,,on ,,laboratory ,,arts ,,by ,,richard ,,threlfall1 ,m4,a4 ,,professor ,,of ,,physics ,,in ,,the ,,university ,,of ,,sydney2 ,,member ,,of ,,the ,,institute ,,of ,,electrical ,,engineers2 ,,associate-,,member ,,of ,,the ,,institute ,,of ,,civil ,,engineers2 ,,member ,,of ,,the ,,physical ,,society ,london ,,macmillan ,,and ,,co1 ,,limited ,,new ,,york3 ,,the ,,macmillan ,,company #ahih ,all rights reserved ,,preface #e ,,chapter ,i #h ,,hints ,,on ,,the ,,manipulation ,,of ,,glass ,,and ,,on ,,glass-,,blowing ,,for ,,laboratory ,,purposes #h #d4 ,soft ,soda ,glass4 #f4 ,flint ,glass4 #i4 ,hard or ,bohemian1 ,glass4 #aj4 ,on the ,choice of ,sizes of ,glass ,tube4 #aa4 ,testing ,glass4 #ac4 ,cleaning ,glass ,tubes4 #ad4 ,the ,blow-pipe4 #ah4 ,the ,table4 #ai4 ,special ,operations4 #bj4 ,closing and blowing out the ,end of a ,tube4 #ba4 ,to make a ,weld4 #bb4 ,to weld two ,tubes of different ,sizes4 #bd4 ,to weld ,tubes of very small ,bore4 #cj4 ,to cut very thick ,tubes4 #ca4 ,to blow a ,bulb at the ,end of a ,tube4 #cb4 ,to blow a bulb in the middle of a tube4 #cc4 ,to make a side ,weld4 #cd4 ,inserted ,joints4 #ce4 ,bending ,tubes4 #cf4 ,spiral ,tubes4 #cg4 ,on ,auxiliary ,operations on ,glass4 #ch4 ,boring small ,holes4 #ci4 ,for boring large holes through thick glass sheets4 #da4 ,operations depending on ,grinding3 ,ground-in ,joints4 #db4 ,use of the ,lathe in ,glass-working4 #df4 ,making ,ground ,glass4 #dg4 ,glass-cutting4 #dh4 ,cementing4 #di4 ,fusing ,electrodes into ,glass4 #ea4 ,the ,art of making ,air-light ,joints4 ,,appendix ,,to ,,chapter ,i4 ,,on ,,the ,,preparation ,,of ,,vacuum ,,tubes ,,for ,,the ,,production ,,of ,,professor ,,roentgen',s ,,radiation4 ,,chapter ,,ii4 ,,glass-,,grinding ,,and ,,opticians' ,,work4 #fa4 ,details of the ,process of ,fine ,grinding4 #fb4 ,polishing4 #fc4 ,centering4 #fe4 ,preparing ,small ,mirrors for ,galvanometers4 #ff4 ,preparation of ,large ,mirrors or ,lenses for ,telescopes4 #fi4 ,the ,preparation of ,flat ,surfaces of ,rock ,salt4 #gj4 ,casting ,specula for ,mirrors4 #ga4 ,grinding and polishing ,specula4 #gb4 ,preparation of ,flat ,surfaces4 #gc4 ,polishing ,flat ,surfaces on ,glass or on ,speculum ,metal4 ,,chapter ,,iii4 ,,miscellaneous ,,processes4 #gd4 ,coating ,glass with ,aluminium and ,soldering ,aluminium4 #ge4 ,the ,use of the ,diamond-cutting ,wheel4 #gf4 ,arming a ,wheel4 #gg4 ,cutting a ,section4 #gh4 ,grinding ,rock ,sections1 or ,thin ,slips of any ,hard ,material4 #gi4 ,cutting ,sections of ,soft ,substances4 #hj4 ,on the ,production of ,quartz ,threads4 #hd4 ,drawing ,quartz ,threads4 #hf4 ,drawing ,threads by the ,catapult4 #hg4 ,drawing ,threads by the ,flame alone4 #hh4 ,properties of ,threads4 #ij4 ,on the ,attachment of ,quartz ,fibres4 #ia4 ,other ,modes of soldering ,quartz4 #ib4 ,soldering4 #id4 ,preparing a ,soldering ,bit4 #ie4 ,soft ,soldering4 #ig4 ,soldering ,zinc4 #ih4 ,soldering other ,metals4 #ii4 ,brazing4 #ajj4 ,silver ,soldering4 #aja4 ,on the ,construction of ,electrical ,apparatus--,insulators4 #ajb4 ,sulphur4 #ajc4 ,fused ,quartz4 #ajd4 ,glass4 #aje4 ,ebonite or ,hard ,rubber4 #ajf4 ,mica4 #ajg4 ,use of ,mica in ,condensers4 #ajh4 ,micanite4 #aji4 ,celluloid4 #aaj4 ,paper4 #aaa4 ,paraffined ,paper4 #aab4 ,paraffin4 #aac4 ,vaseline1 ,vaseline ,oil1 and ,kerosene ,oil4 #aad4 ,imperfect ,conductors4 #aaf4 ,conductors4 #aag4 ,platinoid4 #aai4 ,platinum ,silver4 #abj4 ,platinum ,iridium4 #aba4 ,manganin4 #abb4 ,other ,alloys4 #abc4 ,nickelin4 #abd4 ,patent ,nickel4 #abe4 ,constantin4 #abf4 ,nickel ,manganese ,copper4 ,,chapter ,,iv4 ,,electroplating ,,and ,,allied ,,arts4 #abg4 ,electroplating4 #abh4 ,the ,dipping ,bath4 #acj4 ,scratch-brushing4 #aca4 ,burnishing4 #acb4 ,silver-plating4 #acc4 ,cold ,silvering4 #acd4 ,gilding4 #ace4 ,preparing ,surfaces for ,gilding4 #acf4 ,gilding ,solutions4 #acg4 ,plating with ,copper4 #ach4 ,coppering ,aluminium4 #adj4 ,alkaline ,coppering ,solution4 #ada4 ,nickel-plating4 #adb4 ,miscellaneous ,notes on ,electroplating4 #adc4 ,blacking ,brass ,surfaces4 #add4 ,sieves4 #ade4 ,pottery making in the ,laboratory4 ,,appendix4 ,,platinising ,,glass4 ,,preface ,,experimental work in physical science rests ultimately upon the mechanical arts4 ,it is true that in a well-appointed laboratory1 where apparatus is collected together in greater or less profusion1 the appeal is often very indirect1 and to a student carrying out a set experiment with apparatus provided to his hand1 the temptation to ignore the mechanical basis of his work is often irresistible4 ,it often happens that young physicists are to be found whose mathematical attainments are adequate1 whose observational powers are perfectly trained1 and whose general capacity is unquestioned1 but who are quite unable to design or construct the simplest apparatus with due regard to the facility with which it ought to be constructed4 ,that ultimate knowledge of materials and of processes which by long experience becomes intuitive in the mind of a great inventor of course cannot be acquired from books or from any set course of instruction4 ,there are1 however1 many steps between absolute ignorance and consummate knowledge of the mechanical arts1 and it is the object of the following pages to assist the young physicist in making his first steps towards acquiring a working knowledge of laboratory arts4 ,however humble the ambition may be1 no one can be more keenly alive than the writer to the inadequacy of his attempt2 and it is only from a profound sense of the necessity which exists for some beginning to be made1 that he has had the courage to air his views on matters about which there are probably hundreds or thousands of people whose knowledge is superior to his own4 ,moreover1 nothing has been further from the writer's mind than any idea of instructing any one2 his desire is--if happily it may so befall--to be of assistance1 especially to young physicists or inventors who wish to attain definite mechanical ends with the minimum expenditure of time4 ,most people will agree that one condition essential to success in such an undertaking is brevity1 and it is for this reason that alternative methods as a rule have not been given1 which1 of course1 deprives the book of any pretence to being a treatise4 ,the writer1 therefore1 is responsible for exercising a certain amount of discretion in the selection he has made1 and it is hardly to be hoped that he has in all--or even in the majority of cases--succeeded in recommending absolutely the best method of procedure4 ,this brings another point into view4 ,before all things the means indicated must be definite and reliable4 ,it is for this reason that the writer has practically confined himself to matters lying within his own immediate experience1 and has never recommended any process 7with one or two minor exceptions1 which he has noted7 which he has not actually and personally carried through to a successful issue4 ,this1 although it is a matter which he considers of the highest importance1 and which is his only title to a hearing1 has unfortunately led to a very personal tone in the book4 ,with regard to the arts treated of in the following pages1 matters about which information is easily acquired--such as carpentering1 blacksmithing1 turning1 and the arts of the watchmaker--have been left on one side4 ,with regard to the last1 which is of immense use in the laboratory1 there happen to be at least two excellent and handy books1 viz4 ,saunier's ,watchmakers' ,handbook1 ,tripplin1 ,london1 #ahib2 and ,britton's ,watchmakers' ,dictionary and ,guide4 ,with regard to carpentering1 turning1 and blacksmithing1 almost any one who so desires can obtain a little practical experience in any village4 ,a short chapter has been devoted to glass-blowing1 in spite of there being an excellent and handy book by ,mr4 ,shenstone 7,the ,methods of ,glass-blowing1 ,rivington7 on the subject already in existence4 ,the reason for this exception lies in the fact that the writer's methods differ considerably from those advocated by ,mr4 ,shenstone4 ,the chapter on opticians' work has had to be compressed to an extent which is undesirable in dealing with so complex and delicate an art1 but it is hoped that it will prove a sufficient introduction for laboratory purposes4 ,in this matter the writer is under great obligations to his friend and assistant1 ,mr4 ,james ,cook1 ,f4,r4,a4,s1 who gave him his first lessons in lens-making some twenty years ago4 ,to ,mr4 ,john ,a4 ,brashear of ,allegheny1 ,pa1 thanks are due for much miscellaneous information on optical work1 which is included verbatim in the text1 some of it contained originally in printed papers1 and some most kindly communicated to the writer for the purpose of this book4 ,in particular1 the writer would thank ,mr4 ,brashear for his generously accorded information as to the production of those flat surfaces for which he is so justly famous4 ,the writer is also indebted to ,mr4 ,a4 ,e4 ,kennelly for some information as to ,american practice in the use of insulating material for electrical work1 and to his friends ,mr4 ,j4 ,a4 ,pollock and ,dr4 ,c4 ,j4 ,martin for many valuable suggestions4 ,for the illustrations thanks are due to ,mrs4 ,threlfall and ,mr4 ,james ,cook4 ,with regard to matters which have come to the writer's knowledge by his being specifically instructed in them from time to time1 due acknowledgment is1 it is hoped1 made in the text4 ,with regard to the question as to what matters might be included and what omitted1 the general rule has been to include information which the author has obtained with difficulty1 and to leave on one side that which he has more easily attained4 ,all the unities have been consistently outraged by a deliberate use of the ,english and metric systems side by side4 ,so long as all the materials for mechanical processes have to be purchased to specifications in inches and feet1 it is impossible to use the centimetre consistently without introducing inconvenience4 ,however1 everybody ought to1 and probably does1 use either system with equal facility4 ,no attempt has been made at showing how work can be done without tools4 ,though1 no doubt1 a great deal can be done with inferior appliances where great economy of money and none of time is an object1 the writer has long felt very strongly that ,english physical laboratory practice has gone too far in the direction of starving the workshop1 and he does not wish1 even indirectly1 'to give any countenance to such a mistaken policy4 ,physical research is too difficult in itself1 and students' time is too valuable1 for it to be remunerative to work with insufficient appliances4 ,in conclusion1 the writer would ask his readers to regard the book to some extent as tentative1 and as a means to the procuring and organising of information bearing upon laboratory arts4 ,any information which can be given will be always thankfully received1 and the author hereby requests any reader who may happen to learn something of value from the book to communicate any special information he may possess1 so that it may be of use to others should another edition ever be called for4 ,,chapter ,i ,,hints ,,on ,,the ,,manipulation ,,of ,,glass ,,and ,,on ,,glass-,,blowing ,,for ,,laboratory ,,purposes #a4 ,,the art of glass-blowing has the conspicuous advantage1 from the point of view of literary presentation1 of being to a great extent incommunicable4 ,as in the case of other delightful arts--such as those treated of in the ,badminton ,library1 for instance--the most that can be done by writing is to indicate suitable methods and to point out precautions which experience has shown to be necessary1 and which are not always obvious when the art is first approached4 ,it is not the object of this work to deal with the art of glass-blowing or any other art after the manner befitting a complete treatise1 in which every form of practice is rightly included4 ,on the contrary1 it is my wish to avoid the presentation of alternative methods4 ,i consider that the presentation of alternative methods would1 for my present purpose1 be a positive disadvantage1 for it would swell this book to an outrageous size2 and to beginners--,i speak from experience--too lavish a treatment acts rather by way of obscuring the points to be aimed at than as a means of enlightenment4 ,the student often does not know which particular bit of advice to follow1 and obtains the erroneous idea that great art has to be brought to bear to enable him to accomplish what is1 after all1 most likely a perfectly simple and straightforward operation4 ,this being understood1 it might perhaps be expected that ,i should describe nothing but the very best methods for obtaining any proposed result4 ,such1 of course1 has been my aim1 but it is not likely that ,i have succeeded in every case1 or even in the majority of cases1 for ,i have confined myself to giving such directions as ,i know from my own personal experience will1 if properly carried out1 lead to the result claimed4 ,in the few cases in which ,i have to refer to methods of which ,i have no personal experience1 ,i have endeavoured to give references 7usually taking the form of an acknowledgment71 so that an idea of their value may be formed4 ,all methods not particularised may be assumed by the reader to have come within my personal experience4 #b4 ,returning to glass-blowing1 we may note that two forms of glass-blowing are known in the arts1 ,pot blowing and ,table blowing4 ,in the former case large quantities of fluid metal 7technical term for melted glass7 are assumed to be available1 and as this is seldom the case in the laboratory1 and as ,i have not yet felt the want of such a supply1 ,i shall deal only with table blowing4 ,fortunately there is a convenient book on this subject1 by ,dr4 ,shenstone 7,rivingtons71 so that what ,i have to say will be as brief as possible1 consistent with sufficiency for everyday work4 ,as a matter of fact there is not very much to say1 for if ever there was an art in which manual dexterity is of the first and last importance1 that art is glass-working4 ,i do not think that a man can become an accomplished glass-blower from book instructions merely--at all events1 not without much unnecessary labour1--but he can learn to do a number of simple things which will make an enormous difference to him both as regards the progress of his work and the state of his pocket4 #c4 ,the first thing is to select the glass4 ,in general1 it will suffice to purchase tubes and rods2 in the case where large pieces 7such as the bulbs of ,geissler pumps7 have to be specially prepared by pot-blowing1 the student will have to observe precautions to be mentioned later on4 ,there are three kinds of glass most generally employed in laboratories4 #d4 ,soft ,soda ,glass1 obtained for the most part from factories in ,thuringia1 and generally used in assembling chemical apparatus4--,this glass is cheap1 and easily obtainable from any large firm of apparatus dealers or chemists4 ,it should on no account be purchased from small druggists1 for the following reasons3- 7a7 ,it is usually absurdly dear when obtained in this way4 7b7 ,it is generally made up of selections of different age and different composition1 and pieces of different composition1 even if the difference is slight1 will not fuse together and remain together unless joined in a special manner4 7c7 ,it is generally old1 and this kind of glass often devitrifies with age1 and is then useless for blowpipe work1 though it may be bent sufficiently for assembling chemical apparatus4 ,devitrified glass looks frosty1 or1 in the earlier stages1 appears to be covered by cobwebs1 and is easily picked out and rejected4 #e4 ,it might be imagined that the devitrification would disappear when the glass is heated to the fusing point2 and so it does to a great extent1 but for many operations one only requires to soften the glass1 and the devitrification often persists up to this temperature4 ,my experience is that denitrified glass is also more likely to crack in the flame than good new glass1 though the difference in this respect is not very strongly marked with narrow tubes4 #f4 ,flint ,glass4 ,magnificent flint glass is made both in ,england and ,france4 ,the ,english experimenter will probably prefer to use ,english glass1 and1 if he is wise1 will buy a good deal at a time1 since it does not appear to devitrify with age1 and uniformity is thereby more likely to be secured4 ,i have obtained uniformly good results with glass made by ,messrs4 ,powell of ,whitefriars1 but ,i daresay equally good glass may be obtained elsewhere4 ,for general purposes flint glass is vastly superior to the soft soda mentioned above4 ,in the first place1 it is very much stronger1 and also less liable to crack when heated--not alone when it is new1 but also1 and especially1 after it has been partly worked4 ,apparatus made of flint glass is less liable to crack and break at places of unequal thickness than if made of soda glass4 ,this is not of much importance where small pieces of apparatus only are concerned1 because these can generally be fairly annealed2 and if the work is well done1 the thickness will not be uneven4 ,it is a different matter where large pieces of apparatus1 such as connections to ,geissler pumps1 are concerned1 for the glass has often to be worked partly in situ1 and can only be imperfectly annealed4 ,joints made between specimens of different composition are much more likely to stand than when fashioned in soda glass4 ,indeed1 if it is necessary to join two bits of soda glass of different kinds1 it is better to separate them by a short length of flint glass2 they are more likely to remain joined to it than to each other4 ,a particular variety of flint glass1 known as white enamel1 is particularly suitable for this purpose1 and1 indeed1 may be used practically as a cement4 #g1 ,it is1 however1 when the necessity of altering or repairing apparatus complicated by joints arises that the advantage of flint glass is most apparent4 ,a crack anywhere near to a side1 or inserted joint1 can scarcely ever be repaired in the case of soda glass apparatus1 even when the glass is quite thin and the dimensions small4 ,it should also be mentioned that flint glass has a much more brilliant appearance than soda glass4 ,of course1 there is a considerable difference between different kinds of flint glass as to the melting point1 and this may account for the divergency of the statements usually met with as to its fusibility compared with that of soda glass4 ,the kind of flint glass made by ,messrs4 ,powell becomes distinctly soft soon after it is hot enough to be appreciably luminous in a darkened room1 and at a white heat is very fluid4 ,this fluidity1 though of advantage to the practised worker1 is likely to give a beginner some trouble4 #h4 ,as against the advantages enumerated1 there are some drawbacks4 ,the one which will first strike the student is the tendency of the glass to become reduced in the flame of the blow-pipe4 ,this can be got over by proper adjustment of the flame1 as will be explained later on4 ,a more serious drawback in exact work is the following4 ,in making a joint with lead glass it is quite possible to neglect to fuse the glass completely together at every point2 in fact1 the joint will stand perfectly well even if it be left with a hole at one side1 a thing which is quite impossible with soft soda glass1 or is at least exceedingly unusual4 ,an accident of this kind is particularly likely to happen if the glass be at all reduced4 ,hence1 if a joint does not crack when cold1 the presumption is1 in the case of soda glass1 that the joint is perfectly made1 and will not allow of any leak2 but this is not the case with flint glass1 for which reason all joints between flint glass tubes require the most minute examination before they are passed4 ,if there are any air bubbles in the glass1 especial care must be exercised4 #i4 ,hard or ,bohemian1 ,glass4 ,this is1 of course1 used where high temperatures are to be employed1 and also in certain cases where its comparative insolubility in water is of importance4 ,it is very unusual for the investigator to have to make complicated apparatus from this glass4 ,fused joints may be made between hard glass and flint glass without using enamel1 and though they often break in the course of time1 still there is no reason against their employment1 provided the work be done properly1 and they are not required to last too long4 #aj4 ,on the ,choice of ,sizes of ,glass ,tube4 ,it will be found that for general purposes tubes about one-quarter inch in inside diameter1 and from one-twentieth to one-fortieth of an inch thick1 are most in demand4 ,some very thin soda glass of these dimensions 7so-called cylinder tubes7 will be found very handy for many purposes4 ,for physico-chemical work a good supply of tubing1 from one-half to three-quarters of an inch inside diameter1 and from one-twentieth to one-eighth inch thick1 is very necessary4 ,a few tubes up to three inches diameter1 and of various thicknesses1 will also be required for special purposes4 ,thermometer and barometer tubing is occasionally required1 the latter1 by the way1 making particularly bad barometers4 ,the thermometer tubing should be of all sizes of bore1 from the finest obtainable up to that which has a bore of about one-sixteenth of an inch4 ,glass rods varying from about one-twentieth of an inch in diameter up to1 say1 half an inch will be required1 also two or three sticks of white enamel glass for making joints4 ,to facilitate choice1 there is appended a diagram of sizes from the catalogue of a reliable ,german firm1 ,messrs4 ,desaga of ,heidelberg1 and the experimenter will be able to see at a glance what sizes of glass to order4 ,it is a good plan to stock the largest and smallest size of each material as well as the most useful working sizes4 ,fig4 #a4 #aa4 ,testing ,glass4 ,reject glass which has lumps or knots1 is obviously conical1 or has long drawn-out bubbles running through the substance4 ,if a scratch be made on the surface of a glass tube1 and one end of the scratch be touched by a very fine point of fused glass1 say not more than one-sixteenth inch in diameter1 the tube1 however large it is 7within reason71 ought to crack in the direction of the scratch4 ,if a big crack forms and does not run straight1 but tends to turn longitudinally1 it is a sign that the glass is ill annealed1 and nothing can be done with it4 ,if such glass be hit upon in the course of blow-pipe work1 it is inadvisable to waste time upon it2 the best plan is to reject it at once1 and save it for some experiment where it will not have to be heated4 ,the shortest way of selecting glass is to go to a good firm1 and let it be understood that if the glass proves to be badly annealed it will be returned4 ,though it was stated above that the glass should not be distinctly conical1 of course allowance must be made for the length of the pieces1 and1 on the other hand1 a few highly conical tubes will be of immense service in special cases1 and a small supply of such should be included4 ,the glass1 as it is obtained1 should be placed in a rack1 and covered by a cloth to reduce the quantity of dust finding its way into the tubes4 ,it has been stated by ,professor ,ostwald that tubes when reared up on end tend to bend permanently4 ,i have not noticed this with lead glass well supported4 ,each different supply should be kept by itself and carefully described on a label pasted on to the rack1 and tubes from different lots should not be used for critical welds4 ,this remark is more important in the case of soda than of lead glass4 ,in the case of very fine thermometer tubes it will be advisable to cover the ends with a little melted shellac1 or1 in special cases1 to obtain the tubes sealed from the works4 ,soda glass can generally be got in rather longer lengths than lead glass2 the longer the lengths are the better1 for the waste is less4 ,it is useful to be able to distinguish the different kinds of glass by the colour4 ,this is best observed by looking towards a bright surface along the whole length of the tube and through the glass4 ,lead glass is yellow1 soda glass is green1 and hard glass purple in the samples in my laboratory1 and ,i expect this is practically true of most samples4 ,footnote3 ,some new lead glass ,i have is also almost purple in hue4 ,if any doubt exists as to the kind of glass1 it may be tested at once in the blow-pipe flame1 or by a mixture of oils of different refractive indices1 as will be explained later4 #ab4 ,the question of the solubility of glass in reagents is one of great importance in accurate work1 though it does not always meet with the attention it deserves4 ,it is impossible here to go into the matter with sufficient detail1 and the reader is therefore referred to the ,abstracts of the ,chemical ,society1 particularly for the years #ahhi and #ahib4 ,the memoir by ,f4 ,kohlrausch1 ,wied4 ,ann4 xliv1 should be consulted in the original4 ,the following points may be noted4 ,a method of testing the quality of glass is given by ,mylius 7,c4 ,s4 ,j4 ,abstracts1 #ahhi1 p4 #edi71 and it is stated that the resistance of glass to the action of water can generally be much increased by leaving it in contact with cold water for several days1 and then heating it to #cjj to #djj ,c4 ,this improvement seems to be due to the formation of a layer of moist silica on the surface1 and its subsequent condensation into a resisting layer by the heating4 ,mylius 7,c4 ,s4 ,j4 ,abstracts1 #ahib1 p4 #daa71 and ,weber1 and ,sauer 7,c4 ,s4 ,j4 ,abstracts1 #ahib1 p4 #daj7 have also shown that the best glass for general chemical purposes consists of3 ,silica1 #g to #h parts ,lime1 #a part ,alkali1 #a4e to #a4a parts4 ,this is practically ,bohemian tube glass4 ,the exact results are given in the ,berichte of the ,german ,chemical ,society1 vol4 xxv4 ,an excellent account of the properties of glass will be found in ,grove's edition of ,miller's ,elements of ,chemistry4 #ac4 ,cleaning ,glass ,tubes4 ,this is one of the most important arts in chemistry4 ,if the tubes are new1 they are generally only soiled by dust1 and can be cleaned fairly easily--first by pushing a bit of cotton waste through with a cane1 or pulling a rag through with string--and then washing with sand and commercial hydrochloric acid4 ,i have heard of glass becoming scratched by this process1 and breaking in consequence when heated1 but have never myself experienced this inconvenience4 ,in ,german laboratories little bits of bibulous paper are sometimes used instead of sand2 they soon break into a pulp1 and this pulp has a slightly scouring action4 ,as soon as the visible impurities are removed and the tube when washed looks bright and clean1 it may be wiped on the outside and held perpendicularly so as to allow the water film to drain down4 ,if the tube be greasy 7and perhaps in other cases7 it will be observed that as the film gets thinner the water begins to break away and leave dry spots4 ,for accurate work this grease1 or whatever it is1 must be removed2 and after trying many plans for many years1 ,i have come back to the method ,i first employed1 viz4 boiling out with aqua regia4 ,for this purpose1 close one end of the tube by a cork 7better than a rubber bung1 because cheaper71 and half fill the tube with aqua regia2 then1 having noted the greasy places1 proceed to boil the liquid in contact with the glass at these points1 and in the case of very obstinate dirt--such as lingers round a fused joint which has been made between undusted tubes--leave the whole affair for twelve hours4 ,if the greasiness is only slight1 then simply shaking with hot aqua regia will often remove it1 and the aqua regia is conveniently heated in this case by the addition of a little strong sulphuric acid4 ,the spent aqua regia may be put into a bottle4 ,it is generally quite good enough for the purpose of washing glass vessels with sand1 as above explained4 ,however carefully a tube is cleaned before being subjected to blowpipe operations1 it will be fouled wherever there is an opening during the process of heating1 unless the extreme tip only of an oxidising flame be employed4 ,even this should not be trusted too implicitly unless an oxygas or hydrogen flame is employed4 ,when a tube or piece of apparatus has been cleaned by acid1 so that on clamping it vertically1 dry spaces do not appear1 it may be rinsed with platinum distilled water and left to drain1 the dust being1 of course1 kept out by placing a bit of paper round the top4 ,for accurate work water thus prepared is to be preferred to anything else4 ,when the glass is very clean interference colours will be noticed as the water dries away4 ,carefully-purified alcohol may in some cases be employed where it is desired to dry the tube or apparatus quickly4 ,in this case an alcohol wash bottle should be used1 and a little alcohol squirted into the top of the tube all round the circumference4 ,the water film drags the alcohol after it1 and by waiting a few minutes and then adding a few more drops of alcohol1 the water may be practically entirely removed1 especially if a bit of filter paper be held against the lower end of the tube4 ,it is customary in some laboratories to use ether for a final rinse1 but unless the ether is freshly distilled and very pure1 it leaves a distinct organic residue4 ,when no more liquid can be caused to drain away1 the tube may be dried by heating it along its length1 beginning at the top 7to get the advantage of the reduction of surface tension71 and so on all down4 ,it will then be possible to mop up a little more of the rinsing liquid4 ,when the tube is nearly dry a loose plug of cotton wool may be inserted at the bottom4 ,the wool must be put in so that the fibres lie on an even surface inside the tube1 and the wool must be blown free from dust4 ,ordinary cotton wool is useless1 from being dusty and the fibres short1 and the same remark applies to wadding4 ,use nothing but what is known as medicated cotton wool with a good long fibre4 ,the tube will usually soon dry of itself when the cover is lifted an inch or so4 ,if water has been used1 the air-current may be assisted by means of the water-pump1 the air being sucked from the top1 so that the wool has an opportunity of acting as a dust filter2 a very slow stream of air only must be employed4 ,for connecting the tube to the pump1 a bit of ,india-rubber tube about an inch in diameter1 with a bore of about one-eighth of an inch1 may be employed4 ,the end of the rubber tube is merely pressed against the edge of the glass4 ,these remarks apply1 with suitable modification1 to all kinds of finished apparatus having two openings4 ,for flasks and so on1 it is convenient to employ a blowing apparatus1 dust being avoided by inserting a permanent plug of cotton wool in one of the leading tubes4 ,the efficiency of this method is greatly increased by using about one foot of thin copper tube1 bent into a helix1 and heated by means of a ,bunsen burner2 the hot air 7previously filtered7 is passed directly into the flask1 bottle1 or whatever the apparatus may be4 ,this has proved so convenient that a copper coil is now permanently fastened to the wall in one of the rooms of my laboratory4 ,the above instructions indicate greater refinement than is in general necessary or proper for tubes that have to be afterwards worked by the blow-pipe4 ,in the majority of cases all that is necessary is to remove the dust1 and this is preferably done by a wad of cotton waste 7which does not leave shreds like cotton wool71 followed by a bit of bibulous filter paper4 ,i would especially warn a beginner against neglecting this precaution1 for in the process of blowing1 the dust undergoes some change at the heated parts of the apparatus1 and forms a particularly obstinate kind of dirt4 ,in special cases the methods ,i have advocated for removing dirt and drying without covering the damp surfaces with dust are inadequate1 but an experimenter who has got to that stage will have nothing to learn from such a work as this4 #ad4 ,the ,blow-pipe4 ,i suppose a small book might easily be written on this subject but what ,i have to say--in accordance with the limitation imposed--will be brief4 ,for working lead glass ,i never use anything but an oxygas blow-pipe1 except for very large work1 and should never dream of using anything else4 ,of course1 to a student who requires practice in order to attain dexterity this plan would be a good deal too dear4 ,my advice to such a one is--procure good soda glass1 and work it by means of a modification of a gas blow-pipe1 to be described directly4 ,the ,fletcher's blow-pipes on long stems are generally very inconvenient4 ,the flame should not be more than #e or #f inches from the working table at most1 especially for a beginner1 who needs to rest his arms on the edge of the table to secure steadiness4 ,the kind of oxygas blow-pipe ,i find most convenient is indicated in the sketch4 7,fig4 #b7 ,i like to have two nozzles1 which will slip on and off1 one with a jet of about #j4jce inch in diameter1 the other of about double this dimension4 ,the oxygen is led into the main tube of the blow-pipe by another tube of much smaller diameter1 concentric with the main tube 7,fig4 #c1 at ,a74 ,the oxygen is mixed with the gas during its escape from the inner tube1 which is pierced by a number of fine holes for the purpose1 the extreme end being closed up4 ,the inner tube may run up to within half an inch of the point where the cap carrying the nozzle joins the larger tube4 ,fig4 #b4 ,fig4 #c4 ,if it is desired to use the blow-pipe for working glass which is already fixed in position to a support1 it will be found very advantageous to use a hooked nozzle4 ,the nozzle shown in the sketch is not hooked enough for this work1 which requires that the flame be directed 'backwards towards the worker4 ,with a little practice such a flame may be used perfectly well for blowing operations on the table1 as well as for getting at the back of fixed tubes4 ,to warm up the glass1 the gas supply is turned full on1 and enough oxygen is allowed to pass in to clear the flame4 ,the work is held in front of1 but not touching1 the flame1 until it is sufficiently hot to bear moving into the flame itself4 ,the1 work is exposed to this flame until1 in the case of lead glass1 traces of reduction begin to appear4 ,when this point is reached the oxygen tap is thrown wide open4 ,i generally regulate the pressure on the bags1 so that under these circumstances the flame is rather overfed with oxygen4 ,this condition is easily recognised1 as follows4 ,the flame shrinks down to a very small compass1 and the inner blue cone almost disappears2 also flashes of yellow light begin to show themselves--a thing which does not occur when the proportions of the gases are adjusted for maximum heating effect4 ,for many purposes the small dimensions of the flame render it very convenient1 and the high temperature which can be attained at exact spots enables glass to be fused together after a certain amount of mixing1 which is an enormous advantage in fusing lead glass on to hard glass4 ,the lead glass should not be heated hot enough to burn1 but1 short of this1 the more fluid it is the better for joints between dissimilar samples4 ,it will be noticed that the blow-pipe can be rotated about a vertical axis so as to throw the flame in various directions4 ,this is often indispensable4 #ae4 ,in general the oxygen flame does not require to be delivered under so high a pressure as for the production of a lime light4 ,in ,england1 ,i presume1 most experimenters will obtain their oxygen ready prepared in bottles1 and will not have to undergo the annoyance of filling a bag4 ,if1 however1 a bag is used1 and it has some advantages 7the valves of bottles being generally stiff71 ,i find that a pressure produced by placing about two hundredweight 7conveniently divided into four fifty-six pound weights7 on bags measuring #c' x #b'f x #b' 7at the thicker end7 does very well4 ,to fill such a bag with oxygen1 about #gjj grms of potassium chlorate is required4 ,if the experimenter desires to keep his bag in good order1 he must purify his oxygen by washing it with a solution of caustic soda1 and then passing it through a tower of potash or soda in sticks1 and1 finally1 through a calcium chloride tower4 ,this purifying apparatus should be permanently set up on a board1 so that it may be carried about by the attendant to wherever it is required4 ,oxygen thus purified does not seem to injure a good bag--at least during the first six or seven years3 ,in order to reduce the annoyance of preparing oxygen1 the use of the usual thin copper conical bottle should be avoided4 ,the makers of steel gas bottles provide retorts of wrought iron or steel for oxygen-making1 and these do very well4 ,they have the incidental advantage of being strong enough to resist the attacks of a servant when a spent charge is being removed4 ,the form of retort referred to is merely a large tube1 closed at one end1 and with a screw coupling at the other2 the dimensions may be conveniently about #e inches by #aj4 ,the screw threads should be filled with fireclay 7as recommended by ,faraday7 before the joint is screwed up4 ,before purchasing a bottle the experimenter will do well to remember that unless it is of sufficiently small diameter to go into his largest vice1 he will be inconvenienced in screwing the top on and off4 ,why these affairs are not made with union joints1 as they should be1 is a question which will perhaps be answered when we learn why cork borers are still generally made of brass1 though steel tube has long been available4 ,fig4 #d4 ,these little matters may appear very trivial--and so they are--but the purchaser of apparatus will generally find that unless he looks after details himself1 they will not be attended to for him4 ,whether a union joint is provided or not1 let it be seen that the end of the delivery tube is either small enough to fit a large rubber tube connection going to the wash-bottle1 or large enough to allow of a cork carrying a bit of glass tube for the same purpose to be inserted4 ,this tube should not be less than half an inch in inside diameter4 ,never use a new bottle before it has been heated sufficiently to get rid of grease and carbonaceous dirt4 ,a convenient oxygen-making apparatus is shown in ,fig4 #d1 which is drawn from life4 #af4 ,for large blow-pipe work with lead glass ,i recommend a system of four simple blow-pipes1 in accordance with the sketch annexed4 ,i first saw this system in operation in the lamp factory of the ,westinghouse ,electric ,company at ,pittsburg in #ahhi1 and since then ,i have seen it used by an exceedingly clever trick glass-worker at a show4 ,after trying both this arrangement and the brush flame recommended by ,mr4 ,shenstone1 ,i consider the former the more convenient2 however1 ,i daresay that either can be made to work in competent hands1 but ,i shall here describe only my own choice4 ,footnote3 ,a brush flame is one which issues from the blow-pipe nozzle shaped like a brush1 i4e4 it expands on leaving the jet4 ,it is produced by using a cylindrical air jet or a conical jet with a large aperture1 say one-eighth of an inch4 ,see ,fig4 #be4 ,as will be seen1 the blow-pipe really consists of four simple brass tube blow-pipes about three-eighths of an inch internal diameter and #c inches long1 each with its gas and air tap and appropriate nozzle4 ,each blowpipe can turn about its support 7the gas-entry pipe7 to some extent1 and this possibility of adjustment is of importance1 ,the air jets are merely bits of very even three-sixteenths inch glass tubing1 drawn down to conical points1 the jets themselves being about #j4jce inch diameter4 ,fig4 #e4 ,the flames produced are the long narrow blow-pipe flames used in blow-pipe analysis1 and arranged so as to consist mostly of oxidising flame4 ,the air-supply does not require to be large1 nor the pressure high--#e to #aj inches of water will do--but it must be very regular4 ,the trick glass-blower ,i referred to employed a foot bellows in connection with a small weighted gasometer1 the ,westinghouse ,company used their ordinary air-blast1 and ,i have generally used a large gas-holder with which ,i am provided1 which is supplied by a ,roots blower worked by an engine4 ,i have also used a velocity pump blower1 which may be purchased amongst others from ,gerhardt of ,bonn4 ,the arrangement acts both as a sucking and blowing apparatus1 and is furnished with two manometers and proper taps1 etc4 ,as ,i have reason to know that arrangements of this kind work very ill unless really well made1 ,i venture to add that the ,gerhardt arrangement to which ,i refer is ,no4 #bci in his catalogue1 and costs about three pounds4 ,it hardly gives enough air1 however1 to work four blow-pipes1 and the blast requires to be steadied by passing the air through a vessel covered with a rubber sheet4 ,in default of any of these means being available1 one of ,fletcher's foot-blowers may be employed1 but it must be worked very regularly4 ,a table mounted with one blow-pipe made on this plan1 and worked by a double-acting bellows1 is recommended for students' use4 ,for working flint glass1 the air jet may be one-eighth of an inch in diameter and the pressure higher--this will give a brush flame4 ,see ,fig4 #be4 ,it will be seen1 on looking at the sketch of the blowpipe system1 that the pair of blow-pipes farther from the observer can be caused to approach or recede at will by means of a handle working a block on a slide4 ,it often happens that after using all four blow-pipes at once it is necessary to have recourse to one blow-pipe only1 and to do this conveniently and quickly is rather an object4 ,now1 in my arrangement ,i have to turn off both the gas and air from the farther system1 and then put in a bit of asbestos board to prevent the nozzles being damaged by the flame or flames kept alight4 ,as ,i said before1 when some experience is gained1 glassblowing1 becomes a very simple art1 and work can be done under circumstances so disadvantageous that they would entirely frustrate the efforts of a beginner4 ,this is not any excuse1 however1 for recommending inferior arrangements4 ,consequently1 ,i say that the pipes leading in gas and air should be all branches of one gas and one air pipe1 in so far as the two remote and one proximate blow-pipe are concerned1 and these pipes should come up to the table to the right hand of the operator1 and should have main taps at that point1 each with a handle at least #b inches long4 ,by this arrangement the operator can instantly turn down all the blow-pipes but one1 while1 if the inverse operation is required1 all the three pipes can be started at once4 ,footnote3 ,i find1 since writing the above1 that ,i have been anticipated in this recommendation by ,mr4 ,g4 ,s4 ,ram1 ,the ,incandescent ,lamp and its ,manufacture1 p4 #aad4 ,the separate air and gas taps must be left for permanent regulation1 and must not be used to turn the supply on or cut it off4 ,in some respects this blow-pipe will be found more easy to manage than an oxygas blow-pipe1 for the glass is not so readily brought to the very fluid state1 and this will often enable a beginner who proceeds cautiously to do more than he could with the more powerful instrument4 ,though ,i have mentioned glass nozzles for the air supply1 there is no difficulty in making nozzles of brass4 ,for this purpose let the end of a brass tube of about one-eighth of an inch diameter be closed by a bit of brass wire previously turned to a section as shown 7,fig4 #f71 and then bored by a drill of the required diameter1 say -4#jce inch4 ,it is most convenient to use too small a drill1 and to gradually open the hole by means of that beautiful tool1 the watchmaker's broach4 ,the edges of the jet should be freed from burr by means of a watchmaker's chamfering tool 7see ,saunier's ,watchmaker's ,hand-book1 ,tripplin1 #ahhb1 p4 #bcb1 #cdb71 or by the alternate use of a slip of ,kansas stone and the broach4 ,fig4 #f4 ,the construction of this blow-pipe is so simple1 that in case any one wishes to use a brush flame1 he can easily produce one simply by changing his air jets to bits of the same size 7say one-eighth to one-sixteenth of an inch7 tubing1 cut off clean4 ,to insure success1 the ends of the tubes must be absolutely plane and regular2 the slightest inequality makes all the difference in the action of the instrument4 ,if a jet is found to be defective1 cut it down a little and try again2 a clean-cut end is better than one which has been ground flat on a stone4 ,the end of a tube may1 however1 be turned in a manner hereafter to be described so as to make an efficient jet4 ,several trials by cutting will probably have to be made before success is attained4 ,for this kind of jet the air-pressure must be greatly increased1 and a large ,fletcher's foot-blower or1 better still1 a small double-action bellows worked with vigour will be found very suitable4 ,a fitting for this auxiliary blow-pipe is shown in ,fig4 #e at ,b4 ,professor ,roentgen's discovery has recently made it necessary to give more particular attention to the working of soft soda glass1 and ,i have been obliged to supplement the arrangements described by a table especially intended for work with glass of this character4 ,the arrangement has proved so convenient for general work that ,i give the following particulars4 ,the table measures #e feet long1 #b feet #aa inches wide1 and is #b feet #i inches high4 ,fig4 #g4 ,it is provided with a single gas socket1 into which either a large or small gas tube may be screwed4 ,the larger tube is #e4e inches long and #j4ge of an inch in diameter4 ,the smaller tube is the same length1 and half an inch in diameter4 ,the axis of the larger tube is #c4e inches above the table at the point of support1 and is inclined to the horizontal at an angle of #ab4 ,the axis of the smaller tube is #b4e inches above the surface of the table1 and is inclined to the horizontal at the same angle as the larger one4 ,the air jets are simply pieces of glass tube held in position by corks4 ,the gas supply is regulated by a well-bored tap4 ,the air supply is regulated by treading the bellows--no tap is requisite4 ,the bellows employed are ordinary smiths' bellows1 measuring #bb inches long by #ac inches wide in the widest part4 ,they are weighted by lead weights1 weighing #bf lbs4 ,the treadle is connected to the bellows by a small steel chain1 for the length requires to be invariable4 ,as the treadle only acts in forcing air from the lower into the upper chamber of the bellows1 a weight of #ac lbs4 is hung on to the lower cover1 so as to open the bellows automatically4 ,the air jets which have hitherto been found convenient are3 for the small gas tube 7#a7 a tube #j4ab inch diameter drawn down to a jet of #j4jcb inch diameter for small work2 7#b7 plain tubes not drawn down of #j4ad inch1 #j4abg inch1 and -#j4bde inch diameter1 and for the large gas tube1 plain tubes up to #j4c inch in diameter4 ,the table is placed in such a position that the operator sits with his back to a window and has the black calico screen in front of him and to his right4 ,the object of the screen is to protect the workman against draughts4 ,the table is purposely left unscreened to the left of the workman1 so that long tubes may be treated4 #ag4 ,other appliances which will be required for glass-blowing are of the simplest character4 7#a7 ,small corks for closing the ends of tubes4 7#b7 ,soft wax--a mixture of bees' wax and resin softened by linseed oil to the proper consistency1 easily found by trial1 also used for temporarily closing tubes4 7#c7 ,a bottle of vaseline for lubricating4 7#d7 ,an old biscuit tin filled with asbestos in shreds1 and an asbestos towel or cloth for annealing glass after removal from the flame4 ,as asbestos absorbs moisture1 which would defeat its use as an annealing material1 it must be dried if necessary4 7#e7 ,a ,glass-,cutter's ,knife4 ,this is best made out of a fine three-cornered file1 with the file teeth almost ground out1 but not quite2 it should be about #b inches long4 ,after the surface has been ground several times1 it may be necessary to reharden the steel4 ,this is best done by heating to a full red and quenching in mercury4 ,the grindstone employed for sharpening the knife should be quick1 so as to leave a rough edge4 ,i have tried many so-called glass knives made in ,germany1 but1 with one exception1 they were nothing like so good as a small ,french or ,sheffield file4 ,in this matter ,i have the support of ,mr4 ,shenstone's experience4 7#f7 ,a wire nail1 about #b inches long1 mounted very accurately in a thin cylindrical wooden handle about #e inches long by one-quarter of an inch diameter1 or1 better still1 a bit of pinion wire #f inches long1 of which #a4e inches are turned down as far as the cylindrical core1 ,an old dentists' chisel or filling tool is also a very good form of instrument4 7#g7 ,a bit of charcoal about #c4e inches long and #b wide1 and of any thickness1 will be found very useful in helping to heat a very large tube4 ,the charcoal block is provided with a stout wire handle1 bent in such a manner that the block can be held close above a large glass tube on which the flames impinge4 ,in some cases it is conveniently held by a clip stand4 ,by the use of such a slab of charcoal the temperature obtainable over a large surface can be considerably increased4 ,i have seen a wine-glass 7,venetian sherry-glass7 worked on a table with four blow-pipes1 such as is here described1 with the help of a block of hard wood held over the heated glass1 and helping the attainment of a high temperature by its own combustion4 7#h7 ,several retort stands with screw clips4 7#i7 ,some blocks of wood about #e ,x #b ,x #b with ,v-shaped notches cut in from the top4 7#aj7 ,a strong pair of pliers4 7#aa7 ,an apparatus for cleaning and drying the breath1 when blowing directly by the mouth is not allowable4 ,the apparatus consists of a solid and heavy block of wood supporting a calcium-chloride tube permanently connected with a tube of phosphorus pentoxide divided into compartments by plugs of glass wool4 ,care should be taken to arrange these tubes so as to occupy the smallest space1 and to have the stand particularly stable4 ,the exit tube from the phosphorus pentoxide should be drawn down to form a nozzle1 from1 say1 half an inch to one-eighth of an inch in diameter1 so as to easily fit almost any bit of rubber tube4 ,the entry to the calcium chloride should be permanently fitted to about a yard of fine soft rubber tubing1 as light as possible4 ,the ends of this tube should terminate in a glass mouthpiece1 which should not be too delicate4 ,as an additional precaution against dust1 ,i sometimes add a tube containing a long plug of glass wool1 between the phosphorus pentoxide and the delivery tube1 and also a tube containing stick potash on the entry side of the calcium chloride tube1 but it may safely be left to individual judgment to determine when these additions require to be made4 ,in practice ,i always keep the affair set up with these additions4 ,the communication between all the parts should be perfectly free1 and the tubes should be nearly filled with reagents1 so as to avoid having a large volume of air to compress before a pressure can be got up4 ,the arrangement will be clear by a reference to ,fig4 #h1 which illustrates the apparatus in use for joining two long tubes4 ,i have tried blowing-bags1 etc1 but1 on the whole1 prefer the above arrangement1 for1 after a time1 the skill one acquires in regulating the pressure by blowing by the mouth and lips is such an advantage that it is not to be lightly foregone4 ,fig4 #h4 #ah4 ,the ,table4 ,the system of four blow-pipes is1 of course1 a fixture4 ,in this case the table may be about a yard square1 and may be covered with asbestos mill-board neatly laid down1 but this is not essential4 ,the table should have a rim running round it about a quarter of an inch high4 ,the tools should be laid to the right of the worker1 and for this purpose the blow-pipes are conveniently fixed rather to the left of the centre of the table1 but not so far as to make the leg of the table come so close to the operator as to make him uncomfortable1 for a cheerful and contented spirit ought to be part of the glass-worker's outfit4 ,the most convenient height for a blow-pipe table--with the blow-pipes about #b inches above the table top--is #c feet #b inches4 ,nothing is so convenient to sit upon as a rough music-stool with a good range of adjustment4 ,the advantage of an adjustable seat lies in the fact that for some operations one wants to be well over the work1 while in others the advantage of resting the arms against the table is more important4 #ai4 ,special ,operations4 ,the preliminary to most operations before the blow-pipe1 is to draw down a tube and pull it out to a fine point4 ,this is also the operation on which a beginner should exercise himself in the first instance4 ,i will suppose that it is desired to draw out a tube about one-quarter of an inch in diameter1 with the object of closing it1 either permanently or temporarily1 and leaving a handle for future operations in the shape of the point1 thin enough to cool quickly and so not delay further work4 ,for this simple operation most of the glass-blower's skill is required4 ,the tube must be grasped between the first finger and thumb of both hands1 and held so that the part to be operated on lies evenly between the two hands4 ,the distance between the operator's thumbs may conveniently vary from #b4e to #d inches4 ,releasing the grip of the left hand1 let the operator assure himself of his ability to easily rotate the tube about its axis--by the right thumb and finger--he will incidentally observe by the feel whether the tube is straight or not4 ,a good deal of progress can be made from this point before the tube is heated at all4 ,the operator can acquire a habit of instinctively rotating the tube by both hands1 however the tube itself be moved about in space1 or however it be pushed or pulled4 ,the habit of constant and instinctive rotation is literally about one-third of the whole art of glassblowing4 ,it is unlikely1 however1 that the beginner will discover that he has not got this habit1 until a few failures draw his attention to it4 ,the glass tube being held in position lightly yet firmly1 and the operator being sure that he feels comfortable and at his ease1 and that the blow-pipe flame 7a single flame in this instance7 is well under control1 the preliminary heating may be commenced4 ,with a tube of the dimensions given this is a very simple affair4 ,turn the air partly off1 or blow gently1 to get a partly luminous gas flame2 hold the tube about an inch from the end of this flame1 and turn it round and round till it commences to soften4 ,in the case of soda glass it is usual to employ the gas flame only1 but ,i find that it is better in most cases to use the hot air of a gently-blown flame1 rather than have the disadvantage of the soot deposited in the alternative operation4 ,when the glass begins to soften1 or even before1 it may be moved right into the blow-pipe flame1 and the latter may be properly urged4 ,it is not possible to give quite explicit and definite instructions1 applicable to every case1 as to when the time is ripe for passing the work into the flame1 but the following notes will indicate the general rules to be observed3- 7#a7 ,a thick tube must be warmed more slowly and raised to a higher temperature than a thin tube4 7#b7 ,the same remark applies to a tube of large diameter1 as compared with one of small diameter1 whatever the thickness4 7#c7 ,in the case of very large or thick tubes the hot air is advantageously employed at first1 and to complete the preliminary heating1 the luminous flame alone may be used4 ,the object of this is to enable the operator to judge1 by the presence of soot1 its inability to deposit--or its burning off if deposited--of the temperature of the glass1 and of the equality of this temperature all over the surface1 for a large and thick tube might be heated quite enough to enable it to be safely exposed to the full heat before it is appreciably yielding to the fingers4 ,in general1 when the soot burns off freely1 or lead glass begins to show the faintest sign of reduction1 or soda glass begins to colour the flame1 it is more than safe to proceed4 ,in order to turn on the full flame the operator will form a habit of holding the work in the left hand only1 and he will also take care not to let anything his right hand may be doing cause him to stop rotating the tube with his left thumb and finger4 ,the preliminary adjustment of air or oxygen supply will enable the change to a flame of maximum power to be made very quickly4 ,the tube having been introduced with constant rotation1 it will soon soften sufficiently to be worked4 ,the beginner will find it best to decide the convenient degree of softness by trial4 ,with soda glass it does not much matter how soft the glass becomes1 for it remains viscous1 but with lead glass the viscosity persists for a longer time and then suddenly gives place to a much greater degree of fluidity4 ,footnote3 ,this is only drawn from my impressions acquired in glass-working4 ,i have never explicitly tested the matter experimentally4 ,it is just at this point that a beginner will probably meet with his first difficulty4 ,as soon as the glass gets soft he will find that he no longer rotates the glass at the same speed by the right and left hand1 and1 moreover1 he will probably unconsciously bend the tube1 and even deform it1 by pushing or pulling4 ,the second third of the art of the glass-blower consists in being able to move both hands about1 rotating a tube with each thumb and finger1 and keeping the distance between the hands1 and also the speed of rotation1 constant4 ,nothing but long practice can give this facility1 but it is essential that it be acquired to some extent1 or no progress can be made4 ,some people acquire a moderate proficiency very quickly1 others1 of whom the writer is one1 only become reasonably proficient by months1 or even years1 of practice4 ,supposing that the tube is now ready to be drawn down1 the operator will remove it from the flame1 and will gently pull the ends apart1 interrupting his turning as little as possible4 ,if the tube be pulled too hard1 or if the area heated be too small 7about three-eighths of an inch in length in the case given would be proper71 it will be found that the ends of the two portions of the tube will be nearly closed at a very sharp angle 7nearly a right angle to the length of the tube71 that the ends will be thin1 and that a long length of very fine tube will be produced4 ,to heat a short length of tube and pull hard and suddenly is the proper way to make a very fine capillary tube1 but1 in general1 this is what we want to avoid4 ,if the operation be successfully performed1 the drawn-down tube will have the appearance exhibited1 which is suitable either for subsequently closing or handling by means of the drawn-down portion4 ,the straightness of the point can be obtained by a little practice in feeling the glass when the tube is rotated as it cools just before it loses its viscous condition4 ,when the operation is carried out properly the shoulder of the draw should be perfectly symmetrical and of even thickness1 and its axis regarded as that of a cone should lie in the axis of the tube produced4 ,the operation should be repeated till the student finds that he can produce this result with certainty1 and he should not be discouraged if this takes several days1 or even weeks4 ,of course1 it is probable that within the first hour he will succeed in making a tolerable job1 but it is his business to learn never to make anything else4 ,fig4 #i4 ,fig4 #aj4 ,diagram of a folded end4 #bj4 ,closing and blowing out the ,end of a ,tube4 ,when it is desired to close the end of a particular bit of tube1 this is easily done by heating the end1 and at the same time heating the end of a waste bit of tube or rod2 the ends1 when placed in contact1 stick together1 and a point can be drawn down as before4 ,footnote3 ,point is here used in the technical sense1 i4e4 it is a thin tail of glass produced by drawing down a tube4 ,having got a point1 it will be found that the thin glass cools enough to allow of the point being handled after a few moments4 ,the most convenient way of reducing the point to a suitable length 7say #a4e inch7 is to fuse it off in the flame1 but this must be done neatly2 if a tail is left it may cause inconvenience by catching1 or even piercing the finger and breaking off4 ,the blow-pipe flame being turned down to a suitable size1 and the shoulder of the draw having been kept warm meanwhile1 let the tip of the flame impinge on a point where the diameter is about half that of the undrawn tube1 and let the temperature be very high 7,fig4 #aa74 ,the tube is to be inclined to the flame so that the latter strikes the shoulder normally1 or nearly so4 ,then1 according to circumstances1 little or much of the glass can be removed at will by drawing off the tail 7,fig4 #ab71 till1 finally1 a small drop of melted glass only1 adheres to the end of the now closed tube 7,fig4 #ac74 ,fig4 #aa4 ,fig4 #ab4 ,fig4 #ac4 ,fig4 #ad4 ,when this is satisfactorily accomplished1 heat the extreme end of the tube most carefully and equally1 holding it in such a position that the glass will tend to flow from the bead back on to the tube1 i4e4 hold the closed end up to the flame1 the tube being1 say1 at #de degrees to the horizontal4 ,then when the temperature is such as to indicate complete softness lift the tube to the mouth1 still holding the tube pointing with its closed end a little above the horizontal1 and blow gently4 ,a beginner almost always blows too hard4 ,what is wanted1 of course1 is a continued pressure1 to give the viscous glass time to yield gradually1 if it is uniform2 or else intermittent puffs to enable the thinner parts1 if there are any1 to cool more1 and hence become more resisting than the thicker ones4 ,in any case a little practice will enable the operator to blow out a round and even end--neither thicker nor thinner than the rest of the tube4 #ba4 ,to make a ,weld4 ,to begin with1 try on two bits of glass of the same size1 i4e4 cut a seven-inch length of glass in half by scratching it with the knife1 and pulling the ends apart with a slight inclination away from the scratch4 ,in other words1 combine a small bending moment with a considerable tensional stress4 ,it is important to learn to do this properly4 ,if the proportions are not well observed1 the tube will break with difficulty1 and the section will not be perpendicular to the main length4 ,if the knife is in good order it will make a fine deep scratch--the feel of the glass under the knife will enable the operator to decide when the scratch is made4 ,the operation of cutting large tubes will be treated further on4 ,the two halves of the tube being held one in each hand1 and one tube closed at one end1 the extremities to be united will be warmed1 and then put in the flame as before4 ,fig4 #af4 ,there are many ways of proceeding--perhaps the easiest is as follows4 ,as soon as the glass shows signs of melting at the ends--and care should be taken that much more is not heated--take both bits out of the flame4 ,stop rotating for a moment1 and resting the arms carefully on the edge of the table1 raise the tubes above the flame and bring the ends swiftly and accurately together4 ,this is a case of sudden death no second attempt at making the ends meet can be allowed2 if the tubes join in any other than a perfectly exact manner a kink more or less objectionable will result4 ,in practice the operator will learn to bring the ends together1 commencing at one point2 i4e4 the axes of the tubes will be inclined at first1 so as to cause adherence at one spot only4 ,if this is not quite fair1 then less damage is done in moving one tube slightly up or down to get the contact exact4 ,the tubes will then be closed upon one another as if they were hinged at the joint4 ,this must be done lightly1 yet sufficiently1 to ensure that the glass is actually in contact all round4 ,having gone so far1 replace the tubes--now one--in the flame1 and carefully rotating the glass1 raise the temperature