THE TREASURES 
 OF COAL TAR. 
 
 ALEXANDER FIND LAY 
 

THE TREASURES 
 OF COAL TAR 
 
COAL TAR TREE CHART 
 
 Illustrating the various chemical products de- 
 rived from Coal and Coal Tar, designed in 
 the form of a Genealogical Tree. 34" x 36". 
 Revised Edition. 
 
 BY WALLACE C. NICKELS, F.C.S. 
 
THE TREASURES 
 OF COAL TAR 
 
 BY 
 
 ALEXANDER FINDLAY 
 
 1 1 
 M.A., D.Sc., F.I.C. 
 
 PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF WALES 
 
 AND DIRECTOR OF THE EDWARD DAVIES CHEMICAL LABORATORIES 
 
 UNIVERSITY COLLEGE OF WALES, ABERYSTWYTH 
 
 AUTHOR OF 
 
 1 CHEMISTRY IN THE SERVICE OF MAN ' 
 ETC. 
 
 WITH THREE FIGURES IN THE TEXT 
 
 NEW YORK 
 D. VAN NOSTRAND COMPANY 
 
 1917 
 
Printed in Great Britain 
 fy TurnbuZl &> Sftars, Edinburgh 
 
s* 
 T 
 
 TO 
 
 MY MOTHER 
 
 S820G8 
 
PREFACE 
 
 IN order that the effort now being made to promote 
 the more widespread application of science, and 
 more especially to render this country independent 
 of others for the supply of the dyes necessary for 
 the maintenance of our textile industry, shall not 
 be relaxed, it is essential that the people as a whole 
 should interest themselves in the work, and should 
 gain some knowledge of what has been achieved 
 in the past, and some understanding of the nature 
 and complexity of the problems to be solved. As 
 the matter is urgent, and of vital importance for 
 the welfare of this country, the writer felt that, 
 even in a time of much preoccupation, he could 
 not refuse the invitation of the publishers to dis- 
 cuss in a readily intelligible manner the production 
 and utilisation of coal tar, and to indicate, suffi- 
 ciently fully for the general reader, the almost 
 infinite variety of materials dyes, drugs., perfumes, 
 explosives for the manufacture of which coal tar 
 is the raw material. 
 
 Based on this invaluable by-product of the manu- 
 facture of coke and of illuminating gas, an industry, 
 or rather a series of industries, has been developed ; 
 
 vii 
 
viii THE TREASURES OF COAL TAR 
 
 but although Great Britain played a predominant 
 part in the early stages of this industrial develop- 
 ment, she failed to retain the great advantages 
 she had gained, and the manufacture of synthetic 
 dyes and drugs became increasingly a German 
 monopoly. To such an extent was this the case 
 that, before the outbreak of war, Germany was 
 producing more than three times the quantity of 
 coal-tar products produced by all the rest of the 
 world combined. It is true that dyes were manu- 
 factured in considerable amount in this country, 
 but our manufacturers rested content, in too great 
 a measure, with their dependence on German 
 " intermediates,'^ which, instead of making, they 
 imported and worked up into dyes. In spite of 
 the warnings uttered by our foremost chemists 
 during the past thirty years, in spite of the object- 
 lessons furnished by the destruction of the European 
 madder and the decay of the Indian indigo planta- 
 tions, this country failed to develop its coal-tar 
 chemical industries on a national scale ; and as 
 a result of this failure she found herself, on the 
 outbreak of war, placed in a position of great 
 gravity. Perilously handicapped in our production 
 of the munitions of war, threatened with the de- 
 struction of our textile industry through the cutting 
 off of the supply of the German-made intermedi- 
 ates and dyes, and with the health of our people 
 and army endangered through shortage of those 
 
PREFACE ix 
 
 synthetic drugs with which the German chemical 
 industry had supplied us, we were brought face to 
 face with our past neglect of chemical science and 
 with our failure to encourage the application of 
 that science in our industries. The chemists of 
 the country were hurriedly mobilised, and the 
 production of the essential munitions of war and 
 of a sufficiency of drugs was ensured ; the Govern- 
 ment came to the help of the dye-making industry, 
 and in the past two years, in spite of many handicaps, 
 great progress has been made. 
 
 But what of the future ? Can we feel sure that 
 the lessons of the war have been learned and that 
 the loss and bitter experience of the past three 
 years will be turned to permanent gain ? Have 
 our people acquired that new outlook, that new 
 mentality, which is the only safeguard of our 
 future ? As I have written elsewhere, we are all 
 prone to blame our manufacturers and directors of 
 industry, and to place on them the responsibility 
 for our backwardness in the recognition of the 
 value of science, but we have to remember that they 
 are themselves but a part of the national system, 
 and their organisation and outlook an expression 
 of the national character and habit. Until we 
 realise that our present unfortunate position is 
 the result of a national defect, which shows itself 
 most glaringly in our lack of interest in education 
 and of a desire for knowledge, and until we realise 
 
x THE TREASURES OF COAL TAR 
 
 the necessity for each and all of us gaining a new 
 standpoint and outlook, gaining a new ideal, we 
 cannot hope for a permanent improvement in the 
 attitude of the country and manufacturers towards 
 science and its applications. Lord Moulton has 
 quoted the words of a German industrial chemist : 
 " England talks now not only of holding her own 
 in war, but beating us in our chemical industries. 
 She cannot do it, and that is because the nation 
 is incapable of the moral effort to take up an 
 industry like that which implies study, which im- 
 plies concentration, which implies patience, which 
 implies fixing one's eye on the distant consequences 
 and not considering merely the momentary profit." 
 That is a challenge which this country cannot refuse 
 to take up, but, in taking it up, let us realise that 
 success can be achieved only by a more general 
 appreciation of science, by the cultivation and 
 encouragement of chemical research in an enor- 
 mously higher degree than in the past, and by 
 the continual co-operation between science and 
 technology. And it is important, also, to realise 
 that it is not merely science in its immediate 
 applications to industry that we must cultivate 
 and encourage, but also, and more especially, pure 
 science or " experimental research motivated solely 
 by the desire to increase knowledge." The ac- 
 quisition of knowledge must precede its applica- 
 tion ; chemical invention must follow chemical 
 
PREFACE xi 
 
 discovery. All the great discoveries, all the great 
 advances have been made, not as a result of effort 
 to achieve results of immediate industrial im- 
 portance, but as the result of a patient and per- 
 severing pursuit of knowledge. In developing the 
 coal-tar industries we can succeed if we will ; let 
 us will. 
 
 One point more must be borne in mind. Coal 
 tar is produced not as a primary but as a by-pro- 
 duct in the manufacture of illuminating gas and 
 of metallurgical coke. Its production is therefore 
 dependent on the demand for coal gas and for coke, 
 and the outlet for the latter depends on the develop- 
 ment of our metallurgical industries. A proper 
 balance between output of, and the profitable 
 outlet for, the different products and by-products 
 of the distillation of coal must be established ; 
 and the whole series of interdependent and inter- 
 locking chemical industries must be carefully organ- 
 ised and developed so as to ensure the greatest 
 efficiency and best utilisation of all the products. 
 The question of the production and utilisation of 
 coal tar and coal-tar products is one of great com- 
 plexity as it is also one of great economic import- 
 ance ; and it must be treated as part and parcel 
 of the much larger question of the most effective 
 utilisation of our national reserves of coal. 
 
 My thanks are due to Messrs Longmans, Green 
 & Co. for the use of the block of Figure 3, taken 
 
xii THE TREASURES OF COAL TAR 
 
 from my " Chemistry in the Service of Man " ; 
 to the Comptroller-General of the Department of 
 Commercial Intelligence of the Board of Trade 
 for particulars regarding the production of coal 
 tar ; and to Mr C. M. Whittaker, of British Dyes, 
 Ltd., for information regarding dyes. I am also 
 indebted to my wife for her assistance in passing 
 the book through the press. 
 
 A. F. 
 
 Y GLYN, LLAN PARIAN, 
 
 NR. ABERYSTWYTH, CARDIGANSHIRE, 
 
 September 1917. 
 
CONTENTS 
 
 CHAPTER I 
 
 THE PRODUCTION OF COAL TAR .... I 
 
 CHAPTER II 
 
 THE DISTILLATION OF COAL TAR . . . . 13 
 
 CHAPTER III 
 
 THE CONSTITUENTS OF COAL TAR AND THEIR APPLICA- 
 TIONS IN THE RAW STATE . . . . 22 
 
 CHAPTER IV 
 
 MOLECULAR ARCHITECTURE . .... 31 
 
 CHAPTER V 
 
 THE PRODUCTION OF DYES FROM COAL TAR . . 48 
 
 CHAPTER VI 
 
 AZO-DYES 70 
 
 CHAPTER VII 
 
 ANTHRACENE DYES AND VAT DYES 80 
 
 xiii 
 
xiv THE TREASURES OF COAL TAR 
 
 PAGE 
 
 CHAPTER VIII 
 
 INDIGO AND ITS DERIVATIVES .... 90 
 
 CHAPTER IX 
 
 DRUGS, PERFUMES, AND PHOTOGRAPHIC DEVELOPERS IOO 
 
 CHAPTER X 
 EXPLOSIVES 122 
 
 INDEX 133 
 
THE TREASURES OF 
 COAL TAR 
 
 CHAPTER I 
 
 THE PRODUCTION OF COAL TAR 
 
 COAL, the fossilised and more or less completely 
 carbonised remains of the luxuriant vegetations 
 of a long bygone age, forms at once the source of 
 much of our material wealth and the basis on which 
 our industrial and commercial prosperity has been 
 reared during the past hundred years. The coal 
 mines of this country, worked at least as early as 
 the thirteenth century, have since that time pro- 
 vided us, in ever-increasing amounts, with a valu- 
 able fuel both for domestic and industrial purposes. 
 The introduction of coal, especially as a domestic 
 fuel, was for a long time regarded with disfavour, 
 and, even in the seventeenth century, met with 
 an active boycott on the part of " the nice dames 
 of London," who " would not come into any house 
 or rooms where sea-coales were burned, nor willingly 
 eat of meat that was either sod or roasted with sea- 
 coale fire " doubtless by reason of the pollution 
 of the atmosphere by smoke and of the stench 
 
2 THE TREASURES OF COAL TAR 
 
 produced by the burning coal. At the* present 
 time, however, the normal annual consumption of 
 coal in this country amounts to about 190,000,000 
 tons, of which about 40,000,000 tons are consumed 
 for domestic heating. 
 
 But although it is on its use as a fuel, as a 
 reservoir of energy derived from the sunlight of a 
 long-distant past, that the material comfort and 
 well-being of the people so largely depend, coal 
 yet conceals within itself another wealth long 
 squandered through ignorance and even now but 
 partially utilised which the wizardry of Science 
 has discovered and made available only within 
 comparatively recent years. And it is of this 
 wealth, or part of this wealth, derived from the 
 chemical transformation of coal and from the black 
 and viscid fluid, the coal tar, produced by the " de- 
 structive distillation " of coal, that it is the purpose 
 of this book to treat. 
 
 Although coal had been used as a fuel as early 
 as the beginning of the fourteenth century, it was 
 not till near the end of the seventeenth century 
 that the distillation of coal in closed vessels was 
 carried out, the first English patent being granted 
 in 1681 to J. J. Becher and Henry Serle for " a new 
 way of making pitch and tarre out of pit-coale, 
 never before found out or used by any other." At 
 first an industry of very small proportions, it was 
 not till the early years of the nineteenth century 
 
THE PRODUCTION OF COAL TAR 3 
 
 that the distillation of coal began to be carried 
 out extensively, and then not for the purpose of 
 producing tar and pitch, but, primarily, for the pro- 
 duction of coal gas or illuminating gas. Although 
 the production of an inflammable gas from coal 
 had long been known, it was not till near the end 
 of the eighteenth century that the Scotsman, William 
 Murdoch, developed the process for the production 
 of an illuminating gas for general use. Murdoch 
 was for long associated with the engineering firm 
 of Messrs Boulton & Watt, Birmingham, and it 
 was in their works at Soho that coal gas was first 
 used (in 1798) on a large scale as an illuminant. 
 It was, however, only at a considerably later date 
 that coal gas came into general use for the lighting 
 of streets and public buildings. 
 
 When ordinary or bituminous coal is subjected 
 to " destructive distillation " by heating in retorts 
 out of contact with air, there are produced : (i) the 
 combustible gas which we use for illuminating and 
 heating purposes ; (2) a watery liquor containing 
 ammonia, derived from nitrogen compounds con- 
 tained in the coal ; (3) a thick, dark-coloured liquid, 
 coal tar ; (4) coke, which remains as a solid residue 
 in the retorts. In the manufacture of illuminating 
 gas the coal is heated in large fire-clay retorts at 
 a temperature of about 1000 C. (about 1830 F.), 
 and the products of decomposition are led away by 
 
4 THE TREASURES OF COAL TAR 
 
 a pipe the mouth of which dips under the surface 
 of water contained in what is known as the hydraulic 
 main (Fig. i). Here part of the water and of the 
 coal tar condenses, while the gaseous products pass 
 away to a series of cooling pipes, exposed to the air, 
 in which a further condensation of water vapour 
 and of tar takes place. The ammonia present 
 the gas dissolves for the most part in the water 
 produced, and the remainder is removed by passing 
 the gas through " scrubbers." In this process the 
 ammoniacal liquor, coal tar, and coke are merely 
 by-products, spoken of as " residuals " ; and 
 although the coke has always been a by-product 
 of considerable value which materially affected 
 the price of the illuminating gas, the ammoniacal 
 liquor and coal tar were for a number of years 
 regarded as waste products of a disagreeable kind, 
 the disposal of which involved not a little expense, 
 and thereby retarded to some extent the develop- 
 ment of the gas-producing industry. This con- 
 dition of affairs, however, has been entirely changed, 
 largely owing to the development of the great 
 chemical industries which find their raw material 
 in coal tar, as well as to the increased and increasing 
 employment of ammonia compounds as fertilisers 
 in agriculture, in the production of explosives, 
 dyes, and soda (by the Solvay process), and in many 
 other industries. In 1913, out of a total produc- 
 tion from all sources of 432,000 tons, the gas-works 
 
FIG. i. DIAGRAM OF GAS-MANUFACTURING PLANT. 
 
 A, retort in which coal is heated. 
 
 B, the hydraulic main. 
 
 C, outlet for the tar. 
 
 D, gas pipe. 
 
 E, tank in which the ammoniacal liquor collects. 
 
 F, cooling pipes. 
 
