much as #ae p4c4 of calcium4 ,calcium chloride is soluble in alcohol and absorbs ammonia4 ,a gram molecular weight of calcium chloride in dissolving in an excess of water evolves #ah1gbc calories1 and in dissolving in alcohol #ag1eee units of heat1 according to ,pickering4 ,roozeboom made detailed researches on the crystallo-hydrates of calcium chloride 7#ahhi71 and found that ,ca,cl#b1f;,h#b,o melts at #cjb1 and is formed at low temperatures from solutions containing not more than #ajc parts of calcium chloride per #ajj parts of water2 if the amount of salt 7always to #ajj parts of water7 reaches #abj parts1 then tabular crystals of ,ca,cl#b1d;,h#b,o,greek3 b are formed1 which at temperatures above #chd are converted into the crystallo-hydrates ,ca,cl#b1b;,h#b,o1 whilst at temperatures below #ah the ,greek3 b variety passes into the more stable ,ca,cl#b1d;,h#b,o,greek3 a1 which process is aided by mechanical friction4 ,hence1 as is the case with magnesium sulphate 7,note #bg71 one and the same crystallo-hydrate appears in two forms--the ,greek3 b1 which is easily produced but is unstable1 and the ,greek3 a1 which is stable4 ,the solubility of the above-mentioned hydrates of chloride of calcium1 or amount of calcium chloride per #ajj parts of water1 is as follows3-- #j #bj #cj #dj #fj ,ca,cl#b1f;,h#b,o #fj #ge #ajj 7#ajbh7 ,ca,cl#b1d;,h#b,o,greek3 a -- #ij #aja #aag 7#aedb7 ,ca,cl#b1d;,h#b,o,greek3 b -- #ajd #aad -- ,ca,cl#b1b;,h#b,o -- -- 7#cjhc7 #abh #acg ,the amount of calcium chloride to #ajj parts of water in the crystallo-hydrate is given in brackets4 ,the point of intersection of the curves of solubility lies at about #cj for the first two salts and about #de for the salts with #d;,h#b,o and #b;,h#b,o4 ,the crystals ,ca,cl#b1b;,h#b,o may1 however1 be obtained 7,ditte7 at the ordinary temperature from solutions containing hydrochloric acid4 ,the vapour tension of this crystallo-hydrate equals the atmospheric at #afe1 and therefore the crystals may be dried in an atmosphere of steam and obtained without a mother liquor1 whose vapour tension is greater4 ,this crystallo-hydrate decomposes at about #age into ,ca,cl#b1;,h#b,o and a solution2 this is easily brought about in a closed vessel when the pressure is greater than the atmosphere4 ,this crystallo-hydrate is destroyed at temperatures above #bfj1 anhydrous calcium chloride being formed4 ,neglecting the unstable modification ,ca,cl#b1d;,h#b,o,greek3 b1 we will give the temperatures t at which the passage of one hydrate into another takes place and at which the solution ,ca,cl#b n,h#b,o1 the two solids ,a and ,b and aqueous vapour1 whose tension is given as p in millimetres1 are able to exist together in stable equilibrium1 according to ,roozeboom's determinations3 t n ,a ,b p -#ee #ade ice ,ca,cl#b1f;,h#b,o #j #bih #fa ,ca,cl#b1f;,h#b,o ,ca,cl#b1d;,h#b,o #fh #dec #dg ,ca,cl#b1d;,h#b,o ,ca,cl#b1b;,h#b,o #aah #agee #ba ,ca,cl#b1b;,h#b,o ,ca,cl#b1;,h#b,o #hdb #bfj #ah ,ca,cl#b1;,h#b,o ,ca,cl#b ,several atmospheres ,solutions of calcium chloride may serve as a convenient example for the study of the supersaturated state1 which in this case easily occurs1 because different hydrates are formed4 ,thus at #be solutions containing more than #hc parts of anhydrous calcium chloride per #ajj of water will be supersaturated for the hydrate ,ca,cl#b1f;,h#b,o4 ,on the other hand1 ,hammerl showed that solutions of calcium chloride1 when frozen1 deposit ice if they contain less than #dc parts of salt per #ajj of water1 and if more the crystallo-hydrate ,ca,cl#b1f;,h#b,o separates1 and that a solution of the above composition 7,ca,cl#b1ad;,h#b,o requires #ddj parts calcium chloride per #ajj of water7 solidifies as a cryohydrate at about -#ee4 ,just as for potassium1 ,k #ci 7and sodium1 ,na #bc71 there are the near analogues1 ,rb #he and ,cs #acc1 and also another1 ,li #g1 so in exactly the same manner for calcium1 ,ca #dj 7and magnesium1 ,mg #bd71 there is another analogue of lighter atomic weight1 beryllium1 ,be #i1 besides the near analogues strontium1 ,sr #hg1 and barium1 ,ba #acg4 ,as rubidium and csium are more rarely met with in nature than potassium1 so also strontium and barium are rarer than calcium 7in the same way that bromine and iodine are rarer than chlorine74 ,since they exhibit many points of resemblance with calcium1 strontium and barium may be characterised after a very short acquaintance with their chief compounds2 this shows the important advantages gained by distributing the elements according to their natural groups1 to which matter we shall turn our attention in the next chapter4 ,among the compounds of barium met with in nature the commonest is the sulphate1 ,ba,s,o#d1 which forms anhydrous crystals of the rhombic system1 which are identical in their crystalline form with anhydrite1 and generally occur as transparent and semi-transparent masses of tabular crystals having a high specific gravity1 namely #dde1 for which reason this salt bears the name of heavy spar or barytes4 ,analogous to it is celestine1 ,sr,s,o#d1 which is1 however1 more rarely met with4 ,heavy spar frequently forms the gangue separated on dressing metallic ores from the vein stuff2 this mineral is the source of all other barium compounds2 for the carbonate1 although more easily transformed into the other compounds 7because acids act directly on it1 evolving carbonic anhydride71 is a comparatively rare mineral 7,ba,c,o#c forms the mineral witherite2 ,sr,c,o#c1 strontianite2 both are rare1 the latter is found at ,etna74 ,the treatment of barium sulphate is rendered difficult from the fact that it is insoluble both in water and acids1 and has therefore to be treated by a method of reduction4#ea ,like sodium sulphate and calcium sulphate1 heavy spar when heated with charcoal parts with its oxygen and forms barium sulphide1 ,ba,s4 ,for this purpose a pasty mixture of powdered heavy spar1 charcoal1 and tar is subjected to the action of a strong heat1 when ,ba,s,o#d #d;,c ,ba,s #d;,,co4 ,the residue is then treated with water1 in which the barium sulphide is soluble4#eb ,when boiled with hydrochloric acid1 barium chloride1 ,ba,cl#b1 is obtained in solution1 and the sulphur is disengaged as gaseous sulphuretted hydrogen1 ,ba,s #b;,h,cl ,ba,cl#b ,h#b,s4 ,in this manner barium sulphate is converted into barium chloride1#ec and the latter by double decomposition with strong nitric acid or nitre gives the less soluble barium nitrate1 ,ba7,,no#c7b1ed or with sodium carbonate a precipitate of barium carbonate1 ,ba,c,o#c4 ,both these salts are able to give barium oxide1 or baryta1 ,ba,o1 and the hydroxide1 ,ba7,,ho7#b1 which differs from lime by its great solubility in water1#ee and by the ease with which it forms a crystallo-hydrate1 ,ba,h#b,o#b1h;,h#b,o1 from its solutions4 ,owing to its solubility1 baryta is frequently employed in manufactures and in practical chemistry as an alkali which has the very important property that it may be always entirely removed from solution by the addition of sulphuric acid1 which entirely separates it as the insoluble barium sulphate1 ,ba,s,o#d4 ,it may also be removed whilst it remains in an alkaline state 7for example1 the excess which may remain when it is used for saturating acids7 by means of carbonic anhydride1 which also completely precipitates baryta as a sparingly soluble1 colourless1 and powdery carbonate4 ,both these reactions show that baryta has such properties as would very greatly extend its use were its compounds as widely distributed as those of sodium and calcium1 and were its soluble compounds not poisonous4 ,barium nitrate is directly decomposed by the action of heat1 barium oxide being left behind4 ,the same takes place with barium carbonate1 especially that form of it precipitated from solutions1 and when mixed with charcoal or ignited in an atmosphere of steam4 ,barium oxide combines with water with the development of a large amount of heat1 and the resultant hydroxide is very stable in its retention of the water1 although it parts with it when strongly ignited4#ee bis ,with oxygen the anhydrous oxide gives1 as already mentioned in ,chapters ,,iii4 and ,,iv41 a peroxide1 ,ba,o#b4ef ,neither calcium nor strontium oxides are able to give such a peroxide directly1 but they form peroxides under the action of hydrogen peroxide4 #ea ,the action of barium sulphate on sodium and potassium carbonates is given on p4 #dcg4 #eb ,barium sulphide is decomposed by water1 ,ba,s #b;,h#b,o ,h#b,s ,ba7,,oh7#b 7the reaction is reversible71 but both substances are soluble in water1 and their separation is complicated by the fact that barium sulphide absorbs oxygen and gives insoluble barium sulphate4 ,the hydrogen sulphide is sometimes removed from the solution by boiling with the oxides of copper or zinc4 ,if sugar be added to a solution of barium sulphide1 barium saccharate is precipitated on heating2 it is decomposed by carbonic anhydride1 so that barium carbonate is formed4 ,an equivalent mixture of sodium sulphate with barium or strontium sulphates when ignited with charcoal gives a mixture of sodium sulphide and barium or strontium sulphide1 and if this mixture be dissolved in water and the solution evaporated1 barium or strontium hydroxide crystallises out on cooling1 and sodium hydrosulphide1 ,na,h,s1 is obtained in solution4 ,the hydroxides ,ba,h#b,o#b and ,sr,h#b,o#b are prepared on a large scale1 being applied to many reactions2 for example1 strontium hydroxide is prepared for sugar works for extracting crystallisable sugar from molasses4 ,we may remark that ,boussingault1 by igniting barium sulphate in hydrochloric acid gas1 obtained a complete decomposition1 with the formation of barium chloride4 ,attention should also be turned to the fact that ,grouven1 by beating a mixture of charcoal and strontium sulphate with magnesium and potassium sulphates1 showed the easy decomposability depending on the formation of double salts1 such as ,sr,s1,k#b,s1 which are easily soluble in water1 and give a precipitate of strontium carbonate with carbonic anhydride4 ,in such examples as these we see that the force which binds double salts may play a part in directing the course of reactions1 and the number of double salts of silica on the earth's surface shows that nature takes advantage of these forces in her chemical processes4 ,it is worthy of remark that ,buchner 7#ahic71 by mixing a #dj per cent4 solution of barium acetate with a #fj per cent4 solution of sulphate of alumina1 obtained a thick glutinous mass1 which only gave a precipitate of ,ba,s,o#d after being diluted with water4 #ec ,barium sulphate is sometimes converted into barium chloride in the following manner3 finely-ground barium sulphate is heated with coal and manganese chloride 7the residue from the manufacture of chlorine74 ,the mass becomes semi-liquid1 and when it evolves carbonic oxide the heating is stopped4 ,the following double decompositions proceed during this operation3 first the carbon takes up the oxygen from the barium sulphate1 and gives sulphide1 ,ba,s1 which enters into double decomposition with the chloride of manganese1 ,mn,cl#b1 forming manganese sulphide1 ,mn,s1 which is insoluble in water1 and soluble barium chloride4 ,this solution is easily obtained pure because many foreign impurities1 such as iron1 remain in the insoluble portion with the manganese4 ,the solution of barium chloride is chiefly used for the preparation of barium sulphate1 which is precipitated by sulphuric acid1 by which means barium sulphate is re-formed as a powder4 ,this salt is characterised by the fact that it is unacted on by the majority of chemical reagents1 is insoluble in water1 and is not dissolved by acids4 ,owing to this1 artificial barium sulphate forms a permanent white paint which is used instead of 7and mixed with7 white lead1 and has been termed 'blanc fix' or 'permanent white4' ,the solution of one part of calcium chloride at #bj requires #acf part of water1 the solution of one part of strontium chloride requires #ahh part of water at the same temperature1 and the solution of barium chloride #bhh parts of water4 ,the solubility of the bromides and iodides varies in the same proportion4 ,the chlorides of barium and strontium crystallise out from solution with great ease in combination with water2 they form ,ba,cl#b1b;,h#b,o and ,sr,cl#b1f;,h#b,o4 ,the latter 7which separates out at #dj7 resembles the salts of ,ca and ,mg in composition1 and tard 7#ahib7 obtained ,sr,cl#b1b;,h#b,o from solutions at #ij-acj4 ,we may also observe that the crystallo-hydrates ,ba,br#b1;,h#b,o and ,ba,i#b1g;,h#b,o are known4 #ed ,the nitrates ,sr7,,no#c7b 7in the cold its solutions give a crystallo-hydrate containing #d;,h#b,o7 and ,ba7,,no#c7b are so very sparingly soluble in water that they separate in considerable quantities when a solution of sodium nitrate is added to a strong solution of either barium or strontium chloride4 ,they are obtained by the action of nitric acid on the carbonates or oxides4 #ajj parts of water at #ae dissolve #fe parts of strontium nitrate and #hb parts of barium nitrate1 whilst more than #cjj parts of calcium nitrate are soluble at the same temperature4 ,strontium nitrate communicates a crimson coloration to the flame of burning substances1 and is therefore frequently used for ,bengal fire1 fireworks1 and signal lights1 for which purpose the salts of lithium are still better fitted4 ,calcium nitrate is exceedingly hygroscopic4 ,barium nitrate1 on the contrary1 does not show this property in the least degree1 and in this respect it resembles potassium nitrate1 and is therefore used instead of the latter for the preparation of a gunpowder which is called 'saxifragin powder' 7#gf parts of barium nitrate1 #b parts of nitre1 and #bb parts of charcoal74 #ee ,the dissociation of the crystallo-hydrate of baryta is given in ,chapter ,i41 ,note #fe4 #ajj parts of water dissolve #j #bj #dj #fj #hj ,ba,o #ae #ce #gd #ahh #ijh ,sr,o #jc #jg #ad #c #i ,supersaturated solutions are easily formed4 ,the anhydrous oxide ,ba,o fuses in the oxyhydrogen flame4 ,when ignited in the vapour of potassium1 the latter takes up the oxygen2 whilst in chlorine1 oxygen is separated and barium chloride formed4 #ee bis ,brugellmann1 by heating ,ba,h#b,o#b in a graphite or clay crucible1 obtained ,ba,o in needles1 sp4 gr4 #ecb1 and by heating in