higher than in the operation just described1 in fact the higher the temperature1 short of burning the glass1 the better4 ,take the tube out of the flame and blow into the open end1 turning constantly as before4 ,one puff is enough4 ,then turn and pull the glass apart till it is of the same diameter and thickness throughout1 and feel that it is straight as before4 ,though it is in general of high importance that the joint should be well heated1 the beginner will probably find that he ties up his glass as soon as it gets really soft4 ,if his object is to make one joint--at any cost--then let him be careful to use two bits of exactly the same kind of glass1 and only get the temperature up to the viscous stage4 ,if the joint be then pulled out till it is comparatively thin1 it will probably stand 7if of soda glass72 certainly1 if of lead glass1 though in this case it may not be sound4 ,in any case the joint should be annealed in the asbestos box if practicable1 otherwise 7unless between narrow tubes7 with the asbestos rag4 ,care must be taken that the asbestos is dry4 #bb4 ,to weld two ,tubes of different ,sizes4 ,to do this1 the diameter of the larger tube must be reduced to that of the smaller4 ,the general procedure described in drawing down must be followed1 with the following modification4 ,in general1 a greater length of the tube must be heated1 and it must be made hotter4 ,the tube is to be gradually drawn in the flame with constant turning till the proper diameter and thickness of glass are attained4 ,fig4 #af4 ,for this operation time must be allowed if the operator's hands are steady enough to permit of it2 the shoulder should form partly by the glass sinking in and partly by the process of drawing the hot glass out4 ,a shoulder properly prepared is shown in the sketch4 ,beginners generally make the neck too thin on large tubes1 and too thick on smaller ones4 ,there ought to be no great difference in thickness of glass between the neck on the larger tube1 and the smaller tube4 ,the diameters should be as nearly as possible alike4 ,having drawn down the larger tube to a neck1 take it out of the flame1 and as it cools pull and turn till the neck is of the right thickness and is perfectly straight1 i4e4 make the final adjustment outside the flame1 and to that end have the neck rather too thick 7as to glass7 before it is taken out4 ,it is not necessary to wait till the neck gets cold before the end can be cut off4 ,make a scratch as before--this will probably slightly damage the temper of the file knife1 but that must be put up with4 ,hold the tube against the edge of the table1 so that the scratch is just above the level of the rim1 and strike the upper part a smart blow with the handle of the glass knife rather in the direction of its length4 ,footnote3 ,a bit of hoop iron nailed against the side of the table is a very convenient arrangement1 and it need not project appreciably above the general level of the rim4 ,of course this applies to a tube where economy has been exercised and the end is short4 ,if the tail is long enough to form a handle1 the tube may be pulled apart as before4 ,as a rule a temporary joint between a tube and a rod is not strong enough to enable the shoulder to be broken at the scratch by mere pulling4 ,the ends to be welded must be broken off very clean and true4 ,subsequent operations are to be carried out as already described4 #bc4 ,the above operations will be easily performed on tubes up to half an inch in diameter1 if they are not too long4 ,it is the length of tube1 and consequent difficulty in giving identity of motion with the two hands1 which make the jointing of long tubes difficult4 ,there are also difficulties if the tubes are very thin1 have a very fine bore or a very large diameter4 ,all these difficulties merely amuse a good glass-blower1 but to an experimenter who wants to get on to other things before sufficient skill is acquired 7in the movement of the hands and arms7 the following method is recommended4 ,first1 use flint glass4 ,then1 assuming that any drawing down has to be done1 do it as well as possible1 for on this the success of the method to be described especially depends4 ,be sure that the tubes to be welded are cut off clean and are as nearly as may be of the same size at the point of junction4 ,to fix the description1 suppose it is desired to join two tubes 7see ,fig4 #h71 each about one inch in diameter and a yard long4 ,get four clip stands and place them on a level table4 ,be sure that the stands are firm and have not warped so as to rock4 ,in each pair of clips place a tube1 so that the two tubes are at the same height from the table1 and1 in fact1 exactly abut1 with axes in the same straight line4 ,close one tube by a cork and then fix the blowing apparatus as shown to the other4 ,in such an operation as this the drying apparatus may be dispensed with1 and a rubber tube simply connected to one end of the system and brought to the mouth4 ,take the oxygen blow-pipe and turn the nozzle till the flame issues towards you1 and see that the flame is in order4 ,then turn down the oxygen till it only suffices to clear the smoky flame1 and commence to heat the proposed joint by a current of hot air1 moving the flame round the joint4 ,finally1 bring to bear the most powerful flame you can get out of the blow-pipe1 and carry it round the joint so quickly that you have the latter all hot at once4 ,put down the blow-pipe1 and1 using both hands1 press the tubes together 7which wooden clips will readily allow71 and after seeing that the glass has touched everywhere1 pull the tubes a trifle apart4 ,apply the blow-pipe again1 passing lightly over the thin parts1 if any1 and heating thicker ones2 having the end of the rubber tube in his mouth1 the operator will be able to blow out thick places4 ,when all is hot1 blow out slightly1 and having taken the flame away1 pull the tubes a little apart1 and see that they are straight4 ,throw an asbestos rag over the joint1 loosen one pair of the clamps slightly1 and leave the joint to anneal4 ,it is important that the least possible amount of glass should be heated1 hence the necessity of having the ends well prepared1 and it is also important that the work should be done quickly2 otherwise glass will flow from the upper side downwards and no strong joint will be obtained4 ,fig4 #ag4 ,tube being opened at one end4 #bd4 ,to weld ,tubes of very small ,bore4 ,if the bore is not so small as to prevent the entrance of the point of the iron nail1 get the ends of the tubes hot1 and open the bore by inserting the end of the nail previously smeared over with a trace of vaseline4 ,work the nail round by holding the handle between the thumb and first finger of the right hand1 the tube being similarly placed in the left4 ,the tube and nail should be inclined as shown in the sketch4 ,never try to force the operation2 the nail soon cools the glass1 so that only a very short time is available after each heat2 during this the tube should be rotated against the nail rather than the nail against the tube4 ,be careful not to heat a greater length of tube than is necessary1 or the nail will1 by its component of pressure along the tube1 cause the latter to jump up or thicken and bulge4 ,both ends being prepared1 and if possible1 kept hot1 the weld may be made as before1 and the heating continued till the glass falls in to about its previous thickness1 leaving a bore only slightly greater than before4 ,it is in operations such as this that the asbestos box will be found of great use4 ,as soon as one end of the weld is ready cool it in the flame till soot deposits1 and then plunge it into the asbestos4 ,this will cause it to cool very slowly1 and renders it less likely to crack when again brought into the flame4 ,turned-out ends1 if the glass is at all thick1 are very liable to crack off on reheating1 so that they must be reintroduced 7into the flame7 with especial care4 ,this liability to breakage is reduced1 but not eliminated1 by the asbestos annealing4 ,figs4 #ah and #ai4 #be4 ,when the bore is very fine1 it is best to seal off the tubes1 and blow an incipient bulb near one end of each tube4 ,these bulbs may be cooled in asbestos1 and cut across when cold by means of a scratch touched at one end 7,figs4 #ah and #ai7 by a fine point of highly incandescent glass4 ,for details of this method see p4 #df1 ,fig4 #ba4 ,time is occasionally saved by blowing off the ends of the bulbs4 ,the details of this process will be described when the operation of making thistle-headed tubes is dealt with4 #bf4 ,when the tubes are both of large diameter1 long1 and very thin 7cylinder tubes71 a considerable amount of difficulty will be experienced4 ,on the whole1 it is best to heat each end separately till the glass thickens a little1 anneal in the flame and in asbestos1 and then proceed as in #bb4 ,if the ends are not quite true1 it will be found that quite a thickness of glass may be jumped together at one side of the tubes1 while the edges are still apart at the other4 ,when this looks likely to happen1 incline the tubes as if the joint were a hinge1 and bend back quickly2 do not simply continue to push the tubes together in a straight line1 or an unmanageable lump of glass will be formed on one side4 ,if in spite of these precautions such a lump does form1 proceed as follows4 ,take a rod of glass1 at least one-eighth of an inch thick1 and warm it in the flame at one end4 ,heat the imperfect joint till it softens all round1 and then bring the flame right up to the thick part1 and heat that as rapidly and locally as possible4 ,the oxygas flame does this magnificently4 ,press the heated end of the glass rod against the thick part1 and pull off as much of the lump as it is desired to remove1 afterwards blowing the dint out by a judicious puff4 ,finish off as before4 #bg4 ,occasionally1 when it is seen that in order to produce a joint closed all round1 one side of the tube would be too much thickened1 it is better to patch the open side4 ,for this purpose take a glass rod about one-sixteenth inch in diameter1 and turn the flame to give its greatest effect1 still keeping rather an excess of air or oxygen4 ,see that the side of the joint already made is kept fairly hot--it need not be soft2 interrupt any other work often enough to ensure this4 ,then1 directing the flame chiefly on the thin rod1 begin to melt and pull the glass over the edges of the gap4 ,when the gap is closed get the lump very hot1 so that all the glass is well melted together1 and then1 if necessary1 pull the excess of glass off1 as before described4 ,it must be remembered that this and the method of the previous section are emergency methods1 and never give such nice joints as a manipulation which avoids them1 i4e4 when the ends of the tubes are perfectly straight and true to begin with4 ,also note that1 as the tubes cannot be kept in rotation while being patched1 it is as well to work at as low a temperature as possible1 consistently with the other conditions1 or the glass will tend to run down and form a drop1 leaving a correspondingly thin place behind4 ,fig4 #bj4 #bh4 ,a very common fault in cutting a tube of about an inch in diameter is to leave it with a projecting point1 as shown4 ,this can be slowly chipped off by the pliers1 using the jaws to crush and grind away the edge of the projection2 it is fatal to attempt to break off large pieces of glass all at once4 #bi4 ,it will be convenient here to mention some methods of cutting large tubes4 ,with tubes up to an inch and a half in diameter1 and even over this--provided that the glass is not very thick--we may proceed as follows3 ,make a good scratch about half an inch long1 and pretty deep1 i4e4 pass the knife backwards and forwards two or three times4 ,press a point of melted glass exactly on one end of the scratch2 the glass point even when pressed out of shape should not be as large as a button one-twelfth of an inch in diameter4 ,if this fails at first1 repeat the operation two or three times4 ,fig4 #ba4 ,if a crack does not form1 touch the hot place with the cold end of the nail4 ,if no success is obtained1 try the other end of the scratch4 ,if failure still pursues the operator1 let him make another cut on the opposite side of the tube and try again4 ,in general1 the tube will yield the first or second time the hot drop of glass is applied4 ,never apply the drop at the centre of the scratch1 or a ragged crack1 which may run in any direction1 will result4 ,very often1 with a large tube1 the crack formed by a successful operation will only extend a short distance4 ,in this case it is desirable to entice the crack round the tube1 and not trust to its running straight when the tube is pulled apart4 ,on the whole1 the best method in this case is to employ a flame pencil1 which should be kept ready for use4 ,this merely consists of a bit of glass tube of about the same dimensions as an ordinary lead pencil1 drawn down to a very fine jet at one end4 ,the jet must not be very long or thin1 or the glass will soon fuse up4 ,a few trials will enable the operator to get the proper proportions1 which are such that the tube has the general appearance of a pencil normally sharpened 7say with a cone of #fj'74 ,this tube is best made of hard glass4 ,connect it to a gas supply by light flexible tubing1 and turn down the gas till the flame from the end of the jet is not more than one-tenth of an inch long4 ,then apply the jet1 beginning from the end of the crack1 and gradually draw it 7the crack7 round the tube4 ,the operation will be assisted if a rubber ring is slipped on the tube to begin with1 so that the eye has some guide as to whether the flame is being drawn round properly or not4 ,the ring must1 of course1 be far enough away to escape the effect of the flame4 ,the crack will be found to follow the flame in the most docile manner1 unless the tube is thick or badly annealed4 ,some operators recommend a pencil of glowing charcoal1 but the flame is undoubtedly better4 #cj4 ,to cut very thick ,tubes4 ,a large number of methods have been proposed1 and nearly everybody has his favourite4 ,the following has always succeeded with me4 ,first mark on the tube1 by means of a little dead black spirit paint1 exactly where the cut is to be4 ,then sharpen the glass knife and scratch a quite deep cut all round3 there is no difficulty in making the cut one-twentieth of an inch deep4 ,it will be proper to lubricate the knife with kerosene after the first mark is made4 ,footnote3 ,the edge of the knife may be advantageously saved by using an old file moistened with kerosene for the purpose4 ,i find kerosene is not worse1 but1 if anything1 better than the solution of camphor in turpentine recommended by ,mr4 ,shenstone4 ,if the glass is about one-eighth of an inch thick1 the scratch maybe conveniently about one-twentieth of an inch deep1 but if the glass is anything like one-quarter of an inch thick1 the scratch must be much deeper1 in fact1 the glass may be half cut through4 ,to make a very deep scratch1 a wheel armed with diamond dust1 which will be described later on1 may be used4 ,however1 it is not essential to use a diamond wheel1 though it saves time4 ,when the cut is made to a sufficient depth proceed thus3 ,obtain two strips of bibulous paper or bits of tape and twist them round the tube on each side of the scratch1 allowing not more than one-eighth of an inch between them4 ,then add a few drops of water to each1 till it is thoroughly soaked1 but not allowing water to run away4 ,dry out the scratch by a shred of blotting paper4 ,turn down the oxygas flame to the smallest dimensions1 and then boldly apply it with its hottest part playing right into the nick and at a single point4 ,probably in about two seconds1 or less1 the tube will break4 ,if it does not1 rotate the tube1 but still so that the flame plays in the nick4 ,after making the tube very hot all round--if it has not broken--apply the flame again steadily at one point for a few seconds and then apply a bit of cold iron4 ,if the tube does not break at once during these processes1 let it cool1 and cut the groove deeper2 then try again4 ,footnote3 ,this method is continually being reinvented and published in the various journals4 ,it is of unknown antiquity4 ,fig4 #bb4 ,if the tube breaks after great heating and long efforts1 it will probably leave incipient cracks running away from the break1 or may even break irregularly4 ,a good break is nearly always one that was easily made4 ,if a number of rings have to be cut1 or a number of cuts made on glass tubes of about the same size1 it will be found economical in the end to mount a glazier's diamond for the purpose4 ,a simple but suitable apparatus is figured 7,fig4 #bc74 ,fig4 #bc4 ,the only difficulty is to regulate the position of the diamond so that it cuts4 ,in order to do this1 carefully note its cutting angle by preliminary trials on sheet glass1 and then adjust the diamond by clamps1 or by wriggling it in a fork1 as shown4 ,weight the board very slightly1 so as to give the small necessary pressure1 and produce the cut by rotating the tube by hand4 ,when a cut is nearly completed take great care that the two ends join1 or irregularity will result4 ,this is not always easy to do unless the tube happens to be straight4 ,having got a cut1 start a crack by means of a fine light watchmaker's hammer1 or even a bit of fused glass1 and entice the crack round the cut by tapping with the hammer or by means of the flame pencil4 ,if the cut is a true cut the tube will break at once4 ,as a supply of electrical current for lighting will1 in the near future1 be as much a matter of course for laboratory purposes as a gas supply1 ,i add the following note4 ,to heat a tube round a scratch1 nothing--not even the oxygas blow-pipe--is so good as a bit of platinum or iron wire electrically heated4 ,if the crack does not start by considerable heating of the glass1 stop the current1 unwind the wire1 and touch the glass on the crack either with a bit of cold copper wire or a wet match stem4 ,i prefer the copper wire1 for in my experience the water will occasionally produce an explosion of cracks4 ,on the other hand1 the cold wire frequently fails to start a crack4 ,judging from the appearance of thick tubes as supplied by the dealers1 the factory method of cutting off appears to be to grind a nick almost through the tube1 and right round2 and for really thick glass this is the safest but slowest way2 a thin emery wheel kept wet will do this perfectly4 ,suitable wheels may be purchased from the ,norton ,emery ,wheel ,co4 of ,bedford1 ,mass1 ,u4,s4,a4--in ,england through ,messrs4 ,churchill and ,co4 of ,london1 importers4 #ca4 ,to blow a ,bulb at the ,end of a ,tube4 ,i must admit at once that this is a difficult operation--at all events1 if a large bulb is required4 ,however1 all there is to be said can be said in few words4 ,in general1 when a bulb is required at the end of a tube it will be necessary to thicken up the glass4 ,a professional glass-worker will generally accomplish this by jumping up the tube1 i4e4 by heating it where the bulb is required1 and compressing it little by little until a sufficient amount of glass is collected4 ,the amateur will probably find that he gets a very irregular mass in this way1 and will be tempted to begin by welding on a short bit of wide and thick tubing preparatory to blowing out the bulb4 ,however1 supposing that enough glass is assembled by-either of these methods1 and that it is quite uniform in thickness1 let the thickened part be heated along a circle till it becomes moderately soft1 and let it then be expanded about one-fifth1 say by gently blowing4 ,it is perhaps more important to keep turning the glass during bulb-blowing than in any other operation1 and this both when the glass is in the flame and while the bulb is being blown4 ,it is also very important to avoid draughts4 ,in general1 a bulb is best blown with the tube in a nearly horizontal position1 but sloping slightly upwards from the mouth4 ,if it be noticed that a bulb tends to blow out more at one side than another1 let the side of greatest protuberance be turned down1 so that it is at the lowest point1 reduce the pressure for an instant1 and then blow again4 ,it will be observed that the bulb will now expand at the top4 ,the reason of this is chiefly that the under side cools most rapidly 7according to ,faraday1 ,chemical ,manipulation1 #aaid71 and consequently can expand no further2 but also it is not unlikely that the glass tends to flow somewhat from the upper side1 which remains hot1 and consequently the bulb1 when the next puff reaches it1 will tend to yield at this point4 ,by heating several zones the tube will become gradually expanded4 ,fig4 #bd4 ,fig4 #be4 ,fig4 #bf4 ,when the length of the thickened part of the tube only slightly exceeds its diameter 7,fig4 #be71 let the whole be brought to a temperature which1 with flint glass1 should be just short of that of perfect fluidity2 and then1 holding the tube horizontally and constantly turning it1 let the bulb be blown out to its full size1 noting the appearances and correcting too great protuberance on any side by the means above mentioned4 ,if the bulb appears pear-shaped turn the tube so that the melted mass is directed upwards2 if the bulb have the contrary fault1 correct in the corresponding manner4 ,the bulb when finished may be lightly tapped on the table1 when1 if there is any weak place owing to inequality of thickness1 the bulb will break1 and the operation may be started afresh4 ,a good bulb is round1 set truly on the tube1 and free from lumps of thick glass or places of excessive thinness4 ,when the amateur has succeeded in blowing a bulb two inches in diameter on the end of a strong bit of thermometer tube--say for an air thermometer--he may well seek the congratulations of his friends4 ,in case the bulb is not satisfactory on a first attempt1 it may be melted down again1 if the following precautions are taken4 ,directly creases begin to appear in the bulb let it be withdrawn from the flame1 and gently blown till the creases come out4 ,by alternate heating and blowing the glass can be got back to its original form1 or nearly so1 but unless the operator shows great skill and judgment1 the probability is that the glass will be uneven4 ,by heating and keeping the thicker parts in the higher position1 and blowing a little now and again1 the glass may be got even1 and a new attempt may be made4 ,it must not be supposed that this process can be carried on indefinitely1 for the glass tends to lose its viscous properties after a time1 or1 at all events1 it perishes in some way1 especially if it has been allowed to get very thin2 consequently too frequent attempts on the same glass are unprofitable4 ,two or three trials are as many as it generally pays to make4 ,as a rule the largest possible flame may be used with advantage in this operation4 #cb4 ,to blow a bulb in the middle of a tube1 the procedure is much like that already treated1 but the manipulation is1 if anything1 more difficult1 for the further end of the tube must be carried and turned as well as the end which is held to the lips4 #cc4 ,to make a side ,weld4 ,this is by no means difficult1 but is easier with lead glass than with soda glass4 ,the tube to which it is desired to make a side connection having been selected1 it is closed at one end by rubber tube stops1 or in any other suitable manner4 ,the zone of the proposed connection is noted1 and the tube is brought to near softness round that circle 7if the tube is made actually soft1 inconvenience will arise from the bending1 which is sure to occur74 ,two courses are then open to the operator1 one suitable to a thick tube1 the other to a tube of moderate thickness4 ,taking the former first4 ,provide a piece of glass rod and warm its end4 ,direct a small flame against the spot on the thick tube where the proposed joint is to be4 ,when the glass becomes almost incandescent at this spot1 put the end of the rod against it and draw out a thread of glass till sufficient metal has been removed4 ,then fuse off the thread close to the tube4 ,fig4 #bg4 ,the subsequent procedure is the same as for thin tubes4 ,in this case heat the spot by the smallest flame available1 and get the spot very hot4 ,blow it out gently into a bubble1 perhaps extending to a height equal to its diameter4 ,then heat the top of the bubble till it is incandescent and blow violently4 ,this will produce an opening fringed by glass so thin as to exhibit interference colours4 ,remove the filmy part1 and heat the frayed edges till they cohere and form an incipient tube4 ,if the flame has been of a correct size1 the tube will now be of the same diameter as the tube to be welded on1 and will project perhaps one-sixteenth of an inch from the surface of the main tube 7,fig4 #bh74 ,fig4 #bh4 ,fig4 #bi4 ,when this stage is reached1 again heat the tube all round till it nearly softens1 and by means of the other hand heat the end of the other tube which it is proposed to weld4 ,just before the main tube actually softens1 turn it so as to heat the edges of the aperture1 and at the same time get the end of the side tube very hot4 ,take both out of the flame for an instant1 and press the parts together1 instantly slightly withdrawing the side tube4 ,if the operation is well performed1 it will be found that the point of maximum thickness of glass is now clear of the main tube4 ,the joint is then to be heated all round and blown out--a rather awkward operation1 and one requiring some practice1 but it can be done4 ,fig4 #cj4 ,if great strength is wanted1 heat the main tube all round the joint bit by bit1 and blow each section slightly outwards4 ,if the operator is confident in his skill1 he should then heat the whole joint to the softening point1 blow it out slightly1 and then adjust by pulling and pushing4 ,cool first in the gas flame1 and then plunge the joint into the asbestos and cover it up--or if too large1 throw the asbestos cloth round it4 ,in the case of soda glass this final general heat is almost essential1 but it is not so with flint glass1 and as the general heat is the most difficult part of the job1 it will be found easier to use lead glass and omit the general heating4 ,with soda glass a very small irregularity will cause the joint to break when cold1 but flint glass is much more long-suffering4 ,it is easy to perform the above operation on small tubes4 ,for large ones it will be found best to employ flint glass and use the clip stands as in the case of direct welds1 treated above1 but1 of course1 with suitable modifications4 ,never let the main tube cool after the hole is made until the work is done4 #cd4 ,inserted ,joints4 ,in many instances the performance of apparatus is much improved by joints of this kind1 even when their use is not absolutely essential4 ,there are two ways in which inserted joints may be made4 ,the first method is the easier1 and works well with flint glass2 but when one comes to apply it to soda glass there is a danger of the glass becoming too thick near the joint1 and this often leads to a cracking of the joint as the glass cools4 ,fig4 #ca4 ,suppose it is desired to insert the tube ,b into the tube ,a 7,fig4 #ca74 ,begin by reducing the size of the end of tube ,a till ,b will just slip in quite easily4 ,with ,b about one-quarter inch in diameter1 a clearance of about one-twentieth of an inch1 or less1 in all 7i4e4 one-fortieth of an inch on each side7 will be proper4 ,heat ,b by itself at the proposed zone of junction1 and blow out a very narrow ring2 then compress this slightly so that it forms an almost closed ring of glass4 ,the figure refers to the close of this operation 7,fig4 #ca1 ,b74 ,it does not matter much whether the ring remains a mere flattened bulb1 or whether it is a solid ring1 but it must be one or the other4 ,some judgment must be exercised in preparing the ring4 ,in general1 the beginner will collect too much glass in the ring1 and consequently the joint1 when made1 will either be thick and liable to crack easily1 or it will be blown out into an erratic shape in endeavours to reduce this thickness4 ,accordingly1 the operator will1 if necessary1 thin the tube ,b by drawing slightly1 if he considers it desirable1 before the little enlargement is blown out4 ,in general1 two heats must be used for this operation4 ,fig4 #cb4 ,get the approximating parts of both ,a and ,b up to a temperature just below that at which they will adhere1 and having closed the other end of ,a1 place ,b carefully within it up to the ring1 and if it can be arranged1 have a mica wad in ,a1 with a central hole through which the end of ,b can project4 ,this will very much facilitate the operation1 especially if ,b is long1 but may be dispensed with by the exercise of care and skill4 ,the operation is now simple4 ,fuse the junction and press the tubes lightly together1 being careful not to collect more glass than can be helped2 finally1 blow out the joint and reduce the thickness by mild drawing 7,fig4 #cc74 ,in order to make a really good joint1 two points must be particularly attended to--the rim must be thin and its plane perfectly perpendicular to the axis of tube ,b2 the end of tube ,a must be cut off quite clean and perpendicular to its axis before ,b is inserted4 ,so important are these conditions--especially the latter that the writer has even occasionally used the grindstone to get the end of ,a into a proper condition1 an admission which will probably earn the contempt of the expert glass-worker4 ,fig4 #cc4 ,now for the second method1 which is often practised in ,germany1 where soda glass is chiefly used4 ,with this glass the chief point is to get a very even and not too thick ring at the junction1 and consequently the extra thickening produced by making a rim on ,b is rather a drawback4 ,the method consists in cutting off from ,b the length which it is desired to insert1 slipping this into ,a 7which may be an otherwise closed bulb1 for instance71 and then gradually melting up the open end of ,a till the piece of ,b inside will no longer fall out4 ,by holding the joint downwards so that the inserted portion of ,b rests on the edges of the opening1 a joint may be made with the minimum thickening4 ,the external part of ,b1 previously heated1 is then applied1 and the joint subjected to a general heat and blown out4 ,very nice joints may be made by this method1 and it is perhaps the better one where the external part of ,b is to be less in diameter than the inserted part4 ,it was in this manner that the writer was taught to make glass velocity pumps1 one of which1 of a good design1 is figured as an example4 ,in all cases good annealing should follow this operation4 ,if the inserted part of the inner tube 7,b7 is anything like an inch in diameter1 and especially if it is of any length1 as in some forms of ozone apparatus1 or in a large ,bunsen's ice calorimeter1 the arrangements for supporting the inner part must be very good4 ,a convenient way of proceeding when the inner tube is well supported is to make the mouth of ,a only very little larger than the diameter of ,b1 so that ,b will only just slip in4 ,then the mouth of ,a and the zone of ,b may be heated together1 and ,b blown out upon ,a4 ,this1 of course1 must be arranged for1 if necessary1 by temporarily stopping the inner end of ,b4 ,the inner support of ,b should be removed as soon as practicable after the joint is made1 or1 at all events1 should not be perfectly rigid2 a tightly-fitting cork1 for instance1 is too rigid4 ,the reason is1 of course1 that in cooling there may be a tendency to set ,b a little to one side or the other1 and if it is not free to take such a set1 the joint most probably will give way4 ,good annealing both with flame and asbestos is a sine qua non in all inserted work4 ,fig4 #cd4 #ce4 ,bending ,tubes4 ,i have hitherto said nothing about bending tubes1 for to bend a tube of a quarter of an inch in diameter1 and of ordinary thickness1 is about the first thing one learns in any laboratory1 while to bend large tubes nicely is as difficult an operation as the practice of glass-blowing affords4 ,however1 even in bending a narrow tube it is possible to proceed in the wrong way4 ,the wrong way is to heat a short length of the tube and then bend it rapidly1 holding the plane of the bend horizontal4 ,the right way1 per contra1 is to use a batswing burner to heat1 say1 two inches of the tube with constant turning till it is very soft1 and then1 holding the glass so that the bend will be in a vertical plane passing through one eye 7the other being shut71 to make the bend rather slowly4 ,if an exact angle is required1 it is as well to have it drawn out on a sheet of asbestos board4 ,in this case bend the glass as described till it is approximately right1 and finish by laying it on the asbestos board and bringing it up to the marks4 ,a suitable bit of wood may be substituted for the asbestos on occasion4 ,n4,b3 ,the laboratory table is not a suitable piece of wood4 ,a right-angled bend is often wanted4 ,in this case the corner of a table will serve as a good guide to the eye1 the glass being finished by being held just above it4 ,if great accuracy is wanted1 make a wooden template and suspend it by a screw from the side of the table1 so that the vertex of the gauge for the interior angle projects downwards1 then finish by bending the tube round it4 ,the wood may be about half an inch thick4 ,if a sharp bend is required1 heat the tube in the blow-pipe1 and bend it rapidly1 blowing out the glass meanwhile4 ,the reason why a long bend should be held in a vertical plane is that the hot part tends to droop out of the plane of the bend if the latter be made in a horizontal position4 ,to bend a tube above half an inch in diameter is a more or less difficult operation1 and one which increases in difficulty as the diameter of the tube increases4 ,a ,u-tube1 for instance1 may be made as follows3 ,use the four blow-pipe arrangement so as to heat a fair length of tube1 and get1 say1 two inches of tube very hot--almost fluid1 in fact--by means of the carbon block supported from a stand4 ,remove the tube rapidly from the flame and draw the hot part out to1 say1 three inches4 ,then1 holding the tube so as to make the bend in a vertical plane1 bend it and blow it out together to its proper size4 ,this operation seems to present no difficulties to experienced glass-workers1 even with tubes of about one inch in diameter1 but to the amateur it is very difficult4 ,i always look on a large ,u-tube with feelings of envy and admiration1 which the complex trick work of an elaborate vacuum tube does not excite in the least4 ,it will be noted that this method may1 and often does1 involve a preliminary thickening of the glass4 ,with tubes over an inch in diameter ,i have no idea as to what is the best mode of procedure--whether1 for instance1 a quantity of sand or gas coke might not be used to stuff out the tube during bending1 but in this case there would be the difficulty of removing the fragments1 which would be sure to stick to the glass4 ,of course1 if the bend need not be short1 the tube could be softened in a tube furnace and bent in a kind of way4 ,i must admit that with tubes of even less than one inch in diameter ,i have generally managed best by proceeding little by little4 ,i heat as much of the glass as ,i can by means of a gigantic blow-pipe1 having a nozzle of about an inch in diameter1 and driven by a machine-blower4 ,when ,i find that1 in spite of blowing1 the tube begins to collapse1 ,i suspend operations1 reheat the tube a little farther on1 and so proceed4 ,if by any chance any reader knows a good laboratory method of performing this operation1 ,i hope he will communicate it to me4 ,after all1 the difficulty chiefly arises from laboratory heating appliances being as a rule too limited in scope for such work4 ,the bending of very thin tubes also is a difficulty4 ,i have only succeeded here by making very wide bends1 but of course the blowing method is quite applicable to this case1 and the effect may be obtained by welding in a rather thicker bit of tube1 and drawing and blowing it till it is of the necessary thinness4 ,this is1 however1 a mere evasion of the difficulty4 #cf4 ,spiral ,tubes4 ,these are easily made where good heating apparatus is available4 ,as1 however1 one constantly requires to bend tubes of about one-eighth inch in diameter into spirals in order to make spring connections for continuous glass apparatus1 ,i will describe a method by which this is easily done4 ,provide a bit of iron pipe about an inch and a quarter in outside diameter4 ,cover this with a thick sheath of asbestos cloth1 and sew the edges with iron wire4 ,hammer the wire down so that a good cylindrical surface is obtained4 ,make two wooden plugs for the ends of the iron pipe4 ,bore one to fit a nail1 which may be held in a small retort clip1 and fasten a stout wire crank handle into the other one4 ,support the neck of the handle by means of a second clip4 ,in this way we easily get a sort of windlass quite strong enough for our purpose4 ,fig4 #ce4 ,provide a large blow-pipe1 such as the blow-pipe of a ,fletcher crucible furnace1 ,select a length of tubing and clean it4 ,lash one end to the cylinder by means of a bit of wire1 and hold the other end out nearly horizontally4 ,then start the blow-pipe to play on the tube just where it runs on to the asbestos cylinder1 and at first right up to the lashing4 ,get an attendant to assist in turning the handle of the windlass1 always keeping his eye on the tube1 and never turning so fast as to tilt the tube upwards4 ,by means of the blow-pipe1 which may be moved round the tubing1 heat the latter continuously as it is drawn through the flame1 and lay it on the cylinder in even spirals4 ,if the tubing is thin1 a good deal of care will have to be exercised in order to prevent a collapse4 ,a better arrangement1 which1 however1 ,i have not yet tried1 would1 ,i think1 be to replace the blow-pipe by two bats-wing burners1 permanently fastened to a stand1 one of them playing its flame downwards on to the top of the flame of the other4 ,the angle between the directions of the jets might be1 say1 #acj1 or whatever is found convenient4 ,in this way the glass would not be so likely to get overheated in spots1 and better work would doubtless result4 ,however1 ,i have made numbers of perfectly satisfactory spirals as described4 ,three or four turns only make a sufficiently springy connection for nearly all purposes4 #cg4 ,on ,auxiliary ,operations on ,glass3- ,boring ,holes through ,glass3- ,this is much more easily done than is generally supposed4 ,the best mode of procedure depends on the circumstances4 ,the following three cases will be considered3- #a4 ,boring holes up to one-quarter inch diameter through thick glass 7say over one-eighth inch71 or rather larger holes through thin glass4 #b4 ,boring holes of any size through thick glass4 #c4 ,boring round holes through ordinary window glass4 #ch4 ,boring small ,holes4 ,take a three-cornered file of appropriate dimensions1 and snip the point off by means of a hammer2 grind out most of the file marks to get sharp corners4 ,dip the file in kerosene1 and have plenty of kerosene at hand in a small pot4 ,place the broken end of the file against the glass1 and with considerable pressure begin to rotate it 7the file7 backwards and forwards with the fingers1 very much as one would operate a bradawl against a hard piece of wood4 ,the surface of the glass will shortly be ground away1 and then the file bradawl will make much quicker progress than might be expected4 ,two or three minutes should suffice to bore a bit of sheet window-glass4 ,the following points require attention3 7#a7 ,use any quantity of oil4 7#b7 ,after getting through the skin reduce the pressure on the file4 7#c7 ,be sure to turn the file backwards and forwards through a complete revolution at least4 7#d7 ,when the hole is nearly through reduce the pressure4 7#e7 ,when the hole is through the glass be exceedingly careful not to force the file through too rapidly1 otherwise it will simply act as a wedge and cause a complete fracture4 7#f7 ,in many cases it is better to harden the file in mercury before commencing operations2 both files and glass differ so much in hardness that this point can only be decided by a trial4 ,if it is found necessary to harden the file1 use either a large blow-pipe and a coke or charcoal bed1 or else a small forge4 ,a small blowpipe1 such as is generally found in laboratories1 does more harm than good1 either by burning the end of the file or raising it to an insufficient temperature4 7#g7 ,to sharpen the file1 which is often necessary after passing through the skin of the glass1 put it in a vice so that the point just protrudes clear of the jaws4 ,then1 using a bit of waste iron as an intermediary anvil or punch1 knock off the least bit from the point1 so as to expose a fresh natural surface4 ,the same result may be brought about by the use of a pair of pliers4 ,if several holes have to be bored1 it is convenient to mount the file in the lathe and use a bit of flat hard wood to press up the glass by means of the back rest4 ,a drilling machine1 if not too heavy1 does very well1 and has the advantage of allowing the glass to remain horizontal so that plenty of oil can be kept in the hole4 ,use a very slow speed in either case--much slower than would be used for drilling wrought iron4 ,it is essential that the lubricant should flow on to the end of the file very freely1 either from a pipette or from the regular oil-feed4 ,if a little chipping where the file pierces the back surface is inadmissible1 it is better1 on the whole1 to finish the bore by hand1 using a very taper file4 ,it is not necessary to use a special file for the lathe1 for a well-handled file can be chucked very conveniently in a three-jaw chuck by means of the handle4 ,mr4 ,shenstone recommends a lubricant composed of camphor dissolved in turpentine for general purposes4 ,with the object of obtaining some decisive information as to the use of this lubricant1 and to settle other points1 ,i made the following experiments4 ,using an old three-cornered ,french file1 ,i chipped off the point and adjusted the handle carefully4 ,i also ground out the file marks near the point1 without hardening the file in mercury4 ,using kerosene and turpentine and camphor1 ,i began to bore holes in a hard bit of #c_/cb inch window glass4 ,each hole was bored to about one-eighth inch in diameter in four minutes with either lubricant4 ,after hardening the file in mercury and using kerosene1 ,i also required four minutes per hole4 ,after mounting the file in a lathe which had been speeded to turn up brass rods of about one-half-inch diameter1 and therefore ran too fast1 ,i required one and a half minutes per hole1 and bored them right through1 using kerosene4 ,on the whole1 ,i think kerosene does as well as anything1 and for filing is1 ,i think1 better than the camphor solution4 ,however1 ,i ought to say that the camphor-turpentine compound has probably a good deal to recommend it1 for it has survived from long ago4 ,my assistant tells me he has seen his grandfather use it when filing glass4 ,i beg to acknowledge my indebtedness to ,mr4 ,pye1 of the ,cambridge ,scientific ,instrument ,company1 for showing me in #ahhf 7by the courtesy of the ,company7 the file method of glass-boring2 it is also described by ,faraday in ,chemical ,manipulation1 #abbh4 ,it is not necessary1 however1 to use a file at all1 for the twist drills made by the ,morse ,drill ,company are quite hard enough in their natural state to bore glass4 ,the circumferential speed of the drill should not much exceed #aj feet per minute4 ,in this way the author has bored holes through glass an inch thick without any trouble except that of keeping the lubricant sufficiently supplied4 ,for boring very small holes watchmaker's drills may be used perfectly well1 especially those tempered for boring hardened steel4 ,the only difficulty is in obtaining a sufficient supply of the lubricant1 and to secure this the drill must be frequently withdrawn4 ,my reason for describing the file method at such length is to be found in the fact that a ,morse drill requires to be sharpened after drilling glass before it can be used in the ordinary way1 and this is often a difficulty4 ,i ought to say that ,i have never succeeded in boring the barrel of a glass tap by either of these methods4 ,footnote3 ,i have been lately informed that it is usual to employ a splinter of diamond set in a steel wire holder both for tap boring and for drilling earthenware for riveting4 ,the diamond must1 of course1 be set so as to give sufficient clearance for the wire holder4 ,for methods of using and setting diamond tools see #ee4 ,it will suffice to say here that a steel wire is softened and filed at one end so as to form a fork2 into this the diamond is set by squeezing with pliers4 ,the diamond is arranged so as to present a point in the axis of the wire1 and must not project on one side of the wire more than on the other4 ,it is not always easy to get a fragment satisfying these conditions1 and at the same time suitable for mounting4 ,a drop of solder occasionally assists the process of setting the diamond4 ,in drilling1 the diamond must be held against the work by a constant force1 applied either by means of weight or a spring4 ,i made many trials by this method1 using a watchmaker's lathe and pressing up the work by a weight and string1 which passed over a pulley4 ,i used about #dj ounces1 and drilled a hole #c_/cb in diameter in flint glass at a speed of #ijj revolutions per minute to a depth of one-eighth of an inch in eight minutes4 ,i used soap and water as a lubricant1 and the work was satisfactory4 ,since this was set up1 ,i have been informed by ,mr4 ,hicks of ,hatton ,garden that it is necessary to anneal glass rod by heating it up to the softening point and allowing it to cool very slowly under red-hot sand or asbestos before boring4 ,if this be done1 no trouble will be experienced4 ,the annealing must be perfect4 #ci4 ,for boring large holes through thick glass sheets1 or1 indeed1 through anything where it is necessary to make sure that no accident can happen1 or where great precision of position and form of hole is required1 ,i find a boring tube mounted as shown in the picture 7,fig4 #cf7 is of great service4 ,brass or iron tube borers do perfectly well1 and the end of the spindle may be provided once for all with a small tube chuck1 or the tubes may be separately mounted as shown4 ,a fairly high speed is desirable1 and may be obtained either by foot1 or1 if power is available1 is readily got by connecting to the speed cone of a lathe1 which is presumably permanently belted to the motor4 ,fig4 #cf4 ,after trying tubes armed with diamond dust1 as will be presently explained1 ,i find that emery and thin oil or turpentine1 if liberally supplied below the glass1 will do very nearly as well4 ,the tube should be allowed to rise from the work every few seconds1 so as to allow of fresh emery and oil being carried into the circular grooves4 ,this is done by lifting the hinged upper bearing1 the drill being lifted by a spiral spring between the pulley and the lower bearing shown at ,b4 ,the glass may be conveniently supported on a few sheets of paper if flat1 or held firm in position by wooden clamps if of any other shape4 ,in any case it should be firmly held down and should be well supported4 ,any desired pressure upon the drill is obtained by weighting the hinged board ,a4 #dj4 ,the following method was shown to me by ,mr4 ,wimshurst1 but ,i have not had occasion to employ it myself4 ,it is suitable for boring large holes through such glass as the plates of ,mr4 ,wimshurst's ,influence machines are usually made of4 ,a diamond is mounted as the pencil of a compass1 and with this a circle is drawn on the glass in the desired position4 ,the other leg of the compass of course rests on a suitable washer4 ,to the best of my recollection the further procedure was as follows4 ,a piece of steel rod about one-eighth inch in diameter was ground off flat and mounted in a vice vertically1 so as to cause its plane end to form a small horizontal anvil4 ,the centre 7approximately7 of the diamond-cut circle of the glass was laid on this anvil so as to rest evenly upon it1 and the upper surface 7i4e4 that containing the cut7 was then struck smartly with a hammer1 completely pulverising the glass above the anvil4 ,the hole was gradually extended in a similar manner right up to the diamond cut1 from which1 of course1 the glass broke away4 ,a similar method has long been known to glaziers1 differing from the preceding in that a series of diamond cuts are run across the circle parallel to two mutually perpendicular diameters4 ,a smart tap on the back of the scored disc will generally cause the fragments to tumble out4 ,i have never tried this myself1 but ,i have seen it done4 ,large discs may easily be cut from sheet glass by drawing a circular diamond cut1 and gradually breaking away the outer parts by the aid of additional cuts and a pair of pliers or shanks 7see ,fig4 #dd74 #da4 ,operations depending on ,grinding3 ,ground-in ,joints4 ,the process will be perfectly understood by reference to a simple case4 ,suppose it is desired to grind the end of a tube into the neck of a bottle4 ,if a stoppered bottle is available1 the stopper must be taken out and measured as to its diameter at the top and bottom4 ,select a bit of tube as nearly as possible of the same diameter as the stopper at its thickest part4 ,draw down the glass in the blow-pipe flame rather by allowing it to sink than by pulling it out4 ,after a few trials no difficulty will be experienced in making its taper nearly equal to that of the stopper1 though there will in all probability be several ridges and inequalities4 ,when this stage is reached anneal the work carefully and see that the glass is not too thin4 ,afterwards use emery and water1 and grind the stopper into the bottle4 ,there are six special directions to be note 7#a 7,turn the stopper through at least one revolution in each direction4 7#b7 ,lift it out often so as to give the fresh emery a chance of getting into the joint4 7#c7 ,rotate the bottle as well as the stopper in case there is any irregularity in the force brought to bear1 which might cause one side of the neck to be more ground than another1 or would cause the tube to set rather to one side or the other4 7#d7 ,use emery passing a #ej sieve1 i4e4 a sieve with fifty threads to the inch run 7see #add7 to begin with1 and when the stopper nearly fits1 wash this thoroughly away1 and finish with flour emery1 previously washed to get rid of particles of excessive size2 the process of washing will be fully discussed in the chapter on glass-grinding1 which see4 7#e7 ,any degree of fineness of surface may be obtained by using graded emery1 as will be explained1 but1 in general1 it is unnecessary to attempt a finer surface than can be got with washed flour emery4 ,a superficial and imperfect polish may be given by grinding for a short time with powdered pumice stone4 7#f7 ,if the proper taper is not attained by blowing1 or if ridges are left on the tapered part1 the process may be both hastened and improved by giving the taper a preliminary filing with a three-cornered file and kerosene1 just as one would proceed with iron or brass4 ,a little filing will often save a good deal of grinding and make a better job4 ,if a bottle without a tapered neck is to be employed1 it is as well to do the preliminary grinding by means of a cone turned up from a bit of cast iron4 ,this is put in the lathe and pushed into the mouth of the bottle1 the latter being supported by the hands4 ,use about the same surface speed as would be employed for turning cast iron4 ,in this case the emery is better used with kerosene4 ,if a cylindrical bit of cast iron about an inch in diameter is turned down conically nearly to a point1 it will save a good deal of trouble in making separate cones4 ,if it gets ground into rings1 and it becomes necessary to turn it up1 use a diamond tool until the skin is thoroughly removed2 the embedded emery merely grinds the edge off any ordinary steel tool4 ,for diamond tools see #ee4 #db4 ,use of the ,lathe in ,glass-working4 ,if it is necessary to remove a good deal of glass1 time may be saved by actually turning the glass in a lathe4 ,according to the direction given above for grinding a tube into the neck of a bottle1 very little glass need be removed if the drawing down is well done1 so that for this purpose turning is often unnecessary4 ,if the taper of the stopper be small and it is permissible to use a thick tube1 or if a solid stopper only has to be provided1 or an old stopper quickly altered to a new form1 turning is very useful4 ,the glass may be chucked in any suitable manner1 and run at a speed not exceeding #aj feet per minute4 ,prepare a three-cornered file by mercury-hardening and by grinding the end flat so as to form a cutting angle of about #hj1 and use a moderate amount of kerosene lubrication1 i4e4 enough to keep the glass damp1 but even this is not essential4 ,use the file as an ordinary brass turning tool1 and press much more lightly than for metal turning4 ,the glass will be found to scrape off quite pleasantly4 ,by chucking glass tubes on wooden mandrells the ends may be nicely turned in this manner ready for accurate closing by glass plates4 ,the process of grinding also is made much more rapid--at all events in the earlier stages--by chucking either the stopper or the bottle and holding the other member in the fingers1 or in a wooden vice held in the hands4 ,the finishing touches are best given by hand4 ,i ought to say that ,i think a good deal of glass-grinding1 as practised in laboratories1 might be advantageously replaced by glass turning or filing and certainly will be by any one who will give these methods a trial4 ,if one tube is to be ground into another1 as in grinding a retort into a receiver1 the latter must be drawn down from a larger piece1 few beginners being able to widen a tube by the method explained with sufficient ease and certainty4 ,the other operations are similar to the operations above described4 #dc4 ,funnels often require to be ground to an angle of #fj4 ,for this purpose it is well to keep a cast-iron cone1 tapering from nothing up to four inches in diameter4 ,this may be mounted on a lathe1 and will be found of great use for grinding out the inside of funnels4 ,care must be taken to work the funnel backwards and forwards1 or it will tend to grind so as to form rings1 which interfere with filtering4 ,a rough polish may be given on the lines explained in the next section4 #dd4 ,a rough polish may be easily given to a surface which has been finished by washed flour emery1 in the following manner4 ,turn up a disc of soft wood on the lathe1 and run it at the highest wood-turning speed4 ,rub into the periphery a paste of sifted powdered pumice stone and water4 ,any fairly smooth ground glass surface may be more or less polished by holding it for a moment against the revolving disc4 ,exact means of polishing will be described later on4 ,meanwhile this simple method will be found both quick and convenient1 and is often quite sufficient where transparency1 rather than figure1 is required4 ,i daresay a fine polish may be got on the same lines1 using putty powder or washed rouge 7not jewellers' rouge1 which is too soft1 but glass-polishers' rouge7 to follow the pumice powder1 but ,i have not required to try this4 #de4 ,it is sometimes required to give to ground glass surfaces a temporary transparency4 ,this is to be done by using a film of oil of the same refractive index as the glass4 ,cornu has employed a varnish consisting of a mixture of turpentine and oil of cloves1 but the yellow-brown colour of the latter is often a disadvantage4 ,it will be found that a mixture of nut oil and oil of bitter almonds1 or of bromo-napthalene and acetone1 can be made of only a faint yellow colour2 and by exact adjustment of the proportions will have the same refractive index for any ray as crown glass 7ordinary window glass74 ,procure a sample of the glass and smash it up to small fragments in an iron mortar4 ,sift out the fine dust and the larger pieces2 bits about as large as small beads--say one-sixteenth inch every way--do very well4 ,boil the sifted glass with strong commercial hydrochloric acid to remove iron1 wash with distilled water and a few drops of alcohol1 dry on blotting paper in the sun or otherwise4 ,put the dry glass into a bottle or beaker1 and begin by adding almond oil 7or bromo-napthalene71 then add nut oil 7or acetone7 till the glass practically disappears when examined by sodium light1 or light of any other wave-length1 as may be required4 ,the adjustment of the mixture is a matter of great delicacy1 one drop too much of either constituent1 in1 say1 #ej cubic centimetres1 makes all the difference4 ,the final adjustment is best accomplished by having two mixtures of the oils1 one just too rich in almond1 the other in nut oil2 by adding one or other of these1 the required mixture is soon obtained4 ,it is to be noted 7#a7 ,that adjustment is only perfect for light of one wave-length4 7#b7 ,that adjustment is only perfect at one temperature4 ,on examining a bottle of rather larger fragments of glass immersed in an adjusted mixture by ordinary daylight1 a peculiarly beautiful play of colours is seen4 ,of course1 if it is only desired to make ground glass fairly transparent1 these precautions are unnecessary1 but it seemed better to dispose of the matter once for all in this connection4 ,m4 ,cornu's object was to make a varnish which would prevent reflection from the back of a photographic plate on to the film4 ,i have had occasion to require to do the same when using a scale made by cutting lines through a film of black varnish on a slip of glass4 ,this succeeded perfectly by making the varnish out of ,canada balsam stained with a black aniline dye4 ,mr4 ,russell1 ,government ,astronomer of ,new ,south ,wales1 finds that the halation of star photographs can be prevented by pouring over the back of the plate a film of collodion suitably stained4 #df4 ,making ,ground ,glass4 ,this is easily done by rubbing the surface of polished glass with a bit of cast iron and washed flour of emery4 ,of course1 if the fineness of grain of the surface is of importance1 appropriate sizes of emery must be employed4 ,the iron may be replaced by a bit of glass cut with transverse grooves to allow the emery to distribute itself1 or even by a bit of glass without such grooves1 provided it does not measure more than one or two inches each way4 ,if great speed is an object rather than the fineness of the surface1 use a bit of lead and coarse emery1 say any that will pass a sieve with fifty threads to the inch4 ,it may perhaps be mentioned here that it is a pity to throw away emery which has been used between glass and glass4 ,in the chapter dealing with fine optical work the use of emery of various grades of fineness will be treated1 and the finer grades can only be obtained 7to my knowledge7 from emery which has been crushed in the process of glass or metal grinding1 especially the former4 ,a large jam-pot covered with a cardboard lid does well as a receptacle of washings4 #dg4 ,glass-cutting4 ,this is an art about which more can be learned in five minutes by watching it well practised than by pages of written description4 ,my advice to any one about to commence the practice of the art would be to make friends with a glazier and see it done4 ,what follows is therefore on the supposition that this advice has been followed4 ,after some experience of cutters made of especially hardened steel1 ,i believe better work can generally be got out of a diamond1 provided the cost is not an objection4 ,it is economy to pay a good price for a good diamond4 ,as is well known1 the natural angle of the crystal makes the best point1 and a person buying a diamond should examine the stone by the help of a lens1 so as to see that this condition is fulfilled4 ,the natural angle is generally1 if not always1 bounded by curved edges1 which have a totally different appearance from the sharp edges of a splinter4 ,when a purchase is to be made1 it is as well for the student to take a bit of glass and a foot-rule with him1 and to test the diamond before it is taken away4 ,when a good diamond has been procured1 begin by taking cuts on bits of clean window glass until the proper angle at which to hold the tool is ascertained4 ,never try to cut over a scratch1 if you value your diamond1 and never press hard on the glass2 a good cut is accompanied by an unmistakable ringing sound quite different from the sound made when the diamond is only scratching4 ,perhaps the most important advice that can be given is1 ,never lend the diamond to anybody--under any circumstances4 ,the free use of a diamond is an art which the physicist will do well to acquire1 for quite a variety of apparatus may be made out of glass strips1 and the accuracy with which the glass breaks along a good cut reduces such an operation as glass-box-making to a question of accurate drawing4 #dh4 ,cementing4 ,one of the matters which is generally confused by too great a profusion of treatment is the art of cementing glass to other substances4 ,the following methods will be found to work1 subject to two conditions3 7#a7 ,the glass must be clean2 7#b7 it must be hot enough to melt the cement4 ,for ordinary mending purposes when the glass does not require to be placed in water 7especially if hot7 nothing is better than that kind of glue which is generally called diamond cement4 ,this may be easily made by dissolving the best procurable isinglass in a mixture of #bj per cent water and #hj per cent glacial acetic acid--the exact proportions are not of consequence4 ,first1 the isinglass is to be tightly packed into a bottle with a wide neck1 then add the water1 and let the isinglass soak it up4 ,afterwards pour in the acetic acid1 and keep the mixture near #ajj;,c4 for an hour or two on the water bath--or rather in it4 ,the total volume of acetic acid and water should not be more than about half of the volume of isinglass when the latter is pressed into the bottle as tightly as possible4 ,the proper consistency of the cement may be ascertained by lifting a drop out of the bottle and allowing it to cool on a sheet of glass4 ,in ten minutes it ought not to be more than slightly sticky1 and the mass in the bottle1 after standing a few hours cold1 should not be sticky at all1 and should yield1 jelly-like1 to the pressure of the finger to only a slight degree4 ,if the glue is too weak1 more isinglass may be added 7without any preliminary soaking74 ,a person making the mixture for the first time almost always gets it too weak4 ,it is difficult to give exact proportions by weight1 as isinglass and gelatine 7which may replace it7 differ greatly in quality4 ,this cement is applied like glue1 and will cement nearly anything as well as glass4 ,of course1 as much cement as possible must be squeezed out of any joint where it is employed4 ,the addition of gums1 as recommended in some books1 is unnecessary4 ,ordinary glue will serve perfectly for cementing glass to wood4 ,chipped glass ware is1 ,i understand1 made by painting clean glass with glue4 ,as the glue dries and breaks by contraction1 it chips off the surface of the glass4 ,i have never seen this done4 ,in nearly all cases where alcohol is not to be employed very strong joints may be made by shellac4 ,orange shellac is stronger than the bleached variety4 ,a sine qua non is that the glass be hot enough to melt the shellac4 ,the best way is to heat the glass surfaces and rub on the shellac from a bit of flake2 the glass should not be so hot as to discolour the shellac appreciably1 or its valuable properties will be partly destroyed4 ,both glass surfaces being thus prepared1 and the shellac being quite fluid on both1 they may be brought together and clamped tightly together till cool4 ,shellac that has been overheated1 or dissolved in alcohol1 or bleached1 is of little use as compared with the pale orange flaky product4 ,dark flakes have probably been overheated during the preliminary refining4 ,for many purposes a cement is required capable of resisting carbon