 Page 4 
 
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 11 
 
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THE PRODUCTION OF COAL TAR . 5 
 
 of this country produced 182,180 tons of sulphate 
 of ammonia, the value of which, in some cases, 
 amounted almost to the cost of the coal distilled ; 
 while the production of coal tar in gas-works 
 amounted, in 1910, to 830,000 tons. In all, about 
 20,000,000 tons of coal are now distilled annually in 
 this country, primarily for the production of coal gas. 
 But it is not only for the production of gas that 
 coal is now distilled. Even by the middle of the 
 eighteenth century coke had to a large extent dis- 
 placed wood charcoal and was very generally em- 
 ployed in the smelting of iron ; and as our iron 
 industry advanced and extended, so also the dis- 
 tillation of coal, primarily for the production of 
 the hard and dense coke required for metallurgical 
 purposes, became an industry of ever-increasing 
 importance. For many years the coking of coal 
 was carried out in " beehive " ovens, which are, as 
 the name implies, chambers of beehive shape lined 
 with firebrick. From seven to eight feet high and 
 about twelve feet in diameter, these ovens are now 
 generally arranged side by side in two rows so as 
 to economise heat and allow of the hot products 
 of distillation being carried off through a central 
 flue for the raising of steam (Fig. 2). Through a 
 hole in the top, the oven, still hot from previous 
 use, is charged with coal to a depth of about three 
 feet, the door of the oven being temporarily bricked 
 up. Air in regulated amount is admitted to the 
 
6 THE TREASURES OF COAL TAR 
 
 space above the coal, where combustion of the 
 evolved vapours takes place, and coking or carbon- 
 isation proceeds steadily from above downwards, 
 owing to the heat reflected from the roof and walls 
 of the oven. In this process the heat required for 
 the coking of the coal is derived from the combustion 
 of the gases distilled from the coal as well as from 
 the combustion of part of the charge. 
 
 Although, in these beehive ovens, a hard, dense 
 coke, admirably suited for metallurgical purposes, 
 is produced, the process is a most wasteful one, 
 because not only is a certain amount of coal lost 
 through combustion, but all the volatile products 
 of distillation, amounting to about one-third of 
 the weight of the coal, are lost. In the early days 
 of the industry, when there was little or no outlet 
 for these products, when, at least, their commercial 
 value was small compared with that of the coke, 
 the consciousness of waste was scarcely awakened, 
 and the coke producers made no attempt to recover 
 and utilise the by-products of the coking process. 
 Moreover, the introduction of coking ovens which 
 allow of the recovery of the volatile by-products of 
 distillation was resisted by the iron-makers, as it 
 was thought not, at first, without reason that 
 the coke produced in them was inferior to that 
 produced in the old beehive ovens. But the pre- 
 judice which for long existed, more especially in 
 Great Britain, has now been proved to be ground- 
 
THE PRODUCTION OF COAL TAR 7 
 
 less, and the increasing importance of ammonia 
 and coal tar, and the necessity for greater industrial 
 economy, are leading more or less rapidly to the 
 abolition of the old beehive oven. Whereas in 
 Great Britain in 1900 only ten per cent, of all the 
 metallurgical coke was produced in by-product 
 recovery ovens, in 1913 about sixty per cent, was 
 so produced. In other words, of the 20,000,000 
 tons of coal converted into coke, about 13,500,000 
 tons were coked in by-product recovery ovens and 
 6,500,000 tons in beehive ovens. In the United 
 States, similarly, about one-third of the metal- 
 lurgical coke was produced, in 1914, in beehive 
 ovens, without recovery of the by-products, where- 
 as in Germany, in 1909, only about one-fifth of 
 the coke was produced by this wasteful process. 
 Although, in this respect, this country has lagged 
 considerably in the rear of Germany, fairly rapid 
 progress towards a more economical utilisation of 
 our national resources in coal is being made ; and 
 this will doubtless be accelerated by the experiences 
 of the past three years and the necessities of the 
 future. Although the initial cost of the by-product 
 recovery ovens is greater than in the case of the 
 beehive ovens, it would appear, from evidence 
 given before a Royal Commission on Coal Supplies 
 in England, that the value of the by-products would 
 not only provide a profit on the working of the 
 plant, but would also, within ten years, pay off the 
 
8 THE TREASURES OF COAL TAR 
 
 capital outlay. At the same time it has to be 
 borne in mind that in the future the successful 
 development of the coking industry, with recovery 
 of the by-products, must very largely depend on 
 the development of all those closely interdependent 
 industries more especially chemical industries 
 which afford a remunerative outlet for the by-pro- 
 ducts of distillation, and it is of the highest import- 
 ance that this country shall make a determined 
 and well-directed effort towards this end. 
 
 The recognition of the importance of recovering 
 the volatile matter produced in the coking of coal 
 has, during the past thirty or forty years, led to 
 great activity in the work of designing and construc- 
 tion of ovens adapted for the purpose, and several 
 different types are at present in use. In one type, 
 a modification of the Coppee oven, for example, 
 the oven is formed by a chamber about thirty feet 
 in length, two feet wide, and five feet high, heated 
 by means of gas. A number of these ovens are 
 generally arranged side by side and are charged 
 from hoppers which run on rails over the series of 
 chambers. As the coking proceeds, the volatile 
 by-products pass away through pipes to a hydraulic 
 main and condensers, where the ammoniacal liquor 
 and the tar are collected ; and the gas is then passed 
 to gas-holders whence it is drawn off as required 
 and used for heating the ovens. Since, with im- 
 proved construction, considerably more gas is pro- 
 
THE PRODUCTION OF COAL TAR 9 
 
 duced than is required for this purpose, the coke 
 ovens are now a valuable source of gas supply, the 
 excess gas being employed for heating, for the pro- 
 duction of power, and even for illuminating pur- 
 poses. Thus the city of Leeds, for example, takes 
 a million cubic feet per day of coke-oven gas 
 from the Middleton Estate and Colliery Company, 
 this gas being then " enriched " with carburetted 
 water-gas. 
 
 By the introduction of these ovens great economies 
 have been effected owing to an increase in the yield 
 of coke upwards of seventy per cent, of the weight 
 of coal being obtained as coke and to the recovery 
 of the very valuable by-products, ammonia and 
 coal tar. 
 
 Coal, it must be borne in mind, is not a definite 
 chemical substance, but a complex mixture of sub- 
 stances, the nature of which is not yet definitely 
 known, and doubtless varies considerably in the 
 case of the different kinds of coal. It will there- 
 fore readily be understood that the nature of the 
 products as well as their relative amounts depend 
 on the kind of coal distilled ; and they depend, 
 moreover, in a very marked degree on the general 
 conditions under which the distillation of the coal 
 is carried out for example, on the temperature, 
 size and shape of the retort, and on the time during 
 which the volatile products remain in contact with 
 
io THE TREASURES OF COAL TAR 
 
 the red-hot walls of the retort. As regards the 
 composition, the most important points of difference 
 are found in the nature of the so-called hydrocarbons 
 (compounds of carbon and hydrogen) present in 
 the tar. .When the distillation is carried out at a 
 low temperature (say about 450 C. or 840 F.), the 
 tar contains mainly hydrocarbons belonging to the 
 so-called aliphatic series (p. 38), suitable for use 
 as motor spirit, and as illuminants and lubricants 
 (vaseline). Tars of this description are produced, 
 for example, in the distillation of coal for the produc- 
 tion of coalite, in the manufacture of Mond gas, and 
 in blast furnaces (mainly in Scotland) where raw 
 coal is used in place of coke. When, however, the 
 distillation is carried out at a high temperature (say 
 about 1000 C. or 1830 F.), as is the case when 
 coal is distilled for the production of illuminating 
 gas or of the hard coke used for smelting and other 
 metallurgical purposes, the prevailing hydrocarbons 
 are those belonging to the " aromatic " class (p. 40), 
 e.g. benzene and its derivatives. It is this kind of 
 coal tar which is of such importance as furnishing 
 the raw materials for the production of the innumer- 
 able dyes, drugs, perfumes, explosives, etc., the pro- 
 duction of which now constitutes such an imposing 
 and valuable industry. 
 
 Although the nature of the tar constituents, as 
 well as their relative amounts, depends on the con- 
 ditions under which the distillation is carried out, 
 
THE PRODUCTION OF COAL TAR n 
 
 we may say that under the general conditions met 
 with in gas and coking works, one ton of dry coal 
 will yield, in addition to 11,000-12,000 cubic feet 
 of gas, 20-35 Ibs. of sulphate of ammonia, 56-120 Ibs. 
 of coal tar, and 1400-1800 Ibs. of coke. 
 
 Owing, more especially, to the demand for metal- 
 lurgical coke, the annual production of coal tar has 
 now assumed very large dimensions. This country 
 has always occupied a foremost place in the gas- 
 producing industry, and she was also, for long, the 
 premier producer of coal tar. But it is probable 
 that by 1913 she had already lost to Germany her 
 position of pre-eminence in the tar-producing in- 
 dustry owing to the great development in that 
 country of the iron and steel industry, and the 
 consequent demand for coke. Moreover, owing to 
 the magnitude of the German chemical industries, 
 most of the coal tar formed in the production of 
 this coke was recovered, and Germany thereby 
 made herself largely independent of this country 
 for the supply of coal tar, of which, even in 
 1908, she imported from Great Britain 40,000 tons. 
 Although, up to the outbreak of war, Germany 
 imported from this country considerable amounts 
 of anthracene and of phenol (carbolic acid), the 
 development of the synthetic production of the 
 latter compound from benzene will doubtless 
 render its continued importation unnecessary. 
 
12 THE TREASURES OF COAL TAR 
 
 In 1901 the approximate annual production of 
 coal tar throughout the world has been estimated 
 as follows : 
 
 United Kingdom . . 908,000 tons. 
 
 Germany . . . 590,000 ,, 
 
 United States . . . 272,400 ,, 
 
 France .... 190,680 ,, 
 Belgium, Holland, Sweden, 
 
 and other European 
 
 countries . . . 199,760 ,, 
 
 All other countries . . 227,000 ,, 
 
 2,660,440 
 
 In the following years of the decade the amounts 
 largely increased, as shown by the following approxi- 
 mate figures : 
 
 United Kingdom (1910) 1,380,000 tons. 
 
 Germany (1912) . . 1,082,197 ,, 
 
 United States (1912) . 564,000 ,, 
 
 France (1909) . . 214,800 
 
CHAPTER II 
 
 THE DISTILLATION OF COAL TAR 
 
 CRUDE coal tar, as it is obtained from gas and coking 
 works, although it may vary not a little according 
 to its origin, is a thick, oily, dark-coloured liquid 
 rather heavier than water (specific gravity about 
 1-2). In the early days of the coal-distilling in- 
 dustry this tar was, as has been said, a disagreeable 
 waste product, the disposal of which was the source 
 of much worry and annoyance to the producer no 
 less than to the general public in the neighbourhood. 
 As it was impossible, by reason of the nature of the 
 material, to get rid of the accumulations of tar by 
 running it into streams and rivers, the difficulty 
 of its disposal was solved, to some extent, by 
 burning the tar as a fuel. A certain amelioration 
 was brought about by the use of coal tar as a paint 
 for wood and metal work, and for this purpose the 
 more volatile portions were removed by distilla- 
 tion, the " spirit " so obtained being used either 
 as a substitute for turpentine in making varnishes 
 or as a solvent for rubber in the manufacture of 
 a waterproof material which is still known by 
 the name of the original Glasgow manufacturer, 
 
 13 
 
14 THE TREASURES OF COAL TAR 
 
 Mackintosh. Much of the residue from the dis- 
 tillation was burned for the production of lamp- 
 black, which is used in the manufacture of pigments, 
 blacking, and printer's ink. 
 
 Hitherto, the distillation of coal tar had been 
 carried out only on a comparatively small scale, 
 and the demand for tar lagged far behind the 
 supply, until, in 1838, an entirely new situation 
 was created through the introduction of a process 
 for preserving or " pickling " timber (p. 27) ; and 
 an industry which has now attained to enormous 
 proportions was thereby inaugurated. Moreover, 
 in the year 1845 another great stimulus was given 
 to the coal-tar industry owing to the scientific in- 
 vestigations which were carried out by Professor 
 Hofmann and his students at the newly-founded 
 Royal College of Chemistry in London, investiga- 
 tions which not only led to the isolation from coal 
 tar of some of its main constituents, but were the 
 roots from which the vast modern industry of coal- 
 tar dyes, drugs, and explosives has really grown. 
 Owing to these developments, which we shall dis- 
 cuss more fully in the sequel, a demand was created 
 for the more volatile portions of coal tar which 
 had been rejected by the timber-pickling industry ; 
 and a more complete utilisation of the constituents 
 of the tar was thereby made possible. 
 
 Although, in the past, crude coal tar was largely 
 employed not only as a liquid fuel but also for the 
 
THE DISTILLATION OF COAL TAR 15 
 
 manufacture of roofing felt, the tarring of roads, 
 and other purposes, the water and ammoniacal 
 liquor present in the tar were found to be detri- 
 mental, so that now only a small amount of tar is 
 used in the crude state, except in those cases where 
 it is employed as a fuel. By far the greater pro- 
 portion of the coal tar is now subjected to a process 
 of distillation, a process first carried out systematic- 
 ally by Charles Blachford Mansfield 1 in 1848. 
 
 Coal tar, even after being freed from the water 
 and ammonia with which, when it is received from 
 the gas and coke works, it is intimately mixed, is 
 not a single substance, but an exceedingly complex 
 mixture of over two hundred different compounds, 
 some of which are present, however, only in very 
 minute amount. Although the complete separa- 
 tion and isolation of all these different substances 
 is a matter of the greatest difficulty, and is not 
 attempted in practice, it is possible, by subjecting 
 the tar to a process of distillation, to separate it 
 into a number of portions or " fractions " which 
 distil over at different temperatures. The general 
 principle on which the apparatus employed is con- 
 
 1 Mansfield was a pupil of Hofmann, and, under his direction, 
 was the first to separate coal-tar naphtha into its constituents by 
 fractional distillation. Unfortunately, in 1856, while carrying 
 out this work, the contents of the still boiled over and caught 
 fire. While endeavouring to extinguish the flames Mansfield 
 was so severely burned that death supervened in a few days. 
 
16 THE TREASURES OF COAL TAR 
 
 structed is illustrated by Fig. 3. Here, A is a vessel 
 or " still " in which the liquid is boiled and so con- 
 verted into vapour which passes through the long 
 neck, B, to a spiral " worm " or condenser, C, kept 
 cool by means of flowing water. The condensed 
 vapour issues at D and can be collected in a " re- 
 ceiver/* E is a tube by which water enters and 
 F is the outlet for the warm condenser water. 
 
 FIG. 3. APPARATUS USED FOR DISTILLING LIQUIDS. 
 
 (Illustration from " Chemistry in the Service of Man.") 
 
 T is a thermometer to indicate the temperature 
 of the liquid in the still. In actual practice the still 
 consists of a large iron boiler capable of holding 
 twenty tons or more of tar set in brickwork and 
 heated by a fire. Since the first fractions which 
 distil over are rather volatile liquids at the ordinary 
 temperature, the condensing coil is cooled by means 
 of cold water ; but as the distillation proceeds the 
 substances which pass over solidify on cooling, 
 and so the condenser is kept warm by means of 
 
THE DISTILLATION OF COAL TAR 17 
 
 hot water in order to prevent the choking of 
 the coil. 
 
