,,common ,,commodities ,,and ,,industries ,,series ,each book in crown #hvo1 cloth1 with many illustrations1 charts1 etc41 #b_/f net ,,tea4 ,by ,a4 ,ibbetson ,,coffee4 ,by ,b4 ,b4 ,keable ,,sugar4 ,by ,geo4 ,martineau1 ,c4,b4 ,,oils4 ,by ,c4 ,ainsworth ,mitchell1 ,b4,a41 ,f4,i4,c4 ,,wheat4 ,by ,andrew ,millar ,,rubber4 ,by ,c4 ,beadle and ,h4 ,p4 ,stevens1 ,m4,a41 ,ph4,d41 ,f4,i4,c4 ,,iron ,,and ,,steel4 ,by ,c4 ,hood ,,copper4 ,by ,h4 ,k4 ,picard ,,coal4 ,by ,francis ,h4 ,wilson1 ,m4,inst41 ,m4,e4 ,,timber4 ,by ,w4 ,bullock ,,cotton4 ,by ,r4 ,j4 ,peake ,,silk4 ,by ,luther ,hooper ,,wool4 ,by ,j4 ,a4 ,hunter ,,linen4 ,by ,alfred ,s4 ,moore ,,tobacco4 ,by ,a4 ,e4 ,tanner ,,leather4 ,by ,k4 ,j4 ,adcock ,,knitted ,,fabrics4 ,by ,j4 ,chamberlain and ,j4 ,h4 ,quilter ,,clays4 ,by ,alfred ,b4 ,searle ,,paper4 ,by ,harry ,a4 ,maddox ,,soap4 ,by ,william ,a4 ,simmons1 ,b4,sc4 7,lond471 ,f4,c4,s4 ,,the ,,motor ,,industry4 ,by ,horace ,wyatt1 ,b4,a4 ,,glass ,,and ,,glass ,,making4 ,by ,percival ,marson ,,gums ,,and ,,resins4 ,by ,e4 ,j4 ,parry1 ,b4,sc41 ,f4,i4,c41 ,f4,c4,s4 ,,the ,,boot ,,and ,,shoe ,,industry4 ,by ,j4 ,s4 ,harding ,,gas ,,and ,,gas ,,making4 ,by ,w4 ,h4 ,y4 ,webber ,,furniture4 ,by ,h4 ,e4 ,binstead ,,coal ,,tar4 ,by ,a4 ,r4 ,warnes ,,petroleum4 ,by ,a4 ,lidgett ,,salt4 ,by ,a4 ,f4 ,calvert ,,zinc4 ,by ,t4 ,e4 ,lones1 ,m4,a41 ,,ll4,d41 ,b4,sc4 ,,photography4 ,by ,wm4 ,gamble ,,asbestos4 ,by ,a4 ,leonard ,summers ,,silver4 ,by ,benjamin ,white ,,carpets4 ,by ,reginald ,s4 ,brinton ,,paints ,,and ,,varnishes4 ,by ,a4 ,s4 ,jennings ,,cordage ,,and ,,cordage ,,hemp ,,and ,,fibres4 ,by ,t4 ,woodhouse and ,p4 ,kilgour ,,acids ,,and ,,alkalis4 ,by ,g4 ,h4 ,j4 ,adlam ,,others ,,in ,,preparation ,illustration3 ,copyright by ,messrs ,flatters ,garnett1 ,manchester ,,bacteria ,,nodules ,,on ,,the ,,root ,,of ,,lupin ,,pitman',s ,,common ,,commodities ,,and ,,industries ,,acids1 ,,alkalis ,,and ,,salts ,,by ,g4 ,h4 ,j4 ,,adlam1 ,m4,a41 ,b4,sc41 ,f4,c4,s4 ,editor of 8,the ,school ,science ,review0 ,london ,sir ,isaac ,pitman ,sons1 ,ltd41 #a ,amen ,corner1 ,e4,c4#d ,bath1 ,melbourne and ,new ,york ,printed by ,sir ,isaac ,pitman ,sons1 ,ltd41 ,london1 ,bath1 ,melbourne and ,new ,york ,,preface ,it has often been said1 and still more often implied1 that considerations of utility in education are incompatible with its main object1 which is the training of the mind4 ,extremely divergent views have been expressed on this point4 ,schoolmen have looked askance at some branches of knowledge because they were supposed to be tainted with the possibility of usefulness in after life4 ,on the other hand1 business men and others have complained bitterly of the present state of education because very little that is considered 8useful0 has up to the present been taught in schools4 ,it is possible to err in both directions4 ,a university professor1 lecturing on higher ,mathematics1 is reported to have told his audience that it was a source of great satisfaction to him that the theorem which he was demonstrating could never be applied to anything 8useful40 ,on the other hand1 we have the well-authenticated story of the man who took his son to the ,royal ,school of ,mines to 8learn copper10 and not to waste his time over other parts of ,chemistry1 because 8they would be of no use to him40 ,for narrowness of outlook1 there is nothing to choose between the pedant and the 8practical0 man4 ,national education would deteriorate if its control should ever pass into the hands of extremists of either type1 for nothing worthy of the name of education could ever be given or received in such an irrational spirit4 ,in dealing with the subject of 8,acids1 ,alkalis1 and ,salts10 ,i have endeavoured to give prominence to the commercial and domestic importance of the substances dealt with4 ,i thereby hope to gain the interest of the reader1 since interest stands in the same relation to education that petrol does to the motor-car4 ,it is not education itself1 but it is the source of its motive power4 ,i have also included some considerations of a theoretical nature which may well be taken as a first step towards the continuation of the study of ,chemistry4 ,my sincere thanks are offered to my colleagues1 ,f4 ,w4 ,g4 ,foat1 ,m4,a41 ,d4,litt41 and ,mr4 ,i4 ,s4 ,scarf1 ,f4,i4,c41 for much valuable help and advice2 to ,sir ,edward ,thorpe1 ,c4,b41 ,f4,r4,s41 and ,messrs4 ,william ,collins ,sons for permission to reproduce ,figures #c1 #aa1 and #ad2 to ,messrs4 ,longmans ,co4 for ,figures #d1 #e1 #i1 #ab1 #ac1 #af2 ,messrs4 ,macmillan ,co41 for ,figures #h1 #aj and #ae4 ,i have also availed myself of the assistance of several standard works on ,chemistry4 ,my acknowledgments in this direction take the practical form of the short bibliography which follows-- ,lunge1 ,dr4 ,g4 ,the ,manufacture of ,sulphuric ,acid and ,alkali4 ,vols4 ,i1 ,,ii1 and ,,iii4 ,roscoe ,schorlemmer ,treatise on ,chemistry4 ,vol4 ,i4 ,the ,non-metallic ,elements 7#aiaa74 ,vol4 ,,ii4 ,the ,metals 7#aiac74 ,brannt1 ,w4 ,t4 ,the ,manufacture of ,vinegar and ,acetates4 ,thorp1 ,f4 ,h4 ,outlines of ,industrial ,chemistry 7#aiac74 ,thorpe1 ,t4 ,e4 ,a ,manual of ,inorganic ,chemistry4 ,newth1 ,g4 ,s4 ,a ,text-book of ,inorganic ,chemistry4 ,mellor1 ,j4 ,w4 ,modern ,inorganic ,chemistry4 ,cohen1 ,j4 ,b4 ,theoretical ,organic ,chemistry4 ,g4 ,h4 ,j4 ,a4 ,city of ,london ,school1 ,e4,c4 ,,contents ,,chap4 ,,page ,,preface v ,i4 ,,introduction #a ,,ii4 ,,sulphuric ,,acid ,,and ,,sulphates #aj ,,iii4 ,,nitric ,,acid ,,and ,,nitrates #bh ,,iv4 ,,the ,,halogen ,,acids #dc ,v4 ,,carbonic ,,acid ,,and ,,carbonates #di ,,vi4 ,,phosphoric1 ,,boric1 ,,and ,,silicic ,,acids #ef ,,vii4 ,,organic ,,acids #fg ,,viii4 ,,mild ,,alkali #hj ,,ix4 ,,caustic ,,alkalis #ie ,x4 ,,electrolytic ,,methods #aja ,,index #aji ,,illustrations ,,fig4 ,,page ,,bacteria ,,nodules ,,on ,,the ,,root ,,of ,,lupin ,frontispiece #a4 ,,diagram #g #b4 ,,plan ,,of ,,sulphuric ,,acid ,,works #ac #c4 ,,general ,,view ,,of ,,sulphuric ,,acid ,,works #ae #d4 ,,sulphur ,,trioxide--,,the ,,contact ,,process #ai #e4 ,,preparation ,,of ,,nitric ,,acid #cj #f4 ,,nitrogen ,,cycle 7,,diagram7 #ch #g4 ,,nitric ,,acid ,,from ,,air 7,,diagram7 #da #h4 ,,preparation ,,of ,,hydrochloric ,,acid #de #i4 ,,boric ,,acid #ei #aj4 ,,quick ,,vinegar ,,process #ga #aa4 ,,dutch ,,process ,,for ,,white ,,lead #gd #ab4 ,,salt ,,cake ,,furnace #hc #ac4 ,,black ,,ash ,,furnace #he #ad4 ,,the ,,solvay ,,process #hi #ae4 ,,the ,,electrolysis ,,of ,,salt ,,solution #ajb #af4 ,,the ,,castner ,,process #aje ,,acids1 ,,alkalis1 ,,and ,,salts ,,chapter ,i ,,introduction ,acids4 ,a vague hint from ,nature gave mankind the first indication of the existence of acids4 ,the juice pressed from ripe grapes is a sweetish liquid4 ,if it is kept for some time1 the sweetness goes1 and the liquid acquires a burning taste4 ,if kept still longer1 the burning taste is lost1 and in its place a sharp acid flavour1 not entirely displeasing to the palate1 is developed4 ,the liquid obtained in this way is now called wine vinegar2 the particular substance which gives it its characteristic taste is acetic acid4 ,the strongest vinegar does not contain more than #aj per cent4 of acetic acid1 which is itself a comparatively weak acid4 ,it is1 therefore1 not a very active solvent4 ,nevertheless1 for metals and for limestone rock1 and other substances of a calcareous nature1 its solvent power is greater than that of any other liquid known at the time of its discovery4 ,it was this property which seems to have appealed most strongly to the imagination of the early chemists2 and1 as is very often the case1 the description of its powers was very much exaggerated4 ,livy and ,plutarch1 who have given us an account of ,hannibal's invasion of ,italy by way of the ,alps1 both gravely declare that the ,carthaginian leader cleared a passage for his elephants through solid rocks by pouring vinegar over them6 ,in the ,middle ,ages1 the study of ,chemistry was fostered mainly as a possible means whereby long life and untold riches might be obtained4 ,the 8,philosopher's ,stone10 by the agency of which the base metals were to be changed to gold1 and the 8,elixir of ,life10 which was to banish disease and death1 were eagerly sought for4 ,though these were vain imaginings according to modern ideas1 nevertheless they were powerful incentives towards experimental work4 ,many new substances were discovered in this period1 and among these were nitric acid 7aqua fortis71 hydrochloric acid 7spirit of salt71 and sulphuric acid 7oil of vitriol74 ,acids were then valued above all other substances4 ,the mediaeval chemist 7or alchemist1 as he was called7 clearly saw that unless a body could be dissolved up there was no hope of changing it4 ,nitric acid1 therefore1 which1 in conjunction with hydrochloric acid1 dissolved even gold itself1 was very highly esteemed4 ,oil of vitriol also was scarcely less important1 for it was required for the production of other acids4 ,so far1 taste and solvent power were considered to be the characteristic feature of acids4 ,in the time of ,robert ,boyle 7#afbg-afia71 they were further distinguished from other substances by the change which they produced in the colour of certain vegetable extracts4 ,tincture of red cabbage was first used1 but1 as this liquid rapidly deteriorates on keeping1 it was soon replaced by a solution of litmus1 a colouring matter obtained from ,roccella tinctoria and other lichens4 ,it imparts to water a purple colour1 which is changed to red by the addition of acids4 ,alkalis4 ,wood ashes were valued in very early times because they were found to be good for removing dirt from the skin4 ,mixed with vegetable oil or animal fat1 they formed a very primitive kind of soap1 which was afterwards much improved by using the aqueous extract instead of the ashes themselves1 and also by the addition of a little caustic lime4 ,when plant ashes are treated with water1 about #aj per cent4 dissolves4 ,if the insoluble matter is then allowed to settle down and the clear liquid evaporated to dryness1 a whitish residue is obtained4 ,the soluble matter thus extracted from the ashes of plants which grow in or near the sea is mainly soda2 that from land plants1 mainly potash4 ,formerly no distinction was made1 and the general term 8alkali0 was applied to both4 ,in order to bring the properties of alkalis into contrast with those of acids1 we cannot do better than make a few simple experiments with a weak solution of washing soda4 ,its taste is very different from that of an acid2 it is generally described as caustic4 ,if a little is rubbed between the fingers1 it feels smooth1 almost like very thin oil4 ,it does not dissolve metals or limestone4 ,its action on vegetable colouring matter is just as striking as that of acids4 ,tincture of red cabbage becomes green2 the purple of litmus is changed to a light blue4 ,this colour change is characteristic of alkalis4 ,neutralization4 ,when the colour of litmus solution has been changed to red by the addition of an acid1 the original colour can be restored by adding an alkali4 ,the change can be repeated as often as desired by adding acid and alkali alternately4 ,from this we get a distinct impression of antithesis between the two4 ,in popular language1 an alkali 8kills0 an acid2 in ,chemistry1 the same idea is expressed by the term 8neutralization40 ,salts4 ,both 8neutralization0 and 8killing the acid0 are modes of expression which describe the phenomenon fairly well4 ,when an acid is neutralized1 its characteristic taste1 its solvent power1 and its action on litmus1 are all changed2 in fact1 the acid as an acid ceases to exist1 and so does the alkali4 ,when the neutral solution is evaporated to dryness1 a residue is found which on examination proves to be neither the acid nor the alkali1 but a compound formed from the two4 ,this substance is called a salt4 ,to most people1 salt is the name for that particular substance which is taken as a condiment with food4 ,its use in this connection dates from time immemorial4 ,it is distinctly unfortunate that another and very much wider usage of the term has been introduced into ,chemistry4 ,when the early chemists recognized that other substances1 which they vaguely designated as 8saline bodies10 were similar to common salt in composition1 they took the name of the individual and applied it to the whole class4 ,,other ,,methods ,,of ,,salt ,,formation ,solution of ,metals in ,acids4 ,alkalis are not the only substances which neutralize acids4 ,speaking in a broad and general sense1 we may say that an acid is neutralized when a metal is dissolved in it1 because1 when the point is reached at which no more metal will dissolve1 all the characteristic properties of the acid are destroyed4 ,a salt is formed in this case also4 ,an example will now be given to illustrate this method of salt formation4 ,before two pieces of metal can be united by soldering1 it is necessary to clean the surfaces of the metal and the soldering iron4 ,the liquid used for this purpose is made by adding scraps of zinc to muriatic acid 7hydrochloric acid74 ,the zinc dissolves with effervescence1 which is caused by the escape of hydrogen gas4 ,when effervescence ceases and no more zinc will dissolve1 the liquid is ready for use4 ,the acid has been 8killed0 or neutralized by the metal4 ,a salt called zinc chloride has been formed4 ,this salt can be recovered from the liquid by evaporation4 ,solution of ,oxides in ,acids4 ,the substances most used in commerce with the express purpose of destroying acidity are quicklime1 washing soda1 and powdered chalk4 ,since quicklime is a compound of the metal calcium and the gas oxygen1 its systematic name is calcium oxide2 when it neutralizes an acid1 it forms the corresponding calcium salt2 for example1 if it neutralizes acetic acid1 calcium acetate is formed4 ,an instance of the neutralization of an acid by an oxide of a metal is furnished by one method of preparing blue vitriol 7copper sulphate74 ,copper does not dissolve very quickly in dilute sulphuric acid2 hence1 to make blue vitriol from scrap copper1 the metal is first heated very strongly while freely exposed to air4 ,copper and oxygen of the air combine to form the brownish black powder1 copper oxide1 and this dissolves very readily in sulphuric acid1 making the salt1 copper sulphate4 ,solution of ,carbonates in ,acids4 ,washing soda and chalk belong to a different class of chemical substances4 ,they are carbonates1 that is1 they are salts of carbonic acid4 ,at first it may seem a little perplexing to the reader to learn that a salt can neutralize an acid to form a salt4 ,it must be remembered1 however1 that acids differ from one another in strength1 that is1 in chemical activity1 and that carbonic acid is a weak acid4 ,when a salt of carbonic acid--sodium carbonate or washing soda1 for example--is added to a stronger acid such as sulphuric acid1 sodium sulphate is formed and carbon dioxide liberated4 ,as an example of the neutralization of acids by carbonates1 we may mention here a practical sugar saving device4 ,unripe fruit is very sour because it contains certain vegetable acids dissolved in the juice4 ,these acids are not affected by boiling2 and1 therefore1 to make a dish of stewed fruit palatable1 it is necessary to add sugar in quantity sufficient to mask the sour taste4 ,if a pinch of bicarbonate of soda is added to neutralize the acid1 far less sugar will be necessary for sweetening4 ,insoluble ,salts4 ,the methods given above apply only to those salts which are soluble in water4 ,insoluble salts are obtained by mixing two solutions1 the one containing a soluble salt of the metal1 and the other1 a soluble salt of the acid or the acid itself4 ,the formation of an insoluble salt by the interaction of two soluble substances is well illustrated in the preparation of ,burgundy mixture1 the most effectual remedy yet proposed for checking the spread of potato disease4 ,this mixture contains copper carbonate1 that is1 the copper salt of carbonic acid4 ,for its preparation we require copper sulphate and sodium carbonate 7washing soda71 a soluble carbonate4 ,when these two substances1 dissolved in separate portions of water1 are mixed1 copper carbonate is formed as a pale blue solid which is in such a state of fine subdivision that it remains suspended in the solution of sodium sulphate1 the other product of the reaction4 ,the change is represented diagrammatically below4 ,each circle represents the atom or a group of atoms named therein4 ,at the moment of mixing1 these groups undergo re-arrangement4 ,bordeaux mixture1 which some gardeners prefer1 is a similar preparation containing copper hydroxide instead of copper carbonate4 ,it is made by mixing clear lime water 7a soluble hydroxide7 with copper sulphate4 ,illustration3 ,fig4 #a ,elements and ,compounds4 ,it is scarcely possible to discuss chemical processes without having from time to time to use terms which are not in everyday use4 ,a few preliminary definitions and explanations of terms which will be frequently used may serve to simplify descriptions1 and render it unnecessary to encumber them with purely explanatory matter4 ,among the many different kinds of materials known1 which in the aggregate amount to several hundreds of thousands1 there are about ninety substances which up to the present time have not been broken up into simpler kinds4 ,these primary materials are called 8elements10 the remainder being known as 8compounds40 ,the following is a list of the commonest of these elements1 together with the symbols by which they are represented in ,chemistry4 ,,metals ,aluminium ,al4 ,antimony 7,stibium7 ,sb4 ,barium ,ba4 ,bismuth ,bi4 ,cadmium ,cd4 ,calcium ,ca4 ,chromium ,cr4 ,copper 7,cuprum7 ,cu4 ,gold 7,aurum7 ,au4 ,iron 7,ferrum7 ,fe4 ,lead 7,plumbum7 ,pb4 ,lithium ,li4 ,magnesium ,mg4 ,manganese ,mn4 ,mercury 7,hydrargyrum7 ,hg4 ,nickel ,ni4 ,platinum ,pt4 ,potassium 7,kalium7 ,k4 ,silver 7,argentum7 ,ag4 ,sodium 7,natrium7 ,na4 ,strontium ,sr4 ,tin 7,stannum7 ,sn4 ,zinc ,zn4 ,,non-,,metals ,boron ,b4 ,bromine ,br4 ,carbon ,c4 ,chlorine ,cl4 ,fluorine ,f4 ,hydrogen ,h4 ,iodine ,i4 ,nitrogen ,n4 ,oxygen ,o4 ,phosphorus ,p4 ,silicon ,si4 ,sulphur ,s4 ,the first step in the building-up process consists of the union of a metallic with a non-metallic element4 ,such compounds are binary compounds1 and are distinguished by the termination -ide added to the name of the non-metallic element2 for example1 copper and oxygen unite to form copper oxide1 sodium and chlorine form sodium chloride1 iron and sulphur form iron sulphide or sulphide of iron4 ,a compound containing more than two elements is distinguished by the termination -ate4 ,most salts fall within this category2 thus we speak of acetate of lead and chlorate of potash1 also of sodium sulphate and copper sulphate1 the latter form being the more correct4 ,a difficulty arises when two bodies are composed of the same elements combined in different proportions4 ,then we have to resort to other distinguishing prefixes or suffixes4 ,for this reason we meet with sulphurous acid and sulphuric acid1 the corresponding salts being sulphites and sulphates4 ,crystals and ,water of ,crystallization4 ,when a soluble salt is to be recovered from its solution1 the latter is reduced in bulk by evaporation until1 either by experience or by trial1 it becomes evident that the solid will be formed as the liquid cools4 ,in some cases1 when time is not an important factor1 evaporation is left to take place naturally4 ,under either set of conditions1 the substance generally separates out in particles which have a definite geometrical form4 ,these are spoken of as crystals4 ,crystals often contain a definite percentage of water1 called 8water of crystallization40 ,in washing soda1 this combined water forms nearly #fc per cent4 of the total weight2 in blue vitriol1 it is approximately #cf per cent4 ,on being heated to a moderate temperature1 the water is expelled from the solid2 the substance which is left behind is called the anhydrous 7that is1 the waterless7 salt4 ,,chapter ,,ii ,,sulphuric ,,acid ,,and ,,sulphates ,key ,industries4 ,the importance of the chemical industries depends mainly on the fact that they constitute the first step in a series of operations by which natural products are adapted to our needs4 ,the materials which are found in earth1 air1 and water are both varied in kind and abundant in quantity1 but in their natural state they are not generally available for immediate use4 ,moreover1 very many substances now deemed indispensable are not found ready formed in ,nature4 ,the end product of the chemical manufacturer is often one of the primary materials of some other industry4 ,soda ash and ,glauber's salt are essential for making glass2 soap could not be produced without caustic alkali2 the textile trade would be seriously handicapped if bleaching materials1 mordants1 and dye-stuffs were not forthcoming4 ,considered in this light1 the preparation of chemicals is spoken of as a 8key industry40 ,furthermore1 very few of these indispensable substances can be made without using sulphuric acid4 ,this acid is1 on that account1 just as important to chemical industries as the products of these are to other branches of trade4 ,it may1 therefore1 be looked upon as a master key of industrial life4 ,primary ,materials4 ,the composition of sulphuric acid is not difficult to understand4 ,air is mainly a mixture of oxygen and nitrogen2 and when a combustible body burns1 it is because chemical action between the material and oxygen is taking place4 ,in this way1 sulphur burns to sulphur dioxide4 ,this gas1 dissolved in water1 forms sulphurous acid1 which changes slowly to sulphuric acid by combination with more oxygen4 ,hence1 sulphur1 oxygen1 and water are the primary materials required for making sulphuric acid4 ,sulphur is the familiar yellow solid commonly known as brimstone4 ,it is found native in the earth1 and is fairly abundant in certain localities1 notably in the neighbourhood of active and extinct volcanoes4 ,italy1 ,sicily1 ,japan1 ,iceland1 and parts of the ,united ,states are the principal sulphur-producing countries4 ,though very plentiful and consequently cheap1 only a