bisulphide4 ,this is easily made by adding a little treacle 7say #bj per cent7 to ordinary glue4 ,since the mixture of glue and treacle does not keep1 i4e4 it cannot be satisfactorily melted up again after once it has set1 no more should be made up than will be wanted at the time4 ,if the glue be thick1 glass boxes for carbon disulphide may be easily put together1 even though the edges of the glass strips are not quite smooth1 for1 unlike most cements1 this mixture remains tough1 and is fairly strong in itself4 ,i have found by experiment that most fixed and1 to a less degree1 essential oils have little or no solvent action on shellac1 and ,i suspect that the same remark applies to the treacle-glue mixture1 but ,i have not tried4 ,turpenes act on shellac slightly1 but mineral oils apparently not at all4 ,the tests on which these statements are based were continued for about two years1 during which time kerosene and mineral oils had no observable effect on shellac--fastened galvanometer mirrors4 #di4 ,fusing ,electrodes into ,glass4 ,this art has greatly improved since the introduction of the incandescent lamp2 however1 up to the present1 platinum seems to remain the only substance capable of giving a certainly air-tight result4 ,i have not tried the aluminium-alumina method4 ,many years ago it was the fashion to surround the platinum wire with a drop of white enamel glass in order to cause better adhesion between it and the ordinary glass4 ,footnote3 ,hittorf and ,geissler 7,pogg4 ,ann4 #ahfd1 #ce2 ,english translation1 ,phys4 ,soc4 ,london1 p4 #ach7 found that it was impossible to make air-tight joints between platinum and hard potash glass1 but that soft lead glass could be used with success as a cement4 ,however1 in the case of flint glass1 if one may judge from incandescent lamps1 this is not essential--a fact which entirely coincides with my own experience4 ,on the other hand1 when sealing electrodes into ,german glass ,i have often used a drop of enamel with perfect results1 though this is not always done in ,germany4 ,in all cases1 however1 in which electrodes have to be sealed in--especially when they are liable to heat--,i recommend flint glass1 and in this have the support of ,mr4 ,rain 7,the ,incandescent ,lamp and its ,manufacture1 p4 #aca74 ,the exact details for the preparation of eudiometer tubes are given by ,faraday 7,chemical ,manipulation1 #abjj74 ,in view of what has preceded1 however1 ,i will content myself with the following notes4 ,make the hole through which the wire is to protrude only slightly larger than the wire itself1 and be sure that the latter is clean4 ,allow the glass to cool sufficiently not to stick to the wire when the latter is pushed in4 ,be sure that1 on heating1 the glass does not get reduced1 and that it flows up to the wire all round2 pull and push the wire a little with a pair of pincers1 to ensure this4 ,it is not a bad plan to get the glass exceedingly fluid round the wire--even if the lump has to be blown out a little afterwards--as it cools4 ,the seal should finally be well annealed in asbestos1 but first by gradually moving it into the hot air in front of the flame4 ,it was observed by ,professor ,j4 ,j4 ,thomson and the author some years ago 7,proc4 ,roy4 ,soc4 #dj4 #cca4 #ahhf7 that when very violent discharges are taken through lightly sealed-in electrodes in lead-glass tubes--say from a large battery of ,leyden jars--gas appears to be carried into the tube over and above that naturally given off by the platinum1 and this without there being any apparent want of perfection in the seal4 ,this observation has since been confirmed by others4 ,consequently in experiments on violent discharges in vacuo where certainty is required as to the exclusion of air1 the seals should be protected by a guard tube or cap containing mercury2 this must1 of course1 be put in hot and clean1 on hot and clean glass1 and in special cases should be boiled in situ4 ,a well-known ,german physicist 7,warburg1 ,i think7 recommends putting the seals under water1 but ,i cannot think that this is a good plan1 for if air can get in1 why not water8 which has its surface tension in its favour4 ,the same reasoning prevents my recommending a layer of sulphuric acid above the mercury-a method used for securing air-tightness in mercury joints by ,mr4 ,gimingham1 ,proc4 ,r4 ,s4 #ahgd4 ,further protection may be attained for many purposes by coating the platinum wire with a sheath of glass1 say half an inch long1 fused to the platinum wire to a depth of one-twentieth of an inch all round4 ,in some cases the electrodes must be expected to get very hot1 for instance1 when it is desired to platinise mirrors by the device of ,professor ,wright of ,yale4 ,in this and similar cases ,i have met with great success by using barometer tubes of about one-twelfth of an inch bore1 and with walls1 say1 one-tenth of an inch thick4 ,footnote3 ,barometer tube is merely very thick-walled glass tubing1 and makes particularly bad barometers1 which are sold as weather glasses4 ,this tube is drawn down to a long point--say an inch long by one-eighth of an inch external diameter1 and the wire is fused in for a length1 say1 of three-quarters of an inch1 but only in the narrow drawn--down part of the tube4 ,at different times ,i have tried four such seals1 and though the electrodes were red hot for hours1 ,i have never had an accident--of course they were well annealed4 ,fig4 #cg4 ,for directions as to the making of high vacuum tubes1 see the section dealing with that matter4 #ej4 ,as economy of platinum is often of importance1 the following little art will save money and trouble4 ,platinum is easily caused to join most firmly to copper--with which1 ,i presume1 it alloys--by the following method4 ,hold the platinum wire against the copper wire1 end to end1 at the tip of the reducing flame of a typical blowpipe--or anywhere--preferably in the reducing part of the oxygas flame2 in a moment the metals will fuse together at the point of contact1 when they may be withdrawn4 ,such a joint is very strong and wholly satisfactory1 much better than a soldered joint4 ,if the work is not carried out successfully so that a considerable drop of copper-platinum alloy accumulates1 cut it off and start again4 ,the essence of success is speed1 so that the copper does not get burned4 ,if any considerable quantity of alloy is formed it dissolves the copper1 and weakens it1 so that we have first the platinum wire1 then a bead of alloy1 and then a copper wire fused into the bead1 but so thin just outside the latter that the joint has no mechanical strength4 #ea4 ,the ,art of making ,air-light ,joints4 ,lamp-manufacturers and others have long since learned that when glass is in question not only are fused joints made as easily as others1 but that they afford the only reliable form of joint4 ,an experimenter who uses flint glass1 has a little experience1 an oxygas blow-pipe and a blowing apparatus1 will prefer to make his joints in this way1 simply from the ease with which it may be done4 ,when it comes to making a tight joint between glass and other substances the problem is by no means so easy4 ,thus ,mr4 ,griffiths 7,phil4 ,trans4 #ahic1 p4 #chj7 failed to make air-tight joints by cementing glass into steel tubes1 using hard shellac1 and the tubes fitting closely4 ,these joints were satisfactory at first1 but did not last2 the length of the joint is not stated4 ,the difficulty was finally got over by soldering very narrow platinum tubes into the steel1 and fusing the former into the glass4 ,mr4 ,griffiths has since used an alloy with success as a cement1 but ,i cannot discover what it is made from4 ,many years ago ,professor ,hittorf prepared good high vacuum tubes by plugging the ends of glass tubes with sealing wax merely1 though in all cases the spaces to be filled with wax were long and narrow 7,hittorf1 ,pogg4 ,ann4 #ahfi1 #e1 ,english translation1 ,phys4 ,soc4 p4 #aac74 ,again1 ,regnault habitually used brass ferules1 and cemented glass into them by means of his mastic1 which can still be procured at a low rate from his instrument-makers 7,golan1 ,paris74 ,lenard also1 in his investigations on ,cathode ,rays 7,wied4 ,ann1 vol4 li4 p4 #bbd71 made use of sealing wax covered with marine glue4 ,surely in face of these facts we must admit that cement joints can be made with fair success4 ,i do not know the composition of ,m4 ,regnault's mastic1 but ,faraday 7,manipulations1 #aabc7 gives the following receipt for a cement for joining ferules to retorts1 etc3 ,resin #e parts4 ,beeswax #a part4 ,red ochre or ,venetian red1 finely powdered and sifted #a part4 ,i believe this to be substantially the same as ,regnault's mastic1 though ,i have never analysed the latter4 ,for chemical work the possibility of evolution of gas from such a cement must be taken into account1 and ,i should certainly not trust it for this reason in vacuum tube work1 where the purity of the confined gas could come in question4 ,otherwise it is an excellent cement1 and does not in my experience tend to crack away from glass to the same extent as paraffin or pure shellac4 ,this cracking away from glass1 by the way1 is probably an effect of difference in rate of expansion between the glass and cement which probably always exists1 and1 if the cement be not sufficiently viscous1 must1 beyond certain temperature limits1 either produce cracks or cause separation4 ,professor ,wright of ,yale has used a hard mineral pitch as a cement in vacuum work with success4 ,my attention has been directed to a fusible metal cement containing mercury1 and made according to the following receipt1 given by ,mr4 ,s4 ,g4 ,rawson1 ,journal of the ,society of ,chemical ,industry1 vol4 ix4 7#ahij71 ,p4 #aej3- ,bismuth #dj per cent ,lead #be per cent ,tin #aj per cent ,cadmium #aj per cent ,mercury #ae per cent ,this is practically one form of ,rose's fusible metal with #ae per cent mercury added4 ,it takes nearly an hour to set completely1 and the apparatus must be clean and warm before it is applied4 ,as the result of several trials by myself and friends1 ,i am afraid ,i must dissent from the claim of the author that such a cement will make a really air-tight joint between glass tubes4 ,indeed1 the appearance of the surface as viewed through the glass is not such as to give any confidence1 no matter what care may have been exercised in performing all the operations and cleaning the glass2 besides which the cement is rigid when cold1 and the expansion difficulty comes in4 ,on the other hand1 if extreme air-tightness is not an object1 the cement is strong and easily applied1 and has many uses4 ,i have an idea that if the joints were covered with a layer of soft wax1 the result would be satisfactory in so far as air-tightness is concerned4 ,this anticipation has since been verified4 ,in many cases one can resort to the device already mentioned of enclosing a rubber or tape-wrapped joint between two tubes in a bath of mercury1 but in this case the glass must be clean and hot and the mercury also warm1 dry1 and pure when the joint is put together1 otherwise an appreciable air film is left against the glass1 and this may creep into the joint4 ,perhaps the easiest way of making such a joint is to use an outer tube of thin clean glass1 and bore a narrow hole into it from one side to admit the mercury2 if the mercury is to be heated in vacuo1 it is better to seal on a side joint4 ,it is always better1 if possible1 to boil the mercury in situ1 which involves making the wrapping of asbestos1 but1 after all1 we come back to the position ,i began by taking up1 viz4 that the easiest and most reliable method is by fusion of the glass--all the rest are unsuitable for work of real precision4 ,i should be ungrateful1 however1 were ,i not to devote a few lines to the great convenience and merit of so-called centering cement4 ,this substance has two or three very valuable properties4 ,it is very tough and strong in itself1 and it remains plastic on cooling for some time before it really sets4 ,if for any reason a small tube has to be cemented into a larger one1 which is a good deal larger1 so that an appreciable mass of cement is necessary1 and particularly if the joint requires to have great mechanical strength1 this cement is invaluable4 ,i have even used a plug of it instead of a cork for making the joint between a gas delivery tube and a calcium chloride tower4 7,why are these affairs made with such abominable tubulures87 ,the joint in question has never allowed the tube to sag though it projects horizontally to a distance of #f inches1 and has had to withstand nearly two years of ,sydney temperature4 ,the cement consists of a mixture of shellac and #aj per cent of oil of cassia4 ,the shellac is first melted in an iron ladle1 and the oil of cassia quickly added and stirred in1 to an extent of about #aj per cent1 but the exact proportions are not of importance4 ,great care must be taken not to overheat the shellac4 ,,appendix ,,to ,,chapter ,i ,,on ,,the ,,preparation ,,of ,,vacuum ,,tubes ,,for ,,the ,,production ,,of ,,professor ,,roentgen',s ,,radiation ,footnote3 ,written in ,may #ahif4 ,,when ,professor ,roentgen's discovery was first announced at the end of #ahie much difficulty was experienced in obtaining radiation of the requisite intensity for the repetition of his experiments4 ,the following notes on the production of vacuum tubes of the required quality may therefore be of use to those who desire to prepare their own apparatus4 ,it appears that flint glass is much more opaque to ,roentgen's radiation than soda glass1 and consequently the vacuum tubes require to be prepared from the latter material4 ,fig4 #ci4 ,a form of vacuum tube which has proved very successful in the author's hands is sketched in ,fig4 #ch4 ,it is most easily constructed as follows4 ,a bit of tubing about #b centimetres diameter1 #ae centimetres long1 and #a4e millimetre wall thickness1 is drawn down to a point4 ,the larger bulb1 about #e centimetres in diameter1 is blown at one end of this tube4 ,the thinner the bulb the better1 provided that it does not collapse under atmospheric pressure4 ,a very good idea of a proper thickness may be obtained from the statement that about #d centimetres length of the tubing should be blown out to form the bulb4 ,this would give a bulb of about the thickness of an ordinary fractionating bulb4 ,before going any further it is as well to test the bulb by tapping on the table and by exhausting it by means of an ordinary water-velocity pump4 ,the side tube is next prepared out of narrower tubing1 and is provided with a smaller bulb1 a blowing-out tube1 and a terminal1 to be made as will be described4 ,this side tube is next fused on to the main tube1 special care being taken about the annealing1 and the cathode terminal is then sealed into the main tube4 ,after using clean glass it is in general only necessary to rinse the tube out with clean alcohol1 after which it may be dried and exhausted4 ,the success of the operation will depend primarily on the attention given to the preparation and sealing-in of the electrode facing the large bulb4 ,preparation of ,terminals4 ,some platinum wire of about ,no4 #bf ,b4,w4,g--the exact size is unimportant--must be provided1 also some sheet aluminium about #a millimetre thick1 some white enamel cement glass1 and a cane of flint-glass tube of a few millimetres bore4 ,the electrodes are prepared by cutting discs of aluminium of from #a to #a4e centimetres diameter4 ,the discs of aluminium are bored in the centre1 so as to admit the stems which are made of aluminium wire of about #a millimetre diameter4 ,the stems are then riveted into the discs4 ,the stems are about ,i centimetre long1 and are drilled to a depth of about #c millimetres1 the drill used being about double the diameter of the platinum wire to be used for making the connections4 ,the faces of the electrodes--i4e4 the free surfaces of the aluminium discs--are then hammered flat and brought to a burnished surface by being placed on a bit of highly polished steel and struck by a set provided with a hole to allow of the stem escaping damage4 ,the operation will be obvious after a reference to ,figs4 #ci and #dj2 it is referred to again on page #if4 ,the platinum wires may be most conveniently attached by melting one end of the piece of platinum wire in the oxygas blow-pipe till it forms a bead just large enough to pass into the hole drilled up the stem of the electrode4 ,the junction between the stein and the platinum wire is then made permanent by squeezing the aluminium down upon the platinum wire with the help of a pair of pliers4 ,it is also possible to fuse the aluminium round the platinum1 but as ,i have had several breakages of such joints1 ,i prefer the mechanical connection described4 ,fig4 #ci4 ,sets for striking aluminium electrodes ,fig4 #dj4 i4 ,aluminium electrode4 ii4 ,aluminium electrode connected to platinum wire4 iii4 ,aluminium electrode connected to platinum wire and protected by glass4 iv4 ,detail of fastening platinum wire4 ,the stem and platinum wire may now be protected by covering them with a little flint glass4 ,for this purpose the flint-glass tube is pulled down till it will just slip over the stem and wire1 and is cut off so as to leave about half a centimetre of platinum wire projecting4 ,the flint-glass tube is then fused down upon the platinum wire1 care being taken to avoid the presence of air bubbles4 ,at the close of the operation a single drop of white enamel glass is fused round the platinum wire at a high temperature1 so as to make a good joint with the protecting flint-glass tube4 ,the negative electrode being nearly as large as the main tube1 it must be introduced before the latter is drawn down for sealing4 ,after drawing down the main tube in the usual manner1 taking care not to make it less than a millimetre in wall-thickness1 it is cut off so as to leave a hole not quite big enough for the enamel drop to pass through4 ,by heating and opening1 the aperture is got just large enough to allow the enamel drop to pass into it1 and when this is the case the joint is sealed1 pulled1 and blown out until the electrode occupies the right position--viz4 in the centre of the tube and with its face normal to the axis of the tube4 ,the glass walls near the negative electrode must not be less than a millimetre thick1 and may be rather more with advantage1 the glass must be even1 and the joint between the flint glass and the soda glass1 or between the wire and the soda glass1 must be wholly through the enamel4 ,the seal must be well annealed4 ,it will be found that the sealing-in process is much easier when the stem of the electrode is short and when the glass coating is not too heavy4 ,half a millimetre of glass thickness round the stein is quite sufficient4 ,the diagram1 of the tube shows that the main tube has been expanded round the edges of the cathode4 ,this is to reduce the heating consequent on the projection of cathode rays from the edges of the disc against the glass tube4 ,the anode is inserted into its bulb in a quite similar manner4 ,if desired it may be made considerably smaller1 and does not need the careful adjustment requisite in sealing-in the cathode1 nor does the glass near the entry wire require to be so thick4 ,more intense effects are often got by making the cathode slightly concave1 but in this case the risk of melting the thin glass is considerably increased4 ,no doubt1 ,bohemian glass might be used throughout instead of soft soda glass1 and this would not melt so easily2 the difficulties of manipulating the glass are1 however1 more pronounced4 ,it will be shown directly that the best ,roentgen effects are got with a high vacuum1 and it is for this reason that the glass near the cathode seal requires to be strong4 ,the potential right up to the cathode is strongly positive inside the tube1 and this causes the glass to be exposed to a strong electric stress in the neighbourhood of the seal4 ,although the glass-blowing involved in the making of a so-called focus tube is rather more difficult than in the case just described1 there is no reason why such a difficulty should not be overcome2 ,i will therefore explain how a focus tube may be made4 ,fig4 #da4 ,a bulb about #c inches in diameter is blown from a bit of tube of a little more than #a inch diameter4 ,unless the walls of the tube are about one-eighth of an inch in thickness1 this will involve a preliminary thickening up of the glass4 ,this is not difficult if care be taken to avoid making the glass too hot4 ,the larger gas jet described in connection with the soda-glass-blowing table must be employed4 ,in blowing a bulb of this size it must not be forgotten that draughts exercise a very injurious influence by causing the glass to cool unequally2 this leads to bulbs of irregular shape4 ,in the method of construction shown in ,fig4 #da1 the anode is put in first4 ,this anode simply consists of a square bit of platinum or platinum-iridium foil1 measuring about #j4ge inch by #a inch1 and riveted on to a bent aluminium wire stem4 ,as soon as the anode is fused in1 and while the glass is still hot1 the side tube is put on4 ,the whole of the anode end is then carefully annealed4 ,when the annealing is finished the side tube is bent as shown to serve as a handle when the time comes to mount the cathode4 ,before placing the cathode in position1 and while the main tube is still wide open1 the anode is adjusted by means of a tool thrust in through this open end4 ,this is necessary in view of the fact that the platinum foil is occasionally bent during the operation of forcing the anode into the bulb4 ,the cathode is a portion of a spherical surface of polished aluminium1 a mode of preparing which will be given directly4 ,the cathode having been placed inside the bulb1 the wide glass tube is carefully drawn down and cut off at such a point that when the cathode is in position its centre of curvature will lie slightly in front of the anode plate4 ,for instance1 if the radius of curvature of the cathode be #a4e inches1 the centre of curvature may lie something like an eighth of an inch or less in front of the anode4 ,the cathode as shown in ,fig4 #da is rather smaller than is advantageous4 ,to make it much larger than is shown1 however1 the opening into the bulb would require to be considerably widened1 and though this is not really a difficult operation1 still it requires more practice than my readers are likely to have had4 ,the difficulty is not so much in widening out the entry as in closing it down again neatly4 ,now as to making the anode4 ,a disc of aluminium is cut from a sheet which must not be too thick--one twenty-fifth of an inch is quite thick enough4 ,this disc is bored at the centre to allow of the stem being riveted in position4 ,the disc is then annealed in the ,bunsen flame and the stem riveted on4 ,the curvature is best got by striking between steel dies 7see ,figs4 #ci and #dj74 ,two bits of tool steel are softened and turned on the lathe1 one convex and the other concave4 ,the concave die has a small hole drilled up the centre to admit the stem4 ,the desired radius of curvature is easily attained by cutting out templates from sheet zinc and using them to gauge the turning4 ,the two dies are slightly ground together on the lathe with emery and oil and are then polished1 or rather the convex die is polished--the other one does not matter4 ,the polishing is most easily done by using graded emery and oil and polishing with a rag4 ,the method of grading emery will be described in the chapter on glass-grinding4 ,the aluminium disc is now struck between the dies by means of a hammer4 ,if the radius of curvature is anything more than one inch and the disc not more than one inch in diameter the cathode can be struck at once from the flat as described4 ,for very deep curves no doubt it will be better to make an intermediate pair of dies and to re-anneal the aluminium after the first striking4 ,when the tube is successfully prepared so far as the glassblowing goes it may be rinsed with strong pure alcohol both inside and out1 and dried4 ,the straight part of the side tube is then constricted ready for fusing off and the whole affair is placed on the vacuum pump4 ,in spite of the great improvements made during recent years in the construction of so-called ,geissler vacuum pumps--i4e4 pumps in which a ,torricellian vacuum is continually reproduced--,i am of opinion that ,sprengel pumps are1 on the whole1 more convenient for exhausting ,crooke's tubes4 ,a full discussion of the subject of vacuum pumps will be found in a work by ,mr4 ,g4 ,s4 ,ram 7,the ,incandescent ,lamp and its ,manufacture71 published by the ,electrician ,publishing ,company1 and it is not my intention to deal with the matter here2 the simplest kind of ,sprengel pump will be found quite adequate for our purpose1 provided that it is well made4 ,fig4 #db is intended to represent a modification of a pump based on the model manufactured by ,hicks of ,hatton ,garden1 and arranged to suit the amateur glass-blower4 ,the only point of importance is the construction of the head of the fall tube1 of which a separate and enlarged diagram is given4 ,the fall tubes may have an internal diameter up to #b mm4 7two millimetres7 and an effective length of #abj cm4 ,free use is made of rubber tube connections in the part of the pump exposed to the passage of mercury4 ,the rubber employed should be black and of the highest quality1 having the walls strengthened by a layer of canvas4 ,if such tube cannot be easily obtained1 a very good substitute may be made by placing a bit of ordinary black tube inside another and rather larger bit and binding the outer tube with tape or ribbon4 ,in any case the tubing which comes in contact with the mercury should be boiled in strong caustic potash or soda solution for at least ten minutes to get rid of free sulphur1 which fouls the mercury directly it comes in contact with it4 ,the tubing is well washed1 rinsed with alcohol1 and carefully dried4 ,fig4 #db4 ,the diagram represents what is practically a system of three ,sprengel pumps1 though they are all fed from the same mercury reservoir and run down into the same mercury receiver4 ,it is much easier to make three pumps1 each with separate pinch cocks to regulate the mercury supply1 than it is to make three jets1 each delivering exactly the proper stream of mercury to three fall tubes4 ,sprengel pumps only work at their highest efficiency when the mercury supply is carefully regulated to suit the peculiarities of each fall tube1 and this is quite easily done in the model figured4 ,since on starting the pump the rubber connections have to stand a considerable pressure1 the ends of the tubes must be somewhat corrugated to enable the rubber to be firmly wired on to them4 ,the best binding wire is the purest ,swedish iron wire1 previously annealed in a ,bunsen gas flame4 ,the wire must never be twisted down on the bare rubber1 but must always be separated from it by a tape binding4 ,by taking this precaution the wire maybe twisted very much more tightly than is otherwise possible without cutting the rubber4 ,the only difficulty in making such a pump as is described lies in the bending of the heads of the fall tubes4 ,this bending must be done with perfect regularity and neatness1 otherwise the drops of mercury will not break regularly1 or will break just inside the top of the fall tube1 and so obstruct its entrance that at high vacua no air can get into the tube at all4 ,the connections at the head of the fall tubes must also be well put on and the joints blown out so that the mercury in dropping over the head is not interfered with by the upper surface of the tube4 ,however1 a glance at the enlarged diagram will show what is to be aimed at better than any amount of description4 ,in preparing the fall tubes it is generally necessary to join at least two canes together4 ,the joint must be arranged to occur either in the tube leading the mercury to the head of the fall1 or in that part of the fall tube which remains full of mercury when the highest vacuum is attained4 ,on no account must the joint be made at the fall itself 7at least not by an amateur71 nor in that part of the fall tube where the mercury falls freely1 particularly at its lower end1 where the drops fall on the head of the column of mercury4 ,when a high vacuum is attained the efficacy of the pump depends chiefly on the way in which the drops fall on the head of the column4 ,if the fall is too long the drops are apt to break up and allow the small bubble of air to escape up the tube1 also any irregularity or dirt in the tube at this point makes it more easy for the bubbles of air to escape to the surface of the mercury4 ,any pump in which the supply of mercury to the fall tube can be regulated nicely will pump well until the lowest available pressures are being attained2 a good pump will then continue to hold the air bubbles1 while a bad one will allow them to slip back ,footnote3 ,for special methods of avoiding this difficulty see ,mr4 ,ram's book4 444 ,though three fall tubes are recommended1 it must not be supposed that the pump will produce a ,crooke's vacuum three times more rapidly than one fall tube4 ,until the mercury commences to hammer in the pump the three tubes will pump approximately three times faster than one tube1 but as soon as the major portion of the air collected begins to come from the layer condensed on the glass surface of the tube to be exhausted and from the electrodes1 the rate at which exhaustion will go on no longer depends entirely on the pump4 ,in order that bubbles of air may not slip back up the fall tube it is generally desirable to allow the mercury to fall pretty briskly1 and in this case the capacity of the pump to take air is generally far in excess of the air supply4 ,one advantage of having more than one fall tube is that it often happens that a fall tube gets soiled during the process of exhaustion and no longer works up to its best performance4 ,out of three fall tubes1 however1 one is pretty sure to be working well1 and as soon as the mercury begins to hammer in the tubes the supply may be shut off from the two falls which are working least satisfactorily4 ,thus we are enabled to pump rapidly till a high degree of exhaustion is attained1 having practically three pumps instead of one1 whereas when the final stages are reached1 and three pumps are only a drawback in that they increase the mercury flow1 the apparatus is capable of instant modification to meet the new conditions4 ,the thistle funnels at the head of the fall tubes are made simply by blowing bulbs and then blowing the heads of the bulbs into wider ones1 and finally blowing the heads of the wider bulbs off by vigorous blowing4 ,the stoppers are ground in on the lathe before the tubes are attached to the fall tubes4 ,the stoppers require to be at least half an inch long where they fit the necks1 and must be really well ground in4 ,the stoppers must first be turned up nicely and the necks ground out by a copper or iron cone and emery4 ,the stoppers are rotated on a lathe at quite a slow speed1 say #cj or #dj feet per minute1 and the necks are held against them1 as described in the section dealing with this art4 ,the stoppers must in this case be finished with two seconds emery1 and lastly with pumice dust and water 7see chapter on glass-grinding74 ,unless the stoppers fit exceedingly well trouble will arise from the mercury 7which is poured into the thistle heads to form a seal7 being forced downwards into the pump by atmospheric pressure4 ,the joints between the three fall tubes and the single exhaust main are easily made when the tubes are finally mounted1 the hooked nozzle of the oxygas blow-pipe being expressly made for such work4 ,it is1 on the whole1 advisable to make the pump of flint glass1 or at all events the air-trap tube and the fall tubes4 ,a brush flame from the larger gas tube of the single blowpipe table is most suitable for the work of bending the tubes4 ,the jointing of the long1 narrow bore fall tubes is best accomplished by the oxygas flame1 for in this way the minimum of irregularity is produced2 the blowing tubes will of course be required for the job1 and the narrow tubes must be well cleaned to begin with4 ,the air trap is an important though simple part of the pump4 ,its shoulder or fall should stand rather higher than the shoulders of the fall tubes1 so that the mercury may run in a thin stream through a good ,torricellian vacuum before it passes down to the fall tubes4 ,this is easily attained by regulating the main mercury supply at the pinch cock situated between the tube from the upper reservoir and the air-trap tube1 the other cocks being almost wide open4 ,it might be thought that the mercury would tend to pick up air in passing through the rubber connections to the fall tubes1 but ,i have not found this to be the case in practice4 ,there is1 of course1 no difficulty in eliminating the rubber connections between the fall tubes and the mercury supply from the air trap1 but it impresses a greater rigidity on the structure and1 as ,i say1 is not in general necessary4 ,it must not be forgotten that the mercury always exercises considerable pressure on the rubber joints1 and so there is little tendency for gas to come out of the rubber4 ,the thistle funnels at the head of the fall tubes provide a simple and excellent means of cleaning the fall tubes4 ,for this purpose some pure sulphuric acid which has been boiled with pure ammonium sulphate is placed in each thistle funnel1 and when the fall tube is dirty the connection to the mercury supply is cut off at the pinch cock so as to leave the tube between this entry and the head of the fall tube quite full of mercury1 and the sulphuric acid is allowed to run down the fall tube by raising the stopper4 ,the fall tube should be allowed to stand full of acid for an hour or so1 after which it will be found to be fairly clean4 ,of course the mercury reservoir thus obtains a layer of acid above the mercury1 and as it is better not to run the risk of any acid getting into the pump except in the fall tubes1 the reservoir is best emptied from the bottom1 by a syphon1 if a suitable vessel cannot be procured1 so that clean mercury only is withdrawn4 ,the phosphorus pentoxide tube is best made as shown simply from a bit of wide tube1 with two side connections fused to the rest of the pump4 ,it is no more trouble to cut the tube and fuse it up again when the drying material is renewed than to adjust the drying tube to two fixed stoppers1 which is the alternative4 ,the practice here recommended is rendered possible only by the oxygas blow-pipe with hooked nozzle4 ,the connection between the pump and tube to be exhausted is made simply by a short bit of rubber tube immersed in mercury4 ,the phosphorus pentoxide should be pure1 or rather free from phosphorus and lower oxides2 unless this be the case1 the vapour arising from it is apt to soil the mercury in the pump4 ,the phosphorus pentoxide is purified by distilling with oxygen over red-hot platinum black2 if this cannot be done1 the pentoxide should at least be strongly heated in a tube1 in a current of dry air or oxygen1 before it is placed in the drying tube4 ,the mercury used for the pump must be scrupulously clean4 ,it does not1 however1 require to have been distilled in vacuo4 ,it is sufficient to purify it by allowing it to fall in a fine spray into a large or rather tall jar of #be per cent nitric acid and #ge per cent water4 ,the mercury is then to be washed and dried by heating to1 say1 #aaj ,c4 in a porcelain dish4 ,exhausting a ,roentgen ,tube4 ,with a pump such as has been described there is seldom any advantage in fusing an extra connection to the vacuum tube so as to allow of a preliminary exhaustion by means of a water pump4 ,about half an hour's pumping may possibly be saved by making use of a water pump4 ,the tube to be exhausted is washed and dried by careful heating over a ,bunsen burner and by the passage of a current of air4 ,the exhausting tube is then drawn down preparatory to sealing off1 and the apparatus placed upon the pump4 ,it is best held in position by a wooden clamp supported by a long retort stand4 ,exhaustion may proceed till the mercury in the fall tubes commences to hammer4 ,at this point the tube must be carefully heated by a ,bunsen flame1 the temperature being brought up to1 say1 #djj ,c4 ,the heating may be continued intermittently till little or no effect due to the heating is discernible at the pump4 ,when this stage is reached1 or even before1 the electrodes may be connected up to the coil and a discharge sent through the tube4 ,care must be taken to stop the discharge as soon as a purple glow begins to appear1 because when this happens1 the resistance of the tube is very low1 the electrodes get very hot1 and may easily get damaged by a powerful discharge1 and the platinum of the anode 7if a focus tube is in question7 begins to be distilled on to the glass4 ,the heating and sparking are to be continued till the resistance of the tube sharply increases4 ,this is tested by always having a spark gap1 conveniently formed by the coil terminals1 in parallel with the tube4 ,if the terminals are points1 it is convenient to set them at about one quarter of an inch distance apart4 ,as soon as sparks begin to pass between the terminals of the spark gap it becomes necessary to watch the process of exhaustion very carefully4 ,in the first place1 stop the pump1 but let the coil run1 and note whether the sparks continue to flow over the terminals4 ,if the glass and electrodes are getting gas free1 the discharge will continue to pass by the spark gap1 but if gas is still being freely given off1 then in perhaps three minutes the discharge will return to the tube1 and pumping must be recommenced4 ,the ,roentgen effect only begins to appear when the tube has got to so high a state of exhaustion that the resistance increases rapidly4 ,by pumping and sparking1 the resistance of the tube may be gradually raised till the spark would rather jump over #b inches of air than go through the tube4 ,when this state is attained the ,roentgen effect as tested by a screen of calcium tungstate should be very brilliant4 ,no conclusion as to the equivalent resistance of the tube can be arrived at so long as the discharge is kept going continually4 ,when the spark would rather go over an inch of air in the spark gap than through the tube the pumping and sparking may be interrupted and the tube allowed to rest for1 say1 five minutes4 ,it will generally be found that the equivalent resistance of the tube will be largely increased by this period of quiescence4 ,it may even be found that the spark will now prefer to pass an air gap #c inches long4 ,in any case the sparking should now be continued1 the pump being at rest1 and the variations of tube resistance watched by adjusting the spark gap4 ,if the resistance falls below an equivalent of #b inches of air in the gap the pump must be brought into action again and continued until the resistance as thus estimated remains fairly constant for1 say1 ten minutes4 ,when this occurs the narrow neck of the exhaust tube may be strongly heated till the blow-pipe flame begins to show traces of sodium light4 ,the flame must then be withdrawn and the discharge again tested4 ,this is necessary because it occasionally happens that gas is given off during the heating of the neck to the neighbourhood of its fusion temperature4 ,if all is right the neck may now be fused entirely off and the tube is finished4 ,tubes of the focus pattern with large platinum anodes are in general 7in my experience7 much more difficult to exhaust than tubes of the kind first described4 ,this is possibly to be attributed mainly to the gas given off by the platinum1 but is also1 no doubt1 due to the tubes being much larger and exposing a larger glass surface4 ,the type of tube described first generally takes about two hours to exhaust by a pump made as explained1 while a focus tube has taken as long as nine hours1 eight of which have been consumed after the tube was exhausted to the hammering point4 ,the pressure at which the maximum heating of the anode by the cathode rays occurs is a good deal higher than that at which the maximum ,roentgen effect is produced4 ,there is little doubt that the ,roentgen radiation changes in nature to some extent as the vacuum improves either as a primary or secondary effect4 ,it is therefore of some importance to test the tube for the purpose for which it is to be used during the actual exhaustion4 ,it has been stated1 for instance1 that the relative penetrability of bone and flesh to ,roentgen radiation attains a maximum difference at a certain pressure2 this is very likely the case4 ,whether this effect is a direct function of the density of the gas in the tube1 or whether it is dependent on the voltage or time integral of the current during the discharge1 are questions which still await a solution4 ,the preparation of calcium tungstate for fluorescent screens is very simple4 ,commercial sodium tungstate is fused with dried calcium chloride in the proportion of three parts of the former to two parts of the latter1 both constituents being in fine powder and well mixed together4 ,the fusion is conducted in a ,fletcher's crucible furnace in a clay crucible4 ,the temperature is raised as rapidly as possible to the highest point which the furnace will attain--i4e4 a pure white heat4 ,at this temperature the mixture of salts becomes partly fluid1 or at least pasty1 and the temperature may be kept at its highest point for1 say1 a quarter of an hour4 ,at the end of this time the mass is poured and scraped on to a brick1 and when cold is broken up and boiled with a large excess of water to dissolve out all soluble matter4 ,the insoluble part1 which consists of a gray shining powder1 is washed several times with hot water1 and is finally dried on filter paper in a water oven4 ,in order to prepare a screen the powder is ground slightly with very dilute shellac varnish1 and is then floated over a glass plate so as to get an even covering4 ,unless the covering be very even the screen is useless1 and no pains should be spared to secure evenness4 ,it is not exactly easy to get a regular coat of the fluorescent material1 but it may be done with a little care4 ,,chapter ,,ii ,,glass-,,grinding ,,and ,,opticians' ,,work #eb4 ,as no instructions of any practical value in this art have1 so far as ,i know1 appeared in any book in ,english1 though a great deal of valuable information has been given in the ,english ,mechanic and elsewhere1 ,i shall deal with the matter sufficiently fully for all practical purposes4 ,on the other hand1 ,i do not propose to treat of all the methods which have been proposed1 but only those requisite for the production of the results claimed4 ,the student is requested to read through the chapter before commencing any particular operation4 #ec4 ,the simplest way will be to describe the process of manufacture of some standard optical appliance1 from which a general idea of the nature of the operations will be obtained4 ,after this preliminary account special methods may be considered in detail4 ,i will begin with an account of the construction of an achromatic object glass for a telescope1 not because a student in a physical laboratory will often require to make one1 but because it illustrates the usual processes very well2 and requires to be well and accurately made4 ,a knowledge of the ordinary principles of optics on the part of the reader is assumed1 for there are plenty of books on the theory of lenses1 and1 in any case1 it is my intention to treat of the art rather than of the science of the subject4 ,by far the best short statement of the principles involved which ,i have seen is ,lord ,rayleigh's article on ,optics in the ,encyclopaedia ,britannica1 and this is amply sufficient4 ,the first question that crops up is1 of course1 the subject of the choice of glass4 ,it is obvious that the glass must be uniform in refractive index throughout1 and that it must be free from air bubbles or bits of opaque matter4 ,footnote3 ,the complete testing of glass for uniformity of refractive index can only be arrived at by grinding and polishing a sufficient portion of the surfaces to enable an examination to be made of every part4 ,in the case of a small disc it is sufficient to polish two or three facets on the edge1 and to examine the glass in a field of uniform illumination through the windows thus formed4 ,very slight irregularities will cause a mirage easily recognised4 ,the simplest procedure is to obtain glass of the desired quality from ,messrs4 ,chance of ,birmingham1 according to the following abbreviated list of names and refractive indices1 which may be relied upon3- ,density4 ,refractive ,index4 ,c ,d ,f ,g ,hard crown #b4he #a4eadf #a4eagb #a4ebcb #a4ebhj ,soft crown #b4ee #a4eaai #a4eadf #a4ebaj #a4ebfc ,light flint #c4ba #a4egjj #a4egdj #a4ehci #a4eibb ,dense flint #c4ff #a #fage #a4fbbd #a4fcdh #a4fdec ,extra dense flint #c4he #a4fdej #a4fejd #a4ffdc #a4fgfa ,double extra dense flint #d4de #a4gjcf #a4gajc #a4gbgc 444 ,the above glasses may be had in sheets from #j4be to #a inch thick1 and #f to #ab inches square1 at a cost of1 say1 #gs4 #f;d4 per pound4 ,discs can also be obtained of any reasonable size4 ,discs #b inches in diameter cost about #a per dozen1 discs #c inches in diameter about #ajs4 each4 ,the price of discs increases enormously with the size4 ,a #af-;inch disc will cost about #ajj4 ,for special purposes1 where the desired quality of glass does not appear on the list1 an application may be made to the ,jena ,factory of ,herr ,schott4 ,in order to give a definite example1 ,i may mention that for ordinary telescopic objectives good results may be obtained by combining the hard crown and dense flint of ,chance's list1 using the crown to form a double convex1 and the flint to form a double concave lens4 ,the convex lens is placed in the more outward position in the telescope1 i4e4 the light passes first through it4 ,the conditions to be fulfilled are3 7#a7 ,the glass must be achromatic2 7#b7 it must have a small spherical aberration for rays converging to the principal focus4 ,it is impossible to discuss these matters without going into a complete optical discussion4 ,the radii of curvature of the surfaces1 beginning with the first1 i4e4 the external face of the convex lens1 are in the ratio of #a1 #b1 and #c2 an allowance of #ae inches focal length per inch of aperture is reasonable 7see ,optics in ,ency4 ,brit471 and the focal length is the same as the greatest radius of curvature4 ,thus1 for an object glass #b inches in diameter1 the first surface of the convex lens would have a radius of curvature of #aj inches1 the surface common to the convex and concave lens would have a radius of curvature of #bj inches1 and the last surface a radius of curvature of #cj inches4 ,this would also be about the focal length of the finished lens4 ,the surfaces in contact have1 of course1 a common curvature1 and need not be cemented together unless a slight loss of light is inadmissible4 ,i will assume that a lens of about #b inches diameter is to be made by hand1 i4e4 without the help of a special grinding or polishing machine2 this can be accomplished perfectly well1 so long as the diameter of the glass is not above about #f inches1 after which the labour is rather too severe4 ,the two glass discs having been obtained from the makers1 it will be found that they are slightly larger in diameter than the quoted size1 something having been left for the waste of working4 ,it is difficult to deal with the processes of lens manufacture without entering at every stage into rather tedious details1 and1 what is worse1 without interrupting the main account for the purpose of describing subsidiary instruments or processes4 ,in order that the reader may have some guide in threading the maze1 it is necessary that he should commence with a clear idea of the broad principles of construction which are to be carried out4 ,for this purpose it seems desirable to begin by roughly indicating the various steps which are to be taken4 7#a7 ,the glass is to be made circular in form and of a given diameter4 7#b7 ,called ,rough ,grinding4 ,the surfaces of the glass are to be made roughly convex1 plane1 or concave1 as may be required2 the glass is to be equally thick all round the edge4 ,in this process the glass is abraded by the use of sand or emery rubbed over it by properly shaped pieces of iron or lead called tools4 7#c7 ,the glass is ground with emery to the correct spherical figure as given by a spherometer4 7#d7 ,called ,fine ,grinding4 ,the state of the surface is gradually improved by grinding with finer and finer grades of emery4 7#e7 ,the glass is polished by rouge4 7#f7 ,the glass is figured4 ,this means that it is gradually altered in form by a polishing tool till it gives the best results as found by trial4 ,in processes #b to #e counterpart tool surfaces are required--as a rule two convex and two concave surfaces for each lens surface4 ,these subsidiary surfaces are worked 7i4e4 ground7 on discs of cast iron faced with glass1 or on slate discs2 and discs thus prepared are called tools4 ,taking these processes in the order named1 the mode of manufacture is shortly as follows3- 7#a7 ,the disc of glass1 obtained in a roughly circular form1 is mounted on an ordinary lathe1 being conveniently cemented by ,regnault's mastic to a small face plate4 ,the lathe is rotated slowly1 and the glass is gradually turned down to a circular figure by means 7#a7 of a tool with a diamond point2 or 7#b7 an ordinary hand-file moistened with kerosene1 as described in #db2 or 7#c7 a mass of brass or iron served with a mixture of emery--or sand--and water fed on to the disc1 so that the disc is gradually ground circular4 ,the operation of making a circular disc of given diameter does not differ in any important particular from the similar operation in the case of brass or iron1 and is in fact merely a matter of turning at a slow speed4 7#b and #c7 ,roughing or bringing the surfaces of the glass roughly to the proper convex or concave shape4 ,this is accomplished by grinding1 generally with sand in large works1 or with emery in the laboratory1 where the time saved is of more importance than the value of the emery4 ,discs of iron or brass are cast and turned so as to have a diameter slightly less than that of the glass to be ground1 and are1 say1 half an inch thick4 ,these discs are turned convex or concave on one face according as they are to be employed in the production of concave or convex glass surfaces4 ,the proper degree of convexity or concavity may be approximated to by turning with ordinary turning tools1 using a circular arc cut from zinc or glass 7as will be described7 as a template or pattern4 ,this also is a mere matter of turning4 ,the first approximation to the desired convex or concave surface of the glass is attained 7in the case of small lenses1 say up to three inches diameter7 by rotating the glass on the lathe as described above 7for the purpose of giving it a circular edge7 and holding the tool against the rotating glass1 a plentiful supply of coarse emery and water1 or sand and water1 being supplied between the glass and metal surfaces4 ,the tool is held by hand against the surface of the revolving glass1 and is constantly moved about1 both round its own axis of figure and to and fro across the glass surface4 ,in this way the glass gradually gets convex or concave4 ,the curvature is tested from time to time by a spherometer1 and the tool is increased or decreased in curvature by turning it on a lathe so as to cause it to grind the glass more at the edges or in the middle according to the indications of the spherometer4 ,this instrument1 by the way--so important for lens makers--consists essentially of a kind of three-legged stool1 with an additional leg placed at the centre of the circle circumscribing the other three4 ,this central leg is in reality a fine screw with a very large head graduated on the edge1 so that it is easy to compute the fractions of a turn given to the screw4 ,the instrument is first placed on a flat plate1 and the central screw turned till its end just touches the plate1 a state of affairs which is very sharply discernible by the slight rocking which it enables the instrument to undergo when pushed by the hand4 ,see the sketch4 ,on a convex or concave surface the screw has to be screwed in or out1 and from the amount of screwing necessary to bring all four points into equal contact1 the curvature may be ascertained4 ,let a be the distance between the equidistant feet1 and d the distance through which the screw is protruded or retracted from its zero position on a flat surface4 ,then the radius of curvature rho is given by the formula #brho a#b_/c;d d4 ,fig4 #dc4 ,the process of roughing is not always carried out exactly as described1 and will be referred to again4 7#d7 ,the glass being approximately of the proper radius of curvature on one side1 it is reversed on the chuck and the same process gone through on the other side4 ,after this the glass is usually dismounted from the lathe and mounted by cement on a pedestal1 which is merely a wooden stand with a heavy foot1 so that the glass may be held conveniently for the workman4 ,sometimes a pedestal about four feet high is fixed in the floor of the room1 so that the workman engaged in grinding the lens may walk round and round it to secure uniformity4 ,for ordinary purposes1 however1 a short pedestal may be placed on a table and rotated from time to time by hand1 the operator sitting down to his work4 ,rough iron or brass tools do not succeed for fine grinding--i4e4 grinding with fine emery1 because particles of emery become embedded in the metal so tightly that they cannot be got out by any ordinary cleaning4 ,if we have been using emery passing say a sieve with #fj threads to the inch1 and then go on to some passing say #ajj threads to the inch1 a few of the coarser particles will adhere to the tool1 and go on cutting and scratching all the time grinding by means of the finer emery is in progress4 ,to get over this it is usual to use a rather different kind of grinding tool4 ,a very good kind is made by cementing small squares of glass 7say up to half an inch on the side71 on to a disc of slate slightly smaller than the lens surface to be formed 7,fig4 #ea74 ,the glass-slate tool is then roughed just like the lens surface1 but1 of course1 if the lens has been roughed convex the tool must be roughed concave4 ,the roughed tool is then used to gradually improve the fineness of grinding of the glass4 ,for this purpose grinding by hand is resorted to1 the tool and lens being supplied continually with finer and finer emery4 ,fig4 #eb gives an idea of the way in which the tool is moved across the glass surface4 ,very little pressure is required4 ,the tool is carried in small circular sweeps round and round the lens1 so that the centre of the tool describes a many-looped curve on the lens surface4 ,the tool must be allowed to rotate about its own axis2 and the lens and pedestal must also be rotated from time to time4 ,every few minutes the circular strokes are interrupted1 and simple1 straight1 transverse strokes taken4 ,in no case 7except to correct a1 defect1 as will be explained7 should the tool overhang the lens surface by more than about one quarter the diameter of the latter4 ,after grinding say for an hour with one size of emery fed in by means of a clean stick say every five minutes1 the emery is washed off1 and everything carefully cleaned4 ,the process is then repeated with finer emery1 and so on4 ,the different grades of emery are prepared by taking advantage of the fact that the smaller the particles the longer do they remain suspended in water4 ,some emery mud from a roughing operation is stirred up with plenty of water and left a few seconds to settle1 the liquor is then decanted to a second jug and left say for double the time1 say ten seconds2 it is decanted again1 and so on till four or five grades of emery have been accumulated1 each jug containing finer emery than its predecessor in the process4 ,it is not much use using emery which takes more than half an hour to settle in an ordinary bedroom jug4 ,what remains in the liquid to be decanted is mostly glass mud and not emery at all4 ,the process of fine grinding is continually checked by the spherometer1 and the art consists in knowing how to move the grinding tool so as to make the lens surface more or less curved4 ,in general it may be said that if the tool is moved in small sweeps1 and not allowed to overhang much1 the ,centre of the lens will be more abraded1 while if bold free strokes are taken with much overhanging1 the edges of the lens will be more ground away4 ,by the exercise of patience and perseverance any one will succeed in gradually fine grinding the lens surface and keeping it to the spherometer1 but the skill comes in doing this rapidly by varying the shape of the strokes before any appreciable alteration of curvature has come about4 ,polishing4 ,the most simple way of polishing is to coat the grinding tool with paper1 as will be described1 and then to brush some rouge into the paper4 ,the polisher is moved over the work in much the same way as the fine grinding tool1 until the glass is polished4 ,many operators prefer to use a tool made by squeezing a disc of slate1 armed with squares of warm pitch1 against the lens surface 7finely ground71 and then covering these squares with rouge and water instead of emery and water as in the fine grinding process4 ,the final process is called figuring4 ,it will in general be unnecessary with a small lens4 ,with large lenses or mirrors the final touches have to be given after the optical behaviour of the lens or mirror has been tested with the telescope itself1 and this process is called figuring4 ,a book might easily be written on the optical indications of various imperfections in a mirror or lens4 ,suffice it to say here that a sufficiently skilled person will be able to decide from an observation of the behaviour of a telescope whether a lens will be improved by altering the curvature of one or all of the surfaces4 ,a very small alteration will make a large difference in the optical properties1 so that in general figuring is done merely by using the rouge polishing tool as an abrading tool1 and causing it to alter the curves in the manner already suggested for grinding4 ,there are other methods based on knocking squares out of the pitch-polisher so that some parts of the glass may be more abraded than others4 ,the figuring and polishing may be done by hand just like the grinding4 ,there are machines1 however1 which can be made to execute the proper motions1 and a polisher is set in such a machine1 and the mechanical work done is by no means inconsiderable4 ,in fact for surfaces above six inches in diameter few people are strong enough to work a polisher by hand owing to the intense adhesion between it and the exactly fitting glass surface4 ,such is a general outline of the processes required to produce a lens or mirror4 ,these processes will now be dealt with in much greater detail1 and a certain amount of repetition of the above will unfortunately be necessary3 the reader is asked to pardon this4 ,it will also be advisable for the reader to begin by reading the whole account before he commences any particular operation4 ,the reason for this is that it has been desirable to keep to the main account as far as possible without inserting special instructions for subsidiary operations1 however important they may be2 consequently it may not always be quite clear how the steps described are to be performed4 ,it will be found1 however1 that all necessary information is really given1 though perhaps not always exactly in the place the reader might at first expect4 #ed4 ,all the discs that ,i have seen1 come from the makers already roughly ground on the edges to a circular figure--but occasionally the figure is very rough indeed--and in some cases1 especially if small lenses have to be made1 it is convenient to begin by cutting the glass discs out of glass sheet1 which also may be purchased of suit-able glass4 ,to do this1 the simplest way is to begin by cutting squares and then cutting off the corners with the diamond1 the approximate circular figure being obtained by grinding the edges on an ordinary grindstone4 ,if the pieces are larger1 time and material may be saved by using a diamond compass1 i4e4 an ordinary drawing compass armed with a diamond to cut circles on the glass1 and breaking the superfluous glass away by means of a pair of spectacle-maker's shanks 7,fig4 #dd71 or what does equally well1 a pair of pliers with soft iron jaws4 ,with these instruments glass can be chipped gradually up to any line1 whether diamond-cut or not1 the jaws of the pincers being worked against the edge of the glass1 so as to gradually crush it away4 ,fig4 #dd4 ,assuming that the glass has been bought or made roughly circular1 it must be finished on the lathe4 ,for this purpose it is necessary to chuck it on an iron or hardwood chuck1 as shown in ,fig4 #df4 ,for a lens below say an inch in diameter1 the centering cement may be used2 but for a lens of a diameter greater than this1 sufficient adhesion is easily obtained with ,regnault's mastic1 and its low melting point gives it a decided advantage over the shellac composition4 ,the glass may be heated gradually by placing it on the water bath1 or actually in the water1 and gradually bringing the water up to the boiling-point4 ,the glass1 being taken out1 is rapidly wiped1 and rubbed with a bit of waste moistened1 not wet1 with a little turpentine3 its surface is then rubbed with a stick of mastic previously warmed so as to melt easily4 ,the surface of the chuck being also warm1 and covered with a layer of melted cement1 it is applied to the glass4 ,the lathe is turned slowly by hand1 and the glass pushed gradually into the most central position2 it is then pressed tight against the chuck by the back rest1 a bit of wood being interposed for obvious reasons4 ,when all is cold the turning may be proceeded with4 ,the quickest way is to use the method already described 7i4e4 actual turning by a file tool72 but if the student prefers 7time being no object71 he may accomplish the reduction to a circular form very easily by grinding4 ,fig4 #de4 ,fig4 #df4 ,for this purpose he will require to make the following arrangements 7,fig4 #de74 ,if the lathe has a slide rest1 a piece of stout iron may be bent and cut so as to fit the tool rest1 and project beneath the glass4 ,the iron must be fairly rigid1 for if it springs appreciably beneath the pressure of the glass1 it will not grind the latter really round4 ,the lathe may run rather faster than for turning cast iron of the same size4 ,coarse emery1 passing through a sieve of #hj threads to the inch 7run71 may be fed in between the glass and iron1 and the latter screwed up till the disc just grinds slightly as it goes round4 ,a beginner will generally 7in this as in all cases of grinding processes7 tend to feed too fast--no grinding process can be hurried4 ,if a slide rest is not available1 a hinged board1 carrying a bit of iron1 may 7see ,fig4 #de7 be arranged so as to turn about its hinge at the back of the lathe2 and it may be screwed up readily enough by passing a long set-screw through the front edge1 so that the point of the screw bears upon the lathe bed4 ,i may add that emery behaves as if it were greasy1 and it is difficult to wet it with clean water4 ,this is easily got over by adding a little soap or alcohol to the water1 or exercising a little patience4 ,a good supply of emery and water should be kept between the disc and the iron2 a little putty may be arranged round the point of contact on the iron to form a temporary trough4 ,in any case the resulting emery mud should on no account be thrown away1 but should be carefully kept for further use4 ,the process is complete when the glass is perfectly round and of the required diameter as tested by callipers4 #ee4 ,the next step is to rough out the lens1 and this may easily be done by rotating it more slowly1 i4e4 with a surface speed of ten feet per minute1 and turning the glass with a hard file1 as explained in #db4 ,if it is