 By this process of distillation the tar is separated 
 into a number of portions. First of all, while the 
 temperature of the still gradually rises to 170 C. 
 (338 F.), there distils over a light inflammable 
 liquid known as "light oil." This is followed, 
 between the temperatures of 170 C. and 230 C. 
 (338 F. and 446 F.), by the " carbolic oils," so 
 called because they contain the main portion of the 
 carbolic acid present in the tar. At still higher 
 temperatures, between 230 C. and 270 C. (446 F. 
 and 518 F.), one obtains a complex mixture of 
 substances constituting the " creosote oils " ; and 
 lastly there pass over, between 270 C. and 400 C. 
 (518 F. and 752 F.), the "anthracene oils," the 
 most important constituent of which is the hydro- 
 carbon anthracene. After the different fractions 
 have passed over there remains in the still a residue 
 of pitch. By this process of distillation there are 
 obtained, from one ton of tar, approximately : 
 
 12 gallons of light oils. 
 20 ,, carbolic oils. 
 17 creosote oils. 
 38 anthracene oils. 
 ii hundredweight of pitch. 
 
 The crude coal tar having in this way been sepa- 
 rated into a number of different portions, each of 
 
18 THE TREASURES OF COAL TAR 
 
 these is then subjected to suitable chemical treat- 
 ment and to repeated distillation in order to 
 effect a more complete purification and separation 
 into the different constituents. Thus the light oils 
 are separated into " crude benzol," consisting of 
 a mixture of the hydrocarbons, benzene, toluene, 
 and xylene, " solvent naphtha " and " burning 
 naphtha," consisting of xylene and similar but more 
 complex hydrocarbons. To obtain pure benzene 
 and toluene, such as are required in the manu- 
 facture of dyes, drugs, and explosives, the crude 
 benzol is " rectified " by distillation in a special 
 still. 1 
 
 In order to avoid confusion it may be stated 
 that the hydrocarbons now known to British 
 chemists as benzene (not to be confounded with 
 benzine or benzoline) and toluene, were formerly 
 called benzol (or benzole) and toluol, and these 
 names are still employed commercially. The term 
 benzol, however, is also applied in commerce to 
 various mixtures of hydrocarbons, different grades 
 
 1 It may be mentioned that although benzene and toluene 
 were formerly obtained solely from coal tar, considerable 
 quantities of these compounds are now obtained from coke- 
 oven gas by " scrubbing " it with creosote oil. Indeed, this is 
 now a more important source of crude benzol than coal tar. 
 Since the removal of these hydrocarbons diminishes both the 
 illuminating power and the calorific value of the gas, the above 
 process is not applied in large measure to ordinary coal gas, 
 although, at the present day, owing to the exigencies of war, 
 considerable quantities of these valuable compounds are obtained 
 from this source. 
 
THE DISTILLATION OF COAL TAR 19 
 
 of " benzol " being produced for use in the arts. 
 Thus we have the various grades known as 90 per 
 cent., 50 per cent., and 30 per cent, benzol, these 
 terms being applied to liquids of which 90, 50, or 
 30 per cent, distils over at temperatures up to the 
 boiling-point of water (100 C. or 212 F.). The 
 composition of these three grades of commercial 
 benzol is shown in the following table : 
 
 90 per cent, 
 benzol. 
 
 50 per cent, 
 benzol. 
 
 30 per cent, 
 benzol. 
 
 Benzene 
 
 80-9 
 
 45'4 
 
 13-5 
 
 Toluene 
 
 14-9 
 
 40'3 
 
 73-4 
 
 Xylene 
 Impurities . 
 
 2'2 
 2-0 
 
 12-4 
 1-9 
 
 117 
 1-4 
 
 The portion of the tar distillate known as " car- 
 bolic oils " or " middle oils " is likewise separated 
 by chemical and physical treatment into its chief 
 constituents. On allowing it to cool down there 
 separates out from it a considerable quantity of a 
 hydrocarbon known as naphthalene, and the re- 
 sidual oil is then sold as " crude carbolic acid/' for 
 the manufacture of disinfectants. On gently heat- 
 ing the crude naphthalene it sublimes or passes into 
 vapour which, on cooling, solidifies in the form of large 
 crystalline flakes. In this way it is purified. Much 
 of the crude carbolic acid also is refined by treat- 
 ment with alkali and acid and subsequent distilla- 
 tion. By this means pure carbolic acid or phenol, 
 as it is called by chemists, is obtained, together with 
 
20 THE TREASURES OF COAL TAR 
 
 a mixture of three similar compounds known as 
 cresylic acids or cresols. 
 
 The "creosote oils" or "heavy oils" consist 
 of a number of different compounds which, 
 however, are not separated from each other. 
 These oils are merely " fractionated " in accord- 
 ance with the specifications of the wood-pickling 
 industry. 
 
 From the " anthracene oils " there is obtained, 
 by suitable treatment, the important hydrocarbon 
 anthracene, which is used as the starting substance 
 in the manufacture of alizarin, and of other 
 important dyes. 
 
 Although the amounts of the different compounds 
 obtained vary with the nature of the tar and the 
 treatment to which it is subjected, the following 
 numbers will give a sufficiently exact idea of the 
 relative quantities of the most important con- 
 stituents yielded by one ton of tar : 
 
 Benzene and toluene . 25 Ibs. 
 
 Phenol . . . ii 
 
 Cresols . . 50 
 
 Naphthalene . . 180 
 
 Creosote . . . 200 
 
 Anthracene . . . 6 ,, 
 
 Benzene, first discovered by Faraday in 1825, is 
 a colourless, mobile liquid which boils at 80-5 C. 
 
THE DISTILLATION OF COAL TAR 21 
 
 (176-9 F.), and yields a readily inflammable vapour. 
 Toluene is a similar compound which boils at 111 C. 
 (231-8 F.). Phenol or carbolic acid is, in the pure 
 state, a white crystalline solid which melts at 41 C. 
 (105-8 F.). Owing to the very large amounts of 
 this compound used in the manufacture of dyes, 
 drugs, and explosives, the supply obtained from 
 coal tar is quite insufficient, under present con- 
 ditions, to meet the demand, so that phenol is now 
 manufactured in large amount from benzene. For 
 this purpose benzene is first treated with concen- 
 trated sulphuric acid, and the resulting product then 
 fused with caustic soda. Naphthalene is a white 
 crystalline solid which melts at 80 C. (176 F.), 
 and anthracene is also a white crystalline solid 
 which melts at 213 C. -(415 F.). 
 
CHAPTER III 
 
 THE CONSTITUENTS OF COAL TAR AND THEIR 
 APPLICATIONS IN THE RAW STATE 
 
 IN the later chapters of this book we shall discuss 
 some of the marvellous transformations which 
 chemists have effected in the constituents of coal 
 tar, transformations which are the basis of those 
 great chemical industries of synthetic dyes and 
 drugs whose development during the past half 
 century has so impressed the public mind. But it 
 must not be forgotten that there are other industries 
 dependent on the distillation products of coal tar ; 
 industries which if not, like the chemical ones, 
 suffused with romance, contribute in no small 
 measure to the welfare of man and together make up 
 a large part of the wealth derived from coal tar. 
 Indeed, it is probably to these industries which 
 depend on the use of the coal-tar products in the 
 raw state that the tar distiller mainly looks for the 
 maintenance of his profits, and a brief account of 
 them must not be omitted here. 
 
 Benzol and Naphtha 
 
 Although it is as a raw material in chemical 
 industry, in the manufacture of dyes, drugs, and 
 
THE CONSTITUENTS OF COAL TAR 23 
 
 explosives, that pure benzene and toluene find 
 their chief use, large quantities of the various grades 
 of commercial benzol are now employed as solvents 
 in the preparation of paints and varnishes, and for 
 other purposes. The use of benzol as a solvent 
 goes back, indeed, to the earliest days of coal-tar 
 distillation, although at that time it was the higher 
 boiling fractions which mainly found employment. 
 At the present day, however, the industrial applica- 
 tions of the lower boiling fractions, the higher 
 grades of commercial benzol, have attained a great 
 and increasing importance. By reason of its solvent 
 power, benzol is largely employed as a detergent 
 for the removal of grease, wax, and paint spots 
 (for which purpose it is frequently mixed with 
 alcohol and ammonia), and as a solvent for gums 
 and resins in the manufacture of varnishes and 
 lacs, as well as of enamel, bronze, and aluminium 
 paints, of which a natural gum or resin, such as 
 Damar gum, forms the base. Similarly, by reason 
 of its solvent power for resins, benzol is used in the 
 preparation of paints used in painting resinous 
 woods, the partial solution of the resin by the benzol 
 affording a better penetration or " tooth " to the 
 paint. Of great importance, also, is the use of 
 benzol in the preparation of rubber solutions for 
 use as cements and insulating varnishes, and as a 
 solvent for sulphur monochloride in the cold vulcan- 
 isation of rubber. In recent times benzol has been 
 
24 THE TREASURES OF COAL TAR 
 
 used in large amount, more especially in France 
 and Germany, as a motor fuel, and in the future it 
 will doubtless find, in this direction, a vastly more 
 widespread application. This cannot but exercise 
 a powerful influence on the whole coal-tar industry. 
 
 The higher boiling fractions of the " light oil," 
 the naphthas, find their chief applications as solvents 
 in the preparation of rubber waterproof material, 
 and as illuminants for use in large open spaces ; 
 and the flaring light of the naphtha lamp has cast 
 its beams for many years now over the wares on 
 the costermonger's barrow and the stalls and booths 
 of the open market-place. 
 
 In 1911 nearly 2,000,000 gallons of coal-tar 
 solvents were produced in the United States, and 
 were distributed among the different industries 
 approximately as follows (Weiss) : 
 
 Paint and varnish . . 47 per cent. 
 Rubber and rubber cements 18 
 Imitation leathers . .10 ,, 
 Chemical manufactures . n ,, 
 Miscellaneous . . 14 ,, 
 
 Similar details are, unfortunately, not available 
 with regard to the United Kingdom. 
 
 Carbolic Acid and Naphthalene 
 
 " Crude carbolic acid/' which is separated, as 
 we have already seen, from the " middle oils " 
 
THE CONSTITUENTS OF COAL TAR 25 
 
 obtained in the distillation of coal tar, consists for 
 the most part of various " tar acids," more especially 
 carbolic acid and cresylic acid, the latter being a 
 mixture of three isomeric compounds (see p. 44), 
 known as cresols. Although the pure compounds, 
 more especially pure carbolic acid or phenol (to 
 give it its systematic name), are used to a large 
 extent in chemical industry, they also find a very 
 extensive application in the raw state as antiseptics 
 and disinfectants. As such the cresols are more 
 powerful, and at the same time less poisonous, than 
 the more familiar carbolic acid or phenol. 
 
 Owing to a more widespread knowledge of the 
 causes of disease and to greater efforts made in the 
 promotion of hygiene, the demand for antiseptics 
 and disinfectants has greatly increased during 
 the past two or three decades, and as a con- 
 sequence we now find on the market very many 
 disinfectant preparations of which carbolic and 
 cresylic acids form the basis. Since these acids 
 are not very soluble, the preparation of concen- 
 trated disinfectants which would mix completely 
 with water presented some difficulty ; but this 
 difficulty was overcome by the addition of a certain 
 amount of soft soap (whereby the tar oils present 
 are emulsified), and a large number of disinfecting 
 fluids are now prepared on this general principle. 
 For this purpose use is made not only of the carbolic 
 and cresylic acids obtained from the carbolic oils, 
 
26 THE TREASURES OF COAL TAR 
 
 but also of the lower fractions separated from the 
 creosote oils which are specially rich in cresols and 
 other similar compounds. Thus the well-known 
 liquid lysol consists essentially of a mixture of 
 cresols (about 50 per cent.) with a potash soap 
 (about 20 per cent.) prepared from linseed oil, and 
 a certain amount of glycerin. The cresols also 
 form the essential constituent of Jeyes' Fluid, 
 Cresolin, and other disinfectants. 
 
 Phenol and cresol may also be incorporated in 
 ordinary hard soaps or mixed with various other 
 solid materials ; and many disinfectants of this 
 nature, more or less efficient, are now sold under 
 different names. 
 
 Naphthalene, apart from its important uses in 
 chemical industry, is now employed mainly as a 
 disinfectant and as a preservative against the 
 attack of moths and other insects. 
 
 Creosote 
 
 That tar and pitch are valuable preservatives 
 for wood has long been known, tar having been 
 used for this purpose even in the days of ancient 
 Greek civilisation. But it is only in comparatively 
 recent times that the antiseptic and preservative 
 properties of tar have been applied on an extensive 
 scale. Various antiseptics, such as corrosive sub- 
 limate and copper sulphate, were already in use 
 for the preservation of timber and its protection 
 
THE CONSTITUENTS OF COAL TAR 27 
 
 against the attack of dry-rot and other fungoid 
 growths, but the use of coal tar on a large scale 
 dates only from the introduction of the timber- 
 pickling process by John Bethell in 1838. This 
 industry soon experienced a very rapid develop- 
 ment owing to the growth more especially of railway 
 and telegraph systems throughout this country 
 and the world. As a preservative for wood which 
 is buried in the ground or submerged in water, 
 as a protective even against the formidable Teredo 
 navalis and other marine organisms, coal-tar creosote 
 has been found superior to all other materials. 
 
 In carrying out the " pickling " or " creosoting " 
 of wood, the latter is placed in a large cylindrical 
 boiler and the air is then very thoroughly exhausted 
 by means of a pump. In this way the air is with- 
 drawn from the pores of the wood. Creosote, 
 heated to a temperature of about 100 C. (212 F.), 
 is then allowed to flow into the boiler, the process 
 of exhaustion being still maintained for some time 
 in order that, at the higher temperature, the moisture 
 in the wood may also be withdrawn. On now 
 admitting air into the boiler, the creosote is in- 
 jected into the cells of the timber, and the process 
 of injection is completed by means of a force pump, 
 the pressure within the boiler being raised to eight 
 or ten atmospheres. In other processes the timber is 
 impregnated with creosote under increased pressure, 
 and then maintained for some time under greatly re- 
 
28 THE TREASURES OF COAL TAR 
 
 duced pressure. Under this treatment a cubic foot 
 of wood absorbs about one gallon of creosote oil. 
 
 When one thinks of the countless rows of wooden 
 sleepers which mark out the railway tracks in the 
 different countries of the world, of the never-ending 
 lines of telegraph poles which cany their network 
 of wires across whole continents, or of the wooden 
 piles and wharves exposed to the waters of every 
 ocean, one will understand, in some measure, how 
 important this creosote industry has become. Since 
 by its treatment with tar oil the life of the wood is 
 trebled or quadrupled, it will readily be realised 
 not only that there is an enormous saving effected 
 in the cost of upkeep of railway sleepers, telegraph 
 poles, wooden wharves, etc., but that there is also 
 a consequent great reduction in the consumption 
 of timber a matter of increasing importance in 
 these days when the reserves of timber throughout 
 the world are being rapidly depleted. In this 
 country upwards of 50,000,000 gallons of creosote 
 are produced annually, and most of this is used for 
 the treatment of timber. In 1913, over 36,000,000 
 gallons of creosote, having a value of 592,000, 
 were exported, mainly to the United States, where, 
 owing to the enormous extent of the railway system, 
 the demand for creosote oil is much greater than 
 the supply. 1 It was this wood-pickling industry 
 
 1 " In 1913 the United States consumed, for timber preserva- 
 tion, over 90,000,000 gallons of creosote oil, and of this, 62 per 
 
THE CONSTITUENTS OF COAL TAR 29 
 
 which " saved the situation " in the early days of 
 coal-tar production, and it forms at the present day 
 by far the most important outlet for the coal-tar oils. 
 