relatively small quantity of sulphuric acid is made directly from native sulphur1 because at the time when this industry was started in ,england1 restrictions were placed on the export of sulphur from ,sicily and1 consequently1 the plant which was then established was adapted to the use of iron pyrites4 ,iron pyrites contains about #ec per cent4 of sulphur combined with #dg per cent4 of iron1 and when this is burnt in a good draught1 nearly the whole of the sulphur burns to sulphur dioxide1 leaving a residue of oxide of iron which can be used for making cast iron of a low grade4 ,iron pyrites is often supplemented by the 8spent oxide0 from the gas works4 ,crude coal gas contains sulphur compounds which1 if not removed1 would burn with the gas and form sulphur dioxide4 ,the production of these pungent and suffocating fumes would be a source of great annoyance1 and therefore it is necessary to remove the sulphur compounds4 ,to do this1 the gas is passed through two purifiers1 the first containing slaked lime and the second ferric oxide1 both in a slightly moist condition4 ,after being some time in use1 the purifying material loses its efficacy2 the residue from the lime purifier is sold as 8gas lime10 but that from the ferric oxide purifier is exposed to the air and so 8revived40 ,at length1 however1 it becomes so charged with sulphur that it is of no further use for its original work4 ,it is then passed on to the sulphuric acid maker4 ,evolution of the ,manufacturing ,process4 ,in dealing with the main processes for the manufacture of acids and alkalis1 reference will frequently be made to the methods of bygone times4 ,although as an exact science ,chemistry is comparatively modern1 as a branch of human knowledge its history goes back to the dawn of intelligence in man4 ,it is agreed that the higher types of living things are more easily understood when those of a simpler and more primitive character have been studied4 ,in like manner1 the highly specialized industries of modern times become more intelligible in the light of the efforts of past generations to achieve the same object4 ,basil ,valentine1 who lived in the fifteenth century1 states that the liquid which we now call sulphuric acid was in his day obtained by heating a mixture of green vitriol and pebbles4 ,until quite recent times1 sulphuric acid of a special grade was made by precisely the same method1 except that the pebbles were dispensed with4 ,in passing1 we may remark that the common name 8vitriol10 or 8oil of vitriol10 is accounted for by this connection with green vitriol4 ,the second method1 quoted by ,basil ,valentine1 consisted of the ignition of a mixture of saltpetre and sulphur in the presence of water4 ,this is actually the modern lead chamber process in embryo4 ,illustration3 ,fig4 #b4 ,,plan ,,of ,,sulphuric ,,acid ,,works ,about the middle of the eighteenth century1 8,dr40 ,ward took out a patent for the manufacture of sulphuric acid1 to be carried on at ,richmond in ,surrey4 ,he used large glass bell jars of about #dj-ej galls4 capacity1 in which he placed a little water and a flat stone to support a red-hot iron ladle4 ,a mixture of saltpetre and sulphur was thrown into the ladle and the mouth of the vessel quickly closed4 ,after the vigorous chemical action was over1 the ladle was re-heated and the process repeated until at last fairly concentrated sulphuric acid was produced4 ,the large glass vessels used by ,ward were costly and easily broken4 ,they were soon replaced by chambers about #f ft4 square1 made of sheet lead1 but otherwise the process was just the same4 ,the next advance consisted in making the process continuous instead of intermittent4 ,an enormously increased output was thereby rendered possible1 and the main features of the modern process gradually developed4 ,the ,lead ,chamber ,process4 ,we can now consider the actual working of the lead chamber process1 aided by the diagrammatic plan of the works shown in ,fig4 #b4 ,sulphur dioxide is produced in a row of kilns 7,a-,a7 by burning iron pyrites in a carefully regulated current of air4 ,the mixture of gases which leaves the pyrites burners contains sulphur dioxide1 excess of oxygen1 and a very large quantity of nitrogen4 ,to this is added the vapour of nitric acid1 generated from sodium nitrate and concentrated sulphuric acid contained in the 8nitre pots10 which are placed at ,b4 ,the mixture of gases then passes up the ,glover tower 7,c7 and through the three chambers in succession1 into the first two of which steam is also introduced4 ,sulphuric acid is actually produced in the chambers1 and collects on the floors1 from which it is drawn off from time to time4 ,the residual gas from the last chamber is passed up the ,gay ,lussac tower 7,d71 and after that is discharged into the air by way of the tall chimney 7,j74 ,illustration3 ,fig4 #c4 ,,general ,,view ,,of ,,sulphuric ,,acid ,,works ,the ,oxygen ,carrier4 ,we have seen that sulphur dioxide1 oxygen1 and water are the only substances required to produce sulphuric acid4 ,why1 then1 is the nitric acid vapour added to the mixture8 ,as described in a former paragraph1 the combining of these gases was represented as being a very simple operation4 ,so indeed it is1 for it even takes place spontaneously4 ,yet1 as a commercial process1 it would be quite impracticable without the nitric acid vapour1 for although the gases combine spontaneously1 they do so very slowly1 and it is the nitric acid vapour which accelerates the rate of combination4 ,it is not known with any degree of certainty how the nitric acid acts in bringing about this remarkable change4 ,it has been suggested that reduction to nitrogen peroxide first takes place1 and that sulphur dioxide takes oxygen from this body1 reducing it still further to nitric oxide1 which at once combines with the free oxygen present to form nitrogen peroxide again4 ,so the cycle of changes goes on1 the nitrogen peroxide playing the part of oxygen carrier to the sulphur dioxide2 and since it is continually regenerated1 it remains at the end mixed with the residual gases4 ,recovery of the ,nitrogen ,peroxide4 ,if the gases from the last chamber passed directly into the chimney shaft1 there would be a total loss of the oxides of nitrogen1 and the consequence of this would be that more than #b cwt4 of nitre would have to be used for the production of #a ton of sulphuric acid4 ,this would be a serious item in the cost of production1 and it is therefore essential that this loss should be prevented4 ,the recovery of the oxides of nitrogen is effected in the ,gay ,lussac tower1 a structure about #ej ft4 in height1 built of sheet lead and lined with acid-resisting brick4 ,it is filled with flints1 over which a slow stream of cold concentrated sulphuric acid is delivered from a tank at the top4 ,as the gas from the last chamber passes up this tower1 it meets the stream of acid coming down4 ,this dissolves and retains the nitrogen peroxide4 ,the acid which collects at the bottom of the tower is known as nitrated vitriol4 ,the next step is to bring the recovered nitrogen peroxide again into circulation4 ,the nitrated vitriol is raised by compressed air to the top of the ,glover tower1 and as it trickles down over the flints in this tower it is diluted with water1 while at the same time it meets the hot gases coming from the pyrites burners4 ,under these conditions1 the nitrogen peroxide is liberated and carried along by the current of gas into the first lead chamber4 ,the stream of cold acid coming down the ,glover tower also serves to cool the hot gases before they enter the first chamber4 ,in order to complete the description of the works1 it is necessary to add a note on the lead chambers themselves4 ,the sheet lead used in their construction is of a very substantial character2 it weighs about #g lb4 per square foot4 ,the separate strips are joined together by autogenous soldering1 that is1 by fusing the edges together4 ,in this way the presence of another metal is avoided2 otherwise this would form a voltaic couple with the lead1 and rapid corrosion would take place4 ,the size of the chambers has varied a great deal4 ,in the early years of the nineteenth century1 the capacity of a single chamber was probably not more than #a1jjj cu4 ft42 at the present time1 #ch1jjj cu4 ft4 is an average size1 and there may be three or five of these chambers4 ,the necessity for this large amount of cubic space is easily accounted for4 ,the reaction materials are all gases1 and a gas occupies more than one thousand times as much space as an equal weight of a solid or liquid4 ,moreover1 oxygen constitutes only about one-fifth of the total volume of air used in burning the pyrites2 the other four-fifths is mainly nitrogen1 which1 though it does not enter into the reaction at all1 has to pass through the chambers4 ,modern ,improvements4 ,among the modern innovations in the lead chamber process1 the following are worthy of note4 8,atomized water10 that is1 water under high pressure delivered from a fine jet against a metal plate1 has certain advantages over steam4 ,in order to bring about a more rapid mixing of the gases in the chamber1 it is proposed to make these circular instead of rectangular1 and to deliver the gases tangentially to the sides4 ,another suggestion is to replace the lead chambers by towers containing perforated stoneware plates set horizontally4 ,by this arrangement1 since the holes are not placed opposite one another1 the gases passing up the tower must take a zig-zag course4 ,this makes for more efficient mixing4 ,,the ,,contact ,,process ,sulphur ,trioxide4 ,when elements are combined in different proportions by weight1 they produce different compounds4 ,thus1 in the case of sulphur and oxygen1 there are two well-known compounds1 namely1 sulphur dioxide and sulphur trioxide4 ,in the former1 a given weight of oxygen is combined with an equal weight of sulphur2 in the latter1 this same weight of sulphur is combined with #ej per cent4 more oxygen4 ,on this account1 sulphur trioxide is spoken of as the higher oxide4 ,we can now state in general terms another method by which sulphuric acid can be built up from its elements4 ,sulphur1 as we have seen1 burns in oxygen1 forming sulphur dioxide4 ,this substance can then be made to unite with more oxygen to give sulphur trioxide1 which1 with water1 yields sulphuric acid4 ,there are three steps in this synthesis4 ,the first1 namely1 sulphur to sulphur dioxide1 has already been considered2 the last1 sulphur trioxide to sulphuric acid1 only requires that sulphur trioxide and water shall be brought together3 we can1 therefore1 confine our attention to the intermediate step1 namely1 the conversion of sulphur dioxide into trioxide4 ,this operation1 when carried out in a chemical laboratory1 is a very simple one4 ,fig4 #d shows the necessary apparatus4 ,sulphur dioxide from a siphon of the liquefied gas and air from a gasholder are passed into the ,woulff's bottle ,a1 containing concentrated sulphuric acid2 this removes moisture from the gases4 ,the drying process is completed in the tower ,b1 which contains pumice stone soaked in sulphuric acid4 ,the mixed gases then pass through the tube ,c1 containing platinized asbestos heated to about #djj ,c43 the sulphur trioxide collects in the cooled receiver ,d4 ,illustration3 ,fig4 #d4 ,,sulphur ,,trioxide--,,the ,,contact ,,process ,platinized asbestos is made by soaking long-fibred asbestos in a solution of platinum chloride4 ,the material is then dried and subjected to a gentle heat4 ,in this way1 metallic platinum in an exceedingly fine state of subdivision is deposited on the asbestos fibre1 which merely serves as a convenient support4 ,catalytic or ,contact ,action4 ,the influence of the finely divided platinum is a very important factor in the reaction4 ,it cannot1 however1 be said to cause the union of sulphur dioxide with oxygen1 for the gases combine to a very slight extent when it is not present4 ,what the platinum actually does is to influence the rate of formation to such a degree that1 under favourable conditions1 practically the whole of the sulphur dioxide is changed to sulphur trioxide instead of an exceedingly small fraction of it4 ,the most interesting1 and at the same time the most perplexing1 feature of the reaction is that the platinum itself does not appear to undergo any change4 ,it is not diminished in quantity1 for only a very small amount is necessary for the conversion of a very large amount of the mixed gases4 ,its activity lasts for a very long time1 and even when it does become inactive1 it can be shown that this is due to some external cause1 such as the presence of dust and certain impurities in the gases4 ,many other similar cases are known in which the presence of a small quantity of a third substance greatly influences the course of a chemical reaction without appearing in any other way to be necessary to the reaction4 ,these substances1 which are often metals in a very fine state of subdivision1 are called catalytic or contact agents4 ,the ,contact ,process for making sulphuric acid is nothing more nor less than the simple laboratory operation which we have described above1 carried out on a larger scale4 ,the sulphur dioxide is produced as in the lead chamber process by roasting iron pyrites in a current of air4 ,this gas1 together with the excess of air1 is passed into the contact furnace1 which consists of four tubes1 each containing platinized asbestos1 supported on perforated plates4 ,the union of the two gases is said to be almost complete3 an efficiency of #ih per cent4 of the theoretical value is claimed for this process4 ,the sulphur trioxide1 or 8sulphuric anhydride0#a is either condensed in tin-lined drums or absorbed in ordinary concentrated sulphuric acid4 ,the proposal to manufacture sulphuric acid by this method was first made in #ahca by ,peregrine ,phillips1 of ,bristol4 ,the early attempts were not successful1 and it was not until about forty-four years later that the difficulties arising in the working of the contact process were overcome sufficiently to enable the sulphuric acid produced in this way to be sold at the same price as that made by the lead chamber process4 ,since #ahij1 the total quantity of acid made by the contact method has increased very rapidly1 so that it now furnishes about one-half of the world's supply1 and seems likely in time to displace the lead chamber process altogether4 ,the history of the rise of the contact process is interesting because it illustrates in a striking manner the very great difference that there is between a successful laboratory process and a successful manufacturing process1 though seemingly identical4 ,the first and possibly the most serious difficulty encountered in the working of the contact process was the frequent interruption caused by the loss of activity of the contact substance4 ,iron pyrites always contains arsenic which volatilizes on heating1 and this quickly caused the platinum to lose its activity1 or1 as it was sometimes rather fancifully expressed1 8poisoned the catalyst40 ,dust also is inevitable1 and this1 carried forward mechanically with the stream of gas1 settled on the contact substance and caused the action to cease4 ,to get over this difficulty it is necessary to purify the gases4 ,they are first passed slowly through channels in which the coarser particles of dust settle down4 ,steam is injected into the mixture to wash out the finer particles of solid1 and also to get rid of arsenic1 and then the gases are passed through scrubbers4 ,before being admitted to the contact furnace1 the moist gas is submitted to an optical test4 ,it is passed through a tube1 the ends of which are transparent2 a bright light is placed at one end and viewed from the other through a column of gas of considerable length4 ,if the purification process is working satisfactorily1 there is a complete absence of fog4 ,the gases are then dried by passing through concentrated sulphuric acid and admitted to the contact tubes4 ,in all operations carried out on a large scale1 the regulation of temperature is a matter of some difficulty4 ,in the case which we are considering1 the most suitable temperature range is a rather narrow one1 and the difficulty of keeping within the limits is very much increased by the large amount of heat given out when the sulphur dioxide and oxygen combine4 ,the result of the failure to maintain the temperature at a fairly constant level was that the process worked in a very irregular manner1 for as soon as it was working really well and sulphur trioxide was being formed rapidly1 the heat given out by the reaction itself was also great1 and consequently1 the higher temperature limit was exceeded4 ,the method of controlling the temperature in the contact process is worth noting1 because it is really ingenious4 ,the tubes containing the platinized asbestos are surrounded by wider concentric tubes4 ,the gases which are about to enter the contact furnace pass through the annular space between the two tubes1 and are thereby heated to the required temperature1 while at the same time they serve to cool the inner tubes4 ,the most satisfactory temperature is about #djj ,c4 ,the tubes are first warmed to #cjj ,c4 to start the reaction1 and thereafter the heat evolved by the reaction itself is sufficient to keep it going4 ,the absorption of the sulphur trioxide also caused some difficulty at first4 ,this substance reacts most violently with water1 dissolving with a hissing sound like that produced when a red-hot poker is plunged into water4 ,at the same time great heat is developed1 and consequently1 much of the sulphur trioxide is vaporized1 and in that way lost4 ,this difficulty was got over by using #ih per cent4 sulphuric acid for the absorption1 the acid being kept at this strength by the simultaneous addition of water4 ,the contact process has some very distinct advantages over the older lead chamber process4 ,the plant covers a much smaller area than the bulky lead chambers4 ,although the preliminary purification of the gases is somewhat tedious and costly1 this is in great measure compensated by the purity of the acid produced4 ,no separate plant is required for concentration and purification1 as in the older process4 ,finally1 sulphuric acid of any concentration can be produced at will1 including the fuming acid1 which is required as a solvent for indigo1 and in the manufacture of artificial indigo and other organic chemicals4 ,the lead chamber process produces what is called chamber sulphuric acid very cheaply4 ,although this is only a #fj-gj per cent4 solution and very impure1 nevertheless1 it is quite good enough for the heavy chemical trade1 particularly for the first stage of the ,leblanc soda process1 and for making superphosphate4 ,these two industries alone consume many thousands of tons of this sulphuric acid every year4 ,probably for some years to come the two processes will continue to exist side by side1 but it may be doubted whether new works will now be installed to make sulphuric acid by the lead chamber process4 ,properties of ,sulphuric ,acid4 ,the pure non-fuming acid is a colourless oily liquid whose density is #ahd4 ,it mixes with water in all proportions1 yielding dilute sulphuric acid1 and it also dissolves sulphur trioxide1 yielding the fuming acid4 ,the mixing of sulphuric acid and water is accompanied by an evolution of heat and by contraction in volume4 ,it is an operation which must be carried out with great care1 the acid being always poured into the water1 otherwise the water floats on the heavier acid1 and so much heat is developed at the surface of separation that some of the water will be suddenly converted into steam1 and this1 escaping from the liquid with explosive violence1 may cause the contents of the vessel to be scattered about4 ,strong sulphuric acid chars most organic substances4 ,from substances such as wood1 sugar1 paper1 starch1 it withdraws the elements of water1 liberating carbon4 ,since it acts in the same way upon human flesh1 it is clear that the concentrated acid must be handled with very great care1 for it causes most painful burns4 ,for this reason1 vitriol throwing has always been regarded as a most serious and dastardly offence4 ,a simple first-aid remedy for burns produced by sulphuric acid is the liberal application of an emulsion of linseed oil and lime water4 ,the lime1 being an alkali1 neutralizes the acid1 and the oil excludes air from the wound4 ,the readiness with which sulphuric acid combines with water is often made use of both in the laboratory and in industrial ,chemistry for the purpose of drying gases4 ,one illustration of this use has already been given in describing the contact process4 ,another instance which may be fairly familiar occurs in the case of liquefying air1 where the gas must be thoroughly dried before being passed into the refrigerating apparatus1 otherwise this would soon become blocked with ice4 ,the position which sulphuric acid occupies in ,chemistry is due mainly to three outstanding features4 ,in the first place1 it is a strong mineral acid and displaces all other acids from their salts4 ,secondly1 it has a high boiling point 7#cch ,c471 and consequently1 the displaced acid with the lower boiling point can be distilled from the mixture4 ,lastly1 sulphuric acid can be made very cheaply from materials which are very abundant in ,nature1 and1 therefore1 it meets all the requirements of an acid which is to be used for general purposes4 ,,sulphates ,all the common metals1 except gold and platinum1 dissolve either in concentrated or in dilute sulphuric acid1 forming sulphates4 ,these salts are highly important and interesting substances4 ,they are all soluble in water1 with the exception of the sulphates of calcium1 strontium1 barium1 and lead4 ,ferrous ,sulphate1 also called green vitriol and copperas1 is obtained by dissolving iron in dilute sulphuric acid4 ,the solution is green1 and when it is evaporated1 the crystals which separate out look like bits of green glass4 ,it was because of this that the substance was first called green vitriol 7vitrum glass74 ,it is used very largely in dyeing as a mordant4 ,writing ink and ,prussian blue are also made