desired to employ the slide rest1 it is quicker and better to use a diamond tool--an instrument quite readily made1 and of great service for turning emery wheels and the like1--a thing1 in fact1 which no workshop should be without4 ,a bit of diamond bort1 or even a clear though off-colour stone1 may be employed4 ,an ordinary lathe tool is prepared by drawing down the tool steel to a long cone1 resembling the ordinary practice in preparing a boring tool4 ,the apex of the cone must be cut off till it is only slightly larger than the greatest transverse diameter of the diamond splinter4 ,the latter may have almost any shape--a triangular point1 one side of a three-sided prism is very convenient4 ,a hole is drilled in the steel 7which must have been well softened71 only just large enough to allow the diamond to enter--if the splinter is thicker in the middle than at either end1 so much the better--the diamond is fastened in position by squeezing the soft steel walls tightly down upon it4 ,personally ,i prefer to use a tool holder1 and in this case generally mount the diamond in a bit of brass rod of the proper diameter2 and instead of pinching in the sides of the cavity1 ,i tin them1 and set the diamond in position with a drop of soft solder4 ,fig4 #dg4 ,in purchasing diamond bort1 a good plan is to buy fragments that have been employed in diamond drilling1 and have become too small to reset2 in this case some idea as to the hardness of the bits may be obtained4 ,full details as to diamond tool-making are given in books on watch-making1 and in ,holtzapffell's great work on ,mechanical ,manipulation2 but the above notes are all that are really necessary--it is1 in fact1 a very simple matter4 ,the only advantage of using a diamond tool for glass turning is that one does not need to be always taking it out of the rest to sharpen it1 which generally happens with hard steel1 especially if the work is turned a little too fast4 ,i recommend1 therefore1 that the student should boldly go to work free hand with a hard file2 but if he prefer the more formal method1 or distrust his skill 7which he should not do71 then let him use a diamond point1 even if he has the trouble of making it4 ,when using a diamond it is not necessary to employ a lubricant1 but there is some advantage in doing so4 ,the surface of the lens can be roughly shaped by turning to a template or pattern made by cutting a circular arc 7of the same radius as the required surface7 out of a bit of sheet zinc4 ,another very handy way of making templates of great accuracy is to use a beam compass 7constructed from a light wooden bar7 with a glazier's diamond instead of a pencil4 ,a bit of thin sheet glass is cut across with this compass to the proper curvature--which can be done with considerable accuracy and the two halves of the plate1 after breaking along the cut1 are ground together with a view to avoiding slight local irregularities1 by means of a little fine emery and water laid between the edges4 ,in this process the glass is conveniently supported on a clean board or slate1 and the bits are rubbed backwards and forwards against each other4 #ef4 ,it is not very easy for a beginner to turn a bit of anything--iron1 wood1 or glass--with great accuracy to fit a template1 and consequently time may be saved by the following procedure1 applied as soon as the figure of the template is roughly obtained4 ,a disc of lead or iron1 of the same diameter as the glass1 and of approximately the proper curvature1 is prepared by turning1 and is armed with a handle projecting coaxially from the back of the disc4 ,the glass revolving with moderate speed on the lathe1 the lead tool1 supplied with coarse emery and water1 is held against it1 care being taken to rotate the tool by the handle1 and also to move it backwards and forwards across the disc1 through a distance1 say1 up to half an inch2 if it is allowed to overhang too much the edges of the glass disc will be overground4 ,by the use of such a tool the glass can readily be brought up to the template4 ,the only thing that remains1 so far as the description of this part of the process goes1 is to give a note or two as to the best way of making the lead tools1 and for this purpose the main narrative of processes must be interrupted4 ,the easiest way is to make a set of discs to begin with4 ,for this purpose take the mandrel out of the lathe1 and place it nose downwards in the centre of an iron ring of proper diameter on a flat and level iron plate4 ,the discs are made by pouring lead round the screw-nose of the mandrel4 ,this method1 of course1 leaves them with a hole in the centre2 but this can be stopped up by placing the hot disc 7from which the mandrel has been unscrewed7 on a hot plate1 and pouring in a sufficiency of very hot lead2 or1 better still1 the mandrel can be supported vertically at any desired distance above the plate while the casting is being poured4 ,lead discs prepared in this way are easily turned so as to form very convenient chucks for brass work1 and for use in the case now being treated1 they are easily turned to a template1 using woodturners' tools1 which work better if oiled1 and must be set to cut1 not scrape4 ,if the operator does not mind the trouble of cutting a screw1 or if he has a jaw chuck1 the lead may be replaced by iron with some advantage4 ,the following is a neat way of making concave tools4 ,it is an application of the principle of having the cutting tool as long as the radius of curvature1 and allowing it to move about the centre of curvature4 ,place the disc of iron or lead on the lathe mandrel or in the chuck1 and set the slide rest so that it is free to slide up or down the lathe bed4 ,take a bar of tool steel and cut it a little longer than the radius of curvature required4 ,forge and finish one end of the bar into a pointed turning tool of the ordinary kind4 ,measure the radius of curvature from the point of the tool along the bar1 and bore a hole1 whose centre is at this point1 through the bar from the upper to the lower face4 ,i regard the upper face as the one whose horizontal plane contains the cutting point when the tool is in use4 ,clamp a temporary back centre to the lathe bed1 and let it carry a pin in the vertical plane through the lathe centres1 and let this pin exactly fit the hole in the bar4 ,fig4 #dh4 ,place the radius tool in position for cutting1 and let it be lightly held in the slide rest nearly at the cutting point1 the centre of rotation of the pedestal 7or its equivalent7 passing through the central line of the bar4 ,then adjust the temporary back rest1 so that the tool will take a cut4 ,in the sketch the tool is shown swinging about the back centre instead of about a pin--there is little to choose between the methods unless economy of tool steel is an object4 ,the tool must now be fed across the work4 ,the pedestal must of course be free to rotate1 and the slide rest to slip up and down the bed4 ,in this way a better concave grinding tool can be made than would be made by a beginner by turning to a template--though an expert turner would probably carry out the latter operation so as to obtain an' accuracy of the same order1 and would certainly do it in much less time than would be required in setting up the special arrangements here described4 ,on the other hand1 if several surfaces have to be prepared1 as in the making of an achromatic lens1 the quickest way would be by the use of the radius tool1 bored of course to work at the several radii required4 ,i have tried both methods1 and my choice would depend partly on the lathe at my disposal1 and partly on the number of grinding tools that had to be prepared4 ,having obtained a concave tool of any given radius1 it is easily copied--negatively1 so as to make a convex tool in the following manner4 ,adjust the concave tool already made on the back rest1 so that if it rotated about the line of centres1 it would rotate about its axis of figure4 ,arrangements for this can easily be made1 but of course they will depend on the detailed structure of the lathe4 ,use the slide rest as before1 i4e4 let it grasp an ordinary turning tool lightly1 the pedestal being fixed1 but the rest free to slide up or down the lathe bed4 ,push the back rest up till the butt of the turning tool 7ground to a rounded point7 rests against the concave grinding tool4 ,if the diameter of the convex tool required be very small compared with the radius of curvature of the surface 7the most usual case71 it is only necessary to feed the cutting tool across to copy the concave surface sufficiently nearly4 ,fig4 #di4 ,there seems no reason1 however1 why these methods should not be applied at once to the glass disc by means of a diamond point1 and the rough grinding thus entirely avoided4 ,i am informed that this has been done by ,sir ,henry ,bessemer1 but that the method was found to present no great advantage in practice4 ,a reader with a taste for mechanical experimenting might try radius bar tools with small carborundum wheels rapidly driven instead of a diamond4 ,enough has now been said to enable any one to prepare rough convex or concave grinding tools of iron or lead1 and of the same diameter as the glass to be ground4 ,the general effect of the process of roughing the rotating lens surface is to alter the radius of curvature of both tool and glass2 hence it is necessary to have for each grinding tool another to fit it1 and enable it to be kept 7by working the two together7 at a constant figure4 ,after a little practice it will be found possible to bring the glass exactly up to the required curvature as tested by template or spherometer4 ,the art of the process consists in altering the shape of the grinding tool so as to take off the glass where required1 as described in #ec1 and from this point of view lead has some advantages2 7opinions vary as to the relative advantages of lead and iron tools for this purpose1 however74 ,the subsidiary grinding tool is not actually needed for this preliminary operation1 but it has to be made some time with a view to further procedure1 and occasionally is of service here4 #eg4 ',the glass disc must be ground approximately to the proper curvature on each side before any fine grinding is commenced4 ,it is precisely for this purpose that the previous turning of the disc is recommended1 for it is easy to unmount and recentre a round object1 but not so easy if the object have an indefinite shape4 ,using a cement which is plastic before it sets1 the disc may be easily taken off the chuck and centred by a little handicraft1 i4e4 by rotating the lathe slowly and pushing the disc into such a position that it rotates about its axis4 ,the grinding of the second surface is accomplished exactly as in the former case2 of course on reversing the glass the chuck has to be slightly turned up to fit the convex or concave surface4 #eh4 ,there is1 however1 one point of interest and importance--attention to which will save a good deal of useless labour afterwards4 ,the glass must be ground in such a manner that the thickness at the edge is the same all round4 ,in other words1 the axes of figure of the two surfaces must coincide4 ,this will be the case if the recentering has been accurately performed1 and therefore no pains should be spared to see that it is exactly carried out4 ,any simple form of vernier gauge 7such as ,brown and ,sharpe's vernier callipers7 will serve to allow of a sufficiently accurate measurement of the edge thickness of the lens4 ,if any difference of thickness is observed as the gauge moves round the edge1 one or other of the surfaces must be reground4 ,of course the latitude of error which may be permitted depends so much on the final arrangements for a special finishing process called the centering of the lens--which will be described--that it is difficult to fix a limit1 but perhaps one-thousandth of an inch may be mentioned as a suitable amount for a #b-;inch disc4 ,for rough work1 of course2 more margin may be admitted4 #ei4 ,in a large shop ,i imagine that lenses of only two inches diameter would be ground in nests2 or1 in other words1 a number would be worked at a time1 and centering1 even of a rough kind1 would be left to the last2 but this process will be treated hereafter4 ,at present ,i shall assume that only one lens will be made at a time4 ,consequently we now enter on the stage of fine grinding by hand4 ,a leaden pedestal1 for the sake of stability1 must be provided on which to mount the lens1 so that the surface to be operated on may be nearly horizontal 7,fig4 #ej74 ,before this can be done1 however1 fresh grinding tools 7two for each surface7 must be properly prepared4 ,after trying several plans ,i unhesitatingly recommend that all fine-grinding surfaces should be made of glass4 ,this is easily done by taking two discs of lead1 or iron1 or slate1 cut to a one-tenth inch smaller radius of curvature 7in the case of a convex tool1 and the opposite in the other case7 than the lens surface 7,fig4 #ea1 ,a74 ,on these1 square bits of sheet glass1 one-tenth of an inch thick1 are to be cemented1 so as to leave channels of about one-eighth of an inch between each bit of glass 7,fig4 #eb1 ,b74 ,the mastic cement formerly described may be employed for this purpose4 ,fig4 #ej4 ,the bits of glass ought first to have their edges dressed smooth on the grind-stone4 ,a convex and concave glass surface having been thus roughly prepared1 they must be mounted in turn in the lathe1 and brought to the proper curvature by grinding with the tools formerly employed and tested by the template or spherometer4 ,it is well to control this process by means of a spherometer1 so that the desired radius may be approximately reached4 ,the two glass-grinding tools are then ground together by hand 7see #ec and #fa71 the spherometer being employed from time to time to check the progress of the work4 ,in general1 if large circular sweeps are taken1 greatly overhanging the side of the glass surface to be figured1 both the upper and lower surfaces will be more ground at the edges1 while in the opposite event the centre will be chiefly affected4 ,fig4 #ea4 ,a spherometer capable of measuring a #b-;inch surface may be procured1 having a screw of1 say1 #ej threads to the inch1 and a micrometer surface divided into #bjj parts1 each part easily capable of subdivision--into tenths or even twentieths4 ,to get the full advantage of the spherometer it must screw exceedingly freely 7i4e4 must be well oiled with clock oil71 and must not be fingered except at the milled head4 ,if one of the legs is held by the fingers the expansion is sufficient to throw the instrument quite out of adjustment4 ,the glass-grinding tools being brought to the proper figure1 the next process is to transfer the same to the lens1 and this is done by similar means1 the fellow tool being used to correct the one employed in grinding the lens surface4 ,before the grade of emery is changed all three surfaces must agree1 as nearly1 at least1 as the spherometer will show4 ,in order to prevent confusion the following summary of the steps already taken may be given4 ,the discs of glass are first ground or turned so as to be truly circular4 ,four tools are made for each surface--a rough pair of iron or lead1 and a finishing pair of iron1 lead1 or slate faced by glass squares4 ,for a small lens the iron or lead backing may be used1 for a large one the slate4 ,the rough tools are used to give an approximate figure both to the lens and to the finishing tools4 ,the final adjustment is attained by grinding one of the glass-faced tools alternately upon the lens and upon the fellow glass-faced tool4 ,the spherometer is accepted at all stages of the process as the final arbiter as to curvature4 ,some hints on the form of strokes used in grinding will be given later on 7see #fa74 ,it suffices to state here that the object throughout is to secure uniformity by allowing both the work and the tool to rotate1 and exercising no pressure by the fingers4 ,the tool backing may weigh from one to two pounds for a #b-;inch lens4 #fj4 ,the tools and lens being all of the same curvature1 the state of the surface is gradually improved by grinding with finer and finer emery4 ,the best way of grading the emery is by washing it with clean water1 and allowing the emery 7at first stirred up with the water7 to settle out4 ,the longer the time required for this part of the process the finer will be the emery deposited4 ,an ordinary bedroom jug is a very good utensil to employ during this process2 a large glass jug is even better4 ,the following grades will be found sufficient1 though ,i daresay every operative's practice differs a little on this point4 #ast grade3 ,flour emery1 with the grit washed out1 i4e4 allowed to stand for #b 7sec47 before being poured off4 #bnd grade3 ,stand #e 7secs471 settle in #a' 7min47 #crd grade3 ,stand #a'1 settle in #aj'4 #dth grade3 ,stand #aj'1 settle in #fj'4 ,it is generally advisable to repeat the washing process with each grade4 ,thus1 selecting grade #b for illustration1 the liquor for grade #c must be poured off without allowing any of the sediment to pass over with it4 ,if any sediment at all passes1 one has no security against its containing perhaps the largest particle in the jug4 ,as soon as the liquor for ,no4 #c has been decanted1 jug ,no4 #b is filled up again with clean water 7filtered if necessary71 and after standing #e is decanted into jug ,no4 #b;b1 the sediment is returned to jug ,no4 #a1 and the liquor1 after standing #a'1 is transferred to jug ,no4 #c4 ,the greatest care is necessary at each step of the operation to prevent sediment passing over with liquor4 ,there is a little danger from the tendency which even comparatively large particles of emery have to float1 in consequence of their refusing to get wet1 and the emery worked up on the side of the jug is also a source of danger1 therefore wipe the jug round inside before decanting4 ,in order to get a uniform grade stop the currents of water in the jug1 which may work up coarse particles1 by holding a thin bit of wood in the rotating liquid for a moment1 and then gently withdrawing it in its own plane4 ,these precautions are particularly necessary in the case of grades ,nos4 #b1 #c1 and #d1 especially ,no4 #d1 for if a single coarse particle gets on the tool when the work has progressed up to this point it will probably necessitate a return to grinding by means of ,no4 #b1 and involve many hours' work4 ,the surface of the lens will require to be ground continuously with each grade till it has the uniform state of roughness corresponding to the grade in question4 ,two hours for each grade is about the usual time required in working such a lens as is here contemplated4 ,the coarser grades of emery may be obtained by washing ordinary flour of emery1 but the finer ones have to be got from emery which has been used in the previous processes4 ,it is not a good plan to wash the finer grades of emery out of the proceeds of very rough grinding say with anything coarser than flour of emery--as there is a danger of thereby contaminating the finer grades with comparatively coarse glass particles 7owing to their lightness7 and this may lead to scratching4 ,if the finer grades are very light in colour1 it may be inferred that a considerable portion of the dust is composed of glass1 and this does no good4 ,consequently time may be saved by stirring up the light-coloured mass with a little hydrofluoric acid in a platinum capsule2 this dissolves the finely divided glass almost instantaneously4 ,the emery and excess of hydrofluoric acid may then be thrown into a large beaker of clean water and washed several times4 ,fine emery thus treated has much the same dark chocolate colour as the coarser varieties4 ,the operator should not wear a coat1 and should have his arms bare while working with fine emery1 for a workshop coat is sure to have gathered a good deal of dust1 and increases the chances of coarse particles getting between the surfaces4 #fa4 ,details of the ,process of ,fine ,grinding4 ,a lens of the size selected for description is mounted as before mentioned on a leaden pedestal1 and the operator places the latter on a table of convenient height in a room as free from dust as possible4 ,everything should be as clean as a pin1 and no splashes of emery mud should be allowed to lie about4 ,i have found it convenient to spread clean newspapers on the table and floor1 and to wear clean linen clothes1 which do not pick up dust4 ,i have an idea that in large workshops some simpler means of avoiding scratches must have been discovered1 but ,i can only give the results of my own experience4 ,i never successfully avoided scratches till ,i adopted the precautions mentioned4 ,fig4 #eb4 ,the left hand should be employed in rotating the pedestal either continuously 7though slowly7 or at intervals of1 say1 one minute4 ,this point is rather important4 ,some operators require two hands to work the grinding tool1 and in any case this is the safer practice4 ,under these circumstances the pedestal may be rotated through one-eighth or tenth of a revolution every three minutes1 or thereabouts4 ,the general motion given to the grinding tool should be a series of circular sweeps of about one-fourth the diameter of the glass disc1 and gradually carried round an imaginary circle drawn on the surface of the lens and concentric with it 7,fig4 #eb74 ,the tool may overhang the lens by a quarter of the diameter of the latter as a maximum4 ,the circuit may be completed in from twelve to thirty sweeps4 ,the grinding tool should be lightly held by the fingers and the necessary force applied parallel to the surface4 ,the tool itself must be slowly rotated about its axis of figure4 ,if the tool be lightly held1 it will be found that it tends to rotate by itself4 ,i say tends to rotate1 for if the tool be touching evenly all over the surface it will rotate in a direction opposite to the direction of the circular sweep4 ,for instance1 if the tool be carried round its looped path clockwise1 it will tend to rotate about its own axis of figure counter-clockwise4 ,if it touch more in the middle1 this rotation will be increased1 while if it touches more along the edge1 the rotation will be diminished1 or even reversed in an extreme case4 ,every fifty sweeps or so the tool should be simply ground backwards and forwards along a diameter of the lens surface4 ,this grinding should consist of three or four journeys to and fro along1 say1 eight different diameters4 ,about one-quarter of the whole grinding should be accomplished by short straight strokes1 during which the tool should only overhang about one-quarter of an inch4 ,the object of the straight strokes is to counteract the tendency to a gradual accumulation of the emery in the centre1 which results from the circular grinding4 ,a great deal of the art of the process consists in knowing how to work the tool to produce any given effect4 ,for instance1 if the lens requires to be ground down near the centre1 the epicycloidal strokes must be nearly central2 the tool must never overhang very much4 ,if1 on the other hand1 it is the edges which require attention1 these must be dealt with by wider overhanging strokes4 ,the tool must be frequently tested on its fellow1 and1 indeed1 ground upon it if any marked unevenness of action 7such as that just described7 is required for the lens4 ,a check by spherometer will be applied at intervals according to the judgment of the operator1 but1 in any case1 the fellow tool and lens should be kept at very nearly the same figure4 ,the emery should never be allowed to become anything like dry between the tool and the lens1 for in some way 7probably by capillary action increasing the pressure of the tool7 this seems to lead to scratching and rolling of the emery4 ,the channels in the glass tool between the squares are of the greatest importance in enabling the emery to distribute itself4 ,perhaps the best guide in enabling one to judge as to when it is time to wash off the emery and apply fresh is the feel of the tool2 also when the mud gets light in colour we know that it is full of glass dust1 and proportionately inoperative4 ,new emery may be put on1 say1 every five minutes1 but no absolute rule can be given1 for much depends on the pressure of the tool upon the lens4 ,in the case considered a brass or lead1 or even slate tool1 of an inch1 or even less1 in thickness1 will press quite heavily enough4 ,in washing the lens and tool before new emery is introduced1 a large enamelled iron bucket is very handy2 the whole of the tool should be immersed and scrubbed with a nail-brush4 ,the lens surface may be wiped with a bit of clean sponge1 free from grit1 or even a clean damp cloth4 ,when the time comes to alter the grade of emery1 a fresh lot of newspapers should be put down1 and tools1 lens1 and pedestal well washed and brushed by the nail-brush4 ,the surfaces should be wiped dry by a fresh piece of rag1 and examined for scratches and also for uniformity of appearance2 a good opinion can be formed as to the fit of the surfaces by noting whether--and if so1 to what degree--they differ in appearance from point to point when held so that the light falls on them obliquely4 ,it is necessary to exercise the greatest care in the washing between the application of successive grades of emery1 and this will be facilitated if the edges of the glass squares were dressed on a grindstone before they were mounted4 ,an additional precaution which may be of immense advantage is to allow the tool to dry between the application of successive grades of emery 7of course1 after it has been scrubbed71 and then to brush it vigorously with a hat-brush4 ,it sometimes happens that particles of mud which have resisted the wet scrubbing with the nail-brush may be removed by this method4 ,as my friend ,mr4 ,cook informs me that his present practice differs slightly from the above1 ,i will depart from the rule ,i laid down1 and add a note on an alternative method4 ,consider a single lens surface4 ,this is roughed out as before by an iron tool1 a rough fellow tool being made at the same time4 ,the squares of glass are cemented to the roughing tool1 and this is ground to the spherometer by means of the counterpart tool4 ,the glass-coated tool is then applied to the lens surface and grinding with the first grade of emery commenced4 ,the curvature is checked by the spherometer4 ,two auxiliary tools of1 say1 half the diameter of the lens1 are prepared from slate1 or glass backed with iron1 and applied to grind down either the central part of the lens surface or tool surface1 according to the indications of the spherometer4 ,any changes that may occur during grinding are corrected by these tools4 ,the spherometer is accepted as the sole guide in obtaining the proper curvature4 ,a slate backing is preferred for tools of any diameter over1 say1 #b inches4 #fb4 ,polishing4 ,after the surface has been ground with the last grade of emery1 and commences to become translucent even when dry1 the grinding may be considered to be accomplished1 and the next step is the polishing4 ,there are many ways of carrying out this process1 and the relative suitability of these methods depends on a good many1 so to speak1 accidental circumstances4 ,for instance1 if the intention is to finish the polishing at a sitting1 the polishing tool may be faced with squares of archangel--not mineral or coal-tar--pitch and brought to shape simply by pressing while warm against the face of the lens4 ,a tool thus made is very convenient1 accurate1 and good1 but it is difficult to keep it in shape for any length of time2 if left on the lens it is apt to stick1 and if it overhangs ever so little will1 of course1 droop at the edges4 ,on the whole1 the following will be found a good and sufficient plan4 ,the glass-grinding tool is converted into a polishing tool by pasting a bit of thin paper over its surface2 a bit of woven letter paper of medium thickness with a smooth but not glazed surface does very well4 ,we have found that what is called ,smith's #ba lbs4 ,vellum ,wove is excellent4 ,this is steeped in water till quite pliable and almost free from size4 ,the glass tool is brushed over with a little thin arrowroot or starch paste1 and the paper is laid upon it and squeezed down on the glass squares as well as possible2 if the paper is wet enough and of the proper quality it will expand sufficiently to envelop the tool without creases1 unless the curvature is quite out of the common4 ,this being accomplished1 and the excess of water and paste removed1 the face of the paper is 7for security7 washed with a little clean water and a bit of sponge1 and1 finally1 the tool is slightly pressed on the lens so as to get the paper to take up the proper figure as nearly as possible4 ,after the polishing tool has been thus brought to the proper figure1 it is lifted off and allowed to dry slowly4 ,when the paper is dry it may be trimmed round the edges so as not to project sensibly beyond the glass squares4 ,the next step is to brush the surface over very carefully with polishing rouge 7prepared as is described at the end of this section7 by means of a hat-brush4 ,when the surface of the paper is filled with rouge all excess must be removed by vigorous brushing4 ,fig4 #ec4 ,the tool being placed on the lens1 two or three strokes similar to those used in grinding may be taken1 and the tool is then lifted off and examined4 ,it will be found to be dotted with a few bright points1 produced by the adhesion of glass at the places of contact4 ,these points are then to be removed in the following manner4 ,an old three-cornered file is ground on each side till the file marks disappear1 and sharp edges are produced 7,fig4 #ec74 ,this tool is used as an ink eraser1 and it will be found to scrape the paper of the polishing tool very cleanly and well4 ,the bright spots are the objects of attention1 and they must be erased by the old file1 and the polisher reapplied to the glass4 ,a few strokes will develop other points1 more numerous than before1 and these in turn must be erased4 ,the process is continued till the whole surface of the polishing tool is evenly covered with bright specks1 and then the polishing may be proceeded with4 ,the specks should not be more than about one-eighth of an inch apart1 or the polishing will be irregular4 ,the operation of polishing is similar to that of grinding4 ,a reasonable time for polishing a glass surface is twenty hours2 if more time is required it is a sign that the fine grinding has not been carried far enough4 ,the progress of the operation may be best watched by looking at the surface--not through it4 ,for this purpose a good light is requisite4 ,when the lens is dismounted it may be examined by a beam of sunlight in a dark room1 under which circumstances the faintest signs of grayness are easily discernible4 ,it may be mentioned here that if the surface is in any way scratched the rouge will lodge in the scratches with great persistence1 and an expert can generally tell from the appearance of scratches what kind of polishing powder has been employed4 ,the persistence with which rouge clings to a rough surface of glass is rather remarkable4 ,some glass polishers prefer to use putty powder as a polishing material1 and it is sometimes said to act more quickly than rouge2 from my rather limited experience ,i have not found this to be the case1 but it may have merits that ,i do not know of4 ,is it possible that its recommendation lies in the fact that it does not render scratches so obtrusively obvious as rouge does8 ,rouge is generally made in two or more grades4 ,the softer grade is used for polishing silver1 and is called jewellers' rouge4 ,the harder grade1 suitable for glass polishing1 is best obtained from practical opticians 7not mere sellers of optical instruments74 ,i mean people like ,messrs4 ,cook of ,york4 ,many years ago ,i prepared my own hard rouge by precipitating ferrous sulphate solution by aqueous ammonia1 washing the precipitate1 and heating it to a red heat4 ,the product was ground up with water1 and washed to get rid of large particles4 ,this answered every purpose1 and ,i could not find that it was in any way inferior to hard rouge as purchased4 ,the same precipitate heated to a lower temperature is said to furnish a softer variety of rouge2 at all events1 it gives one more suitable for polishing speculum metal4 ,lord ,rosso used rouge heated to a dull redness for this purpose4 ,rouge1 whether made or bought1 should always be washed to get rid of grit4 ,i ought to add that not the least remarkable fact about the polishing is the extraordinarily small quantity of the polishing material requisite1 which suggests that the process of polishing is not by any means the same as that of exceptionally fine grinding4 ,is it possible that the chief proximate cause of the utility of rouge is to be sought in its curious property of adhering to a rough glass surface1 causing it1 so to speak1 to drag the glass off in minute quantities1 and redeposit it after a certain thickness has been attained on another part of the surface8 #fc4 ,centering4 ,when a lens is ground and polished it will almost always happen that the axis of revolution of its cylindrical edge is inclined to the axis of revolution of its curved surfaces4 ,since in practice lenses have to be adjusted by their edges1 it is generally necessary to adjust the edge to a cylinder about the axis of figure of the active surfaces4 ,this is best done on a lathe with a hollow mandrel44 ,the lens is chucked on a chuck with a central aperture--generally by means of pitch or ,regnault's mastic1 or centering cement for small lenses--and a cross wire is fixed in the axis of revolution of the lathe1 and is illuminated by a lamp4 ,this cross wire is observed by an eye-piece 7with cross wires only in the case of a convex lens1 or a telescope similarly furnished in the case of a concave lens71 also placed in the axis of rotation of the lathe4 ,both cross wires are thus in the axis of revolution of the mandrel1 and the distant one 7,b in the figure7 is viewed through the lens and referred to the fixed cross wires at ,a4 ,in general1 as the lathe is rotated by turning the mandrel the image of the illuminated cross wires will be observed to rotate also4 ,the lens is adjusted until the image remains steady on rotating the mandrel and it is to give time for this operation that a slow-setting cement is recommended4 ,when the image remains stationary we know that the optical centre of the lens is in the axis of revolution1 and that this axis is normal to both lens surfaces1 i4e4 is the principal axis of the lens1 or axis of figure4 ,fig4 #ed4 ,a much readier method1 and one1 in general1 good enough for most purposes1 is to put a candle on the end of the lathe-bed where the back centre generally is1 and observe the images of the flame by reflection from both the lens surfaces4 ,this method is very handy with small lenses2 the mandrel is turned1 and the lens adjusted by hand till the images are immovable4 ,in both cases1 of course1 the edge of the lens is turned or ground till it is truly circular1 the position of the lens remaining undisturbed on the chuck4 ,if the edge gauge has been properly used in the earlier stages of figuring1 it will be found that very little turning or grinding is requisite to produce a true centering4 ,the particular defect due to want of centering in a lens may be observed by using it as the objective of a telescope1 and observing a star slightly out of focus4 ,the interference fringes will not be concentric circles unless the lens is properly centred4 ,i ought to say that ,i have not looked into the theory of this1 but have merely taken it as a generally admitted fact4 ,the diseases of lenses and the modes of treating them are dealt with in a book by ,messrs4 ,cook of ,york1 entitled ,on the ,adjustment and ,testing of ,telescopic ,objectives4 ,the final process of figuring will be dealt with later on 7 #ff and #fg71 as it applies not only to lenses but to mirrors1 prisms1 etc4 ,if the instructions given have been carefully carried out on a #b-;inch lens1 it should perform fairly well1 and possibly perfectly1 without any further adjustment of the glass4 #fd4 ,preparation of ,small ,lenses1 where great ,accuracy is not of the first ,importance4 ,such lenses may generally be made out of bits of good plate or sheet glass1 and are of constant use in the physical laboratory4 ,they may be purchased so cheaply1 however1 that only those who have the misfortune to work in out-of-the-way places need be driven to make them4 ,suitable glass having been obtained and the curves calculated from the index of refraction1 as obtained by any of the ordinary methods applicable to plates 7the microscope method1 in general1 is quite good enough71 squares circumscribing the desired circles are cut out by the help of a diamond4 ,footnote3 ,glazebrook and ,shaw's ,practical ,physics1 p4 #chc 7#dth ed474 ,the squares are roughly snipped by means of a pair of pliers or spectacle-maker's shanks4 ,the rough circles are then mounted on the end of a brass or iron rod of rather greater diameter than the finished lenses are to possess4 ,this mounting is best done by centering cement4 ,the discs are then dressed circular on a grindstone1 the rod serving both as a gauge and handle4 ,a sufficient number of these discs having been prepared1 a pair of brass tools of the form shown in the sketch 7,fig4 #ee71 and of about the proper radius of curvature1 are made4 ,one of these tools is used as a support for the glass discs4 ,fig4 #ee4 ,a compass being set to scribe circles of the same diameter as the glass discs1 centre marks are made on the surface of the appropriate tool1 circles are drawn on this1 and facets are filed or milled 7for which the spiral head of the milling machine is excellent74 ,in the case of concave supporting surfaces1 i4e4 in making concave lenses1 ,i apprehend filing would be difficult1 and the facets would have to be made by a rose cutter or mill2 but if the discs are fairly round1 then1 in fact1 no facets are required4 ,the facets being ready1 the glass discs are cemented to them by centering cement1 which may be used quite generally for small lenses4 ,when the cutting of facets has been omitted on a concave surface1 the best cement is hard pitch4 ,the grinding tool is generally rather larger than the nest of lenses4 ,coarse and fine grinding is accomplished wholly on the lathe--the tool being rotated at a fair speed 7see infra71 and the nest of lenses moved about by its handle so as to grind all parts equally4 ,it must1 of course1 be held anywhere except dead on1 for then the part round the axis would not get ground2 this inoperative portion of the rotating tool must therefore be allowed to distribute its incapable efforts evenly over the nest of lenses4 ,polishing is accomplished by means of the grinding tool1 coated with paper and rouge as before2 or the tool may be coated with very thin cloth and used with rouge as before--in this case the polishing goes on fastest when the surface of the cloth is distinctly damp4 ,in working by this method1 each grade of emery need only be applied from five to ten minutes4 ,the glass does not appear to get scratched when the emery is changed1 provided everything is well washed4 ,a good polish may be got in an hour4 ,the lathe is run as for turning brass of the same diameter as the tool4 ,one side of the lenses being thus prepared1 they are reversed1 and the process gone through for the other side in a precisely similar manner4 ,footnote3 ,unless the radius of curvature is very short and the lenses also convex1 there is no necessity to recess the facets1 provided hard pitch is used as the cement4 ,see note on hard pitch4 ,to save trouble1 it is usual1 to make such lenses of equal curvature on both faces2 but of course this is a matter of taste4 ,fig4 #ef4 ,for very common work1 bits of good plate glass are employed1 and the manufacturer's surface treated as flat 7,fig4 #ef74 ,in this way plano-convex lenses are easily and cheaply made4 ,finally the lenses have to be centred1 an essential operation in this case4 ,this is easily done by the reflection method--the edge being turned off by the file and kerosene and the centering cement being used in making the preliminary adjustment on the chuck4 ,i presume a lens made in this way is worth about a shilling1 so that laboratory manufacture is not very remunerative4 ,fig4 #ef shows the method of mounting small lenses for lathe grinding1 when only one lens is required4 ,the tool is generally rotated in the lathe and the lens held against it4 #fe4 ,preparing ,small ,mirrors for ,galvanometers4 ,to get good mirrors for galvanometers1 ,i have found the best plan is to grind and polish a large number together1 on a disc perhaps #h or #aj inches in diameter4 ,i was led to this after inspecting and rejecting four ounces of microscope cover slips1 a most wearisome process4 ,that regular cover slips should be few and far between is not unlikely1 seeing that they are made 7by one eminent firm at least7 simply by pot blowing a huge thin bulb1 and then smashing it on the floor and selecting the fragments4 ,as in the case of large mirrors1 it is of course only necessary to grind one side of the glass1 theoretically at all events4 ,the objections to this course are3 7#a7 ,a silver surface cannot1 in my experience1 be polished externally 7on a minute object like a cover slip7 to be anything like so bright as the silver surface next the glass2 and1 7#b7 if one side only is ground1 it will be found that the little mirror hopelessly loses its figure directly it is detached from the support on which it has been worked4 ,consequently1 ,i recommend that these small mirrors should be ground and polished on both sides--enough may be made at one operation to last for a very long time4 ,a slate back is prepared of the same radius of curvature as it is desired to impart to the mirrors4 ,bits of thin sheet glass are then ground circular as described in the last section and cemented to this surface by the smallest quantity of clean archangel pitch1 allowed to cool slowly and even to rest for a day before the work is proceeded with4 ,the whole surface is then ground and polished as before4 ,the mirrors are now reversed1 when they ought to nearly fit the tool 7assuming that flats are being made1 and the fellow tool in all other cases71 and are recemented by pitch to the appropriate backing ground1 and polished4 ,if very excellent results are required1 these processes may be preceded by a preliminary rough grinding of one surface1 so that the little discs will sit exactly on the tool surface1 and not run the risk of being strained by capillary forces in the pitch4 ,we have always found this necessary for really good results4 ,on removing such mirrors from the backing1 they generally1 more or less lose their figure1 becoming 7in general fairly uniformly7 more concave or convex4 ,about #e per cent of the mirrors thus prepared will be found almost perfect if the work has been well done1 and the rest will probably be very fair1 unless the diameter is very large as compared with the thickness4 ,the best way of grinding and polishing such large surfaces 7nests #aj inches in diameter7 is on a grinding machine1 such as will be described below4 ,the polishing is best done by means of paper1 as before described4 ,having occasion to require hitherto unapproached lightness and optical accuracy in such mirrors1 ,i got my assistant to try making them of fused quartz1 slices being cut by a diamond wheel from a rod of that material4 ,chips of natural quartz were also obtained from broken pebble spectacles1 and these were worked at the same time4 ,the resulting mirrors were certainly superior to the best we could make from glass1 but the labour of grinding was greater1 and the labour of polishing less1 than in the latter case4 ,the pebble fragments gave practically as good mirrors as the fused slices4 ,for the future it will be better always to make galvanometer mirrors from quartz crystals4 ,these may be easily sliced1 as will be described in #gd4 ,the slices are dressed on a grindstone according to instructions already given for small lenses4 ,the silvering of these mirrors is a point of great importance4 ,after trying nearly every formula published1 we have settled down to the following4 ,a solution of pure crystallised nitrate of silver in distilled water is made up to a strength of #abe grams of the salt per litre4 ,this forms the stock solution and is kept in a dark bottle4 ,let the volume of silvering liquor required in any operation be denoted by #d v4 ,the liquor is prepared as follows3 ,i4 ,measure out a volume v of the stock solution of silver nitrate1 and calculate the weight of salt which it contains2 let this be w4 ,in another vessel dissolve pure ,rochelle salt to the amount of #b4f w1 and make up the solution to the volume v4 ,these two solutions are to be mixed together at a temperature of #ee ,c1 the vessels with their contents being heated to this temperature on the water bath4 ,after mixing the liquids the temperature is to be kept approximately constant for five minutes1 after which the liquor may be cooled4 ,the white precipitate which first forms will become gray or black and very dense as the liquid cools4 ,if it does not1 the liquor must be reheated to #ee ,c1 and kept at that temperature for a few minutes and then again allowed to cool4 ,the solution is in good order when all the precipitate is dense and gray or black and the liquor clear4 ,the blacker and denser the precipitate the better is the solution4 ,the liquor is decanted and filtered from the precipitate and brought up to the volume #b v by addition of some of the wash water4 ,,ii4 ,measure out a volume #j4aah v of the stock solution into a separate vessel1 and add to it a #e per cent solution of ammonium hydrate1 with proper precautions1 so that the precipitate at first formed is all but redissolved after vigorous shaking4 ,it is very important that this condition should be exactly attained4 ,therefore add the latter part of the ammonia very carefully4 ,make up the volume to #b v4 ,mix the solutions ,i4 and ,,ii4 in a separate vessel and pour the mixture into the depositing vessel4 ,the surface to be silvered should face downwards1 and lie just beneath the free surface of the liquid4 ,bubbles must of course be removed4 ,the silver deposit obtained in this manner is exceedingly white and1 bright on the surface next to the glass1 but the back is mat and requires polishing4 ,the detail of the process described above was worked out in my laboratory by ,mr4 ,a4 ,pollock1 to whom my thanks are due4 ,this process gives good deposits when the solutions are freshly prepared1 but the ammonia solution will not keep2 ,the surfaces to be silvered require to be absolutely clean4 ,the process is assisted by a summer temperature1 say #gj ,fahr1 and possibly by the action of light4 ,six or seven hours at least are required for a good deposit2 a good plan is to leave the mirrors in the bath all night4 ,on removal from the bath the mirrors require to be well washed1 and allowed to dry thoroughly in sun heat for several hours before they are touched4 ,care should be taken not to pull the mirrors out of shape when they are mounted for the bath4 ,a single drop of varnish or paint 7a mere speck7 on the centre will suffice to hold them4 ,the back of the deposit requires to be varnished or painted as a rule to preserve the silver4 ,all paints and varnishes thus applied tend to spoil the figure by expanding or contracting4 ,on the whole1 ,i think boiled linseed oil and white or red lead--white or red paint in fact--is less deleterious than other things ,i have tried4 ,shellac varnish is the worst4 ,of course1 the best mirror can be easily spoiled by bad mounting4 ,i have tried a great number of methods and can recommend as fairly successful the following3- ,a little pure white lead1 i4e4 bought as pure as a chemical--not as a paint--is mixed with an equal quantity of red lead and made into a paste with a little linseed oil4 ,i say a paste1 not putty4 ,a trace of this is then worked on to the back of the mirror at the centre as nearly as may be1 and to this is attached the support4 ,the only objection to this is that nearly a week is required for the paste to set4 ,if people must use shellac let it be remembered that it will go on changing its shape for months after it has cooled 7whether it has been dissolved in alcohol or not74 #ff4 ,preparation of ,large ,mirrors or ,lenses for ,telescopes4 ,so much has been written on this subject by astronomers1 generally in the ,english ,mechanic and in the ,philosophical ,transactions for #ahdj1 that it might be thought nothing could be added4 ,i will only say here that the processes already described apply perfectly to this case2 but of course ,i only refer to silver on glass mirrors4 ,for any size over #f inches in diameter1 the process of grinding and polishing by hand1 particularly the latter1 will probably be found to involve too much labour1 and a machine will be required4 ,a description of a modification of ,mr4 ,nasmyth's machine--as made by my assistant1 ,mr4 ,cook--will be found below4 ,there is no difficulty in constructing or working such a machine1 and considered as an all round appliance1 it possesses solid advantages over the simple double pulley and crank arrangement1 which1 however1 from its simplicity deserves a note4 ,two pulleys1 ,a and ,b1 of about #ah inches diameter by #d inches on the face1 are arranged to rotate about vertical axes1 and belted together4 ,the shaft of one of these pulleys is driven by a belt in any convenient manner4 ,each pulley is provided on its upper surface with a crank of adjustable length carrying a vertical crank-pin4 ,each crank-pin passes through a #c,x #b wooden rod1 say #c' #f long1 and these rods are pinned together at their farther extremities1 and this pin carries the grinding or polishing tool1 or rather engages loosely with the back of this tool which lies below the rod4 ,it is clear that if the pulleys are of commensurable diameters1 and are rigidly connected--say by belting which neither stretches nor slips--the polishing tool will describe a closed curve4 ,if1 however1 the belt is arranged to slip slightly1 or if the pulleys are of incommensurable diameters1 the curve traced out by the grinding tool will be very complex1 and in the case of the ratio of the diameters being incommensurable1 will always remain open2 for polishing purposes the consummation to be wished4 ,mirror surfaces are ground spherical1 the reduction to parabolic form being attained in the process of polishing4 ,a very interesting account of the practice of dealing with very large lenses will be found in ,nature1 ,may #ahhf1 or the ,journal of the ,society of ,arts1 same date 7,i presume71 by ,sir ,howard ,grubb4 ,the author considers that the final adjustment of surfaces by figuring--of which more anon--is an art which cannot be learned by inspection1 any more than a man could learn to paint by watching an artist4 ,this is1 no doubt1 the case to some extent2 still1 a person wishing to learn how to figure a lens could not do better than take ,sir ,howard at his word1 and spend a month at his works4 ,meanwhile the following remarks must suffice2 it is not likely that anybody to whom these notes will be of service would embark on such large work as is contemplated by ,sir ,howard ,grubb4 ,fig4 #eg4 ,description of ,polishing ,machine4 ,power is applied through belting to the speed cone ,a4 ,by means of a bevel pinion rotation is communicated to the wheel ,d1 which is of solid metal and carries a ,t-slot1 ,c4 ,a pedestal forming a crank-pin can be clamped so as to have any desired radius of motion by the screw ,e4 ,a train of wheels ,e ,f ,g ,h ,k 7ordinary cast lathe change wheels7 communicate any desired ratio of motion to the tool-holder1 which simply consists of two pins projecting vertically downwards from the spokes of wheel ,k4 ,these pins form a fork1 and each prong engages in a corresponding hole in the back of the slate-grinding tool 7not shown in figure74 ,the connection with the tool is purposely loose4 ,the wheel ,e1 of course1 cannot rotate about the crank-pin ,d4 ,provision for changing the ratio of tool rotation is achieved by mounting the wheels composing the train on pins capable of sliding along a long slot in the bar supporting them4 ,the farther end of this bar is caused to oscillate to and fro very slowly by means of an additional crank-pin ,s and crank-shaft1 the projecting face of the bed-plate ,w being placed so as to allow ,v to slide about easily and smoothly4 ,motion is communicated to this part of the system by means of gears at #j and ,p1 and a belt working from ,p to ,q4 ,thus the vertical shaft ,r is set in motion and communicates by gears with ,s4 ,a pulley placed on the axle of the wheel carrying the crank-pin ,s gives a slow rotation to the work which is mounted on the table ,m4 ,a small but important feature is the tray ,l below the gear ,k4 ,this prevents dirt falling from the teeth of the wheel on to the work4 ,the motion of ,s is of course very much less than of ,b--say #ajj times less4 ,the work can be conveniently adjusted as to height by means of the screw ,n4 ,the machine must be on a steady foundation1 and in a place as free from dust as possible4 ,though it looks complicated it is quite straight-forward to build and to operate4 ,it is explained in ,lord ,rayleigh's article on ,optics in the ,encyclopaedia ,britannica that a very minute change in the form of the curvature of the surface of a lens will make a large difference in the spherical aberration4 ,this is to be expected1 seeing that spherical aberration is a phenomenon of a differential sort1 i4e4 a measure of the difference between the curvature actually attained1 and the theoretical curvature at each point of the lens1 for given positions of point and image4 ,sir ,h4 ,grubb gives an illustration of the minuteness of the abrasion required in passing from a curve of one sort to a curve of another1 say from a spherical to a parabolic curve1 consequently the process of figuring by the slow action of a polishing tool becomes quite intelligible4 ,in making a large mirror or lens all the processes hitherto described under grinding and polishing1 etc1 have to be gone through and in the manner described1 and when all this is accomplished the final process of correcting to test commences4 ,this process is called figuring4 #fg4 ,of the actual operation of this process ,i have no personal knowledge1 and the following brief notes are drawn from the article by ,sir ,h4 ,grubb1 from my assistant's 7,mr4 ,cook7 experience1 and from a small work ,on the ,adjustment and ,testing of ,telescopic ,objectives1 by ,t4 ,cook and ,sons1 ,buckingham ,works1 ,york 7printed by ,ben ,johnson and ,co1 ,micklegate1 ,york74 ,this work has excellent photographs of the interference rings of star images corresponding to various defects4 ,it must be understood that the following is a mere sketch4 ,the art will probably hardly ever be required in laboratory practice1 and those who wish to construct large telescopes should not be above looking up the references4 ,the process is naturally divided for treatment into two parts4 7#a7 ,the detection of errors1 and the cause of these errors4 7#b7 ,the application of a remedy4 7#a7 ,a lens1 being mounted with its final adjustments1 is turned on to a star1 which must not be too bright1 and should be fairly overhead4 ,the following appearances may be noted3- ,a4 ,in focus1 the star appears as a small disc with one or two rings round it2 inside and outside of the focus the rings increase in number1 are round1 concentric with the disc1 and the bright and dark rings are apparently equally wide4 ,the appearance inside the focus exactly resembles that outside when allowance is made for chromatic effects4 ,conclusion3 objective good1 and correctly mounted4 ,b4 ,the rings round the star in focus are not circular1 nor is the star at the centre of the system4 ,in bad cases the fringes are seen at one side only4 ,effects exaggerated outside and inside the focus4 ,conclusion3 the lens is astigmatic1 or the objective is not adjusted to be co-axial with the eyepiece4 ,c4 ,when in focus the central disc is surrounded by an intermittent diffraction pattern1 i4e4 for instance the system of rings may appear along1 and near1 three or more radii4 ,if these shift when the points of support of the lens are shifted1 flexure may be suspected4 ,d4 ,on observing inside and outside the focus1 the rings are not equally bright and dark4 ,this may be due to uncorrected spherical aberration1 particularly to a fault known as zonal aberration1 where different zones of the lens have different foci1 but each zone has a definite focus4 ,e4 ,irregular diffraction fringes point to bad annealing of the glass4 ,this may be checked by an examination of the lens in polarised light4 ,f4 ,if the disc appear blurred and coloured1 however the focus be adjusted1 incomplete correction for chromatic aberration is inferred4 ,if in addition the colouring is unsymmetrical 7in an extreme case the star disc is drawn out to a coloured band71 want of centering is to be inferred4 ,this will also show itself by the interference fringes having the characteristics described in ,c4 7#b7 ,the following steps may be taken in applying a remedy3 ,a4 ,the adjusting screws of the cell mounting the object glass may be worked until the best result is attained2 this requires great care and patience4 ,any errors left over are to be attributed to other causes than the want of collinearity of the axes of object glass and eyepiece4 ,b4 ,astigmatism is detected by rotating the object glass or object glass cell4 ,if the oval fringes still persist and the longer axis follows the lens1 astigmatism may be inferred4 ,similarly1 by rotating one lens on the other1 astigmatism1 or want of centering 7quite a different thing7 may be localised to the lens4 ,c4 ,the presence of flexure may be confirmed by altering the position of the points of support with respect to the eyepiece1 the lens maintaining its original position4 ,the addition of more points of support will in general reduce the ill effects4 ,how to get rid of them ,i do not know2 they are only serious with large lenses4 ,d4 ,spherical aberration may be located by using stops and zonal screens1 and observing the effect on the image4 ,sir ,h4 ,grubb determines whether any point on the lens requires to be raised or lowered1 by touching the glass at that point with a warm hand or cooling it by ether4 ,the effects so produced are the differential results of the change of figure and of refractive index4 ,by observing the effect of the heating or cooling of any part1 the operator will know whether to raise or lower that part1 provided that by a suitable preliminary experiment he has determined the relation between the effect produced by the change of figure1 and that due to the temperature variation of the refractive index4 ,in general it is sufficient to consider the change of shape only and neglect the change in refractive power4 ,e4 ,marked astigmatism has never been noticed by me1 but ,i should think that the whole lens surface would require to be repolished or perhaps reground in this case4 ,f4 ,to decide in which surface faults exist is not easy4 ,by placing a film of oil between the two surfaces nearly in contact these may be easily examined4 ,thus a mixture of nut and almond oil of the right proportion1 to be found by trial1 for the temperature1 will have the same refractive index as the crown glass1 and will consequently reduce any errors of figure in the interior crown surface1 if properly applied between the surfaces4 ,similarly the interior of the flint surface may have its imperfections greatly reduced in effect by using almond oil alone1 or mixed with bisulphide of carbon4 ,the outer surfaces1 ,i presume1 must be examined by warming or cooling over suitable areas or zones4 ,the defects being detected1 a matter requiring a great deal of skill and experience according to ,sir ,h4 ,grubb1 the next step is to remedy them2 and the remedial measures as applied to the glass constitute the process of figuring4 ,there are two ways of correcting local defects1 one by means of small paper or pitch covered tools1 which according to ,sir ,h4 ,grubb is dangerous1 and according to the experience of ,mr4 ,cook1 and ,i think of many ,french opticians1 safe and advantageous4 ,pitch polishing tools are generally used for figuring4 ,they are made by covering a slate backing with squares of pitch4 ,the backing is floated with pitch say one-eighth of an inch thick4 ,this is then scored into squares by a hot iron rod4 ,the tool1 while slightly warm1 is laid upon the lens surface1 previously slightly smeared with dilute glycerine1 until the pitch takes the figure of the glass4 ,the polishing material is rouge and water4 ,small tools are applied locally1 and probably can only be so applied with advantage for grave defects4 ,the other method is longer and probably safer4 ,it consists in furnishing the polishing tool with squares of pitch as before4 ,these being slightly warm1 the lens is placed upon them so that they will flow to the exact figure also as before4 ,i presume that the lens is to be slightly smeared with glycerine1 or some equivalent1 to keep the pitch from sticking4 ,the squares are most thickly distributed where the abrasion is most required1 i4e4 less pitch is melted out by the iron rod4 ,this may be supplemented by taking advantage of differences of hardness of pitch1 making some squares out of harder1 others out of softer pitch4 ,the aim is to produce a polishing tool which will polish unequally so as to remove the glass chiefly from predetermined parts of the lens surface4 ,the tool is worked over the surface of the lens by the polishing machine1 and part of the art consists in adjusting the strokes to assist in the production of the local variations required4 ,a source of difficulty and danger lies in the fact that the pitch squares are rarely of the same hardness1 so that some abrade the glass more rapidly than others4 ,this is particularly likely to occur if the pitch has been overheated4 ,footnote3 ,when pitch is heated till it evolves bubbles of gas its hardness increases with the duration of the process4 ,the reader must be good enough to regard these remarks as of the barest possible kind1 and not intended to convey more than a general idea of the nature of the process of figuring4 #fh4 ,a few remarks on cleaning lenses will fittingly close this part of the subject4 ,there is no need to go beyond the following instructions given by ,mr4 ,brashear in ,popular ,astronomy1 #ahid1 which are reproduced here verbatim4 ,the writer does not advise the use of either fine chamois skin1 tissue paper1 or an old soft silk handkerchief1 nor any other such material to wipe the lenses1 as is usually advised4 ,it is not1 however1 these wiping materials that do the mischief1 but the dust particles on the lenses1 many of them perhaps of a silicious nature1 which are always harder than optical glass1 and as these particles attach themselves to the wiping material they cut microscopic or greater scratches on the surfaces of the objective in the process of wiping4 ,i write this article with the hope of helping to solve this apparently difficult problem1 but which in reality is a very simple one4 ,let us commence by taking the object glass out of its cell4 ,take out the screws that hold the ring in place1 and lift out the ring4 ,placing the fingers of both hands so as to grasp the objective on opposite sides1 reverse the cell1 and with the thumbs gently press the objective squarely out of the cell on to a book1 block of wood1 or anything a little less in diameter than the objective1 which has had a cushion of muslin or any soft substance laid upon it4 ,one person can thus handle any objective up to #ab inches in diameter4 ,before separating the lenses it should be carefully noted how they were put together with relation to the cell1 and to one another1 and if they art not marked they should be marked on the edges conspicuously with a hard lead pencil1 so that when separated they may be put together in the same way1 and placed in the same relation to the cell4 ,with only ordinary precaution this should be an easy matter4 ,setting the objective on edge the two lenses may be readily separated4 ,and now as to the cleaning of the lenses4 ,i have1 on rare occasions1 found the inner surfaces of an object glass covered with a curious film1 not caused directly by moisture but by the apparent oxidation of the tin-foil used to keep the lenses apart4 ,a year or more ago a #g-;inch objective made by ,mr4 ,clark was brought to me to clean4 ,it had evidently been sadly neglected4 ,the inside of the lenses were covered with such a film as ,i have mentioned1 and ,i feared the glass was ruined4 ,when taken apart it was found that the tin-foil had oxidised totally and had distributed itself all over the inner surfaces4 ,i feared the result1 but was delighted to find that nitric acid and a tuft of absorbent cotton cut all the deposit off1 leaving no stains after having passed through a subsequent washing with soap and water4 ,i mention this as others may have a similar case to deal with4 ,for the ordinary cleaning of an objective let a suitable sized vessel1 always a wooden one1 be thoroughly cleaned with soap and water1 then half filled with clean water about the same temperature as the glass4 ,slight differences of temperature are of no moment4 ,great differences are dangerous in large objectives4 ,i usually put a teaspoonful of ammonia in half a pail of water1 and it is well to let a piece of washed 'cheese cloth' lie in the pail1 as then there is no danger if the lens slips away from the hand1 and1 by the way1 every observatory1 indeed every amateur owning a telescope1 should have plenty of 'cheese cloth' handy4 ,it is cheap 7about #c cts4 per yard7 and is superior for wiping purposes to any 'old soft silk handkerchief1' chamois skin1 etc4 ,before using it have it thoroughly washed with soap and water1 then rinsed in clean water1 dried and laid away in a box or other place where it can be kept clean4 ,when you use a piece to clean an objective throw it away1 it is so cheap you can afford to do so4 ,if the lenses are very dirty or 'dusty1' a tuft of cotton or a camel's-hair brush may be used to brush off the loose material before placing the lenses in the water1 but no pressure other than the weight of the cotton or brush should be used4 ,the writer prefers to use the palms of the hand with plenty of good soap on them to rub the surfaces1 although the cheese cloth and the soap answers nicely1 and there seems to be absolutely no danger of scratching when using the hands or the cheese cloth when plenty of water is used2 indeed when ,i wish to wipe off the front surface of an objective in use1 and the lens cannot well be taken out1 ,i first dust off the gross particles and then use the cheese cloth with soap and water1 and having gone over the surface gently with one piece of cloth1 throw it away and take another1 perhaps a third one1 and then when the dirt is1 as it were1 all lifted up from the surface1 a piece of dry cheese cloth will finish the work1 leaving a clean brilliant surface1 and no scratches of any kind4 ,in washing large objectives in water ,i generally use a 'tub' and stand the lenses on their edge4 ,when thoroughly washed they are taken out and laid on a bundle of cheese cloth and several pieces of the same used to dry them4 ,i think it best not to leave them to drain dry2 better take up all moisture with the cloth1 and vigorous rubbing will do no harm if the surfaces have no abrading material on them4 ,i have yet to injure a glass cleaned in this way4 ,this process may seem a rather long and tedious one1 but it is not so in practice1 and it pays4 ,in some places objectives must be frequently cleaned1 not only because they become covered with an adherent dust1 but because that dust produces so much diffused light in the field as to ruin some kinds of telescope work4 ,mr4 ,hale of the ,kenwood ,observatory tells me he cannot do any good prominence photography unless his objective has a clean surface2 indeed every observer of faint objects or delicate planetary markings knows full well the value of a dark field free from diffused light4 ,the object-glass maker uses his best efforts to produce the most perfect polish on his lenses1 aside from the accuracy of the curves1 both for high light value and freedom from diffused light in the field1 and if the surfaces are allowed to become covered with dust1 his good work counts for little4 ,if only the front surface needs cleaning1 the method of cleaning with cheese cloth1 soap and water1 as described above1 answers very well1 but always throw away the first and1 if necessary1 the second cloth1 then wipe dry with a third or fourth cloth2 but if the surfaces all need cleaning ,i know of no better method than that of taking the objective out of its cell1 always using abundance of soap and water1 and keep in a good humor4 #fi4 ,the ,preparation of ,flat ,surfaces of ,rock ,salt4 ,the preliminary grinding is accomplished as in the case of glass1 except that it goes on vastly faster4 ,the polishing process is the only part of the operation which presents any difficulty4 ,the following is an extract from a paper on the subject1 by ,mr4 ,j4 ,a4 ,brashear1 ,pittsburg1 ,pa1 ,u4,s4,a1 from the ,proceedings of the ,american ,association for the ,advancement of ,science1 #ahhe4 ,practically the same method was shown me by ,mr4 ,cook some years earlier1 so that ,i can endorse all that ,mr4 ,brashear says1 with the following exceptions4 ,we consider that for small salt surfaces the pitch is better scored into squares than provided with the holes recommended by ,mr4 ,brashear4 ,mr4 ,brashear's instructions are as follows4 ,after alluding to the difficulty of drying the polished salt surface--which is of course wet--,mr4 ,brashear says3- ,happily ,i have no trouble in this respect now1 and as my method is easily carried out by any physicist who desires to work with rock salt surfaces1 it gives me pleasure to explain it4 ,for polishing a prism ,i make an ordinary pitch bed of about two and one-half or three times the area of the surface of the prism to be polished4 ,while the pitch is still warm ,i press upon it any approximately flat surface1 such as a piece of ordinary plate glass4 ,the pitch bed is then cooled by a stream of water1 and conical holes are then drilled in the pitch with an ordinary counter sink bit1 say one-quarter of an inch in diameter1 and at intervals of half an inch over the entire surface4 ,this is done to relieve the atmospheric pressure in the final work4 ,the upper surface of the pitch is now very slightly warmed and a true plane surface 7usually a glass one1 prepared by grinding and polishing three surfaces in the ordinary way1 previously wetted7 is pressed upon it until the pitch surface becomes an approximately true plane itself4 ,fortunately1 moderately hard pitch retains its figure quite persistently through short periods and small changes of temperature1 and it always pays to spend a little time in the preparation of the pitch bed4 ,the polisher being now ready1 a very small quantity of rouge and water is taken upon a fine sponge and equally distributed over its surface4 ,the previously ground and fined salt surface 7this work is done the same as in glass working7 is now placed upon the polisher and motion instantly set up in diametral strokes4 ,i usually walk around the polisher while working a surface4 ,it is well to note that motion must be constant1 for a moment's rest is fatal to good results1 for the reason that the surface is quickly eaten away1 and irregularly so1 owing to the holes that are in the pitch bed4 ,now comes the most important part of this method4 ,after a few minutes' work the moisture will begin to evaporate quite rapidly4 ,no new application of water is to be made1 but a careful watch must be kept upon the pitch bed1 and as the last vestige of moisture disappears the prism is to be slipped off the polisher in a perfectly horizontal direction1 and if the work has been well done1 a clean1 bright1 and dry surface is the result4 ,the surface is now tested by the well-known method of interference from a perfect glass test plate 7see ,fig4 #agh74 ,if an error of concavity presents itself the process of polishing is gone over again1 using short diametral strokes4 ,if the error is one of convexity1 the polishing strokes are to be made along the chords1 extending over the edge of the polisher4 ,the one essential feature of this method is the fact that the surface is wiped dry in the final strokes1 thus getting rid of the one great difficulty of pitch polishing1 a method undoubtedly far superior to that of polishing on broadcloth4 ,if in the final strokes the surface is not quite cleaned ,i usually breathe upon the pitch bed1 and thus by condensation place enough moisture upon it to give a few more strokes1 finishing just the same as before4 ,in ten minutes ,i have polished prisms of rock salt in this manner that have not only shown the ,d line double1 but ,professor ,langley has informed me that his assistant1 ,mr4 ,keeler 7,j4 ,e471 has seen the nickel line clearly between the ,d lines4 ,this speaks for the superiority of the surfaces over those polished on broadcloth4 ,in polishing prisms ,i prefer to work them on top of the polisher1 as they can be easily held1 but as it is difficult to hold lenses or planes in this way without injuring the surfaces1 ,i usually support them in a block of soft wood1 turned so as to touch only at their edges1 and work the polisher over them4 ,though it takes considerable practice to succeed at first1 the results are so good that it well repays the few hours' work it requires to master the few difficulties it presents4 ,fig4 #eh4 #gj4 ,casting ,specula for ,mirrors4 ,according to ,sir ,h4 ,grubb 7loc4 cit47 the best alloy is made of four atoms of copper and one of tin2 this gives by weight1 copper #beb1 tin #aag4h4 ,the copper is melted first in a plumbago crucible2 the tin is added gradually4 ,of course1 in the process of melting1 even though a little fine charcoal be sprinkled over the copper1 some loss of that metal will occur from oxidation4 ,it is convenient in practice1 therefore to reserve a portion of the tin and test the contents of the crucible by lifting a little of the alloy out and examining it4 ,the following indications may be noted3 ,when the copper is in excess the tint of the alloy is slightly red1 and the structure1 as shown at a fractured surface1 is coarsely crystalline4 ,as the proper proportions are more nearly attained1 the crystalline structure becomes finer1 the colour whiter1 and the crystals brighter4 ,the alloy is ready for use when the maximum brightness is attained and the grain is fine4 ,if too much tin be added1 the lustre diminishes4 ,the correct proportion is1 therefore1 attained when a further small addition of tin produces no apparent increase of brightness or fineness of grain4 ,about three-quarters of the tin may be added at first1 and the other quarter added with testing as described4 ,the alloy is allowed to cool until on skimming the surface the metal appears bright and remains so without losing its lustre by oxidation for a sensible time2 it will still be quite red-hot4 ,fig4 #ei4 ,fig4 #fj4 ,as the speculum alloy is too difficult to work with ordinary tools1 it is best to cast the speculum of exactly the required shape and size4 ,this is done by means of a ring of iron turned inside 7and out7 and on one edge4 ,this ring is laid on a plate of figured iron1 and before the metal is poured the plate 7,g7 7,figs #ei and #fa7 is heated to1 say1 #cjj ,c4 ,in order to avoid the presence of oxide as far as possible1 the following arrangements for pouring are made4 ,a portion of the lower surface of the ring is removed by radial filing until a notch equal to1 say1 one-twentieth of the whole circumference is produced4 ,this is cut to an axial depth of1 say1 half an inch4 ,a bar of iron is then dovetailed loosely into the notch 7,fig4 #fj1 ,b71 so that it will rest on the iron plate1 and half fill the notch4 ,the aperture thus left forms the port of ingress for the hot metal 7see ,fig4 #fa1 ,m74 ,a bit of sheet iron is attached to the upper surface of the ring1 and lies as a sort of flap1 shaped like a deep shovel1 against the outside of the ring overhanging the port 7,figs4 #ei and #fa at ,f74 ,this flap does not quite reach the iron plate1 and its sides are bent so as to be in contact with the ring4 ,a portion of a smaller ring is then applied in such a manner as to form a pouring lip or pool on the outside of the main ring at ,e1 and the metal can only get into the main ring by passing under the edge of the flap and up through the port4 ,this forms an efficient skimming arrangement4 ,the process of casting is carried out by pouring steadily into the lip4 ,to avoid air bubbles it is convenient to cause the metal to spread slowly over the chill1 and ,mr4 ,nasmyth's method of accomplishing this is shown in the figure 7#fa74 ,the chill rests on three pins1 ,a ,b ,c 7,figs4 #ei and #fa74 ,before pouring begins the chill is tilted up off ,c by means of the counterpoise ,d1 which is insufficient to tilt it after the speculum is poured4 ,it is important that the chill should be horizontal at the close of the operation1 in order that the speculum may be of even thickness throughout4 ,this is noted by means of levels placed on the ring 7at ,k for instance74 ,fig4 #fa4 ,this apparatus may appear unnecessarily complex1 but it is worth while to set it up1 for it makes the operation of casting a speculum fairly certain4 ,if the metal is at the right temperature it will form a uniformly liquid disc inside the ring4 ,the mass sets almost directly1 and as soon as this occurs it is pushed to the edge of the plate and the metal in the lip broken off by a smart upward tap with a hammer4 ,the dovetailed bit of iron is knocked downwards and falls off1 and the ring may then be lifted clear of the casting4 ,the object of the dovetail will now be understood1 for without it there is great risk of breaking into the speculum in knocking the tail off4 ,a box of quite dry sawdust is prepared in readiness for the process of annealing before the speculum is cast4 ,the box must be a sound wooden or metal box1 and must be approximately air-tight4 ,for a speculum a foot in diameter the box must measure at least #c feet both ways in plan1 and be #b feet #f inches deep4 ,half the sawdust is in the box and is well pressed down so as to half fill it4 ,the other half must be conveniently ready to hand4 ,as soon as possible after casting1 the speculum is thrown into the box1 covered over with the sawdust1 and the lid is put on4 ,the object in having the box nearly air-tight is to avoid air-currents1 which would increase the rate of cooling4 ,a speculum a foot in diameter may conveniently take about three days to anneal1 and should be sensibly warm when the box is opened on the fourth day4 ,for larger sizes longer times will be required4 ,we will say that the sawdust thickness on each side must be proportional to the dimensions of the speculum1 or may even increase faster with advantage if time is of no moment4 ,the process of annealing may be considered successful if the disc does not fly to pieces in working2 it is to be worked on the chilled side4 ,the object of giving the chill the approximate counterpart form will now appear2 it saves some rough grinding1 and causes the finished surface to be more homogeneous than it would be if the centre were sunk by grinding through the chilled surface4 ,in #ahhi ,i learned from ,mr4 ,schneider1 ,professor ,row-land's assistant at ,baltimore1 that in casting specula for concave gratings a good deal of trouble had been saved by carrying out the operation in an atmosphere consisting mostly of coal gas4 ,it was claimed that in this way the presence of specks of oxide was avoided4 ,i did not see the process in operation1 but the results attained are known and admired by all experimenters4 #ga4 ,grinding and polishing ,specula4 ,the rough grinding is accomplished by means of a lead tool and coarse emery2 the size of grain may be such as will pass a sieve of #fj threads to the inch4 ,the process of grinding is quite similar to that previously described1 but it goes on comparatively quickly4 ,the rough grinding is checked by the spherometer1 and is interrupted when that instrument gives accordant and correct measurements all over the surface4 ,the fine grinding may be proceeded with by means of a glass-faced tool as before described1 or the labour may be reduced in the following manner4 ,a slate tool1 which must be free from green spots 7a source of uneven hardness71 is prepared1 and this is brought nearly to the curvature of the roughly ground speculum1 by turning or otherwise4 ,it is finished on the speculum itself with a little flour of emery4 ,the fine grinding is then carried on by means of slate dust and water1 the slate tool being the grinder4 ,the tool is1 of course1 scored into squares on the surface4 ,if the casting process has been carried out successfully1 the rough grinding may take1 say six hours1 and the fine grinding say thirty hours for a disc a foot in diameter4 ,the greatest source of trouble is want of homogeneity in the casting1 as evidenced by blowholes1 etc4 ,in general1 the shortest way is to discard the disc and start afresh if there is any serious want of perfection in the continuity or homogeneity of the metal4 ,fig4 #fb4 ,the finely ground surface must1 of course1 be apparently correct in so far as a spherometer 7with #c inches between the legs for a disc #a foot in diameter7 will show4 ,polishing and figuring are carried out simultaneously4 ,half an hour's polishing with a slate-backed pitch tool and rouge and water will enable an optical test to be made4 ,the most convenient test is that of ,foucault1 a simple appliance for the purpose being shown in the figure 7#fb74 ,it essentially consists of a small lamp surrounded by an opaque chimney 7,a7 through which a minute aperture 7pin-hole7 is made4 ,a small lens may be used1 of very great curvature1 or even a transparent marble to throw an image of the flame on the pin-hole4 ,a screen 7,b7 is placed close to the source1 and is provided with a rocking or tilting motion 7,c7 in its own plane4 ,the source and screen are partly independent1 and each is provided with a fine adjustment which serves to place it in position near the centre of curvature4 ,the screen is so close to the pin-hole in fact that both the source and a point on the edge of the screen may be said to be at the centre of curvature of the mirror4 ,the mirror is temporarily mounted so as to have its axis horizontal1 in a cellar or other place of uniform temperature4 ,the final focussing to the centre of curvature is made by the fine adjustment screws2 the image may be received on a bit of paper placed on the screen and overlapping the edge nearest the source4 ,the screws are worked till the image has its smallest dimension and is bisected by the edge of the screen4 ,the test consists in observing the appearance of the mirror surface while the screen is tilted to cut off the light1 as seen by an eye placed at the edge of the screen1 a peephole or eye lens being provided to facilitate placing the eye in a correct position4 ,the screen screws are worked so as to gradually cut off the light1 and the observer notes the appearance of the mirror surface4 ,if the curves are perfect and spherical1 the transition from complete illumination to darkness will be abrupt1 and no part of the mirror will remain illuminated after the rest4 ,for astronomical purposes a parabolic mirror is required4 ,in this case the disc may be partially screened by zonal screens1 and the position of the image for different zones noted2 the correctness or otherwise of the curvature may then be ascertained by calculation4 ,a shorter way is to place the source just outside the focus1 to be found by trial1 and then1 moving the extinction screen 7now a separate appliance7 to1 say1 five times the radius of curvature away1 where the image should now appear1 the suddenness of extinction may be investigated4 ,this1 of course1 involves a corresponding modification of the apparatus4 ,whether the tests indicate that a deepening of the ,centre1 i4e4 increase of the curvature1 or a flattening of the edges is required1 at least two remedial processes are available4 ,the chisel and mallet method of altering the size of the pitch1 squares of the polisher may be employed1 or paper or small pitch tools may be used to deepen the centre4 ,the chisel and mallet method merely consists in removing pitch squares from a uniformly divided tool surface by means of the instruments mentioned4 ,this removal is effected at those points at which the abrasion requires to be reduced4 ,when some practice is attained1 ,i understand that it is usual to try for a parabolic form at once1 as soon as the polishing commences4 ,this is done by dividing the pitch surface by ,v-shaped grooves1 the sides of the grooves being radii of the circular surface1 so that the central parts of the mirror get most of the polishing action4 ,if paper tools are used they must not be allowed much overhang1 or the edges of the mirror betray the effects of paper elasticity4 ,most operators sink the middle1 but the late ,mr4 ,lassell1 a most accomplished worker1 always attained the parabolic form by reducing the curvature of the edges of a spherical mirror4 #gb4 ,preparation of ,flat ,surfaces4 ,as ,sir ,h4 ,grubb has pointed out1 this operation only differs from those previously described in that an additional condition has to be satisfied4 ,this condition refers to the mean curvature1 which must be exact 7in the case of flats it is of course zero7 to a degree which is quite unnecessary in the manufacture of mirrors or lenses4 ,a little consideration will show that to get a surface flat the most straightforward method is to carry out the necessary and sufficient condition for three surfaces to fit each other impartially4 ,if they each fit each other1 they must clearly all be flat4 ,to carry out the process of producing a flat surface1 therefore1 two tools are made1 and the glass or speculum is ground first on one and then on the other1 the tools being kept in fit by occasional mutual grinding4 ,the grinding and polishing go on as usual4 ,if paper is employed1 care must be taken that the polisher is about the same size as the object to be polished4 ,there is a slight tendency to polish most at the edges2 but if the sweeps are of the right shape and size1 this may be corrected approximately4 ,the best surfaces which have come under my notice are those prepared as test surfaces by ,mr4 ,brashear of ,alleghany1 ,pa1 ,u4,s4,a4 ,these ,i believe to be pitch polished4 ,a pitch bed is prepared1 ,i presume1 in a manner similar to that described for rocksalt surfaces2 but the working of the glass is an immense art1 and one which ,i believe--if one may judge by results--is only known to ,mr4 ,brashear4 ,in general1 the effect of polishing will be to produce a convex or concave surface1 quite good enough for most purposes1 but distinctly faulty when tested by the interference fringes produced with the aid of the test plate4 ,the following information therefore--which ,i draw from ,mr4 ',cook--will not enable a student to emulate ,mr4 ,brashear1 but will undoubtedly help him to get a very much better surface than he usually buys at a high price1 as exhibited on a spectroscope prism4 ,the only difference between this process and the one described for polishing lenses1 lies in the fact that the rouge is put into the paper surface while the latter is wet with a dilute gum mucilage4 ,it is of course assumed that the object and the two tools have been finely ground and fit each other impartially4 ,the paper is rubbed over with rouge and weak gum water4 ,the tool1 when dry1 is applied to the flat ground surface 7of the object71 and is scraped with the three-cornered file chisel as formerly described4 ,this process must be very carefully carried out4 ,the paper must be of the quality mentioned1 or may even be thinner and harder4 ,the cross strokes should be more employed than in the case of the curved surfaces4 ,a good deal will depend on the method employed for supporting the work2 it is in general better to support the tool1 which may have a slate backing of any desired thickness1 whereby the difficulty resulting from strains is reduced4 ,the work must be mounted in such a way as to minimise the effect of changes of temperature4 ,if a pitch bed is selected1 ,mr4 ,brashear's instructions for rock salt may be followed1 with1 of course1 the obvious necessary modifications4 ,see also next section4 #gc4 ,polishing ,flat ,surfaces on ,glass or on ,speculum ,metal4 ,the above process may be employed for speculum metal1 or pitch may be used4 ,in the latter case a fresh tool must be prepared every hour or so1 because the metal begins to strip and leave bits on the polisher2 this causes a certain amount of scratching to take place4 ,as against this disadvantage1 the process of polishing1 in so far as the state of the surface is concerned1 need not take an hour if the fine grinding has been well done4 ,for the finest work changes of temperature1 as in the case of glass1 cause a good deal of trouble1 and the operator must try to arrange his method of holding the object so as to give rise to the least possible communication of heat from the hand4 ,the partial elasticity of paper1 which is its defect as a polishing backing1 is1 ,i believe1 partly counterbalanced by the difficulty of forming with pitch an exact counterpart tool without introducing a serious rise of temperature 7i4e4 warming the pitch74 ,the rate of subsidence of the latter is very slow at temperatures where it is hard enough to work reliably as a polisher4 ,a student interested in the matter of flat surfaces will do well to read an account of ,lord ,rayleigh's work on the subject1 ,nature1 vol4 xlviii1 #ahic1 pp4 #bab1 #ebf 7or ,b4 ,a4 ,reports1 #ahic74 ,in the first of these communications ,lord ,rayleigh describes the method of using test plates1 and shows how to obtain the interference fringes in the clearest manner4 ,for the ordinary optician a dark room and a soda flame afford all requisite information2 and if a person succeeds in making three glass discs1 say #f inches in diameter1 so flat that1 when superposed in any manner1 the interference fringes are parallel and equidistant1 even to the roughest observation1 he has nothing to learn from any book ever written on glass polishing4 ,lord ,rayleigh has also shown how to use the free clean surface of water as a natural test plate4 ,since the above was written the following details of his exact course of procedure have been sent to me by ,mr4 ,brashear1 and ,i hereby tender my thanks3- ,it really takes years to know just what to do when you reach that point where another touch either gives you the most perfect results attainable1 or ruins the work you have already done4 ,it has taken us a long time to find out how to make a flat surface1 and when we were called upon to make the twenty-eight plane and parallel surfaces for the investigation of the value of the metre of the international standard1 every one of which required an accuracy of one-twentieth of a wave length1 we had a difficult task to perform4 ,however1 it was found that every surface had the desired accuracy1 and some of them went far beyond it4 ,it is an advantage in making flat surfaces to make more than one at a time2 it is better to make at least three1 and in fact we always grind and 'fine' three together4 ,in making speculum plates we get up ten or twelve at once on the lead lap4 ,these speculum plates we can test as we go on by means of our test plane until we get them nearly flat4 ,in polishing them we first make quite a hard polisher1 forming it on a large test plane that is very nearly correct4 ,we then polish a while on one surface and test it1 then on a second and test it1 and after a while we accumulate plates that are slightly concave and slightly convex4 ,by working upon these alternately with the same polisher1 we finally get our polisher into such shape that it approximates more and more to a flat surface1 and with extreme care and slow procedure we finally attain the results desired4 ,all our flats are polished on a machine which has but little virtue in itself1 unmixed with brains4 ,any machine giving a straight diametrical stroke will answer the purpose4 ,the glass should be mounted so as to be perfectly free to move in every direction--that is to say1 perfectly unconstrained4 ,we mount all our flats on a piece of body ,brussels carpet1 so that every individual part of the woof acts as a yielding spring4 ,the flats are held in place by wooden clamps at the edges1 which never touch1 but allow the bits of glass or metal to move slowly around if they are circular2 if they are rectangular we allow them to tumble about as they please within the frame holding them4 ,for making speculum metal plates either plane or concave we use polishers so hard that they scratch the metal all over the surface with fine microscopic scratches4 ,we always work for figure1 and when we get a hard polisher that is in proper shape1 we can do ever so many surfaces with it if the environments of temperature are all right4 ,if we have fifty speculum flats to make1 and we recently made three times that number1 we get them all ready and of accurate surface with the hard polisher4 ,then we prepare a very soft polisher1 easily indented when cold with the thumb nail4 ,a drop of rouge and about three drops of water are put on the plate1 and with the soft polisher about one minute suffices to clean up all the scratches and leave a beautiful black polish on the metal4 ,this final touch is given by hand2 if we do not get the polish in a few minutes the surface is generally ruined for shape1 and we have to resort to the hard polisher again4 ,i assure you that nothing but patience and perseverance will master the difficulty that one has to encounter1 but with these two elements 'you are bound to get there4' ,,chapter ,,iii ,,miscellaneous ,,processes #gd4 ,coating ,glass with ,aluminium and ,soldering ,aluminium4 ,a process of coating glass with aluminium has been lately discovered1 which1 if ,i mistake not1 may be of immense service in special cases where a strongly adherent deposit is required4 ,my attention was first attracted to the matter by an article in the ,archives des ,sciences physiques et naturelles de ,genve1 #ahid1 by ,m4 ,margot4 ,it appears that clean aluminium used as a pencil will leave a mark on clean damp glass4 ,if1 instead of a pencil1 a small wheel of aluminium--say as big as a halfpenny and three times as thick--is rotated on the lathe1 and a piece of glass pressed against it1 the aluminium will form an adherent1 though not very continuous coating on the glass4 ,working with a disc of the size described rotating about as fast as for brass-turning1 ,i covered about two square inches of glass surface in about five minutes4 ,the deposit was of very uneven thickness1 but was nearly all thick enough to be sensibly opaque4 ,by burnishing the brilliance is improved 7,i used an agate burnisher and oil71 but a little of the aluminium is rubbed off4 ,the fact that the burnisher does not entirely remove it is a sign of the strength of the adherence which exists between the aluminium and the glass4 ,in making the experiment1 care must be taken to have the glass quite clean--or at all events free from grease--in order to obtain the best results4 ,m4 ,margot has contributed further information to the ,archives des ,sciences physiques et naturelles 7,february #ahie74 ,he finds that adherence between aluminium and glass is promoted by dusting the glass with powders1 such as rouge4 ,there is no doubt that a considerable improvement is effected in this way2 both rouge and alumina have in my hands greatly increased the facility with which the aluminium is deposited4 ,m4 ,margot finds that zinc and magnesium resemble aluminium in having properties of adherence to glass1 and1 what is more1 carry this property into their alloys with tin4 ,thus an alloy of zinc and tin in the proportions of about #ib per cent tin and #h per cent zinc may be melted on absolutely clean glass1 and will adhere strongly to it if well rubbed by an asbestos crayon4 ,a happy inspiration was to try whether these alloys would1 under similar circumstances1 adhere to aluminium itself1 and a trial showed that this was indeed the case1 provided that both the aluminium and alloy are scrupulously clean and free from oxide4 ,in this way ,m4 ,margot has solved the problem of soldering aluminium4 ,i have satisfied myself by trial of the perfect ease and absolute success of this method4 ,the alloy of zinc and tin in the proportions above mentioned is formed at the lowest possible temperature by melting the constituents together4 ,it is then poured so as to form thin sticks4 ,the aluminium is carefully cleaned by rubbing with a cuttle bone1 or fine sand1 and strong warm potash4 ,it is then washed in water and dried with a clean cloth4 ,the aluminium is now held over a clean flame and heated till it will melt the solder which is rubbed against it4 ,the solder sticks at once1 especially if rubbed with another bit of aluminium 7an aluminium soldering bit7 similarly coated4 ,to solder two bits of aluminium together it is only necessary to tin the bits by this process and then sweat them together4 ,the same process applies perfectly to aluminium caused to adhere to glass by the previously mentioned process1 and enables strong soldered contacts to be made to glass4 ,in one case1 while ,i was testing the method1 the adhesion was so strong that the solder on contracting while cooling actually chipped the surface clean off the glass4 ,in order to get over this ,i have endeavoured to soften the solder by mixing in a little of the fusible metal mercury amalgam2 and though this prevents the glass from being so much strained1 it reduces the adherence of the solder4 ,it is a comfort to be able to solder aluminium after working for so many years by way of electroplating1 or filing under solder4 ,an alternative method of soldering aluminium will be described when the electroplating of aluminium is discussed1 #ach4 ,gilding ,glass4 ,in looking over some volumes of the ,journal fuer praktische ,chemie1 ,i came across a method of gilding glass due to ,boettger 7,journ4 f4 prakt4 ,chem4 #ajc1 p4 #dad74 ,after many trials ,i believe ,i am in a position to give definite instructions as to the best way of carrying out this rather troublesome operation4 ,the films of gold obtained by the process are very thick1 and the appearance of the gold exceedingly fine4 ,the difficulty lies in the exact apportionment of the reducing solution4 ,if too much of the reducing solution be added1 the gold deposits in a fine mud1 and no coating is obtained4 ,if1 on the other hand1 too little of the reducing solution be added1 little or no gold is deposited4 ,the secret of success turns on exactly hitting the proper proportions4 ,the reducing solution consists of a mixture of aldehyde and glucose1 and the difficulty ,i have had in following ,boettger's instructions arose from his specifying commercial aldehyde of a certain specific gravity which it was impossible to reproduce4 ,i did not wish to specify pure aldehyde1 which is not very easily got or stored1 and consequently ,i have had to determine a criterion as to when the proportion of reducing solution is properly adjusted4 ,the aldehyde is best made as required4 ,i employed the ordinary process as described in ,thorpe's ,dictionary of ,applied ,chemistry1 by distilling alcohol1 water1 sulphuric acid1 and manganese dioxide together4 ,the crude product is mixed with a large quantity of calcium chloride 7dry--not fused71 and is rectified once4 ,the process is stopped when the specific gravity of the product reaches #j4hcb at #fj ,f4 ,the specific gravity of pure aldehyde is #j4gi nearly4 ,the following is a modification of ,boettger's formula3- ,solution ,i #a gram of pure gold is converted into chloride--got acid free--i4e4 to the state represented by ,au,cl#c1 and dissolved in #abj cc4 of water4 ,this solution is the equivalent of one containing #f4e grains of trichloride to the ounce of water4 ,solution ,,ii4 #f grams sodium hydrate4 #ajj grams water4 ,solution ,,iii4 #j4b grams glucose 7bought as pure74 #ab4f cubic centimetres #ie per cent alcohol4 #ab4f cubic centimetres water4 #b4j cubic centimetres aldehyde1 sp4 gr4 #j4hcb4 ,to gild glass these solutions are used in the following proportions by volume3- #af parts of ,no4 ,i4 #d parts of ,no4 ,,ii4 #j4h parts of ,no4 ,,iii4 ,the glass is first cleaned well with acid and washed with water3 it is then rinsed with ,solution ,no4 ,,iii4 ,if it is desired to gild the inside of a glass vessel1 ,solution ,no4 ,,iii4 may be placed in the vessel first1 and the walls of the vessel rinsed round carefully4 ,solutions ,i4 and ,,ii4 are mixed separately and then added to ,,iii4--after about two minutes the whole is well shaken up4 ,if it be desired to gild a mirror of glass1 the glass-plate is suspended face downwards in a dish of the mixed solutions--care being taken to rinse the glass with ,solution ,,iii4 first4 ,if the mixture darkens in from #g' to #aj' in diffuse daylight and at #fj;,f4 it will gild well1 and it generally pays to make a few trials in a test tube to arrive at this4 ,if too much reducing solution is present the liquid will get dark more rapidly1 and vice versa4 ,the gilding will require several hours--as much as twelve hours may be needed4 ,the reaction is one of great chemical interest1 being one of that class of reactions which is greatly affected by capillarity4 ,thus it occasionally happens that when the reducing solution is not in the right proportion1 gold will be deposited at the surface of the liquid 7so as to form a gilt ring on the inside of a test tube71 the remainder of the gold going down as mud4 ,the gold deposited is at first transparent to transmitted light and is deeply blue4 ,i thought this might be due to a trace of copper or silver1 but on carefully purifying the gold no change of colour was noted4 ,if the reducing solution is present in slightly greater proportions than that given in the formula1 the gold comes down with a richer colour1 and has a tendency to form a mat surface and to separate from the glass4 ,the gold which is deposited more slowly has a less rich colour but a brighter surface4 ,the operation should be interrupted when a sufficient deposit has been obtained1 because it is found that the thicker the deposit1 the more lightly is it held to the glass surface4 #ge4 ,the ,use of the ,diamond-cutting ,wheel4 ,a matter which is not very well known outside geological circles is the manipulation of the diamond-cutting wheel1 and as this is often of great use in the physical laboratory1 the following notes may not be out of place4 ,i first became acquainted with the art in connection with the necessity which arose for me to make galvanometer mirrors out of fused quartz1 and it was then that ,i discovered with surprise how difficult it is to obtain information on the point4 ,i desire to express my indebtedness to my colleagues1 ,professor ,david and ,mr4 ,smeeth1 for the instruction they have given me4 ,in what follows ,i propose to describe their practice rather than my own1 which has been of a makeshift description4 ,i will therefore select the process of cutting a slice of rock for microscopical investigation4 #gf4 ,arming a ,wheel4 ,fig4 #fc4 ,a convenient wheel is made out of tin-plate1 i4e4 mild steel sheet1 about one-thirtieth of an inch thick and seven inches in diameter4 ,this wheel must be quite flat and true1 as well as round2 too much pains cannot be taken in securing these qualities4 ,after the wheel is mounted1 it is better to turn it quite true by means of a watch-maker's graver or other suitable tool4 ,the general design of a rock-cutting machine will be clear from the illustration 7,fig4 #fc74 ,the wheel being set up correctly1 the next step is to arm it with diamond dust4 ,for this purpose it is before all things necessary that real diamond dust should be obtained4 ,the best plan is to procure a bit of bort which has been used in a diamond drill1 and whose properties have therefore been tested to some extent4 ,this is ground in a diamond mortar--or rather hammered in one--and passed through a sieve having at least #hj threads to the inch4 ,the dust may be conveniently kept in oil4 ,to arm the wheel1 a little dust and oil is taken on the finger1 and laid on round the periphery of the wheel4 ,a bit of flint or agate is then held firmly against the edge of the wheel and the latter is rotated two or three times by hand4 ,the rotation must be quite slow--say one turn in half a minute--and the flint must be held firmly and steadily against the wheel4 ,some operators prefer to hammer the diamond dust into the wheel with a lump of flint1 or agate1 but there is a risk of deforming the wheel in the process4 ,when a new wheel is set up1 it may be necessary to repeat the above process once every half hour or so till the cutting is satisfactory1 but when once a wheel is well armed it will work for a long time without further attention4 #gg4 ,cutting a ,section4 ,a wheel #g inches in diameter may be rotated about #ejj times per minute1 and will give good results at that speed4 ,the work1 as will be seen from the diagram1 is pressed against the edge of the wheel by a force1 which in the case quoted was about the weight of eleven ounces4 ,this was distributed along a cutting arc of three-quarters of an inch4 ,a convenient cutting lubricant is a solution of ,castile soap in water1 and this must be freely supplied2 if the wheel gets dry it is almost immediately spoiled owing to the diamond dust being scraped off4 ,in the figure the lubricant is supplied by a wick running into the reservoir4 ,i have used both clock oil and ordinary gas-engine oil as lubricants1 with equally satisfactory results4 ,as to the speed of cutting1 in the experiment quoted a bit of rather friable gabbro1 measuring three-quarters of an inch on the face by five-eighths of an inch thick1 was cut clean through in six minutes1 or by #cjjj turns of the wheel4 ,the travel of the edge was thus between #ejjj and #fjjj feet1 or say #ijjj feet1 nearly #b miles1 per inch cut4 ,a good solid rock1 like basalt1 can be cut into slices of about #c_/cb inch thick4 ,a very loose rock is best boiled in ,canada balsam1 hard enough to set1 before it is put against the wheel4 ,instead of a grinding machine a lathe may be employed4 ,the disc is1 of course1 mounted on the mandrel1 and the work on the slide-rest4 ,the latter must be disconnected from its feed screws1 and a weight arranged over a pulley so as to keep the work pressed against the wheel by a constant force4 ,it may1 perhaps1 occur to the reader to inquire whether any clearance in the cut is necessary4 ,the answer is that in all probability1 and in spite of every care1 the wheel will wobble enough to give clearance4 ,if it does not1 a little diamond dust rubbed into the side of the wheel1 as well as the edge1 will do all that is required4 ,the edge also1 after two or three armings1 burrs a little1 and thus provides a clearance naturally4 ,it is not unlikely that in the near future the electric furnace will furnish us with a number of products capable of replacing the diamond as abrading agents4 ,the cost of the small amount of diamond dust2 required in a laboratory is so small1 however1 that it2 is doubtful whether any appreciable economy will be1 effected4 #gh4 ,grinding ,rock ,sections1 or ,thin ,slips of any ,hard ,material4 ,a note on this is1 perhaps1 worth making1 for the same reasons as were given for note1 #ge1 which it naturally follows4 ,just as trout-fishing2 is described by ,mr4 ,francis as the art of fine and far off1 ,footnote3 ,in the ,badminton ,library1 volume on ,fishing4 section grinding may be called the art of ,canada balsam cooking1 as follows4 ,a section of rock having been cut from the lump as just described1 it becomes2 necessary to grind it down for purposes of microscopical investigation4 ,for this purpose it is placed on a slip of glass1 and cemented in position by ,canada1 balsam4 ,success in the operation of grinding the mounted section depends almost entirely on the way in which the mounting is done1 and this in its turn depends on the condition to which the ,canada has been brought4 ,to illustrate the operations1 ,i will describe a specific case1 viz4 that of grinding the section of gabbro' above described1 for microscopical purposes4 ,one side of the section is probably sufficiently smooth and plane from the operation of the diamond wheel2 if not1 it must be ground by the finger on a slab of iron or gun-metal with emery and water1 the emery passing a sieve of #hj threads to the inch4 ,the glass base on which the section is to be mounted for grinding is placed on a bit of iron or copper plate over a ,bunsen burner1 and three or four drops of natural ,canada balsam are placed upon it4 ,the section is placed on the plate to heat at the same time4 ,the temperature must not rise so high as to cause any visible change in the ,canada balsam1 except a slight formation of bubbles1 which rise to the surface1 and can be blown off4 ,the heating may require to be continued1 say1 up to twenty minutes4 ,the progress of the operation is tested by examining the balsam as to its viscous properties4 ,an exceedingly simple and accurate way of testing is to dip a pair of ordinary forceps in the balsam1 which may be stirred a little to secure uniformity4 ,the forceps are introduced with the jaws in contact1 and1 as soon as withdrawn1 the jaws are allowed to spring apart1 thus drawing out a balsam thread4 ,in a few moments the thread is cold1 and if the forceps be compressed1 this thread will bend4 ,the ,canada must be heated until it is just in such a state that on bringing the jaws together the thread breaks4 ,the forceps may open to about three-quarters of an inch4 ,if the ,canada is more viscous1 so that the thread does not break1 the section when cemented by it will most probably slip on the slide4 ,on the other hand1 if the balsam is more brittle1 it will crumble away during the grinding4 ,assuming that the proper point has been reached1 the section is mounted with the usual precautions to avoid air bubbles1 i4e4 by dropping one edge on the balsam first4 ,when all is cold1 the surface of the section may be ground on an iron plate with emery passing the #hj sieve1 till it is about #a_/dj inch thick4 ,from this point it must be reduced on ground glass by flours of emery and water2 the rough particles of the former may be washed out for fine work4 ,the process of grinding should not take more than half an hour if the section is properly cut1 etc4 ,beyond this point the allowable thickness must depend on the nature of the rock2 a good general rule is to get the section just so thin that felspars show the yellow of the first order in a polarising1 microscope4 ,the section is then finished with1 say1 two minutes emery or water of ,ayr-stone dust4 ,it is better not to have the surface too smooth4 ,to transfer the section1 the hard ,canada round the sides is scraped away1 and the section itself covered with some fresh ,canada from the bottle4 ,it is then warmed till it will slip off when a pin1 or the invaluable dentist's chisel1 is pressed against one side4 ,if the section be very delicate1 the cover slip should be placed over it before it is moved to the proper slide4 ,the ,canada used for mounting is not quite so hard as that employed in grinding1 but it should be hard when cold1 i4e4 not sticky4 ,the art of preparing ,canada balsam appears to consist in heating it under such conditions as will ensure its being exposed in thin layers4 ,i have wasted a good deal of time in trying to bake ,canada in evaporating basins1 with the invariable result that it was either over or under-baked1 and got dark in colour during the process4 ,on reviewing the process of rock section-cutting and mounting as just described1 ,i cannot help thinking that1 if properly systematised1 it could be made much more rapid by the introduction of proper automatic grinding machinery4 ,it also seems not improbable that a proper overhaul of available gums and cements would be found to lead to a cementing material less troublesome than ,canada balsam4 #gi4 ,cutting ,sections of ,soft ,substances4 ,though this art is fully treated of in books on practical biology1 it is occasionally of use to the physicist1 and the following note treats of that part of the subject which is not distinctly biological4 ,soft materials1 of which thin sections may be required1 generally require to be strengthened before they are cut4 ,for this purpose a variety of materials are available4 ,the one most generally used is hard paraffin4 ,the only point requiring attention is the embedding4 ,the material must be dry4 ,this is accomplished by soaking in absolute alcohol1 i4e4 really absolute alcohol made by shaking up rectified spirit with potassium carbonate1 previously dried1 and then digesting for a day with large excess of quick-lime1 making use of an inverted condenser and finally distilling off the alcohol without allowing it to come in contact with undried air4 ,after soaking for some time in absolute alcohol1 the material may be transferred to oil of bergamot1 or oil of cloves1 or almost any essential oil4 ,after soaking in this long enough to allow the alcohol to diffuse out1 the material may be lifted into a bath of melted paraffin 7melting at1 say1 #ea ,c474 ,the process of soaking is in some cases made to go more rapidly by exhausting1 and1 if the material will stand it1 by raising the temperature over #ajj ,c4 ,the soaking process may require minutes1 hours1 or days1 according to the size and density of the material2 but a few hours are usually sufficient4 ,when cold1 the sections may be cut in any of the ordinary forms of microtome4 ,another way of embedding is to soak in collodion1 and then precipitate the latter in the material and around it by plunging into nearly absolute alcohol4 ,the collodion yields a harder matrix than the paraffin4 ,whatever form of cutting machine is employed1 the art of sharpening the knife is the only one requiring any particular notice4 ,the easiest way of obtaining a knife hard enough to sharpen1 is to use a razor of good quality4 ,if it has to be ground1 it is best to do this on a fine ,turkey stone which is conveniently rested on two bits of rubber tubing1 to avoid jarring the blade4 ,many stones are slightly cracked1 but on no account must the razor be dragged across a crack1 or the edge will suffer4 ,the necessary and sufficient condition is that the razor must be worked in little sweeps over the stone1 and pressed against the latter by little more than its own weight1 and the grinding must be regular4 ,the edge may be inspected under a microscope1 and it must be perfectly smooth and even before it will cut sections4 ,a finishing touch may be given on a leather strap1 but it must be done skilfully1 otherwise it is better omitted4 ,the necessity for providing exceptionally keen and sharp edges arose in the manufacture of phonographs1 where the knife used to turn up the wax cylinders must leave a perfectly smooth surface4 ,in #ahhi this was being accomplished on an ivory lap fed with a trace of very fine diamond dust4 ,i have had this method in mind as a possible solution of the difficulty of razor-grinding1 but have not tried it4 ,i imagine one would use a soft steel or ivory slip rubbed over with fine diamond dust and oil by means of an agate4 ,the lap used in the phonograph works was rotated at a high speed4 #hj4 ,on the ,production of ,quartz ,threads4 ,footnote3 ,since this was written an article on the same subject by ,mr4 ,boys appeared in the ,electrician for #ahif4 ,the instructions therein given are in accordance with what ,i had written1 and ,i have made no alteration in the text4 ,in #ahhg the important properties of fused quartz were discovered by ,mr4 ,vernon ,boys 7,philosophical ,magazine1 ,june #ahhg1 p4 #dhi1 ,on the ,production1 ,properties1 and ,some ,suggested ,uses of the ,finest ,threads74 ,a detailed study of the properties of quartz threads was made by ,mr4 ,boys and communicated to the ,society of ,arts in #ahhi 7,journal of the ,society of ,arts1 #ahhi74 ,an independent study of the subject was made by the present writer in #ahhi 7,philosophical ,magazine1 ,july #ahij1 ,on the ,elastic ,constants of ,quartz ,threads 74 ,there is also a paper in the ,philosophical ,magazine for #ahid 7vol4 xxxvii4 p4 #dfc71 by ,mr4 ,boys1 on ,the ,attachment of ,quartz ,fibres4 ,this paper also appeared in the ,journal of the ,physical ,society at about the same date1 together with an interesting discussion of the matter4 ,in the ,american ,journal1 ,electric ,power1 for #ahid1 there is a series of articles by ,professor ,nichols on ,galvanometers1 in which a particular method of producing quartz threads is recommended4 ,the method was originally discovered by ,mr4 ,boys1 but he seems to have made no use of it4 ,a hunt through ,french and ,german literature on the subject has disclosed nothing of interest--nothing indeed which cannot be found in the papers mentioned4 #ha4 ,quartz fibres have two great advantages over other forms of suspension when employed for any kind of torsion balance1 from an ordinary more or less astatic galvanometer to the ,cavendish apparatus4 ,in the first place the actual strength of the fibres under longitudinal stress is remarkably high1 ranging from fifty to seventy tons weight per square inch of section1 and even more than this in the case of very fine threads2 the second and more important point in favour of quartz depends on the wide limits within which cylindrical threads of this material obey the simplest possible law of torsion1 i4e4 the law that for a given thread carrying a given weight at a given temperature and having one end clamped1 the twist about the axis of figure produced by a turning moment applied at the free end is proportional simply to the moment of the twisting forces1 and is independent of the previous history of the thread4 ,it is to be noted1 however1 that the torsional resilience of quartz as tested by the above law is not so perfect but that our instrumental means allow us to detect its imperfections1 and thus to satisfy ourselves that threads made of quartz are not things standing apart from all other materials1 except in the sense that the limits within which they may be twisted without deviating in their behaviour from the law of strict proportionality by more than some unassigned small quantity1 are phenomenally wide4 ,a torsion balance--if we except the case of certain spiral springs--is almost always called upon for information as to the magnitude of very small forces1 and for this purpose it is not essential merely that some law of twisting should be exactly obeyed1 but also that the resistance to twisting of the suspension should be small4 ,now1 regarded merely as a substance possessing elastic rigidity1 quartz is markedly inferior to the majority of materials1 for it is very stiff indeed2 its utility depends as much as anything upon its great strength1 for this allows us to1 use threads of exceeding fineness4 ,in addition to this it must be possible1 and moreover readily possible1 to obtain threads of uniform section over a sufficient length1 or the rate of twist per unit length of the thread will vary in practice from point to point1 so that the limits of allowable twist averaged over the whole thread may not be exceeded1 and yet they may be greatly overpassed at particular points of the thread4 ,it is interesting to note that in the case of quartz we not only have a means for readily producing very uniform cylindrical threads1 but that the limits of allowable rate of twist are so wide that a small departure from uniformity of section produces much less inconvenience than in the case of any other known substance4 #hb4 ,there are three methods generally in use for drawing quartz fibres1 all depending on the fact that quartz when fused is so viscous that it may be drawn into threads of great length1 without these threads breaking up into drops1 or indeed without their showing any sign of doing so4 ,the surface tension of the melted quartz must1 however1 be very considerable1 as may be seen by examining the shape of a drop of the molten material1 and this suffices to impress a rigidly cylindrical form upon the thread1 the great viscosity apparently damping down all oscillation4 ,the first method is the one originally employed by ,mr4 ,boys4 ,a needle of quartz is melted somewhere in its length and is then drawn out rapidly by a light arrow1 to which one end of the needle is attached1 and which is projected from a kind of crossbow4 ,a modification of this method1 which the writer has found of service when very thick threads are required1 is to replace the bow and arrow by a kind of catapult4 ,the third method1 which yields threads of almost unmanageable fineness1 depends on the experimental fact that when a fine point of quartz is held in a high pressure oxygen gas blow-pipe flame1 the friction of the flame gases suffices to overcome the tendency of the capillary forces to produce a spherical drop1 and actually causes a fine thread to be projected outwards in the direction of the flame4 #hc4 ,a preliminary operation to any method is the production of a stick of fused quartz4 ,this is managed as follows4 ,a rock crystal or quartz pebble is selected and examined4 ,it must be perfectly white1 transparent1 and free from dirt4 ,surface impurity can of course be got rid of by means of a grindstone4 ,the crystal is placed in a perfectly clean ,stourbridge clay crucible1 furnished with a cover1 and heated to bright redness for about an hour in a clean fire or in a ,fletcher's gas furnace4 ,the contents of the crucible are turned out when sufficiently cool on to a clean brick or bit of slate4 ,it will be found that the crystal is completely broken up and the fragments must be examined in case any of them have become contaminated by the crucible1 but this will not have happened if the temperature did not rise beyond a bright red heat4 ,the heap of fragments being found satisfactory1 the next thing is to fuse some of the pieces together4 ,unless the preliminary heating has been efficiently carried out this will prove an annoying task1 because a rock crystal generally contains so much water that it splinters under the blow-pipe in a very persistent manner4 ,there are two ways of assembling the fragments4 ,one is to place two tiles or bricks on edge about the heap of quartz lying upon a third tile1 so that the heap occupies the angular corner or nook formed by the tiles 7,fig4 #fd74 ,the oxygas blow-pipe previously described is adjusted to give its hottest flame1 the bags being weighted by at least two hundredweight1 if of the size described 7see #ae74 ,the tip of the inner cone of the blow-pipe is brought to bear directly upon one of the fragments1 and if the operation is performed boldly it will be found that the surface of the fragment can be fused1 and the fragment thus caused to hold together before the lower side gets hot enough to suffer any contamination from the tile or brick4 ,a second fragment may be treated in the same way1 and then a third1 and so on4 ,finally1 the fragments may be fused together slightly at the corners1 and a stick may thus be formed4 ,of course a good deal of cracking and splitting of the fragments takes place in the process2 the best pieces to operate upon are those which are well cracked to begin with1 and that in such a way that the little fragments are interlocked4 ,an alternative method which has some advantages is to arm a pair of forceps with two stout platinum jaws1 say an inch and a half long1 and flattened a little at the ends4 ,the fragments are held in these platinum forceps and the blow-pipe applied as before4 ,this method works very well in adding to a rod which has already been partly formed1 but the jaws require constant renewals4 ,the first fragment which is fused sufficiently to cohere may also be fused to a bit of tobacco pipe1 or hard glass tube or rod1 and the quartz stick gradually built up by fusing fresh pieces on to the one already in position4 ,fig4 #fd4 ,since the glass or pipeclay will contaminate the quartz which has been fused on to it1 it is necessary to discard the end pieces at the close of the operation4 ,a string of fragments having been collected and stuck together1 the next step is to fuse them down into a uniform rod4 ,this is easily done by holding the string in the blow-pipe flame and allowing it to fuse down4 ,twisting the fused part has a good effect in assisting the operation4 ,it is desirable to use a large jet and as powerful a flame as can be obtained during this part of the operation4 ,the final result should be a rod1 say two or three inches long and one-eighth of an inch thick1 which will in most cases contain a large number of air bubbles4 ,since the presence of drawn-out bubbles cannot be advantageous1 it is often desirable to get rid of them1 and this can conveniently be done at the present stage4 ,the process at best is rather tedious2 it consists in drawing the quartz down very fine before an intense flame1 in order to allow the bubbles to get close enough to the surface to burst4 ,a considerable loss of material invariably occurs during the process2 for whenever the thin rod separates into two bits the process of flame-drawing of threads goes on1 and entails a certain waste2 moreover1 the quartz in fine filaments is probably partially volatilised4 ,sooner or later1 however1 a sufficient length of bubble-free quartz can be obtained4 ,it must not be supposed that it is always necessary to eliminate bubbles as perfectly as is contemplated in the foregoing description of the treatment1 but for special purposes it may be essential to do so1 and in any case the reader's attention is directed to a possible source of error4 ,it may be mentioned in connection with this matter that crystals of quartz may look perfectly white and clear1 and yet contain impurity4 ,for instance1 traces of sodium are generally present1 and lithium was found in large spectroscopic quantity in five out of six samples of the purest crystals in my laboratory4 ,the presence of lithium in rock crystal has also been detected by ,tegetmeier 7,vied4 ,ann1 xli4 p4 #ai1 #ahij74 ,after some practice in preparing rods and freeing them of bubbles the operator will notice a distinct difference in the fusibility of the samples of quartz he investigates1 though all may appear equally pure to the unaided eye4 ,it should be mentioned1 however1 that high infusibility cannot always be taken as a test of purity1 for the most infusible1 or rather most viscous1 sample examined by the writer contained more lithium than some less viscous samples4 ,fig4 #fe4 ,during the process of freeing the quartz from bubbles the lithium and sodium will be found to burn away1 or at all events to disappear4 ,a rod of quartz1 say three inches long1 one-sixteenth of an inch in diameter1 and free from bubbles for half an inch of its length1 even when examined by a strong lens1 is suitable for drawing into threads4 ,the rod is manipulated exactly in the manner described under glass-blowing1 and is finally drawn down at the bubble free part into a needle1 say #j4jb inch in diameter 7,no4 #be on the ,birmingham wire gauge71 and #b inches long4 ,fig4 #ff4 ,there is one peculiarity about fused quartz which renders its manipulation easier than that of glass--it is impossible to break fused quartz1 however suddenly it be thrust into the blow-pipe flame4 ,a rod having a diameter of three-sixteenths of an inch--and perhaps much more--may be brought right up to the tip of the inner cone of the oxy-gas flame and held there-till one side fuses1 the other being comparatively cool1 without the slightest fear of precipitating a smash4 ,in seven years' experience ,i have never seen a bit of once fused quartz broken by sudden heating2 whether it might be done if sufficient precautions were taken ,i do not know4 ,the reason of the fortunate peculiarity of quartz in this respect is1 ,i presume1 to be found in the fact that quartz once it has been fused is really a very strong