 The antiseptic and disinfecting properties of 
 creosote oil, which are not entirely due to the 
 presence of carbolic, cresylic, and other tar acids, 
 have also led to the extensive use of creosote in the 
 preparation of cattle washes, sheep dips, and general 
 disinfectants. In this case the oil is generally 
 mixed with a quantity of soft soap, whereby, owing 
 to the emulsifying action of the soap, a very fine 
 emulsion can be obtained with water. 
 
 Creosote oil, suitably fractionated by distillation, 
 also finds application as a liquid fuel for internal 
 combustion (Diesel) engines, as an illuminant (in 
 the " Lucigen " lamp), in the production of lamp- 
 blafck, for softening hard pitch, and for " scrubbing " 
 coal and coke-oven gas for the recovery of benzene 
 and toluene. 
 
 Refined Tars and Pitch 
 
 In recent years owing largely to improved methods 
 of road construction and to the desirability, in view 
 of the great increase of motor traffic, of obtaining 
 dust-free roads, an increased demand for refined 
 tars and pitch has sprung up. For some time, it 
 
 cent, was imported from Europe. Between 60 and 70 per cent, 
 of the total quantity of oil consumed was used for the treatment 
 of railway ties, some 25,000,000 being thus treated " (E. Stans- 
 field and F. E. Carter : Report to Department of Mines, Canada) . 
 
30 THE TREASURES OF COAL TAR 
 
 is true, there existed considerable prejudice against 
 the practice of sprinkling the roads with tar owing 
 to the supposed harmful effects of the tar on sur- 
 rounding vegetation and the irritating action of 
 the dust from such roads on the eyes. But the 
 fear of harmful effects has been shown to be with- 
 out real foundation, and the tar-sprinkling of many 
 of our main thoroughfares has proved a great boon. 
 The tar used for this purpose must be fractionated 
 so as to satisfy the requirements of the Road Board, 
 the specification of which lays it down that : " The 
 tar shall be free from water, and on distillation shall 
 yield no distillate below 140 C. (284 F.), nor more 
 than 5 per cent, of distillate up to 220 C. (428 F.), 
 which distillate shall remain clear and free from 
 solid matter (crystals of naphthalene, etc.), when 
 maintained at a temperature of 30 C. (86 F.), for 
 half an hour. Between 140 and 300 C. (284 and 
 572 F.) it shall yield not less than 15 per cent, 
 nor more than 21 per cent, of the weight of the 
 tar." 
 
 The various grades, also, of hard and soft pitch, 
 obtained as residues from the distillation of coal 
 tar, have found valuable applications as road- 
 binding material, in the preparation of tar-mac, 
 for filling the joints between paving stones, for 
 the manufacture of roofing felt, for making coal 
 briquettes, for electrical insulation and for other 
 important purposes. 
 
CHAPTER IV 
 
 MOLECULAR ARCHITECTURE 
 
 IN the preceding chapters we have seen how from 
 the black, unsavoury liquid, coal tar, various sub- 
 stances have been isolated and have found import- 
 ant applications in the general structure of our 
 modern civilisation. But the benzene, the toluene, 
 and the other materials to which reference has 
 already been made exist as such in the tar, and 
 their separation from this liquid and their applica- 
 tions, important as they are, are not such as to 
 make any powerful appeal to the intellect or arouse 
 a feeling of wonder in the mind. Far otherwise is 
 it, however, with those marvellous transforma- 
 tions which have been brought about in these sub- 
 stances by chemists ; transformations which have 
 produced from some eight or nine colourless liquids 
 or solids, dyes in infinite variety which rival Nature's 
 products in range of colour and delicacy of tone ; 
 drugs and anaesthetics which purge the blood of 
 its evil humours and give relief from pain ; the 
 sweet-smelling essences of flowers ; and explosives 
 which give power and strength to the arm of man 
 in peace as well as in war. These are triumphs 
 
 31 
 
32 THE TREASURES OF COAL TAR 
 
 of the human intellect and as such command the 
 admiration and wonder of thinking men. Not 
 only has the chemist prepared numberless com- 
 pounds hitherto unknown, but he has entered into 
 competition with Nature herself and has success- 
 fully broken the monopoly which heretofore she 
 had enjoyed in the production of many important 
 compounds. So successful, indeed, has the chemist 
 been, that these artificial products have, in some 
 cases at least, driven the natural products entirely 
 out of the market. But this rivalry with Nature, 
 the task of building up or synthesising numerous 
 highly complex compounds from the simple materials 
 contained in coal tar, would have been hopeless 
 without the aid of some guiding principle. It is 
 necessary, therefore, before passing to the discussion 
 of the substances which have been evolved by 
 chemists from the constituents of coal tar, to give 
 a short account of the theory of molecular structure 
 by which chemists have directed their labours. 
 The understanding of the processes by which these 
 compounds are produced will thereby be facilitated. 
 
 For convenience in representing chemical elements 
 and compounds, the Swedish chemist Berzelius 
 introduced, a century ago, a system of symbols, 
 each of which consists of one or two letters and 
 represents one atom of the particular element. 
 Thus, C, H, O, N, for example, represent one atom 
 
MOLECULAR ARCHITECTURE 33 
 
 or smallest particle of the elements carbon, hydro- 
 gen, oxygen, and nitrogen respectively. But a com- 
 pound can be regarded as being formed by the 
 combination or uniting of the atoms of the con- 
 stituent elements in certain definite proportions, 
 , and so we can conveniently represent the molecule, 
 or smallest particle of a compound, by writing the 
 symbols of the constituent elements side by side. 
 Thus, CO represents a compound of carbon and 
 oxygen, the molecule of which contains one atom 
 of carbon and one atom of oxygen ; and NO, 
 similarly, represents a compound of nitrogen and 
 oxygen. Frequently, however, the molecule of a 
 compound is formed by the combination of elements 
 in more than one atomic proportion, and so we 
 write, for example, H 2 O, which is the formula, as 
 it is called, for water. This formula indicates that 
 the molecule of water contains two atoms of hydro- 
 gen and one atom of oxygen. The formula NH 3 , 
 similarly, which is the formula for ammonia, indi- 
 cates that the molecule of this compound contains 
 three atoms of hydrogen united with one atom of 
 nitrogen. 
 
 It might, perhaps, be thought that an infinite 
 number of compounds could be formed by the 
 combination of the atoms of two elements in differ- 
 ent proportions, e.g. HO, H 2 O, H 3 O, etc. But 
 although no a priori reason can be given against 
 the possibility, it has been found that, as a matter 
 
34 THE TREASURES OF COAL TAR 
 
 of fact, elementary atoms do not possess this un- 
 limited power of combination ; and the recognition 
 of this important fact is embodied in the doctrine 
 of valency, a doctrine which we owe to the late Sir 
 Edward Frankland. As no element is known which 
 has a lower combining power than hydrogen, this 
 element is taken as the standard of reference and is 
 said to have unit combining power or unit valency, 
 or to be univalent. Oxygen, one atom of which 
 can combine with two atoms of hydrogen (as in 
 water, H 2 O), is said to be bivalent, and carbon, 
 one atom of which can combine with four atoms 
 of hydrogen, is said to be quadrivalent. Since an 
 atom of carbon is never found to combine with more 
 than four atoms of hydrogen, the carbon is, in this 
 case, said to be saturated ; and the compound 
 CH 4 , which represents methane or marsh gas, is 
 spoken of as a saturated hydrocarbon. 
 
 Although there is, of course, no material link or 
 bond between the atoms, we can, nevertheless, re- 
 present union between atoms as if it were material, 
 by means of a line or lines, according to the valency 
 of the atom. Thus we can represent the molecule 
 of methane by the diagrammatic or graphic formula, 
 
 H 
 H C H 
 
 But the element carbon is remarkable among all the 
 
MOLECULAR ARCHITECTURE 35 
 
 elements in its property of combining also with other 
 atoms of carbon and so forming " chains " of carbon 
 atoms ; and we therefore obtain a series of com- 
 pounds which may be represented by the diagrams : 
 
 H H H 
 
 H C C C H ; etc. 
 
 Ill 
 H H H 
 
 C 3 H 8 (Propane) 
 
 This series of hydrocarbons is generally known 
 as the methane series, and to it belong gasoline, 
 petrol, vaseline, and paraffin wax. 
 
 There are also other hydrocarbons which contain 
 a lower proportion of hydiogen and are therefore 
 said to be unsaturated. Thus, if we take away two 
 hydrogen atoms from each of the compounds of 
 the methane series, we obtain hydrocarbons which 
 can be represented by the formulae : 
 
 H H 
 
 H\ /H H\ | | 
 
 >C = C< ; >C = C C H ; etc. 
 H/ \H H/ | 
 
 H 
 
 (Ethylene) C 3 H 6 (Propylene) 
 
 These constitute another series of hydrocarbons 
 known by the name of the first member, ethylene. 
 
 These diagrammatic formulae, it should be em- 
 phasised, are not intended to represent the spatial 
 arrangement of the atoms ; there is, indeed, reason 
 to believe that these " chains " of carbon atoms 
 would form a spiral in space. These formulae, 
 
36 THE TREASURES OF COAL TAR 
 
 rather, are intended merely to indicate that in the 
 molecule of a compound the constituent atoms are 
 not present in disordered array but are associated 
 in some definite manner, certain atoms being 
 attached, as it were, to certain other atoms, although 
 by no material bond or connection, just as a satellite 
 may be said to be attached to a planet. We can, 
 therefore, also write the above formulae in a some- 
 what more compact form, and represent propane, 
 for example, by the formula CH 3 -CH 2 -CH 3 (the 
 " bond " between the carbon atoms being now 
 represented by a dot), and propylene by 
 
 CH 2 : CH-CH 3 . 
 
 This theory of chemical structure, which depends, 
 as we see, on the recognition of the quadrivalency of 
 the carbon atom, is due to August von Kekule, and 
 was put forward by him in 1858. The origin of 
 the theory has been recounted by Kekule himself. 
 During a period of residence in London he was 
 returning from a visit paid at Islington to where 
 he stayed at Clapham. " One fine summer even- 
 ing," he relates, " I was returning by the last 
 omnibus, ' outside ' as usual, through the deserted 
 streets of the metropolis, which are at other times 
 so full of life. I fell into a reverie, and lo ! the 
 atoms were gambolling before my eyes ! When- 
 ever, hitherto, these diminutive beings had appeared 
 to me, they had always been in motion ; but up 
 
MOLECULAR ARCHITECTURE 37 
 
 to that time I had never been able to discern the 
 nature of their motion. Now, however, I saw how, 
 frequently, two smaller atoms united to form a 
 pair ; how a larger one embraced two smaller ones ; 
 how still larger ones kept hold of three or even four 
 of the smaller ; whilst the whole kept whirling in 
 a giddy dance. I saw how the larger ones formed 
 a chain," . . . And then he adds : "I spent part 
 of the night in putting on paper at least sketches 
 of these dream-forms." From these sketches were 
 developed the structural formulae of which ex- 
 amples have just been given. 
 
 The saturated hydrocarbons, methane, ethane, 
 etc., may be regarded as the parents of a countless 
 brood of other compounds derived from them by 
 the substitution or replacement, direct or indirect, 
 of one or more hydrogen atoms by the atoms of 
 other elements or by groups of elements (" radicles ") 
 which pass from compound to compound like single 
 atoms. Thus, by substituting one atom of hydrogen 
 in the saturated hydrocarbons by an atom of iodine, 
 we get a series of iodides, CH 3 -I, C 2 H 5 -I, C 3 H 7 -I, 
 etc. ; or by substituting one atom of hydrogen 
 by the group OH (hydroxyl), we obtain a series 
 of compounds known as alcohols (" alcohol " in 
 chemistry is a generic name), thus, CH 3 -OH, methyl 
 alcohol or " wood-spirit " ; C 2 H 5 -OH, ethyl alcohol 
 or " spirits of wine " ; and so on, the groups of 
 atoms CH 3 , C 2 H 5 , being known as methyl and ethyl. 
 
38 THE TREASURES OF COAL TAR 
 
 It will readily be understood from this that the 
 possible number of compounds is exceedingly large, 
 and, for this reason, the study of the compounds 
 of carbon the number of which at the present 
 day exceeds 150,000 has developed into a special 
 branch of chemistry known as organic chemistry. 
 
 Since the hydrocarbons of the methane series 
 are formed of " chains " of carbon atoms, so also 
 are the compounds derived from them ; and since 
 the natural animal and vegetable fats and oils are 
 amongst these compounds, the term " fatty " or 
 " aliphatic " (&Xet<pa,p = fat) has been applied to 
 the whole group or class of compounds. 
 
 In studying the carbon compounds we meet with 
 a phenomenon which, although not unknown in the 
 compounds of the other elements, is found with 
 extraordinary frequency amongst the former. This 
 is the phenomenon to which the name of isomerism 
 has been given. 
 
 One of the fundamental laws of chemistry states 
 that the composition of a compound that is, the 
 nature and number of the atoms present in the 
 molecule is constant and definite (Law of Constant 
 Proportions), and for long it was believed that 
 the converse statement also was true, namely, 
 that only a single compound could exist correspond- 
 ing with a particular composition. As the number 
 of compounds became multiplied, it began to be 
 
MOLECULAR ARCHITECTURE 39 
 
 observed more and more frequently that the same 
 elements might be united in the same proportions 
 and yet yield compounds with entirely different 
 properties. It is to this phenomenon that the term 
 isomerism is applied. Just as the same set of bricks 
 can, by varying their arrangement, be formed into 
 structures of totally different kinds, so also the same 
 atoms can, by varying their arrangement within 
 the molecule, give rise to different atomic structures, 
 or different compounds. We are led, therefore, 
 to the recognition of the fact that the properties of 
 a compound depend not merely on its composition, 
 but also on its internal structure, or the arrange- 
 ment of the atoms within the molecule. A know- 
 ledge of this atomic arrangement or constitution of 
 the molecule is of the highest importance, and is, 
 indeed, essential for the successful building up or 
 synthesis of a compound from simpler materials, 
 such as we shall discuss in the following chapters. 
 It is because the theory of Kekule enables one to 
 represent molecular constitution and to foresee the' 
 possible existence of isomeric compounds that it 
 has exercised such an important influence on the 
 development of organic chemistry. Thus, if we 
 have the compound CH 3 -CH 2 -CH 3 , it is clear that 
 we can replace one atom of hydrogen in this com- 
 pound by an atom, say, of iodine, in two ways, 
 so that we should obtain either the compound 
 CH 3 -CH 2 -CH 2 I, or the compound CH 3 -CHI-CH 3 , 
 
40 THE TREASURES OF COAL TAR 
 
 the iodine being attached, in the former case, to 
 a terminal carbon atom, and in the latter case to 
 the intermediate carbon atom. Accordingly, there 
 should exist two and only two different compounds 
 having the composition C 3 H 7 I ; and as a matter of 
 fact two compounds and only two are known. 
 