from it4 ,the ,alums are double sulphates4 ,they are made by crystallizing solutions of potassium1 sodium1 or ammonium sulphate together with solutions of iron 7ferric71 chromium1 or aluminium sulphates4 ,in this way1 we may have potassium aluminium alum1 or iron ammonium alum1 and so on1 but whichever combination of elements is present1 the salt which is formed always crystallizes in octahedra4 ,the chief use of the alums1 as also of aluminium sulphate1 is as mordants in dyeing4 ,since a great many metallic salts1 particularly acetates and sulphates1 are used in the dye industry as mordants1 it may be well to explain here very briefly what a mordant is4 ,it must be remembered that almost all the dyes are solids which dissolve in water1 yielding intensely coloured solutions4 ,hence1 in most cases1 if a fabric is merely dipped in the dye and then dried1 the colouring is not permanent1 but can be washed out with water4 ,in order to fix the colouring matter1 the material is first dipped in the mordant1 usually a bath of some metallic salt1 and then1 generally after exposure to air or after steaming1 into the dye bath1 with the result that the colour becomes fixed4 ,the first part of the process is called 8mordanting0 the material4 ,the mordant either adheres to or combines with the fibres1 and the dye forms with the mordant a coloured compound called a 8lake10 which resists the action of water4 ,the colour is then said to be 8fast10 that is1 firmly fixed4 ,for printing on calico1 the mordant is thickened with gum arabic or other glutinous substance4 ,the design is then stamped on the material with the thickened mordant liquor4 ,the subsequent treatment consists of dipping the material in the dye and afterwards in water1 when the colour comes away from those parts which have not received the impress of the mordant4 ,sodium ,sulphate1 or ,glauber's salt1 is made from common salt by the action of concentrated sulphuric acid4 ,it is one of the raw materials used in making glass4 ,ammonium ,sulphate4 7,see p4 #ii47 ,calcium ,sulphate1 or gypsum1 occurs in large quantities in ,nature4 ,the salt contains #bji per cent4 of combined water1 and when carefully heated to #abj ,c1 it loses about two-thirds of this water1 yielding a white powder known as plaster of ,paris4 ,this substance1 when made into a paste with water1 gradually sets to a hard mass1 because the partially dehydrated gypsum re-combines with the water4 ,lead ,sulphate1 the chief impurity of commercial oil of vitriol1 is a white powder which is very often used for making white paint in place of lead carbonate 7white lead74 ,the sulphate has the advantage over the carbonate in not being so readily discoloured2 its disadvantage is that it lacks 8body40 ,copper ,sulphate1 or blue vitriol1 is frequently found in the drainage of copper mines1 where it is formed by the oxidation of copper pyrites4 ,it is made on a large scale by roasting sulphide ores of copper in a current of air4 ,oxygen combines with copper sulphide1 forming copper sulphate1 which is extracted with water and crystallized4 ,it forms large blue crystals containing #cf per cent4 of water4 ,this salt is put to many different uses4 ,very large quantities are used for dyeing and calico printing2 some of the green pigments1 such as ,schweinfurt green1 are made from it4 ,,chapter ,,iii ,,nitric ,,acid ,,and ,,nitrates ,nitric acid1 the aqua fortis of the alchemists1 must be placed next to sulphuric acid in the scale of relative importance1 because of the variety of its uses4 ,it is indispensable for making explosives1 and is used for the preparation of drugs and fine chemicals1 including the coal-tar dyes4 ,the acid also dissolves many metals1 forming nitrates1 which are put to several uses4 ,silver nitrate is the basis of marking ink1 and it is also the substance from which the light-sensitive silver compounds required for the photographic industry are made4 ,the important pigments1 chrome yellow and chrome red1 are prepared from lead nitrate4 ,the solvent action of nitric acid on copper is made use of in etching designs on copper plates4 ,over and above all this1 it must be mentioned that an adequate supply of 8nitrate0 is required for artificial manure4 ,thus it can be said that with the uses of this acid and its salts are associated our supply of daily bread1 our freedom from foreign oppression1 and many of the refinements and conveniences of life4 ,we shall begin the study of nitric acid by taking stock1 as it were1 of the natural sources of supply4 ,the free acid is not found in ,nature except for very small traces in the air after thunderstorms4 ,we have1 therefore1 to rely entirely on that which can be obtained artificially4 ,until quite recently1 it could be said that there was only one method of making the acid1 namely1 by the distillation of a mixture of potassium or sodium nitrates and concentrated sulphuric acid4 ,now1 however1 nitric acid is being made from the air1 though as yet only in small quantity1 notwithstanding the great development of this method owing to war requirements2 hence1 we are still mainly dependent on the naturally occurring nitrates just mentioned4 ,potassium ,nitrate 7nitre1 saltpetre1 sal prunella7 is found in the soil of hot countries1 especially in the neighbourhood of towns and villages where the sanitary arrangements are primitive4 ,in very favourable circumstances1 it may even appear as a whitish1 mealy efflorescence on the surface of the ground4 ,to obtain the salt1 it is only necessary to agitate the surface soil with water and1 after the insoluble matter has settled down1 to evaporate the clear solution4 ,potassium nitrate is required for making gunpowder1 which1 until quite recent times1 was the only explosive used in warfare4 ,continental countries that could not afford to rely entirely on sea-borne nitre had to make their own4 ,the refuse of the farmyard1 mixed with lime and ashes1 was made up into a heap of loose texture1 which was periodically moistened with the drainage from the stables4 ,in the course of years1 saltpetre and calcium nitrate were formed in the surface layers1 from which they were extracted from time to time4 ,the farmer was then allowed to pay part of his taxes in nitrates4 ,sodium ,nitrate1 also called caliche1 ,chili-saltpetre1 or ,chili-nitrate1 comes mainly from ,south ,america4 ,the beds extend for a distance of about #bbj miles in ,chili1 ,peru1 and ,bolivia1 between the ,andes mountains and the sea4 ,the deposit is about #e ft4 thick1 and its average breadth #e miles4 ,the crude material is treated with water in steam-heated wooden vats4 ,the clear solution is evaporated1 and the residue obtained is washed with the mother liquor and dried4 ,this product may contain as much as #ih per cent4 of the nitrate4 ,illustration3 ,fig4 #e4 ,,preparation ,,of ,,nitric ,,acid ,nitric ,acid4 ,chili-nitrate is always used for making nitric acid4 ,it is the more abundant of the two naturally occurring nitrates1 and therefore cheaper2 moreover1 weight for weight1 it yields more nitric acid than the corresponding potassium compound4 ,a mixture of sodium nitrate and sulphuric acid is heated in a large cast-iron retort 7,c1 ,fig4 #e74 ,the retort is entirely surrounded by flame and hot gases to prevent the condensation of the acid on the upper parts4 ,if this precaution were not taken1 the acid would dissolve the iron and the life of the retort would not be long2 moreover1 the product would contain ferric nitrate as an impurity4 ,the vapour of the acid is led away by the tube ,d into a series of two-necked earthenware receivers called bonbonnes 7,e71 and there condenses to a liquid4 ,the lower figure shows how the leading tube of the retort is protected from corrosion by the clay tube a1 b2 and how it is connected to the first receiver by the glass tube e1 which is luted on at f4 ,the percentage strength of the acid which distils over depends upon that of the sulphuric acid used and on the purity of the sodium nitrate4 ,pure nitric acid is a colourless liquid #aeei times as heavy as water1 volume for volume4 ,it fumes strongly in air1 and is a very corrosive liquid4 ,the pure acid of commerce is obtained by distillation of a less concentrated acid4 ,it is #fh per cent4 pure4 ,it is rendered free from dissolved oxides of nitrogen by blowing air through it4 ,when kept exposed to light1 the colour changes at first to yellow and then to brown1 because light causes a certain amount of decomposition4 ,red fuming nitric acid owes its colour to the great quantity of oxides of nitrogen dissolved in it4 ,it is made by distilling sodium nitrate that has been thoroughly dried with the strongest sulphuric acid2 the distillation is carried out at a high temperature1 with the express purpose of decomposing some of the nitric acid to furnish the oxides of nitrogen4 ,sometimes a little powdered starch is also added to facilitate the formation of these oxides4 ,this variety of nitric acid is particularly active and is used in many operations1 especially in making dyes1 explosives1 and other organic chemicals4 ,nitric acid has all the general properties of an acid1 that is1 it has a sour taste even in very dilute solution1 it changes the colour of litmus to red1 and dissolves carbonates and many metals4 ,when the vapour of nitric acid is passed through a red-hot tube1 and also when a nitrate is strongly heated1 oxygen gas is given off4 ,analysis shows that the oxygen combined in pure nitric acid amounts to #gf per cent4 of its weight1 while that in sodium and potassium nitrates is #ef and #ej per cent4 respectively4 ,nitric acid and the nitrates are1 therefore1 highly oxygenated compounds2 moreover1 under favourable circumstances1 they are rather easily broken up4 ,pure nitric acid will set fire to warm1 dry sawdust1 and a piece of charcoal or sulphur thrown on the surface of molten nitre takes fire spontaneously and is quickly consumed1 giving out a very vivid light4 ,the explanation of this is that the supply of oxygen is abundant2 it is also readily available and concentrated in a small space4 ,we can vary the experiment4 ,when a mixture of #ge parts by weight of finely-powdered saltpetre1 with #ae of charcoal dust and #aj of ground sulphur1 is ignited1 it burns very vigorously1 and is soon consumed4 ,this mixture is1 indeed1 home-made gunpowder4 ,explosives4 ,gunpowder was discovered in very early times by the ,chinese1 but for many years the secret of its composition did not get outside the ,great ,wall4 ,in the fifth century ,a4,d41 it was apparently re-discovered at ,constantinople1 and that city was for a long time defended by the use of what is known in history as ,greek ,fire1 an incendiary mixture very similar to1 if not actually the same as1 gunpowder4 ,but again the secret of its composition was jealously guarded1 and it was not until the thirteenth century that it was discovered1 apparently for the third time1 and introduced to ,western ,europe by ,roger ,bacon4 ,it was used in siege cannon early in the fourteenth century and in field guns at ,crcy2 but it was apparently not employed for blasting until about #afbg1 although in #afje1 ,guy ,fawkes and his fellow-conspirators were able to obtain it in large quantity4 ,from the battle of ,crcy in #acdf to the beginning of the ,south ,african campaign in #ahhi1 gunpowder was the only explosive used in warfare4 8,villainous saltpetre0 has therefore played a very important part in shaping the course of events in the world's history4 ,at the present day1 gunpowder has become 8old-fashioned40 ,in warfare1 it has been superseded by 8smokeless0 powders of much greater power2 while for mining operations1 explosives with a much greater shattering effect have long since taken its place4 ,the composition of gunpowder may vary1 but on the average it contains #ge parts by weight of saltpetre to #ae of charcoal and #aj of sulphur4 ,it is1 therefore1 a mixture of two combustible substances1 with a large quantity of a third very rich in oxygen4 ,the separate constituents are very finely ground and afterwards thoroughly incorporated4 ,when the mixture is ignited1 charcoal and sulphur burn very fiercely in the oxygen supplied by the saltpetre4 ,the secret of the action of gunpowder lies in the extraordinary rapidity with which combustion1 started at one point1 is propagated through the whole mass4 ,moreover1 the products of combustion are mainly gases1 and these occupy several thousand times the volume of the solid from which they are produced4 ,in a confined space1 a gas may exert enormous pressure when its normal tendency to expand is resisted4 ,propellants4 ,although combustion is propagated through a quantity of gunpowder with very great rapidity1 it is not done instantaneously4 ,the time required is about one-hundredth of a second under ordinary conditions1 and this interval1 short though it is1 is very important4 ,when the object is to throw a projectile1 the inertia of the latter has to be overcome1 that is1 a certain amount of force has to be applied before the heavy body begins to move4 ,in order that the strain on the breech of the gun may be as small as possible1 the pressure must be gradually developed and must reach its maximum just as the projectile begins to move4 ,the time factor in the explosion constitutes the difference between what we now call 8propellants0 and 8high explosive40 ,propellants are explosives which develop pressure gradually1 and are therefore used to launch the projectile2 high explosive develops pressure instantaneously1 and is therefore used as the bursting charge inside the shell1 bomb1 or torpedo1 and also in blasting operations4 ,cordite1 or smokeless powder1 is the propellant now most used4 ,it is made by macerating guncotton and nitroglycerine with their common solvent acetone4 ,a pulp is thus made to which #e per cent4 of vaseline is added4 ,the mixture is then forced through a die1 and in this way it is formed into threads or rods1 which harden as the acetone evaporates4 ,cordite produces no smoke1 because all the products of its combustion are invisible gases4 ,high ,explosive4 ,nitroglycerine and ,guncotton are both explosives of the instantaneous kind4 ,the former is made by forcing glycerine1 under pressure in a very fine stream1 into a mixture of fuming nitric and concentrated sulphuric acids2 the latter by soaking cotton-wool in a similar mixture4 ,both products are washed with water until quite free from acid1 and subsequently dried4 ,nitroglycerine is a colourless oil with a burning taste4 ,the oil itself is very dangerous to handle1 for it is liable to explode spontaneously even when the utmost care has been taken in its preparation4 ,a mere spot on a filter paper explodes with a deafening report when gently hammered on an anvil2 and one drop1 when heated on a stout iron plate1 blows a hole through the plate4 ,no use could be made of this substance for many years after its discovery because it was so liable to explode during transportation2 now1 however1 it is made safer by mixing with absorbent infusorial earth or kieselguhr4 ,this mixture is known as dynamite4 ,blasting gelatine1 like cordite1 is a mixture of nitroglycerine and guncotton4 ,trinitrotoluene 7,t4,n4,t47 is made from toluene and nitric acid1 and is a type of the modern high explosive4 ,it is a yellow crystalline substance which melts at #gi-hae ,c41 and is poured into the shell in a molten condition4 ,it is a remarkably stable substance1 which burns quickly when heated to #ahj ,c42 it cannot be exploded even by hammering4 ,explosion is only brought about by that of a subsidiary substance called the detonator4 ,the percentage composition of ,t4,n4,t4 is as follows-- ,carbon #cce ,hydrogen #bc ,nitrogen #aie ,oxygen #ddg #ajjj ,the oxygen present is only just sufficient to burn the whole of the carbon to carbon monoxide2 but since carbon dioxide is also formed1 which requires twice as much oxygen for the same weight of carbon1 and since the hydrogen and nitrogen may also be oxidized1 the combustion of the carbon is not complete2 and therefore the explosion of ,t4,n4,t4 is accompanied by a dense black smoke1 consisting of finely divided particles of carbon4 ,the explosive known as ammonal is a mixture of ,t4,n4,t41 aluminium powder1 and ammonium nitrate2 the function of the latter substance is to supply more oxygen to render the combustion of the carbon of ,t4,n4,t4 complete4 ,nitrates and the ,food ,supply4 ,chemical analysis shows that compounds of nitrogen enter largely into the composition of the living tissues of all plants and animals2 hence1 either nitrogen itself or some of its compounds must be assimilated by all living organisms to provide for growth and development1 and to repair wastage4 ,air1 since it contains approximately four-fifths of its volume of free nitrogen1 is the most obvious source of supply4 ,at every breath1 a mixture of oxygen and nitrogen is inhaled by animals1 but only part of the oxygen is used4 ,practically the whole of the nitrogen is returned to the atmosphere unchanged2 it serves only to dilute the oxygen4 ,from this it is clear that animals do not build up their nitrogenous constituents from elementary nitrogen4 ,with plants it is very much the same1 for1 although they obtain their principal food1 namely1 carbon1 from the carbon dioxide which is present in air1 it is only in a few exceptional cases that free nitrogen is assimilated4 ,the exceptions will be considered first1 because it was through these that we first began to learn something definite about the great importance of nitrogen in agriculture4 ,virgil1 who was born in #gj ,b4,c41 wrote a poem in praise of agriculture4 ,almost in the opening lines he deals with the treatment of corn land4 ,he advises that1 in alternate years1 this should either be left fallow or sown with pulse1 vetch1 or lupin2 but not with flax or oats1 because they exhaust the land4 ,from this we learn that rotation of crops was one of the established principles of good husbandry even at the beginning of the ,christian era4 ,it was not until the later years of the nineteenth century that any explanation as to why rotation of crops is beneficial was put forward4 ,let us first state the facts more precisely4 ,peas1 beans1 vetches1 clover1 and other members of the natural order called ,leguminosae1 which includes about #g1jjj species1 produce fruits rich in complex nitrogen compounds without being dependent in any way upon nitrogen compounds in the soil4 ,moreover1 they do not exhaust the land as far as these compounds are concerned2 hence wheat and other grain can be grown on the same land the following year4 ,it is now known that leguminous plants assimilate atmospheric nitrogen with the help of certain bacteria4 ,if anyone will dig up a lupin root1 he will observe#b conspicuous wrinkled swellings or nodules at various points on the roots4 ,these1 when examined with a high-power microscope1 are found to contain colonies of bacteria4 ,it is these minute vegetable organisms which assimilate nitrogen and pass on nitrogen compounds to the larger plant4 ,other plants cannot assimilate what we might call raw nitrogen2 they require soluble nitrates4 ,these they build up into complex organic nitrogen compounds suitable for the feeding of animals which can assimilate neither free nitrogen nor nitrates4 ,the ,nitrogen ,cycle4 ,the supply of nitrates in the soil needs continually to be renewed by the addition of decaying vegetable matter1 stable or farmyard manure1 or ,chili saltpetre4 ,the natural manures contain organic nitrogen compounds which were built up during the life of some animal or plant4 ,they are not immediately available as food for other plants1 because they are1 as it were1 the end products of life1 and are not soluble in water4 ,but ,nature provides for this4 ,the manures decay1 forming humus1 and ultimately ammonia1 one of the simplest of inorganic nitrogen compounds4 ,ammonia is then transformed to nitrites by certain bacteria present in the soil1 while other bacteria change nitrites into nitrates4 ,both of these organisms are quite distinct from the root nodule bacteria of the ,leguminosae4 ,the nitrates pass into the plant in solution1 and then begins again that wonderful cycle of changes which we have described4 ,this is perhaps made clearer by the following diagram4 ,illustration3 ,fig4 #f4 ,,the ,,nitrogen ,,cycle ,it now remains to show why artificial manures also are necessary4 ,let us consider what happens to a piece of ground which is left uncultivated4 ,although nothing is taken from it in the way of a crop1 yet it very quickly deteriorates1 and the soil becomes infertile through the loss of nitrogen compounds4 ,this is explained by the fact that nitrates are soluble in water1 and so they get washed away from the top soil4 ,in addition to this1 the nitrogen which is returned to the land forms quite an insignificant fraction of that which is taken from it1 for we waste a great deal of organic nitrogen4 ,the difference on both these accounts has1 therefore1 to be made up by the addition of artificial manures containing soluble nitrates4 ,the natural supply of nitrate is very limited4 ,according to a report of the ,chilian ,government published in #aiji1 the nitre beds of that country were expected to last for less than a century at the current rate of consumption4 ,wheat1 above all things1 will not grow to perfection on soil which is deficient in nitrate4 ,in #aijh1 ,sir ,william ,crookes called attention to the difficulty which might be experienced in the near future in supplying the people of the world with bread4 ,statistics showed that wheat was grown on #aei1jjj1jjj acres out of a possible #bfj1jjj1jjj4 ,the average yield is #abg bushels per acre4 ,by #aica1 it is calculated that the population of the world will be #a1gdf1jjj1jjj2 and to supply these with bread1 wheat would have to be grown on #bfd1jjj1jjj acres1 that is1 #d1jjj1jjj acres beyond the total available wheat land4 ,the remedy which ,sir ,william ,crookes suggested in order to avoid famine was to raise the average yield from #abg to #bj bushels per acre by the application of an additional #ab1jjj1jjj tons of ,chili saltpetre per annum4 ,in view of the possible exhaustion of the supply of this substance1 this