material indeed1 and is also probably the least expansible substance known4 ,from some experiments of the writer upon the subject1 it may be concluded that at the most quartz which has been fused expands only about one-fifth as fast as flint-glass1 at all events between #bj and #gj ,c4 #hd4 ,drawing ,quartz ,threads4 ,the thick end of the rod of quartz is held in the fingers or occasionally in a clip4 ,the end of the fine point is attached to a straw arrow by means of a little sealing-wax4 ,the arrow is laid on the stock of a crossbow in the proper position for firing4 ,see ,figs4 #fg and #fh1 which practically explain themselves4 ,the needle is heated by the blow-pipe till a minute length is in a state of uniform fusion2 the arrow is then let fly1 when it draws a thread out with it4 ,the arrow is preferably allowed to strike a wooden target placed1 say1 #cj feet away from the bow1 and a width of black glazed calico is laid under the line of fire to catch the thread or arrow if it falls short4 ,the general arrangements will be obvious from the figure4 ,the bow is of pine in the case where very long thin threads are required1 though for ordinary purposes ,i have found a bow of lance-wood succeed quite as well4 ,the trigger of the bow consists of a simple pin passing through the stock and fastened at its lower end to a string connected with a board which can be depressed by foot4 ,in the figure an ordinary trigger is shown1 but the pin does just as well4 ,fig #fg4 ,the arrow is made out of about #f inches of straw1 plugged up aft by a small plug of pine or willow fastened in with sealing-wax1 and projecting backwards one-eighth of an inch4 ,this projection serves a double purpose3 it gives a point of attachment for the quartz needle1 and on firing the bow it forms a resisting anvil on which the string of the bow impinges4 ,the head of the arrow is formed by a large needle stuck in with sealing-wax1 and heavy enough to bring the centre of gravity of the arrow forward of one-third of its length1 the condition of stability in flight4 ,fig4 #fh4 ,it is not necessary to employ any feathering for these arrows2 though ,i have occasionally used feathers or mica to wing the shaft no advantage has resulted therefrom4 ,to get fine threads a high velocity is essential4 ,this is obtained by considering 7and acting upon7 the principles involved4 ,the bow may be regarded as a doubly-tapering rod clamped at the middle4 ,after deflection it returns towards its equilibrium position at a rate depending in general terms on the elastic forces brought into play1 directly1 and on the effective moment of inertia of the rod1 inversely 7see ,rayleigh1 ,sound1 vol4 ii4 chap4 viii47 ,if the mass of the arrow is negligible compared with the bow1 the rate at which the arrow moves is practically determined by that attained by the end of the bow1 which is a maximum in crossing its equilibrium position4 ,the extent to which the arrow profits by this velocity depends on the way the bow is strung4 ,it will be greatest when the string is perpendicular to the bow when passing its equilibrium position2 or in other words1 when the string is infinitely long4 ,since the string has mass1 however1 it is not permissible to make it too long1 or its weight begins to make itself felt1 and a point is soon reached at which the geometrical gain in string velocity is compensated for by the total loss of velocity due to the inertia of the string4 ,in practice it is sufficient to use a string #aj per cent longer than the bow4 ,it is well to use a light fiddle string1 served with waxed silk at the trigger catch2 if this be omitted the gut gets worn through very quickly4 ,in order to decide how far it is permissible to bend the bow1 the quickest way is to make a rough experiment on a bit of the same plank from which the bow is to be cut1 and then to allow a small factor of safety4 ,in the figure the bow is of lance-wood and is more bent than would be suitable for pine4 ,the bow itself is tapered from the middle outwards just like any other bow4 ,if thick threads are required1 the above considerations are modified by the fact that quartz opposes a considerable resistance to drawing1 and that consequently the arrow must not only have a high velocity1 but a fair supply of energy as well2 in other words1 it must be heavy4 ,a thin pine arrow instead of a straw generally does very well1 but in this case the advantage of using pine for the bow vanishes2 and in fact lance-wood does better1 owing to the greater displacement which it will stand without breaking4 ,this of course only means that a greater store of energy can be accumulated at one bending4 ,i had occasion to investigate whether the unavoidable spin of an arrow about its axis produces any effect on the thread1 and for this purpose made arrows with inertia bars thrust through the head1 i4e4 an arrow with a bit of wire run through it1 perpendicular to its length--forming a cross in fact--the arms of the cross being weighted at the extreme ends by shot4 ,this form of arrow has a considerable moment of inertia about its longer axis1 and consequently rotates less than a mere straw1 provided that the couples tending to produce rotation are not increased by the cross arm1 or the velocity too much reduced4 ,shooting one of these arrows slowly1 ,i could see that it did not rotate1 and when fired at a high velocity1 it generally arrived at the target 7placed at varying distances front bow7 with the arms nearly horizontal1 thus showing that it probably did not rotate much4 ,i did not succeed in this at the first trial1 by any means4 ,the threads got in this way were no better than those made with a single straw1 whence we may conclude very provisionally that the spin of the arrow has only a small effect1 if any1 on the quality of the threads4 ,feathering the arrow1 in my experience1 tends1 if anything to make it spin more2 for one thing1 because it is practically impossible to lay the feathering on straight4 ,after the arrow is shot1 it remains to gather in the thread1 and if the latter is at all thin1 we have a rather troublesome job4 ,in a thread thirty or forty feet long1 the most uniform part generally lies in the middle if the thread is thin1 i4e4 of the order of a ten-thousandth of an inch in diameter4 ,if the thread is thick the most uniform part may be anywhere4 ,the part of the thread required is generally best isolated by passing a slip of paper under it at each end and cementing the thread to the paper by means of a little paraffin or soft wax1 and then cutting off the outer portions4 ,one bit of paper may then be lifted off the calico1 and the thread will carry the other bit4 ,in this way the thread may be taken to a blackened board1 where it may be mounted for stock4 ,by passing the two ends of the thread under a microscope1 or rather by breaking bits off the two ends and examining them together1 it is easy to form an ,opinion as to uniformity4 ,mr4 ,boys has employed an optical method of examining threads1 but the writer has invariably found a high-power microscope more convenient and capable of giving more exact information as to the diameter of the threads4 ,the beginner--or indeed the practised hand--need not expect to get a thread of the exact dimensions required at the first shot4 ,a little experience is necessary to enable one to judge of the right thickness of the needle for a thread of given diameter4 ,the threads are so easily shot1 however1 that a few trials take up very little time and generally afford quite sufficient experience to enable a thread of any required diameter to be prepared4 ,it is no use attempting to heat an appreciable length of needle2 if this be done the thread almost invariably has a thick part about the middle of its length44 ,it is sufficient to fuse at most about one-twentieth of an inch along the needle before firing off the bow4 ,this can be done by means of the smaller oxygas blow-pipe jet described in the article on blow-pipes for glass-blowing1 #ad4 ,the flame must of course be turned down so as to be of a suitable size4 ,a sufficiently small flame may be got from almost any jet4 ,if the needle be not equally heated all round1 the thread tends to be curly2 indeed by means of the catapult1 threads may be pulled which1 when broken1 tend to coil up like the balance-springs of watches1 if only care be taken to have one side of the needle much hotter than the other4 #he4 ,when examining bits of threads1 say thicker than the two-thousandth of an inch1 under the microscope it is convenient to use a film of glycerine stained with some kind of dye1 in order to render the thread more sharply visible4 ,the thread is mounted beneath a cover slip1 and a drop of the stained glycerine allowed to run in4 ,such a treatment gives the image of the thread a sharply defined edge #c and the contrast between the whiteness of the thread and the colour of the background allows measurements to be made with great ease4 ,on the whole the easiest way of measuring the diameter of a thick thread is to use a measuring microscope1 i4e4 one in which the lens system can be displaced along a plane bed by means of a finely cut micrometer screw4 ,the instruments made by the ,cambridge ,scientific ,instrument ,company do fairly well4 ,direct measurements up to #j4jjja inch are easily made by means of a microscope provided with a ,zeiss ,a objective1 and rather smaller differences of thickness can be made out by it4 ,for thin threads the method next to be described is more fitting1 because higher powers can be more conveniently used4 ,in this method an ordinary microscope is employed together with a scale micrometer1 and either an eyepiece micrometer1 or a camera and subsidiary scale4 ,the eyepiece micrometer is the more convenient4 ,if a camera be employed1 i4e4 such an one as is supplied by ,zeiss1 it is astonishing how the accuracy of observation may be increased by attending carefully to the illumination of both the subsidiary scale and of the thread4 ,the two images should be as far as possible of equal brightness1 and for this purpose it will be found requisite to employ small screens4 ,the detail of making a measurement by means of the micrometer eyepiece is very simple4 ,the thread is arranged on the stage so as to point towards the observer1 and the apparent diameter is read off on the eyepiece scale4 ,in order to calibrate the latter it is only necessary to replace the thread by the stage micrometer1 and to observe the number of stage micrometer divisions occupying the space in the eyepiece micrometer formerly occupied by the thread4 ,it is essential that both thread and stage micrometer should occupy the same position in the field1 for errors due to unequal distortion may otherwise become of importance4 ,for this reason it is best to utilise the centre of the field only4 ,the same remark applies to measurements by means of the camera1 where the image of the thread is projected against the reflected image of the subsidiary scale laid alongside the microscope4 ,in this case the value of the subsidiary scale divisions must be obtained from the divisions of the stage micrometer1 coinciding as nearly as possible with the position occupied by the thread4 ,before commencing a measurement the screens are moved about till both images appear equally bright4 ,threads up to about one twenty-thousandth of an inch in diameter may be sufficiently well measured by means of a ,zeiss #d centimetre apochromatic object-glass and an eyepiece ,no4 #f with sixteen centimetre tube length4 ,footnote3 ,the objective certainly had #d cm4 marked on it1 but the focal length appeared to be about ,i4#e mm4 only4 #hf4 ,drawing ,threads by the ,catapult4 ,the bow-and-arrow method fails when threads of a greater diameter than about #j4jjae inch are required--at least if any reasonable uniformity be demanded1 and no radical change in the bow and arrow be carried out4 ,thus in the writer's laboratory a thread of about this diameter1 within #a_/ajjjj of an inch-#ac inches long and free from air bubbles--was required4 ,a fortnight's work by a most skilful operator only resulted in the production of two lengths satisfying the conditions4 ,the greatest loss of time occurs in the examination of the thread by means of the microscope4 ,threads for galvanometer suspensions are conveniently from #j4jjja to #j4jjjd inch in diameter1 and are much more easily made and got uniform than thicker threads1 to the production of which the catapult method applies4 ,a reference to the diagram will make the construction of the instrument quite clear4 ,the moving end of the quartz is attached to a small boxwood slider working on a tubular girder or between wires4 ,the quartz is secured in position by clamps shown at ,a and ,b1 and motion is imparted to the slider by a stretched piece of catapult elastic 7,c74 ,an easy means of regulating the pull of the elastic is to hold it back by a loop of string whose length can be varied by twisting it round a pin4 ,fig4 #fi4 ,footnote3 ,for greater clearness of drawing1 the tube carrying the slider is shown somewhat higher above the base than is convenient in practice2 and the slide itself is shown too thin in the direction of the hole through it4 ,since it is not permissible to allow the slider to rebound at the end of its journey1 some such arrangement of breaks as is shown must be adopted4 ,in the diagram the bottom of the slider runs on to a brass spring between the girder and the base of the appliance1 and so gets jammed2 the spiral spring acts merely as an additional guard4 ,the diagram does not show the lower spring very clearly2 it is a mere strip lying in the groove4 ,a rod of quartz1 with a needle at one end1 is prepared as before and secured in the clamps4 ,during the operation of fastening down the clamps1 there is some danger of breaking the needle1 and consequently it is advisable to soften the latter before and while adjusting the second clamp4 ,the process of drawing a thread by this method is exactly similar to the operation already described in connection with the arrow method4 ,though short thick threads form the product generally obtained from the catapult1 it must not be supposed that thin threads cannot be obtained in this way4 ,if a short length of a very fine needle be heated1 it will be found to yield threads quite fine enough for ordinary suspension purposes1 but naturally not so uniform as those obtained from the #dj-;foot lengths obtainable by the bow-and-arrow method4 ,it is easy to make spiral quartz springs resembling watch balance-springs by means of the catapult4 ,all that is necessary is to see that the quartz is rather unequally heated before the shot is fired4 ,in the future it is by no means impossible that such springs may have a real value1 for the rigidity of quartz is known to increase as temperature rises4 ,hence it is probable that the springs would become stiffer as temperature rises1 even though they work chiefly by bending1 and little or not at all by twisting4 ,as this is the kind of temperature variation required to compensate an uncompensated watch balance wheel1 it may turn out to have some value4 #hg4 ,drawing ,threads by the ,flame alone4 ,a stick of quartz is drawn down to a fine point1 and the tip of this point is held in the blow-pipe flame in the position shown in ,fig4 #gj4 ,fig4 #gj4 ,the friction of the flame gases is found to be sufficient to carry forward the fused quartz and to draw it into threads in spite of the influence of the capillary forces4 ,if a sheet of paper be suspended at a distance of two or three feet in front of the blow-pipe flame1 it will be found to be covered with fine threads tangled together into a cobwebby mass4 ,as this method is an exceedingly simple one of obtaining threads1 ,i have endeavoured to reduce it to a systematic operation4 ,a sheet of cardboard1 about two feet square1 is painted dead black and suspended horizontally1 painted side downwards 7,fig4 #gj1 ,a71 at a height of about two feet above the blow-pipe flame4 ,the latter is adjusted so as to point almost vertically upwards and towards the centre of the cardboard4 ,a few half-inch pins are thrust through the card from the upper surface and pushed home2 about one dozen pins scattered over the surface will be sufficient4 ,their object is to prevent the threads being carried away round the edge of the screen4 ,the flame from the jet described so often is fed from gas bags weighted to about eighty pounds per square foot of 7one7 surface1 i4e4 #d-;foot bags require from three to four hundredweight to give an advantageous pressure4 ,footnote3 ,the resulting threads were really too fine for convenient manipulation1 so that unless extremely fine threads are required it will be better to reduce the pressure of the gases considerably4 ,two sticks of quartz are introduced and caused to meet just in front of the inner cone--the hottest part of the flame4 ,they are then drawn apart so as to form a fine neck1 which softens and is bent in the direction of motion of the flame gases4 ,when fusion is complete the neck separates into two parts1 and a thread is drawn from each of them4 ,by alternately lightly touching the rods together1 and drawing them apart1 quite a mass of threads may be obtained in two or three minutes1 when the process should be stopped4 ,if too many threads get entangled in the pins1 one gives one's self the unnecessary trouble of separating them4 ,on taking down the card it will be found that the threads have been caught by the pins2 but the card now being laid black side upwards1 the former easily slip off the points4 ,threads at least a foot long1 and perhaps vastly longer1 may be obtained by this method1 and are extraordinarily fine4 ,when ,i first read ,professor ,nichols' statement 7,electric ,power1 #ahid7 as to the value of these fibres for galvanometer purposes1 ,i was rather sceptical on the ground that the threads would tend to get annealed by being drawn gradually1 instead of suddenly1 from a place of intense heat to regions of lower temperature4 ,now annealing threads by a ,bunsen makes them rotten4 ,the threads being immersed in the hot flame gases could only cool at the same rate as the gas1 and it was not--and is not--clear to me that annealing of the threads can be avoided4 ,on the other hand1 it may be possible that a thread cooled slowly from the first does not suffer in the same way as a cold thread would do when annealed in a ,bunsen flame4 ,again the velocity of the gases is beyond doubt exceedingly high1 so that the annealing1 even supposing it to be deleterious1 might not be carried very far4 ,threads drawn by this method and measured dry1 i4e4 by mounting them on a slide without the addition of any liquid1 turned out to have a diameter of about #a_/bjjjj of an inch4 ,i do not think ,i could manage to mount such fine threads without very special trouble4 ,all the threads lying on the board1 however1 were found in reality to consist of three or four separate threads1 and there is no reason why several threads should not be mounted in parallel1 provided1 of course1 that they are equally stretched and touching each other4 ,equality of tension in the mounting could be secured by making one attachment good1 then cementing the other attachment to the other end of the threads1 and drawing the two attachments slightly apart at the moment the cement commences to set4 ,this method may turn out to be very valuable1 for1 so far as ,i can see1 the carrying power would be increased without an increase of torsional stiffness of anything like so high an order as would be the case were one thread only employed4 ,on the other hand1 the law of torsion could hardly be quite so simple1 at all events1 to the second order of approximations4 #hh4 ,properties of ,threads4 ,a large number of experiments on the numerical values of the elastic constants of quartz threads have been made by ,mr4 ,boys and his students1 and by the writer4 ,as the methods employed were quite distinct and the results wholly independent1 and yet in good agreement with each other1 a rounded average may be accepted with considerable confidence4 ,,tenacity ,,of ,,quartz ,,fibres 7,,boys74 ,diameter of ,thread4 ,tenacity in ,tons' ,weight per ,square ,inch of ,section4 ,tenacity in ,dynes per ,square ,centimetre4 ,inches ,centimetres #j4jjjfi #j4jjage #ea4g #h ,x #aji #j4jjjai #j4jjjdh #gd4e #aa4e ,x #aji ,rounded mean of ,boys' and ,threlfall's results3 ,young's ,modulus at #bj ,c1 #e4f ,x #ajaa ,c4,g4,s4 ,modulus of ,simple ,rigidity at #bj ,c1 #b4fe ,x #ajaa ,c4,g4,s4 ,modulus of ,incompressibility1 #a4d ,x #ajaa ,c4,g4,s4 ,modulus of ,torsion1 #c4g ,x #ajaa ,c4,g4,s4 ,approximate coefficient of linear expansion of quartz per degree between #hj ,c4 and #cj ,c4 is #j4jjjjjag 7,threlfall loc4 cit474 ,this must be regarded with some suspicion1 as the data were not concordant4 ,there is no doubt1 however1 about the extreme inexpansibility of quartz4 ,temperature coefficient of modulus of torsional rigidity per degree centigrade1 #bb to #ih ,c1 #j4jjjacc ,ditto1 absolute simple rigidity1 #j4jjjabh 7,threlfall74 ,limit of allowable rate of twist in round numbers is1 one-third turn per centimetre1 in a fibre #j4ja cm4 diameter4 ,the limiting rate is probably roughly inversely as the diameter4 ,attention must be called to the rapid increase in the torsional rigidity of these threads as the temperature rises4 ,a quartz spiral spring-balance will be appreciably stronger in hot weather4 #hi4 ,in the majority of instances in which quartz threads are applied in the laboratory1 it is desirable to keep the coefficient of torsion as small as possible1 and hence threads are used as fine as possible4 ,it is convenient to remember that a thread #j4jjad cm4 or #j4jjjg inch in diameter breaks with a weight of about ten grammes1 and may conveniently be employed to carry1 say1 five grammes4 ,with threads three times finer the breaking strength per unit area increases1 say1 #ej per cent4 ,in ordinary practice--galvanometric work for instance--where it is desirable to use a thread as fine and short as possible to sustain a weight up to1 say1 half a gramme1 it will be found that fibres five centimetres long or over give no trouble through defect of elastic properties4 ,a factor of safety of two is a fair allowance when loading threads4 ,no difficulty will be experienced in mounting threads having a diameter of #j4jjjb inch or over4 ,with finer threads it is necessary to employ very dark backgrounds 7,mr4 ,boys uses the darkness of a slightly opened drawer71 or the threads cannot be sufficiently well seen4 ,in the case of instruments in which threads remain highly twisted for long periods of time1 the above rule as to the safe limit of twist does not allow of a sufficient margin2 it is only applicable to galvanometric and similar purposes4 ,the cause of the increase in tenacity as the diameter diminishes is at present unknown4 ,it is due neither to an effect of annealing 7annealed threads are rotten71 nor is it a skin effect1 nor is it due to the cooling of the thread under higher capillary pressure4 ,it is1 however1 possible that it may be associated with some kind of permanent set taken by the fibres during the stage of passage from the liquid to the solid state4 #ij4 ,on the ,attachment of ,quartz ,fibres4 ,for many purposes it is sufficient to cement the fibres in position by means of ordinary yellow shellac1 but where very great accuracy is aimed at1 the shellac 7being itself imperfectly elastic and exposed to shearing stress7 imposes its imperfections on the whole system4 ,this source of error can be got over by soldering the threads in position4 ,attempts were made by the writer in this direction1 with fair success1 in #ahhi1 but as ,mr4 ,boys has carried the art to a high degree of perfection1 ,i will suppress the description of my own method and describe his in preference4 ,it has1 of course1 been frequently repeated in my laboratory4 ,in many cases1 however1 if not in all1 it may be replaced by ,margot soldering1 as already described1 a note on the application of which to this purpose will follow4 ,a thread of the proper diameter having been selected1 it is cut to the right length4 ,with fine threads this is not always a perfectly easy matter4 ,the best way is for the operator to station himself facing a good light1 not sunlight1 which is too tiring to the eye1 but bright diffused light4 ,the thread will be furnished with bits of paper stuck on with paraffin at both ends1 as already described4 ,a rough sketch of the apparatus--or1 at all events1 two lines showing the exact length which the free part of the thread must have--are marked on a smooth board1 and this is supported with its plane vertical4 ,the thread is held against the board1 and the upper piece of paper is stuck lightly to the board with a trace of soft wax1 so that the lower edge of the paper is at any desired height above the upper mark4 ,this distance is measured1 and forms the length of thread allowed to overlap the support4 ,a second bit of paper is attached below the lower mark1 a margin for the attachment of the lower end being measured and left as before4 ,the thread will be most easily seen if the board is painted a dead black4 ,if it is desired to attach the thread to its supports merely by shellac1 this is practically all that needs to be done4 ,the supports should resemble large pins4 ,the upper support will be a brass wire in most cases1 and will require to be filed away as shown in the sketch 7,fig4 #ga74 ,it is then coated with shellac by heating and rubbing upon the shellac4 ,as previously noted1 the shellac must not be overheated4 ,the thread is cut off below the lower slip of paper1 and the upper support being conveniently laid in a horizontal position on another dead-black surface1 the thread is carried to it and laid as designed against the shellac1 which is now cold4 ,when the thread is in place1 a soldering iron is put against the brass wire1 and the shellac gradually melted till it closes over the thread4 ,fig4 #ga4 ,the iron is then withdrawn and the thread pulled away from the point for one-twentieth of an inch or less4 ,this ensures that the thread makes proper contact with the cement1 and also that it is free from kinks2 of course1 it must leave the cement in the proper direction4 ,a similar process is next carried out with respect to the lower attachment1 and the ends of the thread are neatly trimmed off4 ,both ends of the thread being secured1 the next step is to transfer the upper support to a clip stand1 the suspended parts being held by hand1 so that the weight comes on the thread very gradually4 ,in this way it will be easily seen whether the thread is bent where it enters the shellac1 and should this be the case1 a hot iron must be brought up to the shellac and the error rectified4 ,when both the support and the suspended parts are brought nearly to the required bearing1 the hot iron is held for a moment close up to each attachment1 the hand being held close below but not touching the suspended parts1 and both attachments are allowed to straighten themselves out naturally4 ,these details may appear tiresome1 and so they are when written out at length1 but the time occupied in carrying them out is very short1 and quartz threads break easily1 unless the pull upon them is accurately in the direction of their length at all points4 ,in the event of its being decided to attach the thread by soldering1 the process is rather more expensive in time1 but not otherwise more troublesome4 ,fig4 #gb4 ,fig4 #gc4 ,the thread being cut as before to the proper length1 little bits of aluminium foil are smeared all over with melted shellac and suspended from the thread replacing the paper slips before described4 ,it is important that no paraffin should be allowed to touch the thread anywhere near a point intended to be soldered4 ,the thread is hung up from a clip stand by one of the bits of foil1 and the lower end is washed by dipping it into strong nitric acid for a moment and thence into water4 ,the object of smearing the foils all over with shellac is to prevent them being acted upon by the acid4 ,the threads are not very easily washed acid free1 but the process may be assisted by means of a fine camel's-hair pencil4 ,some silvering solution made as described 7 #fe7 is put into a test tube2 the thread1 after rinsing with distilled water1 is lowered into the solution so far as is required1 and is allowed to receive a coating of silver4 ,it has been observed that the coating of silver must not be too thick--not sufficiently thick to be opaque4 ,a watch may be kept on the process by immersing a minute strip of mica alongside the thread4 ,the silvered thread is rinsed with distilled water and allowed to dry4 ,meanwhile the other end of the thread may be silvered4 ,when both ends are silvered the process of coppering by electro deposit is commenced4 ,a test tube is partially filled with a ten per cent solution of sulphate of copper1 and several copper wires are dipped into it to form an anode4 ,the thread is lowered carefully into the solution so as not to introduce air bubbles1 and the silvered part is allowed to project far enough above the surface of the solution to come in contact with a fine copper wire4 ,the circuit is closed through a ,leclanch cell and a resistance box4 ,it is as well to begin with a fair resistance1 say #ajj ohms out in the box1 and the progress of the deposit is watched by means of a low-power microscope set up in front of the thread4 ,if the copper appears to come down in a granular form1 the resistance is too small and must be increased2 if no headway appears to be made1 the resistance must be diminished4 ,as soon as a fair coat of copper has come down1 i4e4 when the diameter of the thread is about doubled1 the process is interrupted4 ,the thread is withdrawn1 washed1 dipped in a solution of chloride of zinc1 and carefully tinned by dragging it over a small clean drop of solder on a soldering bit4 ,during this part of the process the shellac is apt to get melted if the iron is held too close1 so that it is advisable to begin by making the thread somewhat over long4 ,the end of the thread must only be trimmed off at the conclusion of the operation1 i4e4 after the thread is soldered up4 ,the thread is attached to the previously tinned supports much in the same way as has been described under the head of shellac attachments4 ,it does not very much matter whether both ends are coppered before one is soldered up or not4 ,at the conclusion of the whole process the superfluous copper and silver are dissolved off by a little hot strong nitric acid applied on a glass hair pencil4 ,this is best done by holding the thread horizontally with the assistance of clip stands4 ,if the thread is too delicate to bear brushing1 the nitric acid may be applied by pouring out a big drop into a bit of platinum foil and holding this below the thread so as to touch it lightly4 ,the dissolving of the copper and silver is1 of course1 followed by copious washing with hot water4 ,this process is more laborious than might be imagined1 but it may be shortened by heating the platinum foil supporting the water 7,fig4 #gd74 ,fig4 #gd ,the washing part of the process is1 in the opinion of the writer1 the most difficult part of the whole business1 and it requires to be very thorough1 or the thread will end by drawing out of the solder4 ,in many cases it is better to try to do without any application of nitric acid at all1 but1 of course1 this involves silvering and coppering to exact distances from the ends of the thread--at all events1 in apparatus where the effective length of the thread is narrowly prescribed4 ,it is important not to leave the active parts of the thread appreciably silvered1 for the sake of avoiding zero changes due to the imperfect elasticity of the silver4 ,in this soldering process ordinary tinman's solder may be employed2 it must be applied very free from dust or oxide4 #ia4 ,other ,modes of soldering ,quartz4 ,thick rods of quartz may be treated for attachment by solder in the same way as glass was treated by ,professor ,kundt to get a foundation for his electrolytically deposited prisms4 ,footnote3 ,see ,appendix at end of book4 ,the application of a drop of a strong solution of platinum tetrachloride to the rod will1 on drying1 give rise to a film of the dry salt1 and this may be reduced in the luminous gas flame4 ,during the process1 however1 the quartz is apt to get rotten1 especially if the temperature has been anything approaching a full red heat4 ,the resulting platinum deposit adheres very strongly to the quartz1 and may be soldered to as before4 ,this method has been employed by the writer with success since #ahhg1 and may even be extended to thick threads4 ,it was also found that fusible metal either stuck to or contracted upon clean quartz so as to make a firm joint4 ,in the light of ,m4 ,margot's researches 7already described71 it occurred to me that perhaps my experience was only a special case of the phenomena of adhesion investigated with so much success by ,m4 ,margot4 ,i therefore tried whether the alloy of tin and zinc used for soldering aluminium would stick to quartz1 and instantly found that this was indeed the case4 ,adhesion between the alloy and perfectly clean quartz takes place almost without rubbing4 ,a rod of quartz thus tinned can be soldered up to anything to which solder will stick1 at once4 ,on applying the method to thick quartz threads1 success was instantaneous 7the threads were some preserved for ordinary galvanometer suspensions72 but when the method was applied to very fine threads1 great difficulty in tinning the threads was experienced4 ,the operation is best performed by having the alloy on the end of an aluminium soldering bit1 and taking care that it is perfectly free from oxide before the thread is drawn across it4 ,there was no difficulty in soldering a thread tinned in this manner to a copper wire with tinman's solder1 and the joint appeared perfect1 the thread breaking finally at about an inch away from the joint4 ,i allow ,mr4 ,boys' method to stand as ,i have written it1 simply because ,i have not had time as yet to make thorough tests of the durability of ,margot joints on the finest threads2 but ,i have practically no doubt as to its perfect applicability1 provided always that the solder can be got clean enough when melted on the bit4 ,very fine threads will require to be stretched before tinning1 in order to enable them to break through the capillary barrier of the surface of the melted solder4 #ib4 ,soldering4 ,it is almost unfair to the arts of the glass-blower or optician to describe them side by side with the humble trade of soldering4 ,nevertheless1 no accomplishment of a mechanical kind is so serviceable to the physicist as handiness with the soldering bit2 and1 as a rule1 there is no other exercise in which the average student shows such lamentable incapacity4 ,the following remarks on the subject are therefore addressed to persons presumably quite ignorant of the way in which soldering is carried out1 and do not profess to be more than of the most elementary character4 ,for laboratory purposes three kinds of solder are in general sufficient4 ,one is the ordinary tinman's solder composed of lead and tin4 ,the second is spelter1 or soft fusible brass1 and the third is an alloy of silver and brass called silver solder4 ,tinman's solder is used for most purposes where high temperatures are not required1 or where the apparatus is intended to be temporary4 ,the spelter1 which is really only finely granulated fusible brass1 is used for brazing iron joints4 ,the silver solder is convenient for most purposes where permanency is required1 and is especially suited to the joining of small objects4 #ic4 ,soft tinman's solder is made by melting together two parts of grain tin and one of soft lead--the exact proportions are not of consequence--but1 on the other hand1 the purer the constituents the better the solder4 ,within certain limits1 the greater the proportion of tin the cleaner and more fusible is the solder4 ,it is usually worth while to prepare the solder in the laboratory1 for in this way a uniform and dependable product is assured4 ,good soft lead is melted in an iron ladle and skimmed2 the temperature is allowed to rise very little above the melting-point4 ,the tin is then added little by little1 the alloy stirred vigorously and skimmed1 and sticks of solder conveniently cast by sweeping the ladle over a clean iron plate1 so as to pour out a thin stream of solder4 ,if the solder be properly made it will have a mat and bright mottled surface1 and will crackle when held up to the ear and bent4 ,perhaps the chief precaution necessary in making solder is to exclude zinc4 ,the presence of a very small percentage of this metal entirely spoils the solder for tinman's work by preventing its running or flowing smoothly under the soldering bit4 ,fig4 #ge4 ,fig4 #gf4 ,fig4 #gg4 #id4 ,preparing a ,soldering ,bit4 ,the wedge-shaped edge of one of the forms of bit shown in the sketch is filed to shape and the bit heated in a fire or on a gas heater4 ,a bit of rough sandstone1 or even a clean soft brick1 or a bit of tin plate having some sand sprinkled over it1 is placed in a convenient position and sprinkled with resin4 ,as soon as the bit is hot enough to melt solder it is withdrawn and a few drops of solder melted on to the brick or its equivalent4 ,the iron or bit is then rubbed to and fro over the solder and resin till the former adheres to and tins the copper head4 ,it will be found advisable to tin every side of the point of the bit and to carry the tinning back at least half an inch from the edge4 ,if the solder obstinately refuses to adhere1 the cause is to be sought in the oxidation of the copper1 or of the solder1 or both--in either case the result of too high a temperature or too prolonged heating4 ,the simple remedy is to get the iron hot1 and then to dress it with an old file1 so as to expose a bright surface1 which is instantly passed over the resin as a means of preserving it from oxidation4 ,if the process above described be now carried out1 it will be found that the difficulty disappears4 ,before using the iron1 wipe off any soot or coke or burned resin by means of an old rag4 ,an iron tinned in this way is much to be preferred to one tinned by means of chloride of zinc4 ,a shorter and more usual method is carried out as follows3 ,the solution of chloride of zinc is prepared by adding bits of zinc to some commercial hydrochloric acid diluted with a little 7say #be per cent7 of water4 ,the acid may conveniently be placed in a small glazed white jar 7a jam pot does excellently71 and this should only be filled to about one-quarter of its capacity4 ,an excess of zinc may be added4 ,it may be fancy1 but ,i prefer a soldering solution made in this way to a solution of chloride of zinc bought as a chemical product4 ,the jar is generally mounted on a heavy leaden base1 so as to avoid any danger of its getting knocked over1 for nothing is so nasty or bad for tools as a bench on which this noxious liquid has been upset 7,fig4 #gh74 ,fig4 #gh4 ,to tin a soldering bit1 a little of the fluid is dipped out of the jar on to a bit of tin plate bent up at the edges--a few drops is sufficient--and the iron is heated and rubbed about in the liquid with a drop of solder4 ,if the iron is anything like clean it will tin at once and exhibit a very bright surface1 but quite dirty copper may be tinned by dipping it for a moment in the liquid in the pot and then working it about over the solder4 ,an iron so tinned remains covered with chloride of zinc1 and this must be carefully wiped off if it is intended to use the iron with a resin or tallow flux in lead soldering4 ,one disadvantage of this process is that the copper bit soon gets eaten into holes and requires to be dressed up afresh4 ,on the other hand1 an iron so tinned always presents a nice clean solder surface until the next time it is heated1 when it generally becomes very dirty and requires to be carefully wiped before using4 ,in my experience also an iron so tinned is more easily spoiled as to the state of its surface1 detinned1 in fact1 by overheating than when the tinning is carried out by resin and friction4 ,when this happens1 the shortest way out of the difficulty is the application of the old file so as to obtain a perfectly fresh surface4 ,no one who knows his business ever uses an iron that is not perfectly clean and well tinned4 ,the iron may be cleaned from time to time by heating it red hot and quenching it in water to get rid of the oxide1 which scales off in the process4 #ie4 ,soft ,soldering4 ,in the laboratory the chief application of the process is to copper soldering during the construction of electrical apparatus and to zinc soldering for general purposes4 ,in ninety-nine cases out of every hundred where difficulties occur their origin is to be traced to dirt4 ,there seems to be some inexplicable kink in the human mind which renders it callous to repeated proofs of the necessity for cleaning surfaces which it is intended to solder4 ,the slightest trace of albuminous or gelatinous matter or shellac will prevent solder adhering to most metals and the same remark applies in a measure to the presence of oxides1 although these may be removed by chloride of zinc or prevented from forming by resin or tallow4 ,a touch with an ordinarily dirty hand--,i refer to a solderer's hand--will often soil work sufficiently to make the adherence of solder difficult4 ,the fluxes most generally employed are tallow for lead1 resin or ,venice turpentine for copper1 chloride of zinc for anything except lead1 which never requires it4 ,the latter flux has the property 7also possessed by borax at a red heat7 of dissolving any traces of oxide which may be formed1 as well as acting as a protecting layer to the metal4 ,we may now turn to the consideration of a simple case of soldering1 say the joining of two copper wires4 ,the wires are first cleaned either by dipping in a bath of sulphuric and nitric acids--a thing no laboratory should be without--or by any suitable mechanical means4 ,the cleaned wires are then twisted together--there is a regulation way of doing this1 but it presents no advantage in laboratory practice--and the joint is sprinkled over with resin1 or painted with a solution of resin in alcohol4 ,the iron1 being heated and floated with solder1 is held against the joint1 the latter being supported on a brick1 and the solder is allowed to sweat into the joint4 ,enough solder must be present to penetrate right through the joint4 ,nothing is gained by rubbing violently with the iron4 ,if the copper is clean it will tin1 and if it is dirty it won't1 and there the matter ends4 ,beginners generally use too small or too cold a bit1 and produce a ragged1 dirty joint in consequence4 ,if the saving of time be an object1 the joint may be twisted together on ordinarily dirty oxidised wires and heated to1 say1 #bjj ,c4 ,it is then painted with chloride of zinc and soldered with the bit4 ,there is a difference of opinion as to the relative merits of chloride of zinc and of resin as a flux in soldering copper4 ,thus the standing ,german practice is1 or was1 to employ the former flux in every case for soldering electric light wires1 while in ,england the custom used to be to specify that soldering should be done by resin1 and this custom may still prevail2 it lingers in ,australia at all events4 ,however1 it is agreed on all hands that when chloride of zinc is used it must be carefully washed off4 ,i have known of an electrical engineer insisting on his workmen licking joints with their tongues to ensure the total removal of chloride of zinc2 it has a horrible taste2 and ,i have occasionally pursued the same plan myself when the soldering of fine wires was in question4 ,in any case1 it is very certain that chloride of zinc left in a joint will ruin it sooner or later by loosening the contact between copper and solder4 ,very often it is requisite to solder together two extensive flat surfaces--for instance1 in chucking certain kinds of brass work4 ,the surfaces to be soldered must be carefully tinned1 most conveniently by the help of the blow-pipe and chloride of zinc4 ,after tinning1 the surfaces are laid together and heated so as to sweat them together2 the phrase1 though inelegant1 is expressive4 #if4 ,soldering ,tin ,plate4 ,if the plate be new and clean1 a little resin or its solution in alcohol is all that is necessary as a flux4 ,if the tin plate is rusty the rust must be removed and the clean iron1 or rather mild steel1 surface exposed4 ,the use of chloride of zinc is practically essential in this case4 ,tin plate is often spotted with rust long before it becomes rusty as a whole1 when1 of course1 it may be regarded as worn out1 and such rust spots are most conveniently removed by means of the plumber's shave-hook4 ,the shave-hook is merely a peculiarly shaped hard steel scraping knife on a handle 7,fig4 #gi74 ,fig4 #gi ,with tin plate the soldering of long joints is often necessary4 ,the plate must be temporarily held in position either by binding with iron wire1 fastening by clamps1 or holding by an assistant4 ,the flux is applied and the iron run slowly along the joint4 ,enough solder is used to completely float the tip of the iron4 ,by arranging the joint so that it slopes downward slightly1 and commencing at the upper end1 the solder may be caused to flow after the iron1 and will leave a joint with the minimum permissible amount of solder in it4 ,by regulating the slope1 heat of iron1 etc1 any desired quantity of solder may be run into the joint4 #ig4 ,soldering ,zinc4 ,zinc alloys with soft solder very easily1 and by so doing entirely spoils it1 making1 it crumbly1 dirty1 and preventing it running4 ,consequently1 in soldering up zinc great care must be taken to prevent the solder becoming appreciably contaminated by the zinc4 ,to this end the zinc surfaces are cleaned by means of a little hydrochloric acid1 which is painted on instead of chloride of zinc4 ,plenty of solder is melted on to the work1 and is drawn along over the joint by a single slow motion of the soldering bit4 ,the iron must be just hot enough to make the solder flow freely1 and it must never be rubbed violently on the zinc or allowed to linger in one spot2 the result of the latter action will be to melt a hole through the zinc1 owing to the tendency of this metal to form an easily fusible alloy with the solder4 ,the art of soldering zinc is a very useful one in the laboratory4 ,the majority of physicists appear to overlook the advantages of zinc considered as a material for apparatus construction4 ,it is light1 fairly strong1 cheap1 easily fusible1 and yet hard and elastic when cold4 ,it may be worked as easily as lead at a temperature of1 say1 #aej to #bjj ,c1 and slightly below the melting-point 7#dbc ,c47 it is brittle and may powdered4 ,the property of softening at a moderate temperature is invaluable as a means of flattening zinc plate or shaping it in any way4 ,during the work it may be held by means of an old cloth4 ,zinc sheet which has been heated between iron plates and flattened by pressure retains its flatness very fairly well after cooling4 #ih4 ,soldering other ,metals4 ,iron4 ,the iron must be filed clean and then brushed with chloride of zinc solution4 ,some people add a little sat ammoniac to the chloride of zinc1 but the improvement thus made is practically inappreciable4 ,if the iron is clean it tins quite easily1 and the process of soldering it is perfectly easy and requires no special comment4 ,brass4 ,the same method as described for iron succeeds perfectly4 ,the brass1 if not exceedingly dirty1 may be cleaned by heating to the temperature at which solder melts 7below #bjj ,c471 and painting it over with chloride of zinc1 or dipping it in the liquor4 ,if now the brass be heated again in the blow-pipe flame1 it will be found to tin perfectly well when rubbed over with solder4 ,german ,silver1 ,platinoid1 ,silver1 and ,platinum are treated like iron4 ,with regard to silver and platinum the same precautions as recommended in the case of zinc must be observed1 for both these metals form fusible alloys with solder4 ,gold when pure requires no flux4 ,standard gold1 which contains copper1 solders better with a little chloride of zinc4 ,lead must be pared absolutely clean and then soldered quickly with a hot iron1 using tallow as a flux4 ,since solder if over hot will adhere to lead almost anywhere1 plumbers are in the habit of specially soiling those parts to which it is not intended that solder shall adhere4 ,the soiling paint consists of very thin glue1 called size1 mixed with lampblack2 on an emergency a raw potato may be cut in half1 and the work to be soiled may be rubbed over with the cut surface of the potato4 ,hard ,carbon or gas coke may be soldered after coating with copper by an electrolytic process1 as will be described4 #ii4 ,brazing4 ,soldering at a red heat by means of spelter is called brazing4 ,spelter is soft brass1 and is generally made from zinc one part1 copper one part2 an alloy easily granulated at a red heat2 it is purchased in the granular form4 ,the art of brazing is applied to metals which will withstand a red heat1 and the joints so soldered have the strength of brass4 ,the pieces to be jointed by this method must be carefully cleaned and held in their proper relative positions by means of iron wire4 ,it is generally necessary to soften iron wire as purchased by heating it red hot and allowing it to cool in the air2 if this is not done the wire is usually too hard to be employed satisfactorily for binding4 ,very thin wire--i4e4 above ,no4 #bj on the ,birmingham wire gauge--does not do1 for it gets burned through1 and perhaps allows the work to fall apart at a critical moment4 ,the work being securely fastened1 the next step is to cover the cleaned parts with flux in order to prevent oxidation4 ,for this purpose glass borax is employed4 ,glass borax is simply ordinary borax which has been fused for the purpose of getting rid of water of crystallisation4 ,the glass borax is reduced to powder in an iron mortar1 for it is very hard1 and is then made up into a cream with a little water4 ,this cream is painted on to the parts of the work which are destined to receive the solder4 ,the next step is to prepare the spelter1 and this is easily done by mixing it with the cream1 taking care to stir thoroughly with a flattened iron wire till each particle of spelter is perfectly covered with the borax4 ,the mixture should not be too wet to behave as a granular mass1 and may then be lifted on to the work by means of the iron spatula4 ,care must be taken to place the spelter on those parts only which are intended to receive it1 and when this is done1 the joint may be lightly powdered over with the dry borax1 and will then be ready for heating4 ,if the object is of considerable size it is most conveniently heated on the forge2 if small the blowpipe is more convenient4 ,in the latter case1 place the work on a firebrick1 and arrange two other bricks on edge about it1 so that it lies more or less in a corner4 ,a few bits of coke may also be placed on and about the work to increase the temperature by their combustion1 and to concentrate the flame and prevent radiation4 ,the temperature is gradually raised to a bright red heat1 when the spelter will be observed to fuse or run1 as it is technically said to do4 ,if the cleaning and distribution of flux has been successful1 the spelter will run along the joint very freely1 and the work should be tapped gently to make sure that the spelter has really run into the joint4 ,the heating may be interrupted when the spelter is observed to have melted into a continuous mass4 ,as soon as the work has fallen below a red heat it may be plunged into water1 a process which has the effect of cracking off the glass-like layer of borax4 ,there is1 however1 some risk of causing the work to buckle by this violent treatment1 which must of course be modified so as to suit the circumstances of the case4 ,if the joint is in such a position that the borax cannot be filed off1 a very convenient instrument for its removal by scraping is the watchmaker's graver1 a square rod of hard steel ground to a bevelled point 7,fig4 #hj74 ,fig4 #hj4 ,several precautions require to be mentioned4 ,in the first place1 spelter is merely rather soft brass1 and consequently it often cannot be fused without endangering the rest of the work4 ,a good protection is a layer of fireclay laid upon the more delicate parts1 such for instance as any screwed part4 ,gun-metal and tap-metal do not lend themselves to brazing so readily as iron or yellow brass1 and are usually more conveniently treated by means of silver solder4 ,spelter tends to run very freely when it melts1 and if the brass surface in the neighbourhood of the joint is at all clean1 may run where it is not wanted4 ,of course some control may be exercised by soiling with fireclay or using an oxidising flame2 but the erratic behaviour of spelter in this respect is the greatest drawback to its use in apparatus construction4 ,the secret of success in brazing lies in properly cleaning up the work to begin with1 and in disposing the borax so as to prevent subsequent oxidation4 #ajj4 ,silver ,soldering4 ,this process resembles that last described1 but instead of spelter an alloy of silver1 copper1 and zinc is employed4 ,the solder1 as prepared by jewellers to meet special cases1 varies a good deal in composition1 but for the laboratory the usual proportions are3 ,for soft silver solder ,fine silver #b parts ,brass wire #a part ,for hard silver solder ,sterling silver #c parts ,brass wire #a part ,the latter is1 perhaps1 generally the more convenient4 ,silver solders may1 of course1 be purchased at watchmakers' supply shops1 and as thus obtained1 are generally in thin sheet4 ,this is snipped fine with a pair of shears preparatory to use4 ,as odds and ends of silver 7from old anodes and silver residues7 generally accumulate in the laboratory1 it is often more convenient to make the solder one's self4 ,in this case it must be remembered in making hard solder by the second receipt that standard silver contains about one-twelfth of its weight of copper--exactly #ah parts copper to #bbj silver4 ,the silver is first melted in a plumbago crucible in a small furnace together with a little borax2 if any copper is required this is then added1 and finally the brass is introduced4 ,when fusion is complete1 the contents of the crucible are poured into any suitable mould4 ,the quickest and most convenient way of preparing the alloy for use is to convert it into filings with the assistance of a coarse file1 or by milling it1 if a milling machine is available4 ,equal volumes of filings and powdered glass borax are made into a thin paste with water1 and applied in an exactly similar manner to that described under the head of brazing4 ,in fact all the processes there described may be applied equally to the case under discussion1 the substitution of silver for spelter being the only variation4 ,the silver solder is more manageable than spelter1 and does not tend to run wild over the work3 a property which makes it much more convenient both for delicate joints and in cases where it is desired to restrict the solder to a single point or line4 ,small objects are almost invariably soldered with silver solder1 and are held by forceps or on charcoal in the pointed flame of an ordinary blow-pipe4 #aja4 ,on the ,construction of ,electrical ,apparatus3 ,insulators4 ,it is not intended to deal in any way with the design of special examples of electrical apparatus1 but merely to describe a rather miscellaneous set of materials and processes constantly required in its construction4 ,it is not known whether there is such a thing as a perfect insulator1 even if we presuppose ideal circumstances4 ,materials as they exist must be regarded merely as of high specific resistance1 that is if we allow ourselves to use such a term in connection with substances1 conduction through which is neither independent of electromotive force per unit length1 nor of previous history4 ,even the best of these substances generally get coated with a layer of moisture when exposed to the air1 and this as a rule conducts fairly well4 ,very pure crystalline sulphur and fused quartz suffer from this defect less than any other substances with which the writer is acquainted1 but even with them the surface conductivity soon grows to such an extent as totally to mask the internal conduction4 ,it is proposed to give a brief account of the properties of some insulating substances and their application in electrical construction1 and at the same time to indicate the appliances and methods requisite for working them4 ,with regard to the specific resistances which will be quoted1 the numbers must not be taken to mean too much1 partly for the reason already given4 ,it is also in general doubtful whether sufficient care has been taken to distinguish the body from the surface conductivity1 and consequently numerical estimates are to be regarded with suspicion4 ,the question of sampling also arises1 for it must be remembered that a change in composition amounting to1 say1 #a_/ajjjj per cent may be accompanied by a million-fold change in specific resistance4 #ajb4 ,sulphur4 ,this element exists in several allotropic forms1 which have very different electric properties4 ,after melting at about #abe ,c1 and annealing at #aaj for several hours1 the soluble crystalline modification is formed4 ,after keeping for some days--especially if exposed to light--the crystals lose their optical properties1 but remain of the same melting-point1 and are perfectly soluble in carbon bisulphide4 ,the change is accompanied by a change in colour1 or rather in brightness1 as the transparency changes4 ,the specific resistance of sulphur in this condition is above #ajbh ,c4,g4,s4,e4,m4 units1 or #ajac megohms per cubic centimetre for an electric intensity of say #ab1jjj volts per centimetre4 ,this is at ordinary temperatures4 ,at #ge ,c4 the specific resistance falls to about #ajbe under similar conditions as to voltage4 ,in all cases the conductivity appears to increase with the electric intensity1 or at all events with an increase in voltage1 the thickness of the layer of sulphur remaining the same4 ,the specific inductive capacity is #c4afb at ordinary temperatures1 and increases very slightly with rise of temperature4 ,footnote3 ,march #ahig4--;,it is now the opinion of the writer that though the specific inductive capacity of a given sample of a solid element is perfectly definite1 yet it is very difficult to obtain two samples having exactly the same value for this constant1 even in the case of a material so well defined as sulphur4 ,the total residual charge1 after ten minutes' charging with an intensity of #ab1jjj volts per centimetre1 is not more than #d parts in #aj1jjj of the original charge4 ,in making this measurement the discharge occupied a fraction of a second4 ,the electric strength for a homogeneous plate of crystalline sulphur is not less than #cc1jjj volts per centimetre1 and probably a good deal more4 ,if the sulphur is contaminated with up to #c per cent of the amorphous variety1 as is the case if it is cooled fairly quickly from a temperature of #agj ,c4 or over1 the specific resistance falls to from #ajbe to #ajbf at ordinary temperatures2 and the specific inductive capacity increases up to #c4ge1 according to the amount of insoluble sulphur present4 ,the residual charge under circumstances similar to those described above1 but with an intensity of about #djjj volts per centimetre is1 say1 #b per cent of the initial charge4 ,so far as the writer is aware sulphur is the only solid non-conductor which can be easily obtained in a condition of approximate purity and in samples sufficiently exactly comparable with one another2 it is the only one1 therefore1 that repays any detail of description4 ,very pure sulphur can be bought by the ton if necessary from the ,united ,alkali ,company of ,newcastle-on-,tyne4 ,it is recovered from sulphur waste by the ,chance process1 which consists in converting the sulphur into hydrogen sulphide1 and burning the latter with insufficient air for complete combustion4 ,the sulphur is thrown out of combination1 and forms a crystalline mass on the walls and floor of the chamber4 ,the sulphur which comes into the market consists of this mass broken up into convenient fragments4 ,in order to purify it sufficiently for use as an insulator1 the sulphur may be melted at a temperature of #abj to #adj ,c1 and filtered through a plug of glass wool in a zinc funnel2 as thus prepared it is an excellent insulator4 ,to obtain the results mentioned in the table it is1 however1 necessary to conduct a further purification 7chiefly from water7 by distillation in a glass retort4 ,the sulphur thus obtained may be cast of any desired form in zinc moulds1 the castings and moulds being immediately removed to an annealing oven at a temperature of from #ajj to #aaj ,c1 where they are left for several hours4 ,if the sulphur is kept melted for some time at #abe ,c4 the annealing is not so important4 ,the castings may be removed from the mould by slightly heating the latter1 but many breakages result4 ,insulators made on this plan are much less affected by the condensation of moisture from the air than anything except fused quartz4 ,they are1 however1 very weak mechanically1 and apt to crack by exposure to such changes of temperature as go on from day to day4 ,it is clear1 however1 that in spite of this their magnificent electrical properties fit them for many important uses4 ,if the sulphur be cooled rapidly from #agj ,c4 or over1 a mixture of the crystalline and amorphous varieties of sulphur is obtained4 ,this mixture is very much stronger and tougher than the purely crystalline substance1 and may be worked with ordinary hardwood tools into fairly permanent plates1 rods1 etc4 ,sheets of pure thick filter paper may also be dipped into sulphur at #agj ,c1 at which temperature air and moisture are mostly expelled1 and such sheets show a very considerable insulating power4 ,the sulphur does not penetrate the paper1 which therefore merely forms a nucleus4 ,cakes of the crystalline or mixed varieties may be made by grinding up some purified sulphur1 moistening it with redistilled carbon bisulphide1 or toluene1 or even benzene 7,c#f;,h#f71 and pressing it in a suitable mould under the hydraulic press4 ,the plates thus formed are porous1 but are splendid insulators1 especially if made from the crystalline variety of sulphur1 and they appear to keep their shape very well1 and do not crack with ordinary temperature changes4 ,the metals which resist the action of sulphur best are gold and aluminium2 while platinum and zinc are practically unacted upon at temperatures below a red heat--in the former case1--and below the boiling-point of sulphur in the latter4 ,a very convenient test of the purity of sulphur is the colour assumed by it when suddenly cooled from the temperature at which it is viscous4 ,quite pure sulphur remains of a pale lemon yellow under this treatment1 but the slightest trace of impurity1 such as arises from dust containing organic matter1 stains the sulphur1 and renders it darker in colour4 #ajc4 ,fused ,quartz4 ,this is on the whole the most reliable and most perfect insulator for general purposes4 ,no exact numerical data have been obtained1 but the resistivity must certainly be of the same order as that of pure sulphur at its best4 ,the influence of the moisture of the air also reaches its minimum in the case of quartz1 as was originally observed by ,boys4 ,as yet1 however1 the material can only be obtained in the form of rods or threads4 ,for most purposes rods of about one-eighth of an inch in diameter are the most convenient4 ,these rods may be used as insulating supports1 and succeed perfectly even if they interpose less than an inch of their length to electrical conduction4 ,the sketch 7,figs4 #ha and #ha;,a7 shows 7to a scale of about one-quarter full size7 a complete outfit for elementary electrostatic experiments1 such as has been in use in the writer's laboratory for five years4 ,with these appliances all the fundamental experiments may be performed1 and the apparatus is always ready at a moment's notice4 ,fig4 #ha4 ,though quartz does not condense moisture or gas to form a conducting layer of anything like the same conductivity as in the case of glass or ebonite1 still it is well to heat it if the best results are to be obtained4 ,for this purpose a small pointed blow-pipe flame may be used1 and the rods may be got red-hot without the slightest danger of breaking them4 ,they then remain perfectly good and satisfactory for several hours at least1 even when exposed to damp and dusty air4 ,the rods are conveniently held in position by small brass ferrules1 into which they are fastened by a little plaster of ,paris4 ,sealing-wax must be avoided1 on account of the inconvenience it causes when the heating of the rods is being carried out4 ,one useful application of fused quartz is to the insulation of galvanometer coils 7,fig4 #hb71 another to the manufacture of highly insulating keys 7,fig4 #hc72 while as an insulating suspension it has all the virtues4 ,if it is desired to render the threads conducting they may be lightly silvered1 and will be found to conduct well enough for electrometer work before the silver coating is thick enough to sensibly impair their elastic properties4 ,fig4 #ha;,a4 ,fig4 #hb is a full-size working drawing of a particular form of mounting for galvanometer coils4 ,the objects sought to be attained are3 7#a7 high insulation of the coils1 7#b7 easy adjustment of the coils to the suspended system4 ,the first object is attained as follows4 ,the ebonite ring ,a is bored with four radial holes1 through which are slipped from the inside the fused quartz bolt-headed pins ,b4 ,the coil already soaked in hard paraffin is placed concentrically in the ring ,a by means of a special temporary centering stand4 ,the space between the coil and the ring is filled up with hard paraffin1 and this holds the quartz pins in position4 ,the system of ebonite ring1 coil1 and pins is then fastened into the gun-metal coil carrier1 which is cut away entirely1 except near the edges1 where it carries the pin brackets ,c4 ,these brackets can swivel about the lower fastening at ,e before the latter is tightened up4 ,the coil is now adjusted in the adjusting stand to be concentric with the axis of symmetry of the coil carrier1 and the supporting pins are slipped into slot holes cut in the brackets1 the brackets being swivelled as much as necessary to allow of this4 ,when the pins are all inserted the brackets are screwed up by the screws at ,e4 ,the pins are then cemented firmly to the brackets by a little plaster of ,paris4 ,the coil carrier can now be adjusted to the galvanometer frame by means of screws at ,d1 which pass through wide holes in the carrier and bold the latter in position by their heads4 ,in the sectional plan the parts of the galvanometer frame are shown shaded4 ,the front of the frame at ,f ,f is of glass1 and the back of the frame is also made of glass1 though this is not shown in the section4 ,a represents an ebonite ring into which the wire coil is cemented by means of paraffin4 ,b ,b ,b ,b are quartz pins1 with heads inside the ebonite ring4 ,c ,c ,c are slotted brackets adjustable to the pins and capable of rotation by releasing the screws ,e ,e4 ,d ,d are the screws holding the coil carriage to the galvanometer framework4 ,these screws pass through large holes in the carriage so as to allow of some adjustment4 ,fig4 #hb4 ,fig4 #hc4 #ajd4 ,glass4 ,when glass is properly chosen and perfectly dry it has insulating properties possibly equal to those possessed by quartz or crystalline sulphur4 ,for many purposes1 however1 its usefulness is seriously reduced by the persistence with which it exhibits the phenomena of residual charge1 and the difficulty that is experienced in keeping it dry4 ,the insulating power of white flint glass is much in excess of that of soft soda glass1 which is a poor insulator1 and of ordinary green bottle glass4 ,the jars of ,lord ,kelvin's electrometers1 which insulate very well1 are made of white flint glass manufactured in ,glasgow1 but it is found that occasionally a particular jar has to be rejected on account of its refusing to insulate1 and this1 if ,i understand aright1 even when it exhibits no visible defects4 ,a large number of varieties of glass were tested by ,dr4 ,hopkinson at ,messrs4 ,chance ,bros4 ,works1 in #ahge and #ahgf 7,phil4 ,trans1 #ahgg71 and in #ahhg 7,proc4 ,roy4 ,soc4 xli4 #dec71 chiefly with a view to the elucidation of the laws regulating the residual charge2 and incidentally some extraordinarily high insulations were noted among the flint glasses4 ,the glass which gave the smallest residual charge was an opal glass2 and flint glasses were found to insulate #aje times as well as soda lime glasses4 ,the plates of ,wimshurst machines are made of ordinary sheet window glass1 but as the insulating property of this material appears to vary1 it is generally necessary to clean1 dry1 and test a sheet before using it4 ,with regard to hard ,bohemian glass1 this is stated by ,koeller 7,wien ,bericht7 to insulate ten times as well as the ordinary ,thuringian soft soda glass4 ,on the whole the most satisfactory laboratory practice is to employ good white flint glass4 ,the only point requiring attention is the preparation of the glass by cleaning and drying4 ,of course all grease or visible dirt must be removed as described in an earlier chapter 7 #ac71 but this is only a beginning4 ,the glass after being treated as described and got into such a state as to its surface that clean water no longer tends to dry off unequally1 must be subjected to a further scrub with bibulous paper and a clear solution of oleate of soda4 ,the glass is then to be well rinsed with distilled water and allowed to drain on a sheet of filter paper4 ,a very common cause of failure lies in the contamination of the glass with grease from the operator's fingers4 ,before setting out to clean the glass the student will do well to wash his hands with soap and water1 then with dilute ammonia and finally with distilled water4 ,in the case of an electrometer jar which has become conducting but is not perceptibly dirty1 rubbing with a little oleate of soda and a silk ribbon1 followed1 of course1 by copious washing1 does very well4 ,if there is any tin-foil on the jar1 great care must be taken not to allow the glass surface to become contaminated by the shellac varnish or gum used to stick the tin-foil in position4 ,finally1 the glass should be dried by radiant heat and raised to a temperature of #ajj ,c4 at least1 and kept at it for at least half an hour4 ,before drying it is of course advisable to allow the water to drain away as far as possible1 and if the water is only the ordinary distilled water of the laboratory1 the glass is preferably wiped with a clean bit of filter paper2 any hairs which may be left upon the glass will brush off easily when the glass is dry4 ,in order to obtain satisfactory results the glass must be placed in dry air before it has appreciably cooled4 ,this is easily done in the case of electrometer jars1 and so long as the air remains perfectly dry through the action of sulphuric acid or phosphorus pentoxide1 the jar will insulate4 ,the slightest whiff of ordinarily damp air will1 however1 enormously reduce the insulating power of the glass1 so that unvarnished glass surfaces must be kept for apparatus which is practically air-tight4 ,for outside or imperfectly protected uses the glass does better when varnished4 ,it is a fact1 however1 that varnished glass is rarely if ever so good as unvarnished glass at its best4 ,too much care cannot be taken over the preparation of the varnish2 ,french polish1 or carelessly made shellac varnish1 is likely to do more harm than good4 ,the best orange shellac must be dissolved in good cold alcohol by shaking the materials together in a bottle4 ,the alcohol is made sufficiently pure by starting with rectified spirit and digesting it in a tin flask over quick-lime for several days1 a reversed condenser being attached4 ,a large excess of lime must be employed1 and this leads to a considerable loss of alcohol1 a misfortune which must be put up with4 ,after1 say1 thirty hours' digestion1 the alcohol may be distilled off and employed to act on the shellac4 ,in making varnish1 time and trouble are saved by making a good deal at one operation--a ,winchester full is a reasonable quantity4 ,the bottle may be filled three-quarters full of the shellac flakes and then filled up with alcohol2 this gives a solution of a convenient strength4 ,the solution1 however1 is by no means perfect1 for the shellac contains insoluble matter1 and this must be got rid off4' ,one way of doing this is to filter the solution through the thick filtering paper made by ,schleicher and ,schuell for the purpose1 but the filtering is a slow process1 and hence requires to be conducted by a filter paper held in a clip 7not a funnel7 under a bell jar to avoid evaporation4 ,another and generally more convenient way in the laboratory is to allow the muddy varnish to settle--a process requiring at least a month--and to decant the clear solution off into another bottle1 where it is kept for use4 ,the muddy residue works up with the next lot of shellac and alcohol1 which may be added at once for future use4 ,the glass to be varnished is warmed to a temperature of1 say1 #ej ,c1 and the varnish put on with a lacquering brush2 a thin uniform coat is required4 ,the glass is left to dry long enough for the shellac to get nearly hard and to allow most of the alcohol to evaporate4 ,it is then heated before a fire1 or even over a ,bunsen1 till the shellac softens and begins to yield its fragrant characteristic smell4 ,if the coating is too heavy1 or if the heating is commenced before the shellac is sufficiently dry1 the latter will draw up into tears1 which are unsightly and difficult to dry properly4 ,on no account must the shellac be allowed to get overheated4 ,if the varnish is not quite hard when cold it may be assumed to be doing more harm than good4 ,in varnishing glass tubes for insulating purposes it must be remembered that the inside of the tube is seldom closed perfectly as against the external air1 and consequently it also requires to be varnished4 ,this is best done by pouring in a little varnish considerably more dilute than that described1 and allowing it to drain away as far as possible1 after seeing that it has flooded every part of the tube4 ,during this part of the process the upper end of the tube must be closed1 or evaporation will go on so fast that moisture will be deposited from the air upon the varnished surface4 ,afterwards the tube may be gently warmed and a current of air allowed to pass1 so as to prevent alcohol distilling from one part of the tube to another4 ,the tube is finally heated to the softening point of shellac1 and if possible closed as far as is practicable at once4 #aje4 ,ebonite or ,hard ,rubber4 ,this exceedingly useful substance can be bought of a perfectly useless quality4 ,much of the ebonite formerly used to cover induction coils for instance1 deteriorates so rapidly when exposed to the air that it requires to have its surface renewed every few weeks4 ,the very best quality of ebonite obtainable should be solely employed in constructing electric works4 ,it is possible to purchase good ebonite from the ,silvertown ,rubber ,company 7and probably from other firms71 but the price is necessarily high1 about four shillings per pound or over4 ,at ordinary temperatures ebonite is hard and brittle and breaks with a well-marked conchoidal fracture4 ,at the temperature of boiling water the ebonite becomes somewhat softened1 so that it is readily bent into any desired shape2 on cooling it resumes its original hardness4 ,this property of softening at the temperature of boiling water is a very valuable one4 ,the ebonite to be bent or flattened is merely boiled for half an hour or so in water1 taken out1 brought to the required shape as quickly as possible1 and left to cool clamped in position4 ,the sheet ebonite as it comes from the makers is generally far from flat4 ,it is often necessary to flatten a sheet of ebonite1 and of course this is the more easily accomplished the smaller the sheet4 ,consequently a bit of ebonite of about the required size is first cut from the stock sheet by a hack-saw such as is generally used for metals4 ,this piece is then boiled and pressed between two planed iron plates previously warmed to near #ajj ,c4 ,with pieces of ebonite such as are used for the tops of resistance boxes1 measuring1 say1 #bj ,x #h ,x #aa inches1 very little trouble is experienced4 ,the sheets when cold are found to retain the flatness which has been forced upon them perfectly well4 ,it is otherwise with large thin sheets such as are used for ,holtz machines4 ,i have succeeded fairly1 but only fairly1 by pressing them in a gluing press1 consisting of heavy planed iron slabs previously heated to #ajj ,c4 ,i do not know exactly how best to flatten very thin and large sheets4 ,it is easy to make large tubes out of sheet ebonite by taking advantage of the softening which ebonite undergoes in boiling water4 ,a wooden mandrel is prepared of the proper size and shape4 ,the ebonite is softened and bent round it2 this may require two or three operations1 for the ebonite gets stiff very quickly after it is taken out of the water4 ,finally the tube is bound round the mandrel with sufficient force to bring it to the proper shape and boiled in water1 mandrel and all4 ,the bath and its contents are allowed to cool together1 so that the ebonite cools uniformly4 ,tubes made in this way are of course subject to the drawback of having an unwelded seam1 but they do well enough to wind wire upon if very great accuracy of form is not required4 ,if very accurate spools are needed the mandrel is better made of iron or slate and the spool is turned up afterwards4 ,the seam may be strapped inside or at the ends by bits of ebonite acting as bridges1 and the seam itself may be caulked with melted paraffin or anthracene4 ,working ,ebonite4 ,ebonite is best worked as if it were brass1 with ordinary brass-turning or planing tools4 ,these tools should be as hard as possible1 for the edges are apt to suffer severely1 and blunt tools leave a very undesirable woolly surface on the ebonite4 ,in turning or shaping ebonite sheets it is as well to begin by taking the skin off one side first1 and then reversing the sheet1 finishing the second side1 and then returning to the first4 ,this is on account of the fact that ebonite sometimes springs a little out of shape when the skin is removed4 ,turned work in ebonite1 if well done1 requires no sand-papering1 but may be sufficiently polished by a handful of its own shavings and a little vaseline4 ,the advantage of using a polished ebonite surface is that such a surface deteriorates more slowly under the influence of light and air than a surface left rough from the tool4 ,if very highly polished surfaces are required1 the ebonite after tooling is worked with fine pumice dust and water1 applied on felt1 or where possible by means of a felt buff on the lathe1 and finally polished with rouge and water1 applied on felt or cloth4 ,ebonite works particularly well under a spiral milling cutter1 and sufficiently well under an ordinary rounded planing tool1 with cutting angle the same as for brass1 and hardened to the lightest straw colour4 ,it is not possible1 on the other hand1 to use the carpenter's plane with success1 for the angle of the tool is too acute and causes the ebonite to chip4 ,in boring ebonite the drill should be withdrawn from the hole pretty often and well lubricated1 for if the borings jam1 as they are apt to do1 the heat developed is very great and the temper of the drill gets spoiled4 ,ebonite will spoil a drill by heating as quickly as anything known2 on the other hand1 it may be drilled very fast if proper precaution is taken4 ,it is advisable to expose ebonite to the light as little as possible1 especially if the surface is unpolished1 for under the combined action of light and air the sulphur at the surface of the ebonite rapidly oxidises1 and the ebonite becomes covered with a thin but highly conducting layer of sulphurous or sulphuric acid or their compounds4 ,when this happens the ebonite may be improved by scrubbing with hot water1 or washing freely with alcohol rubbed on with cotton waste in the case of apparatus that cannot be dismounted4 ,a complete cure1 however1 can only be effected by scraping off the outer layer of ebonite so as to expose a fresh surface4 ,for this purpose a bit of sheet glass broken so as to leave a curved edge is very useful1 and the ebonite is then scraped like a cricket bat4 ,in designing apparatus for laboratory use it is as well to bear in mind that sooner or later the ebonite parts will require to be taken down and scraped up4 ,rods or tubes are1 of course1 most quickly treated on the lathe with rough glass cloth1 and may be finished with fine sandpaper1 then pumice dust and water1 applied on felt4 ,after cleaning the pumice off by means of water and a rag1 the final touch may be given by means of vaseline1 applied on cloth or on ebonite shavings4 #ajf4 ,mica4 ,a great variety of minerals go under this name4 ,speaking generally1 the ,russian micas coming into commerce are potash micas1 and mica purchased in ,england may be taken to be potash mica1 especially if it is in large sheets4 ,at ordinary temperatures mica of the kind found in commerce is an excellent insulator4 ,schultze 7,wied4 ,ann4 vol4 xxxvi4 p4 #fee7 comes to the conclusion that both at high and at low temperatures mica 7of all kinds87 is a better insulator than white mirror glass1 the composition of which is not stated4 ,the experiments of the author referred to were apparently left unfinished1 and altogether too much stress must not be laid on the results obtained1 one of which was that mica conducts electrolytically to some extent at high temperatures4 ,bouty 7,journal de ,physique1 #ahij #i1 #bhh7 and ,j4 ,curie 7,thse de ,doctorat1 ,paris1 #ahhh7 agree in making the final conductivity of the mica used in ,carpentier's condensers exceedingly small--at all events at ordinary temperatures4 ,bearing in mind that for such substances the term specific resistance has no very definite meaning1 ,m4 ,bouty considers it is not less than #c4ai x #ajbh ,e4,m4 units at ordinary temperatures4 ,m4 ,bouty gives a note or illustration of what such numbers mean--a precaution not superfluous in cases where magnitudes are denoted logarithmically4 ,referring to the value quoted1 viz4 #c4ai x #ajbh1 ,m4 ,bouty says1 ,ce serait la resistance d'une colonne de mercure de #ammq de section et de longueur telle que la lumire se propageant dans le vide1 mettrait plus de #cjjj ans ,a se transmettre d'une extrmit ,i'autre de la colonne4 ,m4 ,bouty returns to the study of mica 7muscovite7 in the ,journal de ,physique for #ahib1 p4 #e1 and there deals with the specific inductive capacity1 which for a very small period of charge he finds has the value #h--;an enormous value for such a good insulator1 and one that it would be desirable to verify by some totally distinct method4 ,this remark is enforced by the fact that ,m4 ,klemencic finds the number #f for the same constant4 ,the temperature coefficient of this constant was too small for ,m4 ,bouty to determine4 ,the electric intensity was of the order of #ajj volts per centimetre1 and the experiments seem to indicate that the specific inductive capacity would be only slightly less if referred to a period of charge indefinitely short4 ,i have found that the residual charge in a mica condenser1 made according to ,carpentier's method 7to be described below71 is about #a per cent of the original charge under the following circumstances4 ,voltage #cjj volts on a plate #j4b mm4 thick1 duration of charge ten minutes1 temperature about #bj ,c4 ,to get this result the mica must be most carefully dried4 ,this and other facts indicate that the so-called residual charge on ordinary condensers is1 to a very large extent1 due to the creeping of the charge from the armatures over the more or less conducting varnished surfaces of the mica1 and its slow return on discharge4 ,this source of residual charge was carefully guarded against by ,rowland and ,nichols 7,phil4 ,mag4 #ahha7 in their work on quartz1 and is referred to by ,m4 ,bouty1 who adduces some experiments to show that his own results are not vitiated by it4 ,on the other hand1 ,m4 ,bouty shows that a small rise in temperature enormously affects the state of a mica surface1 and that the surface gets changed in such a way as to become very fairly conducting at #cjj ,c4 ,also anybody can easily try for himself whether exposing a mica condenser plate which has been examined in presence of phosphorus pentoxide to ordinary air for five minutes will not enormously increase the residual charge1 as has always been the case in the writer's experience1 and if so1 it is open to him to suggest some cause other than surface creeping as an explanation4 ,m4 ,bouty1 using less perfectly dried mica1 did not get so good a result as to smallness of residual charge as the one above quoted4 ,the chief use of mica for laboratory purposes depends on the ease with which it can be split1 and also upon the fact that it may be considerably crumpled and bent without breaking4 ,it therefore makes an excellent dielectric in so far as convenience of construction is concerned in the preparation of condensers1 and lends itself freely to the construction of insulating washers or separators of any kind4 ,its success as a fair insulator at moderate temperatures has led to its use in resistance thermometers1 where it appears to have given satisfaction up to1 at all events1 #djj ,c4 ,it is worth a note that according to ,werner ,siemens1 who had immense experience 7,wied4 ,ann4 vol4 clix471 soapstone is the only reliable insulator at a red heat1 but1 no doubt1 a good deal depends on the particular specimen investigated4 #ajg4 ,use of ,mica in ,condensers4 ,if good results are desired it is essential to select the mica very carefully4 ,pieces appreciably stained1--particularly if the stain is not uniformly distributed1--cracked pieces1 and pieces tending to flake off in patches should be rejected4 ,the best samples of mica that have come under the writer's observation are those sheets sold for the purpose of giving to silver photographic prints that hideous glazed surface which some years ago was so popular4 ,sheets of mica about #j4a to #j4b mm4 thick form good serviceable condenser plates1 and will certainly stand a pressure of #cjj volts1 and most likely a good deal more4 ,the general practice in ,england seems to have been to build up condensers of alternate sheets of varnished or paraffined-mica and tin-foil4 ,this practice is open to several objections4 ,in the first place1 the capacity of a condenser made in this way varies with the pressure binding the plates together4 ,in the second place1 the amount of mica and tin-foil required is often excessive in consequence of the imperfect contact of these substances4 ,again1 the inevitable air film between the mica and tin-foil renders condensers so made unsuitable for use with alternating currents1 owing to the heating set up through air discharges1 and which is generally1 though often 7if not always7 wrongly1 attributed to dielectric hysteresis4 ,these imperfections are to a great extent got over by ,m4 ,carpentier's method of construction1 which is1 however1 rather more costly both in material and labour4 ,on the other hand1 wonderful capacities are obtained with quite small amounts of mica4 ,m4 ,bouty mentions a condenser of one microfarad capacity weighing #aejj grms4 and contained in a square box measuring #ab centimetres on the side1 and about #c centimetres thick4 ,the relation between the capacity and surface of doubly-coated plates is in electro-static units3 ,capacity 7sp4 ind4 capacity ,x area of one surface7_/7#dpi ,x thickness7 ,this may be reduced to electro-magnetic units by dividing by #ix#ajbj1 and to microfarads by further multiplying by #ajae4 ,m4 ,carpentier begins1 of course1 by having his mica scrupulously clean and well selected4 ,it is then silvered by one of the silvering processes 7 #fe7 on both sides1 for which purpose the sheets may be suspended in a paraffined wood rack1 so as to lie horizontally in the silvering solution1 a space of about half an inch being allowed between the sheets4 ,the silvering being finished1 the sheets are dipped along two parallel edges in #ge per cent nitric acid4 ,with regard to the third and fourth edges of the sheet1 the silver is removed on one side only1 using a spun glass brush2 if we agree to call the two surfaces of the mica ,a and ,b respectively1 and the two edges in question ,c and ,d1 then the silver is removed from the ,a side along edge ,c1 and from the ,b side along edge ,d4 ,the silvered part is shown shaded in ,fig4 #hd4 ,by this arrangement the silver and mica plates may be built up together so as to form the same mutual arrangement of contacts as in an ordinary mica tin-foil condenser4 ,fig4 #hd4 ,it need hardly be said that the sheets require very complete washing after treatment with nitric acid1 followed by a varnishing of the edges as already described in the case of glass1 and baking at a temperature of #adj ,c4 in air free from flame gases1 till the shellac begins to emit its characteristic odour and is absolutely hard when cold4 ,the plates are then built up so as to connect the sheets which require to be connected1 and to insulate the other set4 ,general contact is1 if necessary1 secured by means of a little silver leaf looped across from plate to plate--a part of the construction which requires particular attention and clean hands1 for it is by no means so easy to make an unimpeachable contact as might at first appear4 ,the condenser1 having been built up1 may be clamped solid and placed in its case2 the capacity will not depend appreciably on the tightness of the clamp screws--a great feature of the construction4 ,such a condenser will not give its best results unless absolutely dry4 ,i have kept one very conveniently in a vacuum desiccator over phosphorus pentoxide1 but if of any size1 the condenser deserves a box to itself1 and this must be air-tight and provided with a drying reagent1 so arranged that it can be removed through a manhole of some sort4 ,contact to the brass-work on the lid may be made by pressing spring contacts tightly down upon the ends of the silver foils and carrying the connections through the lid4 ,this also serves to secure the condenser in position4 #ajh4 ,micanite4 ,this substance1 though probably comparing somewhat unfavourably with the insulators already enumerated1 and being subject to the uncertainties of manufacture1 has during the last few years achieved a considerable success in ,american electrical engineering construction4 ,it is composed of scrap mica and shellac varnish worked under pressure to the desired shape1 and may be obtained in sheets1 plates1 and rods1 or in any of the forms for which a die happens to have been constructed4 ,of course1 in special cases it would be worth while to prepare a die1 and then the attainable forms would be limited by moulding considerations only4 ,the writer's experience is very limited in this matter1 but ,dr4 ,kennelly1 with whom he communicated on the subject1 was good enough to reply in favour of micanite for engineering work4 #aji4 ,celluloid4 ,this material is composed of nitrocellulose and camphor4 ,it has fair insulating properties1 and may be obtained in a variety of forms1 but has now been generally abandoned for electrical work on account of its inflammability4 #aaj4 ,paper4 ,pure white filter paper1 perfectly dry1 is probably a very fair insulator2 the misfortune is that in practice it cannot be kept dry4 ,under the most favourable circumstances its specific resistance may approach #ajbd ,e4,m4 units4 ,it must therefore be considered rather as a partial conductor than as an insulator4 ,the only case of the use of dry paper as an insulator in machine construction which has come under the writer's notice is in building up the commutators of the small motors which used to drive the ,edison phonographs4 ,its advantages in this connection are to be traced to the fact that a commutator so built up is durable and keeps a clean surface4 ,of course1 the use of paper as an insulator for telephone wires is well known1 but its success in this direction depends less upon its insulating properties than upon the fact that it can be arranged in such a way as to allow of the wires being partially air insulated1 an arrangement tending to reduce the electrostatic capacity of the wire system4 ,at one time it was the custom of instrument makers to employ ordinary printed paper in the shape of leaves torn from books or the folios of old ledgers to form the dielectric of the condensers used in connection with the contact breakers of induction coils4 ,this practice has nothing but economy to recommend it1 for cases often occur in which the paper1 by gradual absorption of moisture from the air1 comes to insulate so badly that it practically short circuits the spark gap1 and so stops the action of the coil4 ,three separate cases have come within the writer's experience4 ,some measurements of the resistance of paper have been made by ,f4 ,uppenborn 7,centralblatt fuer ,electrotechnik1 ,vol4 xi4 p4 #bae1 #ahhi74 ,there is an abstract of the paper also in ,wiedemann's ,beiblaetter 7#ahhi1 vol4 xiii4 ,p4 #gaa74 ,uppenborn examined the samples of paper under normal conditions as to moisture and obtained the following results3- ,description of ,paper ,i ,pressure ,intensity ,,ii4 ,specific ,resistance corresponding to pressures as in ,column ,i4 ,ohms4 ,,iii ,pressure ,intensity4 ,,iv4 ,specific ,resistance corresponding to ,column ,,iii4 ,ohms4 ,common cardboard #b4c mm4 thick #j4je kilo4 per #fjjj sq4 mm4 #d4he x #ajae #bj kg4 per #fjjj sq4 mm4 #d4g x #ajad ,gray paper1 #j4bf mm4 thick #j4je kilo4 per #ejjj sq4 mm4 #c4a x #ajae #bj kg4 per #ejjj sq4 mm4 #h x #ajad ,yellow parchment paper-#ji mm4 thick #j4je kilo4 per #ecjj sq4 mm4 #c4je x #ajaf #bj kg4 per #ecjj sq4 mm4 #h x #ajaf ,linen tracing cloth #j4je kilo4 per #fjjj sq4 mm4 #a4ce x #ajaf #bj kg4 per #cc1jjj sq4 mm4 #a4hf x #ajae #aaa4 ,paraffined ,paper4 ,like wood and other semiconductors1 paper can be vastly improved as an insulator by saturating it with melted paraffin4 ,to get the best results a pure paper free from size must be employed--gray ,swedish filter paper does well4 ,this is dried at a temperature above #ajj ,c4 for1 say1 half an hour1 and the sheets are then floated on the top of paraffin1 kept melted at #adj ,c4 or thereabout in a baking dish4 ,as soon as the paper is placed upon the melted paraffin the latter begins to soak through1 in virtue of capillary action1 and drives before it the air and moisture1 causing a strongly marked effervescence4 ,after about one minute the paper may be thrust below the paraffin to soak4 ,when a sufficient number of papers have accumulated1 and when no more gas comes off1 the tray may be placed in a vacuum box 7,fig4 #he71 and the pressure reduced by the filter pump4 ,as the removal of the air takes time1 provision must be made for keeping the bath hot4 ,a vacuum may be maintained for about an hour1 and air then readmitted4 ,repeated exhaustions and readmissions of air1 which appear to improve wood1 do not give anything like such a good result with paper4 ,in using a vacuum box provision must be made in the shape of a cool bottle between the air pump and the box4 ,if this precaution be omitted1 and if any paraffin splashes on to the hot surface of the box1 it volatilises with decomposition and the products go to stop up the pump4 ,paraffin with a melting-point of #ej ,c4 or upwards does well4 ,the bath should be allowed to cool to about #fj ,c4 before the papers are removed1 so that enough paraffin may be carried out to thoroughly coat the paper and prevent the entrance of air4 ,fig4 #he4 ,fig4 #he is a section of a vacuum vessel which has been found very convenient4 ,it measures about two feet in diameter at the top4 ,it is round1 because it is much easier to turn one circular surface than to plane up four surfaces1 which has to be done if the box is square4 ,both the rim of the vessel and the approximating part of the cover require to be truly turned and smoothly finished4 ,a very good packing is made of solid indiarubber core about half an inch thick4 ,this is carefully spliced--cemented by means of a solution of rubber in naphtha1 and the splice sewed by thick thread4 ,the lid ought to have a rim fitting inside the vessel1 for this keeps the rubber packing in place2 the rim has been accidentally omitted in ,fig4 #he4 ,the bolts should not be more than five inches apart1 and should lie at least half an inch in diameter1 and the rim and lid should be each half an inch thick4 ,condensers may now be built up of sheets of this prepared paper interleaved with tin-foil in the ordinary way4 ,if good results are required1 the condenser when finished is compressed between wooden or glass end-pieces by means of suitable clamps4 ,it can then be put in a box of melted paraffin1 heated up to #adj ,c1 and exhausted by means of the water pump for several hours4 ,in this process the air rushes out from between the paper and foils with such vehemence that the paraffin is generally thrown entirely out of the box4 ,to guard against this the box must be provided with a loosely fitting and temporary lid1 pierced with several holes4 ,the real test as to when exhaustion is complete would be the cessation of any yield of air or water4 ,since it is not generally convenient to make the vacuum box so air-tight that there are absolutely no leaks at all1 and as the paraffin itself is1 ,i think1 inclined to crack slightly at the temperature of #adj ,c1 this test or criterion cannot be conveniently applied4 ,two exhaustions1 each of about two hours' duration1 have1 however1 in my experience succeeded very well1 provided1 of course1 that the dielectric is prepared as suggested4 ,at the end of the exhaustion process the clamping screws are tightened as far as possible1 the condenser remaining in its bath until the paraffin is pasty4 ,condensers made in this way resist the application of alternating currents perfectly1 as the following tests will show4 ,the dielectric consisted of about equal parts of hard paraffin and vaseline4 ,a condenser of about #j4abc microfarads capacity and an insulation resistance of #bjjj megohms1 ,footnote3 ,as tested by a small voltage4 having a tin-foil area of #d4bc square metres 7about71 and double papers each about #j4b mm4 thick1 designed to run at #bjjj volts with a frequency of #fc complete periods1 was tested at this frequency4 ,the condenser was thoroughly packed all round in cotton-wool to a thickness of #f inches1 and its temperature was indicated more or less by a thermometer plunged through a hole in the lid of the containing box and of the condenser box1 and resting on the upper surface of one set of tin-foil electrodes1 from which the soft paraffin mixture had been purposely scraped away4 ,the following were the results of a four hours' run at a voltage #ej per cent higher than that for which the condenser was designed--i4e4 #cjjj volts4 ,times4 ,voltage ,temperature ,temperature ,difference in ,condenser4 in ,air4 ,hrs4 ,min4 #b #aj #bgej #bb4h ,c4 #bc4j ,c4 #j4b #c #aj #bgjj #bc4j ,c4 #bc4c ,c4 #j4c #c #ah #cbjj #bc4a ,c4 #bc4j ,c4 -#j4a #d #aj #cbjj #bc4c ,c4 #bc4g ,c4 #j4d #e #aj #cajj #bc4f ,c4 #bc4d ,c4 -#j4b #f #aj #cjjj #bc4h ,c4 #bc4ce ,c4 -#j4de ,an idea of the order of the amount of waste may be formed from the following additional experiment4 ,a condenser similar to the one described was filled with oil of a low insulating power4 ,it was tested calorimetrically1 and also by the three voltmeter method1 which1 however1 proved to be too insensitive4 ,the temperature rise in the non-conducting box in air was about #j4c ,c4 per hour1 and the loss of power was found to be less than #j4a per cent4 ,in the present case the actual rise was only #a in four hours1 and the integral give and take between the condenser and the air is practically nothing2 consequently we may consider with safety that the rate of rise is certainly less than #a degree per three hours4 ,the voltage and frequency were about the same in both experiments1 consequently the energy passed is about proportional to the capacity used in the two experiments4 ,from this it follows that since the specific heat of both condensers was the same 7nearly71 the loss in the present case is a good deal less than one-tenth per cent4 ,the residual charge is also much less than when the condenser is simply built up of paper paraffined in an unsystematic manner1 and from which the air and water have been imperfectly extracted1 as by baking the condenser first1 and then immersing it in paraffin or oil4 ,it is usual to consider that the phenomena of residual charge and heating in condensers1 to which alternating voltages are applied1 are closely allied4 ,this is true1 but the alliance is not one between cause and effect--at all events1 with regard to the greater part of the heating4 ,the imperfect exclusion of air and moisture1 particularly the latter1 certainly increases the residual charge by allowing surface creeping to occur2 but it also acts directly in producing heating1 both by lowering the insulation of the condenser and by allowing of air discharges between the condenser plates4 ,of these causes of heating1 the discharges in air or water vapour are probably the more important4 ,long ago a theory of residual charge was given by ,maxwell1 based on the consideration of a laminated dielectric1 the inductivity and resistance of which varied from layer to layer4 ,it was shown that such an arrangement1 and hence generally any want of homogeneity in a direction inclined to the lines of force leading to a change of value of the product of specific resistance and specific inductive capacity1 would account for residual charge4 ,this possible explanation has been generally accepted as the actual explanation1 and many cases of residual charge attributed to want of homogeneity1 which are certainly to be explained in a simpler manner4 ,for instance1 the residual charge in a silvered mica plate condenser1 carefully dried1 can be increased at least tenfold by an exposure of a few minutes to ordinarily damp air4 ,the same result occurs with condensers of paraffined or sulphured paper2 and these are the residual changes generally observed4 ,the greater part must be due to creeping4 #aab4 ,paraffin4 ,this substance has long enjoyed great popularity in the physical laboratory4 ,its specific resistance is given by ,ayrton and ,perry as more than #ajbe1 but it is probably much higher in selected samples4 ,the most serviceable kind of paraffin is the hardest obtainable1 melting at a temperature of not less than #eb ,c4 ,it is a good plan to remelt the commercial paraffin and keep it at a temperature of1 say1 #abj ,c4 for an hour1 stirring it carefully with a glass rod so that it does not get overheated2 this helps to get rid of traces of water vapour4 ,hard paraffin1 when melted1 has an enormous rate of expansion with temperature1 so great1 indeed1 that care must be taken not to overfill the vessels in which it is to be heated4 ,castings can only be prepared by cooling the mould slowly from the bottom1 keeping the rest of the mould warm1 and adding-paraffin from time to time to make up for the contraction4 ,the cooling is gradually allowed to spread up to the free surface4 ,the chief use of paraffin in the laboratory is in the construction of complicated connection boards1 which are easily made by drilling holes in a slab of paraffin1 half filling them with mercury1 and using them as mercury cups4 ,since paraffin is a great collector of dust1 it should be screened by paper1 otherwise the blocks require to be scraped at frequent intervals1 which1 of course1 electrifies them considerably4 ,this electrification is often difficult to remove without injuring the insulating power of the paraffin4 ,a light touch with a clean ,bunsen flame is the readiest method1 and does not appear to reduce the insulation so much as might be expected4 ,the safest way1 however1 is to leave the key covered by a clean cloth1 which1 however1 must not touch the surface1 for a sufficient time to allow of the charges getting away4 ,the paraffin often becomes electrified itself by the friction of the key contacts1 so that in electrometer work it is often convenient to form the cups by lining them with a little roll of copper foil twisted up at the bottom4 ,in this case the connecting wires should1 of course1 be copper4 ,small steel staples are convenient for fastening the collecting wires upon the paraffin2 or1 in the case where these wires have to be often removed and changed about1 drawing-pins are very handy4 ,with mercury cups simply bored in paraffin great trouble will often be experienced in electrometer work1 owing to a potential difference appearing between the cups--at all events when the contacts are inserted and however carefully this be done4 ,a few drops of very pure alcohol poured in above the mercury often cures this defect4 ,the surface of paraffin is by no means exempt from the defect of losing its insulating power when exposed to damp air1 but it is not so sensitive as glass1 nor does the insulating power fall so far4 ,two useful appliances are figured4 ,fig4 #hf4 ,fig4 #hg4 ,one1 in which paraffin appears as a cement1 is an insulating stand made out of a bit of glass or ebonite tube cemented into an ,erlenmeyer flask1 having its neck protected from dust when out of use by a rubber washer1 the other a petticoat insulator made by cementing a flint glass bottle into a glass dish with paraffin4 ,in course of time the paraffin will be found to have separated from the glass4 ,when this occurs the apparatus may be melted together again by placing it on the water bath for a few minutes4 #aac4 ,vaseline1 ,vaseline ,oil1 and ,kerosene ,oil4 ,these1 when dry1 insulate almost1 but not quite as well as solid paraffin4 ,h4 ,koeller 7,wien ,berichte1 #ih1 ii4 #bja1 #ahhi2 ,beibl4 ,wied4 ,ann4 #ahij1 p4 #ahf71 working with very small voltages1 places the final787 specific resistance of commercial petroleum1 ether1 and vaseline oil at about #b ,x #ajbg ,c4,g4,s4 ,this is ten times higher than the value assigned to commercial benzene 7,c#f;,h#f71 and a hundred times higher than the value for commercial toluene4 ,all these numbers mean little or nothing1 but the petroleum and vaseline oil were the best fluid insulators examined by ,koeller4 ,by mixing vaseline with paraffin a soft wax may be made of any desired degree of softness1 and by dissolving vaseline in kerosene an insulating liquid of any degree of viscidity may be obtained4 ,hard paraffin may be softened somewhat by the addition of kerosene1 and an apparently homogeneous mass cast from the mixture4 ,it will be found1 however1 that in course of time the kerosene oozes out1 unless present in very small quantity4 ,koeller has found 7loc4 cit47 that some samples of vaseline oil conducted vollstaendig gut1 but ,i have not come across such samples4 ,vaseline oil1 however1 is sold at a price much above its value for insulating purposes4 ,kerosene oil is best obtained dry by drawing it directly from a new tin and exposing it to air as little as possible4 ,of course1 it may be dried by chemical means and distillation1 but this is usually 7or always7 unnecessary4 ,fig #hh4 ,there is some danger of kerosene containing minute traces of sulphuric acid1 and it and other oils may be conveniently tested for insulation in the following manner4 ,the quartz electroscope is taken1 and the insulating rod heated in the blow-pipe4 ,the electroscope will now insulate well enough to show no appreciable collapse of the leaves in one or two hours' time4 ,upon the plate of the electroscope is put a platinum or copper cylinder1 and this is filled with kerosene 7say7 up to a fixed mark4 ,the electroscope is placed on a surface plate1 or1 at all events1 on a sheet of plate glass1 and a scribing block is placed along side it and the scriber adjusted to dip into the kerosene to any required depth4 ,this is done by twisting a bit of wire round the scribing point and allowing it to project downwards4 ,the point itself serves to give an idea of the height to which the vessel may be filled4 ,the liquid is adjusted till its surface is in contact with the end of the scribing point1 and the wire then projects into the liquid and forms an electrode of constant area of surface4 ,the scribing block is put to earth4 ,a charge is given to the electroscope1 and the time required for a given degree of collapse of the leaves noted4 ,the kerosene is then removed and its place taken by vaseline or paraffin1 known to insulate well as a standard for comparison4 ,the experiment is then repeated1 and the time noted for the same degree of collapse4 ,this test1 though of course rough1 is generally quite sufficient for workshop purposes1 and is easily applied4 ,moreover1 it does not require correction for electrometer leakage1 as generally happens when more elaborate appliances are used4 ,the actual resistance of insulating oils depends so much on the electrical intensity1 on the duration of that intensity1 and on the previous history of the oil as to the direction of the voltage to which it has been subjected--to say nothing of the effects of traces of moisture--that quantitative experiments are of no value unless they are extremely elaborate4 ,i shall therefore only append the following numbers due to ,bouty1 ,ann4 de ,chemie et de ,physique 7#f71 vol4 xxvii4 p4 #fb1 #ahib1 in which the effect of the conductivity on the determination of the specific inductive capacity was properly allowed for3- ,carbon ,bisulphide4 ,turpentine4 ,benzene 7,c#f;,h#f7 at #bj ,c4 ,benzene at #fj ,c4 ,specific inductive capacity #b4gae #b4cad #b4ba #b4bb ,specific resistance in ohms per cubic centimetre #a4e x #ajac1 #a4ge x #ajab #a4ef x #ajaa #g4i x #ajaa ,footnote3 ,professor ,j4 ,j4 ,thomson1 and ,newall 7,phil4 ,proc4 #ahhf7 consider that carbon bisulphide showed traces of a residual charge effect2 hence1 until this point is cleared up1 we must regard ,bouty's value as corresponding only to a very short1 but not indefinitely short1 period of charge4 ,on this point the paper must be consulted4 ,march #ahig--,the writer has investigated this point by an independent method1 but found no traces of residual charge4 ,information as to the specific inductive capacity of a large number of oils may be found in a paper by ,hopkinson1 ,phil4 ,proc4 #ahhg1 and in a paper by ,quincke in ,wiedemann's ,annalen1 #ahhc4 #aad4 ,imperfect ,conductors4 ,under this heading may be grouped such things as wood1 slate1 marble1 etc--in fact1 materials generally used for switchboard insulation4 ,an examination of the insulating power of these substances has recently been made by ,b4 ,o4 ,peirce 7,electrical ,review1 #aath ,january #ahie7 with quite sufficient accuracy1 having in view the impossibility of being certain beforehand as to the character of any particular sample4 ,the tests were made by means of holes drilled in slabs of the material to be examined4 ,these holes were three-eighths of an inch in diameter1 and from five-eighths to three-quarters of an inch deep1 and one inch apart1 centre to centre4 ,a voltage of about #ae volts was employed4 ,the following general results were arrived at3- 7#a7 ,heating in a paraffin bath always increases the resistance of wood1 though only slightly if the wood be hard and dense4 7#b7 ,frequent exhaustion and readmission of air above the surface of the paraffin always has a good effect in increasing the resistance of wood4 7#c7 ,when wood is once dry1 impregnating it with paraffin tends to keep it dry4 7#d7 ,red vulcanised fibre1 like wood1 absorbs paraffin1 but it cannot be entirely waterproofed in this way4 7#e7 ,the resistance of wood with stream lines along the grain is twenty to fifty per cent lower than when the stream lines cross the grain4 7#f7 ,the contact resistance between slabs of wood pressed together is always very high4 ,the following table will sufficiently illustrate the results obtained4 ,the stone was dried in the sun for three weeks in the summer 7,united ,states71 and the wood is described as having been well seasoned3- ,,current ,,with ,,the ,,grain ,lowest ,resistance ,highest ,resistance ,lowest ,specific ,highest ,specific between two ,cups between two ,cups ,resistance in ,resistance in in ,megohms4 in ,megohms4 ,megohms4 ,megohms4 ,ash4 #eej #ibj #chj #gjj ,cherry #aajj #djjj #bhjj #fjjj ,mahogany #dcj #gcj #caj #faj ,oak #bbj #dbj #ajej #bbjj ,pine4 #ccj #fcj #cfj #adgj ,hard pine4 #aj #dh #ag #ajej ,black walnut #aajj #cjjj #cbj #bajj ,red fibre #b #d #c #fj ,slate #ahd #bhj ,soapstone4 #ccj #ejj ,white marble #bjjj #hhjj #aae4 ,as to working the materials very little need be said4 ,fibre is worked like wood1 but has the disadvantage of rapidly taking the edge off the tools4 ,in turning it1 therefore1 brass-turning tools1 though leaving not quite such a perfect finish as wood-turning tools1 last much longer1 and really do well enough4 ,fibre will not bear heating much above #ajj;,c--at all events in paraffin4 ,at #adj ,c4 it becomes perfectly brittle4 ,its chief merit lies in its great strength4 ,so far as insulation is concerned1 ,mr4 ,peirce's experiments show that it is far below most kinds of wood4 ,slate4 ,this is a vastly more useful substance than it is generally credited with being4 ,it is very easily worked at a slow speed1 either on the shaping machine or on the lathe1 with tools adjusted for cutting brass1 and it keeps its figure1 which is more than can be said for most materials4 ,it forms a splendid base for instruments1 especially when ground with a little emery by iron or glass grinders1 fined with its own dust1 and ,french polished in the ordinary way4 ,spools for coils of considerable radial dimension may be most conveniently made of slate instead of wood or brass1 both because it keeps its shape1 and because it insulates sufficiently well to stop eddy currents--at all events1 sufficiently for ordinary practice4 ,an appreciable advantage is that slate may be purchased at a reasonable rate in large slabs of any desired thickness4 ,it is generally cut in the laboratory by means of an old cross-cut saw1 but it does not do much damage to a hard hack saw such as is used for iron or brass4 ,marble4 ,according to ,holtzapffell1 marble may be easily turned by means of simple pointed tools of good steel tempered to a straw colour4 ,the cutting point is ground on both edges like a wood-turning tool1 and held up to the work at an angle of twenty or thirty degrees 78with the horizontal74 ,the marble is cut wet to save the tool4 ,as soon as the point gets1 by grinding1 to be about one-eighth of an inch broad it must either be drawn down or reground2 a flat tool will not turn marble at all4 ,a convenient saw for marble is easily made on the principle of the frame saw4 ,a bit of hoop iron forms a convenient blade1 and is sharpened by being hammered into notches along one edge1 using the sharp end of a hammer head4 ,the saw is liberally supplied with sand and water--or emery and water1 where economy of time is an object4 ,the sawing of marble is thus really a grinding process1 but it goes on rapidly4 ,marble is ground very easily with sand and water2 it is fined with emery and polished with putty powder1 which1 ,i understand1 is best used with water on cloth or felt4 ,as grinding processes have already been fully described1 there is no need to go into them here4 ,i have no personal knowledge of polishing marble4 #aaf4 ,conductors4 ,the properties of conductors1 more particularly of metals1 have been so frequently examined1 that the literature of the subject is appallingly heavy4 ,in what follows ,i have endeavoured to keep clear of what might properly appear in a treatise on electricity on the one hand1 and in a wiring table on the other4 ,the most important work on the subject of the experimental resistance properties of metals has been done by ,matthieson1 ,phil4 ,trans4 #ahfj and #ahfb1 and ,british ,association ,reports 7#ahfd72 ,callender1 ,phil4 ,trans4 vol4 clxxiii2 ,callender and ,griffiths1 ,phil4 ,trans4 vol4 clxxxii2 ,the ,committee of the ,british ,association on ,electrical ,standards from #ahfb to ,present ,time2 ,dewar and ,fleming1 ,phil4 ,mag4 vol4 xxxvi4 7#ahic72 ,klemencic1 ,wiener ,sitzungsberichte 7,denkschrift71 #ahhh1 vol4 xcvii4 p4 #hch2 ,feussner and ,st4 ,lindeck1 ,zeitsch4 fuer ,inst4 ',kunde1 ix4 #ahhi1 p4 #bcc1 and ,b4 ,a4 ,reports1 #ahib1 p4 #aci4 ,of these1 ,matthieson1 and ,dewar and ,fleming treat of resistance generally1 the latter particularly at low temperatures4 ,footnote3 ,the following is a list of ,dr4 ,matthieson's chief papers on the subject of the electrical resistance of metals and alloys3 ,phil4 ,mag4 xvi4 #aheh1 pp4 #bai-bbc2 ,phil4 ,trans4 #aheh1 pp4 #chc-chh ,phil4 ,trans4 #ahfj1 pp4 #afa-agf2 ,phil4 ,trans4 #ahfb1 pp4 #a-bg ,phil4 ,mag4 xxi4 7#ahfa71 pp4 #ajg-aae2 ,phil4 ,mag4 xxiii4 7#ahfb71 pp4 #aga-agi2 ,electrician1 iv4 #ahfc1 pp4 #bhe-bif2 ,british ,association ,reports1 #ahfc1 p4 #cea4 ,matthieson1 and ,matthieson and ,hockin1 ,klemencic1 ,feussner1 and ,st4 ,lindeck deal with the choice of metals for resistance standards4 ,callender's1 and ,callender and ,griffiths' work is devoted to the study of platinum for thermometric purposes4 ,the bibliography referring to special points will be given later4 ,the simplest way of exhibiting the relative resistances of metals is by means of a diagram published by ,dewar and ,fleming 7loc4 cit471 which is reproduced on a suitable scale on the opposite page4 ,for very accurate work1 in which corrections for the volumes occupied by the metals at different temperatures are of importance1 the reader is referred to the discussion in the original paper1 which will be found most pleasant reading4 ,from this diagram both the specific resistance and the temperature coefficient may be deduced with sufficient accuracy for workshop purposes4 ,in interpreting the diagram the following notes will be of assistance4 ,the diagram is drawn to a scale of so-called platinum temperatures--that is to say1 let ,r#j1 ,r#ajj1 ,rt be the resistances of pure platinum at #j1 #ajj1 and t ,c4 respectively1 then the platinum temperature pt is defined as pt #ajj ,x 7,rt-,r#j7_/7,r#ajj-,r#j7 ,this amounts to making the temperature scale such that the temperature at any point is proportional to the resistance of platinum at that point4 ,consequently on a resistance temperature diagram the straight line showing the relation between platinum resistance and platinum temperature will run out at the platinum absolute zero1 which coincides more or less with the thermodynamic absolute zero1 and also with the perfect gas absolute zero4 ,platinum temperatures may be taken for workshop purposes over ordinary ranges as almost coinciding with air thermometer temperatures4 ,the metals used by ,professors ,dewar and ,fleming were1 with some exceptions1 not absolutely pure1 but in general represent the best that can be got by the most refined process of practical metallurgy4 ,we may note further that the specific resistance is only correct for a temperature of about #ae ,c1 since no correction for the expansion or contraction of material has been applied4 ,the following notes on alloys suitable for resistance coils will probably be found sufficient4 #aag4 ,platinoid4 ,this substance1 discovered by ,martino and described by ,bottomley 7,phil4 ,proc4 ,roy4 ,soc4 #ahhe71 is an alloy of nickel1 zinc1 copper1 and #a per cent to #b per cent of tungsten1 but ,i have not been able to obtain an analysis of its exact composition4 ,it appears to be difficult to get the tungsten to alloy1 and it has to be added to part of the copper as phosphide of tungsten1 in considerably greater quantity than is finally required4 ,the nickel is added to part of the copper and the phosphide of tungsten1 then the zinc1 and then the rest of the copper4 ,the alloy requires to be remelted several times1 and a good deal of tungsten is lost by oxidation4 ,the alloy is of a fine white colour1 and is very little affected by air--in fact1 it is to some extent untarnishable4 ,the specific resistance will be seen to be about one and a half times greater than that of ,german silver1 and the temperature coefficient is about #j4jba per cent per degree ,c4 7i4e4 about nineteen times less than copper1 and half that of ,german silver74 ,to all intents and purposes it may be regarded as ,german silver with #a per cent to #b per cent of tungsten4 ,it does not appear to have been particularly examined for secular changes of resistance4 #aah4 ,german ,silver4 ,this material has been exhaustively examined of late years by ,klemencic and by ,feussner and ,st4 ,lindeck4 ,everybody agrees that ,german silver1 as ordinarily used for resistances1 and composed of copper four parts1 zinc two parts1 nickel one part1 is very ill-fitted for the purpose of making resistance standards4 ,this is due 7#a7 to its experiencing a considerable increase in resistance on winding4 ,feussner and ,st4 ,lindeck found an increase of #a per cent when ,german silver was wound on a core of ten wire diameters4 7#b7 ,to the fact that the change goes on1 though with gradually decreasing rate1 for months or years2 7#c7 to the fact that the resistance is permanently changed 7increased7 by heating to #dj ,c4 or over4 ,by artificially ageing coils of ,german silver by heating to #aej ,c1 say for five or six hours1 its permanency is greatly improved1 and it becomes fit for ordinary resistance coils where changes of1 say1 #a_/ejjj do not matter4 ,it is a remarkable property of all nickel alloys containing zinc that their specific resistance is permanently increased by heating1 whereas alloys which do not contain zinc suffer a change in the opposite direction4 ,the manufacturers of ,german silver appear to take very little care as to the uniformity of the product put on the market2 some so-called ,german silver is distinctly yellow1 while other samples are bright and white4 ,it is noted by ,price 7,measurements of ,electrical ,resistance1 p4 #bd7 that ,german silver wire is apt to exhibit great differences of resistance within quite short lengths4 ,this has been my own experience as well1 and is a great drawback to the use of ,german silver in the laboratory1 for it makes it useless to measure off definite lengths of wire with a view to obtaining an approximate resistance4 ,in ,england ,german silver coils are generally soaked in melted hard paraffin4 ,in ,germany1 at all events at the ,charlottenburg ,institute1 according to ,st4 ,lindeck--coils are shellac-varnished and baked4 ,in any case it appears to be essential to thoroughly protect the metal against atmospheric influence4 #aai4 ,platinum ,silver4 ,in the opinion of ,matthieson and of ,klemencic the #aj per cent silver1 #ij per cent platinum alloy is the one most suitable for resistance standards4 ,at all events1 it has stood the test of time1 for1 with the following exceptions1 all the ,british ,association coils constructed of it from #ahfg to the present day have continued to agree well together4 ,the exceptions were three one-ohm coils1 which permanently increased between #ahhh and #ahij1 probably through some straining when immersed in ice4 ,one coil changed by #j4jjjf in #a between the years #ahfg and #ahia4 ,according to ,klemencic1 absolute permanency is not to be expected even from this alloy4 ,its recommendation as a standard depends on its chemical inertness1 its small temperature coefficient 7#j4jjjbg per degree71 and its small thermo-voltage against copper1 as the following table 7taken from ,klemencic7 will show3- ,thermo-voltages in ,micro-volts per degree against ,copper over the ,range #j to #ag ,c4 ,platinum iridium #g4ad micro-volts per degree ,c4 ,platinum silver #f4fb micro-volts per degree ,c4 ,nickelin #bh4e micro-volts per degree ,c4 ,german silver #aj4dc micro-volts per degree ,c4 ,manganin 7,st4 ,lindeck7 #a4e micro-volts per degree ,c4 ,mechanically1 the platinum silver is weak1 and is greatly affected as to its resistance by mechanical strains--in fact1 ,klemencic considers it the worst substance he examined from this point of view--a conclusion rather borne out by ,mr4 ,glazebrook's experience with the ,british ,association standards already referred to 7,b4 ,a4 ,reports1 #ahia and #ahib74 ,taking everything into account1 it will probably be well to construct standards either with oil insulation only1 or to bake the coils in shellac before testing1 instead of soaking in paraffin4 ,fig4 #hi illustrates a form of an oil immersed standard which is in use in my laboratory1 and through which a considerable current may be passed4 ,the oil is stirred by means of a screw propeller4 ,fig4 #hi4 ,fig4 #hi represents a standard resistance for making ,clerk cell comparisons by the silver voltameter method4 ,the framework on which the coils are wound consists of a base and top of slate4 ,the pillars are of flint glass tube surrounding brass bolts1 and cemented to the latter by raw shellac4 ,grooves are cut in the glass sleeves to hold the wires well apart4 ,these grooves were cut by means of a file working with kerosene lubrication4 ,a screw stirrer is provided1 and the whole apparatus is immersed in kerosene in the glass box of a storage cell4 ,the apparatus is aged to begin with by heating to a temperature a good deal higher than any temperature it is expected to reach in actual work4 ,after this the rigidity of the frame is intended to prevent any further straining of the wire4 ,the apparatus as figured is not intended to be cooled to #j ,c1 so that it is put in as large a box as possible to gain the advantage of having a large volume of liquid4 ,the top and bottom slates measure seven inches by seven inches1 and the distance between them is seven inches4 ,the inner coil is wound on first1 and the loop which constitutes the end of the winding is brought up to a suitable position for adjustment4 ,the insulation of the heavy copper connectors is by means of ebonite4 #abj4 ,platinum ,iridium4 ,platinum #ij per cent1 iridium #aj per cent4 ,this material was prepared in some quantity at the cost of the ,french ,government1 and distributed for test about #ahhf4 ,klemencic got some of it as representing ,austria1 and found it behaved very like the platinum silver alloy just discussed4 ,the temperature coefficient is1 however1 higher than for platinum silver 7#j4jjabf as against #j4jjjbg74 ,the mechanical properties of the alloy are1 however1 much better than those of the silver alloy2 and in view of the experience with ,b4 ,a4 standards above quoted1 it remains an open question whether1 on the whole1 it would not be the better material for standards1 in spite of its higher price4 ,improvements in absolute measurements of resistance1 however1 may render primary standards superfluous4 #aba4 ,manganin4 ,discovered by ,weston--at all events as to its application to resistance coils4 ,a glance at the diagram will exhibit its unique properties1 on account of which it has been adopted by the ,physikalisch ,technischen ,reichsanstalt for resistance standards4 ,the composition of the alloy is copper #hd per cent1 manganese #ab per cent1 nickel #d per cent1 and it is described as of a steel-gray colour4 ,unfortunately it is apt to oxidise in the air1 or rather the manganese it contains does so1 so that it wants a very perfect protection against the atmosphere4 ,like ,german silver1 manganin changes in resistance on winding1 and coils made of it require to be artificially aged by heating to #aej for five hours before final adjustment4 ,the annealing cannot be carried out in air1 owing to the tendency to oxidation4 ,the method adopted by ,st4 ,lindeck 7at all events up to #ahib7 is to treat the coil with thick alcoholic shellac varnish till the insulation is thoroughly saturated1 and then to bake the coil as described4 ,the baking not only anneals the wire1 but reduces the shellac to a hard and highly insulating mass4 ,whether stresses of sufficient magnitude to produce serious mechanical effects can be set up by