 Although a number of hydrocarbons belonging 
 to the methane series are found in the tar which 
 is produced by distilling coal at a low temperature, 
 they are not met with in the ordinary gas or coke- 
 oven tar, and they are of only secondary importance 
 in the manufacture of coal-tar dyes and other pro- 
 ducts. The most important compounds occurring 
 in gas and coke-oven tar are, as we have seen, 
 benzene, toluene, xylene, phenol, cresol, naphthalene, 
 and anthracene, these being the compounds from 
 which, for the most part, the endless array of dyes 
 and other coal-tar products has been derived. 
 These compounds, however, belong to quite a 
 different class from those already described ; they 
 possess a totally different constitution, and belong, 
 as it were, to a different type of molecular 
 architecture. From the fact that many of the 
 compounds which occur naturally and belong to 
 this group possess a distinct odour or " aroma," 
 the term " aromatic " has been given to the com- 
 pounds belonging to this division of organic 
 chemistry. 
 
MOLECULAR ARCHITECTURE 41 
 
 Just as we have seen that methane may be re- 
 garded as the first parent of the compounds be- 
 longing to the aliphatic group, so benzene (C 6 H 6 ) 
 may be called the parent of the aromatic compounds ; 
 and it is to Kekule also that we owe the elucidation 
 of the structure of this important hydrocarbon. 
 Again Kekule had a dream. He was now (1865) 
 in Ghent and dozed before the fire. Again he saw 
 the atoms gambolling before his eyes, the chains 
 twining and twisting in snake-like motion. " But 
 look ! What was that ? One of the snakes had 
 seized hold of its own tail, and the form whirled 
 mockingly before my eyes. As if by a flash of 
 lightning I awoke " ; . . . but the picture Kekule 
 had seen of the snake which had seized hold of its 
 own tail gave him the clue to one of the most puzzling 
 molecular structures, the structure of the benzene 
 molecule, a ring of six carbon atoms to each of which 
 a hydrogen atom is attached. Thus we obtain the 
 structural formula of the benzene molecule : 
 
 H 
 
 HC C-H 
 
 II 
 C-H 
 
 HC 
 
 H 
 
 the " ring " of carbon atoms being written in the 
 form of a hexagon instead of in the form of a circle. 
 Since this structure occurs very frequently in the 
 
42 THE TREASURES OF COAL TAR 
 
 formulae of coal-tar products, it is generally simpli- 
 fied to the skeleton form by omitting the symbols 
 for carbon and hydrogen. We thus obtain as the 
 diagrammatic representation of the benzene mole- 
 cule the simple hexagon : 
 
 From methane, as we saw, many other compounds 
 could be derived by replacing one or more atoms of 
 hydrogen by the atoms of other elements and by 
 radicles or groups of elements. So also from benzene 
 whole series of compounds can be similarly derived. 
 Thus, if we replace one atom of hydrogen by the 
 group or radicle CH 3 (methyl), we obtain the hydro- 
 carbon toluene, the formula of which, C 6 H 5 -CH 3 , 
 will be represented by the diagram : 
 
 CH 3 
 
 Similarly phenol or carbolic acid is derived from 
 benzene by the replacement of one atom of hydro- 
 gen by the group OH (hydroxyl), and so we obtain 
 
 the formula C 6 H 5 -OH or 
 
 OH 
 
MOLECULAR ARCHITECTURE 
 
 43 
 
 In the case of naphthalene, which is a hydrocarbon 
 having the formula C 10 H 8 , we have two benzene 
 " rings " joined together thus : 
 
 H H 
 
 ^X/ 6 
 H-C C 
 
 \H 
 
 or, more simply, 
 H-C C C-H 
 
 H H 
 
 whereas, in the case of anthracene, C 14 H 10 , we have 
 three " rings " : 
 
 H H H 
 
 x^ CN 
 H-C C 
 
 H 
 
 H 
 
 \H 
 C-H 
 
 H 
 
 In the case of the aliphatic compounds we saw how, 
 according to the theory of Kekule, the existence of 
 isomeric compounds could be foreseen and explained. 
 In the case of the compounds derived from benzene, 
 similarly, isomerism can occur, but this isomerism 
 is found only when more than one atom of hydrogen 
 is substituted or replaced. Thus, for example, 
 when one atom of hydrogen is substituted by the 
 methyl group, CH 3 , and another by the hydroxyl 
 
44 THE TREASURES OF COAL TAR 
 
 group, OH, we can obtain three different arrange- 
 ments, represented by the formulas : 
 
 CH 3 
 
 'OH 
 
 OH 
 
 and these three different arrangements correspond 
 with three distinct isomeric compounds known 
 respectively as ortho-cresol, meta-cresol, and para- 
 cresol, the terms ortho-, meta-, and para- referring 
 to the relative positions of the two substituting 
 groups. These three isomeric cresols occur, as we 
 have seen, in coal tar, and have powerful antiseptic 
 properties. Ortho-cresol melts at 31 C. (87-8 F.), 
 meta-cresol at 5 C. (41 F.), and para-cresol at 
 36 C. (96-8 F.). 
 
 It may seem, perhaps, to some that whatever 
 psychological or speculative interest the dreams 
 and theories of Kekule might possess, they could 
 have no importance for the practical life of the 
 people. And yet it is just these theories which 
 form the very basis and fundament of those great 
 chemical industries which command the wonder 
 and respect of all. For the advance and develop- 
 ment of organic chemistry,- described by Wohler as 
 a " tropical forest primeval, full of the strangest 
 growths, an endless and pathless thicket, in which 
 
MOLECULAR ARCHITECTURE 45 
 
 a man may well dread to wander," the theories 
 of molecular structure were as important as is a 
 map to a traveller in an unknown land. Without 
 them we could not have witnessed what is, perhaps, 
 the crowning achievement of organic chemistry, 
 the synthesis of many of Nature's own products 
 as well as of the innumerable dyes, therapeutic 
 agents, and other materials which are regarded as 
 necessaries in our modern civilisation and in respect 
 of which this country is now endeavouring to make 
 herself independent of outside supplies. It was, 
 indeed, largely if not mainly owing to her neglect 
 of pure science and of scientific theory, and owing 
 to the fact that " the English manufacturer has 
 considered that a knowledge of the benzol market 
 was of greater importance than a knowledge of the 
 benzol theory," that this country lost the pre- 
 eminence in the coal-tar industry which in the early 
 days, when that industry was controlled by 
 chemists (like Perkin and Nicholson), she so fully 
 enjoyed. 
 
 If there are any among the readers of this book 
 who feel some dismay at the aspect of the formulae 
 which have been introduced in this chapter and which 
 will be employed more frequently in the sequel, 
 I would ask them to believe that if they will but 
 make the slightest effort they will find nothing of 
 which to be afraid, especially if they will bear in 
 
46 THE TREASURES OF COAL TAR 
 
 mind that these formulas need not be memorised. 
 No reader of ordinary intelligence will refrain from 
 reading a work on architecture because of the 
 drawings of pillar capitals, or of arches, or of the 
 plans of buildings which accompany the text. On 
 the contrary, without these drawings, how could the 
 reader form a true mental picture of even a simple 
 structure or understand the mutual relationships 
 of its parts ? So is it with the formulae which we 
 shall employ here. These formulae are the plans, 
 so to say, of molecular structures by which the read- 
 ing and understanding of the text may be made 
 more easy. By looking at these diagrams, these 
 formulae, one sees at a glance how the different 
 compounds are related ; how, for example, from 
 benzene, the parent hydrocarbon, there have been 
 evolved numerous other compounds of much more 
 complex structure. We may, indeed, regard the 
 hexagon, the diagram which we have used to re- 
 present the structure of the benzene molecule, as 
 representing, so to say, a simple house to which 
 succeeding owners may add at their pleasure one 
 a bow window, another a turret, a third an additional 
 room, and so on so that the original building be- 
 comes completely transformed. By means of our 
 formulae we shall be able readily to follow the suc- 
 cessive changes which take place. The contem- 
 plation of these formulae, moreover, has a value 
 for the layman in that he will thus gain some idea 
 
MOLECULAR ARCHITECTURE 47 
 
 of the complexity of the compounds and may come 
 to appreciate more fully the ability of the chemists 
 who have not only succeeded in unravelling the 
 intricate details of molecular constitution, but have 
 also built up these complex structures from more 
 simple materials. The ordinary person is impressed 
 by the grandeur or magnitude of some engineering 
 triumph and may be overwhelmed by statistics 
 of the number of bolts or nuts, or the weight of 
 metal employed ; but although the intricate struc- 
 ture of the molecule cannot be seen by the eye, 
 it must nevertheless impress the mind of every 
 thoughtful person. Indeed, until the imagination 
 of our people can be fired by the mental contem- 
 plation of these great chemical achievements we 
 shall never be able to gain for chemistry and for 
 chemical study that measure of interest, respect, 
 recognition, and encouragement which alone will 
 enable this country to hold her own in the industrial 
 competition of the world. 
 
CHAPTER V 
 
 THE PRODUCTION OF DYES FROM COAL TAR 
 
 FROM time immemorial, men, denied by nature the 
 more varied and gorgeous colourings of the animals, 
 have delighted in staining their bodies or dyeing 
 their garments by means of the various colouring 
 matters with which the animal and vegetable 
 creation supplied them the colouring matter of 
 logwood ; the animal dye carmine or cochineal 
 which was used in this country to dye the scarlet 
 tunics of our soldiers ; the blue dye, indigo or woad, 
 one of the oldest of dyes ; the . red dye, alizarin, 
 obtained from the root of the madder- plant and 
 employed in the production of Turkey red ; and 
 the costliest and most famous dye of the ancient 
 world, Tyrian purple, obtained from a shell-fish 
 found on the eastern shores of the Mediterranean. 
 Until the middle of the nineteenth century these 
 and some other dyes, mainly of animal or vegetable 
 origin, were practically the only dyes with which 
 man was acquainted. But in 1856 a new chapter, 
 and one of the highest importance in the history 
 of dyes, commenced with the discovery of the once 
 favourite synthetic dye, mauve, which found its last 
 
 48 
 
PRODUCTION OF DYES FROM COAL TAR 49 
 
 use for colouring the postage stamps of the late 
 Victorian era. This dye was prepared from crude 
 aniline, which was in turn produced from the benzole 
 derived from coal tar, and it was the first of a long 
 list of synthetic dyes prepared by chemists from 
 the constituents of coal tar. Starting from benzene, 
 toluene, phenol, naphthalene, anthracene, and a 
 few other constituents of the thick black liquid, 
 coal tar, which less than a hundred years ago was 
 a useless waste material and a nuisance to the gas 
 manufacturer, synthetic dyes, to the estimated 
 value of nearly 20,000,000, are now manufactured 
 annually, more than two- thirds of this amount being 
 produced, in 1913, in Germany. These dyes have, 
 by reason of their almost infinite variety and 
 applicability, their range of colour and delicacy 
 of tone, ousted the natural dyes to a very large 
 extent from the dye-works. 
 
 It is, of course, now universally known that these 
 numerous coal-tar dyes are not present as such in 
 the coal tar, but that they are obtained from the 
 constituents of coal tar by a more or less complex 
 series of chemical reactions. Thus, from some eight 
 or nine primary constituents of coal tar (benzene, 
 toluene, xylene, phenol, cresol, naphthalene, anthra- 
 cene, etc.), there are produced, by the action of 
 various chemical reagents nitric acid, sulphuric 
 acid, chlorine, caustic soda, etc. some two hundred 
 and ninety " intermediate " compounds, and from 
 
50 THE TREASURES OF COAL TAR 
 
 these " intermediates/' by their mutual combina- 
 tions and interactions, the finished dyes are pre- 
 pared, of which upwards of nine hundred actually 
 find application at the present time. The production 
 of a dye, therefore, is by no means a simple operation, 
 except in a very few cases, and is, in most cases, a 
 very complex process involving, it may be, fifteen 
 or twenty distinct and separate chemical reactions. 
 That the industry of dye-manufacture is a very 
 intricate one will, therefore, be readily understood, 
 and if success is to be attained, each step in the 
 process of manufacture must be scientifically con- 
 trolled and carried out with the highest degree of 
 efficiency. But a further very serious complication 
 is introduced owing to the fact that in the prepara- 
 tion of many of the intermediates, " by-products " 
 are produced in varying amounts, and for these by- 
 products a remunerative outlet must be obtained. 
 Even when all the products formed in the manu- 
 facture of a given intermediate can be used up in 
 the manufacture of dye-stuffs, the demand for the 
 dye-stuffs thus obtained may differ very greatly, 
 and by no means always in the same direction or 
 in the same measure as the intermediates. The 
 problem of working up, completely and remuner- 
 atively, without waste and without over-production, 
 all the by-products formed in the manufacture of 
 the intermediates, is one of the utmost importance 
 for the success of the industry. Owing to the 
 
PRODUCTION OF DYES FROM COAL TAR 51 
 
 enormous development of her organic chemical 
 industry, embracing the manufacture of dyes, drugs, 
 perfumes, and " fine " (organic) chemicals generally, 
 the solution of this problem has become more easy 
 for Geimany than for any other country. 
 
 Although Great Britain is one of the largest 
 producers of coal tar, she has, hitherto, manu- 
 factured only a small number of the intermediates 
 required for the production of the finished dyes, 
 and has contented herself with importing many of 
 the most important intermediates from Germany. 
 If, therefore, this country is to gain her independ- 
 ence in respect of the manufacture and supply of 
 dyes, she must undertake, in the future, the pro- 
 duction of the necessary intermediates on a greatly 
 more extensive scale than in the past. In this 
 direction very marked advance has been made since 
 the outbreak of the war. 
 
 The value, in round figures, of the estimated 
 production of coal-tar dyes in 1912 is given in the 
 following table : 
 
 Germany .... 13,500,000 
 Switzerland .... 1,290,000 
 Great Britain . . . 1,190,000 
 France .... 1,000,000 
 
 United States . . . 750,000 
 Other countries . . . 2,000,000 
 
52 THE TREASURES OF COAL TAR 
 
 In 1913 Great Britain imported dyes to the value 
 of 1,946,224, the value of the dyes obtained from 
 Germany amounting to about 1,800,000. Of all 
 the dyes used in this country in the textile, fur, 
 feather, paint, and other industries, only about 
 10 per cent, were of home manufacture. 
 
 In the year 1845, largely owing to the efforts of 
 the Prince Consort and of the Queen's physician, 
 Sir James Clark, there was founded the Royal 
 College of Chemistry in London, and A. W. Hofmann, 
 a young German chemist who had been trained 
 under the renowned Justus von Liebig at Giessen, 
 was appointed Professor of Chemistry. For some 
 time chemists had been interesting themselves in 
 the nature and composition of coal tar, and Hofmann 
 and his students engaged energetically in the work 
 of investigation. One of the earliest results to be 
 obtained was the isolation from coal tar of the hydro- 
 carbon benzene, a compound which was first dis- 
 covered by Michael Faraday in 1825 x ; and after 
 the work of Mansfield (p. 15), coal tar became the 
 chief source of the compound. As early as 1834 
 Mitscherlich had shown that when benzene is treated 
 
 1 In 1815 oil-gas was introduced as an illuminant, and was 
 supplied to the consumers in cylinders into which the gas had 
 been pumped under a pressure of thirty atmospheres. Under 
 this pressure a portion of the gas condensed to a liquid, and from 
 this liquid Faraday isolated benzene, or bi-carburet of hydrogen 
 as he called it. 
 