would only mean a postponement of the evil day4 ,the position1 however1 is now modified to a great extent because undeveloped deposits of sodium nitrate are known to exist in ,upper ,egypt1 and the making of nitric acid from the air1 which in #aijh was put forward as a suggestion1 is now an accomplished fact4 ,nitric ,acid from ,air4 ,the supply of nitrogen in the air is truly inexhaustible2 it amounts to about #g tons for every square yard of the earth's surface1 which is about #bjj1jjj1jjj square miles4 ,it is quite evident that anything man may do in the way of taking nitrogen from the air will make no perceptible difference to its composition4 ,every time a flash of lightning passes between a cloud and the earth1 oxygen and nitrogen combine in the path of the spark1 producing oxides of nitrogen4 ,these dissolve in water1 and are washed into the earth as a very dilute solution of nitric acid4 ,as long ago as #aghe1 ,h4 ,cavendish imitated this natural phenomenon4 ,a reference to the diagram 7,fig4 #g7 will show how nitric acid can be made from the air on a small scale4 ,the globe contains air under slightly increased pressure4 ,the platinum wires or carbon rods are connected with the terminals of an induction coil1 which in its turn is connected to accumulators supplying the current required4 ,when the coil is put into action1 a spark passes across the gap between the ends of the carbon rods4 ,with a larger coil and a more powerful battery1 there is an arching flame which can be blown out and re-lighted4 ,this is actually nitrogen burning in oxygen4 ,the result in either case is the same2 the air in the globe sooner or later acquires a reddish-brown colour due to oxides of nitrogen1 which1 when shaken with water1 form a very dilute solution of nitric acid4 ,the same process is now carried out on a large scale4 ,air is driven by fans through a very powerful electric arc1 whereby #ae to #b per cent4 is converted into nitric oxide4 ,this combines spontaneously with more oxygen to form nitrogen peroxide1 which1 when dissolved in water1 gives a very dilute solution of nitrous and nitric acids4 ,illustration3 ,fig4 #g4 ,,nitric ,,acid ,,from ,,air ,the absorption of the oxides of nitrogen is carried out systematically4 ,the mixed gases1 after passing through the arc1 are passed through a series of towers filled with acid-resisting material over which a stream of water is flowing4 ,the solution of nitric acid so obtained is very dilute1 but by using the liquid over and over again1 a moderately strong solution is ultimately produced4 ,this is collected in granite tanks and neutralized with lime1 forming calcium nitrate or ,norwegian saltpetre1 as it is now called4 ,this is a new industry and a rapidly-growing one2 in the course of five years 7#aije-aiji7 the annual output of ,norwegian or 8air0 saltpetre increased from #aae to #i1dbb tons4 ,mountainous countries like ,norway and ,switzerland are perhaps in a specially favoured position with respect to this industry4 ,rapid streams and waterfalls1 in conjunction with turbines1 are used for driving the dynamos1 and in this way electricity is produced at very low cost4 ,it is interesting1 however1 to note that a plant for the manufacture of nitric acid from air has now been established in ,manchester4 ,,chapter ,,iv ,,the ,,halogen ,,acids ,a group of acids1 namely1 hydrochloric1 hydrofluoric1 hydrobromic1 hydriodic1 must now be considered together with their corresponding salts4 ,in appearance and in other physical properties they resemble one another very closely2 they are1 therefore1 called by the general name 8halogen acids40 ,this name is derived from the ,greek word meaning 8sea-salt10 which is a mixture of the salts of these acids1 and from which the acids themselves can be obtained by treatment with oil of vitriol4 ,hydrochloric ,acid4 ,when concentrated sulphuric acid is added to common salt1 a gas is liberated which has a very pungent acid smell and taste4 ,this is a compound of the elements hydrogen and chlorine1 and therefore called hydrogen chloride4 ,it is extremely soluble in water2 a given volume of water dissolves as much as #ejj times its own volume of the gas4 ,the solution produced in this way is now called hydrochloric acid1 but formerly it was known as spirits of salt1 or muriatic acid4 ,hydrochloric acid has all the general properties of acids4 ,it dissolves many metals1 such as zinc1 iron1 aluminium1 and magnesium2 hydrogen gas is given off1 and the chloride of the metal is formed4 ,it also dissolves limestone1 marble1 and all forms of calcium carbonate2 carbon dioxide gas is liberated1 and a solution of calcium chloride remains4 ,the hydrochloric acid of commerce is obtained as a by-product in the manufacture of washing soda from common salt by the method proposed by ,nicholas ,leblanc towards the end of the eighteenth century4 ,in the first stage of this process1 salt is mixed with sulphuric acid2 this causes the liberation of hydrogen chloride gas1 which1 when dissolved in water1 produces hydrochloric acid4 ,the past history of this branch of chemical industry is interesting4 ,until about #ahgj1 there was no very great demand for hydrochloric acid1 and in the early days of the working of the ,leblanc process the soda manufacturer took no pains to recover more than he could actually sell4 ,consequently1 a large quantity of hydrogen chloride gas was allowed to escape into the air1 with results which can well be imagined4 ,for miles around1 great damage was frequently sustained by the growing crops2 when it rained in the neighbourhood of the works1 the gas was washed out of the air and1 speaking quite literally1 it rained dilute hydrochloric acid1 which rapidly corroded all stone and metal work4 ,it is not1 therefore1 surprising to learn that alkali makers were frequently involved in litigation1 and chemical works were regarded as a great nuisance4 ,by the ,alkali ,act of #ahfc1 chemical manufacturers were compelled to prevent the escape of more than #e per cent4 of hydrochloric acid gas2 and by a subsequent ,act1 this limit was lowered to #jb grain per cubic foot4 ,the provisions of the ,acts were not difficult to carry out1 because hydrogen chloride is extremely soluble in water4 ,the gases coming from the pans in which the salt was decomposed were led into towers 7see ,fig4 #h7 built of bricks or ,yorkshire flags soaked in tar4 ,these towers were filled up with coke or other acid-resisting material1 which was kept moist by water flowing from the tank ,f4 ,in this way1 hydrogen chloride gas was removed and hydrochloric acid collected in tanks 7not shown in the figure7 at the bottom of the towers4 ,even then1 there was no market for the greater part of the recovered acid1 consequently much of it found its way into drains and streams1 and so carried on its work of destruction in a less obtrusive way4 ,illustration3 ,fig4 #h4 ,,preparation ,,of ,,hydrochloric ,,acid ,by another piece of legislation1 which at first sight seems to be wholly unconnected with ,chemistry1 hydrochloric acid acquired a greatly enhanced value4 ,in #ahfa1 the tax on paper was removed1 and in the next twenty years the demand for that commodity increased so much that raw material both cheaper and more abundant than rag had to be found4 ,esparto grass and eventually wood pulp proved successful substitutes4 ,there is really very little difference in composition between cotton and linen rag on the one hand and wood fibre on the other1 for both are mainly composed of cellulose1 which is a definite chemical compound4 ,wood fibre is the less pure1 and it is also coloured1 and therefore has to be bleached before it can be used for making white paper4 ,it was this circumstance which led to the greatly increased demand for hydrochloric acid4 ,at the beginning of this chapter1 it was mentioned1 in passing1 that hydrogen chloride gas is a compound of hydrogen and chlorine4 ,the latter element is a very active bleaching agent1 and is most easily obtained by treating hydrogen chloride or its solution in water with pyrolusite 7black oxide of manganese71 whereby the hydrogen is oxidized1 forming water1 and chlorine gas is set free4 ,being a gas1 chlorine is not convenient to handle in large quantities2 it is1 therefore1 converted into bleaching powder1 commonly but wrongly called chloride of lime4 ,bleaching ,powder4 ,the manufacture of bleaching powder is carried out in the following way4 ,slaked lime to the depth of #c or #d in4 is spread over the floor of a special chamber which can be made gas-tight4 ,the lime is raked up into ridge and furrow1 and the chamber is filled with chlorine4 ,at the end of about twenty-four hours1 the greater part of this chlorine will have been absorbed by the lime4 ,the chamber is then opened1 the lime is raked over to expose a fresh surface1 and the process of chlorination is repeated4 ,generally this is sufficient2 the bleaching powder should then contain about #ce per cent4 of available chlorine4 ,the demand for bleaching powder is great and steadily increasing4 ,the price of #ce per cent4 bleaching powder has never been less than about #e a ton1#c so that it is perhaps unnecessary to add that the absorption of hydrogen chloride gas is now made so complete that it is well within the requirements of the second ,alkali ,act4 ,chlorides4 ,the salts of hydrochloric acid are called chlorides1 and the most important of these is sodium chloride or common salt--a body that is so well known that it need not be described here4 ,although the quantity of this substance required for domestic purposes is very large1 it is1 nevertheless1 small by comparison with that which is used for industrial purposes4 ,it has already been mentioned that salt is the starting-point for the manufacture of washing soda by the ,leblanc process1 and1 in addition to this1 it is employed in the glass industry to produce whiteness and transparency in certain kinds of glass2 in pottery1 for glazing earthenware2 in soap-making1 for salting out the crude soap2 and in the dye trade as a mordant1 and also for improving the quality of certain colours4 ,a full account of the salt industry is given in another volume of this series4 ,hydrofluoric ,acid4 ,when calcium fluoride 7fluorspar1 ,derbyshire spar1 or blue-john7 is warmed with concentrated sulphuric acid in a leaden dish1 hydrogen fluoride gas is evolved1 and this1 when dissolved in water1 gives hydrofluoric acid4 ,the peculiar property of this substance is that it has a very marked corrosive action on glass4 ,it cannot1 therefore1 be kept in glass vessels1 but must be stored in bottles made of hardened caoutchouc4 ,on the other hand1 it is this same property which gives it its place in commerce4 ,as far back as #afgj it was used for etching on glass4 ,the process is a very simple one4 ,the article is first coated with wax1 which is then removed in places by a sharp pointed tool4 ,when exposed to the action of the gas or its solution1 corrosion takes place only where the glass has been laid bare1 the other parts being protected by the wax4 ,after a short interval1 the wax can be melted off1 and the design made more distinct by rubbing in some opaque cement4 ,for general trade purposes1 such as the stamping of lamp chimneys or electric light bulbs1 a quicker method is required4 ,in this case1 a preparation of hydrofluoric acid which can be applied with a rubber stamp is used4 ,fluorspar or calcium fluoride is the most important salt of hydrofluoric acid4 ,it is a commonly occurring mineral1 and besides its use for the preparation of the acid1 it is employed in many metallurgical operations to form a fusible slag4 ,hydrobromic and ,hydriodic ,acids are not much used1 but their salts1 the bromides and iodides respectively1 are of great technical importance4 ,silver chloride1 bromide1 and iodide1 are sensitive to light1 and mixed with gelatine they form the emulsion which is spread over photographic plates and papers4 ,potassium bromide and iodide are also well known to photographers4 ,when the halogen salts of silver are exposed to light1 an extremely subtle chemical change takes place1 which is only made apparent when the plate or paper is developed4 ,then the silver salts on which the light has fallen are reduced to metallic silver1 and this reduction is greatest where the light was most intense1 and in other places is proportional to the light intensity4 ,a very faint image may appear on the plate while it is in the developer1 but generally the image is only brought out clearly when the plate1 film1 or paper is placed in 8hypo0 solution1 which dissolves out the silver salts which have not been changed1 leaving the metallic silver unaffected4 ,,chapter ,v ,,carbonic ,,acid ,,and ,,carbonates ,carbon4 ,when any product of animal or vegetable life is strongly heated in a vessel from which all air currents are excluded1 a mixture of gases and liquids is driven off1 and a charred mass remains4 ,this residue1 from whatever source obtained1 is composed mainly of the element carbon4 ,it sometimes happens that a loaf of bread or a cake is left in the oven and forgotten4 ,in popular language it is then said to be 8burnt to a cinder02 in reality1 the surface layers have been converted into carbon4 ,carbonic ,acid4 ,if carbon is heated in an open vessel provided with a good draught1 it glows and in time disappears1 because it combines with oxygen to form an invisible gas1 carbon dioxide or carbonic acid gas1 which1 when dissolved in water1 forms carbonic acid4 ,compared with the acids which have been described in the foregoing chapters1 this is a very feeble acid2 it changes the colour of litmus to a wine red1 not a bright pink2 its taste is just pleasantly acid1 and its solvent action on metals and limestone is very small indeed4 ,the solution of the acid1 obtained by passing carbon dioxide into water1 is1 of course1 very dilute1 and it cannot be concentrated by evaporation1 since this only results in expelling the carbon dioxide from solution1 leaving pure water4 ,soda ,water4 ,in the case of most gases1 the weight which dissolves in a given quantity of water is proportional to the pressure4 ,this is true for carbonic acid gas4 ,under a pressure of #d atmospheres1 the weight of gas which dissolves is four times as great as under a pressure of one atmosphere4 ,soda water is water charged with carbon dioxide under pressure4 ,this pressure is maintained from the time it leaves the manufacturer to the time it reaches the consumer by the strong walls of the syphon or bottle4 ,immediately this pressure is released1 the greater part of the excess gas escapes1 producing effervescence4 ,it is1 however1 curious to note that all the gas which ought to escape when the pressure is reduced does not do so at once4 ,if soda water is allowed to stand in an open glass until it becomes 8flat10 a brisk effervescence can be started again by dropping a lump of sugar into the quiescent liquid4 ,soda water remains supersaturated with gas for some time after the pressure has been released4 ,calcium ,carbonate4 ,the salts of carbonic acid are called carbonates4 ,calcium carbonate is one of the most abundant substances in ,nature4 ,the white cliffs of the east and south coasts of ,england1 and those of ,france across the intervening sea1 are the exposed parts of enormous beds of chalk or calcium carbonate4 ,whole mountain ranges in various parts of the world are composed of limestone1 which in some cases is mainly calcium carbonate1 and in others a mixture of this substance with magnesium carbonate4 ,marble1 whether white1 black1 or variegated1 is almost pure calcium carbonate1 the differences of colour being due to insignificant traces of iron and other foreign matter4 ,in ,iceland spar and calc spar1 sometimes called dog-tooth spar1 we have two transparent crystalline forms of this same substance4 ,connected with the animal kingdom there are forms of calcium carbonate no less varied in appearance4 ,egg shells are composed of this substance1 and so are oyster shells and the hard external coverings of some of the lower animals4 ,the mother-of-pearl lining of the oyster shell1 and also the pearl itself1 are secretions of calcium carbonate4 ,the beauty of the last-named variety is due to the external form and to minute inequalities of the surface1 which cause the resolution of white light into colours seen in the spectrum or in the rainbow4 ,the coral reefs or atolls of the ,southern oceans1 which may be miles in breadth and hundreds of miles in length1 are all composed of calcium carbonate1 which a tiny marine animal has formed for its own support and protection4 ,it is perhaps somewhat surprising at first to be told that all these forms are composed of the same chemical substance1 yet on this point the evidence is definite and unmistakable4 ,all the varieties dissolve readily in dilute hydrochloric acid with effervescence caused by the escape of carbon dioxide gas2 moreover1 if any of the purer forms1 such as pearl1 marble1 or ,iceland spar1 are heated to redness for some time1 they all lose about #dd per cent4 by weight1 leaving a residue which is pure lime4 ,quicklime4 ,the making of lime from limestone or chalk is called lime burning4 ,the operation is carried out in a structure called a lime kiln1 which is usually a barrel-shaped vertical shaft surrounded by substantial brickwork4 ,there are two main methods of procedure1 the one continuous and the other intermittent4 ,in the continuous process1 the kiln is filled up with limestone and fuel 7generally coke7 in alternate layers4 ,combustion is started at the bottom and maintained by a regulated draught4 ,as the charge works down1 the addition of limestone and fuel is continued from the top1 while the lime is removed from the bottom of the kiln4 ,the lime produced by this method has the ashes of the fuel mixed with it4 ,to avoid this1 the more modern type of kiln has four lateral fire grates outside the actual kiln4 ,for the intermittent method1 a kiln is required which has a fireplace at the bottom4 ,over this a rough arch is built of large pieces of limestone1 laid dry1 and then the kiln is filled up with pieces of limestone which decrease in size from below upwards4 ,the fire is kindled beneath the arch and urged by a regulated draught4 ,the heating is maintained for three days and nights1 after which time the charge is allowed to cool down4 ,carbonic ,acid ,gas in ,nature4 ,although the solvent action of carbonic acid is very small compared with that of strong acids1 it is nevertheless great in comparison with that of water4 ,this is shown especially in its action on limestone1 an action from which several important consequences arise4 ,rain1 as it falls through the air1 dissolves a little carbon dioxide and1 although this is only an exceedingly dilute solution of a very weak acid1 its cumulative effect1 especially in limestone districts1 is very great2 it hollows out enormous caves and causes the formation of those fantastic creations in stone known as stalactites and stalagmites4 ,when a drop of water charged with carbonic acid gas falls on limestone1 it dissolves a little of that substance1 forming calcium bicarbonate1 which may be regarded as a compound of calcium carbonate1 carbon dioxide1 and water4 ,little by little1 the solid rock is hollowed out and a cave1 or perhaps an underground watercourse1 is formed4 ,again1 the drop of water charged with calcium bicarbonate may find its way to the roof of a cave4 ,as it hangs from the roof while it gathers strength to fall1 a little of the carbon dioxide escapes1 and a minute quantity of calcium carbonate is deposited4 ,in this way1 a stalactite looking like an icicle in stone gradually grows downwards4 ,when the drop reaches the floor of the cave1 a little time elapses before it sinks into the ground2 again a little carbon dioxide escapes1 and a small quantity of calcium carbonate is formed4 ,little is added to little1 and in the course of ages the stalagmite grows upward from the floor and ultimately meets the stalactite to form a continuous column of glistening crystallized calcium carbonate4 ,hard and ,soft ,water4 ,water that is used for domestic or manufacturing purposes is described as either hard or soft4 ,soft water produces a soap lather almost at once2 hard water forms at first a scum or curd which has no detergent properties1 and only after a time gives the soap lather which is required4 ,the difference is due to the relative amount of dissolved solid contained in the water4 ,only distilled water or rain water collected in the open country is perfectly soft1 for this is the only kind of water which on being evaporated to dryness leaves no solid residue4 ,in districts where the underlying strata are composed of hard insoluble rock1 such as granite or millstone grit1 the water contains very little dissolved matter and is relatively soft4 ,in a limestone or chalk country1 water is very hard and in many cases has to be softened either before delivery or before use4 ,the chief impurities which cause hardness are the chlorides1 sulphates1 and bicarbonates of magnesium and calcium4 ,the chlorides and sulphates are not affected in any way by boiling1 and the hardness which is due to them is said to be 8permanent40 ,the bicarbonates1 on the other hand1 are decomposed when the water is boiled1 and then they cease to cause the water to be hard4 ,this part of the hardness is spoken of as 8temporary0 hardness4 ,let us now consider what calcium bicarbonate is and how it is formed4 ,it is a compound of calcium carbonate and carbonic acid1 and is formed by the solvent action of carbonic acid on limestone or chalk4 ,the compound is soluble in water2 but when the solution is boiled1 the carbonic acid is broken up1 carbonic acid gas is expelled from the solution1 and calcium carbonate is formed4 ,temporary hardness is the more troublesome4 ,in the first place1 the bicarbonates1 especially that of calcium1 often form the greater part of the dissolved impurity4 ,moreover1 when the water is boiled1 although the hardness is removed1 the insoluble calcium carbonate is a source of trouble1 for it gradually settles down into the hard mass known as 8fur0 in kettles and 8scale0 in boilers4 ,it is perhaps necessary at this point to