a platinum crucible--in crystals belonging to the cubical system1 sp4 gr4 #egd4 ,sr,o is obtained in the latter form from the nitrate4 ,the following are the specific gravities of the oxides from different sources3-- ,mg,o ,ca,o ,sr,o from ,,rn#b,o#f #cch #cbe #dge ,,rco#c #cdh #cbf #dde ,,rh#b,o#b #cda #cbe #deg #ef ,the property of barium oxide of absorbing oxygen when heated1 and giving the peroxide1 ,ba,o#b1 is very characteristic for this oxide 7see ,chapter ,,iii41 ,note #g74 ,it only belongs to the anhydrous oxide4 ,the hydroxide does not absorb oxygen4 ,peroxides of calcium and strontium may be obtained by means of hydrogen peroxide4 ,barium peroxide is insoluble in water1 but is able to form a hydrate with it1 and also to combine with hydrogen peroxide1 forming a very unstable compound having the composition ,ba,h#b,o#d 7obtained by ,professor ,schne71 which in course of time evolves oxygen 7,chapter ,,iv41 ,note #ba74 ,barium oxide is decomposed when heated with potassium2 fused barium chloride is decomposed1 as ,davy showed1 by the action of a galvanic current1 forming metallic barium2 and ,crookes 7#ahfb7 obtained an amalgam of barium from which the mercury could easily be driven off1 by heating sodium amalgam in a saturated solution of barium chloride4 ,strontium is obtained by the same processes4 ,both metals are soluble in mercury1 and seem to be non-volatile or only very slightly volatile4 ,they are both heavier than water2 the specific gravity of barium is #cf1 and of strontium #be4 ,they both decompose water at the ordinary temperature1 like the metals of the alkalis4 ,barium and strontium as saline elements are characterised by their powerful basic properties1 so that they form acid salts with difficulty1 and scarcely form basic salts4 ,on comparing them together and with calcium1 it is evident that the alkaline properties in this group 7as in the group potassium1 rubidium1 csium7 increase with the atomic weight1 and this succession clearly shows itself in many of their corresponding compounds4 ,thus1 for instance1 the solubility of the hydroxides ,,rh#b,o#b and the specific gravity#eg rise in passing from calcium to strontium and barium1 while the solubility of the sulphates decreases1#eh and therefore in the case of magnesium and beryllium1 as metals whose atomic weights are still less1 we should expect the solubility of the sulphates to be greater1 and this is in reality the case4 #eg ,even in solutions a gradual progression in the increase of the specific gravity shows itself1 not only for equivalent solutions 7for instance1 ,r,cl#b #bjj;,h#b,o71 but even with an equal percentage composition1 as is seen from the curves giving the specific gravity 7water #d #aj1jjj7 at #ae 7for barium chloride1 according to ,bourdiakoff's determinations73 ,be,cl#b 3 ,s #i1iib #fgbap #jaaap#b ,ca,cl#b 3 ,s #i1iib #hjbdp #jdgfp#b ,sr,cl#b 3 ,s #i1iib #heegp #jgccp#b ,ba,cl#b 3 ,s #i1iib #hfefp #jhacp#b #eh ,one part of calcium sulphate at the ordinary temperature requires about #ejj parts of water for solution1 strontium sulphate about #g1jjj parts1 barium sulphate about #djj1jjj parts1 whilst beryllium sulphate is easily soluble in water4 ,just as in the series of the alkali metals we saw the metals potassium1 rubidium1 and csium approaching near to each other in their properties1 and allied to them two metals having smaller combining weights--namely1 sodium1 and the lightest of all1 lithium1 which all exhibited certain peculiar characteristic properties--so also in the case of the metals of the alkaline earths we find1 besides calcium1 barium1 and strontium1 the metal magnesium and also beryllium or glucinum4 ,in respect to the magnitude of its atomic weight1 this last occupies the same position in the series of the metals of the alkaline earths as lithium does in the series of the alkali metals1 for the combining weight of beryllium1 ,be or ,gl #i4 ,this combining weight is greater than that of lithium 7#g71 as the combining weight of magnesium 7#bd7 is greater than that of sodium 7#bc71 and as that of calcium 7#dj7 is greater than that of potassium 7#ci71 c4#ei ,beryllium was so named because it occurs in the mineral beryl4 ,the metal is also called glucinum 7from the ,greek word ,greek3 glykys1 'sweet'71 because its salts have a sweet taste4 ,it occurs in beryl1 aquamarine1 the emerald1 and other minerals1 which are generally of a green colour2 they are sometimes found in considerable masses1 but as a rule are comparatively rare and1 as transparent crystals1 form precious stones4 ,the composition of beryl and of the emerald is as follows3 ,al#b,o#c1c;,be,o1#f,si,o#b4 ,the ,siberian and ,brazilian beryls are the best known4 ,the specific gravity of beryl is about #bg4 ,beryllium oxide1 from the feebleness of its basic properties1 presents an analogy to aluminium oxide in the same way that lithium oxide is analogous to magnesium oxide4#fj ,owing to its rare occurrence in nature1 to the absence of any especially distinct individual properties1 and to the possibility of foretelling them to a certain extent on the basis of the periodic system of the elements given in the following chapter1 and owing to the brevity of this treatise1 we will not discuss at any length the compounds of beryllium1 and will only observe that their individuality was pointed out in #agih by ,vauquelin1 and that metallic beryllium was obtained by ,whler and ,bussy4 ,whler obtained metallic beryllium 7like magnesium7 by acting on beryllium chloride1 ,be,cl#b1 with potassium 7it is best prepared by fusing ,k#b;,be,f#d with ,na74 ,metallic beryllium has a specific gravity #afd 7,nilson and ,pettersson74 ,it is very infusible1 melting at nearly the same temperature as silver1 which it resembles in its white colour and lustre4 ,it is characterised by the fact that it is very difficultly oxidised1 and even in the oxidising flame of the blowpipe is only superficially covered by a coating of oxide2 it does not burn in pure oxygen1 and does not decompose water at the ordinary temperature or at a red heat1 but gaseous hydrochloric acid is decomposed by it when slightly heated1 with evolution of hydrogen and development of a considerable amount of heat4 ,even dilute hydrochloric acid acts in the same manner at the ordinary temperature4 ,beryllium also acts easily on sulphuric acid1 but it is remarkable that neither dilute nor strong nitric acid acts on beryllium1 which seems especially able to resist oxidising agents4 ,potassium hydroxide acts on beryllium as on aluminium1 hydrogen being disengaged and the metal dissolved1 but ammonia has no action on it4 ,these properties of metallic beryllium seem to isolate it from the series of the other metals described in this chapter1 but if we compare the properties of calcium1 magnesium1 and beryllium we shall see that magnesium occupies a position intermediate between the other two4 ,whilst calcium decomposes water with great ease1 magnesium does so with difficulty1 and beryllium not at all4 ,the peculiarities of beryllium among the metals of the alkaline earths recall the fact that in the series of the halogens we saw that fluorine differed from the other halogens in many of its properties and had the smallest atomic weight4 ,the same is the case with regard to beryllium among the other metals of the alkaline earths4 #ei ,we refer beryllium to the class of the bivalent metals of the alkaline earths--that is1 we ascribe to its oxide the formula ,be,o1 and do not consider it as trivalent 7,be #ace1 ,chapter ,,vii41 ,note #ba71 although that view has been upheld by many chemists4 ,the true atomic composition of beryllium oxide was first given by the ,russian chemist1 ,avdeff 7#ahai71 in his researches on the compounds of this metal4 ,he compared the compounds of beryllium to those of magnesium1 and refuted the notion prevalent at the time1 of the resemblance between the oxides of beryllium and aluminium1 by proving that beryllium sulphate presents a greater resemblance to magnesium sulphate than to aluminium sulphate4 ,it was especially noticed that the analogues of alumina give alums1 whilst beryllium oxide1 although it is a feeble base1 easily giving1 like magnesia1 basic and double salts1 does not form true alums4 ,the establishment of the periodic system of the elements 7#ahfi71 considered in the following chapter1 immediately indicated that ,avdeff's view corresponded with the truth--that is1 that beryllium is bivalent1 which therefore necessitated the denial of its trivalency4 ,this scientific controversy resulted in a long series of researches 7#ahgj-hj7 concerning this element1 and ended in ,nilson and ,pettersson--two of the chief advocates of the trivalency of beryllium--determining the vapour density of ,be,cl#b #dj1 7,chapter ,,vii41 ,note #ba71 which gave an undoubted proof of its bivalency 7see also ,note #c74 #fj ,beryllium oxide1 like aluminium oxide1 is precipitated from solutions of its salts by alkalis as a gelatinous hydroxide1 ,be,h#b,o#b1 which1 like alumina1 is soluble in an excess of caustic potash or soda4 ,this reaction may be taken advantage of for distinguishing and separating beryllium from aluminium1 because when the alkaline solution is diluted with water and boiled1 beryllium hydroxide is precipitated1 whilst the alumina remains in solution4 ,the solubility of the beryllium oxide at once clearly indicates its feeble basic properties1 and1 as it were1 separates this oxide from the class of the alkaline earths4 ,but on arranging the oxides of the above-described metals of the alkaline earths according to their decreasing atomic weights we have the series ,ba,o1 ,sr,o1 ,ca,o1 ,mg,o1 ,be,o1 in which the basic properties and solubility of the oxides consecutively and distinctly decrease until we reach a point when1 had we not known of the existence of the beryllium oxide1 we should expect to find in its place an oxide insoluble in water and of feeble basic properties4 ,if an alcoholic solution of caustic potash be saturated with the hydrate of ,be,o1 and evaporated under the receiver of an air pump1 it forms silky crystals ,be,k#b,o#b4 ,another characteristic of the salts of beryllium is that they give with aqueous ammonia a gelatinous precipitate which is soluble in an excess of ammonium carbonate like the precipitate of magnesia2 in this beryllium oxide differs from the oxide of aluminium4 ,beryllium oxide easily forms a carbonate which is insoluble in water1 and resembles magnesium carbonate in many respects4 ,beryllium sulphate is distinguished by its considerable solubility in water--thus1 at the ordinary temperature it dissolves in an equal weight of water2 it crystallises out from its solutions in well-formed crystals1 which do not change in the air1 and contain ,be,s,o#d1d;,h#b,o4 ,when ignited it leaves beryllium oxide1 but this oxide1 after prolonged ignition1 is re-dissolved by sulphuric acid1 whilst aluminium sulphate1 after a similar treatment1 leaves aluminium oxide1 which is no longer soluble in acids4 ,with a few exceptions1 the salts of beryllium crystallise with great difficulty1 and to a considerable extent resemble the salts of magnesium2 thus1 for instance1 beryllium chloride is analogous to magnesium chloride4 ,it is volatile in an anhydrous state1 and in a hydrated state it decomposes1 with the evolution of hydrochloric acid4 ,in addition to the above characteristics of the compounds of the metals of the alkaline earths1 we must add that they1 like the alkali metals1 combine with nitrogen and hydrogen1 and while sodium nitride 7obtained by igniting the amide of sodium1 ,chapter ,,xii41 ,note #dd bis7 and lithium nitride 7obtained by heating lithium in nitrogen1 ,chapter ,,xiii41 ,note #ci7 have the composition ,r#c,n1 so the nitrides of magnesium 7,note #ad71 calcium1 strontium1 and barium have the composition ,r#c,n#b1 for example1 ,ba#c,n#b1 as might be expected from the diatomicity of the metals of the alkaline earths and from the relation of the nitrides to ammonia1 which is obtained from all of these compounds by the action of water4 ,the nitrides of ,ca1 ,sr1 and ,ba are formed directly 7,maquenne1 #ahib7 by heating the metals in nitrogen4 ,they all have the appearance of an amorphous powder of dark colour2 as regards their reactions1 it is known that besides disengaging ammonia with water1 they form cyanides when heated with carbonic oxide2 for instance1 ,ba#c,n#b #b;,,co ,ba7,,cn7#b #b;,ba,o4#fa #fa ,thus in the nitrides of the metals we have substances by means of which we can easily obtain from the nitrogen of the air1 not only ammonia1 but also with the aid of ,,co1 by synthesis1 a whole series of complex carbon and nitrogen compounds4 ,the metals of the alkaline earths1 just like ,na and ,k1 absorb hydrogen under certain conditions1 and form pulverulent easily oxidisable metallic hydrides1 whose composition corresponds exactly to that of ,na#b;,h and ,k#b;,h1 with the substitution of ,k#b and ,na#b by the atoms ,be1 ,mg1 ,ca1 ,sr1 and ,ba4 ,the hydrides of the metals of the alkaline earths were discovered by ,c4 ,winkler 7#ahia7 in investigating the reducibility of these metals by magnesium4 ,in reducing their oxides by heating them with magnesium powder in a stream of hydrogen1 ,winkler observed that the hydrogen was absorbed 7but very slowly71 i4e4 at the moment of their separation all the metals of the alkaline earths combine with hydrogen4 ,this absorptive power increases in passing from ,be to ,mg1 ,ca1 ,sr1 and ,ba1 and the resultant hydrides retain the combined hydrogen#fb when heated1 so that these hydrides are distinguished for their considerable stability under heat1 but they oxidise very easily4#fc #fb ,as the hydrides of calcium1 magnesium1 c4 are very stable under the action of heat1 and these metals and hydrogen occur in the sun1 it is likely that the formation of their hydrides may take place there4 7,private communication from ,prof4 ,winkler1 #ahid47 ,it is probable that in the free metals of the alkaline earths hitherto obtained a portion was frequently in combination with nitrogen and hydrogen4 #fc ,thus1 for instance1 a mixture of #ef parts of ,ca,o and #bd parts of magnesium powder is heated in an iron pipe 7placed over a row of gas burners as in the combustion furnace used for organic analysis7 in a stream of hydrogen4 ,after being heated for #a_/b hour the