PRODUCTION OF DYES FROM COAL TAR 53 
 with concentrated nitric acid, it is converted into 
 
 * 
 
 an oily liquid, nitro-benzene, C 6 H 5 -NO 2 , which, 
 even before the introduction of the coal-tar dyes, 
 was manufactured in small quantity and used, 
 under the name of Essence of Mirbane, for scenting 
 soap. Nitrobenzene, in its turn, as was found by 
 Bechamp in 1854, could be converted into aniline, 1 
 C 6 H 5 NH 2 , by acting on it with a mixture of acetic 
 acid and finely divided iron. We see, then, that 
 coal tar became not only a convenient source of 
 supply of benzene, but also, through the chemical 
 transformation of this substance, of the compound 
 aniline. Entering into this heritage of knowledge, 
 W. H. Perkin, who had, as one of Hermann's students, 
 been trained in an atmosphere of purely scientific 
 investigation, made his important discovery of the 
 first coal-tar dye. It was in 1856, while engaged 
 in an attempt to produce the naturally-occurring 
 alkaloid quinine from simpler substances, that 
 Perkin treated a solution of aniline in dilute sul- 
 phuric acid with potassium dichromate. As a 
 result, there separated out from the liquid a dark- 
 coloured, resinous mass, and from this unpromising 
 material Perkin separated the first-known aniline 
 dye, which he somewhat later manufactured and 
 sold under the name of " aniline purple," or " Tyrian 
 Purple," or " mauve," the name given to it by the 
 
 1 Derived from anil, the Portuguese name for indigo, from 
 which aniline was first obtained in 1826. 
 
54 THE TREASURES OF COAL TAR 
 
 French dyers to whom, as we learn, the industrial 
 application of this dye was largely due. " I 
 distinctly remember/' said Sir William Perkin 
 at a later date, " the first time I induced a 
 calico-printer to make trials of this colour that 
 the only report I obtained was that it was too 
 dear, and it was not until nearly two years after- 
 wards, when French printers put aniline purple 
 into their patterns, that it began to interest 
 British printers/' 
 
 The successful industrial production of mauve 
 depends on the successful production of nitro- 
 benzene from benzene, and of aniline from nitro- 
 benzene ; and although these two " intermediates " 
 had already been prepared in small quantities, their ' 
 production on a large scale presented a number of 
 difficulties to the pioneers in this industry. In- 
 stead of the glass flasks in which, hitherto, nitro- 
 benzene had been prepared by the action of fuming 
 nitric acid on benzene, Perkin employed a large 
 cast-iron cylinder, capable of holding between thirty 
 and forty gallons of liquid, and furnished with a 
 stirrer which could be worked by a handle. Since a 
 sufficiently large supply of fuming nitric acid could 
 not at that time be obtained, Perkin used a mixture 
 of sodium nitrate and concentrated sulphuric acid, 
 and, later, a mixture of concentrated nitric and 
 sulphuric acids. The conversion of nitro-benzene 
 into aniline was effected in large iron stills by means 
 
PRODUCTION OF DYES FROM COAL TAR 53 
 
 of iron filings and acetic acid, this acid, however, 
 being replaced at a later date by the cheaper hydro- 
 chloric acid or muriatic acid. By the action of 
 the acid on the iron, hydrogen is produced, and this 
 " reduces " the nitrobenzene, or replaces its oxygen 
 by hydrogen, and so yields aniline. The methods 
 used at the present day for the manufacture of these 
 two very important compounds are essentially 
 those which were introduced by Perkin ; and in 
 thus working out the details of the process of manu- 
 facture of aniline, now perhaps the most important 
 of all the coal-tar " intermediates," Perkin per- 
 formed a service of the highest value to the coal- 
 tar colour industry. 
 
 The success which attended the introduction of 
 mauve, the vogue of which among the women of 
 1859 became so " epidemic " that Punch referred 
 to it as " The Mauve Measles," naturally led 
 chemists to try the action on aniline of other 
 oxidising agents (substances capable of giving up 
 oxygen to other substances) than the potassium 
 dichromate used by Perkin ; and although they 
 did not succeed in displacing the latter for the pre- 
 paration of mauve, their efforts led to the discovery 
 of a new dye, aniline red, magenta, or fuchsine. 
 The formation of this red dye had been observed by 
 several chemists, even as early as 1856, and although 
 it was manufactured in Small quantity in France 
 in 1858-9, by a process due to Verguin, the greatest 
 
56 THE TREASURES OF COAL TAR 
 
 success in its manufacture was achieved, in 1860, 
 by two English chemists, Medlock and Nicholson, 
 former pupils of Hofmann, who prepared it by the 
 action of arsenic acid on commercial aniline. The 
 manufacture of this important dye was carried out 
 by Messrs Simpson, Maule and Nicholson, and the 
 " crown " of magenta crystals prepared by this 
 firm was one of the most notable exhibits of the 
 International Exhibition of 1862. Stirred by this 
 and by the other exhibits of English dye manu- 
 facturers, Hofmann was prompted to make the 
 prediction : " England will, beyond question, at 
 no distant day become herself the greatest colour- 
 producing country in the world, nay, by the strangest 
 of revolutions, she may, ere long, send her coal- 
 derived blues to indigo-growing India, her tar- 
 distilled crimsons to cochineal-producing Mexico, 
 and her fossil substitutes for quercitron and safflower 
 to China, Japan, and the other countries whence 
 these articles are now derived." At that time 
 England was pre-eminent in the industrial pro- 
 duction of the coal-tar dyes as she was pre-eminent 
 in the production of the raw materials of their 
 manufacture, but in the subsequent years, largely 
 through her failure to recognise the vital importance 
 of persistent chemical research, she had to yield 
 pride of place to Germany. Let us, however, still 
 hope that the faith in science which has been 
 awakened in the people of this country during the 
 
PRODUCTION OF DYES FROM COAL TAR 57 
 
 years of war will yet enable us in the years of 
 peace to regain our lost position and so realise the 
 prophecy made by Hofmann in 1862. 
 
 The brilliancy of the new aniline dyes and the 
 great success they achieved owing, partly, to the 
 simplification which their use brought about in 
 the process of dyeing, made a very powerful appeal 
 to the imagination of the scientific chemist no less 
 than to the business instincts of the manufacturer. 
 " A new world was disclosed full of magic promise, 
 and all joined eagerly in the search, the manufac- 
 turer and the professor, the business man and the 
 adventurer ; for the one a new gold-mine, for the 
 other new opportunities of fruitful investigation/' 
 To such an extent, indeed, were the energies of 
 chemists directed along this one channel that it 
 was feared by some that the general progress 
 of chemical science would be gravely prejudiced. 
 But the check, if any, was but temporary, for 
 the co-operation which then existed between the 
 scientific investigator and the chemical technolo- 
 gist, a co-operation which this country must 
 endeavour once more to re-establish, proved most 
 beneficial ; and the new materials which the 
 manufacturer soon placed at the disposal of the 
 investigator led to a far greater extension of 
 chemical science than would otherwise have been 
 possible. 
 
 Just as aniline formed the basis of manufacture 
 
58 THE TREASURES OF COAL TAR 
 
 of rnauve and of magenta, so magenta became, in 
 its turn, the starting-point for the preparation of 
 a series of new dyes, the number of which now began 
 rapidly to increase. In 1861 Girard and de Laire 
 prepared aniline blue or Lyons blue by heating 
 magenta with aniline in presence of benzoic acid ; 
 and by treating this dye with concentrated sulphuric 
 acid, E. C. Nicholson, in 1862, produced the more 
 valuable Nicholson's blue or water blue, which 
 possessed the great advantage of being soluble 
 in water and in solutions of alkalies, and was 
 better adapted for dyeing wool than the dyes pre- 
 viously prepared. Hofmann, also, prepared brilliant 
 but not very stable violet dyes, Hofmann violets, 
 by acting on magenta with methyl iodide and ethyl 
 iodide. 
 
 But although the preparation of new dyes and 
 the perfecting of their industrial production were 
 carried on with much vigour along the lines 
 opened up by W. H. Perkin, chemists were not 
 unmindful of the need of more theoretical in- 
 vestigations for the purpose of determining the 
 composition and unravelling the constitution of 
 these important new substances. Without such 
 knowledge the dye industry could not be placed 
 on a secure scientific basis and its further 
 development ensured. In this work of investiga- 
 tion Hofmann took a leading part, and in 1862 
 he showed that the dye magenta was the salt of 
 
PRODUCTION OF DYES FROM COAL TAR 59 
 
 a base l which he called rosaniline. Moreover, in 
 1864 he confirmed what had already been dis- 
 covered by Nicholson, that magenta cannot be 
 obtained by the oxidation of pure aniline but only 
 of commercial aniline which contained the two 
 isomeric substances, ortho- and para-toluidine, as 
 impurities. 
 
 We have already seen that coal-tar contains not 
 only benzene but also several other similar hydro- 
 carbons, more especially toluene ; and in the early 
 days of the industry the separation of these was not 
 carried out very effectively. In other words, the 
 benzol obtained by the distillation of the coal-tar 
 always contained larger or smaller amounts of 
 toluene. When this commercial benzol was treated 
 with a mixture of nitric and sulphuric acids there 
 were produced not only nitrobenzene but also two 
 isomeric nitro-toluenes, namely, ortho- and para- 
 nit rot oluene (see p. 44) : 
 
 CH 3 CH 3 
 
 N0 2 
 Ortho-nitrotoluene Para-nitrotoluene 
 
 and when these nitrotoluenes are " reduced " by 
 
 1 Chemists are accustomed to classify substances into acids, 
 bases, and salts. An acid is a substance with a sharp, sour, or 
 acid taste (e.g. vinegar or acetic acid), which can combine with 
 or neutralise a base (e.g. ammonia or soda), with production of 
 a " neutral " substance, a salt. 
 
60 THE TREASURES OF COAL TAR 
 
 means of iron and hydrochloric acid, the two cor- 
 responding toluidines are produced : 
 
 CH 3 CH 3 
 
 NH 2 
 Ortho-toluidine Para-toluidine 
 
 It was to the presence of these compounds in the 
 aniline employed that the discovery of magenta, 
 as, indeed, also of mauve, was due. 
 
 But although Hofmann succeeded in determining 
 the composition of magenta and of some of the 
 other dyes then known, the true relationships 
 which existed between these dyes could not be 
 understood without a knowledge of the structure 
 or constitution of the molecule. This knowledge, 
 made possible by the theories of structure put 
 forward by Kekule (p. 36), was finally obtained 
 in 1878 by the two German chemists Emil and 
 Otto Fischer, who showed that the parent of rosani- 
 line, magenta, and a number of other dyes derived 
 from aniline, is a hydrocarbon called triphenyl- 
 methane. This compound can be regarded as 
 
 H 
 arising from methane, H-C-H, by the replacement 
 
 H 
 
 of three of the hydrogen atoms by the group 
 phenyl or C 6 H 5 , that is, a molecule of benzene from 
 which one hydrogen atom has been removed. We 
 
PRODUCTION OF DYES FROM COAL TAR 61 
 
 therefore obtain as the formula of this parent 
 hydrocarbon : 
 
 / 
 
 H-C < y or H-C C 6 H 5 
 
 When a mixture of aniline, ortho-toluidine, and 
 para-toluidine is oxidised, rosaniline is produced : 
 
 CH 
 
 NH 2 
 
 NH 2 or HO-C C 6 H 4 -NH 2 
 NH 2 
 
 (the relation of which to the parent hydrocarbon, 
 triphenyl-methane, is readily seen), and this base 
 unites with hydrochloric acid, with elimination of 
 water, to yield rosaniline hydrochloride or magenta : 
 
 ,C 6 H 3 (CH 3 )-NH 2 
 C C 6 H 4 'NH 2 
 
 When a mixture of aniline and para-toluidine 
 is oxidised, another base, para-rosaniline, is 
 obtained, and this also gives rise to dyes similar 
 
62 THE TREASURES OF COAL TAR 
 
 to those given by rosaniline, to which they are, 
 structurally, closely related, as the formulae show : 
 
 // C 6 H 4 -NH 2 / /C 6 H 4 -NH 2 
 
 HO'C C 6 H 4 'NH 2 C C 6 H 4 'NH 2 
 
 X 6 H 4 -NH 2 ^C 6 H 4 -NH 2 C1 
 
 Para-rosaniline Para-rosaniline hydrochloride 
 
 (red dye) 
 
 The theory of the structure of the benzene mole- 
 cule put forward by Kekule, and the elucidation 
 of the constitution of rosaniline and para-rosaniline 
 which it rendered possible, not only enabled one to 
 understand the exact relations between the different 
 dyes, the Hofmann violets, aniline blue, etc., which 
 had already been prepared, but new and better 
 processes for the synthesis of these and other 
 dyes could be introduced. Thus the old process for 
 the manufacture of para-rosaniline and rosaniline 
 (magenta) has given place to the " New Fuchsine " 
 process in which para-rosaniline is prepared from 
 formaldehyde and aniline, while rosaniline is pre- 
 pared from formaldehyde, aniline, and ortho- 
 toluidine. 
 
 The hydrogen atoms of the three NH 2 -groups 
 present in rosaniline and para-rosaniline can be 
 replaced by various groups, such as methyl (CH 3 ), 
 ethyl (C 2 H 5 ), phenyl (C 6 H 5 ), benzyl (C 6 H 5 -CH 2 ), 
 etc., by acting on the compounds with methyl 
 chloride, ethyl chloride, aniline, benzyl chloride, 
 etc. In this way whole series of dyes can be 
 
PRODUCTION OF DYES FROM COAL TAR 63 
 
 obtained, like the Hofmann violets, which were 
 prepared by replacing three hydrogen atoms in 
 rosaniline by methyl or ethyl groups ; and aniline 
 blue, which is derived from magenta by the replace- 
 ment of three hydrogen atoms by phenyl groups. 
 Since, therefore, a varying number of hydrogen 
 atoms can be replaced by different groups, it will 
 readily be understood that from the parent dye, 
 magenta, quite a considerable number of derived 
 dyes can be prepared. 
 