emphasize the fact that matter suspended in water does not make it hard1 and it is only matter which is dissolved which makes any difference in this respect4 ,since the softening of temporary hard water by boiling has the undesirable feature of introducing solid matter into the boiler1 it is customary now to treat this water chemically4 ,the following is the process most generally used4 ,quicklime or slaked lime is stirred into the water until the mixture gives a faint brown coloration when a drop of silver nitrate is added to a small test portion4 ,unsoftened water is then added until a sample just ceases to give this test4 ,the temporary hardness has then been removed1 and it is only necessary to allow the suspended matter to settle4 ,the explanation of the method is as follows4 ,the lime which is added neutralizes the carbonic acid combined with the calcium bicarbonate1 and the result is the same as in the former case where this carbonic acid was decomposed by heating1 for calcium carbonate is thrown out of solution4 ,for domestic purposes1 water is softened by the addition of washing soda4 ,since this reacts with all the calcium and magnesium compounds forming the insoluble carbonates1 all hardness1 both temporary and permanent1 is removed4 ,,chapter ,,vi ,,phosphoric1 ,,boric1 ,,and ,,silicic ,,acids ,the acids which are grouped in this chapter are not in themselves of much interest1 though some of their salts are extremely important compounds4 ,bone4 ,much of the refuse bone1 sooner or later1 reaches the marine store1 and from that point starts on a career of usefulness in the industrial world4 8,green bone10 as it is then called1 may have fat adhering to it or confined in its hollow interior as marrow4 ,this is recovered by treatment with benzine1 and after that the bone is subjected to the action of superheated steam in order to convert cartilage into glue4 ,in some cases1 the residue is then ground up to make bone meal1 which is valuable as a manure because of the calcium phosphate which it contains4 ,in this way1 the phosphate returns again to the animal kingdom1 for it supplies plants with the phosphates that they require1 and from the vegetable kingdom it passes to animals and helps to build up bone again4 ,calcium ,phosphate and ,bone ,black4 ,instead of being ground up1 bone may be heated in a retort in much the same way as coal is treated for the manufacture of coal gas2 bone oil is distilled off1 and a non-volatile residue1 called bone black or animal charcoal1 remains4 ,this contains about #ij per cent4 of calcium phosphate and #aj per cent4 of finely divided carbon disseminated throughout the mass4 ,it has the peculiar property of absorbing colouring matter1 and is used for this purpose in the sugar industry and in the preparation of fine chemicals4 ,phosphoric ,acid4 ,after being some time in use1 bone black loses the property of absorbing colouring matter2 and though it can be 8revived0 several times by heating it strongly in a closed retort1 it ultimately becomes spent and of no further use to the sugar refiner4 ,it is then heated again1 this time in an open vessel1 until all the carbon is burnt away4 ,the residue is now a greyish solid consisting mainly of calcium phosphate4 ,this1 supplemented with native phosphate1 which is probably fossilized bone1 is used for the preparation of phosphoric acid4 ,the salt is decomposed by sulphuric acid in wooden vats2 calcium sulphate is formed1 and ultimately settles on the bottom of the vat1 leaving a clear supernatant liquid1 which is a dilute solution of phosphoric acid4 ,this liquid is drawn off and evaporated to a syrup4 ,this is 8syrupy0 phosphoric acid4 ,on being still more strongly heated1 the syrup loses still more water1 and a semi-transparent glassy-looking substance1 called metaphosphoric acid1 remains4 ,superphosphate4 ,all fertile soils1 especially those on which wheat is to be grown1 must contain a certain amount of phosphate4 ,with this1 as with all other plant foods1 the actual percentage weight required in the soil is very small indeed1 but it is necessary that it should be disseminated throughout the soil4 ,even distribution is very difficult to secure in the case of a substance like calcium phosphate1 which is practically insoluble in water4 ,to get over this difficulty1 calcium phosphate is converted into a mixture known as 8superphosphate0 by the following process4 ,bone ash or the mineral phosphate is finely ground and thoroughly mixed by machinery with two-thirds its weight of sulphuric acid from the lead chambers4 ,after a time1 this mixture sets to a hard mass1 containing principally gypsum and calcium tetrahydrogen phosphate4 ,it is then ground up finely and is ready for use4 ,the special modification of calcium phosphate contained in superphosphate is soluble in water4 ,it is1 therefore1 carried into the soil in solution1 and in this way very evenly distributed4 ,in the soil it reacts with the lime or chalk which is present1 and is gradually reconverted into insoluble calcium phosphate4 ,the manufacture of superphosphate is a very important industry4 ,the weight of the substance produced annually in ,great ,britain alone is not far below a million tons4 ,basic ,slag4 ,in the ,bessemer process for converting iron into steel1 cast iron is melted up in a vessel called a converter and1 by the aid of a powerful blast blown through the molten iron1 most of the impurities are burnt off4 ,if1 however1 phosphorus and sulphur are present1 they are not removed if the converter has a silica 7acid7 lining4 ,the original ,bessemer process was1 therefore1 modified by ,thomas and ,gilchrist1 and the converter for this kind of iron is lined with dolomite and lime 7basic lining74 ,phosphorus is then converted into phosphate and retained by the lining1 which is subsequently removed1 ground up finely1 and sold as 8basic slag40 ,boric ,acid1 or boracic acid1 is familiar because it is used in medicine as a mild antiseptic2 it is also employed as a preservative for food4 ,it is a white crystalline compound1 sparingly soluble in water4 ,it has no well-marked taste1 and causes only a partial change in the colour of litmus solution2 it is1 therefore1 one of the weak acids4 ,it does not dissolve metals1 but it displaces carbon dioxide from carbonates1 forming salts4 ,borax1 the best known salt of boric acid1 is used in laundry work and also for making some enamels1 for when it is strongly heated it loses water1 and ultimately melts down to a clear 8glass0 in which the oxides of metals will dissolve1 yielding transparent substances which are beautifully coloured according to the nature of the oxide used4 ,this property is often made use of in chemical analysis in what is known as the 8borax-bead0 test4 ,illustration3 ,fig4 #i4 ,,boric ,,acid ,boric acid is a natural product2 the method by which it is obtained is of some interest1 because it is so simple1 and because it shows how mere traces can be gradually accumulated until a very fair total is ultimately obtained4 ,moreover1 the method is copied directly from ,nature4 ,in the early years of the nineteenth century1 certain jets of natural steam1 called suffioni1 which issue from the earth in ,tuscany1 were found to contain the vapour of boric acid4 ,these jets of steam are of volcanic origin4 ,the quantity of boric acid in the vapour is very small indeed2 nevertheless1 by the method which is adopted1 it can be profitably recovered1 and more than a ton of the solid is daily produced4 ,in the same country there are many lagoons1 the water of which contains boric acid4 ,it was rightly conjectured that this boric acid came from jets of steam which issued from the earth in the bed of the lagoon4 ,this suggested the idea of building up an artificial lagoon around a group of jets4 ,series of about five of these collecting basins 7,fig4 #i7 are formed1 each one at a slightly lower level than the one which precedes it4 ,the first basin is filled with water from an adjacent spring1 and this is allowed to remain for twenty-four hours4 ,a sluice is then opened and the liquid contained in the first basin flows down to the second1 where it remains for another day1 and so on until it reaches the last basin of the series4 ,the liquid by this time is almost fully charged with boric acid1 but it contains only about #b per cent41 because the acid is so sparingly soluble in water4 ,from the last basin 7,a71 the liquid runs into large vats 7,b1 ,d71 where the suspended impurities settle down4 ,the solution of boric acid is then concentrated by causing it to flow over a broad inclined plane made of corrugated lead or through a series of shallow vessels heated by jets of natural steam4 ,the hot liquid flows into another vat 7,c71 and1 as it cools1 boric acid crystallizes out and is removed by perforated ladles4 ,the mother liquor from which the crystals have been withdrawn is1 of course1 a cold saturated solution of the acid1 and this is returned to the top of the incline to flow down again and lose more water4 ,the boric acid is finally transferred to drying chambers1 which are also heated by the natural steam4 ,native borax or 8tinkal0 comes from ,thibet and also from ,ceylon4 ,in ,california1 a large quantity of borax is obtained from a borax lake1 and also from the mud of marshes in its neighbourhood4 ,silica4 ,the element silicon does not occur in the free state in ,nature1 neither has any particular use been found for it1 and therefore it is not often isolated except to provide a lecture specimen4 ,the compounds of silicon1 however1 are both plentiful and important1 especially silica1 the oxide1 and the silicates or salts of silicic acid4 ,the commonest forms of silica are sand1 flint1 and quartz4 ,silver sand is composed of small crystals of pure silica1 while flint is the amorphous variety of the same substance4 ,quartz1 or rock crystal1 is a very hard and transparent mineral4 ,it forms six-sided prisms ending in pyramids4 ,it is distinguished from other common transparent minerals1 such as calcspar1 by the fact that it cannot be scratched even with a good knife or file1 and that a drop of hydrochloric acid has no action on it4 ,the melting point of silica is very high4 ,sometimes silica is very delicately coloured with minute traces of metallic oxides and other substances1 and these forms1 because of their rarity and beauty1 are more highly valued4 ,smoky quartz1 cat's-eye1 and amethyst are some of the coloured varieties of quartz4 ,opal1 agate1 jasper1 onyx1 and chalcedony are1 in the chemist's classification1 merely coloured flints4 ,in recent years1 chemical apparatus has been made from pure fused silica4 ,this can only be worked in the oxy-hydrogen blow-pipe flame or in the electric furnace2 nevertheless1 crucibles1 flasks1 beakers1 and retorts can be made4 ,silica ware has several advantages over glass1 notably1 that water has no action upon it at all2 moreover1 its coefficient of expansion is so very small that a piece of apparatus made of silica can be suddenly heated or cooled without risk of fracture2 indeed1 it can be made red-hot and cooled immediately by plunging into cold water4 ,quartz or silica fibres1 used for suspending magnets and other bodies in very delicate physical apparatus1 are made in the following way4 ,molten silica is attached to the bolt of a crossbow1 which is then released above a carpet of black velvet4 ,as the bolt flies forward1 it draws out the silica into a filament1 which is so fine that it would be difficult to find were it not for the velvet background4 ,silicic ,acid itself is only of theoretical interest4 ,it is obtained by adding hydrochloric acid to a solution of potassium or sodium silicate4 ,it is a gelatinous substance of somewhat indefinite composition4 ,it has no effect on litmus1 no taste1 and no solvent action2 in fact1 it is only recognizable as an acid because it dissolves in alkalis1 forming salts called silicates4 ,it is one of the weakest acids known4 ,the natural silicates are very abundant and varied2 orthoclase or potash felspar1 and albite or soda felspar1 are those which most commonly occur4 ,the former is potassium aluminium silicate1 and the latter1 sodium aluminium silicate4 ,iron is generally present in both as an impurity4 ,the weathering of the felspars1 in conjunction with the action of water1 has produced the clays4 ,in this way1 pure white ,china clay has been formed from felspars which contain little or no iron1 and the coarser kinds of clay from others containing a greater proportion of foreign substances4 ,mica1 which is used for making lamp chimneys1 is a potassium aluminium silicate4 ,asbestos1 meerschaum1 beryl1 garnet1 jade1 and hornblende are all silicates of various metals4 ,glass is a complex mixture of insoluble silicates with excess of silica4 ,the varieties in common use are soda glass1 ,bohemian glass1 and lead glass 7which is also called flint glass74 ,soda glass is mainly a mixture of calcium and sodium silicates1 and is distinguished by its low melting point1 which makes it easy to work at moderate temperatures4 ,it appears in commerce as plate glass1 window glass1 and common bottles4 ,bohemian glass contains calcium and potassium silicates1 and has a high melting point4 ,it is used for making chemical apparatus4 ,lead or flint glass contains the silicates of lead and potassium2 this is a dense glass1 but at the same time rather soft4 ,it takes a high polish and is used for making ornamental or cut-glass ware4 ,remembering that glass is composed of the salts of silicic acid1 the reader will readily understand that the mixture from which it is made must contain acidic and basic constituents4 ,the acidic or acid-forming material is in every case silica or sand4 ,this must be pure1 and for all but the commonest kind of bottle or window glass1 it must be free from iron1 otherwise the glass will have a more or less pronounced greenish colour4 ,it must also be fine and even grained4 ,formerly1 the glass sands used in this country came from ,holland and ,belgium1 but now supplies from several ,british sources are being successfully used4 ,the basic portion of the glass mixture differs according to the kind of glass required4 ,an average mixture for soda glass contains sand1 #bj parts2 salt cake 7sodium sulphate71 #aj parts2 quicklime1 #e parts2 charcoal1 #a part4 ,for ,bohemian glass1 pearl ash 7potassium carbonate7 takes the place of salt cake1 and no charcoal is necessary because the materials used are finer4 ,for lead glass1 the mixture is still further modified by the use of litharge1 or more often red lead1 in place of lime4 ,the ingredients are well mixed and thoroughly dried4 ,waste glass from a previous batch is also added4 ,the mixture is heated to about #abjj ,c4 in large pots made of ,stourbridge clay1 and the heating is continued for as much as sixteen hours1 and until the whole of the material in the pot is molten and fairly mobile4 ,scum or glass-gall is removed1 and when gas bubbles have disappeared1 the temperature is allowed to fall to #gjj-hjj1 when the glass becomes sufficiently viscous for subsequent working4 ,the semi-fluid mass is then blown1 moulded1 or drawn1 according to the kind of article that is required4 ,the physical properties of glass will now be considered in order that we may be able to account for its extended use4 ,such an inquiry as this1 especially in the case of materials in common use1 is often interesting1 because it frequently happens that the special property upon which we set so much value is an abnormal one and1 consequently1 the feature which we take for granted is precisely the one into which we should inquire most closely4 ,the most striking feature of glass is its transparency4 ,this property is abnormal1 if glass is a solid4 ,consider what happens in most cases4 ,a substance like nitre melts easily and in the molten state is perfectly transparent2 when it cools1 crystals form and1 though these individually may be transparent1 yet the solid mass is opaque4 ,the reason for this is that the solid is not optically homogeneous1 and therefore a ray of light cannot pass through it in a straight line4 ,at each facet of a crystal light is deviated and reflected1 and in the end is almost wholly scattered4 ,consequently1 an object1 even if it can be seen at all1 can be discerned only in a blurred and indistinct fashion through such a medium4 ,there are very good reasons1 however1 for supposing that glass is not a true solid but an extremely viscous liquid4 ,if glass is heated1 it softens and begins to flow very sluggishly at first1 but afterwards more readily4 ,there is no abrupt change1 as there generally is in passing from the solid to the liquid state4 ,similarly in cooling1 there is no point at which it is possible to say that the glass is solidifying4 ,the view that this substance is really a liquid is perhaps a little startling at first1 but it becomes less so when we observe that a long glass rod supported at its ends in a horizontal position sags in the middle and is permanently deformed4 ,to avoid that change which would be technically called solidification by a scientist1 the article which has been fashioned in glass is cooled down very slowly and gradually4 ,this part of the process is called annealing2 it may occupy some days in extreme cases1 and it points to the fact that experience has shown that it is necessary to guard against some change which would normally take place if this precaution were neglected4 ,the change in glass which annealing is intended to prevent is known as devitrification4 ,in spite of all precautions1 this does occur sometimes1 and specimens of old window glass are often seen to have lost their transparency completely and to have an opalescent sheen4 ,in these cases1 the silicates have crystallized4 ,an extreme case of badly annealed glass is illustrated by ,rupert's drops1 a scientific curiosity of very old standing4 ,these are 8tears0 of glass made by dropping the molten substance into water4 ,when the tail of the drop is nipped off1 the whole thing is shattered to powder with something like explosive violence4 ,clearly there is a very great internal strain1 due to the fact that the outer parts have solidified and contracted1 while the inner part is still warm and dilated4 ,another remarkable feature of glass is the ease and simplicity with which it can be fashioned into articles of various shapes4 ,as a plastic material1 molten glass almost ranks with clay4 ,this again is due to the property of passing through a viscous state1 that is1 one which is intermediate between a solid and a liquid4 ,water ,glass1 or soluble glass1 is mainly sodium silicate4 ,it is made by fusing sand or powdered flint with caustic or with mild soda2 sometimes1 by digesting crushed flint or chert with caustic soda solution under considerable pressure in autoclaves or specially constructed boilers4 ,in the latter case1 no extraction is necessary2 but in the former1 the residue is treated with water and the solution evaporated until it becomes a viscous transparent liquid4 ,this liquid is used in various ways in industry4 ,it is added to the cheaper varieties of yellow soap1 and is employed as a mordant in dyeing and printing calico4 ,an artificial sandstone is made by mixing sand1 calcium chloride1 and sodium silicate2 the two last-named substances interact to form calcium silicate1 which is insoluble in water4 ,for domestic purposes1 water glass is best known in connection with the preserving of eggs4 ,when the film of water glass dries on the surface of the egg shell1 the latter becomes impervious to air4 ,,chapter ,,vii ,,organic ,,acids ,organic ,chemistry4 ,about a century ago1 when the science of ,chemistry was still in its infancy1 several substances were known which could then only be obtained from animals or plants4 ,the composition of these substances was not understood1 and they were not classified2 moreover1 since none of them had ever been prepared artificially1 it was supposed that it was impossible to do this--the reason given was that 8vital force0 was necessary for their production4 ,in time1 however1 some of the most typical animal and vegetable products were prepared in the laboratory1 and the belief in vital force disappeared4 ,in later times it was proved that substances like sugar1 starch1 urea1 indigo1 and a great many more1 all contain the element carbon4 ,at the present time1 more than #ajj1jjj compounds of this element are known2 and since they resemble one another1 and at the same time differ in several important respects from the compounds of other elements1 it is both natural and convenient that they should be classed together and studied separately4 ,this branch of ,chemistry is called organic4 ,it must not1 however1 be supposed that all organic compounds are necessarily produced by some living organism4 ,a great many are1 but there are many more which are purely synthetic products4 ,inorganic ,chemistry includes all the other elements and their derivatives4 ,the element carbon1 and also some of its simpler compounds1 such as carbon monoxide1 carbon dioxide1 carbonic acid1 and carbonates1 are more appropriately placed in the inorganic section4 ,the acids which have been considered up to this point are all inorganic acids1 and those which follow are organic4 ,sulphuric1 nitric1 and hydrochloric acids are often distinguished as the mineral acids in contradistinction to oxalic1 citric1 tartaric1 and some others which were first obtained from unripe fruits and therefore called vegetable acids4 ,organic acids have all the general properties of the class1 but they are much weaker than the mineral acids mentioned above4 ,this is shown by their solvent action on metals1 oxides1 and carbonates1 which is in all cases slight4 ,vinegar is the trade name for what is essentially a dilute solution of acetic acid which has been made by the acetous fermentation of saccharine fluids containing weak alcohol4 ,in addition to acetic acid1 vinegar contains minute quantities of a large number of compounds4 ,some of these help to produce that agreeable flavour and aroma which distinguishes vinegar from diluted acetic acid4 ,the nature and quantity of the flavouring constituents depend mainly upon the nature of the alcoholic