mixture is found to absorb hydrogen 7it no longer passes over the mixture1 but is retained by it74 ,the product1 which is light grey1 and slightly coherent1 disengages a mass of hydrogen when water is poured over it1 and burns when heated in air4 ,the resultant mass contains #cc per cent4 ,ca,h1 about #bh per cent4 ,ca,o1 and about #ch per cent4 ,mg,o4 ,neither ,ca,h nor any other ,,mh has yet been obtained in a pure state4 ,the acetylene derivatives of the metals of the alkaline earths ,c#b,m 7,chapter ,,viii41 ,note #ab bis71 for instance1 ,c#b;,ba1 obtained by ,maquenne and ,moissan1 belong to the same class of analogous compounds4 ,it must here be remarked that the oxides ,,mo of the metals of the alkaline earths1 although not reducible by carbon at a furnace heat1 yet under the action of the heat attained in electrical furnaces1 not only give up their oxygen to carbon 7probably partly owing to the action of the current71 but also combine with carbon4 ,the resultant compounds1 ,c#b,m1 evolve acetylene1 ,c#b;,h#b1 with ,h,cl1 just as ,n#b,m#c give ammonia4 ,we may remark moreover that the series of compounds of the metals of the alkaline earths with hydrogen1 nitrogen and carbon is a discovery of recent years1 and that probably further research will give rise to similar unexpected compounds1 and by extending our knowledge of their reactions prove to be of great interest4 ,thus the analogies and correlation of the metals of these two groups are now clearly marked1 not only in their behaviour towards oxygen1 chlorine1 acids1 c41 but also in their capability of combining with nitrogen and hydrogen4 99 99 99 99 99 ,,end ,,of ,,the ,,first ,,volume ,,printed ,,by ,,spottiswoode ,,and ,,co41 ,,new-,,street ,,square ,,london 99 99 99 99 99 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#bs4 #f;d4 ,,garrett ,,and ,,harden4--,,an ,,elementary ,,course ,,of ,,practical ,,organic ,,chemistry4 ,by ,f4 ,c4 ,,garrett1 ,m4,sc4 7,vict4 et ,dunelm471 ,assistant ,lecturer and ,demonstrator in ,chemistry1 the ,durham ,college of ,science1 ,newcastle-on-,tyne2 and ,,arthur ,,harden1 ,m4,sc4 7,vict471 ,ph4,d41 ,assistant ,lecturer and ,demonstrator in ,chemistry1 the ,owens ,college1 ,manchester4 ,,hjelt4--,,principles ,,of ,,general ,,organic ,,chemistry4 ,by ,professor ,e4 ,,hjelt1 of ,helsingfors4 ,translated from the ,german by ,j4 ,,bishop ,,tingle1 ,ph4,d41 ,assistant in the ,laboratory of the ,heriot ,watt ,college1 ,edinburgh4 ,crown #hvo41 #fs4 #f;d4 ,,jago4--,works by ,w4 ,,jago1 ,f4,c4,s41 ,f4,i4,c4 ,,inorganic ,,chemistry1 ,,theoretical ,,and ,,practical4 ,with an ,introduction to the ,principles of ,chemical ,analysis1 ,inorganic and ,organic4 ,with #fc ,woodcuts and numerous ,questions and ,exercises4 ,fcp4 #hvo41 #bs4 #f;d4 ,,an ,,introduction ,,to ,,practical ,,inorganic ,,chemistry4 ,crown #hvo41 #as4 #f;d4 ,,inorganic ,,chemistry1 ,,theoretical ,,and ,,practical4 ,a ,manual for ,students in ,advanced ,classes of the ,science and ,art ,department4 ,with ,plate of ,spectra and #gh ,woodcuts4 ,crown #hvo41 #ds4 #f;d4 ,,kolbe4--,a ,,short ,,text-,,book ,,of ,,inorganic ,,chemistry4 ,by ,dr4 ,,hermann ,,kolbe4 ,translated and ,edited by ,t4 ,s4 ,,humpidge1 ,ph4,d4 ,with #ff ,illustrations4 ,crown #hvo41 #hs4 #f;d4 ,,mendel,,eff4--,,the ,,principles ,,of ,,chemistry4 ,by ,d4 ,,mendel,,eff4 ,translated from the ,russian 7,sixth ,edition7 by ,,george ,,kamensky1 ,a4,r4,s4,m41 of the ,imperial ,mint1 ,st4 ,petersburg2 and ,edited by ,t4 ,a4 ,,lawson1 ,b4,sc41 ,ph4,d41 ,fellow of the ,institute of ,chemistry4 ,with #if ,diagrams and ,illustrations4 #b vols4 #hvo41 #cfs4 ,,meyer4--,,outlines ,,of ,,theoretical ,,chemistry4 ,by ,,lothar ,,meyer1 ,professor of ,chemistry in the ,university of ,tbingen4 ,translated by ,professors ,p4 ,,phillips ,,bedson1 ,d4,sc41 and ,w4 ,,carleton ,,williams1 ,b4,sc4 #hvo41 #is4 ,,miller4--,,introduction ,,to ,,the ,,study ,,of ,,inorganic ,,chemistry4 ,by ,w4 ,,allen ,,miller1 ,m4,d41 ,,ll4,d4 ,with #ga ,woodcuts4 ,fcp4 #hvo41 #cs4 #f;d4 ,,muir4--,works by ,m4 ,m4 ,p4 ,,muir1 ,m4,a41 ,fellow and ,prlector in ,chemistry of ,gonville and ,caius ,college1 ,cambridge4 ,a ,,course ,,of ,,practical ,,chemistry4 7#c ,parts47 ,part ,i41 ,elementary4 ,crown #hvo41 #ds4 #f;d4 ,,tables ,,and ,,directions ,,for ,,the ,,qualitative ,,chemical ,,analysis ,,of ,,moderately ,,complex ,,mixtures ,,of ,,salts4 ,crown #hvo1 #as4 #f;d4 ,,newth4--,works by ,g4 ,s4 ,,newth1 ,f4,i4,c41 ,f4,c4,s41 ,demonstrator in the ,royal ,college of ,science1 ,london2 ,assistant ,examiner in ,chemistry1 ,science and ,art ,department4 ,,chemical ,,lecture ,,experiments4 ,with #bbd ,diagrams4 ,crown #hvo41 #ajs4 #f;d4 ,a ,,text-,,book ,,of ,,inorganic ,,chemistry4 ,with #adf ,illustrations4 ,crown #hvo41 #fs4 #f;d4 ,,elementary ,,practical ,,chemistry3 a ,laboratory ,manual for ,use in ,organised ,science ,schools4 ,with #ajh ,illustrations and #bed ,experiments4 ,crown #hvo41 #bs4 #f;d4 ,,ostwald4--,,solutions4 ,by ,w4 ,,ostwald1 ,professor of ,chemistry in the ,university of ,leipzig4 ,being the ,fourth ,book1 with some additions1 of the ,second ,edition of ,oswald's ',lehrbuch der allgemeinen ,chemie'4 ,translated by ,m4 ,m4 ,,pattison ,,muir1 ,fellow and ,prlector in ,chemistry of ,gonville and ,caius ,college1 ,cambridge4 #hvo41 #ajs4 #f;d4 ,,payen4--,,industrial ,,chemistry4 ,a ,manual for use in ,technical ,colleges and ,schools1 based upon a ,translation of ,stohmann and ,engler's ,german ,edition of ,,payen',s ,prcis de ,chimie ,industrielle4 ,edited by ,b4 ,h4 ,,paul1 ,ph4,d4 ,with #fih ,woodcuts4 #hvo41 #dbs4 ,,reynolds4--,,experimental ,,chemistry ,,for ,,junior ,,students4 ,by ,j4 ,,emerson ,,reynolds1 ,m4,d41 ,f4,r4,s41 ,professor of ,chemistry1 ,university of ,dublin2 ,examiner in ,chemistry1 ,university of ,london4 ,fcp4 #hvo41 with numerous ,woodcuts4 ,part ,i4 ,introductory4 ,fcp4 #hvo41 #as4 #f;d4 ,part ,,ii4 ,non-,metals1 with an ,appendix on ,systematic ,testing for ,acids4 ,fcp4 #hvo41 #bs4 #f;d4 ,part ,,iii4 ,metals1 and ,allied ,bodies4 ,fcp4 #hvo41 #cs4 #f;d4 ,part ,,iv4 ,carbon ,compounds4 ,fcp4 #hvo41 #ds4 ,,shenstone4--,works by ,w4 ,a4 ,,shenstone1 ,lecturer on ,chemistry in ,clifton ,college4 ,,the ,,methods ,,of ,,glass-,,blowing4 ,for the use of ,physical and ,chemical ,students4 ,with #db ,illustrations4 ,crown #hvo41 #as4 #f;d4 ,a ,,practical ,,introduction ,,to ,,chemistry4 ,intended to give a ,practical acquaintance with the ,elementary ,facts and ,principles of ,chemistry4 ,with #be ,illustrations4 ,crown #hvo41 #bs4 ,,thorpe4--,works by ,t4 ,e4 ,,thorpe1 ,b4,sc4 7,vict471 ,ph4,d41 ,f4,r4,s41 ,professor of ,chemistry in the ,royal ,college of ,science1 ,south ,kensington4 ,assisted by ,eminent ,contributors4 ,a ,,dictionary ,,of ,,applied ,,chemistry4 #c vols4 #hvo4 ,vols4 ,i4 and ,,ii41 #dbs4 each4 ,vol4 ,,iii41 #fcs4 ,,quantitative ,,chemical ,,analysis4 ,with #hh ,woodcuts4 ,fcp4 #hvo41 #ds4 #f;d4 ,,thorpe ,,and ,,muir4--,,qualitative ,,chemical ,,analysis ,,and ,,laboratory ,,practice4 ,by ,t4 ,e4 ,,thorpe1 ,ph4,d41 ,d4,sc41 ,f4,r4,s41 and ,m4 ,m4 ,,pattison ,,muir1 ,m4,a4 ,with ,plate of ,spectra and #eg ,woodcuts4 ,fcp4 #hvo41 #cs4 #f;d4 ,,tilden4--,works by ,,william ,a4 ,,tilden1 ,d4,sc4 ,london1 ,f4,r4,s41 ,professor of ,chemistry in the ,royal ,college of ,science1 ,south ,kensington4 ,,introduction ,,to ,,the ,,study ,,of ,,chemical ,,philosophy4 ,the ,principles of ,theoretical and ,systematic ,chemistry4 ,with #e ,woodcuts4 ,with or without the ,,answers of ,problems4 ,fcp4 #hvo41 #ds4 #f;d4 ,,practical ,,chemistry4 ,the principles of ,qualitative ,analysis4 ,fcp4 #hvo41 #as4 #f;d4 ,,hints ,,on ,,the ,,teaching ,,of ,,elementary ,,chemistry ,,in ,,schools ,,and ,,science ,,classes4 ,with #g ,illustrations4 ,crown #hvo41 #bs4 ,,watts' 7,h47 ,,dictionary ,,of ,,chemistry4 ,revised and entirely ,re-written by ,h4 ,,forster ,,morley1 ,m4,a41 ,d4,sc41 ,fellow of1 and lately ,assistant-,professor of ,chemistry in1 ,university ,college1 ,london2 and ,m4 ,m4 ,,pattison ,,muir1 ,m4,a41 ,f4,r4,s4,e41 ,fellow1 and ,prlector in ,chemistry1 of ,gonville and ,caius ,college1 ,cambridge4 ,assisted by ,eminent ,contributors4 #d vols4 #hvo4 ,vols4 ,i4 and ,,ii41 #dbs4 each4 ,vol4 ,,iii41 #ejs4 ,vol4 ,,iv41 #fcs4 ,,whiteley4--,works by ,r4 ,,lloyd ,,whiteley1 ,f4,i4,c41 ,principal of the ,municipal ,science ,school1 ,west ,bromwich4 ,,chemical ,,calculations4 ,with ,explanatory ,notes1 ,problems and ,answers1 specially adapted for use in ,colleges and ,science ,schools4 ,with a ,preface by ,professor ,f4 ,,clowes1 ,d4,sc4 7,lond471 ,f4,i4,c4 ,crown #hvo41 #bs4 ,,organic ,,chemistry3 the ,fatty ,compounds4 ,with #de ,illustrations4 ,crown #hvo41 #cs4 #f;d4 ,,physics1 ,,etc4 ,,earl4--,,the ,,elements ,,of ,,laboratory ,,work3 a ,course of ,natural ,science4 ,by ,a4 ,g4 ,,earl1 ,m4,a41 ,f4,c4,s41 late ,scholar of ,christ's ,college1 ,cambridge4 ,with #eg ,diagrams and numerous ,exercises and ,questions4 ,crown #hvo41 #ds4 #f;d4 ,,ganot4--,works by ,,professor ,,ganot4 ,translated and ,edited by ,e4 ,,atkinson1 ,ph4,d41 ,f4,c4,s4 ,,elementary ,,treatise ,,on ,,physics1 ,experimental and ,applied4 ,with #i ,coloured ,plates and ,maps1 and #ajbh ,woodcuts1 and ,appendix of ,problems and ,examples with ,answers4 ,crown #hvo41 #aes4 ,,natural ,,philosophy ,,for ,,general ,,readers ,,and ,,young ,,persons2 ,with #g ,plates1 #fbd ,woodcuts1 and an ,appendix of ,questions4 ,crown #hvo41 #gs4 #f;d4 ,,glazebrook ,,and ,,shaw4--,,practical ,,physics4 ,by ,r4 ,t4 ,,glazebrook1 ,m4,a41 ,f4,r4,s41 and ,w4 ,n4 ,,shaw1 ,m4,a4 ,with #acd ,woodcuts4 ,fcp4 #hvo41 #gs4 #f;d4 ,,guthrie4--,,molecular ,,physics ,,and ,,sound4 ,by ,f4 ,,guthrie1 ,ph4,d4 ,with #ia ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,henderson4--,,elementary ,,physics4 ,by ,,john ,,henderson1 ,b4,sc4 7,edin471 ,a4,i4,e4,e41 ,lecturer in ,physics1 ,manchester ,municipal ,technical ,school4 ,crown #hvo41 #bs4 #f;d4 ,,helmholtz4--,,popular ,,lectures ,,on ,,scientific ,,subjects4 ,by ,,hermann ,,von ,,helmholtz4 ,translated by ,e4 ,,atkinson1 ,ph4,d41 ,f4,c4,s41 formerly ,professor of ,experimental ,science1 ,staff ,college4 ,with #fh ,illustrations4 #b vols41 crown #hvo41 #cs4 #f;d4 each4 ,,contents4--,vol4 ,i4--,the ,relation of ,natural ,science to ,science in ,general--,goethe's ,scientific ,researches--,the ,physiological ,causes of ,harmony in ,music--,ice and ,glaciers--,the ,interaction of the ,natural ,forces--,the ,recent ,progress of the ,theory of ,vision--,the ,conservation of ,force--,the ,aim and ,progress of ,physical ,science4 ,,contents4--,vol4 ,,ii4--,gustav ,magnus4 ,in ,memoriam--,the ,origin and ,significance of ,geometrical ,axioms--,the ,relation of ,optics to ,painting--,the ,origin of the ,planetary ,system--,thought in ,medicine--,academic ,freedom in ,german ,universities--,hermann ,von ,helmholtz--,an ,autobiographical ,sketch4 ,,van ,t',,hoff4--,,the ,,arrangement ,,of ,,atoms ,,in ,,space4 ,by ,j4 ,h4 ,,van ,t',,hoff4 ,second1 ,revised1 and ,enlarged ,edition4 ,with a ,preface by ,,johannes ,,wislicenus1 ,professor of ,chemistry at the ,university of ,leipzig2 and an ,appendix ',stereo-chemistry among ,inorganic ,substances1' by ,,alfred ,,werner1 ,professor of ,chemistry at the ,university of ,zrich4 ,translated and ,edited by ,,arnold ,,eiloart4 ,,watson4--,,elementary ,,practical ,,physics3 a ,laboratory ,manual for ,use in ,organised ,science ,schools4 ,by ,w4 ,,watson1 ,b4,sc4 ,demonstrator in ,physics in the ,royal ,college of ,science1 ,london2 ,assistant ,examiner in ,physics1 ,science and ,art ,department4 ,with #aii ,illustrations and #aic ,exercises4 ,crown #hvo41 #bs4 #f;d4 ,,worthington4--,a ,,first ,,course ,,of ,,physical ,,laboratory ,,practice4 ,containing #bfd ,experiments4 ,by ,a4 ,m4 ,,worthington1 ,m4,a41 ,f4,r4,s4 ,with ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,wright4--,,elementary ,,physics4 ,by ,,mark ,r4 ,,wright1 ,professor of ,normal ,education1 ,durham ,college of ,science4 ,with #bdb ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,mechanics1 ,,dynamics1 ,,statics1 ,,hydrostatics1 ,,etc4 ,,ball4--,a ,,class-,,book ,,of ,,mechanics4 ,by ,sir ,r4 ,s4 ,,ball1 ,,ll4,d4 #hi ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,geldard4--,,statics ,,and ,,dynamics4 ,by ,c4 ,,geldard1 ,m4,a41 formerly ,scholar of ,trinity ,college1 ,cambridge4 ,crown #hvo41 #es4 ,,goodeve4--,works by ,t4 ,m4 ,,goodeve1 ,m4,a41 formerly ,professor of ,mechanics at the ,normal ,school of ,science1 and the ,royal ,school of ,mines4 ,,the ,,elements ,,of ,,mechanism4 ,with #ceg ,woodcuts4 ,crown #hvo41 #fs4 ,,principles ,,of ,,mechanics4 ,with #bec ,woodcuts and numerous ,examples4 ,crown #hvo41 #fs4 ,a ,,manual ,,of ,,mechanics3 an ,elementary ,text-,book for ,students of ,applied ,mechanics4 ,with #ach ,illustrations and ,diagrams1 and #ahh ,examples taken from the ,science ,department ,examination ,papers1 with ,answers4 ,fcp4 #hvo41 #bs4 #f;d4 ,,grieve4--,,lessons ,,in ,,elementary ,,mechanics4 ,by ,w4 ,h4 ,,grieve1 late ,engineer1 ,r4,n41 ,science ,demonstrator for the ,london ,school ,board1 etc4 ,stage #a4 ,with #afe ,illustrations and a large number of ,examples4 ,fcp4 #hvo41 #as4 #f;d4 ,stage #b4 ,with #abb ,illustrations4 ,fcp4 #hvo41 #as4 #f;d4 ,stage #c4 ,with #ajc ,illustrations4 ,fcp4 #hvo41 #as4 #f;d4 ,,magnus4--,works by ,,sir ,,philip ,,magnus1 ,b4,sc41 ,b4,a4 ,,lessons ,,in ,,elementary ,,mechanics4 ,introductory to the study of ,physical ,science4 ,designed for the ,use of ,schools1 and of ,candidates for the ,london ,matriculation and other ,examinations4 ,with numerous ,exercises1 ,examples1 ,examination ,questions1 and ,solutions1 etc41 from #ahgj-ahie4 ,with ,answers1 and #aca ,woodcuts4 ,fcp4 #hvo41 #cs4 #f;d4 ,key for the use of ,teachers only1 price #es4 #c-a_/b;d4 ,,hydrostatics ,,and ,,pneumatics4 ,fcp4 #hvo41 #as4 #f;d42 or1 with ,answers1 #bs4 ,the ,worked ,solutions of the ,problems1 #bs4 ,,robinson4--,,elements ,,of ,,dynamics 7,kinetics and ,statics74 ,with numerous ,exercises4 ,a ,text-book for ,junior ,students4 ,by the ,rev4 ,j4 ,l4 ,,robinson1 ,b4,a4 ,crown #hvo41 #fs4 ,,smith4--,works by ,j4 ,,hamblin ,,smith1 ,m4,a4 ,,elementary ,,statics4 ,crown #hvo41 #cs4 ,,elementary ,,hydrostatics4 ,crown #hvo41 #cs4 ,,key ,,to ,,statics ,,and ,,hydrostatics4 ,crown #hvo41 #fs4 ,,tate4--,,exercises ,,on ,,mechanics ,,and ,,natural ,,philosophy4 ,by ,,thomas ,,tate1 ,f4,r4,a4,s4 ,fcp4 #hvo41 #bs4 ,key1 #cs4 #f;d4 ,,taylor4--,works by ,j4 ,e4 ,,taylor1 ,m4,a41 ,b4,sc4 7,lond471 ,head ,master of the ,central ,higher ,grade and ,science ,school1 ,sheffield4 ,,theoretical ,,mechanics1 including ,hydrostatics and ,pneumatics4 ,with #age ,diagrams and ,illustrations1 and #ebb ,examination ,questions and ,answers4 ,crown #hvo41 #bs4 #f;d4 ,,theoretical ,,mechanics--,,solids4 ,with #afc ,illustrations1 #abj ,worked ,examples and over #ejj ,examples from ,examination ,papers1 etc4 ,crown #hvo41 #bs4 #f;d4 ,,theoretical ,,mechanics4--,,fluids4 ,with #abb ,illustrations1 numerous ,worked ,examples1 and about #ejj ,examples from ,examination ,papers1 etc4 ,crown #hvo41 #bs4 #f;d4 ,,thornton4--,,theoretical ,,mechanics--,,solids4 ,including ,kinematics1 ,statics1 and ,kinetics4 ,by ,,arthur ,,thornton1 ,m4,a41 ,f4,r4,a4,s4 ,with #bjj ,illustrations1 #acj ,worked ,examples1 and over #ijj ,examples from ,examination ,papers1 etc4 ,crown #hvo41 #ds4 #f;d4 ,,twisden4--,works by the ,rev4 ,,john ,f4 ,,twisden1 ,m4,a4 ,,practical ,,mechanics2 an ,elementary ,introduction to their ,study4 ,with #hee ,exercises1 and #ahd ,figures and ,diagrams4 ,crown #hvo41 #ajs4 #f;d4 ,,theoretical ,,mechanics4 ,with #agb ,examples1 numerous ,exercises1 and #aed ,diagrams4 ,crown #hvo41 #hs4 #f;d4 ,,williamson4--,,introduction ,,to ,,the ,,mathematical ,,theory ,,of ,,the ,,stress ,,and ,,strain ,,of ,,elastic ,,solids4 ,by ,,benjamin ,,williamson1 ,d4,sc41 ,f4,r4,s4 ,crown #hvo41 #es4 ,,williamson ,,and ,,tarleton4--,,an ,,elementary ,,treatise ,,on ,,dynamics4 ,containing ,applications to ,thermodynamics1 with numerous ,examples4 ,by ,,benjamin ,,williamson1 ,d4,sc41 ,f4,r4,s41 and ,,francis ,a4 ,,tarleton1 ,,ll4,d4 ,crown #hvo41 #ajs4 #f;d4 ,,worthington4--,,dynamics ,,of ,,rotation3 an ,elementary introduction to ,rigid ,dynamics4 ,by ,a4 ,m4 ,,worthington1 ,m4,a41 ,f4,r4,s4 ,crown #hvo41 #ds4 #f;d4 ,,optics ,,and ,,photography4 ,,abney4--,a ,,treatise ,,on ,,photography4 ,by ,captain ,w4 ,,de ,,wiveleslie ,,abney1 ,f4,r4,s41 ,director for ,science in the ,science and ,art ,department4 ,with #aae ,woodcuts4 ,fcp4 #hvo41 #cs4 #f;d4 ,,glazebrook4--,,physical ,,optics4 ,by ,r4 ,t4 ,,glazebrook1 ,m4,a41 ,f4,r4,s41 ,fellow and ,lecturer of ,trinity ,college1 ,demonstrator of ,physics at the ,cavendish ,laboratory1 ,cambridge4 ,with #ahc ,woodcuts of ,apparatus1 etc4 ,fcp4 #hvo41 #fs4 ,,wright4--,,optical ,,projection3 a ,treatise on the ,use of the ,lantern in ,exhibition and ,scientific ,demonstration4 ,by ,,lewis ,,wright1 ,author of ',light3 a ,course of ,experimental ,optics'4 ,with #bcb ,illustrations4 ,crown #hvo41 #fs4 ,,sound1 ,,light1 ,,heat1 ,,and ,,thermodynamics4 ,,cumming4--,,heat ,,treated ,,experimentally4 ,by ,,linn,,us ,,cumming1 ,m4,a4 ,with #aib ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,day4--,,numerical ,,examples ,,in ,,heat4 ,by ,r4 ,e4 ,,day1 ,m4,a4 ,fcp4 #hvo41 #cs4 #f;d4 ,,emtage4--,,light4 ,by ,w4 ,t4 ,a4 ,,emtage1 ,m4,a4 ,with #bcb ,illustrations4 ,crown #hvo41 #fs4 ,,helmholtz4--,,on ,,the ,,sensations ,,of ,,tone ,,as ,a ,,physiological ,,basis ,,for ,,the ,,theory ,,of ,,music4 ,by ,,hermann ,,von ,,helmholtz4 ,royal #hvo41 #bhs4 ,,madan4--,,an ,,elementary ,,text-,,book ,,on ,,heat4 ,for the ,use of ,schools4 ,by ,h4 ,g4 ,,madan1 ,m4,a41 ,f4,c4,s41 ,fellow of ,queen's ,college1 ,oxford2 late ,assistant ,master at ,eton ,college4 ,crown #hvo41 #is4 ,,maxwell4--,,theory ,,of ,,heat4 ,by ,j4 ,,clerk ,,maxwell1 ,m4,a41 ,f4,r4,,ss41 ,l4 and ,e4 ,with ,corrections and ,additions by ,lord ,,rayleigh4 ,with #ch ,illustrations4 ,fcp4 #hvo41 #ds4 #f;d4 ,,smith4--,,the ,,study ,,of ,,heat4 ,by ,j4 ,,hamblin ,,smith1 ,m4,a41 of ,gonville and ,caius ,college1 ,cambridge4 ,crown #hvo41 #cs4 ,,tyndall4--,works by ,,john ,,tyndall1 ,d4,c4,l41 ,f4,r4,s4 ,see p4 #bg4 ,,wormell4--,a ,,class-,,book ,,of ,,thermodynamics4 ,by ,,richard ,,wormell1 ,b4,sc41 ,m4,a4 ,fcp4 #hvo41 #as4 #f;d4 ,,wright4--,works by ,,mark ,r4 ,,wright1 ,hon4 ,inter4 ,b4,sc41 ,london4 ,,sound1 ,,light1 ,,and ,,heat4 ,with #afj ,diagrams and ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,advanced ,,heat4 ,with #acf ,diagrams and numerous ,examples and ,examination ,papers4 ,crown #hvo41 #ds4 #f;d4 ,,steam1 ,,oil1 ,,and ,,gas ,,engines4 ,,bale4--,a ,,handbook ,,for ,,steam ,,users2 being ,rules for ,engine ,drivers and ,boiler ,attendants1 with ,notes on ,steam ,engine and ,boiler ,management and ,steam ,boiler ,explosions4 ,by ,m4 ,,powis ,,bale1 ,m4,i4,m4,e41 ,a4,m4,i4,c4,e4 ,fcp4 #hvo41 #bs4 #f;d4 ,,bolton4--,,motive ,,powers ,,and ,,their ,,practical ,,selection4 ,by ,,reginald ,,bolton1 ,associate ,member of the ,institution of ,civil ,engineers1 etc4 ,crown #hvo41 #fs4 #f;d4 net4 ,,clerk4--,,the ,,gas ,,and ,,oil ,,engine4 ,by ,,dugald ,,clerk1 ,associate ,member of the ,institution of ,civil ,engineers1 ,fellow of the ,chemical ,society1 ,member of the ,royal ,institution1 ,fellow of the ,institute of ,patent ,agents4 #hvo41 #aes4 ,,holmes4--,,the ,,steam ,,engine4 ,by ,,george ,c4 ,v4 ,,holmes1 ,whitworth ,scholar1 ,secretary of the ,institution of ,naval ,architects4 ,with #bab ,woodcuts4 ,fcp4 #hvo41 #fs4 ,,norris4--,a ,,practical ,,treatise ,,on ,,the ',,otto' ,,cycle ,,gas ,,engine4 ,by ,,william ,,norris1 ,m4,i4,mech4,e4 ,with #bjg ,illustrations4 #hvo41 #ajs4 #f;d4 ,,ripper4--,works by ,,william ,,ripper1 ,professor of ,mechanical ,engineering in the ,sheffield ,technical ,school4 ,,steam4 ,with #adb ,illustrations4 ,crown #hvo1 #bs4 #f;d4 ,,steam ,,engineering4 ,in the press4 ,,sennett4--,,the ,,marine ,,steam ,,engine4 ,a ,treatise for the ,use of ,engineering ,students and ,officers of the ,royal ,navy4 ,by ,,richard ,,sennett1 ,r4,n41 late ,engineer-in-,chief of the ,royal ,navy4 ,with #bfa ,illustrations4 #hvo41 #bas4 ,,stromeyer4--,,marine ,,boiler ,,management ,,and ,,construction4 ,being a ,treatise on ,boiler ,troubles and ,repairs1 ,corrosion1 ,fuels1 and ,heat1 on the properties of ,iron and ,steel1 on ,boiler ,mechanics1 ,workshop ,practices1 and ,boiler ,design4 ,by ,c4 ,e4 ,,stromeyer1 ,member of the ,institute of ,naval ,architects1 etc4 #hvo41 #ahs4 net4 ,,building ,,construction4 ,,advanced ,,building ,,construction4 ,by the ,author of ',rivingtons' ,notes on ,building ,construction4 ,with #che ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,burrell4--,,building ,,construction4 ,by ,,edward ,j4 ,,burrell1 ,second ,master of the ,people's ,palace ,technical ,school1 ,london4 ,with #cjc ,working ,drawings4 ,crown #hvo41 #bs4 #f;d4 ,,seddon4--,,builder',s ,,work ,,and ,,the ,,building ,,trades4 ,by ,col4 ,h4 ,c4 ,,seddon1 ,r4,e41 ,superintending ,engineer1 ,h4,m4's ,dockyard1 ,portsmouth2 ,examiner in ,building ,construction1 ,science and ,art ,department1 ,south ,kensington2 with numerous ,illustrations4 ,medium #hvo41 #afs4 ,,rivingtons' ,,course ,,of ,,building ,,construction4 ,,notes ,,on ,,building ,,construction4 ,arranged to meet the requirements of the syllabus of the ,science and ,art ,department of the ,committee of ,council on ,education1 ,south ,kensington4 ,medium #hvo4 ,part ,i4 ,first ,stage1 or ,elementary ,course4 ,with #eeb ,woodcuts1 #ajs4 #f;d4 ,part ,,ii4 ,commencement of ,second ,stage1 or ,advanced ,course4 ,with #dgi ,woodcuts1 #ajs4 #f;d4 ,part ,,iii4 ,materials4 ,advanced ,course1 and ,course for ,honours4 ,with #ahh ,woodcuts1 #bas4 ,part ,,iv4 ,calculations for ,building ,structures4 ,course for ,honours4 ,with #eig ,woodcuts1 #aes4 ,,electricity ,,and ,,magnetism4 ,,cumming4--,,electricity ,,treated ,,experimentally4 ,for the ,use of ,schools and ,students4 ,by ,,linn,,us ,,cumming1 ,m4,a4 ,with #bdb ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,day4--,,exercises ,,in ,,electrical ,,and ,,magnetic ,,measurements1 with ,answers4 ,by ,r4 ,e4 ,,day4 #abmo41 #cs4 #f;d4 ,,du ,,bois4--,,the ,,magnetic ,,circuit ,,in ,,theory ,,and ,,practice4 ,by ,dr4 ,h4 ,,du ,,bois1 ,privatdocent in the ,university of ,berlin4 ,translated by ,e4 ,,atkinson1 ,ph4,d4 ,with #id ,illustrations4 #hvo41 #abs4 net4 ,,ebert4--,,magnetic ,,fields ,,of ,,force3 an ,exposition of the ,phenomena of ,magnetism1 ,electro-,magnetism and ,induction1 based on the ,conception of ,lines of ,force4 ,by ,h4 ,,ebert1 ,professor of ,physics in the ,university of ,kiel4 ,translated by ,c4 ,v4 ,,burton1 ,d4,sc4 ,part ,i4 ,with #ic ,illustrations4 #hvo41 #ajs4 #f;d4 net4 ,,gore4--,,the ,,art ,,of ,,electro-,,metallurgy1 including all known ,processes of ,electro-,deposition4 ,by ,g4 ,,gore1 ,,ll4,d41 ,f4,r4,s4 ,with #ef ,woodcuts4 ,fcp4 #hvo41 #fs4 ,,jenkin4--,,electricity ,,and ,,magnetism4 ,by ,,fleeming ,,jenkin1 ,f4,r4,s4,s41 ,l4 and ,e41 ,m4,i4,c4,e4 ,with #agg ,illustrations4 ,fcp4 #hvo41 #cs4 #f;d4 ,,joubert4--,,elementary ,,treatise ,,on ,,electricity ,,and ,,magnetism4 ,founded on ,,joubert',s ',trait lmentair d',electricit'4 ,by ,g4 ,c4 ,,foster1 ,f4,r4,s41 and ,e4 ,,atkinson1 ,ph4,d4 ,with #cha ,illustrations4 ,crown #hvo41 #gs4 #f;d4 ,,joyce4--,,examples ,,in ,,electrical ,,engineering4 ,by ,,samuel ,,joyce1 ,a4,i4,e4,e4 ,crown #hvo41 #es4 ,,larden4--,,electricity ,,for ,,public ,,schools ,,and ,,colleges4 ,by ,w4 ,,larden1 ,m4,a4 ,with #bae ,illustrations1 and a ,series of ,examination ,papers1 with ,answers4 ,crown #hvo41 #fs4 ,,merrifield4--,,magnetism ,,and ,,deviation ,,of ,,the ,,compass4 ,for the ,use of ,students in ,navigation and ,science ,schools4 ,by ,,john ,,merrifield1 ,,ll4,d41 ,f4,r4,a4,s41 #ahmo41 #bs4 #f;d4 ,,poyser4--,works by ,a4 ,w4 ,,poyser1 ,m4,a41 ,grammar ,school1 ,wisbech4 ,,magnetism ,,and ,,electricity4 ,with #bce ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,advanced ,,electricity ,,and ,,magnetism4 ,with #cag ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,slingo ,,and ,,brooker4--,works by ,w4 ,,slingo and ,a4 ,,brooker4 ,,electrical ,,engineering ,,for ,,electric ,,light ,,artisans ,,and ,,students4 ,with #cdf ,illustrations4 ,crown #hvo41 #abs4 ,,problems ,,and ,,solutions ,,in ,,elementary ,,electricity ,,and ,,magnetism4 ,embracing a ,complete ,set of ,answers to the ,south ,kensington ,papers for the years #ahhe-ahid1 and a ,series of ,original ,questions4 ,with #fg ,original ,illustrations4 ,crown #hvo41 #bs4 ,,tyndall4--,works by ,,john ,,tyndall1 ,d4,c4,l41 ,f4,r4,s4 ,see p4 #bg4 ,,telegraphy ,,and ,,the ,,telephone4 ,,bennett4--,,the ,,telephone ,,systems ,,of ,,continental ,,europe4 ,by ,a4 ,r4 ,,bennett1 ,member of the ,institute of ,electrical ,engineers2 late ,general ,manager in ,scotland of the ,national ,telephone ,company1 and ,general ,manager and ,electrician of the ,mutual and ,new ,telephone ,companies4 ,with #afi ,illustrations4 ,crown #hvo41 #aes4 ,,culley4--,a ,,handbook ,,of ,,practical ,,telegraphy4 ,by ,r4 ,s4 ,,culley1 ,m4,i4,c4,e41 late ,engineer-in-,chief of ,telegraphs to the ,post ,office4 ,with #ace ,woodcuts and #ag ,plates4 #hvo41 #afs4 ,,preece ,,and ,,sivewright4--,,telegraphy4 ,by ,w4 ,h4 ,,preece1 ,c4,b41 ,f4,r4,s41 ,v4,p4,inst41 ,c4,e41 etc41 ,engineer-in-,chief and ,electrician ,post ,office ,telegraphs2 and ,sir ,j4 ,,sivewright1 ,k4,c4,m4,g41 ,general ,manager1 ,south ,african ,telegraphs4 ,with #beh ,woodcuts4 ,fcp4 #hvo41 #fs4 ,,engineering1 ,,strength ,,of ,,materials1 ,,etc4 ,,anderson4--,,the ,,strength ,,of ,,materials ,,and ,,structures3 the ,strength of ,materials as depending on their ,quality and as ascertained by ,testing ,apparatus4 ,by ,sir ,j4 ,,anderson1 ,c4,e41 ,,ll4,d41 ,f4,r4,s4,e4 ,with #ff ,woodcuts4 ,fcp4 #hvo41 #cs4 #f;d4 ,,barry4--,,railway ,,appliances3 a ,description of ,details of ,railway ,construction subsequent to the completion of the ,earthworks and ,structures4 ,by ,sir ,,john ,,wolfe ,,barry1 ,k4,c4,b41 ,f4,r4,s41 ,m4,i4,c4,e4 ,with #bah ,woodcuts4 ,fcp4 #hvo41 #ds4 #f;d4 ,,smith4--,,graphics1 or the ,art of ,calculation by ,drawing ,lines1 applied especially to ,mechanical ,engineering4 ,by ,,robert ,h4 ,,smith1 ,professor of ,engineering1 ,mason ,college1 ,birmingham4 ,part ,i4 ,with separate ,atlas of #bi ,plates containing #ig ,diagrams4 #hvo41 #aes4 ,,stoney4--,,the ,,theory ,,of ,,the ,,stresses ,,on ,,girders ,,and ,,similar ,,structures4 ,with ,practical ,observations on the ,strength and other ,properties of ,materials4 ,by ,,bindon ,b4 ,,stoney1 ,,ll4,d41 ,f4,r4,s41 ,m4,i4,c4,e4 ,with #e ,plates and #adc ,illustrations in the ,text4 ,royal #hvo41 #cfs4 ,,unwin4--,works by ,,william ,,cawthorne ,,unwin1 ,f4,r4,s41 ,b4,s4,c4 ,,the ,,testing ,,of ,,materials ,,of ,,construction4 ,embracing the description of ,testing ,machinery and ,apparatus ,auxiliary to ,mechanical ,testing1 and an ,account of the most ,important ,researches on the ,strength of ,materials4 ,with #ada ,woodcuts and #e ,folding-out ,plates4 #hvo41 #bas4 ,,on ,,the ,,development ,,and ,,transmission ,,of ,,power ,,from ,,central ,,stations3 being the ,howard ,lectures delivered at the ,society of ,arts in #ahic4 ,with #ha ,diagrams4 #hvo41 #ajs4 net4 ,,warren4--,,engineering ,,construction ,,in ,,iron1 ,,steel1 ,,and ,,timber4 ,by ,,william ,,henry ,,warren1 ,challis ,professor of ,civil and ,mechanical ,engineering1 ,university of ,sydney4 ,with #ac ,folding ,plates1 and #cge ,diagrams4 ,royal #hvo41 #afs4 net4 ,,machine ,,drawing ,,and ,,design4 ,,low ,,and ,,bevis4--,a ,,manual ,,of ,,machine ,,drawing ,,and ,,design4 ,by ,,david ,,allan ,,low 7,whitworth ,scholar71 ,m4,i4,mech4,e41 ,professor of ,engineering1 ,east ,london ,technical ,college1 ,people's ,palace1 ,london2 and ,,alfred ,,william ,,bevis 7,whitworth ,scholar71 ,m4,i4,mech4,e41 ,director of ,manual ,training to the ,birmingham ,school ,board4 ,with #gjj ,illustrations4 #hvo41 #gs4 #f;d4 ,,low4--,works by ,,david ,,allan ,,low1 ,professor of ,engineering1 ,east ,london ,technical ,college4 ,,improved ,,drawing ,,scales4 #d;d4 in case4 ,,an ,,introduction ,,to ,,machine ,,drawing ,,and ,,design4 ,with #ig ,illustrations and ,diagrams4 ,crown #hvo41 #bs4 ,,mechanical ,,engineer',s ,,pocket-,,book4 ,in the press4 ,,unwin4--,,the ,,elements ,,of ,,machine ,,design4 ,by ,w4 ,,cawthorne ,,unwin1 ,f4,r4,s41 ,professor of ,engineering at the ,central ,institute of the ,city and ,guilds of ,london ,institute4 ,part ,i4 ,general ,principles1 ,fastenings1 and ,transmissive ,machinery4 ,with #cjd ,diagrams1 etc4 ,fcp4 #hvo41 #fs4 ,part ,,ii4 ,chiefly on ,engine ,details4 ,with #agd ,woodcuts4 ,fcp4 #hvo41 #ds4 #f;d4 ,,longmans' ,,civil ,,engineering ,,series4 ,edited by the ,author of ',notes on ,building ,construction'4 ,,tidal ,,rivers3 their 7#a7 ,hydraulics1 7#b7 ,improvement1 7#c7 ,navigation4 ,by ,w4 ,h4 ,,wheeler1 ,m4,inst4,c4,e41 author of ',the ,drainage of ,fens and ,low ,lands by ,gravitation and ,steam ,power'4 ,with #ge ,illustrations4 ,medium #hvo41 #afs4 net4 ,,notes ,,on ,,docks ,,and ,,dock ,,construction4 ,by ,c4 ,,colson1 ,m4,inst4,c4,e41 ,assistant ,director of ,works1 ,admiralty4 ,with #cfe ,illustrations4 ,medium #hvo41 #bas4 net4 ,,principles ,,and ,,practice ,,of ,,harbour ,,construction4 ,by ,,william ,,shield1 ,f4,r4,s4,e41 ,m4,inst4,c4,e41 and ,executive ,engineer1 ,national ,harbour of ,refuge1 ,peterhead1 ,n4,b4 ,with #ig ,illustrations4 ,medium #hvo41 #aes4 net4 ,,calculations ,,for ,,engineering ,,structures4 ,by ,t4 ,,claxton ,,fidler1 ,m4,i4,c4,e41 ,professor of ,engineering in the ,university of ,dundee2 ,author of ',a ,practical ,treatise on ,bridge ,construction'4 ,in preparation4 ,,principles ,,and ,,practice ,,of ,,civil ,,engineering4 ,by ,l4 ,f4 ,,vernon-,,harcourt1 ,m4,inst4,c4,e41 ,professor of ,civil ,engineering at ,university ,college1 ,london4 ,in preparation4 ,,railway ,,construction4 ,by ,w4 ,h4 ,,mills1 ,m4,i4,c4,e41 ,engineer-in-,chief1 ,great ,northern ,railway1 ,ireland4 ,in preparation4 ,,workshop ,,appliances1 ,,etc4 ,,northcott4--,,lathes ,,and ,,turning1 ,simple1 ,mechanical and ,ornamental4 ,by ,w4 ,h4 ,,northcott4 ,with #cch ,illustrations4 #hvo41 #ahs4 ,,shelley4--,,workshop ,,appliances1 including ,descriptions of some of the ,gauging and ,measuring ,instruments1 ,hand-cutting ,tools1 ,lathes1 ,drilling1 ,plaining1 and other ,machine ,tools used by ,engineers4 ,by ,c4 ,p4 ,b4 ,,shelley1 ,m4,i4,c4,e4 ,with an additional ,chapter on ,milling by ,r4 ,r4 ,,lister4 ,with #cbc ,woodcuts4 ,fcp4 #hvo41 #es4 ,,mineralogy1 ,,metallurgy1 ,,etc4 ,,bauerman4--,works by ,,hilary ,,bauerman1 ,f4,g4,s4 ,,systematic ,,mineralogy4 ,with #cgc ,woodcuts and ,diagrams4 ,fcp4 #hvo41 #fs4 ,,descriptive ,,mineralogy4 ,with #bcf ,woodcuts and ,diagrams4 ,fcp4 #hvo41 #fs4 ,,gore4--,,the ,,art ,,of ,,electro-,,metallurgy1 including all known ,processes of ,electro-,deposition4 ,by ,g4 ,,gore1 ,,ll4,d41 ,f4,r4,s4 ,with #ef ,woodcuts4 ,fcp4 #hvo41 #fs4 ,,huntington ,,and ,m',,millan4--,,metals3 their ,properties and ,treatment4 ,by ,a4 ,k4 ,,huntington1 ,professor of ,metallurgy in ,king's ,college1 ,london1 and ,w4 ,g4 ,m',,millan1 ,lecturer on ,metallurgy in ,mason's ,college1 ,birmingham4 ,with #abb ,illustrations4 ,fcp4 #hvo41 #gs4 #f;d4 ,,rhead4--,,metallurgy4 ,an ,elementary ,text ,book4 ,by ,e4 ,c4 ,,rhead1 ,lecturer on ,metallurgy at the ,municipal ,technical ,school1 ,manchester4 ,with #id ,illustrations4 ,fcp4 #hvo41 #cs4 #f;d4 ,,rutley4--,,the ,,study ,,of ,,rocks3 an ,elementary ,text-book of ,petrology4 ,by ,f4 ,,rutley1 ,f4,g4,s4 ,with #f ,plates and #hh ,woodcuts4 ,fcp4 #hvo41 #ds4 #f;d4 ,,astronomy1 ,,navigation1 ,,etc4 ,,abbott4--,,elementary ,,theory ,,of ,,the ,,tides3 the ,fundamental ,theorems ,demonstrated without ,mathematics and the ,influence on the ,length of the ,day ,discussed4 ,by ,t4 ,k4 ,,abbott1 ,b4,d41 ,fellow and ,tutor1 ,trinity ,college1 ,dublin4 ,crown #hvo41 #bs4 ,,ball4--,works by ,sir ,,robert ,s4 ,,ball1 ,,ll4,d41 ,f4,r4,s4 ,,elements ,,of ,,astronomy4 ,with #acj ,figures and ,diagrams4 ,fcp4 #hvo41 #fs4 #f;d4 ,a ,,class-,,book ,,of ,,astronomy4 ,with #da ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,clerke4--,,the ,,system ,,of ,,the ,,stars4 ,by ,,agnes ,m4 ,,clerke4 ,with #f ,plates1 and numerous ,illustrations4 #hvo41 #bas4 ,,goodwin4--,,azimuth ,,tables ,,for ,,the ,,higher ,,declinations4 7,limits of ,declination #bd to #cj1 both inclusive47 ,between the ,parallels of ,latitude #j and #fj4 ,with ,examples of the ,use of the ,tables in ,english and ,french4 ,by ,h4 ,b4 ,,goodwin1 ,naval ,instructor1 ,royal ,navy4 ,royal #hvo41 #gs4 #f;d4 ,,herschel4--,,outlines ,,of ,,astronomy4--,by ,sir ,,john ,f4 ,w4 ,,herschel1 ,bart41 ,k4 ,h41 etc4 ,with #i ,plates1 and numerous ,diagrams4 #hvo41 #abs4 ,,lowell4--,,mars4 ,by ,,percival ,,lowell1 ,fellow ,american ,academy1 ,member ,royal ,asiatic ,society1 ,great ,britain and ,ireland1 etc4 ,with #bd ,plates4 #hvo41 #abs4 #f;d4 ,,martin4--,,navigation ,,and ,,nautical ,,astronomy4 ,compiled by ,staff ,commander ,w4 ,r4 ,,martin1 ,r4,n4 ,royal #hvo41 #ahs4 ,,merrifield4--,a ,,treatise ,,on ,,navigation4 ,for the ,use of ,students4 ,by ,j4 ,,merrifield1 ,,ll4,d41 ,f4,r4,a4,s41 ,f4,m4,s4 ,with ,charts and ,diagrams4 ,crown #hvo41 #es4 ,,parker4--,,elements ,,of ,,astronomy4 ,with ,numerous ,examples and ,examination ,papers4 ,by ,,george ,w4 ,,parker1 ,m4,a41 of ,trinity ,college1 ,dublin4 ,with #hd ,diagrams4 #hvo41 #es4 net4 ,,webb4--,,celestial ,,objects ,,for ,,common ,,telescopes4 ,by the ,rev4 ,t4 ,w4 ,webb1 ,m4,a41 ,f4,r4,a4,s4 ,fifth ,edition1 ,revised and greatly ,enlarged by the ,rev4 ,t4 ,e4 ,,espin1 ,m4,a41 ,f4,r4,a4,s4 7,two ,volumes47 ,vol4 ,i41 with ,portrait and a ,reminiscence of the ,author1 #b ,plates1 and numerous ,illustrations4 ,crown #hvo41 #fs4 ,vol4 ,,ii41 with numerous ,illustrations4 ,crown #hvo41 #fs4 #f;d4 ,,works ,,by ,,richard ,a4 ,,proctor4 ,,old ,,and ,,new ,,astronomy4 ,with #ba ,plates and #dgb ,illustrations in the ,text4 #dto41 #bas4 ,,the ,,moon3 ,her ,motions1 ,aspect1 ,scenery1 and ,physical ,condition4 ,with many ,plates and ,charts1 ,wood ,engravings1 and #b ,lunar ,photographs4 ,crown #hvo41 #cs4 #f;d4 ,,the ,,universe ,,of ,,stars3 ,researches into1 and ,new ,views respecting the ,constitution of the ,heavens4 ,with #bb ,charts 7#d ,coloured71 and #bb ,diagrams4 #hvo41 #ajs4 #f;d4 ,,other ,,worlds ,,than ,,ours3 the ,plurality of ,worlds ,studied ,under the ,light of ,recent ,scientific ,researches4 ,with #ad ,illustrations2 ,map1 ,charts1 etc4 ,crown #hvo41 #cs4 #f;d4 ,,our ,,place ,,among ,,infinities3 a ,series of ,essays contrasting our ,little ,abode in ,space and ,time with the ,infinities around us4 ,crown #hvo41 #cs4 #f;d4 ,,myths ,,and ,,marvels ,,of ,,astronomy4 ,crown #hvo41 #cs4 #f;d4 ,,light ,,science ,,for ,,leisure ,,hours3 ,familiar ,essays on ,scientific ,subjects4 ,natural ,phenomena1 etc4 #c vols41 crown #hvo41 #es4 each4 ,,the ,,orbs ,,around ,,us2 ,essays on the ,moon and ,planets1 ,meteors and ,comets1 the ,sun and ,coloured ,pairs of ,suns4 ,crown #hvo41 #cs4 #f;d4 ,,the ,,expanse ,,of ,,heaven3 ,essays on the ,wonders of the ,firmament4 ,crown #hvo41 #cs4 #f;d4 ,,other ,,suns ,,than ,,ours3 a ,series of ,essays on ,suns--,old1 ,young1 and ,dead4 ,with other ,science ,gleanings4 ,two ,essays on ,whist1 and ,correspondence with ,sir ,john ,herschel4 ,with #i ,star-,maps and ,diagrams4 ,crown #hvo41 #cs4 #f;d4 ,,half-,,hours ,,with ,,the ,,telescope3 a ,popular ,guide to the ,use of the ,telescope as a means of ,amusement and ,instruction4 ,with #g ,plates4 ,fcp4 #hvo41 #bs4 #f;d4 ,,new ,,star ,,atlas ,,for ,,the ,,library1 the ,school1 and the ,observatory1 in ,twelve ,circular ,maps 7with ,two ,index-,plates74 ,with an ,introduction on the ,study of the ,stars4 ,illustrated by #i ,diagrams4 ,crown #hvo41 #es4 ,,the ,,southern ,,skies3 a ,plain and ,easy ,guide to the ,constellations of the ,southern ,hemisphere4 ,showing in #ab ,maps the position of the principal ,star-,groups night after night throughout the year4 ,with an ,introduction and a separate ,explanation of each ,map4 ,true for every ,year4 #dto41 #es4 ,,half-,,hours ,,with ,,the ,,stars3 a ,plain and ,easy ,guide to the ,knowledge of the ,constellations4 ,showing in #ab ,maps the position of the principal ,star-,groups night after night throughout the year4 ,with ,introduction and a separate ,explanation of each ,map4 ,true for every ,year4 #dto41 #cs4 #f;d4 ,,larger ,,star ,,atlas ,,for ,,observers ,,and ,,students4 ,in ,twelve ,circular ,maps1 showing #fjjj ,stars1 #aejj ,double ,stars1 ,nebul1 etc4 ,with #b ,index-,plates4 ,folio1 #aes4 ,,the ,,stars ,,in ,,their ,,seasons3 an ,easy ,guide to a ,knowledge of the ,star-,groups4 ,in #ab ,large ,maps4 ,imperial #hvo41 #es4 ,,rough ,,ways ,,made ,,smooth4 ,familiar ,essays on ,scientific ,subjects4 ,crown #hvo41 #cs4 #f;d4 ,,pleasant ,,ways ,,in ,,science4 ,crown #hvo41 #cs4 #f;d4 ,,nature ,,studies4 ,by ,r4 ,a4 ,,proctor1 ,,grant ,,allen1 ,a4 ,,wilson1 ,t4 ,,foster1 and ,e4 ,,clodd4 ,crown #hvo41 #cs4 #f;d4 ,,leisure ,,readings4 ,by ,r4 ,a4 ,,proctor1 ,e4 ,,clodd1 ,a4 ,,wilson1 ,t4 ,,foster1 and ,a4 ,c4 ,,ranyard4 ,crown #hvo41 #cs4 #f;d4 ,,manufactures1 ,,technology1 ,,etc4 ,,bell4--,,jacquard ,,weaving ,,and ,,designing4 ,by ,f4 ,t4 ,,bell1 ,medallist in ,honours and ,certificated ,teacher in ',linen ,manufacturing' and in ',weaving and ,pattern ,designing1' ,city and ,guilds of ,london ,institute4 ,with #aii ,diagrams4 #hvo41 #abs4 net4 ,,lupton4--,,mining4 ,an ,elementary ,treatise on the ,getting of ,minerals4 ,by ,,arnold ,,lupton1 ,m4,i4,c4,e41 ,f4,g4,s41 etc4 ,with #eif ,diagrams and ,illustrations4 ,crown #hvo41 #is4 net4 ,,morris ,,and ,,wilkinson4--,,the ,,elements ,,of ,,cotton ,,spinning4 ,by ,,john ,,morris and ,f4 ,,wilkinson4 ,with a ,preface by ,sir ,b4 ,a4 ,,dobson1 ,c4,e41 ,m4,i4,m4,e4 ,with #afi ,diagrams and ,illustrations4 ,crown #hvo1 #gs4 #f;d4 net4 ,,sharp4--,,bicycles ,,and ,,tricycles3 an ,elementary ,treatise on their ,design and ,construction4 ,with ,examples and ,tables4 ,by ,,archibald ,,sharp1 ,b4,sc41 ,whitworth ,scholar2 ,associate ,member of the ,institution of ,civil ,engineers4 ,with #efe ,illustrations and ,diagrams4 ,crown #hvo41 #aes4 ,,taylor4--,,cotton ,,weaving ,,and ,,designing4 ,by ,,john ,t4 ,,taylor4 ,with #cgc ,diagrams4 ,crown #hvo41 #gs4 #f;d4 net4 ,,watts4--,,an ,,introductory ,,manual ,,for ,,sugar ,,growers4 ,by ,,francis ,,watts1 ,f4,c4,s41 ,f4,i4,c4 ,with #bj ,illustrations4 ,crown #hvo41 #fs4 ,,physiography ,,and ,,geology4 ,,bird4--,works by ,,charles ,,bird1 ,b4,a4 ,,elementary ,,geology4 ,with ,geological ,map of the ,british ,isles1 and #bdg ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,advanced ,,geology4 ,a ,manual for ,students in ,advanced ,classes and for ,general ,readers4 ,with over #cjj ,illustrations1 a ,geological ,map of the ,british ,isles 7coloured71 and a set of ,questions for ,examination4 ,crown #hvo41 #gs4 #f;d4 ,,thornton4--,works by ,j4 ,,thornton1 ,m4,a4 ,,elementary ,,practical ,,physiography 7for ,section ,i4 of the ,new ,syllabus of the ,science and ,art ,department74 ,with #bae ,illustrations4 ,crown #hvo1 #bs4 #f;d4 ,,elementary ,,physiography3 an ,introduction to the ,study of ,nature4 ,with #ab ,maps and #bdg ,illustrations4 ,with ,appendix on ,astronomical ,instruments and ,measurements4 ,crown #hvo41 #bs4 #f;d4 ,,advanced ,,physiography4 ,with #f ,maps and #bjc ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,health ,,and ,,hygiene4 ,,brodribb4--,,manual ,,of ,,health ,,and ,,temperance4 ,by ,t4 ,,brodribb1 ,m4,a4 ,with ,extracts from ,gough's ',temperance ,orations'4 ,revised and ,edited by the ,rev4 ,w4 ,,ruthven ,,pym1 ,m4,a41 ,member of the ,sheffield ,school ,board4 ,crown #hvo41 #as4 #f;d4 ,,buckton4--,,health ,,in ,,the ,,house2 ,twenty-five ,lectures on ,elementary ,physiology4 ,by ,mrs4 ,c4 ,m4 ,,buckton4 ,with #da ,woodcuts and ,diagrams4 ,crown #hvo41 #bs4 ,,corfield4--,,the ,,laws ,,of ,,health4 ,by ,w4 ,h4 ,,corfield1 ,m4,a44 ,m4,d4 ,fcp4 #hvo41 #as4 #f;d4 ,,notter ,,and ,,firth4--,works by ,j4 ,l4 ,,notter1 ,m4,a41 ,m4,d41 and ,r4 ,h4 ,,firth1 ,f4,r4,c4,s4 ,,hygiene4 ,with #ie ,illustrations4 ,crown #hvo41 #cs4 #f;d4 ,,practical ,,domestic ,,hygiene4 ,with #hc ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,poore4--,works by ,,george ,,vivian ,,poore1 ,m4,d4 ,,essays ,,on ,,rural ,,hygiene4 ,crown #hvo41 #fs4 #f;d4 ,,the ,,dwelling-,,house4 ,with #cf ,illustrations4 ,crown #hvo41 #cs4 #f;d4 ,,wilson4--,a ,,manual ,,of ,,health-,,science3 adapted for use in ,schools and ,colleges4 ,by ,,andrew ,,wilson1 ,f4,r4,s4,e41 ,f4,l4,s41 etc4 ,with #gd ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,natural ,,history4 ,,furneaux4--,works by ,,william ,s4 ,,furneaux1 ,f4,r4,g4,s4 ,,the ,,outdoor ,,world2 or1 ,the ,young ,collector's ,handbook4 ,with #ah ,plates1 #af of which are coloured1 and #edi ,illustrations in the ,text4 ,crown #hvo41 #gs4 #f;d4 ,,life ,,in ,,ponds ,,and ,,streams4 ,with #h ,coloured ,plates and #cca ,illustrations in the ,text4 ,crown #hvo41 #gs4 #f;d4 ,,butterflies ,,and ,,moths 7,british74 ,with #ab ,coloured ,plates and #bda ,illustrations in the ,text4 #gs4 #f;d4 ,,hudson4--,,british ,,birds4 ,by ,w4 ,h4 ,,hudson1 ,c4,m4,z4,s4 ,with #h ,coloured ,plates from ,original ,drawings by ,a4 ,,thorburn1 and #h ,plates and #ajj ,figures by ,c4 ,e4 ,,lodge1 and #c ,illustrations from ,photographs4 ,crown #hvo41 #gs4 #f;d4 ,,stanley4--,a ,,familiar ,,history ,,of ,,birds4 ,by ,e4 ,,stanley1 ,d4,d41 formerly ,bishop of ,norwich4 ,with #afj ,illustrations4 ,crown #hvo1 #cs4 #f;d4 ,,medicine ,,and ,,surgery4 ,,ashby4--,,notes ,,on ,,physiology ,,for ,,the ,,use ,,of ,,students ,,preparing ,,for ,,examination4 ,by ,,henry ,,ashby1 ,m4,d4 ,lond41 ,f4,r4,c4,p41 ,physician to the ,general ,hospital for ,sick ,children1 ,manchester2 formerly ,demonstrator of ,physiology1 ,liverpool ,school of ,medicine4 ,sixth ,edition1 thoroughly ,revised4 ,with #ada ,illustrations4 ,fcp4 #hvo41 #es4 ,,ashby ,,and ,,wright4--,,the ,,diseases ,,of ,,children1 ,,medical ,,and ,,surgical4 ,by ,,henry ,,ashby1 ,m4,d41 ,lond41 ,f4,r4,c4,p41 ,physician to the ,general ,hospital for ,sick ,children1 ,manchester2 and ,g4 ,a4 ,,wright1 ,b4,a41 ,m4,b4 ,oxon41 ,f4,r4,c4,s41 ,eng41 ,assistant ,surgeon to the ,manchester ,royal ,infirmary1 and ,surgeon to the ,children's ,hospital4 ,enlarged and ,improved ,edition4 ,with #aib ,illustrations4 #hvo41 #bes4 ,,bennett4--,works by ,,william ,h4 ,,bennett1 ,f4,r4,c4,s41 ,surgeon to ,st4 ,george's ,hospital2 ,member of the ,board of ,examiners1 ,royal ,college of ,surgeons of ,england4 ,,clinical ,,lectures ,,on ,,varicose ,,veins ,,of ,,the ,,lower ,,extremities4 ,with #c ,plates4 #hvo41 #fs4 ,,on ,,varicocele2 ,a ,,practical ,,treatise4 ,with #d ,tables and a ,diagram4 #hvo41 #es4 ,,clinical ,,lectures ,,on ,,abdominal ,,hernia3 chiefly in relation to ,treatment1 including the ,radical ,cure4 ,with #ab ,diagrams in the ,text4 #hvo41 #hs4 #f;d4 ,,bentley4--,a ,,text-,,book ,,of ,,organic ,,materia ,,medica4 ,comprising a ,description of the ,vegetable and ,animal ,drugs of the ,british ,pharmacopoeia1 with some others in common use4 ,arranged ,systematically1 and ,especially ,designed for ,students4 ,by ,,robert ,,bentley1 ,m4,r4,c4,s4 ,eng41 ,f4,l4,s4 ,with #fb ,illustrations on ,wood4 ,crown #hvo41 #gs4 #f;d4 ,,brodie4--,,the ,,essentials ,,of ,,experimental ,,physiology4 ,for the ,use of ,students4 ,by ,t4 ,g4 ,,brodie1 ,m4,d41 ,lecturer on ,physiology1 ,st4 ,thomas's ,hospital ,medical ,school4 ,in the press4 ,,cabot4--,a ,,guide ,,to ,,the ,,clinical ,,examination ,,of ,,the ,,blood ,,for ,,diagnostic ,,purposes4 ,by ,,richard ,c4 ,,cabot1 ,m4,d4 ,with #c ,coloured ,plates and #bh ,illustrations in the ,text4 #hvo41 #afs4 ,,clarke4--,works by ,j4 ,,jackson ,,clarke1 ,m4,d4 ,lond41 ,f4,r4,c4,s41 ,assistant ,surgeon at the ,north-west ,london and ,city ,orthopdic ,hospitals1 late ,senior ,demonstrator of ,anatomy1 ,demonstrator of ,bacteriology1 and ,curator of the ,museum in ,st4 ,mary's ,hospital ,medical ,school1 and ,pathologist to ,st4 ,mary's ,hospital4 ,,surgical ,,pathology ,,and ,,principles4 ,with #aid ,illustrations4 ,crown #hvo41 #ajs4 #f;d4 ,,post-,,mortem ,,examinations ,,in ,,medico-,,legal ,,and ,,ordinary ,,cases4 ,with ,special ,chapters on the ,legal ,aspects of ,post ,mortems1 and on ,certificates of ,death4 ,fcp4 #hvo41 #bs4 #f;d4 ,,coats4--,a ,,manual ,,of ,,pathology4 ,by ,,joseph ,,coats1 ,m4,d41 ,professor of ,pathology in the ,university of ,glasgow4 ,third ,edition4 ,revised throughout4 ,with #ejg ,illustrations4 #hvo41 #cas4 #f;d4 ,,cooke4--,works by ,,thomas ,,cooke1 ,f4,r4,c4,s4 ,eng41 ,b4,a41 ,b4,sc41 ,m4,d41 ,paris1 ,senior ,assistant ,surgeon to the ,westminster ,hospital4 ,,aphorisms ,,in ,,applied ,,anatomy ,,and ,,operative ,,surgery4 ,including #ajj ,typical viv voce ,questions on ,surface ,marking1 etc4 ,crown #hvo41 #cs4 #f;d4 ,,dissection ,,guides4 ,aiming at ,extending and ,facilitating such ,practical work in ,anatomy as will be specially useful in connection with an ordinary ,hospital ,curriculum4 #hvo41 #ajs4 #f;d4 ,,dakin4--,a ,,handbook ,,of ,,midwifery4 ,by ,,william ,,radford ,,dakin1 ,m4,d41 ,f4,r4,c4,p41 ,obstetric ,physician and ,lecturer on ,midwifery at ,st4 ,george's ,hospital1 etc4 ,with #cid ,illustrations4 ,large crown #hvo41 #ahs4 ,,dickinson4--,works by ,w4 ,,howship ,,dickinson1 ,m4,d4 ,cantab41 ,f4,r4,c4,p41 ,physician to1 and ,lecturer on ,medicine at1 ,st4 ,george's ,hospital1 ,consulting ,physician to the ,hospital for ,sick ,children4 ,,on ,,renal ,,and ,,urinary ,,affections4 ,with #ab ,plates and #abb ,woodcuts4 ,three ,parts4 #hvo41 #c #ds4 #f;d4 ,,the ,,tongue ,,as ,,an ,,indication ,,of ,,disease2 being the ,lumleian ,lectures delivered at the ,royal ,college of ,physicians in ,march1 #ahhh4 #hvo41 #gs4 #f;d4 ,,occasional ,,papers ,,on ,,medical ,,subjects1 #ahee-ahif4 #hvo41 #abs4 ,,duckworth4--,,the ,,sequels ,,of ,,disease3 being the ,lumleian ,lectures delivered in the ,royal ,college of ,physicians1 #ahif4 ,together with ,observations on ,prognosis in ,disease4 ,by ,sir ,,dyce ,,duckworth1 ,m4,d41 ,,ll4,d41 ,fellow and ,treasurer of the ,royal ,college of ,physicians1 etc4 #hvo41 #ajs4 #f;d4 ,,erichsen4--,,the ,,science ,,and ,,art ,,of ,,surgery2 a ,treatise on ,surgical ,injuries1 ,diseases1 and ,operations4 ,by ,sir ,,john ,,eric ,,erichsen1 ,bart41 ,f4,r4,s41 ,,ll4,d4 ,edin41 ,hon4 ,m4,ch4 and ,f4,r4,c4,s4 ,ireland1 late ,surgeon ,extraordinary to ,h4,m4 the ,queen4 ,illustrated by nearly #ajjj ,engravings on ,wood4 #b vols4 ,royal #hvo41 #dhs4 ,,fowler ,,and ,,goodlee4--,,the ,,diseases ,,and ,,injuries ,,of ,,the ,,lungs ,,and ,,pleura4 ,by ,,james ,,kingston ,,fowler1 ,m4,a41 ,m4,d41 ,f4,r4,c4,b41 ,physician and ,lecturer on ,pathological ,anatomy1 ,middlesex ,hospital1 etc42 and ,,rickman ,j4 ,,goodlee1 ,b4,a4 ,lond41 ,m4,b41 ,f4,r4,c4,s41 etc4 ,with ,illustrations4 ,in the press4 ,,garrod4--,works by ,,sir ,,alfred ,,baring ,,garrod1 ,m4,d41 ,f4,r4,s41 etc41 ,physician ,extraordinary to ,h4,m4 the ,queen2 ,consulting ,physician to ,king's ,college ,hospital2 late ,vice-,president of the ,royal ,college of ,physicians4 ,a ,,treatise ,,on ,,gout ,,and ,,rheumatic ,,gout 7,,rheumatoid ,,arthritis74 ,third ,edition1 thoroughly ,revised and ,enlarged4 ,with #f ,plates1 comprising #ba ,figures 7#ad ,coloured71 and #bg ,illustrations engraved on ,wood4 #hvo41 #bas4 ,,the ,,essentials ,,of ,,materia ,,medica ,,and ,,therapeutics4 ,the ,thirteenth ,edition1 ,revised and ,edited1 under the supervision of the ,author1 by ,,nestor ,,tirard1 ,m4,d4 ,lond41 ,f4,r4,c4,p41 ,professor of ,materia ,medica and ,therapeutics in ,king's ,college1 ,london1 etc4 ,crown #hvo41 #abs4 #f;d4 ,,gray4--,,anatomy1 ,,descriptive ,,and ,,surgical4 ,by ,,henry ,,gray1 ,f4,r4,s41 late ,lecturer on ,anatomy at ,st4 ,george's ,hospital4 ,the ,fourteenth ,edition1 re-edited by ,t4 ,,pickering ,,pick1 ,surgeon to ,st4 ,george's ,hospital1 ,inspector of ,anatomy in ,england and ,wales1 late ,member of the ,court of ,examiners1 ,royal ,college of ,surgeons of ,england4 ,with #gje large ,woodcut ,illustrations1 a large proportion of which are ,coloured1 the ,arteries being coloured red1 the ,veins blue1 and the ,nerves yellow4 ,the attachments of the muscles to the bones1 in the section on ,osteology1 are also shown in coloured outline4 ,royal #hvo41 #cfs4 ,,halford4--,,the ,,life ,,of ,,sir ,,henry ,,halford1 ,bart41 ,g4,c4,h41 ,m4,d41 ,f4,r4,s41 ,president of the ,royal ,college of ,physicians1 ,physician to ,george ,,iii41 ,george ,,iv41 ,william ,,iv41 and to ,her ,majesty ,queen ,victoria4 ,by ,,william ,,munk1 ,m4,d41 ,f4,s4,a41 ,fellow and late ,vice-,president of the ,royal ,college of ,physicans of ,london4 ,with #b ,portraits4 #hvo41 #abs4 #f;d4 ,,halliburton4--,works by ,w4 ,d4 ,,halliburton1 ,m4,d41 ,f4,r4,s41 ,m4,r4,c4,p41 ,professor of ,physiology in ,king's ,college1 ,london2 ,lecturer on ,physiology at the ,london ,school of ,medicine for ,women4 ,a ,,text-,,book ,,of ,,chemical ,,physiology ,,and ,,pathology4 ,with #ajd ,illustrations4 #hvo41 #bhs4 ,,essentials ,,of ,,chemical ,,physiology4 #hvo41 #es4 999999 ,this is a book suitable for medical students4 ,it treats of the subject in the same way as ,prof4 ,,schafer',s ,essentials treats of ,histology4 ,it contains a number of elementary and advanced practical lessons1 followed in each case by a brief descriptive account of the facts related to the exercises which are intended to be performed by each member of the class4 ,,lang4--,,the ,,methodical ,,examination ,,of ,,the ,,eye4 ,being ,part ,i4 of a ,guide to the ,practice of ,ophthalmology for ,students and ,practitioners4 ,by ,,william ,,lang1 ,f4,r4,c4,s4 ,eng41 ,surgeon to the ,royal ,london ,ophthalmic ,hospital1 ,moorfields1 etc4 ,with #ae ,illustrations4 ,crown #hvo41 #cs4 #f;d4 ,,liveing4--,,handbook ,,on ,,diseases ,,of ,,the ,,skin4 ,with especial reference to ,diagnosis and ,treatment4 ,by ,,robert ,,liveing1 ,m4,a4 and ,m4,d41 ,cantab41 ,f4,r4,c4,p4 ,lond41 etc41 ,physician to the ,department for ,diseases of the ,skin at the ,middlesex ,hospital1 etc4 ,fcp4 #hvo41 #es4 ,,longmore4--,works by ,surgeon-,general ,sir ,t4 ,,longmore ,c4,b41 ,f4,r4,c4,s41 late ,professor of ,military ,surgery in the ,army ,medical ,school1 ,officer of the ,legion of ,honour4 ,,the ,,illustrated ,,optical ,,manual2 ,,or1 ,,handbook ,,of ,,instructions ,,for ,,the ,,guidance ,,of ,,surgeons ,,in ,,testing ,,quality ,,and ,,range ,,of ,,vision1 ,,and ,,in ,,distinguishing ,,and ,,dealing ,,with ,,optical ,,defects ,,in ,,general4 ,illustrated by #gd ,drawings and ,diagrams by ,inspector-,general ,dr4 ,,macdonald1 ,r4,n41 ,f4,r4,s41 ,c4,b4 ,fourth ,edition4 #hvo41 #ads4 ,,gunshot ,,injuries4 ,their ,history1 ,characteristic ,features1 ,complications1 and ,general ,treatment2 with ,statistics concerning them as they have been met with in ,warfare4 ,with #gh ,illustrations4 #hvo41 #cas4 #f;d4 ,,luff4--,,text-,,book ,,of ,,forensic ,,medicine ,,and ,,toxicology4 ,by ,,arthur ,p4 ,,luff1 ,m4,d41 ,b4,sc4 7,lond471 ,physician in ,charge of ,out-,patients and ,lecturer on ,medical ,jurisprudence and ,toxicology in ,st4 ,mary's ,hospital2 ,examiner in ,forensic ,medicine in the ,university of ,london2 ,external ,examiner in ,forensic ,medicine in the ,victoria ,university2 ,official ,analyst to the ,home ,office4 ,with #ac full-page ,plates 7#a in colours7 and #cc ,illustrations in the ,text4 #b vols4 ,crown #hvo41 #bds4 ,,newman4--,,on ,,the ,,diseases ,,of ,,the ,,kidney ,,amenable ,,to ,,surgical ,,treatment4 ,lectures to ,practitioners4 ,by ,,david ,,newman1 ,m4,d41 ,surgeon to the ,western ,infirmary ,out-,door ,department2 ,pathologist and ,lecturer on ,pathology at the ,glasgow ,royal ,infirmary2 ,examiner in ,pathology in the ,university of ,glasgow2 ,vice-,president ,glasgow ,pathological and ,clinical ,society4 #hvo41 #hs4 ,,owen4--,a ,,manual ,,of ,,anatomy ,,for ,,senior ,,students4 ,by ,,edmund ,,owen1 ,m4,b41 ,f4,r4,s4,c41 ,senior ,surgeon to the ,hospital for ,sick ,children1 ,great ,ormond ,street1 ,surgeon to ,st4 ,mary's ,hospital1 ,london1 and co-,lecturer on ,surgery1 late ,lecturer on ,anatomy in its ,medical ,school4 ,with #baj ,illustrations4 ,crown #hvo41 #abs4 #f;d4 ,,poole4--,,cookery ,,for ,,the ,,diabetic4 ,by ,w4 ,h4 and ,mrs4 ,,poole4 ,with ,preface by ,dr4 ,,pavy4 ,fcap4 #hvo41 #bs4 #f;d4 ,,quain4--,a ,,dictionary ,,of ,,medicine2 ,including ,general ,pathology1 ,general ,therapeutics1 ,hygiene1 and the ,diseases of ,women and ,children4 ,by ,various ,writers4 ,edited by ,,richard ,,quain1 ,bart41 ,m4,d4 ,lond41 ,,ll4,d4 ,edin4 7,hon47 ,f4,r4,s41 ,physician ,extraordinary to ,h4,m4 the ,queen1 ,president of the ,general ,medical ,council1 ,member of the ,senate of the ,university of ,london1 etc4 ,assisted by ,,frederick ,,thomas ,,roberts1 ,m4,d4 ,lond41 ,b4,sc41 ,fellow of the ,royal ,college of ,physicians1 ,fellow of ,university ,college1 ,professor of ,materia ,medica and ,therapeutics1 ,university ,college1 c42 and ,j4 ,,mitchell ,,bruce1 ,m4,a4 ,abdn41 ,m4,d4 ,lond41 ,fellow of the ,royal ,college of ,physicians of ,london1 ,physician and ,lecturer on the ,principles and ,practice of ,medicine1 ,charing ,cross ,hospital1 c4 ,new ,edition1 ,revised throughout and ,enlarged4 #b ,vols4 ,medium #hvo41 #djs4 net4 ,,quain4--,,quain',s 7,,jones7 ,,elements ,,of ,,anatomy4 ,the ,tenth ,edition4 ,edited by ,,edward ,,albert ,,sch,,fer1 ,f4,r4,s41 ,professor of ,physiology and ,histology in ,university ,college1 ,london2 and ,,george ,,dancer ,,thane1 ,professor of ,anatomy in ,university ,college1 ,london4 999999 ,the several parts of this work form ,,complete ,,text-,,books ,,of ,,their ,,respective ,,subjects4 ,,vol4 ,i41 ,,part ,i4 ,,embryology4 ,by ,e4 ,a4 ,,sch,,fer1 ,f4,r4,s4 ,with #bjj ,illustrations4 ,royal #hvo41 #is4 ,,vol4 ,i41 ,,part ,,ii4 ,,general ,,anatomy ,,or ,,histology4 ,by ,e4 ,a4 ,,sch,,fer1 ,f4,r4,s4 ,with #bia ,illustrations4 ,royal #hvo41 #abs4 #f;d4 ,,vol4 ,,ii41 ,,part ,i4 ,,osteology4 ,by ,g4 ,d4 ,,thane4 ,with #afh ,illustrations4 ,royal #hvo41 #is4 ,,vol4 ,,ii41 ,,part ,,ii4 ,,arthrology--,,myology--,,angeiology4 ,by ,g4 ,d4 ,,thane4 ,with #bee ,illustrations4 ,royal #hvo41 #ahs4 ,,vol4 ,,iii41 ,,part ,i4 ,,the ,,spinal ,,cord ,,and ,,brain4 ,by ,e4 ,a4 ,,sch,,fer1 ,f4,r4,s4 ,with #aci ,illustrations4 ,royal #hvo41 #abs4 #f;d4 ,,vol4 ,,iii4 ,,part ,,ii4 ,,the ,,nerves4 ,by ,g4 ,d4 ,,thane4 ,with #ajb ,illustrations4 ,royal #hvo41 #is4 ,,vol4 ,,iii41 ,,part ,,iii4 ,,the ,,organs ,,of ,,the ,,senses4 ,by ,e4 ,a4 ,,sch,,fer1 ,f4,r4,s4 ,with #agh ,illustrations4 ,royal #hvo41 #is4 ,,vol4 ,,iii41 ,,part ,,iv4 ,,splanchnology4 ,by ,e4 ,a4 ,,sch,,fer1 ,f4,r4,s41 and ,,johnson ,,symington1 ,m4,d4 ,with #ccg ,illustrations4 ,royal #hvo41 #afs4 ,,appendix4 ,,superficial ,,and ,,surgical ,,anatomy4 ,by ,professor ,g4 ,d4 ,,thane and ,professor ,r4 ,j4 ,,godlee1 ,m4,s4 ,with #bi ,illustrations4 ,royal #hvo41 #fs4 #f;d4 ,,richardson4--,,vita ,,medica3 ,chapters of ,medical ,life and ,work4 ,by ,sir ,b4 ,w4 ,,richardson1 ,m4,a41 ,,ll4,d41 ,f4,r4,s4 #hvo41 #afs4 ,,sch,,fer4--,,the ,,essentials ,,of ,,histology4 ,descriptive and ,practical4 ,for the ,use of ,students4 ,by ,e4 ,a4 ,,sch,,fer1 ,f4,r4,s41 ,jodrell ,professor of ,physiology in ,university ,college1 ,london2 ,editor of the ,histological ,portion of ,quain's ',anatomy'4 ,illustrated by more than #cjj ,figures1 many of which are new4 ,fourth ,edition1 ,revised and ,enlarged4 #hvo41 #gs4 #f;d4 7,interleaved1 #ajs47 ,,schenk4--,,manual ,,of ,,bacteriology4 ,for ,practitioners and ,students4 ,with especial reference to ,practical ,methods4 ,by ,dr4 ,s4 ,l4 ,,schenk1 ,professor 7,extraordinary7 in the ,university of ,vienna4 ,translated from the ,german1 with an ,appendix1 by ,w4 ,r4 ,,dawson1 ,b4,a41 ,m4,d41 ,univ4 ,dub42 late ,university ,travelling ,prizeman in ,medicine4 ,with #ajj ,illustrations1 some of which are coloured4 #hvo41 #ajs4 net4 ,,smale ,,and ,,colyer4 ,,diseases ,,and ,,injuries ,,of ,,the ,,teeth1 including ,pathology and ,treatment3 a ,manual of ,practical ,dentistry for ,students and ,practitioners4 ,by ,,morton ,,smale1 ,m4,r4,c4,s41 ,l4,s4,a41 ,l4,d4,s41 ,dental ,surgeon to ,st4 ,mary's ,hospital1 ,dean of the ,school1 ,dental ,hospital of ,london1 etc42 and ,j4 ,f4 ,,colyer1 ,l4,r4,c4,p41 ,m4,r4,c4,s41 ,l4 ,d4,s41 ,assistant ,dental ,surgeon to ,charing ,cross ,hospital1 and ,assistant ,dental ,surgeon to the ,dental ,hospital of ,london4 ,with #ccd ,illustrations4 ,large ,crown #hvo41 #aes4 ,,smith 7,h4 ,f474 ,,the ,,handbook ,,for ,,midwives4 ,by ,,henry ,,fly ,,smith1 ,b4,a41 ,m4,b4 ,oxon41 ,m4,r4,c4,s4 ,second ,edition4 ,with #da ,woodcuts4 ,crown #hvo41 #es4 ,,stevenson4--,,wounds ,,in ,,war3 the ,mechanism of their ,production and their ,treatment4 ,by ,surgeon-,colonel ,w4 ,f4 ,,stevenson 7,army ,medical ,staff71 ,a4,b41 ,m4,b41 ,m4,ch4 ,dublin ,university1 ,professor of ,military ,surgery1 ,army ,medical ,school1 ,netley4 ,with #hf ,illustrations4 #hvo41 #ahs4 ,,tirard4--,,diphtheria ,,and ,,antitoxin4 ,by ,,nestor ,,tirard1 ,m4,d4 ,lond41 ,fellow of the ,royal ,college of ,physicians2 ,fellow of ,king's ,college1 ,london2 ,professor of ,materia ,medica and ,therapeutics at ,king's ,college2 ,physician to ,king's ,college ,hospital2 and ,senior ,physician to the ,evelina ,hospital for ,sick ,children4 #hvo41 #gs4 #f;d4 ,,wakley4--,,the ,,life ,,and ,,times ,,of ,,thomas ,,wakley1 ,founder and ,first ,editor of the ,lancet1 ,member of ,parliament for ,finsbury1 and ,coroner for ,west ,middlesex4 ,by ,,squire ,,sprigge1 ,m4,b4 ,cantab4 ,with #b ,portraits4 #hvo41 #ahs4 ,,waller4--,works by ,,augustus ,d4 ,,waller1 ,m4,d41 ,lecturer on ,physiology at ,st4 ,mary's ,hospital ,medical ,school1 ,london2 late ,external ,examiner at the ,victorian ,university4 ,,an ,,introduction ,,to ,,human ,,physiology4 ,third ,edition1 ,revised4 ,with #cad ,illustrations4 #hvo41 #ahs4 ,,lectures ,,on ,,physiology4 ,first ,series4 ,on ,animal ,electricity4 #hvo41 #es4 net4 ,,exercises ,,in ,,practical ,,physiology4 ,part ,i4 ,elementary ,physiological ,chemistry4 ,by ,,augustus ,d4 ,,waller and ,w4 ,,legge ,,symes4 #hvo41 #as4 net4 ,part ,,ii4 in the press4 ,part ,,iii4 ,physiology of the ,nervous ,system2 ,electro-,physiology4 #hvo41 #bs4 #f;d4 net4 ,,weichselbaum4--,,the ,,elements ,,of ,,pathological ,,histology4 ,with ,special ,reference to ,practical ,methods4 ,by ,dr4 ,,anton ,,weichselbaum1 ,professor of ,pathology in the ,university of ,vienna4 ,translated by ,w4 ,r4 ,,dawson1 ,m4,d4 7,dub471 ,demonstrator of ,pathology in the ,royal ,college of ,surgeons1 ,ireland1 late ,medical ,travelling ,prizeman of ,dublin ,university1 etc4 ,with #bba ,figures1 partly in ,colours1 a ,cromo-lithographic ,plate1 and #g ,photographic ,plates4 ,royal #hvo41 #bas4 net4 ,,wilks ,,and ,,moxon4--,,lectures ,,on ,,pathological ,,anatomy4 ,by ,sir ,,samuel ,,wilks1 ,bart41 ,m4,d41 ,f4,r4,s41 ,president of the ,royal ,college of ,physicians1 and ,physician ,extraordinary to ,h4,m4 the ,queen1 and the late ,,walter ,,moxon1 ,m4,d41 ,f4,r4,c4,p41 ,physician to1 and some time ,lecturer on ,pathology at1 ,guy's ,hospital4 ,third ,edition1 thoroughly ,revised4 ,by ,sir ,,samuel ,,wilks1 ,bart41 ,m4,d41 ,,ll4,d41 ,f4,r4,s4 #hvo41 #ahs4 ,,veterinary ,,medicine1 ,,etc4 ,,steel4--,works by ,,john ,,henry ,,steel1 ,f4,r4,c4,v4,s41 ,f4,z4,s41 ,a4,v4,d41 late ,professor of ,veterinary ,science and ,principal of ,bombay ,veterinary ,college4 ,a ,,treatise ,,on ,,the ,,diseases ,,of ,,the ,,dog2 being a ,manual of ,canine ,pathology4 ,especially adapted for the use of ,veterinary ,practitioners and ,students4 ,with #hh ,illustrations4 #hvo41 #ajs4 #f;d4 ,a ,,treatise ,,on ,,the ,,diseases ,,of ,,the ,,ox2 being a ,manual of ,bovine ,pathology4 ,especially adapted for the use of ,veterinary ,practitioners and ,students4 ,with #b ,plates and #aag ,woodcuts4 #hvo41 #aes4 ,a ,,treatise ,,on ,,the ,,diseases ,,of ,,the ,,sheep2 being a ,manual of ,ovine ,pathology for the use of ,veterinary ,practitioners and ,students4 ,with ,coloured ,plate and #ii ,woodcuts4 #hvo41 #abs4 ,,outlines ,,of ,,equine ,,anatomy2 a ,manual for the use of ,veterinary ,students in the ,dissecting ,room4 ,crown #hvo41 #gs4 #f;d4 ,,fitzwygram4--,,horses ,,and ,,stables4 ,by ,major-,general ,sir ,f4 ,,fitzwygram1 ,bart4 ,with #ef pages of ,illustrations4 #hvo41 #bs4 #f;d4 net4 ,,schreiner4--,,the ,,angora ,,goat 7published under the auspices of the ,south ,african ,angora ,goat ,breeders' ,association71 and a ,paper on the ,ostrich 7reprinted from the ,zoologist for ,march1 #ahig74 ,by ,s4 ,c4 ,,cronwright ,,schreiner4 #hvo4 ',,stonehenge4'--,,the ,,dog ,,in ,,health ,,and ,,disease4 ,by ',,stonehenge'4 ,with #gh ,wood ,engravings4 #hvo41 #gs4 #f;d4 ,,youatt4--,works by ,,william ,,youatt4 ,,the ,,horse4 ,revised and ,enlarged by ,w4 ,,watson1 ,m4,r4,c4,v4,s4 ,with #eb ,wood ,engravings4 #hvo41 #gs4 #f;d4 ,,the ,,dog4 ,revised and ,enlarged4 ,with #cc ,wood ,engravings4 #hvo41 #fs4 ,,physiology1 ,,biology1 ,,bacteriology1 ,,and ,,zoology4 7,and see ,,medicine ,,and ,,surgery47 ,,ashby4--,,notes ,,on ,,physiology1 for the ,use of ,students ,preparing for ,examination4 ,by ,,henry ,,ashby1 ,m4,d4 ,with #ada ,illustrations4 ,fcp4 #hvo41 #es4 ,,barnett4--,,the ,,making ,,of ,,the ,,body3 a ,children's ,book on ,anatomy and ,physiology1 for ,school and ,home ,use4 ,by ,mrs4 ,s4 ,a4 ,,barnett4 ,with #aac ,illustrations4 ,crown #hvo41 #as4 #i;d4 ,,bidgood4--,a ,,course ,,of ,,practical ,,elementary ,,biology4 ,by ,,john ,,bidgood1 ,b4,sc41 ,f4,l4,s4 ,with #bbf ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,bray4--,,physiology ,,and ,,the ,,laws ,,of ,,health1 in ,easy ,lessons for ,schools4 ,by ,mrs4 ,,charles ,,bray4 ,fcp4 #hvo41 #as4 ,,curtis4--,,essentials ,,of ,,practical ,,bacteriology4 ,by ,,henry ,j4 ,,curtis1 ,m4,d4 ,with numerous ,illustrations4 ,in the press4 ,,frankland4--,,micro-,,organisms ,,in ,,water4 ,together with an ,account of the ,bacteriological ,methods involved in their ,investigation4 ,specially designed for the use of those connected with the ,sanitary ,aspects of ,water-,supply4 ,by ,,percy ,,frankland1 ,ph4,d41 ,b4,sc4 7,lond471 ,f4,r4,s41 and ,mrs4 ,,percy ,,frankland4 ,with #b ,plates and ,numerous ,diagrams4 #hvo41 #afs4 net4 ,,furneaux4--,,human ,,physiology4 ,by ,w4 ,,furneaux1 ,f4,r4,g4,s4 ,with #bah ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,hudson ,,and ,,gosse4--,,the ,,rotifera1 or ',,wheel-,,animacules'4 ,by ,c4 ,t4 ,,hudson1 ,,ll4,d41 and ,p4 ,h4 ,,gosse1 ,f4,r4,s4 ,with #cj ,coloured and #d ,uncoloured ,plates4 ,in #f ,parts4 #dto41 #ajs4 #f;d4 each ,supplement #abs4 #f;d4 ,complete in #b vols41 with ,supplement1 #dto41 #d #ds4 ,,leumann4--,,notes ,,on ,,micro-,,organisms ,,pathogenic ,,to ,,man4 ,by ,surgeon-,captain ,b4 ,h4 ,s4 ,,leumann1 ,m4,b41 ,indian ,medical ,service4 #hvo41 #cs4 ,,macalister4--,works by ,,alexander ,,macalister1 ,m4,d41 ,professor of ,comparative ,anatomy and ,zoology1 ,university of ,dublin4 ,with #da ,diagrams4 ,,an ,,introduction ,,to ,,the ,,systematic ,,zoology ,,and ,,morphology ,,of ,,vertebrate ,,animals4 #hvo41 #ajs4 #f;d4 ,,zoology ,,of ,,the ,,invertebrate ,,animals4 ,with #ei ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,zoology ,,of ,,the ,,vertebrate ,,animals4 ,with #gg ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,morgan4--,,animal ,,biology3 an ,elementary ,text-,book4 ,by ,c4 ,,lloyd ,,morgan4 ,with #ajc ,illustrations4 ,crown #hvo41 #hs4 #f;d4 ,,schenk4--,,manual ,,of ,,bacteriology1 for ,practitioners and ,students1 with ,especial ,reference to ,practical ,methods4 ,by ,dr4 ,s4 ,l4 ,,schenk4 ,with #ajj ,illustrations1 some ,coloured4 #hvo41 #ajs4 net4 ,,thornton4--,,human ,,physiology4 ,by ,,john ,,thornton1 ,m4,a4 ,with #bfg ,illustrations1 some ,coloured4 ,crown #hvo41 #fs4 ,,botany4 ,,aitken4--,,elementary ,,text-,,book ,,of ,,botany4 ,by ,,edith ,,aitken1 late ,scholar of ,girton ,college4 ,with #djj ,diagrams4 ,crown #hvo41 #ds4 #f;d4 ,,bennett ,,and ,,murray4--,,handbook ,,of ,,cryptogamic ,,botany4 ,by ,,alfred ,w4 ,,bennett1 ,m4,a41 ,b4,sc41 ,f4,l4,s41 ,lecturer on ,botany at ,st4 ,thomas's ,hospital2 and ,,george ,,murray1 ,f4,l4,s41 ,keeper of ,botany1 ,british ,museum4 ,with #cgh ,illustrations4 #hvo41 #afs4 ,,cross ,,and ,,bevan4--,,cellulose3 an ,outline of the ,chemistry of the ,structural ,elements of ,plants4 ,with ,reference to their ,natural ,history and ,industrial ,uses4 ,by ,,cross and ,,bevan 7,c4 ,f4 ,cross1 ,e4 ,j4 ,bevan1 and ,c4 ,beadle74 ,with #ad ,plates4 ,crown #hvo41 #abs4 net4 ,,curtis4--,a ,,text-,,book ,,of ,,general ,,botany4 ,by ,,carlton ,c4 ,,curtis1 ,a4,m41 ,ph4,d41 ,tutor in ,botany in ,columbia ,university1 ,u4,s4,a4 ,with #hg ,illustrations4 #hvo41 #abs4 net4 ,,edmonds4--,works by ,,henry ,,edmonds1 ,b4,sc41 ,london4 ,,elementary ,,botany1 ,theoretical and ,practical4 ,with #cai ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,botany ,,for ,,beginners4 ,with #he ,illustrations4 ,fcp4 #hvo41 #as4 #f;d4 ,,kitchener4--,a ,,year',s ,,botany4 ,adapted to ,home and ,school ,use4 ,by ,,frances ,a4 ,,kitchener4 ,with #aie ,illustrations4 ,crown #hvo41 #es4 ,,lindley ,,and ,,moore4--,,the ,,treasury ,,of ,,botany4 ,edited by ,j4 ,,lindley1 ,m4,d41 ,f4,r4,s41 and ,t4 ,,moore1 ,f4,l4,s4 ,with #bj ,steel ,plates and numerous ,woodcuts4 ,two parts4 ,fcp4 #hvo41 #abs4 ,mc,n,a,b4--,,class-,,book ,,of ,,botany4 ,by ,w4 ,r4 ,,mcnab4 ,two ,parts4 ,,morphology ,,and ,,physiology4 ,with #db ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,classification ,,of ,,plants4 ,with #aah ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,sorauer4--,a ,,popular ,,treatise ,,on ,,the ,,physiology ,,of ,,plants4 ,for the use of ,gardeners1 or for ,students of ,horticulture and of ,agriculture4 ,by ,dr4 ,,paul ,,sorauer4 ,translated by ,f4 ,e4 ,,weiss1 ,b4,sc41 ,f4,l4,s4 ,with #cc ,illustrations4 #hvo41 #is4 net4 ,,thom ,,and ,,bennett4--,,structural ,,and ,,physiological ,,botany4 ,by ,,otto ,,wilhelm ,,thom and by ,,alfred ,w4 ,,bennett1 ,m4,a41 ,b4,sc41 ,f4,l4,s4 ,with ,coloured ,map and #fjj ,woodcuts4 ,fcp4 #hvo41 #fs4 ,,tubeuf4--,,diseases ,,of ,,plants ,,induced ,,by ,,cryptogamic ,,parasites4 ,introduction to the ,study of ,pathogenic ,fungi1 ,slime ,fungi1 ,bacteria and ,alg4 ,by ,dr4 ,,karl ,,freiherr ,,von ,,tubeuf1 ,privatdocent in the ,university of ,munich4 ,english ,edition by ,,william ,g4 ,,smith1 ,b4,sc41 ,ph4,d41 ,lecturer on ,plant ,physiology1 ,university of ,edinburgh4 ,with #ccj ,illustrations4 ,royal #hvo41 #ahs4 net4 ,,watts4--,a ,,school ,,flora4 ,for the use of ,elementary ,botanical ,classes4 ,by ,w4 ,,marshall ,,watts1 ,d4,sc4 ,lond4 ,crown #hvo41 #bs4 #f;d4 ,,agriculture4 ,,addyman4--,,agricultural ,,analysis4 ,a ,manual of ,quantitative ,analysis for ,students of ,agriculture4 ,by ,,frank ,t4 ,,addyman1 ,b4,sc4 7,lond471 ,f4,i4,c4 ,with #di ,illustrations4 ,crown #hvo41 #es4 net4 ,,coleman ,,and ,,addyman4--,,practical ,,agricultural ,,chemistry4 ,by ,j4 ,,bernard ,,coleman1 ,a4,r4,c4,sc41 ,f4,i4,c41 and ,,frank ,t4 ,,addyman1 ,b4,sc4 7,lond471 ,f4,i4,c4 ,with #bd ,illustrations4 ,crown #hvo41 #as4 #f;d4 net4 ,,cooke4--,,the ,,foundations ,,of ,,scientific ,,agriculture4 ,by ,,samuel ,,cooke1 ,a4,m41 ,a4,m4,i4,c4,e41 ,f4,i4,c41 ,f4,g4,s41 ,cor4,m4,r4,h4,s41 ,principal and ,professor of ,chemistry and ,geology1 ,college of ,science1 ,poona4 ,with #he ,illustrations and a ,series of ,examination ,questions4 ,crown #hvo41 #ds4 #f;d4 ,,webb4--,works by ,,henry ,j4 ,,webb1 ,ph4,d41 ,b4,sc4 7,lond472 late ,principal of the ,agricultural ,college1 ,aspatria4 ,,elementary ,,agriculture4 ,a ,text-,book specially adapted to the requirements of the ,science and ,art ,department1 the ,junior ,examination of the ,royal ,agricultural ,society1 and other ,elementary ,examinations4 ,with #cd ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,agriculture4 ,a ,manual for ,advanced ,science ,students4 ,with #ajj ,illustrations4 ,crown #hvo41 #gs4 #f;d4 net4 ,,works ,,by ,,john ,,tyndall1 ,d4,c4,l41 ,,ll4,d41 ,f4,r4,s4 ,,fragments ,,of ,,science3 a ,series of ,detached ,essays1 ,addresses1 and ,reviews4 #b vols4 ,crown #hvo41 #afs4 ,vol4 ,i4--,the ,constitution of ,nature--,radiation--,on ,radiant ,heat in ,relation to the ,colour and ,chemical ,constitution of ,bodies--,new ,chemical ,reactions produced by ,light--,on ,dust and ,disease--,voyage to ,algeria to observe the ,eclipse--,niagara--,the ,parallel ,roads of ,glen ,roy--,alpine ,sculpture--,recent ,experiments on ,fog-,signals--,on the ,study of ,physics--,on ,crystalline and ,slaty ,cleavage--,on ,paramagnetic and ,diamagnetic ,forces--,physical ,basis of ,solar ,chemistry--,elementary ,magnetism--,on ,force--,contributions to ,molecular ,physics--,life and ,letters of ,,faraday--,the ,copley ,medallist of #ahgj--,the ,copley ,medallist of #ahga--;,death by ,lightning--,science and the ,spirits4 ,vol4 ,,ii4--,reflections on ,prayer and ,natural ,law--,miracles and ,special ,providences--,on ,prayer as a ,form of ,physical ,energy--,vitality--,matter and ,force--,scientific ,materialism--,an ,address to ,students--,scientific ,use of the ,imagination--,the ,belfast ,address--,apology for the ,belfast ,address--,the ,rev4 ,,james ,,martineau and the ,belfast ,address--,fermentation1 and its ,bearings on ,surgery and ,medicine--,spontaneous ,generation--,science and ,man--,professor ,,virchow and ,evolution--,the ,electric ,light4 ,,new ,,fragments4 ,crown #hvo41 #ajs4 #f;d4 ,,contents4--,the ,sabbath--,goethe's ',farbenlehre'--,atoms1 ,molecules1 and ,ether ,waves--,count ,rumford--,louis ,pasteur1 his ,life and ,labours--,the ,rainbow and its ,congeners--,address delivered at the ,birkbeck ,institution on ,october #bb1 #ahhd--,thomas ,young--,life in the ,alps--,about ,common ,water--,personal ,recollections of ,thomas ,carlyle--,on ,unveiling the ,statue of ,thomas ,carlyle--,on the ,origin1 ,propagation1 and ,prevention of ,phthisis--,old ,alpine ,jottings--,a ,morning on ,alp ,lusgen4 ,,lectures ,,on ,,sound4 ,with ,frontispiece of ,fog-,syren1 and #bjc other ,woodcuts and ,diagrams in the ,text4 ,crown #hvo41 #ajs4 #f;d4 ,,heat1 ,a ,,mode ,,of ,,motion4 ,with #abe ,woodcuts and ,diagrams4 ,crown #hvo41 #abs4 ,,lectures ,,on ,,light ,,delivered ,,in ,,the ,,united ,,states ,,in #ahgb ,,and #ahgc4 ,with ,portrait1 ,lithographic ,plate1 and #ei ,diagrams4 ,crown #hvo41 #es4 ,,essays ,,on ,,the ,,floating ,,matter ,,of ,,the ,,air ,,in ,,relation ,,to ,,putrefaction ,,and ,,infection4 ,with #bd ,woodcuts4 ,crown #hvo41 #gs4 #f;d4 ,,researches ,,on ,,diamagnetism ,,and ,,magnecrystallic ,,action2 including the ,question of ,diamagnetic ,polarity4 ,crown #hvo41 #abs4 ,,notes ,,of ,a ,,course ,,of ,,nine ,,lectures ,,on ,,light1 delivered at the ,royal ,institution of ,great ,britain1 #ahfi4 ,crown #hvo41 #as4 #f;d4 ,,notes ,,of ,a ,,course ,,of ,,seven ,,lectures ,,on ,,electrical ,,phenomena ,,and ,,theories1 delivered at the ,royal ,institution of ,great ,britain1 #ahgj4 ,crown #hvo41 #as4 #f;d4 ,,lessons ,,in ,,electricity ,,at ,,the ,,royal ,,institution #ahge-ahgf4 ,with #eh ,woodcuts and ,diagrams4 ,crown #hvo41 #bs4 #f;d44 ,,the ,,glaciers ,,of ,,the ,,alps3 being a ,narrative of ,excursions and ,ascents4 ,an ,account of the ,origin and ,phenomena of ,glaciers1 and an ,exposition of the ,physical ,principles to which they are related4 ,with numerous ,illustrations4 ,crown #hvo41 #fs4 #f;d4 net4 ,,faraday ,,as ,a ,,discoverer4 ,crown #hvo41 #cs4 #f;d4 ,,text-,,books ,,of ,,science4 ,,photography4 ,by ,captain ,w4 ,,de ,,wiveleslie ,,abney1 ,c4,b41 ,f4,r4,s4 ,with #aje ,illustrations4 ,fcp4 #hvo41 #cs4 #f;d4 ,,the ,,strength ,,of ,,material ,,and ,,structures4 ,by ,sir ,j4 ,,anderson1 ,c4,e41 etc4 ,with #ff ,illustrations4 ,fcp4 #hvo41 #cs4 #f;d4 ,,railway ,,appliances4 ,by ,sir ,,john ,,wolfe ,,barry1 ,k4,c4,b41 ,m4,i4,c4,e4 ,with #bah ,illustrations4 ,fcp4 #hvo41 #ds4 #f;d4 ,,introduction ,,to ,,the ,,study ,,of ,,inorganic ,,chemistry4 ,by ,,william ,,allen ,,miller1 ,m4,d41 ,,ll4,d41 ,f4,r4,s4 ,with #gb ,illustrations4 #cs4 #f;d4 ,,quantitative ,,chemical ,,analysis4 ,by ,t4 ,e4 ,,thorpe1 ,f4,r4,s41 ,ph4,d4 ,with #hh ,illustrations4 ,fcp4 #hvo41 #ds4 #f;d4 ,,qualitative ,,analysis ,,and ,,laboratory ,,practice4 ,by ,t4 ,e4 ,,thorpe1 ,ph4,d41 ,f4,r4,s41 and ,m4 ,m4 ,,pattison ,,muir1 ,m4,a4 and ,f4,r4,s4,e4 ,with ,plate of ,spectra and #eg ,illustrations4 ,fcp4 #hvo41 #cs4 #f;d4 ,,introduction ,,to ,,the ,,study ,,of ,,chemical ,,philosophy4 ,by ,,william ,a4 ,,tilden1 ,d4,sc41 ,london1 ,f4,r4,s4 ,with #e ,illustrations4 ,with or without ,answers to ,problems4 ,fcp4 #hvo41 #ds4 #f;d4 ,,elements ,,of ,,astronomy4 ,by ,sir ,r4 ,s4 ,,ball1 ,,ll4,d41 ,f4,r4,s4 ,with #acj ,illustrations4 ,fcp4 #hvo41 #fs4 #f;d4 ,,systematic ,,mineralogy4 ,by ,,hilary ,,bauerman1 ,f4,g4,s4 ,with #cgc ,illustrations4 ,fcp4 #hvo41 #fs4 ,,descriptive ,,mineralogy4 ,by ,,hilary ,,bauerman1 ,f4,g4,s41 etc4 ,with #bcf ,illustrations4 ,fcp4 #hvo41 #fs4 ,,metals3 ,,their ,,properties ,,and ,,treatment4 ,by ,a4 ,k4 ,,huntington and ,w4 ,g4 ,,mcmillan4 ,with #abb ,illustrations4 ,fcp4 #hvo41 #gs4 #f;d4 ,,theory ,,of ,,heat4 ,by ,j4 ,,clerk ,,maxwell1 ,m4,a41 ,,ll4,d41 ,edin41 ,f4,r4,,ss41 ,l4 ,e4 ,with #ch ,illustrations4 ,fcp4 #hvo41 #ds4 #f;d4 ,,practical ,,physics4 ,by ,r4 ,t4 ,,glazebrook1 ,m4,a41 ,f4,r4,s41 and ,w4 ,n4 ,,shaw1 ,m4,a4 ,with #acd ,illustrations4 ,fcp4 #hvo41 #gs4 #f;d4 ,,preliminary ,,survey ,,and ,,estimates4 ,by ,,theodore ,,graham ,,gribble1 ,civil ,engineer4 ,including ,elementary ,astronomy1 ,route ,surveying1 ,tacheometry1 ,curve-ranging1 ,graphic ,mensuration1 ,estimates1 ,hydrography and ,instruments4 ,with #acc ,illustrations4 ,fcp4 #hvo41 #gs4 #f;d4 ,,algebra ,,and ,,trigonometry4 ,by ,,william ,,nathaniel ,,griffin1 ,b4,d4 #cs4 #f;d4 ,notes on1 with ,solutions of the more difficult ,questions4 ,fcp4 #hvo41 #cs4 #f;d4 ,,the ,,steam ,,engine4 ,by ,,george ,c4 ,v4 ,,holmes1 ,secretary of the ,institution of ,naval ,architects4 ,with #bab ,illustrations4 ,fcp4 #hvo41 #fs4 ,,electricity ,,and ,,magnetism4 ,by ,,fleeming ,,jenkin1 ,f4,r4,,ss41 ,l4 ,e4 ,with #agg ,illustrations4 ,fcp4 #hvo41 #cs4 #f;d4 ,,the ,,art ,,of ,,electro-,,metallurgy4 ,by ,g4 ,,gore1 ,,ll4,d41 ,f4,r4,s4 ,with #ef ,illus4 ,fcp4 #hvo41 #fs4 ,,telegraphy4 ,by ,w4 ,h4 ,,preece1 ,c4,b41 ,f4,r4,s41 ,m4,i4,c4,e41 and ,sir ,j4 ,,sivewright1 ,m4,a41 ,k4,c4,m4,g4 ,with #bee ,illustrations4 ,fcp4 #hvo41 #fs4 ,,physical ,,optics4 ,by ,r4 ,t4 ,,glazebrook1 ,m4,a41 ,f4,r4,s4 ,with #ahc ,illustrations4 ,fcp4 #hvo41 #fs4 ,,technical ,,arithmetic ,,and ,,mensuration4 ,by ,,charles ,w4 ,,merriefield1 ,f4,r4,s4 #cs4 #f;d4 ,key1 by the ,rev4 ,,john ,,hunter1 ,m4,a4 ,fcp4 #hvo41 #cs4 #f;d4 ,,the ,,study ,,of ,,rocks4 ,by ,,frank ,,rutley1 ,f4,g4,s4 ,with #f ,plates and #hh ,illustrations in the ,text1 ,fcp4 #hvo41 #ds4 #f;d4 ,,workshop ,,appliances1 including ,descriptions of some of the ,machine ,tools used by ,engineers4 ,by ,c4 ,p4 ,b4 ,,shelley1 ,m4,i4,c4,e4 ,with #cbc ,illustrations4 ,fcp4 #hvo41 #es4 ,,elements ,,of ,,machine ,,design4 ,by ,w4 ,,cawthorne ,,unwin1 ,f4,r4,s41 ,b4,sc41 ,m4,i4,c4,e4 ,,part ,i4 ,general ,principles1 ,fastenings and ,transmissive ,machinery4 ,with #cjd ,illustrations4 #fs4 ,,part ,,ii4 ,chiefly on ,engine ,details4 ,with #agd ,illustrations4 ,fcp4 #hvo41 #ds4 #f;d4 ,,structural ,,and ,,physiological ,,botany4 ,by ,,otto ,,wilhelm ,,thom1 and ,a4 ,w4 ,,bennett1 ,m4,a41 ,b4,sc41 ,f4,l4,s4 ,with #fjj ,illustrations4 ,fcp4 #hvo41 #fs4 ,,plane ,,and ,,solid ,,geometry4 ,by ,h4 ,w4 ,,watson1 ,m4,a4 ,fcp4 #hvo41 #cs4 #f;d4 ,,advanced ,,science ,,manuals4 999999 ,written specially to meet the requirements of the ,,advanced ,,stage of ,science ,subjects as laid down in the ,syllabus of the ,directory of the ,,science ,,and ,,art ,,department1 ,,south ,,kensington4 ,,building ,,construction4 ,by the ,author of ',rivington's ,notes on ,building ,construction'4 ,with #che ,illustrations and an ,appendix of ,examination ,questions4 ,crown #hvo41 #ds4 #f;d4 ,,theoretical ,,mechanics4 ,solids1 including ,kinematics1 ,statics1 and ,kinetics4 ,by ,a4 ,,thornton1 ,m4,a41 ,f4,r4,a4,s41 ,with #bbj ,illustrations1 #acj ,worked ,examples1 and over #ijj ,examples from ,examination ,papers1 etc4 ,crown #hvo41 #ds4 #f;d4 ,,heat4 ,by ,,mark ,r4 ,,wright1 ,hon4 ,inter4 ,b4,sc4 7,lond474 ,with #acf ,illustrations and numerous ,examples and ,examination ,papers4 ,crown #hvo41 #ds4 #f;d4 ,,light4 ,by ,w4 ,j4 ,a4 ,,emtage1 ,m4,a4 ,with #bcb ,illustrations4 ,cr4 #hvo41 #fs4 ,,magnetism ,,and ,,electricity4 ,by ,,arthur ,,william ,,poyser1 ,m4,a4 ,with #cag ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,inorganic ,,chemistry1 ,,theoretical ,,and ,,practical4 ,a ,manual for ,students in ,advanced ,classes of the ,science and ,art ,department4 ,by ,,william ,,jago1 ,f4,c4,s41 ,f4,i4,c4 ,with ,plate of ,spectra and #gh ,woodcuts4 ,crown #hvo41 #ds4 #f;d4 ,,geology3 a ,manual for ,students in ,advanced ,classes and for ,general ,readers4 ,by ,,charles ,,bird1 ,b4,a4 7,lond471 ,f4,g4,s4 ,with over #cjj ,illustrations1 a ,geological ,map of the ,british ,isles 7coloured71 and a set of ,questions for ,examination4 ,crown #hvo41 #gs4 #f;d4 ,,human ,,physiology3 a ,manual for ,students in advanced ,classes of the ,science and ,art ,department4 ,by ,,john ,,thornton1 ,m4,a4 ,with #bfh ,illustrations1 some of which are ,coloured1 and a set of ,questions for ,examination4 ,crown #hvo41 #fs4 ,,physiography4 ,by ,,john ,,thornton1 ,m4,a4 ,with #f ,maps1 #bjc ,illustrations1 and ,coloured ,plate of ,spectra4 ,crown #hvo41 #ds4 #f;d4 ,,agriculture4 ,by ,dr4 ,,henry ,j4 ,,webb1 ,ph4,d41 ,b4,sc4 ,with #ajj ,illustrations4 ,crown #hvo41 #gs4 #f;d4 net4 ,,hygiene4 ,by ,j4 ,,lane ,,notter1 ,m4,a41 ,m4,d41 ,professor of ,hygiene in the ,army ,medical ,school1 ,netley1 ,surgeon-,colonel1 ,army ,medical ,staff2 and ,r4 ,h4 ,,firth1 ,f4,r4,c4,s41 ,assistant ,professor of ,hygiene in the ,army ,medical ,school1 ,netley1 ,surgeon-,major1 ,army ,medical ,staff4 ,with #ie ,illustrations4 ,crown #hvo41 #cs4 #f;d4 ,,elementary ,,science ,,manuals4 999999 ,written specially to meet the requirements of the ,,elementary ,,stage ,,of ,,science ,,subjects as laid down in the ,syllabus of the ,directory of the ,,science ,,and ,,art ,,department4 ,,practical1 ,,plane1 ,,and ,,solid ,,geometry1 including ,graphic ,arithmetic4 ,by ,i4 ,h4 ,,morris4 ,fully ,illustrated with ,drawings4 ,crown #hvo41 #bs4 #f;d4 ,,geometrical ,,drawing ,,for ,,art ,,students4 ,embracing ,plane ,geometry and its ,applications1 the ,use of ,scales1 and the ,plans and ,elevations of ,solids1 as required in ,section ,i4 of ,science ,subject ,i4 ,by ,i4 ,h4 ,,morris4 ,crown #hvo41 #as4 #f;d4 ,,text-,,book ,,on ,,practical1 ,,solid1 ,,or ,,descriptive ,,geometry4 ,by ,,david ,,allan ,,low 7,whitworth ,scholar74 ,part ,i4 ,crown #hvo41 #bs4 ,part ,,ii4 ,crown #hvo41 #cs4 ,,an ,,introduction ,,to ,,machine ,,drawing ,,and ,,design4 ,by ,,david ,,allan ,,low4 ,with #ig ,illustrations4 ,crown #hvo41 #bs4 ,,building ,,construction4 ,by ,,edward ,j4 ,,burrell4 ,with #cjh ,illustrations and ,working ,drawings4 ,crown #hvo41 #bs4 #f;d4 ,,an ,,elementary ,,course ,,of ,,mathematics4 ,containing ,arithmetic2 ,euclid 7,book ,i41 with ,deductions and ,exercises72 and ,algebra4 ,crown #hvo41 #bs4 #f;d4 ,,theoretical ,,mechanics4 ,including ,hydrostatics and ,pneumatics4 ,by ,j4 ,e4 ,,taylor1 ,m4,a41 ,b4,sc4 ,with numerous ,examples and ,answers1 and #age ,diagrams and ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,theoretical ,,mechanics--,,solids4 ,by ,j4 ,e4 ,,taylor1 ,m4,a41 ,b4,sc4 7,lond474 ,with #afc ,illustrations1 #abj ,worked ,examples1 and over #ejj ,examples from ,examination ,papers1 etc4 ,crown #hvo41 #bs4 #f;d4 ,,theoretical ,,mechanics--,,fluids4 ,by ,j4 ,e4 ,,taylor1 ,m4,a41 ,b4,sc4 7,lond474 ,with #abb ,illustrations1 numerous ,worked ,examples1 and about #ejj ,examples from ,examination ,papers1 etc4 ,crown #hvo41 #bs4 #f;d4 ,a ,,manual ,,of ,,mechanics4 ,with #ach ,illustrations and ,diagrams1 and #ahh ,examples taken from ,examination ,papers1 with ,answers4 ,by ,t4 ,m4 ,,goodeve1 ,m4,a4 ,crown #hvo41 #bs4 #f;d4 ,,sound1 ,,light1 ,,and ,,heat4 ,by ,,mark ,r4 ,,wright4 ,with #afj ,diagrams and ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,metallurgy3 an ,elementary ,text-,book4 ,by ,e4 ,l4 ,,rhead4 ,with #id ,illustrations4 ,crown #hvo41 #cs4 #f;d4 ,,physics4 ,alternative ,course4 ,by ,,mark ,r4 ,,wright4 ,with #bdb ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,problems ,,and ,,solutions ,,in ,,elementary ,,electricity ,,and ,,magnetism4 ,by ,w4 ,,slingo and ,a4 ,,brooker4 ,with #fg ,illustrations4 ,crown #hvo41 #bs4 ,,magnetism ,,and ,,electricity4 ,by ,a4 ,w4 ,,poyser1 ,m4,a4 ,with #bce ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,organic ,,chemistry3 the ,fatty ,compounds4 ,by ,r4 ,,lloyd ,,whiteley1 ,f4,i4,c41 ,f4,c4,s4 ,with #de ,illustrations4 ,crown #hvo41 #cs4 #f;d4 ,,inorganic ,,chemistry1 ,,theoretical ,,and ,,practical4 ,by ,,william ,,jago1 ,f4,c4,s41 ,f4,i4,c4 ,with #fc ,illustrations and numerous ,questions and ,exercises4 ,fcp4 #hvo41 #bs4 #f;d4 ,,an ,,introduction ,,to ,,practical ,,inorganic ,,chemistry4 ,by ,,william ,,jago1 ,f4,c4,s41 ,f4,i4,c4 ,crown #hvo41 #as4 #f;d4 ,,practical ,,chemistry3 the ,principles of ,qualitative ,analysis4 ,by ,,william ,a4 ,,tilden1 ,d4,sc4 ,fcp4 #hvo41 #as4 #f;d4 ,,elementary ,,inorganic ,,chemistry1 ,by ,w4 ,s4 ,,furneaux1 ,f4,r4,g4,s4 ,crown #hvo41 #bs4 #f;d4 ,,elementary ,,geology4 ,by ,,charles ,,bird1 ,b4,a41 ,f4,g4,s4 ,with ,coloured ,geological ,map of the ,british ,islands1 and #bdg ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,human ,,physiology4 ,by ,,william ,s4 ,,furneaux1 ,f4,r4,g4,s4 ,with #bah ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,a ,,course ,,of ,,practical ,,elementary ,,biology4 ,by ,j4 ,,bidgood1 ,b4,sc4 ,with #bbf ,illustrations4 ,crown #hvo41 #ds4 #f;d4 ,,elementary ,,botany1 ,,theoretical ,,and ,,practical4 ,by ,,henry ,,edmonds1 ,b4,sc4 ,with #cai ,woodcuts4 ,crown #hvo1 #bs4 #f;d4 ,,steam4 ,by ,,william ,,ripper1 ,member of the ,institution of ,mechanical ,engineers4 ,with #adb ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,elementary ,,physiography4 ,by ,j4 ,,thornton1 ,m4,a4 ,with #ab ,maps and #bdg ,illustrations4 ,with ,appendix on ,astronomical ,instruments and ,measurements4 ,crown #hvo41 #bs4 #f;d4 ,,agriculture4 ,by ,,henry ,j4 ,,webb1 ,ph4,d4 ,with #cd ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,the ,,london ,,science ,,class-,,books4 ,edited by ,g4 ,,carey ,,foster1 ,f4,r4,s41 and by ,sir ,,philip ,,magnus1 ,b4,sc41 ,b4,a41 of the ,city and ,guilds of ,london ,institute4 ,,astronomy4 ,by ,sir ,,robert ,,stawell ,,ball1 ,,ll4,d41 ,f4,r4,s4 ,with #da ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,mechanics4 ,by ,sir ,,robert ,,stawell ,,ball1 ,,ll4,d41 ,f4,r4,s4 ,with #hi ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,the ,,laws ,,of ,,health4 ,by ,w4 ,h4 ,,corfield1 ,m4,a41 ,m4,d41 ,f4,r4,c4,p4 ,with #bb ,illustrations4 ,fcp4 #hvo41 #as4 #f;d4 ,,molecular ,,physics ,,and ,,sound4 ,by ,,frederick ,,guthrie1 ,f4,r4,s4 ,with #ia ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,geometry1 ,,congruent ,,figures4 ,by ,o4 ,,henrici1 ,ph4,d41 ,f4,r4,s4 ,with #ada ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,zoology ,,of ,,the ,,invertebrate ,,animals4 ,by ,,alexander ,,macalister1 ,m4,d4 ,with #ei ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,zoology ,,of ,,the ,,vertebrate ,,animals4 ,by ,,alexander ,,macalister1 ,m4,d4 ,with #gg ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,hydrostatics ,,and ,,pneumatics4 ,by ,sir ,,philip ,,magnus1 ,b4,sc41 ,b4,a4 ,with #gi ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 7,to be had also with ,answers1 #bs47 ,the ,worked ,solutions of the ,problems4 #bs4 ,,botany4 ,outlines of the ,classification of ,plants4 ,by ,w4 ,r4 ,,mcnab1 ,m4,d4 ,with #aah ,diagrams4 ,fcp4 #hvo1 #as4 #f;d4 ,,botany4 ,outlines of ,morphology and ,physiology4 ,by ,w4 ,r4 ,,mcnab1 ,m4,d4 ,with #db ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,thermodynamics4 ,by ,,richard ,,wormell1 ,m4,a41 ,d4,sc4 ,with #da ,diagrams4 ,fcp4 #hvo41 #as4 #f;d4 ,,practical ,,elementary ,,science ,,series4 ,,elementary ,,practical ,,physiography4 7,section ,i47 ,by ,,john ,,thornton1 ,m4,a41 ,head ,master of the ,central ,higher ,grade ,school1 ,bolton4 ,with #bae ,illustrations and a ,coloured ,spectrum4 ,crown #hvo41 #bs4 #f;d4 ,,practical ,,domestic ,,hygiene4 ,by ,j4 ,,lane ,,notter1 ,m4,a41 ,m4,d41 ,professor of ,hygiene in the ,army ,medical ,school1 ,netley1 ,surgeon-,colonel1 ,army ,medical ,staff2 and ,r4 ,h4 ,,firth1 ,f4,r4,c4,s41 ,assistant ,professor of ,hygiene in the ,army ,medical ,school1 ,netley1 ,surgeon-,major ,army ,medical ,staff4 ,with #hc ,illustrations4 ,crown #hvo41 #bs4 #f;d4 ,,elementary ,,practical ,,chemistry3 a ,laboratory ,manual for ,use in ,organised ,science ,schools4 ,by ,g4 ,s4 ,,newth1 ,f4,i4,c41 ,f4,c4,s41 ,demonstrator in the ,royal ,college of ,science1 ,london2 ,assistant ,examiner in ,chemistry1 ,science and ,art ,department4 ,with #ajh ,illustrations and #bed ,experiments4 ,crown #hvo41 #bs4 #f;d4 ,,elementary ,,practical ,,physics3 a ,laboratory ,manual for ,use in ,organised ,science ,schools4 ,by ,w4 ,,watson1 ,b4,sc41 ,demonstrator in ,physics in the ,royal ,college of ,science1 ,london2 ,assistant ,examiner in ,physics1 ,science and ,art ,department4 ,with #aai ,illustrations and #aic ,exercises4 ,crown #hvo41 #bs4 #f;d4 ,,other ,,volumes ,,in ,,preparation4 99 99 99 99 99 ,transcriber's note3 ,the half-title has been deleted4 ,obvious misprints were corrected4 ,inconsistant hyphenation was kept as it is4