 The production of the Hofmann violets, we have 
 seen, involves the introduction of new reagents, 
 methyl and ethyl iodide or chloride, and for the 
 preparation of these, methyl alcohol or " wood 
 spirit " and ethyl alcohol or " spirits of wine " are 
 required. The development of the dye industry, 
 therefore, depended on and was accompanied by 
 the development of other chemical industries ; 
 and the industrial production of the large number 
 of different compounds used in the manufacture of 
 dyes becomes a matter of supreme importance for 
 the success of dye-manufacture. By the introduc- 
 tion of new reagents, moreover, other new com- 
 pounds, other intermediates, can be prepared, and 
 these, in turn, may become the starting-point for 
 other series of dyes, the dye industry thereby grow- 
 ing rapidly in diversity as a tree grows by the rami- 
 fication of its branches. Thus, by heating aniline 
 with methyl chloride, the compound dimethyl- 
 
64 THE TREASURES OF COAL TAR 
 
 aniline is produced, and by acting on this with an 
 oxidising agent a beautiful violet dye, known as 
 methyl violet, is obtained, and is largely used as a 
 staining liquid in microscopy and in the manu- 
 facture of indelible pencils. It consists of a mixture 
 of dyes which may be regarded as derived from 
 para-rosaniline by the replacement of four, five, 
 and six atoms of hydrogen by methyl groups. The 
 pure hexamethyl derivative of para-rosaniline (with 
 six atoms of hydrogen replaced by methyl groups) 
 is obtained from dimethyl-aniline and phosgene 
 (from carbon monoxide and chlorine), and is known 
 as crystal violet. By the action of methyl chloride 
 on methyl violet, a brilliant green dye, methyl 
 green, is obtained. 
 
 The production of new dyes by the replace- 
 ment of hydrogen by different groups of atoms 
 leads us to the consideration of a subject of the 
 highest importance, the relation between colour 
 and constitution. 
 
 Put in general terms, a substance possesses colour 
 only when it has the power of absorbing light of a 
 certain wave-length while allowing light of other 
 wave-length to pass through. When a substance 
 absorbs the green rays, for example, the light which 
 passes through will show the complementary colour, 
 namely, purple red. In other words, the substance 
 will appear of this colour. 
 
PRODUCTION OF DYES FROM COAL TAR 65 
 
 From the study of a large number of substances 
 the conclusion has been reached that colour is 
 associated with the presence of certain groups 
 or arrangements of atoms in the molecule such 
 groups being known as " chromophors " or colour- 
 bearing groups. 1 In the dyes which we have 
 already mentioned the colour is believed to be due 
 to a modification of one of the benzene rings. But 
 although the character of a dye-stuff is derived 
 from its chromophor, the actual colour and shade 
 may be very distinctly altered by the introduction 
 of different groups into the molecule. Thus, if we 
 write down the different primary colours, namely, 
 
 Greenish-yellow, 
 
 Yellow, 
 
 Orange, 
 
 Red, 
 
 Purple, 
 
 Violet, 
 
 Indigo, 
 
 Cyanide-blue, 
 
 Bluish-green, 
 
 it is found that the introduction of methyl and 
 ethyl groups, and still more the introduction of 
 
 1 When a chromophor is present in a compound the latter is 
 said to be a " chromogen," and may or may not be coloured. 
 To convert the latter into a coloured substance, it is necessary 
 to introduce certain groups (especially OH and NH 2 ), called 
 " auxochromes." 
 
66 THE TREASURES OF COAL TAR 
 
 phenyl, benzyl, and other groups derived from 
 benzene, produces a change of colour in the direc- 
 tion shown by the arrow. This fact is well illus- 
 trated by the dyes which have already been men- 
 tioned, for by introducing three methyl or ethyl 
 groups into the molecule of magenta (red), Hofmann 
 obtained violet dyes. Moreover, by increasing the 
 number of such groups the violet shade becomes 
 bluer, as is shown in the case of methyl violet and 
 crystal violet, which contain five or six methyl 
 groups. But, as we have said, the phenyl group 
 has a more powerful effect than the methyl or ethyl 
 group, and so we find that when it is introduced 
 into a molecule in place of hydrogen, the alteration 
 of shade or colour is much greater. This is illus- 
 trated by aniline blue, which is obtained by replacing 
 three hydrogen atoms in magenta by three phenyl 
 groups. Since, by the introduction of certain other 
 groups, the shade or colour can be altered in the 
 opposite direction to that indicated by the arrow, 
 it will readily be understood how it becomes possible 
 not only to prepare a considerable number of parent 
 dyes, but also, from these dyes, to produce, at will, 
 a great variety of shades and colours simply by the 
 introduction of different groups into the molecule 
 of the parent dye. 
 
 To attempt a full discussion of all the dye deriva- 
 tives of triphenyl-methane would be impossible 
 within the limits of space available here, and would, 
 
PRODUCTION OF DYES FROM COAL TAR 67 
 
 moreover, be a very tedious matter for the general 
 reader. But reference may be made to one other 
 dye of importance, discovered in 1878. It has 
 already been noted that as a result of the intro- 
 duction into the dye industry of methyl chloride 
 and iodide, the preparation of dimethyl-aniline 
 [C 6 H 5 -N(CH 3 ) 2 ] became possible. Similarly, in place 
 of the two methyl (CH 3 ) groups, other groups can 
 be introduced into aniline, as has already been 
 indicated, and from these derivatives of aniline 
 other series of dyes can be obtained by the action 
 of benzaldehyde or bitter almond oil. Thus, 
 dimethyl-aniline gives rise to the well-known dye, 
 malachite green or Victoria green, and diethyl- 
 aniline [C 6 H 5 -N(C 2 H 5 ) 2 ] to the dye brilliant green. 
 
 The synthetic production of these two and other 
 dyes necessitating the use of benzaldehyde depended 
 for industrial success on the discovery of a cheaper 
 and better source of this compound than that 
 already existing. Hitherto, benzaldehyde had been 
 obtained by the fermentation of bitter almonds. 
 In this process the compound, amygdalin, present 
 in the almonds, undergoes a decomposition with 
 production of benzaldehyde and certain other sub- 
 stances. At the present day, however, this com- 
 pound, formerly obtainable only from vegetable 
 sources, is manufactured in large quantities from 
 the coal-tar hydrocarbon, toluene ; and from this 
 
68 THE TREASURES OF COAL TAR 
 
 same source is also obtained the compound benzoic 
 acid, which, as we have seen, is used in the manu- 
 facture of aniline blue (p. 58). Thus the dye 
 industry goes on increasing in diversity, drawing 
 into its service one compound after another, and 
 depending, therefore, not only on the persistent and 
 untiring work of the research chemist but also on 
 the success with which the technologist can carry 
 out on an industrial scale the discoveries of the 
 investigator. And any country which aims at 
 developing a dye-making industry which will render 
 it independent of other countries must learn to 
 produce, with the utmost economy, the large 
 number of " intermediates " of which that industry 
 makes use. 
 
 The dyes which have so far been mentioned will 
 dye silk and wool directly, but will dye cotton only 
 with the aid of a mordant. Although not character- 
 ised by great fastness to light, the dyes of the tri- 
 phenyl-methane series possess, for the most part, 
 great brilliance, and are, in consequence, much 
 esteemed for the dyeing of those materials ribbons, 
 for example for which brilliancy is more valued 
 than fastness. Owing to the introduction of other 
 series of dyes, however, the further development 
 of the triphenyl-methane dyes has practically 
 ceased. 
 
 The relations between the different dyes men- 
 
PRODUCTION OF DYES FROM COAL TAR 69 
 
 tioned in this chapter are shown in the following 
 diagram l : 
 
 Crystal Methyl 
 Violet Violet 
 I 1 
 
 Malachite 
 Green 
 1 1 
 
 lehyde 
 
 Dime 
 ani 
 
 thyl- 
 ine 
 Hofmann 
 Violets 
 
 Aniline 
 Blue 
 1 1 
 
 
 Magenta Benzoic Benzal< 
 | | Acid 
 
 Ani 
 
 Nitrob 
 
 Ben 
 
 line 
 enzene 
 
 sene 
 
 Toluidines 
 1 
 Nitrotoluenes 
 I 
 
 
 I 
 Toluene 
 
 1 For a more complete representation of the relationships 
 between the coal-tar crudes, intermediates, and finished dyes, 
 the reader is referred to the Chart drawn up by Thomas H. 
 Norton and published by Messrs George Allen & Unwin, Ltd., 
 40 Museum Street, London, W.C. 
 
CHAPTER VI 
 
 AZO-DYES 
 
 SOME years after the epoch-making discovery of 
 mauve an observation was made which led in time 
 to the development of an entirely new class of com- 
 pounds which, in number and importance, now 
 occupy a foremost place among coal-tar dyes. In 
 1860 it was found by Dr Peter Griess, chemist in 
 the brewery of Messrs Allsopp & Sons, Burton-on- 
 Trent, that when nitrous acid acts on aniline, or 
 on any other derivative of benzene containing the 
 amino-group (NH 2 ), an unstable compound a so- 
 called diazo-compound is produced. The diazo- 
 compounds which were thus obtained possessed 
 the very important property of combining or 
 " coupling " with aromatic amines (compounds con- 
 taining the NH 2 -group), or with phenols (compounds 
 containing the OH-group). In this way were pro- 
 duced the so-called azo-dyes, which have found their 
 special application as wool-dyes, and which owe 
 their colour to the presence of the chromophoric 
 azo-group or pair of linked nitrogen atoms, N : N . 
 Even in 1863, although its constitution was not then 
 known, an azo-dye, aniline yellow, had been prepared 
 
 70 
 
AZO-DYES 71 
 
 and put on the market, with only a limited success ; 
 but it was not till 1876, and after the constitution of 
 the compounds had been elucidated, that the pro- 
 duction of azo-dyes began to undergo a rapid de- 
 velopment. Many hundreds, even thousands, of 
 azo-dyes have now been prepared, and a very con- 
 siderable number have been found suitable for use 
 as dyes. 
 
 When aniline is " diazotised " by the action of 
 nitrous acid, and the resulting compound then 
 " coupled " with a molecule of aniline, a compound 
 is obtained which can be represented by the graphic 
 formula 
 
 & 
 
 This is a basic substance and can form a salt with 
 hydrochloric acid, yielding thereby the dye aniline 
 yellow, a dye which, on account of its fugitive 
 nature, is now no longer used except in the manu- 
 facture of other dyes. If, however, aniline yellow 
 is treated with a highly concentrated sulphuric 
 acid, two sulphonic acid groups (SO 2 OH) enter the 
 molecule, and a more stable dye, acid yellow, is 
 obtained. This process of sulphonation, as was 
 pointed out by the late Sir W. H. Perkin, is one 
 of the highest importance in the production of 
 stable dyes. 
 
 If, after diazotising aniline one couples the pro- 
 duct, not with aniline but with a benzene deriva- 
 
72 THE TREASURES OF COAL TAR 
 
 live containing two ammo-groups, namely, meta- 
 phenylene diamine, 
 
 NH 2 
 
 J NH 2 
 
 one obtains the compound 
 
 N <f >NH 2 
 
 the salt of which with hydrochloric acid constitutes 
 the orange-red dye chrysoidine. Or, again, if one 
 diazotises meta-phenylene diamine and couples 
 the product with another molecule of this compound, 
 there is formed 
 
 >N : N<T >NH 2 
 
 NH a NH a 
 
 the salt of which with hydrochloric acid constitutes 
 Bismarck brown. 
 
 But we can couple the diazo-compounds not only 
 with compounds containing the amino-group, but 
 also with compounds containing the hydroxyl- 
 group (OH), e.g. phenol, cresol, salicylic acid, etc., 
 and in this way another series of azo-dyes is obtained 
 known as the tropaeolines. Thus, for example, 
 if one diazotises not aniline itself but the sulphonic 
 acid derivative of aniline, known as sulphanilic 
 
 acid, SO 2 OH<^ /> N H a , and if one then couples the 
 
AZO-DYES 73 
 
 product with the compound resorcinol, <(^ _/ OH 
 
 OH 
 
 (a substance derived from benzene), there is obtained 
 the compound 
 
 N : N OH 
 
 OH 
 
 a dye known as tropseoline 0. 
 
 Not only can one produce azo-dyes from aniline, 
 toluidine, phenol, etc., and from their derivatives, 
 but one may also use similar compounds derived 
 from other hydrocarbons. In this connection the 
 coal-tar hydrocarbon naphthalene has been found of 
 especial value, and very many dyes have now been 
 produced from the amino- and hydroxyl-derivatives 
 of this compound. By thus drawing naphthalene 
 within the sphere of the dye industry, an important 
 outlet was secured for this coal-tar product other- 
 wise but little used and at the same time a large 
 number of valuable new dyes were obtained. 
 
 Just as we have seen that phenol and aniline are 
 derived from benzene by the replacement of a 
 hydrogen atom by a hydroxyl- or amino-group, so 
 from naphthalene one can obtain similar com- 
 pounds naphthol and naphthylamine. Owing 
 to the peculiar constitution of naphthalene, how- 
 ever, two isomeric naphthols and naphthylamines 
 can be obtained, known as alpha-naphthol and 
 beta-naphthol, alpha-naphthylamine and beta- 
 
74 THE TREASURES OF COAL TAR 
 
 naphthylamine. These very important com- 
 pounds, as well as their sulphonic acid derivatives, 
 are now manufactured in large quantities and used 
 in the production, more especially, of azo-dyes. 
 By diazotising aniline, toluidine, xylidine, etc., 
 and coupling the products with the sulphonic acid 
 derivative of beta-naphthol, one obtains a series 
 of red dyes known as Ponceaux, the shade varying 
 according to the amino-compound (aniline, toluidine, 
 etc.) employed. And, similarly, other dyes can 
 be obtained by diazotising the naphthylamines or 
 their derivatives, and coupling the products with 
 various amino-compounds, phenols, naphthols, etc. 
 Moreover, in those cases where a diazo-compound 
 is coupled with an amino-compound, the process 
 of diazotisation can be repeated, and dyes contain- 
 ing two azo-groups, e.g. Biebrich scarlet, can thus 
 be obtained. Many of these are of great importance. 
 From what has now been said, some idea will be 
 gained not only of the large number of azo-dyes 
 which can be obtained, but also of the increasing 
 number of " intermediates " made use of by the 
 dye industry. Moreover, for the production of all 
 these azo-dyes, nitrous acid is necessary for carry- 
 ing out the first step in the process, that is to 
 say, the diazotising of the initial amino-compound. 
 This nitrous acid is produced from sodium nitrite, 
 and this, in turn, is obtained by heating sodium 
 nitrate with lead. Until recently, one was depend- 
 
AZO-DYES 75 
 
 ent for sodium nitrate on the large deposits of this 
 salt in Chile, but in the past decade the production 
 of nitric acid directly from the air has opened up 
 a new source of supply of this important salt. In 
 this matter of the utilisation of atmospheric nitrogen 
 for the production of nitric acid and other com- 
 pounds of nitrogen, this country has lagged behind 
 not only Germany but nearly every other civilised 
 country in the world. There seems, however, to be 
 now some hope that this past neglect will be repaired. 
 
 Although most of the dyes to which reference has 
 already been made, dye silk and wool directly, 
 vegetable fibres, e.g. linen and cotton, must first 
 be mordanted before they will take up the dye 
 from the bath. It was therefore an event of the 
 first importance when, in 1884, the German chemist 
 Bottinger discovered a new group of azo-dyes 
 which were able to dye cotton and linen directly 
 without requiring a mordant. In this discovery 
 is to be found undoubtedly one of the reasons for 
 the outstanding importance of the azo-dyes. 
 