solution used4 ,since the acetic acid in vinegar is always produced by fermentation1 all processes for the manufacture of vinegar are essentially arrangements for promoting the vigorous growth and development of ,mycoderma aceti1 the organism which produces the vinegar ferment4 ,like all other plants1 ,mycoderma aceti will flourish only under certain favourable conditions4 ,in the first place1 it requires nourishment1 and therefore certain nitrogen compounds and salts must be present in the alcoholic solution4 ,these are contained in wines1 beer1 cider1 and malt liquors1 but not in spirits of wine1 which is pure alcohol distilled from liquids which have undergone vinous fermentation4 ,if the plant is placed in dilute spirits of wine1 only a very little acetic acid is formed1 and then the action ceases because the solution does not contain the necessary food substances4 ,temperature also has a very marked effect on growth1 the most favourable range being between #fh and #ie ,f4 ,alcohol is changed to acetic acid by the process of oxidation1 and therefore1 in making vinegar1 arrangements have to be made to bring together weak alcohol and air in the presence of the plant4 ,the ferment which is secreted by the plant then causes an acceleration of the reaction4 ,there is a considerable amount of similarity between fermentation and contact action4 ,in this connection1 it is interesting to note that the conversion of alcohol into acetic acid can also be brought about by exposing a mixture of alcohol vapour and air to the action of platinum black2 in fact1 there is one process for making vinegar in this way4 ,french ,vinegar4 ,new wine1 especially that which contains a low percentage of alcohol1 is liable to many kinds of 8sickness40 ,it may turn bitter1 it may turn sour1 or it may undergo what is called lactic fermentation4 ,in either case1 it becomes unsaleable as a beverage4 ,wine which has turned sour is the best material for making vinegar1 and when this is done by the ,french or slow process1 a product with a very fine bouquet is obtained4 ,the methods adopted are very simple4 ,formerly1 the wine was poured into barrels leaving the top portion empty1 and providing for a current of air over the surface4 ,the barrels were often set up in rows in the open air in an enclosure which was then known as a 8vinegar field40 ,the process of souring which had already begun went on naturally1 and in the course of a few months1 nearly the whole of the alcohol was converted into acetic acid4 ,the process now in use in some of the ,french factories is somewhat similar4 ,large casks holding about #ajj gallons are set up in a room1 and provision is made for keeping the temperature uniform4 ,each cask is first acidulated by allowing strong vinegar to stand in it until the vinegar plant has developed on the surface4 ,the casks are then filled up very gradually by adding a few gallons of wine every eight or ten days4 ,when the cask is full1 a fraction of the contents is drawn off and replaced by wine4 ,the process then becomes continuous1 until it is necessary to clean out the generator and start again4 ,in recent times1 the manufacture of wine vinegar has been carried out on more scientific principles4 ,the vinegar plant is actually cultivated and examined microscopically before being used1 in order to make sure of the absence of moulds and bacteria1 which set up other fermentations1 producing substances which affect adversely the taste and aroma of the finished product4 ,the cultivated ferment is then added to the wine in shallow vessels and the process is carried on as described above4 ,malt ,vinegar4 ,a dilute solution of alcohol which is made from malt by fermentation with yeast contains the nutritive substances necessary for the growth of the vinegar plant1 and can therefore be used as a starting-point for the manufacture of vinegar4 ,sprouted barley or malt is mixed with oats1 barley1 rice1 or other starch-containing material4 ,the mixture is mashed with warm water and then fermented with yeast1 giving what is called 8raw spirit40 ,this is converted into vinegar by the 8quick0 process4 ,the vinegar generator 7,fig4 #aj7 is a large barrel divided into three compartments by two perforated partitions4 ,the lower disc is fixed about one-third of the way up the barrel1 and near it holes are bored to admit air4 ,the upper disc1 fixed near the top of the barrel1 is perforated with a large number of small holes which are partially stopped up with short threads or wicks1 which hang from the under side4 ,the space between the two discs is packed with specially prepared beech shavings1 which have been left to stand in strong vinegar until they are covered with the vinegar plant4 ,illustration3 ,fig4 #aj4 ,,quick ,,vinegar ,,process ,the weak spirit is delivered into the upper portion of the barrel and is distributed in very small drops by the threads2 it then passes slowly over the vinegar plant1 to which the air also has free access4 ,when it reaches the bottom1 it overflows into a reservoir and is again passed through the generator2 this is repeated until the product contains the desired amount of acetic acid4 ,the principle of the quick vinegar process is the same as that employed in making wine vinegar4 ,the speed of the reaction is1 however1 greatly increased by having the ferment spread over a very large surface and by the free circulation of air4 ,it is possible to make wine vinegar by the quick process1 but it is not done1 because the product is inferior in taste and aroma to that made by the slow process4 ,both wine vinegar and malt vinegar when freshly prepared have a stupefying and unpleasant odour4 ,before the product is ready for the market1 it has to be matured in barrels4 ,during this process1 a small quantity of alcohol which still remains in the vinegar combines slowly with some of the acetic acid1 producing acetic ester1 a substance which has a pleasant fruity odour4 ,the colour of wine vinegar is natural1 but vinegar which is produced by the quick process is colourless or only faintly coloured4 ,since the public has a preference for vinegar which is brown in colour1 the product of the quick process is coloured artificially1 either by adding caramel or by preparing the weak spirit from malt which has been slightly charred in drying4 ,industrial ,acetic ,acid4 ,the solutions of acetic acid dealt with above would be too dilute for any industrial purpose2 moreover1 the acid can be obtained much more cheaply by the distillation of wood4 ,when wood is subjected to a high temperature1 it is converted into charcoal and1 at the same time1 an inflammable gas1 an acid liquid1 and tar are given off1 and can be collected in suitable vessels4 ,the following table1 on page #gc1 gives the relative amounts of the various substances obtained from wood by dry distillation4 ,the quantities are those derived from one cord1 that is1 #abe cu4 ft4 ,charcoal ,alcohol ,calcium ,tar in ,wood oil ,turpentine in in acetate gallons4 in gallons4 bushels4 gallons4 in lbs4 gallons4 ,hard #dj-ej #h-ab #aej-bjj #h-bj woods ,resinous #be-dj #b-d #ej-ajj #cj-fj #cj-fj ,heavy woods woods #ab-be ,light woods #b-aj ,sawdust #be-ce #b-d #de-ge ,the aqueous liquid that distils over contains methyl alcohol 7wood spirit71 acetone1 and acetic acid4 ,the crude mixture is known as pyroligneous acid4 ,this is neutralized with milk of lime or soda ash1 which converts acetic acid into calcium or sodium acetate1 but has no action on the methyl alcohol and acetone which are also present4 ,the mixture is then distilled1 when methyl alcohol1 acetone1 and water pass over into the distillate1 leaving the acetate in the retort4 ,to obtain the free acid from the acetate1 the latter is well dried and then distilled with concentrated sulphuric acid4 ,acetic acid1 being the more volatile of the two acids1 distils over1 and is nearly pure4 ,the method of removing the last traces of water depends upon the fact that acetic acid solidifies at #ag ,c4 ,the acid1 which is nearly1 but not quite1 free from water1 is cooled until a portion solidifies4 ,the part which still remains liquid is poured away1 and the process is repeated until a residue is obtained which solidifies as a whole4 ,this is glacial acetic acid1 so called because it is a mass of glistening plates which look like newly-formed ice4 ,the ,acetates ,aluminium ,acetate1 made by dissolving alumina in acetic acid1 is the 8red liquor0 which is used as a mordant in dyeing4 ,it is a colourless liquid1 but is called 8red liquor0 because it is used with dyes which give a red colour4 ,ferrous ,acetate1 made in a similar way from scrap iron and acetic acid1 is the 8black liquor0 used in dyeing4 ,verdigris1 or basic copper acetate1 is a valuable pigment4 ,it is made by interposing cloths soaked in vinegar between plates of copper4 ,after the action has been allowed to go on for a long time1 the plates are washed with water and the verdigris is scraped off4 ,the finest verdigris is made in ,france in the wine-producing district around ,montpellier4 ,here1 instead of cloths soaked in vinegar1 the solid residue from the wine presses is spread in layers between the copper plates4 ,the product made in this way is called vert de ,montpellier4 ,illustration3 ,fig4 #aa4 ,,dutch ,,process ,,for ,,white ,,lead ,verdigris1 like all the copper compounds1 is extremely poisonous4 ,it is very liable to be formed on the surface of copper utensils used for cooking purposes4 ,lead ,acetate1 or sugar of lead1 is used in large quantities in the colour industry for making various reds and yellows4 ,it is prepared by dissolving the metal or its oxide 7litharge7 in acetic acid4 ,the slow action which acetic acid vapour has upon the metal lead finds a very interesting application in what is known as the ,dutch process for the manufacture of white lead#d for paint4 ,the metal is cast into grids or spirals1 which are placed on the shoulders of the specially made pots sketched in ,fig4 #aa4 ,a little dilute acetic acid is poured into each of the pots1 which are then arranged side by side on a thick layer of tan bark1 stable manure1 or other material which will heat by fermentation4 ,the first layer of pots is then boarded over2 another layer of pots is placed upon this1 and so on1 tier upon tier1 until the shed is quite full4 ,the heat developed by the fermenting material vaporizes the acetic acid1 and this vapour corrodes the lead1 forming basic lead acetate4 ,the carbon dioxide which is also produced during fermentation converts the acetate into the carbonate1 which falls as a heavy white powder into the pots4 ,future ,supply of ,acetic ,acid4 ,when all the operations involved in the production of acetic acid from wood1 from the felling of the tree to the final separation of the glacial substance1 are taken into consideration1 it will be readily understood how it is that this acid has never been cheap when compared with other acids used on an equally large scale4 ,in addition to this1 the competition for wood for paper-making and for the very numerous cellulose industries is rapidly increasing4 ,it is1 therefore1 not surprising to learn that chemists have turned their attention towards the discovery of newer and cheaper methods of making acetic acid4 ,such a process seems to have been worked out in ,germany4 ,the starting-point is acetylene gas made by the action of water on calcium carbide4 ,when this gas is passed through sulphuric acid containing suspended mercuric oxide or dissolved mercury salt1 the acetylene is oxidized first to aldehyde and then to acetic acid4 ,if this process should prove to be successful1 it will form the starting-point of a new and important industry1 for1 apart from the large amount of acetic acid which is used in commerce1 there is the production of the very important solvent known as acetone1 which can be made from acetic acid by a very simple operation4 ,tartaric ,acid4 ,grape juice contains a large quantity of potassium hydrogen tartrate dissolved in it2 when the liquid is fermented and alcohol is formed1 this salt crystallizes out because it is not soluble in alcohol4 ,after the new wine has been poured off1 the salt is found as a brownish crystalline residue adhering to the sides of the vat4 ,also the salt goes on crystallizing after the wine is put into barrels1 and forms an incrustation on the sides4 ,this is called the lees or sediment of wine4 ,in commerce1 the substance is known as argol 7sometimes spelt argal71 and also tartar of wine4 ,crude argol is purified by dissolving it in water and destroying the colour by boiling with animal charcoal4 ,when the clear liquid obtained from this mixture by filtration is evaporated1 a white crystalline substance separates out4 ,this is potassium hydrogen tartrate or cream of tartar4 ,tartaric acid is obtained from cream of tartar4 ,the salt is dissolved in water and nearly neutralized with milk of lime4 ,insoluble calcium tartrate is precipitated1 and potassium tartrate remains in solution4 ,a further quantity of calcium tartrate is obtained by adding calcium chloride to the solution just mentioned4 ,the two precipitates of calcium tartrate are then mixed and decomposed by dilute sulphuric acid1 and after the calcium sulphate is filtered off1 tartaric acid is obtained as a solid by evaporating the clear liquid4 ,the general properties of tartaric acid are well known4 ,it is soluble in water1 giving a solution which has a pleasantly acid taste4 ,citric ,acid4 ,the sharp flavour of many unripe fruits is due to the presence of citric acid2 the juice of lemons contains #e-f per cent4 of the acid4 ,the free acid is obtained in a manner precisely similar in principle to that described for tartaric acid4 ,oxalic ,acid4 ,oxalic acid and its salts1 the oxalates1 are very widely distributed in the vegetable kingdom4 ,these compounds are present in wood sorrel 7,oxalis acetosella71 in rhubarb1 in dock1 and in many other plants4 ,the acid is made on a large scale by mixing pine sawdust to a stiff paste with a solution containing caustic soda and potash4 ,the paste is spread out on iron plates and heated1 care being taken not to heat the mixture to the point at which it chars4 ,the mass is then allowed to cool1 and is mixed with a small quantity of water to dissolve out the excess of alkali4 ,this is recovered and used again4 ,sodium oxalate1 which is the main product of the reaction described above1 is dissolved in water and treated with milk of lime1 whereby insoluble calcium oxalate is obtained1 which is subsequently decomposed with sulphuric acid1 yielding oxalic acid4 ,potassium hydrogen oxalate is sometimes called salts of sorrel1 and potassium quadroxalate1 salts of lemon4 ,the most familiar use of the latter substance is in the removal of ink stains4 ,oxalic acid and its salts are poisonous4 ,the free acid has sometimes been mistaken for sugar with fatal results4 ,formic ,acid 7,l4 formica1 an ant7 is found both in the vegetable and in the animal kingdom4 ,if the leaf of a stinging nettle is examined with a microscope1 it is seen to be covered with long pointed hairs having a gland at the base4 ,this gland contains formic acid4 ,when the nettle is touched lightly1 the fine point of the hair punctures the skin1 and a subcutaneous injection of formic acid is made1 which quickly raises a blister4 ,the inconvenience which arises from the stings of bees and wasps1 also from the fluid ejected by ants when irritated1 is due to formic acid4 ,the remedy in each case is the same2 the acid must be neutralized as quickly as possible with mild alkali1 such as washing soda4 ,formic acid was first made by distilling an infusion of red ants4 ,it is now made from glycerine and oxalic acid4 ,the ,fatty ,acids4 ,animal fats and vegetable oils are similarly constituted bodies4 ,they are composed mainly of three chemical compounds known as stearin1 palmitin1 and olen4 ,of these1 stearin and palmitin are solids at ordinary temperatures1 while olen is a liquid4 ,hard fats like those of mutton and beef are composed mainly of stearin2 fats of medium hardness contain stearin1 palmitin1 and some olen2 while oils such as cod-liver oil and olive oil are nearly pure olen4 ,stearin1 palmitin1 and olen are analogous in composition to salts4 ,their proximate constituents are glycerine and certain organic acids1 stearic1 palmitic1 and olec respectively4 ,in order to obtain the fat free from tissue which it contains in its natural state1 it is tied up in a muslin bag and heated in boiling water4 ,the fat is squeezed out through the meshes of the fabric and floats on the surface of the water as an oil which solidifies on cooling4 ,this clarified fat is called tallow4 ,all fats and vegetable oils can be resolved into their two constituents1 the acid and the glycerine4 ,this can be brought about by heating the fat with water to about #bjj ,c4 ,this operation must be carried out in a vessel capable of withstanding pressure and closed with a safety valve2 otherwise1 the requisite temperature could not be obtained4 ,after this treatment1 there is left in the vessel an oily layer which solidifies on cooling and an aqueous layer which contains the glycerine4 ,the solidified oily layer is the fatty acid4 ,in the case of mutton or beef tallow1 it would be mainly a mixture of stearic and palmitic acids4 ,this mixture is used to make 8stearin0 candles4 ,the acids themselves are wax-like solids without any distinctive taste4 ,stearic acid melts at #fi ,c4 and palmitic at #fb ,c4 ,they have no perceptible action on the colour of litmus1 neither have they any solvent action on metals or carbonates4 ,we should not recognize these substances as acids at all were it not for the fact that they combine with alkalis1 forming salts4 ,the salts of the fatty acids are called soaps4 ,to make soap1 the fat is boiled with caustic alkali or caustic lye1 as it is more often called4 ,this breaks the fat up primarily into the acid and glycerine2 but in this case1 instead of obtaining the acid as the final product as we did above by heating with water under pressure1 we get the sodium or potassium salt of the acid according to the alkali used4 ,when caustic soda is used1 the product is a hard soap2 when caustic potash is used1 it is a soft soap4 ,the treatment of fats in this way with caustic alkalis is called 8saponification40 ,,chapter ,,viii ,,mild ,,alkali ,caustic and ,mild4 ,there are two classes of alkalis distinguished by the terms caustic and mild4 ,if a piece of all-wool material is boiled with a solution of caustic soda or potash1 it dissolves completely1 giving a yellow solution4 ,mild alkali will not dissolve flannel1 though it may have some slight chemical action causing shrinkage4 ,partly for this reason1 and partly because commercial washing soda often contains a little caustic soda1 woollen garments must not be boiled or even washed in hot soda water4 ,the disintegrating action of the caustic alkalis is also illustrated by the use of caustic soda in the preparation of wood pulp for paper making4 ,tree trunks are first torn up and shredded by machinery2 but notwithstanding the power of modern machinery1 the fibre is not nearly fine enough for the purpose until it has been 8beaten0 with a solution of caustic soda1 whereby the pulp is brought to a smooth and uniform consistency like that of thin cream4 ,mild ,soda and ,potash4 ,until the middle of the eighteenth century1 it was thought that the soluble matter extracted from the ashes of all plants was the same4 ,in #ageb it was shown that the substance obtained in this way from plants which grew in or near the sea differed from that from land vegetation by producing a golden yellow colour when introduced into the non-luminous flame of a spirit lamp1 while that from land plants gave to the flame a pale lilac tinge4 ,the former substance is now known as mild soda1 and the latter as mild potash4 ,at this point it is well to make it clear to the reader that there are two bodies commonly called soda1 and two called potash4 ,one of each pair is caustic and one mild4 ,by a simple chemical test it is easy to distinguish a mild from a caustic alkali4 ,when a little dilute acid is added to the former1 there is a vigorous effervescence caused by the escape of carbon dioxide1 but no gas is given off when a caustic alkali is treated in the same way4 ,the liberation of carbon dioxide on the addition of acids shows that the mild alkalis are carbonates4 ,washing ,soda is so well known1 that very little description of its external characteristics is necessary4 ,it is a crystalline substance1 easily soluble in water4 ,the crystals1 when freshly prepared1 are semi-transparent2 but after exposure to air for some time1 they are found to lose their transparency and to become coated with an opaque white solid which crumbles easily4 ,this change in appearance is accompanied by a loss in weight4 ,crystals of soda melt very easily on the application of heat and1 on continued heating1 the liquid seems to boil4 ,when this operation is carried out in a vessel attached to a condenser1 the vapour that is given off from the melted soda condenses to a clear colourless liquid which1 on examination1 proves to be water4 ,when no more water collects in the receiver1 the vessel contains a dry1 white solid1 which by any chemical test that may be applied is shown to be the same as washing soda1 but it contains no water of crystallization and has a different crystalline form4 ,this substance is anhydrous sodium carbonate1 or soda ash as it is called in commerce4 ,when soda ash is mixed with water1 it combines with about twice its own weight of that liquid1 forming soda crystals again4 ,washing soda1 then1 contains nearly two-thirds of its weight of water4 ,some of this water is given off spontaneously when the soda is exposed to air2 the water may even be said to evaporate4 ,this accounts for the loss of weight observed and also for the formation of the white layer of partially dehydrated soda over the surface of the crystal4 ,the property of losing water in this way is common to most crystals containing a high percentage of water of crystallization4 ,the phenomenon is known as 8efflorescence40 ,it may here be observed that crystals of washing soda which have become