unequal expansion of wire and shellac during heating and cooling is not yet known1 but so far as tested 7and it must be presumed that the ,reichsanstalt tests are thorough7 no difficulty seems to have been met with4 ,in course of time1 however1 probably the best shellac coating will crack1 and then adieu to the permanency of the coil6 ,this might1 of course1 be obviated by keeping the coil in kerosene1 which has no action on shellac1 but which decomposes somewhat itself4 ,the method of treatment above described suffices to render coils of manganin constant for at least a year 7in #ahib the tests had only been made for this time7 within a few thousands per cent4 ,manganin can be obtained in sheets1 and from this material standards of #aj-b1 #aj-c1 and #aj-d ohms are made by soldering strips between stout copper bars1 and these are adjusted by gradually increasing their resistance by boring small holes through them4 ,the solder employed is said to be silver4 ,mr4 ,griffiths 7,phil4 ,trans4 vol4 clxxxiv4 #ahic1 ,a1 p4 #cij7 has had some experience with manganin carrying comparatively heavy currents1 under which circumstances its resistance when immersed in water was found to rise in spite of the varnish which coated it4 ,other experiments in which the manganin wire was immersed in paraffin oil did not exhibit this effect1 though stronger currents were passed4 ,on the whole1 manganin appears to be the best material for coil boxes and secondary resistance standards4 ,whether it is fit to rank with the platinum alloys as regards permanency must be treated as an open question4 #abb4 ,other ,alloys4 ,the following tables1 taken from the work of ,feussner and ,st4 ,lindeck1 ,zeitschrift fuer ,instrumenten ,kunde1 #ahhi1 vol4 ix4 p4 #bcc1 together with the following notes1 will suffice4 #abc4 ,nickelin4 ,this is only ,german silver with a little less zinc1 a little more nickel1 and traces of cobalt and manganese4 ,it behaves like ,german silver1 but is an improvement on the latter in that all the faults of ,german silver appear upon a reduced scale in nickelin4 #abd4 ,patent ,nickel4 ,practically a copper nickel alloy1 used to some extent by ,siemens and ,halske4 ,it stands pretty well in the same relation to nickelin as the latter does to ,german silver4 ,after annealing as for manganin it can be made into serviceable standards which do not change more than a few thousandths per cent4 ,i have not come across a statement of its thermo-voltage against copper4 #abe4 ,constantin4 ,another nickel copper alloy containing #ej per cent of each constituent4 ,it appears to be a serviceable substance1 having a temperature coefficient of #j4jjc per cent per degree only1 but an exceedingly high thermo-voltage1 viz4 #dj micro-volts per degree against copper4 #a #b #c #d #e #f #g #h ,german ,nickelin made ,rheo- ,patent ,nickel ,manga- ,nickel ,silver by ,obermaier tane nese ,manga- ,dia- ,dia- ,dia- ,dia- ,copper nese meter meter meter meter ,copper #a4jmm #j4amm #j4fmm #a4jmm ,copper #fj4af #fa4fc #ed4eg #ec4bh #gd4da #gd4ga #gj #gc ,zinc #be4cg #ai4fg #bj4dd #af4hi #j4bc #j4eb 444 444 ,tin 444 444 444 444 trace 444 444 ,nickel #ad4jc #ah4df #bd4dh #be4ca #be4aj #bd4ad 444 #c ,iron #j4cj #j4bd #j4fd #d4df #j4db #j4gj 444 444 ,cobalt trace #j4ai 444 444 trace trace 444 444 ,mang- trace #j4ah #j4bg #j4cg #j4ac #j4ag #cj #bd anese4 #ii4hf #ajj4cg #ajj4dj #ajj4ca #ajj4bd #ajj4bd 444 444 ,specific resistance #cj4j #cc4b #dd4h #eb4e #cd4b #cb4h #ajj4f #dg4g ,temperature coefficient #j4jjjcf #j4jjjcj #j4jjjcc #j4jjjda #j4jjjai #j4jjjba #j4jjjjd #j4jjjjc ,the specific resistance is in microhms1 i4e4 #aj-f ohms per cubic centimetre1 and the temperature coefficient in degrees centigrade4 #abf4 ,nickel ,manganese ,copper4 ,i can find no other reference with regard to this alloy mentioned by ,lindeck4 ,nicholls1 however 7,silliman's ,journal #c1 #ci1 #aga1 #ahij71 gives some particulars of alloys of copper and ferromanganese4 ,the following table is taken from ,wiedemann's ,beiblatter 7abstract of ,nicholl's paper1 #ahij1 p4 #haa74 ,all these alloys appear to require annealing at a red heat before their resistances are anything like constant4 ,let x be percentage of copper1 then #ajj--x is percentage of ferromanganese4 ,values of x4 #ajj #ii4bf #ia 4#hh #hf4ih #hj4d #gj4fe ,specific resistance with respect to copper 78 pure7 #a #a4ai #aa4bh #bj4d #bg4e #de4a ,temperature coefficient per degree x #ajf7;hard7 #cbjb #bafg #ach #af #bb -#bd ,ditto 7soft7 444 444 #ahd #hj #ff #ba ,if nickel is added1 alloys of much the same character are obtained1 some with negative temperature coefficients--for instance1 one containing #eb4ea per cent copper1 #ca4bg per cent ferromanganese1 and #af4bb nickel4 ,a detailed account of several alloys will be found in a paper by ,griffiths 7,phil4 ,trans4 #ahid1 p4 #cij71 but as the constants were determined to a higher order of accuracy than the composition of the material--or1 at all events1 to a higher degree of accuracy than that to which the materials can be reproduced--there is no advantage in quoting them here4 ,,chapter ,,iv ,,electroplating ,,and ,,allied ,,arts #abg4 ,electroplating4 ,this is an art which is usually deemed worthy of a treatise to itself1 but for ordinary laboratory purposes it is a very simple matter--so simple1 indeed1 that the multiplicity of receipts as given in treatises are rather a source of embarrassment than otherwise4 ,the fundamental principles of the art are3- 7#a7 ,dirty work cannot be electroplated4 7#b7 ,electroplated surfaces may be rougher1 but will not be smoother than the original unplated surface4 7#c7 ,the art of electroplating being in advance of the science1 it is necessary to be careful as to carrying out instructions in detail4 ,this particularly applies to the conditions which determine whether a metallic deposit shall come down in a reguline or in a crystalline manner4 #abh4 ,the ,dipping ,bath4 ,an acid dipping bath is one of the most useful adjuncts to the laboratory1 not only for cleansing metals for electroplating1 but for cleaning up apparatus made out of bits of brass tube and sheet1 and particularly for quickly cleaning binding screws1 etc1 where it is necessary to ensure good electrical contact4 ,the cheapest and most satisfactory way in the end is to make up two or three rather large baths to begin with4 ,the glass boxes of storage batteries do very nicely for the purpose1 and being generally ground pretty flat at the top1 they may be covered by sheets of patent plate glass1 and thus preserved from the action of the air4 ,first ,bath4 ,a #cj or #dj per cent solution of commercial caustic soda4 ,objects may be cleansed from grease in this bath by heating them as hot as is consistent with individual circumstances1 and plunging them into it4 ,it is a considerable advantage to begin by removing grease from articles subsequently to be dipped in an acid bath1 both because it saves time and acid1 and because more uniform results are obtainable when this is done than when it is omitted4 ,it is a great advantage to have the caustic soda solution hot4 ,this is always done in factories where nickel-plating is carried on1 but it is inconvenient in the laboratory4 ,the articles after dipping in the alkali are swilled with water1 and may even be scrubbed with a brush1 so as to remove greasy matters that have been softened but not entirely removed4 ,acid ,bath4 ,a convenient bath for laboratory purposes is made by mixing two volumes of strong commercial nitric acid with one of strong sulphuric acid in a cell measuring1 say1 #ab ,x #aj ,x #ae inches4 ,copper or brass articles are dipped in this bath for a few seconds1 then rinsed with water1 then dipped again for a second or two1 or until they appear equally white all over1 and then withdrawn as rapidly as possible and plunged into a large quantity of clean water4 ,care must be taken to transfer the articles from the bath to the water as quickly as possible1 for if time be allowed for gas to be evolved1 the surfaces become mat instead of bright4 ,in order to save acid it is advisable to make up a third bath1 using those odds and ends of acids which gradually accumulate in the laboratory4 ,sulphuric acid from the balance cases1 for instance1 mixed with its own volume of commercial nitric acid1 does very well4 ,the objects to be dipped receive a preliminary cleansing by a dip in this bath1 the strong bath being reserved for the final dip4 ,sheet brass and drawn tube1 as it comes from the makers1 possesses a really fine surface1 though this is generally obscured by grease and oxide4 ,work executed in these materials1 cleaned in alkali1 and dipped in really strong acid1 will be found to present a much better appearance than work which has been filed1 unless the latter be afterwards elaborately polished4 ,on no account must paraffin be allowed to get into any of the baths4 ,when the final bath gets weak it must be relegated to a subordinate position and a new bath set up4 ,a weak acid bath leaves an ugly mottled surface on brass work4 #abi4 ,a metallic surface which it is intended to electroplate must1 as has been mentioned1 be scrupulously clean4 ,if the metal is not too valuable or delicate1 cleaning by dipping is easy and effectual4 ,the following notes will be found to apply to special cases which often occur4 7#a7 ,silver ,surfaces intended to be gilt4 ,these are first washed clean with soap and hot water1 and polished with whitening4 ,they are then dipped for a moment in a boiling solution of potassium cyanide4 ,a #bj per cent solution of common commercial cyanide does well1 but the exact strength is quite immaterial4 ,the cyanide is washed away in a large volume of soft water1 and the articles are kept under water till they are scratch-brushed4 ,mat surfaces are readily produced on standard silver by dipping in hot strong sulphuric acid4 ,the appearance of new silver coins1 which is familiar to everybody1 is obtained by this process4 7#b7 ,finely turned and finished ,brass ,work4 ,if it is intended to nickel-plate such work1 and if it is desirable to obtain brightly polished nickel surfaces1 the work must be perfectly polished to begin with4 ,full details as to polishing may be found in workshop books or treatises on watch-making4 ,it will suffice here to say that the brass work is first smoothed by the application of successive grades of emery and oil1 or by very fine dead smooth files covered with chalk4 ,polishing is carried out by means of rotten stone and oil applied on leather4 ,in polishing turned work care must be taken to move the file1 emery1 or rotten stone to and fro over the work with great regularity1 or the surface will end by looking scratchy and irregular4 ,the first process of cleaning is1 of course1 to remove grease1 and this is accomplished best by dipping in a bath of strong hot caustic soda solution1 and less perfectly by heating the work and dipping it in the cold caustic soda bath4 ,during this process a certain amount of chemical action often occurs leading to the brass surface exhibiting some discoloration4 ,the best way of remedying this is to dip the brass into a hot bath of cyanide of potassium solution4 ,if it is inconvenient to employ hot baths or to heat the brass work1 good results may be obtained by rubbing the articles over with a large rough cork plentifully lubricated with a strong solution of an alkali4 ,if the surfaces are very soiled or dirty1 a paste of alkali and fine slaked lime may be applied on a cork rubber1 and this in my experience has always been most effective and satisfactory in every way1 except that it is difficult to get into crevices4 ,if the alkali stains the work1 a little cyanide of potassium may be rubbed over the surface in a similar manner4 ,brass work treated by either of these methods is to be washed in clean water till the alkali is entirely removed1 and may then be nickel-plated without any preliminary scratch-brushing4 ,the treatment in hot baths of alkali and cyanide is the method generally employed in ,american factories as a preliminary to the nickelling of small brass work for sewing machines1 etc4 7#c7 ,copper either for use as the kathode in electrolysis calibration experiments or otherwise is most conveniently prepared by dipping in the acid bath1 rinsing quickly in cold water1 scratch-brushing under cold water1 and transferring at once to the plating bath4 ,in the case where the copper plates require to be weighed they are dipped into very hot distilled water after scratch-brushing1 and then dried at once by means of a clean glass cloth4 7#d7 ,aluminium 7which1 however1 does not readily lend itself to plating operations ,footnote3 ,this difficulty has now been overcome4 ,see note1 section #ach4 7 is best treated by alkali rubbed on with a cork1 or by a hot alkaline carbonate where rubbing is inexpedient4 ,the clean aluminium is scratch-brushed under water1 and at once transferred to the plating bath4 7#e7 ,iron for ,nickel-plating4 ,according to ,dr4 ,gore 7,electra-metallurgy1 p4 #cai7 the best bath for cleaning iron is made as follows3 ,one gallon of water and one pound of sulphuric acid are mixed with one or two ounces of zinc 7which of course dissolves72 to this is added half a pound of nitric acid4 ,the writer has been accustomed to clean iron by mechanical means1 to deprive it of grease by caustic alkali1 and to finish it off by1 means of a hard scratch brush4 ,this process has always worked satisfactorily4 7#f7 ,articles soldered with soft solder containing lead and tin do not readily lend themselves to electrolytic processes1 the solder generally becoming black and refusing to be coated with the electro-deposit4 ,moreover1 if soldered articles are boiled for any length of time in caustic alkali during the preliminary cleansing1 enough tin will dissolve to form a solution of stannate of potash or soda--strong enough to deposit tin on brass or copper4 ,a method of coppering soldered articles will be described later on4 #acj4 ,scratch-brushing4 ,this process is generally indispensable1 and to its omission is to be traced most laboratory failures in electroplating4 ,scratch-brushes may be bought at those interesting shops where watchmakers' supplies are sold4 ,it will be well1 therefore1 to purchase a selection of scratch brushes1 for they are made to suit particular kinds of work4 ,they are all made of brass wire1 and vary both in hardness and in the fineness of the wire4 ,the simplest kind of scratch brush consists merely of a bundle of wires bound up tightly by another wire1 and somewhat frizzed out at the ends 7,fig4 #ij74 ,a more useful kind is made just like a rotating brush1 and has to be mounted on a lathe 7,fig4 #ia74 ,fig4 #ij4 ,fig4 #ia4 ,the scratch brush is generally1 if not always1 applied wet2 the lubricant generally recommended is stale beer1 but this may be replaced by water containing a small quantity of glue1 or any other form of gelatine in solution--a mere trace 7say 4#a per cent7 is quite sufficient4 ,very fair results may be got by using either pure or soapy water4 ,the rotating brushes require to be mounted on a lathe1 and may be run at the same speed as would be employed for turning wooden objects of the same dimensions4 ,since the brush has to be kept wet by allowing water or its equivalent to drip upon it1 it is usual to make a tin trough over which the brush can revolve1 and to further protect this by a tin hood to keep the liquid from being thrown all over the room4 ,in many works the brush is arranged to lie partly in the liquid1 and this does very well if the hood is effective4 ,there is a superstition that electro-deposits stick better to scratch-brushed surfaces than to surfaces which have not been so treated1 and consequently it is usual to scratch-brush surfaces before electro-deposit4 ,however this may be1 there is no doubt that adherence and solidity are promoted by frequent scratch-brushing during the process of depositing metal1 especially when the latter tends to come down in a spongy manner4 ,gilt surfaces--if the gilding is at all heavy--are generally dull yellow1 or even brown1 when they come from the bath1 and require the scratch brush to cause the gold to brighten1 an office which it performs in a quite striking manner4 ,the same remark applies to silvered surfaces1 which generally leave the bath a dead white--at all events if the deposit is thick1 and if ordinary solutions are employed4 ,in either case the touch of the scratch brush is magical4 #aca4 ,burnishing4 ,burnishers of steel1 agate1 or bloodstone can be bought at the shops where scratch brushes are sold1 and are used to produce the same brightening effect as can be got by scratch-brushing4 ,the same solutions are employed1 but rather stronger1 and the burnisher is swept over the surface so as to compress the deposited metal4 ,burnishing is rather an art1 but when well done gives a harder and more brilliant 7because smoother7 surface than the scratch brush4 ,on the whole1 steel burnishers are the most convenient if in constant use4 ,if the burnishing tools have to lie about1 steel is apt to rust1 unless carefully protected by being plunged in quicklime or thickly smeared with vaseline1 and the least speck of rust is fatal to a burnisher4 ,in any case the steel requires to be occasionally repolished by rouge and water on a bit of cloth or felt4 ,the process of burnishing is necessarily somewhat slow and tedious1 and as a rule is not worth troubling about except in cases where great permanence is required4 ,the burnisher is moved over the work somewhat like a pencil with considerable pressure1 and care is taken to make the strokes as uniform in direction as possible2 otherwise the surface looks non-uniform1 and has to be further polished by tripoli1 whitening1 etc1 before it is presentable4 #acb4 ,silver-plating4 ,the most convenient solution for general purposes is an #h to #aj per cent solution of the double cyanide of silver and potassium together with #a or #b per cent of free potassium cyanide4 ,great latitude is permissible in the strength of solution and density of current4 ,as commercial cyanide of potassium generally contains an unknown percentage of other salts1 which1 however1 do not interfere with its value for the purpose of silver-plating1 the simplest procedure is as follows4 ,for every #ajj c4c4 of plating solution about #g grms4 of dry crystallised silver nitrate are required4 ,the equivalent amount of potassium cyanide 7if dry and pure7 is #e4b grms1 but commercial cyanide may contain from #ej per cent upwards to #if per cent in the best fused cyanide made from ferrocyanide only4 ,an approximate idea of the cyanide content can be obtained from the dealers when the salt is purchased1 and this is all that is required4 ,a quantity slightly in excess of the computed amount of cyanide is dissolved in distilled water1 and this is cautiously added to the solution of the silver nitrate till precipitation is just complete4 ,the supernatant liquors are then drained away1 and the precipitate dissolved by adding a sufficiency of the remaining cyanide2 this process is assisted by warming and stirring4 ,an allowance of about one-tenth of the whole cyanide employed may be added to form free cyanide1 and the solution made up to the strength named4 ,it is advisable to begin with the cyanide in a moderately strong solution1 for the sake of ease in dissolving the precipitate4 ,this solution will deposit silver upon articles of copper or brass immersed in it even without the battery1 but the coat will be thin4 ,the solution is used cold1 with a current density of about #aj to #bj ampres per square foot4 ,the articles to be silvered are scratch-brushed1 washed1 and electroplated1 till they begin to look undesirably rough4 ,they are then taken out of the bath1 rebrushed1 and the process continued till a sufficiency of silver is deposited4 ,four grammes weight of silver 7nearly7 is deposited per ampre hour4 ,it is best to use a fine silver anode1 so that the solution1 does not get contaminated by copper4 ,in most factories it is usual to quicken the objects to be silvered before placing them in the electrolysis vats1 because the deposit is said to adhere better in consequence of this treatment4 ,i have never found it any improvement for laboratory purposes1 but it is easy to do4 ,a dilute 7say #b per cent7 solution of cyanide of mercury is required containing a little free cyanide4 ,the objects to be quickened are scratch-brushed and dipped into the cyanide of mercury solution till they are uniformly white2 it is generally agreed that the less the mercury deposited the better1 so long as a perfect coating is obtained4 ,the objects are rinsed after quickening1 and put in the depositing bath at once4 ,the mat surface of silver obtained by electrolysis of the cyanide is very beautiful--one of the most beautiful things in nature--shining with incomparable crystalline whiteness4 ,so delicate is it1 however1 for so great is the surface it exposes1 that it is generally rapidly deteriorated by exposure to the air4 ,it may be protected to some extent by lacquering with pale lacquer1 but it loses some of its brilliancy and purity in the process4 ,the deposit is generally scratch-brushed or burnished down to a regular reflecting surface4 #acc4 ,cold ,silvering4 ,a thin but brilliant coat of silver may be readily applied to small articles of brass or copper in the following way4 ,a saturated solution of sodium sulphite 7neutral7 is prepared1 and into this a #aj per cent solution of nitrate of silver is poured so long as the precipitate formed is redissolved4 ,a good deal of silver may be got into solution in this way4 ,articles to be silvered need only to be cleaned1 brushed1 and dipped in this solution till a coat of the required thickness is obtained4 ,i must admit1 however1 that the coating thus laid on does not appear to be so permanent as one deposited by simple immersion from the cyanide solution1 even though it is thicker4 ,the cyanide plating solution will itself give a good coat of silver if it is used boiling1 and if a little potassium cyanide be added4 ,for purposes of instrument construction1 however1 a thin coat of silver is seldom to be recommended1 on account of its liability to tarnish and its rapid destruction when any attempt is made to repolish it4 ,for these reasons1 nickel or gold plating is much to be preferred4 #acd4 ,gilding4 ,this art deserves to be much more widely practised than is usual in laboratories4 ,regarded as a means of preserving brass1 copper1 or steel1 it is not appreciably more time robbing than lacquering1 and gives infinitely better results4 ,moreover1 it is not much more expensive4 ,strange as it may seem1 the costliness of gilding seldom lies in the value of the gold deposited2 the chief cost is in the chemicals employed to clean the work1 and in interest on the not inconsiderable outlay on the solution and anode4 ,the easiest metal to gild is silver1 and it is not unusual to give base metals a thin coating of silver or copper1 or both1 one after the other1 before gilding1 in order to secure uniformity4 ,to illustrate the virtue of a thin layer of gold1 ,i will mention the following experiment4 ,about three years ago ,i learned for the first time that to clean the silver used in a small household required at least an hour's labour per diem4 ,i further ascertained that most of this time is spent on the polishing part of the process4 ,as this seemed a waste of labour1 ,i decided to try the effect of gilding4 ,in order to give the proposal a fair trial ,i gilt the following articles3 half a dozen table spoons and forks1 a dozen dessert forks and spoons1 and a dozen tea spoons4 ,these were all common electroplated ware4 ,they were weighed before and after gilding1 and it was with difficulty that the increase of weight was detected1 even though a fine bullion balance was employed4 ,on calculating back to money1 it appeared that the value of the gold deposited was about threepence4 ,assuming that an equal weight of silver had been accidentally dissolved by the free cyanide during the plating--which is unlikely--the total amount of gold deposited would be worth1 say1 sixpence4 ,after three years' continuous use the gilding is still perfect1 except at the points on which the spoons and forks rest1 where it is certainly rather shabby4 ,meanwhile the gold plate only requires to be washed with hot water and soap to keep it in perfect order1 a much more cleanly and expeditious process than that of silver cleaning4 #ace4 ,preparing ,surfaces for ,gilding4 ,ordinary brass work--rough or smooth--may for purposes of preservation be dipped1 scratch-brushed1 and gilt at once4 ,seven years ago the writer gilt the inside of the head of a copper water still1 and simply scratch-brushed it2 it is to-day in as good order as when it was first done4 ,if it is intended to gild work from the first1 with the view of making an exceptionally fine job of it1 gilding metal1 i4e4 brass containing one to one and a quarter ounces of zinc to the pound of copper may be specified4 ,from its costliness1 however1 this is only desirable for small work4 ,iron and steel are generally given a preliminary coating of copper1 but this may be dispensed with though with no advantage--by using a particular process of gilding4 ,base metals1 zinc1 pewter1 lead1 etc1 are first coppered in a cyanide of copper solution1 as will be described under the head of ,copper-plating4 ,if it is intended to gild soldered articles1 the preliminary coating of copper is essential4 ,the most convenient vessel for holding a gilding solution is undoubtedly one formed of enamelled iron4 ,particularly useful are the buckets and billies 7i4e4 cylindrical cans7 made of this material4 ,these vessels may be heated without any fear of a smash1 and do not appear to be appreciably affected by gilding solutions--at all events during several days or weeks4 ,the avoidance of all risk of breakage when twenty or thirty pounds' worth of solution is in question is a matter of importance4 ,under no circumstances is it desirable to use anything but the purest gold and best fused cyanide 7called gold cyanide7 in the preparation of the solutions4 ,the appearance of a pure gold deposit is far richer than of one containing silver1 and its resistance to the atmosphere is perfect2 moreover1 in chemico-physical processes one has the satisfaction of knowing what one is dealing with4 #acf4 ,gilding ,solutions4 ,the strength of solution necessary for gilding brass1 copper1 and silver is not very material4 ,about one to two pounds of gold potassium cyanide 78 #if per cent ,,kcn7 per gallon does very well4 ,the gold is best introduced by electrolysing from a large to a small gold electrode4 ,one purchases a plate of pure gold either from the mint or from reliable metallurgists 7say ,messrs4 ,johnson and ,matthey of ,london71 and from this electrodes are cut4 ,the relative areas of the electrodes do not really much matter4 ,i have used an anode of four times the area of the cathode4 ,the solution is preferably heated to a temperature of about #ej ,c1 and a strong current is sent through it1 say twenty amperes to the square foot of anode4 ,the electrodes must be suspended below the surface of the solution by means of platinum wires4 ,if the gold plates are only partly immersed1 they dissolve much more rapidly where they cut the surface1 possibly on account of the effect of convection currents1 though so far as the writer is aware no proper explanation has yet been given4 ,after a time gold begins to be deposited on the cathode in a powdery form1 for which reason it is a good plan to begin by wrapping the latter in filter paper4 ,the process has gone on for a sufficient time when a clean bit of platinum foil immersed in the place of the cathode becomes properly gilt at a current density of about ten amperes per square foot4 ,the powdery gold deposited on the cathode while preparing the solution can be scraped off and melted for further use1 or the whole cathode may now be used as an anode4 ,the platinum foil testing cathode may also be stripped by making it an anode1 and is for this reason preferable to ,german silver or copper1 which would contaminate the solution while the stripping process was in progress4 ,for general purposes a current density of say ten to fifteen amperes per square foot may be used1 but this may be considerably varied1 so long as the upper limit is not greatly overpassed4 ,during gold-plating there is a considerable advantage in keeping the electrodes moving or the solution stirred4 ,after immersing the cleaned and scratch-brushed articles1 depositing may go on for about three minutes1 after which they are removed from the bath and examined1 in order to detect any want of uniformity in the deposit4 ,the articles should be entirely immersed2 if this is not done1 irregularity is apt to appear at the surface4 ,platinum wires employed as suspenders1 and coated along with the articles to be gilt1 may also be cleaned without loss by making them anodes4 ,if1 on examination1 all is found to be going on well1 reimmerse the cathodes1 and continue plating till they appear of a dull yellowish brown 7this will occur in about four minutes71 then remove them1 rinse and scratch-brush them1 and replace them in the bath4 ,when a second coat appears to be getting rather brown than yellowish brown1 i4e4 of the colour of wet wash-leather1 the removal1 followed by scratch-brushing1 may be repeated1 and for nearly all laboratory purposes1 the articles are now fully gilt4 ,the coating of gold deposited from a hot cyanide solution is spongy in the extreme1 and if the maximum wear-resisting effect is to be obtained1 it is advisable to burnish the gold rather than to rely upon the scratch brush alone4 ,if the area of the cathode exceeds that of the anode the solution is said to grow weaker1 and vice versa4 ,this may be remedied in the former case by an obvious readjustment2 the latter introduces no difficulty so far as ,i know except when plating iron or steel4 ,the student need not be troubled at the poor appearance of the deposit before it is scratch-brushed4 ,heavy gold deposits are almost always dull1 not to say dirty1 in appearance till the burnisher or scratch brush is applied4 ,on the other hand1 the deposit ought not to get anything like black in colour4 ,the following indications of defects may be noted--they are taken from ,gore4 ,i have never been really troubled with them4 ,the deposit is blackish4 ,this is caused by too strong a current in too weak a bath4 ,this may be remedied to some extent by stirring or keeping the cathode in motion4 ,the obvious remedy is to add a little cyanide of gold4 ,the gold anode gets incrusted4 ,this is a sign that the bath is deficient in potassium cyanide4 ,the gold anode gets black and gives off gas4 ,the solution is deficient in cyanide1 and too large a current is being passed4 ,if a bright surface is desired direct from the bath1 some caustic potash 7say #b per cent7 may1 according to ,gore1 be added1 or the articles may be plated only slightly by using a weak current and taking them out directly they show signs of getting dull4 ,by a weak current ,i mean one of about five amperes per square foot4 ,the deposit is said to be denser if the solution be heated as directed2 but the bath will gild1 though not quite so freely when cold4 ,to gild iron or steel directly1 dilute the bath as above recommended some five or six times1 add about #a per cent of potassium cyanide1 and gild with a very weak current 7say two or three amperes per square foot7 in the cold4 ,frequent scratch-brushing will be found requisite to secure proper adherence4 ,it is generally recommended to gild brass or ,german silver in solutions which are rather weak1 but in the small practice which occurs in the laboratory a solution prepared as suggested does perfectly for everything except iron or steel4 ,the scratch-brushing should be done over a large photographic developing dish to avoid loss of gold4 ,it is a good plan to rinse the articles after leaving the bath in a limited quantity of distilled water1 which is afterwards placed in a residue bottle1 and then to scratch-brush them by hand over the dish to catch fine gold4 ,when any loose dust is removed the articles may be scratched in the lathe without appreciable further loss4 ,silver-gilt articles tend to get discoloured by use1 but this discoloration can be removed by soap and water4 ,after long use a gold cyanide bath tends to alter greatly in composition1 ,in general1 the bath tends to grow weaker1 from the fact that there is a strong temptation to gild as many articles at once as possible4 ,it is therefore a good plan to keep a rough profit and loss account of the gold in order to find the quantity in solution4 ,fifty dwts4 per gallon 7or #gh grms4 per #d4e litres7 is recommended4 ,a gallon of solution of this strength is worth about eleven pounds sterling in gold and cyanide1 and a serviceable anode will be worth about #aj pounds4 7,fine gold is worth nominally four pounds four shillings and eleven pence ha'penny per oz47 ,gold may be easily obtained containing less impurity than one part in ten thousand4 #acg4 ,plating with ,copper4 ,copper may be deposited from almost any of its salts in reguline form1 the sulphate and nitrate being most usually employed4 ,in the laboratory a nearly saturated solution of sulphate of copper with #a or #b per cent of sulphuric acid will answer most purposes4 ,a current density of1 at most1 fifteen amperes per square foot may be used1 either for obtaining solid deposits for constructional purposes or for calibrating current measuring instruments by electrolysis4 ,a copper anode is of course employed4 ,when coppering with a view to obtaining thick deposits it is a good plan to place the electrodes several inches apart1 and1 if possible1 to keep the liquid stirred1 as there is a considerable tendency on the part of copper deposits to grow out into mossy masses wherever the current density exceeds the limit mentioned4 ,as the masses grow towards the anode the defect naturally tends to increase of itself1 hence the necessity for care4 ,the phenomenon is particularly marked at the edges and corners of the cathode4 ,if the deposit becomes markedly irregular1 the best plan is to stop the process and file the face of the deposit down to approximate smoothness4 ,in coppering it is of the utmost importance that the cathode be clean and free from grease2 it must never be touched 7by the finger1 for instance7 from the time it is scratch-brushed till it is immersed in the plating bath4 ,any grease or oxidation tends to prevent the copper deposit adhering properly4 ,a copper deposit oxidises very easily when exposed to the air4 ,consequently if the surface be required free from oxide1 as1 for instance1 when it is to be silvered or gilt1 it must be quickly washed when withdrawn from the coppering bath1 scratch-brushed1 and transferred immediately to the silvering or gilding bath4 ,if the surface is to be dried1 as in electrolysis calibrations1 it must be rinsed quickly with boiling water and pressed between sheets of filter paper4 ,another method which has been recommended is to rinse the copper in--water slightly acidulated with sulphuric acid 7which prevents oxidation71 then in distilled water1 and to dry by blotting paper and in front of a fire1 taking care not to make the plate too hot4 ,the wash water is sufficiently acidulated by the addition of two or three drops of acid per litre4 ,so far as ,i know1 the method of washing in acidulated water was first proposed by ,mr4 ,t4 ,gray4 #ach4 ,coppering ,aluminium4 ,a good adherent deposit of copper on aluminium used to be considered a desideratum in the days when it afforded the only means of soldering the latter4 ,many receipts have been published from time to time1 and ,i have tried1 ,i think1 most of them4 ,on no occasion1 however1 till this year 7#ahif71 have ,i succeeded in obtaining a deposit which would not strip after it was tinned and soldered1 though it is not difficult to get apparently adherent deposits so long as they are not operated upon by the soldering iron4 ,the best of the many solutions which have been proposed in years gone by is very dilute cupric nitrate with about #e per cent of free nitric acid4 ,the problem of electroplating aluminium which ,i have indicated as awaiting a solution has at last found one4 ,in the ,archives des ,sciences physiques et naturelles de ,genve for ,december #ahie 7vol4 xxxiv4 p4 #efc7 there is a paper by ,m4 ,margot on the subject1 which discloses a perfectly successful method of plating aluminium with copper4 ,the paper itself deals in an interesting way with the theory of the matter--however1 the result is as follows4 7#a7 ,the aluminium articles are boiled for a few minutes in a strong solution of ordinary washing soda4 ,the aluminium surface is thus corroded somewhat1 and rendered favourable to the deposit of an adherent film of copper4 ,after removal from the soda solution the aluminium is well washed and brushed in running water4 7#b7 ,the articles are dipped for thirty seconds or so in a hot #e per cent solution of pure hydrochloric acid4 7#c7 ,after dipping in the hydrochloric acid1 the work is instantly plunged into clean water for about one second1 so as to remove nearly1 but not quite1 all of the aluminium chloride4 7#d7 ,the work is transferred to a cold dilute 7say #e per cent7 solution of cupric sulphate slightly acidulated with sulphuric acid4 ,the degree of acidulation does not appear to be very important1 but about one-tenth per cent of strong acid does well4 ,if the preliminary processes have been properly carried out the aluminium will become coated with copper1 and the process is accompanied by the disengagement of gas4 ,it appears to be a rule that if gas is not given off1 the film of copper deposited is non-adherent4 ,the work must be left in the copper sulphate solution till it has received a uniform coating of copper4 7#e7 ,when this is the case the work is removed--well washed so as to get rid of the rest of the aluminium chloride1 and then electroplated by the battery in the ordinary copper sulphate bath4 ,if the operation 7#d7 does not appear to give a uniform coat1 or if gas is not evolved from every part of the aluminium surface1 ,i find that operations 7#b7 and 7#c7 may be repeated without danger1 provided that the dip in the hydrochloric acid is shortened to two or three seconds4 ,the copper layer obtained by ,margot's method is perfectly adherent--even when used as a base for ordinary solder--though in this case it can be stripped if sufficient force is applied4 ,since the solder recommended by ,m4 ,margot for aluminium contains zinc1 it does not run well when used to unite aluminium to copper1 brass1 iron1 etc4 ,in this case1 therefore1 ,i have found the most advantageous method of soldering to be by way of a preliminary copper-plating4 ,the success of ,m4 ,margot's method depends in my experience on obtaining just the proper amount of aluminium chloride in contact with the aluminium when the latter is immersed in the copper sulphate solution4 #aci4 ,the process of copper-plating from sulphate or nitrate may1 according to ,mr4 ,swan 7,journal of the ,royal ,institution1 #ahib1 p4 #fcj71 be considerably accelerated by the addition of a trace of gelatine to the solution4 ,as success appears to depend upon hitting the exact percentage amount of the gelatine1 which must in any case be but a fraction of one per cent1 and as ,mr4 ,swan refrains from stating what the amount is1 ,i am unable to give more precise instructions4 ,a few experiments made on the subject failed1 doubtless through the gelatine content not having been rightly adjusted4 ,mr4 ,swan claims to be able to get a hard deposit of copper with a current density of #ajjj amperes per square foot1 but seems to recommend about one-tenth of that amount for general use4 ,the solution employed is a mixture of nitrate of copper and ammonium chloride--proportions not stated4 ,electrolytic copper1 as generally prepared1 is very pure1 but this is a mere accident depending on the impurities which1 as a rule1 have to be got rid of4 ,electrolysis seems to have no effect in purifying from arsenic1 for instance4 ,roughly speaking1 about #aa grms4 of copper are deposited per ampere hour from cupric salt solutions4 ,when the current density is too high the anode suffers by oxidation1 and this introduces a large and very variable resistance into the circuit4 #adj4 ,alkaline ,coppering ,solution ,coppering ,base ,metals4 ,it is often desirable to coat lead1 zinc1 pewter1 iron1 etc1 with a firm and uniform layer of copper preparatory to gilding or silvering4 ,if copper or brass articles are soldered with soft solder it is found that the solder does not become silvered or gilt along with the rest of the material1 but remains uncoated and of an ugly dark colour4 ,this defect is got over by giving a preliminary coating of copper4 ,this is done in an alkaline solution1 generally containing cyanogen and ammonia4 ,the following method has succeeded remarkably well with me4 ,the receipt was taken originally from ,gore's ,electro-metallurgy1 p4 #bjh4 ,a solution is made of #ej grms4 of potassium cyanide 7ordinary commercial1 say1 #ge per cent7 and #cj grms4 of sodium bisulphite in ,i4#e litres of water4 ,thirty-five grammes of cupric acetate are dissolved in a litre of water1 and #bj cubic centimetres of the strongest liquid ammonia are added4 ,the precipitate formed must be more or less dissolved to a strong blue solution4 ,the cyanide and bisulphite solution is then added with warming till the blue colour is destroyed4 ,this usually requires the exact amount of cyanide and bisulphite mentioned1 but ,i have not found it essential to entirely destroy the colour4 ,the solution contains cuprocyanide of sodium and ammonium 7871 which is not very soluble1 and this salt tends to be deposited in granular crystalline masses on standing4 ,however1 at a temperature of #ej ,c4 the above receipt gives an excellent coppering liquid1 which will coat zinc with a fine reguline deposit4 ,brass or copper partly smeared with solder will receive a deposit of copper on the latter as well as on the former1 and1 moreover1 a deposit which appears to be perfectly uniform4 ,in using the bath the anode tends1 as a rule1 to become incrusted1 and this rapidly increases the resistance of the cell1 so that the current falls off quickly4 ,the articles should be scratch-brushed and plated for about two minutes with a current density of about ten ampres per square foot4 ,as soon as the deposit begins to look red the articles are to be removed and rebrushed1 after which the process may be continued4 ,about five minutes' plating will give a copper deposit quite thick enough after scratch-brushing to allow of a very even gilding or silvering4 ,aluminium appears to be fairly coated1 but1 as usual1 the copper strips after soldering4 ,iron receives an excellent and adherent coat4 ,i do not think that the formation of a crust upon the anode can be entirely prevented4 ,according to ,gore1 its formation is due to the solution being too poor in copper1 but ,i have added a solution of the acetate of copper and ammonium till the colour was bright blue without in any way reducing the incrustation4 ,if the solutions become violently blue it is perhaps as well to add a little more cyanide and bisulphite1 but ,i have not found such an addition necessary4 ,the process is one of the easiest and most satisfactory in electro-metallurgy4 #ada4 ,nickel-plating4 ,an examination of several ,american samples of nickel-plated goods has disclosed that the coating of nickel is1 as a rule1 exceedingly thin4 ,this is what one would expect from laboratory repetition of the processes employed4 ,commercial practice in the matter of the composition of nickelling solutions appears to vary a good deal4 ,thin coatings of nickel may be readily given in a solution of the double sulphate of nickel and ammonia1 which does rather better if slightly alkaline4 ,deposits from this solution1 however1 become gray if of any thickness1 and1 moreover1 are-apt to flake off the work4 ,the following solution has given very good results with me4 ,it is mentioned1 together with others1 in the ,electrical ,review1 #gth ,june #ahie4 ,the ingredients are3- ,nickel sulphate #e parts ,ammonia sufficient to neutralise the nickel salt4 ,ammonium tartrate #c4ge parts ,tannin #j4jbe parts ,water #ajj parts ,the nickel sulphate and ammonia are dissolved in half the water1 the ammonium tartrate in the other half with the tannin4 ,the solutions are mixed and filtered at about #dj ,c4 ,this solution works well at ordinary temperatures1 or slightly warm1 with a current density of ten ampres per square foot4 ,in an experiment made for the purpose ,i found that plating may go on for an hour in this solution before the deposit begins to show signs of flaking off4 ,the deposit is of a fine white colour4 ,the resistance of the bath is rather high and rather variable1 consequently it is as well to have a current indicator in circuit1 and it may well happen that five or six volts will be found requisite to get the current up to the value stated4 ,for nickelling small objects of brass1 such as binding screws1 etc1 it is very necessary to be careful as to the state of polish and uniformity of their surfaces before placing them in the plating bath4 ,a polished surface will appear when coated as a polished surface1 and a mat surface as a mat surface2 moreover1 any local irregularity1 such as a speck of a foreign metal1 will give rise to an ugly spot in the nickelling bath4 ,for this reason it is often advisable to commence with a coat of copper laid on in an alkaline solution and scratch-brushed to absolute uniformity4 ,an examination of the work will1 however1 disclose whether such a course is desirable or not2 it is not done in ,american practice1 at all events for small brass objects4 ,these are cleaned in alkali and in boiling cyanide1 which does not render a polished surface mat1 as weak acid is apt to do1 and are then coated with a current density of about ten ampres per square foot4 ,in spite of what is to be found in books as to the ease with which nickel deposits may be polished1 ,i find that the mat surface obtained by plating on an imperfectly polished cathode of iron is by no means easily polished either by fine emery1 tripoli1 or rouge4 ,consequently1 as in the case of brass1 if a polished surface is desired1 it must be first prepared on the unplated cathode4 ,in this case1 even if the deposit appears dull1 but not gray1 it may be easily polished by tripoli and water1 using a cork as the polisher4 ,scratch-brushing with brass wire1 however1 though possibly not with ,german silver wire1 brightens the deposit1 but discolours it4 ,when the deposit becomes gray ,i have not succeeded in polishing it satisfactorily4 ,soldered brass or iron may be satisfactorily coated with nickel by giving it a preliminary coating of copper in the cyanide bath4 ,on the whole1 ,i recommend in general that iron be first coated with copper in the alkaline bath1 scratch-brushed1 and then nickel-plated1 and this whether the iron appears to be uniform or not4 ,much smoother1 thicker1 and stronger coats of nickel are obtained upon the copper-plated surface than on the iron one1 and the coating does not become discoloured 78 by iron rust7 in the same way that a coating on bare iron does4 ,the copper surface may be plated for at least an hour at a density of ten ampres per square foot without scaling4 ,scales or circles divided on brass may be greatly improved in durability by nickel--plating4 ,for this purpose the brass must be highly polished and divided before it is nickelled4 ,the plating should be continued for a few minutes only1 when a very bright but thin coat of nickel will be deposited2 it then only remains to wash and dry the work1 and this must be done at once4 ,if the nickel is deposited before the scale or circle is engraved1 very fine and legible divisions are obtained1 but there is a risk that flakes of nickel may become detached here and there in the process of engraving4 #adb4 ,miscellaneous ,notes on ,electroplating4 ,occasionally it is desirable to make a metallic mould or other object of complex shape4 ,the quickest way to do this is to carve the object out of hard paraffin1 and then copy it by electrotyping4 ,electrotype moulds can be made in many ways4 ,the easiest way perhaps is to take a casting in plaster of ,paris1 or by means of pressure in warm gutta-percha4 ,in cases where the mould will not draw1 recourse must be had to the devices of iron-founders1 i4e4 the plaster cast must be made in suitable pieces1 and these afterwards fitted together4 ,this process can occasionally be replaced by another in which the moulding material is a mixture of treacle and glue4 ,the glue is soaked in cold water till it is completely soft4 ,the superfluous water thrown away1 one-fourth part by volume of thick treacle is added1 and the mixture is melted on the water bath2 during which process stirring has to be resorted to1 to produce a uniform mixture4 ,this liquid forms the moulding mixture1 and it is allowed to flow round the object to be copied1 contained in a suitable box1 whose sides have been slightly oiled4 ,the object to be copied should also be oiled4 ,after some hours1 when the glue mixture has set1 it will be found to be highly elastic1 so that it may be pulled away from the mould1 and afterwards resume very nearly its original form4 ,one drawback to the use of these moulds lies in the fact that the gelatine will rarely stand the plating solution without undergoing change1 but this may be partially obviated by dipping it for a few seconds in a #aj per cent solution of bichromate of potash1 exposing it to the sunlight for a few minutes1 and then rinsing it4 ,in order to render the surface conducting1 it is washed over with a solution of a gold or silver salt1 and the latter reduced in situ to metal by a suitable reagent4 ,a solution of phosphorus is the most usual one 7see ,gore1 ,electro-metallurgy1 p4 #baf74 ,such a mould may be copper-plated in the sulphate bath1 connection being made by wires suitably thrust into the material4 ,plaster of ,paris moulds require to be dried and waxed by standing on a hot plate in melted wax before they are immersed in the plating bath4 ,in this case the surface is best made conducting either by silvering it by the silvering process used for mirrors1 or by brushing it over with good black lead rendered more conducting by moistening with an ethereal solution of chloride of gold and then drying in the sun4 ,the brushing requires a stiff camel's-hair pencil of large size cut so that the hairs project to a distance of about a quarter of an inch from the holder4 ,the brushing must continue till the surface is bright1 and is often a lengthy process4 ,the same process of blackleading may be employed to get a coat of deposited metal which will strip easily from the cathode4 ,in all cases where extensive deposits of copper are required1 the growth takes place too rapidly at the corners4 ,consequently it is often desirable to localise the action of the deposit4 ,a stopping of ordinary copal varnish seems to be the usual thing1 but a thin coat of wax or paraffin or photographic 7black7 varnish does practically as well4 ,i do not propose to deal with the subject of electrotyping to any extent1 for if practised as an art1 a good many little precautions are required1 as the student may read in ,gore's ,electro-metallurgy4 ,the above instructions will be found sufficient for the occasional use of the process in the construction of apparatus1 etc4 ,there is no advantage in attempting to hurry the process1 a current density of about ten ampres per square foot being quite suitable and sufficiently low to ensure a solid deposit4 #adc4 ,blacking ,brass ,surfaces4 ,a really uniform dead-black surface is difficult to produce on brass by chemical means4 ,a paste of nitrate of copper and nitrate of silver heated on the brass is said to give a dead-black surface1 but ,i have not succeeded in making it act uniformly4 ,for optical purposes the best plan is to use a paint made up of drop black1 ground very fine with a little shellac varnish1 and diluted for use with alcohol4 ,no more varnish than is necessary to cause the black to hold together should be employed4 ,in general1 if the paint be ground to the consistency of very thick cream with ordinary shellac varnish it will be found to work well when reduced by alcohol to a free painting consistency4 ,a very fine gray and black finish1 with a rather metallic lustre1 may be easily given to brass work4 ,for this purpose a dilute solution of platinum tetrachloride 7not stronger than #a per cent7 is prepared by dissolving the salt in distilled water4 ,the polished brass work is cleaned by rubbing with a cork and strong potash till all grease has disappeared1 as shown by water standing uniformly on the metal and draining away without gathering into drops4 ,after copious washing the work is wholly immersed in a considerable volume of the platinum tetrachloride solution at the ordinary temperature4 ,after about a quarter of an hour the brass may be taken out and washed4 ,the surface will be found to be nicely and uniformly coated if the above instructions have been carried out1 but any finger-marks or otherwise dirty places will cause irregularity of deposit4 ,if the process has been successful it will be found that the deposit adheres perfectly1 hardly any of it being removed by vigorous rubbing with a cloth4 ,if the deposit is allowed to thicken--either by leaving the articles in the solution too long or heating the solution1 or having it too strong--it will merely rub off and leave an irregular surface4 ,this process succeeds well with yellow brass and ,muntz metal1 either cast or rolled1 but it does not give quite such uniform 7though still good7 results with gun-metal1 on which1 however1 the deposit is darker and deader in appearance4 ,a book might be written 7several have been written7 on the art of metal colouring1 but though doubtless a beautiful and delicate art1 it is of little service in the laboratory4 ,for further information the reader may consult a work by ,hiorns4 #add4 ,sieves4 ,properly graded sieves with meshes of a reliable size are often of great use4 ,they should be made out of proper bolting cloth1 a beautiful material made for flour-millers4 ,messrs4 ,henry ,simon and ,company of ,manchester have kindly furnished me with the following table of materials used in flour-milling4 ,sieves made of these materials will be found to work much more quickly and satisfactorily than those made from ordinary muslin or wire gauze4 ,relative ,bolting ,value of ,silk1 ,wire1 and ,grit ,gauze ,threads per inch ,trade ,no4 ,trade ,no4 ,trade ,no4 of ,approximate4 of ,silk4 of ,wire4 ,grit ,gauze4 #ah #jjjj #ah #af #bb #jjj #bj #bj #bh #jj #bf #bf #ch #j #cb #cd #dh #a #dj #dd #eb #b #de #ej #ef #c #ej #ed #fj #d #ef #eh #fd #e #fj #fj #gb #f #fd #ff #hj #g #gj #gj #hd #h #hj #hj #id #i #ajf #aj #aad #aa #abd #ab #acj #ac #aci #ad #adh #ae #aef #af #afc #ag #afg #ah #agj #ai #agc #bj #ade4 ,pottery making in the ,laboratory4 ,when large pieces of earthenware of any special design are required1 recourse must be had to a pottery4 ,small vessels1 plates1 parts of machines1 etc1 can often be made in the laboratory in less time than it would take to explain to the potter what is required4 ,for this purpose any good pipeclay may be employed4 ,i have used a white pipe-clay dug up in the laboratory garden with complete success4 ,the clay should be kneaded with water and squeezed through a cloth to separate grit4 ,it is then mixed with its own volume or thereabouts of powdered porcelain evaporating basins1 broken basins being kept for this purpose4 ,the smoothness of the resulting earthenware will depend on the fineness to which the porcelain fragments have been reduced4 ,i have found that fragments passing a sieve of sixty threads to the inch run1 do very well1 though the resulting earthenware is decidedly rough4 ,the porcelain and clay being thoroughly incorporated by kneading1 the articles are moulded1 it being borne in mind that they will contract somewhat on firing4 ,footnote3 ,the contraction depends on the temperature attained as well as on the time4 ,an allowance of one part in twelve will be suitable in the case considered4 ,the clay should be as stiff as is convenient to work1 and after moulding must be allowed to get thoroughly dry by standing in an airy place2 the drying must not be forced1 especially at first1 or the clay will crack4 ,small articles are readily fired in a ,fletcher's crucible furnace supplied with a gas blow-pipe2 the furnace is heated gradually to begin with4 ,when a dull red heat is attained1 the full power of the blast may be turned on1 and the furnace kept at its maximum temperature for three or four hours at least1 though on an emergency shorter periods may be made to do4 ,the articles are supported on a bed of white sand2 after firing1 the crucible furnace must be allowed to cool slowly4 ,it must be remembered that the furnace walls will get hot externally after the first few hours1 consequently the furnace must be supported on bricks1 to protect the bench4 ,the pottery when cold may be dressed on a grindstone if necessary4 ,this amateur pottery will be found of service in making small fittings for switch-boards1 commutators1 and in electrical work generally4 ,pottery made as described is very hard and strong1 the hardness and strength depending in a great degree on the proportion of powdered porcelain added to the clay1 as well1 of course1 as on the quality of both of these materials4 ,it is a good plan to knead a considerable quantity of the mixture1 which may then be placed in a well-covered jar1 and kept damp by the addition of a little water4 ,pottery thus made does not require to be glazed1 but1 of course1 a glaze can be obtained by any of the methods described in works on pottery manufacture4 ,the following glaze has been recommended to me by a very competent potter3- ,litharge #g parts by weight ,ground flint #b parts by weight ,cornish stone or felspar #a parts by weight ,these ingredients are to be ground up till they will pass the finest sieve--say #ahj threads to the inch4 ,they are then mixed with water till they form a paste of the consistency of cream4 ,they must1 of course1 be mixed together perfectly4 ,the ware to be glazed is dipped into the cream after the first firing2 it is then dried as before and refired4 ,the glaze will melt at a bright red heat1 but it will crack if not fired harder2 the harder it is fired the less likely is it to crack4 ,if colouring matters are added they must be ground in a mill free from iron till they are so fine that a thick blanket filter will not filter them when suspended in water4 ,this remark applies particularly to oxide of cobalt4 ,,appendix ,,platinising ,,glass ,,in the ,philosophical ,magazine for ,july #ahhh 7vol4 xxvi4 p4 #a7 there is a paper by ,professor ,kundt translated from the ,sitzungsberichte of the ,prussian ,academy4 ,this paper deals with the indices of refraction of metals4 ,thin prisms were obtained by depositing metals electrolytically on glass surfaces coated with platinum4 ,the preparation of these surfaces is troublesome4 ,kundt recounts that no less than two thousand trials were made before success was attained4 ,a detailed account of the preparation of these surfaces is not given by ,kundt1 but one is promised--a promise unfortunately unfulfilled so far as ,i am able to discover4 ,a hunt through the literature led to the discovery of the following references3 ,central ,zeitung fuer ,optik und ,mechanik1 p4 #adb 7#ahhh72 ,dingler's ,polytechnik ,journal1 ,vol4 cxcv4 p4 #dfd2 ,comptes ,rendus1 vol4 lxx4 7#ahgj74 ,the original communication is a paper by ,jouglet in the ,comptes ,rendus1 of which the other references are abstracts4 ,the account in ,dingier is a literal translation of the original paper1 and the note in the ,central ,zeitung is abbreviated sufficiently to be of no value4 ,the details are briefly as follows3- ,one hundred grams of platinum are dissolved in aqua regia and the solution is dried on the sand bath1 without1 however1 producing decomposition4 ,though the instructions are not definite1 ,i presume that the formation of ,pt,cl#d is contemplated4 ,the dried salt is added little by little to rectified oil of lavender1 placed on a glass paint-grinding plate1 and the salt and oil are ground together with a muller4 ,care is required to prevent any appreciable rise of temperature which would decompose the compound aimed at1 and it is for this reason that the salt is to be added gradually4 ,of course the absorption of water from the air must be prevented from taking place as far as possible4 ,finally1 the compound is diluted by adding oil of lavender up to a total weight of #adjj grams 7of oil74 ,the liquid is poured into a porcelain dish and left absolutely at rest for eight days4 ,it is then decanted and filtered1 left six days at rest1 and again decanted 7if necessary74 ,the liquid should have a specific gravity of #e on the acid hydrometer4 7,if by this the ,baum scale is intended1 the corresponding specific gravity would be #a4jcg47 ,a second liquid is prepared by grinding up #be grams of litharge with #be grams of borate of lead and #h to #aj grams of oil of lavender4 ,the grinding must be thoroughly carried out4 ,this liquid is to be added to the one first described1 and the whole well mixed4 ,the resulting fluid constitutes the platinising liquid1 and is applied as follows3- ,a sheet of clean glass is held vertically1 and the liquid is painted over it1 carrying the brush from the lower to the upper edge4 ,the layer of oil dries slowly1 and when it is dry the painting is again proceeded with1 moving the brush this time from right to left2 and similarly the process is repeated twice1 the brush being carried from top to bottom and left to right4 ,this is with the object of securing great uniformity in the coating4 ,nothing is said as to the manner in which the glass is to be dried4 ,the dried glass is finally heated to a temperature of dull redness in a muffle furnace4 ,the resinous layer burns away without running or bubbling1 and leaves a dull metallic surface4 ,as the temperature rises this suddenly brightens1 and we obtain the desired surface 7which probably consists of an alloy of lead and platinum74 ,it is bright only on the surface away from the glass4 ,i have not had an opportunity of trying this process since ,i discovered the detailed account given by ,jouglet2 but many modifications have been tried in the laboratory of the ,sydney ,university by ,mr4 ,pollock1 starting from the imperfect note in the ,central ,zeitung1 which led to no real success4 ,it was found that it is perfectly easy to obtain brilliant films of platinum by the following process1 provided that the presence of a few pin-holes does not matter4 ,the platinum salt employed is what is bought under the name of platinic chloride2 it is1 however1 probably a mixture of this salt and hydro-chloro-platinic acid1 and has all the appearance of having been obtained by evaporating a solution of platinum in aqua regia to dryness on the water bath4 ,a solution of this salt in distilled water is prepared2 the strength does not seem to matter very much1 but perhaps one of salt to ninety-nine water may be regarded as a standard proportion4 ,to this solution is added a few drops of ordinary gum water 7i4e4 a solution of dextrin74 ,the exact quantity does not matter1 but perhaps about #b per cent may be mentioned as giving good results4 ,the glass is painted over with this solution and dried slowly on the water bath4 ,when the glass is dry1 and covered with a uniform hard film of gum and platinum salt free from bubble holes1 it is heated to redness in a muffle furnace4 ,the necessary and sufficient temperature is reached as soon as the glass is just sensibly red-hot4 ,the mirrors obtained in this way are very brilliant on the free platinum surface4 ,if the gum be omitted1 the platinum will have a mat surface2 and if too much gum be used1 the platinum will get spotty by bubbles bursting4 ,there does not appear to be any advantage in using lead4 ,it is quite essential that the film be dry and hard before the glass is fired4