coated over in this way contain relatively more soda than those which are transparent4 ,natural ,soda4 ,in ,egypt1 ,thibet1 and ,utah1 there are tracts of country where the soil is so impregnated with soda that the land is desert4 ,the separation of the soda from the earth is a simple operation1 for it is only necessary to agitate the soil with water and1 after the insoluble matter has settled down1 to evaporate the clear solution until the soda crystallizes out4 ,in addition to alkali deserts1 there are also alkali lakes4 ,those in ,egypt are small1 nevertheless1 about #cj1jjj tons of soda per annum are exported from ,alexandria4 ,owens ,lake in ,california is said to contain sufficient soda to supply the needs of ,north ,america2 while in the ,east ,african ,protectorate1 beneath the shallow waters of ,lake ,magadi 7discovered in #aiaj71 there is a deposit of soda estimated at #bjj1jjj1jjj tons4 ,the ,leblanc ,process4 ,at the present time1 the greater part of the world's supply of soda is made from common salt by two processes4 ,the older of these1 which is known as the ,leblanc process1 was introduced in ,france towards the end of the eighteenth century4 ,in those days soda was very dear1 for the main supply came from the ashes of seaweeds2 wherefore the ,french ,academy of ,sciences1 in #agge1 offered a prize for the most suitable method of converting salt into soda on a manufacturing scale4 ,the prize was won by ,nicholas ,leblanc1 who in #agia started the first soda factory near ,paris4 ,these were the days of the ,french ,revolution2 the 8,comit de ,sret ,gnral0 abolished monopolies and ordered citizen ,leblanc to publish the details of his process4 ,illustration3 ,fig4 #ab4 ,,salt ,,cake ,,furnace ,the first alkali works were established in ,great ,britain in #ahad4 ,the total amount of soda now made in this country every year is about #a1jjj1jjj tons1 of which nearly one-half is still made by the ,leblanc process4 ,salt ,cake4 ,the first stage of the ,leblanc process consists in mixing a charge of salt weighing some hundredweights with the requisite amount of 8chamber0 sulphuric acid4 ,the operation is carried out in a circular cast-iron pan 7,d1 ,fig4 #ab7 about #i ft4 in diameter and #b ft4 deep4 ,the pan is covered over with a dome of brickwork1 leaving a central flue 7,e7 for the escape of hydrochloric acid gas which is produced4 ,at first1 the reaction takes place without the application of heat1 but towards the end the mass is heated for about one hour4 ,the contents of the pan are then raked out on to the hearth of a reverberatory furnace 7a1 b7 and more strongly heated4 ,more hydrochloric acid gas is given off1 and the reaction is completed4 ,the solid product which remains is impure ,glauber's salt 7sodium sulphate71 and is known in the trade as 8salt cake40 ,black ,ash4 ,in the second stage of the ,leblanc process1 salt cake is converted into black ash4 ,the salt cake is crushed and mixed with an equal weight of powdered limestone or chalk and half its weight of coal dust4 ,this mixture is introduced into a reverberatory furnace 7,fig4 #ac7 by the hopper ,k1 and heated to about #ajjj ,c4 by flames and hot gases from a fire at a4 ,during this operation1 the mass is kept well mixed1 and after some time it is transferred to h where the temperature is higher4 ,the mixture then becomes semi-fluid and carbon monoxide gas is given off4 ,illustration3 ,fig4 #ac4 ,,black ,,ash ,,furnace ,the formation of carbon monoxide within the semi-solid mass renders it porous4 ,this is an advantage1 because it greatly facilitates the subsequent operation of dissolving out the soluble sodium carbonate4 ,the appearance of the flames of carbon monoxide at the surface of the black ash indicates the end of the process4 ,the product is then worked up into balls and removed from the furnace4 ,the chemical changes which take place in making black ash are probably as follows3 ,carbon 7coal dust7 removes oxygen from sodium sulphate1 which is thus changed to sodium sulphide4 ,this substance then reacts with the limestone 7calcium carbonate71 forming sodium carbonate 7soda7 and calcium sulphide4 ,extraction of ,soda4 ,it now only remains to dissolve out the soda from the insoluble impurities with which it is mixed in the black ash4 ,it is evident that the smaller the amount of water used for this purpose the better1 because the water has subsequently to be got rid of by evaporation4 ,the process of extraction is1 therefore1 carried out systematically4 ,the black ash is treated with water in a series of tanks which are fitted with perforated false bottoms4 ,the soda solution1 which is heavier than water1 tends to sink to the bottom and1 after passing through the perforations1 is carried away by a pipe to the second tank1 and so on throughout the series4 ,the fresh water is brought first into contact with the black ash from which nearly all the soda has been extracted4 ,the method of finishing off the black ash liquor differs according to the final product which the manufacturer desires to obtain1 for the liquor contains caustic soda as well as mild soda4 ,for the present1 we will suppose that the end product is to be washing soda4 ,in this case1 carbon dioxide is passed into the liquor to convert what caustic soda there is into mild soda4 ,the clarified soda liquor is then evaporated until crystals of soda separate out4 ,the first part of this process is carried out in large shallow pans 7,p4 ,fig4 #ac71 using the waste heat of the black ash furnace1 and finally in vats containing steam-heated coils4 ,as the crystals separate out1 they are removed1 drained1 and dried4 ,alkali ,waste4 ,black ash contains less than half its weight of soda1 so that for every ton of soda produced there is from a ton and a half to two tons of an insoluble residue which collects in the lixiviating and settling tanks4 ,this residue is known as alkali waste4 ,alkali waste is of no particular value4 ,it is not even suitable as a dressing for the land1 and since it is not soluble in water there is no convenient means of disposing of it4 ,consequently1 it is just accumulated at the works and1 as the heap grows at an alarming rate1 it cumbers much valuable ground4 ,moreover1 it contains sulphides from which1 under the influence of air and moisture1 sulphuretted hydrogen is liberated4 ,alkali waste1 therefore1 has a very unpleasant odour4 ,the whole of the sulphur which was contained in the sulphuric acid used in the first stage of the process remains in the alkali waste1 mainly as calcium sulphide4 ,a plant for the recovery of this sulphur is established in some of the larger works4 ,the alkali waste is mixed with water to the consistency of a thin cream1 in tall1 vertical cylinders4 ,carbon dioxide under pressure is forced into the mixture1 and this converts the calcium sulphide into calcium carbonate and sets free hydrogen sulphide1 which1 when burnt with a limited supply of air1 yields sulphur4 ,by this process1 the most unpleasant feature of alkali waste1 namely1 the smell1 is removed4 ,the calcium carbonate which remains is of very little value4 ,some of it is used in making up fresh charges for the black ash process and some for preparing ,portland cement1 for which finely-ground calcium carbonate is required2 the remainder is thrown on a heap4 ,bicarbonate of ,soda4 ,bicarbonate of soda can be easily distinguished from washing soda4 ,it is a fine1 white powder similar in appearance to the efflorescence on soda crystals4 ,it does not contain any water of crystallization4 ,when bicarbonate of soda is heated1 it does not melt1 and1 as far as its external appearance is concerned1 it does not seem to undergo any change4 ,if1 however1 suitable arrangements are made1 water and carbon dioxide gas can be collected1 and the sodium bicarbonate will be found to have lost #cfi per cent4 of its weight4 ,the substance which remains is identical with that obtained by heating soda crystals1 that is1 anhydrous sodium carbonate4 ,sodium bicarbonate is1 therefore1 a compound of sodium carbonate and carbonic acid4 ,the most familiar use of this compound is indicated by its common names 8baking-soda0 and 8bread-soda40 ,it is mixed with dough or other similar material in order to keep this from settling down to a hard solid mass in baking4 ,the way in which bicarbonate of soda prevents this will be readily understood when it is remembered that an ounce of this substance liberates more than #b1cjj cu4 in4 of carbon dioxide when it is heated4 ,when the bicarbonate of soda is well mixed with the ingredients of the cake or loaf and disseminated throughout the mass1 each particle will furnish 7let us say7 its bubble of gas4 ,since these cannot escape1 a honey-combed structure is produced4 ,illustration3 ,fig4 #ad4 ,,the ,,solvay ,,process ,baking powder is a mixture of bicarbonate of soda and ground rice2 the latter substance is merely a solid diluent4 ,the ,solvay ,process4 ,soda ash is one of the principal forms of mild alkali used in commerce4 ,large quantities of this substance are made by heating bicarbonate of soda4 ,we shall now consider another alkali process in which this substance is the primary product4 ,for the greater part of the first century of its existence1 the ,leblanc soda process had no rival1 although another method1 known as the ammonia-soda process1 was patented as early as #ahch4 ,in this case1 however1 as in many others1 expectations based on the experiments carried out in the laboratory were not realized when the method came to be tried under manufacturing conditions4 ,it was not until #ahgb that ,ernest ,solvay1 a ,belgian chemist1 had so far solved the difficulties1 that a new start could be made4 ,in that year1 about #c1jjj tons of soda were produced by the ammonia-soda or ,solvay process1 as it has now come to be known4 ,since then1 however1 the quantity produced annually has been steadily increasing1 until at the present time it amounts to more than half of the world's supply4 ,the ,solvay process is very simple in theory4 ,purified brine is saturated first with ammonia gas and then with carbon dioxide4 ,water1 ammonia1 and carbon dioxide combine1 forming ammonium bicarbonate1 which reacts with salt 7sodium chloride71 producing sodium bicarbonate and ammonium chloride4 ,the principal reaction is carried out in a tower 7,fig4 #ad 7#a71 a1 a7 from #ej to #fe ft4 in height and about #f ft4 in diameter4 ,at intervals of about #c ft4 throughout its length1 the tower is divided into sections by pairs of transverse discs1 one flat with a large central hole1 and one hemispherical and perforated with small holes 7,fig4 #ad 7#b774 ,the discs are kept in position by a guide rod ,g4 ,fig4 #ad 7#c7 shows a better arrangement of the guide rods4 ,in modern works1 the space between the discs is kept cool by pipes conveying running water4 ,the ammoniated brine is led into the tower near its middle point4 ,the carbon dioxide is forced in at ,e in the lowest segment1 and as it passes up the tower it is broken up into small bubbles by the sieve plates4 ,sodium bicarbonate separates out as a fine powder1 which makes its way to the bottom of the tower suspended in the liquid4 ,the perforated plates are necessary for the proper distribution of carbon dioxide through the brine4 ,they are1 however1 a source of trouble1 because the holes quickly become blocked up with sodium bicarbonate1 and every ten days or so it is necessary to empty the tower and clean it out with steam or boiling water4 ,recovery of ,ammonia4 ,the production of #a ton of soda ash by the ,solvay process involves the use of a quantity of ammonia which costs about eight times as much as the price realized by selling the soda4 ,it is evident that the success of the process as a commercial venture depends largely on the completeness with which the ammonia can be recovered4 ,during the process1 ammonia is converted into ammonium chloride1 which remains dissolved in the residual liquor4 ,from this ammonia gas is set free by adding quicklime and by blowing steam through the mixture4 ,it is now claimed that #ii per cent4 of the ammonia used in one operation is recovered4 ,soda ,ash4 ,the bicarbonate of soda produced by the ,solvay process is moderately pure4 ,for all ordinary purposes1 it is only necessary to wash it with cold water to remove unchanged salt1 and after drying1 it is ready to be placed on the market if it is to be sold as bicarbonate4 ,the greater part of the ,solvay product1 however1 is converted into soda ash by the application of heat4 ,if soda crystals are required1 the soda ash is dissolved in water and crystallized4 ,in many ways1 the ,solvay process compares very favourably with the older method4 ,it is an advantage to start with brine1 for that is the form in which salt is very often raised from the mines4 ,the end product is relatively pure2 moreover1 it is quite free from caustic soda1 which for some purposes for which soda ash is used is a great recommendation4 ,there is no unpleasant smelling alkali waste4 ,on the other hand1 the efficiency of the ,solvay process is not high1 for only about one-third of the salt used is converted into soda4 ,this would make the process impossible from the commercial point of view were it not for the cheapness of salt4 ,the ,leblanc process1 too1 has its advantages4 ,in the next chapter we shall see that it is adaptable for the production of caustic as well as mild alkali4 ,the chlorine which is recovered in the ,leblanc process is a very valuable by-product4 ,in the ,solvay process1 chlorine is lost1 for hitherto no practicable method has been found for its recovery from calcium chloride4 ,the position with regard to the future supply of alkali is very interesting4 ,the competition between the ,leblanc and the ,solvay processes for supremacy in the market is very keen4 ,at the same time1 both processes are in some degree of danger of being supplanted by the newer electrical methods1 which will be mentioned in the last chapter4 ,the following table shows very clearly the rapid progress made by the ,solvay process in ten years4 ,the quantities are given in tonnes 7#a tonne #jihdb ton74 #ahhd4 #ahid4 ,leblanc ,solvay ,leblanc ,solvay soda4 soda4 soda4 soda4 ,great ,britain #chj1jjj #eb1jjj #cdj1jjj #aha1jjj ,germany #ef1ejj #dd1jjj #dj1jjj #baj1jjj ,france #gj1jjj #eg1jjj #bj1jjj #aej1jjj ,united ,states -- #a1ajj #bj1jjj #hj1jjj ,austria-,hungary #ci1jjj #a1jjj #bj1jjj #ge1jjj ,russia -- -- #aj1jjj #ej1jjj ,belgium -- #h1jjj #f1jjj #cj1jjj #ede1ejj #afc1ajj #def1jjj #ggf1jjj ,mild ,potash4 ,potassium carbonate 7mild potash7 was formerly obtained from wood ashes4 ,the clear aqueous extract was evaporated to dryness in iron pots1 and the substance was on this account called potashes2 later1 potash4 ,a whiter product was obtained by calcining the residue1 and this was distinguished as pearl-ash4 ,chemically pure potassium carbonate was formerly obtained by igniting cream of tartar 7potassium hydrogen tartrate7 with an equal weight of nitre4 ,it is for this reason that potassium carbonate is sometimes called 8salt of tartar40 ,about the middle of last century1 natural deposits of potassium chloride were discovered in ,germany4 ,the beds of rock salt near ,stassfurt are covered over with a layer of other salts1 and for many years these were removed and cast aside as 8waste salts0 7abraumsalze74 ,when at a later date they were examined more carefully1 they were found to contain valuable potassium compounds1 notably the chloride4 ,after that discovery1 mild potash was made by the ,leblanc process41 and ,germany controlled the world's markets for all potassium compounds4 ,at the outbreak of war1 the ,german supplies of potassium compounds ceased as far as the allied nations were concerned1 and an older method of making potassium chloride from orthoclase or potash-felspar was revived4 ,this involves the heating of the powdered mineral to a high temperature after mixing it with calcium chloride1 lime1 and a little fluorspar4 ,the potassium chloride is then extracted from the fused mass with water4 ,this method has been worked with great success in ,america1 and it is claimed that potassium chloride can be made in that country at a cost which is lower than that formerly paid for the imported article4 ,mild potash and soda are so very similar in chemical properties that in most cases it is immaterial which compound is used4 ,in all cases in which there is this choice1 soda is employed1 both because it is cheaper and because it is more economical1 for #ajf parts of soda ash are equivalent to #ach parts of potash4 ,there are1 however1 some occasions when soda cannot be substituted1 notably for the manufacture of hard glass and soft soap1 and for the preparation of caustic potash1 potassium dichromate1 and other potassium salts4 ,potassium ,bicarbonate4 ,this resembles the corresponding sodium salt in nearly every respect4 ,it is1 however1 much more readily soluble in water1 so much so1 that it is not possible to obtain this substance by the ,solvay method4 ,it is made from potassium carbonate by saturating a strong aqueous solution of that substance with carbon dioxide4 ,,chapter ,,ix ,,caustic ,,alkalis ,the ,alkali ,metals4 ,the discovery of current electricity in #agij furnished the chemist with a very powerful agency for bringing about the decomposition of compounds4 ,hydrogen and oxygen were soon obtained by passing an electric current through acidulated water2 and in #ahjg1 ,sir ,humphry ,davy1 who is perhaps better remembered for his invention of the miners' lamp1 isolated the metals sodium and potassium by subjecting caustic soda and caustic potash respectively to the action of the current4 ,sodium and potassium are very remarkable metals4 ,they are only a little harder than putty1 and can easily be cut with a knife or moulded between the fingers4 ,when exposed to the air1 they rust or oxidize very rapidly1 so much so that they have to be preserved in some mineral oil or in airtight tins4 ,they are lighter than water1 which they decompose with the liberation of hydrogen1 and under favourable circumstances the hydrogen takes fire so that the metals appear to burn on the surface of the water4 ,after the reaction is over and the sodium or potassium has disappeared1 a clear colourless liquid remains which has a strongly alkaline reaction1 and when this is evaporated until the residue solidifies on cooling1 caustic soda or potash is obtained4 ,for very special purposes1 the caustic alkalis are sometimes made by the action of the metals on water1 but for production on a large scale1 less expensive methods are adopted4 ,caustic ,alkali is obtained from the corresponding mild alkali in the following way4 ,the substance--washing soda1 for example--is dissolved in water and the solution is warmed4 ,lime is stirred into this solution1 and from time to time a small test portion of the clear supernatant liquid is removed and mixed with a dilute mineral acid4 ,when this ceases to cause effervescence1 the change is complete4 ,the clear liquid is now separated from the solid matter 7excess of lime together with calcium carbonate7 and evaporated in a metal dish4 ,since the caustic alkalis are extremely soluble in water1 they do not crystallize as do most of the compounds previously described4 ,evaporation is1 therefore1 carried on until the liquid which remains solidifies when cold4 ,caustic ,soda4 ,to describe the process by which caustic soda is manufactured1 we must return to the making of black ash4 ,the mixture from which black ash is made contains limestone4 ,it is heated to #ajjj ,c41 which is a sufficiently high temperature to convert limestone into lime4 ,when the black ash is subsequently treated with water1 the lime which is present converts some of the mild alkali to caustic2 consequently1 black ash liquor always contains both alkalis4 ,when the manufacturer intends to make caustic soda and not soda crystals1 the composition of the black ash mixture is varied by adding a larger proportion of limestone1 so that there may be an excess of lime in the black ash produced4 ,the treatment with water is carried out as described under washing soda1 and then more lime is added to convert the mild soda into caustic soda4 ,after the excess of lime and other suspended matter has settled down1 the clear caustic liquor is evaporated in iron kettles until it becomes molten caustic1 which will solidify on being allowed to cool4 ,there are various grades of caustic soda on the market differing one from another in purity4 ,the soap manufacturer uses caustic liquor or lye containing about #dj per cent4 of caustic soda4 ,for other purposes1 the solid containing from #fj to #gh per cent4 is used4 ,sometimes the product is whitened by blowing air through the strong caustic liquor or by the addition of a little potassium nitrate4 ,finally1 for analytical purposes1 caustic soda is purified by dissolving it in alcohol and subsequently evaporating the clear liquid4 ,caustic ,potash4 ,the methods for the preparation of the corresponding potassium compound are precisely the same as those described for caustic soda2 in fact1 wherever the words sodium and soda occur in this chapter1 the reader can always substitute potassium and potash respectively4 ,caustic ,lime4 ,apart from its use in making mortar and cement1 lime is very often employed to neutralize acids4 ,for this purpose1 a suspension in water1 called milk of lime1 is generally used1 for lime itself is not very soluble4 ,probably it is only the soluble part which reacts2 nevertheless1 as soon as this is used up1 more of the solid dissolves1 and in this way the action goes on as if all the lime were in solution4 ,lime is also a very valuable substance in agriculture1 especially on damp1 boggy land1 where there is much decaying vegetable matter1 and on land which has been liberally manured4 ,the soil in these cases is very likely to become acid and is then unproductive4 ,lime is added to 8sweeten0 the soil2 in other words1 to neutralize the acid4 ,ammonia4 ,the pungent smelling liquid popularly known as 8spirits of hartshorn0 is a solution of ammonia gas in water4 ,it is a caustic alkali and1 as such1 is sometimes used to remove grease spots4 ,here1 however1 we shall consider ammonia only in connection with ammonium salts1 some of which are used in very large quantity as fertilizers4 ,the principal source of ammonia at the present time is the ammoniacal liquor obtained as a by-product in the manufacture of gas for heating and lighting4 ,coal contains about #a per cent4 of nitrogen1 and when it is distilled1 some of this nitrogen is given off as ammonia1 which dissolves in the water produced at the same time4 ,this liquid is condensed in the hydraulic main and in other parts of the plant where the gas is cooled down4 ,gas liquor contains chiefly the carbonate1 sulphide1 sulpho-cyanide1 and chloride of ammonia1 together with many other substances1 some of which are of a tarry nature4 ,it would not be practicable to evaporate this liquid with a view to obtaining the ammonium salts1 because it is only a very dilute solution4 ,hence1 after the removal of tar1 the liquor is treated in such a way that ammonia is set free4 ,in some cases the liberation of ammonia is accomplished by blowing superheated steam into the liquor1 which sets free the ammonia which is combined as carbonate1 sulphide1 and sulpho-cyanide1 but not that which is present as chloride4 ,in other works1 the gas liquor is mixed with milk of lime1 which liberates all the combined ammonia4 ,the ammonia is then expelled from the mixture by a current of steam or air and steam4 ,in both cases1 the gas which is given off is passed into sulphuric acid1 whereby ammonium sulphate is formed in solution and afterwards obtained as a solid by evaporation4 ,ammonium ,salts ,ammonium ,chloride4 ,like all other alkalis1 ammonia solution neutralizes acids1 forming salts4 ,with hydrochloric acid1 it produces the white solid known as sal ammoniac or ammonium chloride4 ,this compound is familiar as the one required to make the liquid used in a ,leclanch cell1 which is generally used as the current generator for electric bells4 ,ammonium ,carbonate1 which is also called stone ammonia and salt of hartshorn1 is made by subliming a mixture containing two parts chalk and one part ammonium sulphate4 ,it is a white solid which gives off ammonia slowly and is1 therefore1 used as the basis for smelling salts4 ,ammonium ,nitrate is obtained by passing ammonia gas into nitric acid until it is neutralized4 ,it is a white solid1 which melts easily on being heated1 and breaks up into water and nitrous oxide 7laughing gas71 which is the 8gas0 administered by dentists4 ,ammonium nitrate is also used in the composition of some explosives3 for example1 8ammonite0 is said to contain #hj per cent4 of this substance4 ,ammonium ,sulphate is used chiefly as an artificial manure2 the amount required for this purpose throughout the world is over #a1ejj1jjj tons every year4 ,synthetic ,ammonia4 ,though the soluble compounds of nitrogen are fairly abundant1 the supply is by no means equal to the demand1 because such enormous quantities are required for agricultural purposes4 ,it has been already said that ammonia is obtained as a by-product in the distillation of coal1 and it has been repeatedly pointed out that our coal supplies are far from inexhaustible2 moreover1 coal gas may not always be used for lighting and heating4 ,it1 therefore1 becomes a very important question as to how the future supply of ammonium salts is to be maintained4 ,ammonia is a very simple compound formed from the elements nitrogen and hydrogen1 and1 as before mentioned1 the supply of free nitrogen in the air is literally inexhaustible4 ,in recent years1 the efforts of chemists have been directed towards finding a method of converting the free nitrogen of the air into some simple soluble compound4 ,this problem is usually spoken of as the 8fixation of nitrogen40 ,in the ,haber process1 nitrogen obtained by the fractional distillation of liquid air is mixed with three times its volume of hydrogen1 and this mixture is heated to between #ejj;,c4 and #gjj;,c4 under a pressure of #aej atmospheres 7nearly #a ton to the square inch7 and in the presence of a contact agent4 ,under these conditions1 nitrogen and hydrogen combine to form ammonia1 which is condensed by passing the mixed gases into a vessel cooled with liquid air1 any unchanged nitrogen and hydrogen being passed back again over the contact substance4 ,the problem of making ammonia from the air is closely connected with that of making nitric acid from the same source4 ,in some experiments the two are combined1 and ammonium nitrate is produced directly4 ,ammonia made by the ,haber process1 or some modification1 is mixed with atmospheric oxygen and passed through platinum gauze heated to low redness4 ,this results in the formation of nitric oxide1 which is further oxidized by atmospheric oxygen2 and finally1 from a mixture of oxides of nitrogen1 water vapour1 and ammonia1 synthetic ammonium nitrate is obtained4 ,,chapter ,x ,,electrolytic ,,methods ,one of the most noteworthy developments of modern chemical industry has been the increasing use of electricity as an agent for bringing about changes in matter4 ,this has followed naturally from the reduction in the cost of electricity1 due in great measure to the utilization of natural sources of energy which for untold ages had been allowed to run to waste4 ,this last achievement is likely to produce such a change in economic conditions that it is worth while giving a little thought to what may be called a newly-discovered asset of civilization4 ,one example will make this clear4 ,in the bed of the ,niagara river1 which flows from ,lake ,erie to ,lake ,ontario1 there is a sudden drop of #afg ft4 over which the water rushes with tremendous force and expends its energy in producing heat which cannot be utilized4 ,this is a waste of energy1 but it cannot be circumvented because no method has yet been found to control the waters of the ,falls themselves4 ,nevertheless1 by leading the head waters through suitable channels from the high level to the low1 it is possible to use the energy to drive turbines1 which1 in their turn1 drive dynamos which produce the current4 ,this is merely the conversion of the energy of running water into electrical energy2 and while the sun remains1 this supply of energy will be forthcoming in undiminished quantity1 because by the heat of the sun the water is lifted again as vapour1 which descends as rain to replenish the sources from which the ,niagara flows4 ,electricity is employed in chemical industry in two ways4 ,in the first place1 it may be used to produce very high temperatures required for the reduction of some metallic ores1 for melting highly-refractory substances1 and for making steel4 ,it is1 however1 rather with the second method1 called electrolysis1 that we are here mainly concerned4 ,illustration3 ,fig4 #ae4 ,,the ,,electrolysis ,,of ,,salt ,,solution ,solutions of acids1 bases1 and salts1 and in some cases the fused substances themselves1 conduct the electric current2 but at the same time they suffer decomposition4 ,this method of decomposing a substance is known as electrolysis1 or a breaking up by the agency of electricity4 ,the apparatus required in a very simple case is shown in ,fig4 #ae4 ,it merely consists of some suitable vessel to contain the liquid2 two plates--one to lead the current into the solution1 the other to lead it away again--and wires to connect the plates to the poles of a battery1 storage-cell1 or dynamo4 ,each plate is called an electrode1 and distinguished as positive or negative according as it is joined to the positive or negative pole of the current generator4 ,by convention1 electricity is supposed to 8flow0 from the positive pole of the battery to the positive electrode or anode1 and then through the solution to the negative electrode or cathode1 and so back to the negative pole of the generator1 thus completing the circuit external to the battery4 ,when acids1 alkalis1 and salts are dissolved in water1 there is strong evidence to show that they break up to a greater or less extent into at least two parts called ions4 ,these are atoms1 or groups of atoms1 which have either acquired or lost one or more electrons4#e ,they move about quite independently of one another and in any direction until the electrodes are placed in the liquid4 ,then they are constrained to move in two opposing streams--those which have acquired electrons all move towards the negative electrode1 and those which have lost electrons towards the other4 ,at the electrodes themselves1 the former give up and the latter take up electrons1 and become atoms again4 ,let us now consider a concrete example4 ,common salt is composed of atoms of sodium and atoms of chlorine paired4 ,when a small quantity of this substance is dissolved in a large quantity of water1 the pairing no longer obtains4 ,the chlorine atoms move away independently accompanied by an extra satellite or electron1 and the sodium atoms move away also but with their electron strength one below par4 ,when the current is introduced into the liquid1 the sodium ions travel towards the cathode and chlorine ions towards the anode1 and when they reach the goal1 sodium ions gain one electron and chlorine ions lose one1 and both become atoms again4 ,chlorine atoms combine in pairs forming molecules and escape from the solution in the greenish yellow cloud that we call chlorine gas4 ,the sodium atoms react immediately with water1 forming caustic soda with the liberation of hydrogen4 ,to return now to practical considerations4 ,the electrolysis of salt solution appears to be an ideally simple method of obtaining caustic soda and chlorine from sodium chloride4 ,as a manufacturing process1 it would seem to be perfect1 for the salt is broken up directly into its elements and a secondary reaction gives caustic soda automatically4 ,there is no 8waste0 as in the ,leblanc process1 and it does not require the use of any expensive intermediary substance afterwards to be recovered1 as in the ,solvay process4 ,but1 as very often happens when working on a large scale1 difficulties arise1 and these up to the present have only been partially overcome4 ,some of the chlorine remains dissolved in the liquid and reacts with the caustic soda1 forming other substances which1 though valuable1 are not easy to separate from the caustic soda4 ,it is possible to get over this difficulty to some extent by placing a porous partition between the anode and the cathode1 and in that way dividing the cell into cathodic and anodic compartments4 ,as long as the partition is porous to liquids1 it will allow the current to pass1 but at the same time it will greatly retard the mixing of the contents of the two compartments4 ,porous partitions or cells which are in common use for batteries are made of 8biscuit0 or unglazed porcelain4 ,it must be remembered1 however1 that porous partitions only retard the mixing of liquids2 they do not prevent it4 ,moreover1 a further difficulty arises from the fact that chlorine is a most active substance1 and therefore it is difficult to find a material which will resist its corrosive action for any length of time1 and the same difficulty arises in the case of the anode where the chlorine is given off4 ,castner ,process for ,caustic ,soda4 ,the following is the most successful electrical process for the manufacture of caustic soda yet devised4 ,it was introduced in #ahib1 and is known as the ,castner process4 ,it should be noted that the use of the porous partition has been avoided in a very ingenious way4 ,illustration3 ,fig4 #af4 ,,the ,,castner ,,process ,the cell 7see ,fig4 #af7 is a closed1 rectangular-shaped tank divided into three compartments by two non-porous partitions fixed at one end to the top of the tank1 while the other end is free and fits loosely into a channel running across the tank4 ,the floor of the tank is covered with a layer of mercury of sufficient depth to seal the separate compartments4 ,the two end compartments contain the brine in which are the carbon anodes2 the middle compartment contains water or very dilute caustic soda in which the cast-iron cathode is immersed4 ,the current enters the end compartments by the carbon anodes and passes through the salt solution to the mercury layer which in these compartments are the cathodes4 ,the current then passes through the mercury to the middle compartment1 and then through the solution to the cathode1 thence back to the dynamo4 ,it is important to note that in the middle compartment the mercury becomes the anode4 ,chlorine is liberated at the carbon electrodes1 and when no more can dissolve in the liquid it escapes and is conveyed away by the pipe ,p4 ,sodium atoms are formed at the surface of the mercury cathodes in the outside compartments and dissolve instantly in the mercury1 forming sodium amalgam4 ,while the current is passing1 a slight rocking motion is given to the tank by the cam ,e4 ,this is sufficient to cause the mercury containing the dissolved sodium to flow alternately into the middle compartment1 and there the sodium amalgam comes into contact with water2 the sodium is dissolved out of the mercury and caustic soda is formed4 ,water in a regulated stream is constantly admitted to the middle compartment1 and a solution of caustic soda of about #bj per cent4 strength overflows4 ,the production of caustic soda by an electrical method still remains to be fully developed4 ,a process which gives only a #bj per cent4 solution cannot be looked upon as final4 ,in the meantime1 other methods have been tried1 in some of which fused salt is used in place of brine in order to give caustic soda in a more concentrated form4 ,for a description of these methods1 the reader must consult some of the larger works mentioned in the preface4 ,here we can only say that very great difficulties have been encountered1 particularly in the construction of a satisfactory porous diaphragm or1 alternately1 in devising methods in which this can be dispensed with4 ,another interesting application of electrolysis is furnished by the use of copper sulphate in industry4 ,when this salt is dissolved in water1 it breaks up into copper ions 7positive7 and an equal number of negative ions1 composed of #a atom of sulphur and #d atoms of oxygen 7,,so#d74 ,under the influence of the current copper ions travel to the cathode1 and there by the gain of two electrons become copper atoms4 ,now1 since copper is not soluble in copper sulphate solution1 and is not volatile except at very high temperatures1 it is deposited on the cathode in a perfectly even and continuous film when the strength of the current is suitably adjusted4 ,this film continues to grow in thickness as long as the conditions for its deposition are maintained4 ,if the current employed is not suitable1 the metallic film is not coherent1 and the copper may appear as a red powder at the bottom of the cell4 ,any other metal or impurity which might be present in the unrefined copper falls to the bottom of the tank4 ,other metals are deposited electrolytically in exactly the same way4 ,the metal to be deposited is joined to the positive pole and the article to be plated to the negative pole of the battery4 ,both are suspended in a solution of salt1 generally the sulphate1 of the metal which is to be deposited4 ,thus1 for nickel plating1 a piece of sheet nickel would be used in conjunction with a solution of sulphate of nickel or1 better1 a solution of nickel ammonium sulphate1 made by crystallizing ammonium and nickel sulphates together4 ,the current required is small2 indeed1 if it is too strong1 the deposit adheres loosely to the article1 and the result is1 therefore1 not satisfactory4 ,electrotype blocks are also made by a similar process4 ,an impression of the article to be reproduced is made in wax1 or some suitable plastic material1 and polished with very fine graphite or black lead1 in order to give a conducting surface4 ,it is then suspended in a solution of copper sulphate and joined to the negative pole of the battery2 a plate of copper connected with the positive pole is suspended in the same solution4 ,when a weak current is passed1 copper is deposited on the black-leaded surface and grows gradually in thickness1 until at length it can be stripped off1 giving a positive replica of the object4 ,,index ,a ,acetic acid 7glacial71 #gc ,acids1 early notions of1 #a ----1 fatty1 #gh ----1 mineral1 #fh ----1 vegetable1 #fh ,agate1 #fa ,air-saltpetre1 #db ,alkali ,acts1 #dd ----1 caustic1 #if ----1 metals1 #ie ----1 mild1 #hj ---- waste1 #hg ,alkalis1 properties1 #c ,aluminium acetate1 #gc ,alums1 the1 #bf ,amethyst1 #fa ,ammonal1 #cf ,ammonia1 #ig ----1 synthetic1 #ii ,ammonite1 #ii ,ammonium carbonate1 #ii ---- chloride1 #ih ---- nitrate1 #ii ---- sulphate1 #ii ,anhydride1 an1 #ba ,anode1 #ajc ,argol1 #gf ,asbestos1 #fc ----1 platinized1 #ai ,ash1 black1 #hd ----1 pearl1 #ic ----1 soda1 #aj1 #ib ,atolls1 #ea ,atomized water1 #ah ,b ,bacon1 ,roger1 #cb ,basic slag1 #eh ,basil ,valentine1 #ab ,beryl1 #fc ,black liquor1 #gd ,blasting gelatine1 #ce ,bleaching powder1 #df ,blue-john1 #dg ,boiler scale1 #ed ,bonbonnes1 #ca ,bone1 #ef ---- ash1 #eg ---- black1 #ef ---- meal1 #ef ,borax1 #ei ,bordeaux mixture1 #g ,boric acid1 #eh ,boyle1 ,robert1 #b ,burgundy mixture1 #f ,c ,calcium acetate1 #e ---- bicarbonate1 #ed ---- carbonate1 #ej ---- fluoride1 #dg ---- nitrate1 #bi ---- phosphate1 #ef ---- sulphate1 #bg ,calc spar1 #ej ,caliche1 #bi ,calico printing1 #bf ,carbon1 #di ,carbonic acid1 #di ---- ---- gas1 #di ,castner process1 #aje ,catalytic action1 #bj ,cathode1 #ajc ,cat's-eye1 #fa ,cavendish1 ,h41 #dj ,cellulose1 #df ,chalcedony1 #fa ,chalk1 #ej ,chert1 #ff ,chili-saltpetre1 #bi1 #ci ,china clay1 #fb ,citric acid1 #gg ,chlorides1 #dg ,chlorine1 #df ,chrome yellow1 #bh ---- red1 #bh ,compound1 #g ,compounds1 binary1 #h ,contact action1 #bj ---- process1 #ah ,copper refining1 #ajg ---- sulphate1 #e1 #bg ,coral reefs1 #ea ,cordite1 #cd ,cream of tartar1 #gf ,crops1 rotation of1 #cg ,crystallization1 water of1 #i ,crystals1 #i ,d ,davy1 ,sir ,humphry1 #ie ,derbyshire spar1 #dg ,devitrification1 #fe ,dynamite1 #ce ,e ,efflorescence1 #hb ,electrode1 #ajc ,electrolysis1 #ajb ,electrons1 #ajc ,electrotype blocks1 #ajg ,element1 definition of1 #g ,elements1 list of1 #h ,explosives1 #cb ,f ,felspars1 #fb ,ferrous acetate1 #gd ---- sulphate1 #be ,flint1 #fa ,fluorspar1 #dh ,formic acid1 #gh ,fur in kettles1 #ed ,g ,garnet1 #fc ,gas1 laughing1 #ii ---- lime1 #ab ---- liquor1 #ih ,gay ,lussac tower1 #af ,glass1 #fd ----1 annealing of1 #fe ----1 ,bohemian1 #fc ----1 etching on1 #dg ----1 flint1 #fc ----1 lead1 #fc ----1 soda1 #fc ----1 water1 #ff ,glauber's salt1 #aj ,glover tower1 #ag ,glue1 #ef ,graphite1 #ajh ,greek fire1 #cb ,guncotton1 #cd ,gunpowder1 #cb ,gypsum1 #bg ,h ,haber process1 #ajj ,halogen1 #dc ,hardness1 permanent1 #ec ----1 temporary1 #ec ,hartshorn1 salt of1 #ii ----1 spirits of1 #ig ,hornblende1 #fc ,hydriodic acid1 #dh ,hydrobromic acid1 #dh ,hydrochloric acid1 #dc ,hydrofluoric acid1 #dg ,i ,iceland spar1 #ej ,ions1 #ajc ,iron pyrites1 #aa ,j ,jade1 #fc ,jasper1 #fa ,k ,key industries1 #aj ,l ,lake1 #bf ,lead acetate1 #ge ---- chambers1 #ag ---- chamber process1 #ad ----1 sugar of1 #ge ---- sulphate1 #bg ----1 white1 #ge ,leblanc soda process1 #hb ,leguminosae1 #cg ,lemon1 salts of1 #gg ,lime burning1 #ea ----1 caustic1 #ig ---- kiln1 #ea ,limestone1 #ej ,litmus1 #b ,lupin root1 #cg ,m ,marble1 #ej ,marking ink1 #bh ,meerschaum1 #fc ,mica1 #fc ,mordants1 #bf ,mycoderma aceti1 #fh ,n ,neutralization1 example of1 #d ----1 explanation of1 #c ,niagara1 #aja ,nitre1 #bi ---- pots1 #ad ,nitric acid1 #cj ---- ----1 from air1 #dj ---- ----1 importance of1 #bh ---- ---- manufacture of1 #cj ---- ----1 properties1 #ca ---- ----1 red fuming1 #ca ---- oxide1 #af ,nitrogen cycle1 #cg ----1 fixation of1 #ajj ---- peroxide1 #af ,nitroglycerine1 #cd ,o ,olein1 #gh ,onyx1 #fa ,opal1 #fa ,orthoclase1 #fb ,oxalic acid1 #gg ,p ,palmitin1 #gh ,pearls1 #ea ,peregrine ,phillips1 #ba ,philosopher's stone1 #b ,phosphoric acid1 #eg ,plaster of ,paris1 #bg ,potash1 caustic1 #ig ----1 mild1 #ic ,potassium1 #ie ---- bicarbonate1 #id ---- nitrate1 #bi ,propellants1 #cc ,prussian blue1 #be ,pyrites burners1 #ad ,pyroligneous acid1 #gc ,q ,quartz1 #fa ---- fibres1 #fb ----1 smoky1 #fa ,quicklime1 #e1 #ea ,r ,red liquor1 #gc ,rock crystal1 #fa ,rupert's drops1 #fe ,s ,sal ammoniac1 #ii ---- prunella1 #bi ,salt cake1 #hd ----1 common1 #dg ----1 formation of a1 #d ,saltpetre1 #bi ,salts1 from carbonates1 #e ----1 from oxides1 #e ----1 from metals1 #d ----1 insoluble1 #f ,sandstone1 artificial1 #ff ,saponification1 #gi ,schweinfurt green1 #bg ,shells1 egg1 #ea ----1 oyster1 #ea ,silica1 #fa ---- ware1 #fb ,silicic acid1 #fb ,silver bromide1 #dh ---- chloride1 #dh ---- iodide1 #dh ---- nitrate1 #bh ---- sand1 #fa ,soap1 hard1 #gi ----1 soft1 #gi ,soda1 baking1 #hh ----1 bicarbonate of1 #f1 #hh ----1 bread1 #hh ----1 caustic1 #if ----1 mild1 #hj ----1 natural1 #hb ----1 washing1 #c1 #e1 #ha ---- water1 #di ,sodium1 #ie ---- nitrate1 #bi ---- sulphate1 #bg ,soil bacteria1 #ch ,solvay process1 #ij ,sorrel1 salts of1 #gg ,spent oxide1 #aa ,stalactite1 #ec ,stalagmite1 #ec ,stearin1 #gh ---- candles1 #gi ,stone ammonia1 #ii ,suffioni1 #fj ,sulphur1 #aa ---- dioxide1 #aa ---- trioxide1 prep4 of1 #ai ,sulphuric acid1 properties1 #bj1 #bd ---- anhydride1 #ba ,sulphurous acid1 #aa ,superphosphate1 #eg ,t ,tallow1 #gi ,tartaric acid1 #gf ,tinkal1 #fa ,trinitrotoluene1 #ce ,v ,verdigris1 #gd ,vert de ,montpellier1 #gd ,vinegar1 #fh ----1 malt1 #gj ----1 wine1 #gj ,vitriol1 blue1 #e ----1 nitrated1 #af ----1 oil of1 #ab ,w ,ward1 ,dr41 #ab ,water1 hard1 #ec ----1 soft1 #ec ----1 softening of1 #ed ,wood ashes1 source of potash1 #c ---- ----1 used as soap1 #b ,z ,zinc chloride1 #e ,,the ,,end ,footnotes #a;,an anhydride is a substance which unites with water to form an acid4 #b,see ,frontispiece4 #c,now #ac a ton4 #d;,basic lead carbonate4 #e;,an electron is probably an 8atom0 of negative electricity detached from matter4 ,printed by ,sir ,isaac ,pitman ,sons1 ,ltd4 ,bath1 ,england 7v--#adfh;c7 ,transcriber's ,notes --,silently corrected several palpable typographical errors4 --,retained publication information from the original source4 --,in the text versions1 included italicized text in underscores4