 These direct cotton dyes contain two azo-groups 
 or two pairs of nitrogen atoms, and are derived 
 from compounds similar to benzidine and tolidine, 
 in which there are two benzene rings joined together, 
 thus: 
 
 NH '<Z>-<Z> NHa NH.OO^H, 
 
 Benzidine Tolidine 
 
76 THE TREASURES OF COAL TAR 
 
 When these compounds, which are prepared from 
 benzene and toluene respectively, are acted on by 
 nitrous acid, both NH 2 -groups are diazotised, and 
 the product can then couple with two molecules 
 of amine or phenol. When benzidine, for example, 
 is diazotised and the product coupled with two 
 molecules of sulphonated naphthylamine, the red 
 dye, Congo red, is obtained. This was the first 
 direct cotton dye to be synthesised, and led to 
 the preparation of a large number of similarly 
 constituted dyes, covering the whole range of 
 colour, all of which possess the valuable property 
 of dyeing cotton without the aid of a mordant. 
 The chart on the following page indicates the 
 relationship of some of the azo-dyes to the 
 hydrocarbon benzene. 
 
 The simplest azo-dyes are yellow, but, as has 
 already been pointed out, the shade and colour 
 can be altered very greatly by the introduction of 
 different groups. In this way dyes of deeper and 
 deeper tone, from the vivid scarlets known as 
 xylidine scarlet and Biebrich scarlet successful 
 rivals of the natural dye cochineal to blue, violet, 
 brown, and black, have been produced ; and this 
 change of colour is effected, more especially, by the 
 introduction of groups derived from naphthalene. 
 By the introduction of nitro-groups (NO 2 ), green 
 dyes, e.g. diamine green, are obtained. 
 
 As a result of the introduction of the azo-dyes 
 
AZO-DYES 
 
 77 
 
78 THE TREASURES OF COAL TAR 
 
 an important development in the art of dyeing has 
 taken place. If the fabric to be dyed is first im- 
 pregnated with an alkaline solution of beta-naphthol 
 and then immersed in a solution of diazotised 
 
 para-nitraniline (NO 2 <^ yNH 2 ), the dye, para- 
 
 nitr aniline red or para- red, is produced on the fibre. 
 Dyeing with this pigment dye as a dye produced 
 on the fibre is called is carried out on a very large 
 scale, nearly two thousand tons of para-nitraniline 
 being produced annually for the purpose. This 
 process of dyeing can, similarly, be carried out with 
 dyes containing an amino-group which can be 
 diazotised on the fibre. Thus there is a very com- 
 plex dye primuline, discovered by Professor A. G. 
 Green in 1887, which will dye cotton directly of a 
 yellow shade. This dye is somewhat fugitive, and 
 is, consequently, of comparatively little value in 
 itself. If, however, a piece of calico, dyed with 
 primuline, is passed through a cold, dilute solution 
 of nitrous acid, the primuline (which contains an 
 amino-group), is diazotised ; and if the fabric is 
 then passed through a solution of an amine or 
 phenol, a dye is produced or " developed " on the 
 fibre. Thus, with beta-naphthol, primuline red; 
 with resorcinol, primuline orange ; with meta- 
 phenylene-diamine, primuline brown ; and with 
 salicylic acid, oriol yellow is obtained. To these 
 colours developed on the fibre the name of " in- 
 
AZO-DYES 79 
 
 grain dyes " is given ; or, since the solution of the 
 diazo-compound has, on account of its instability, 
 to be kept cool by means of ice, the name " ice 
 colours " is also sometimes applied to this group 
 of dyes. 
 
 Another pigment dye to which we may here refer, 
 a dye which carries us back again to the substance, 
 aniline, from which the first artificial colouring 
 matter was prepared, is the very important black 
 dye, aniline black. This dye, which has a very 
 complex structure, and is not an azo-dye, is 
 obtained by a modification of the process first 
 used by Perkin in the production of mauve, the 
 dye being formed, however, directly on the fibre. 
 Thus, if a piece of cotton is first steeped in 
 a solution of aniline in hydrochloric acid and 
 afterwards immersed in a cold solution of sodium 
 bichromate, a fast black colour is developed 
 on the fibre. Various improvements have more 
 recently been introduced for the purpose both of 
 cheapening the dye and of obtaining more readily 
 a black which will not turn green ; and it has been 
 found by Professor A. G. Green that in the presence 
 of certain substances the oxidation of the aniline 
 may even be effected by atmospheric oxygen. 
 
CHAPTER VII 
 
 ANTHRACENE DYES AND VAT DYES 
 
 Ws have already seen how, owing to the introduc- 
 tion of the azo-dyes, the coal-tar hydrocarbon 
 naphthalene, through its derivatives the naph- 
 thylamines, the naphthols, and their sulphonic 
 acids, became one of the important raw materials 
 in the coal-tar dye industry. In a like manner, as 
 a result of the all-transforming genius of the chemist, 
 another coal-tar hydrocarbon, anthracene, has also 
 been made to play a role of the highest importance 
 in the production of colouring matters. This hydro- 
 carbon, as we have already seen (p. 43), has the 
 formula C 14 H 10 , and can be represented graphically 
 by three rings joined together, thus : 
 
 By oxidation this compound is readily converted 
 into an orange-coloured substance, anthraquinone, 
 
 CH CO CH r> 
 
 I -II II I 
 HC C C CH 
 
 or 
 
ANTHRACENE DYES AND VAT DYES 81 
 
 This structure forms the nucleus of a considerable 
 number of important dyes, and more especially 
 of alizarin, the colouring matter of the madder. 
 This dye, which is capable of dyeing cotton of a 
 bright red colour the so-called Turkey red is one 
 of the oldest and best -known dye-stuffs employed 
 by man (to which, indeed, its use in the dyeing 
 of Egyptian mummy-cloths bears witness) ; and, 
 although now partly displaced by the azo-dyes, it 
 is still very extensively used for the dyeing of cotton 
 goods. Fifty years or so ago, in the South of France 
 and extending eastwards to Asia Minor, great 
 tracts of land, about 400,000 acres in extent, were 
 devoted to the cultivation of the madder plant 
 (Rubia tinctoria], and produced about 80,000 tons 
 annually of madder roots. When these roots are 
 crushed and allowed to ferment certain compounds 
 which they contain, known as glucosides, undergo 
 decomposition with production of the sugar glucose 
 and various colouring matters, of which the most 
 important are alizarin so called from the name, 
 alizari, given by the Arabs to the madder root 
 and purpurin, dyes which were first isolated in 
 1826 by the French chemists Robiquet and Colin. 
 But in 1868 far-reaching economic changes were 
 initiated by the discovery, due to Graebe and 
 Liebermann, of the chemical nature of alizarin and 
 by its artificial production from what was then a 
 waste by-product of the distillation of coal, the 
 F 
 
82 THE TREASURES OF COAL TAR 
 
 hydrocarbon anthracene ; and in 1869 the com- 
 mercial production of alizarin from anthracene was 
 commenced by Perkin in England. Since that 
 time the natural dye-stuff has been completely 
 superseded by the synthetic, and the widespread- 
 ing lands over which the madder once bloomed 
 are covered now with other crops ; and an industry 
 which was valued at about 4,000,000 annually 
 has passed from the field to the factory. By this 
 revolution, also, anthracene, which once tar dis- 
 tillers did not trouble to separate from the " last 
 runnings " of the stills, and which was either burnt 
 or used as a lubricant, became greatly in demand 
 and its value was enhanced from pence to pounds. 
 
 The preparation of alizarin is simple. Anthracene 
 is first converted into anthraquinone by heating 
 with potassium bichromate and sulphuric acid, 
 and the anthraquinone then converted into its 
 sulphonic acid derivative by heating with concen- 
 trated sulphuric acid. When this compound is 
 fused with caustic soda, in presence of a quantity 
 of potassium or sodium chlorate, alizarin or di- 
 hydroxy-anthraquinone (anthraquinone with two 
 
 hydroxyl groups) 
 
 o OH 
 
 )H 
 
 o 
 s formed. In this process we see the first 
 
ANTHRACENE DYES AND VAT DYES 83 
 
 triumphant success of the chemist in the artificial 
 production of a natural colouring matter, and in 
 this way the madder dye can be manufactured 
 much more cheaply than Nature can produce it. 
 
 Alizarin is a compound which is insoluble in cold 
 water, and is generally put on the market in the 
 form of a paste containing 10 or 20 per cent, of 
 alizarin. Over 2000 tons of this dye-stuff are now 
 manufactured annually, and of this and other 
 anthracene dyes the United Kingdom imported, 
 in 1913, over 3000 tons, of the value of about 
 270,000. 
 
 The main importance of alizarin lies in its wide- 
 spread use in cotton dyeing and printing. Unlike 
 some of the azo-dyes to which reference has been 
 made, alizarin will not dye either vegetable or animal 
 fibres directly, but only with the aid of mordants. 
 As mordants, substances are mainly used which 
 give rise to oxides of metals with which the dye 
 forms an insoluble compound or lake, and the colour 
 produced on the fibre depends on the metallic oxide 
 used. With alumina as mordant, alizarin gives a 
 bright red colour (as in Turkey red) ; with oxide 
 of chromium maroon is obtained ; whereas with 
 oxide of iron alizarin produces a violet shade. 
 Orange shades can also be produced by using tin 
 salts as mordants. 
 
 Besides alizarin, a large number of other deriva- 
 tives of anthraquinone are used as dyes. By intro- 
 
84 THE TREASURES OF COAL TAR 
 
 ducing three, four, five, and six OH-groups into 
 the molecule of anthraquinone, one obtains pur- 
 purin (which is also found along with alizarin 
 in the madder root), alizarin bordeaux, alizarin 
 cyanine R, and anthracene blue, which give 
 red, violet, and blue shades. By the introduction 
 of the nitro-group (NO 2 ) into alizarin, one obtains 
 alizarin orange and alizarin brown, 
 
 O OH O OH 
 
 H 
 
 O O N0 a 
 
 Alizarin Orange Alizarin Brown 
 
 and by the introduction of the sulphonic acid group 
 (SO 2 OH), alizarin red S. The introduction of still 
 other groups into the molecule of anthraquinone 
 gives rise to dyes of widely varying shades browns, 
 greens, blues, and violets the shade obtained 
 depending both on the nature of the group intro- 
 duced and on its position in the molecule (cf. p. 65). 
 
 Closely related to the anthracene dyes are the 
 acridine dyes, some of which are used mainly for 
 dyeing leather of a yellow colour. One of these 
 dyes, trypa-flavine or acri-flavine, has recently 
 found important application as an antiseptic (p. no). 
 
 One of the most important developments in 
 recent years has been the production of a series of 
 dyes known as the indanthrenes, a series of dye- 
 
ANTHRACENE DYES AND VAT DYES 85 
 
 stuffs derived from arithraquinone and possessing 
 exceptional fastness to light and to cleansing agents. 
 Although first discovered only in 1901, by R. Bohn 
 of the Badische Anilin- und Soda-Fabrik, these dyes 
 are now manufactured in twelve or thirteen different 
 shades covering the whole range of colours from 
 red to blue. On account of their fastness they are 
 largely employed in the dyeing of Sundour and 
 other guaranteed " fadeless " fabrics. The first 
 and one of the most important of these dyes, in- 
 danthrene blue, was obtained by fusing an amino- 
 derivative of anthraquinone, 
 o 
 
 NH 2 
 
 with caustic alkali, whereby two molecules of this 
 compound were caused to join up and yield indan- 
 threne blue, thus : 
 
86 THE TREASURES OF COAL TAR 
 
 Other indanthrene dyes, derivatives of anthraquinone, 
 have also been obtained, such as, indanthrene yellow 
 or flavanthrene, indanthrene red, indanthrene 
 green, etc., as well as the dyes known as algol 
 yellow, algol red, helindon yellow, etc. Until 
 recently, none of these dyes had been manufactured 
 in England, but in the present year (1917) indan- 
 threne blue was manufactured by British Dyes, Ltd., 
 and placed on the market under the name of 
 chloranthrene blue, and a second blue anthracene 
 dye-stuff for wool and silk has also been placed on 
 the market under the name of alizarine delphinol. 
 Some years after the introduction of indanthrene 
 blue there was discovered, by a strange mishap, 
 a new method of making this compound and others 
 of a similar kind. It befell in this way. For the 
 production of another dye derived from anthra- 
 quinone, a dye called sky-blue alizarine, the neces- 
 sary ingredients were heated for some time in a 
 vessel made of iron. In the course of time new 
 apparatus had to be installed ; and with this 
 apparatus no sky-blue alizarine was obtained, but 
 something entirely different. What could be the 
 reason of the failure ? The process was carried 
 out in the same way as before and under the same 
 direction. The apparatus, certainly, was new, but 
 it was exactly the same as the old apparatus. And 
 yet, no ; it was not exactly the same. The new 
 apparatus, instead of being entirely of iron, had a 
 
After this book had passed through the press, 
 the production of a number of anthracene dyes 
 was announced by Sol way Dyes Co., Carlisle (an 
 offshoot of Morton Sundour Fabrics, Ltd.), the 
 production of such dyes having been begun, on a 
 commercial scale, as early as February 1915. So 
 far, the manufacture of the following dyes has 
 been taken up by the above firm, the Company's 
 trade name for the dye being given in brackets : 
 Indanthrene yellow G. (Caledon yellow) ; indan- 
 threne blue (Caledon blue) ; indanthrene dark blue 
 B.O. (Caledon purple) ; indanthrene green B. 
 (Caledon green) ; indanthrene brown B.B. (Caledon 
 brown) ; indanthrene red B.N. (Caledon red) ; 
 indanthrene pink B. (Caledon pink) ; indanthrene 
 violet R. Extra (Caledon violet) ; alizarine sapph- 
 irole (Solway blue) ; alizarine cyanine green 
 (Kymric green). 
 
 Page 86 
 
ANTHRACENE DYES AND VAT DYES 87 
 
 copper lid. But surely that could not be the cause 
 of the different behaviour. Yet so it was, for the 
 small trace of copper derived from the lid exerted 
 a powerful catalytic influence, as it is called, on the 
 reaction. Merely by its presence the copper greatly 
 accelerated the reaction in one particular direction, 
 and so led to the production not of sky-blue alizarine 
 but of an indanthrene dye. By utilising this 
 property of copper indanthrene blue and other 
 valuable dyes, belonging to this and similar series, 
 could be obtained. To some the discovery of this 
 process may appear merely as a " lucky chance," 
 but it must be remembered that it is only he who 
 has the discerning eye and the understanding mind 
 who can turn the " lucky chance " to profit. It is, 
 however, part of the romance of scientific investi- 
 gation that the " lucky chance " is also one of the 
 rewards that come to those who actively and per- 
 sistently till the virgin soil of science. 
 
 Besides those already mentioned, several other 
 series of dyes are known derived from the constitu- 
 ents of coal-tar, but although a number of these 
 dyes are of much importance (e.g. the rhodamines, 
 the saffranines, etc.), a discussion of them would 
 lead beyond the limits allowable. To one important 
 series of dyes, however, known as the sulphur or 
 sulphide dyes, a brief reference must be made. 
 These dyes, which have only recently been exten- 
 
88 THE TREASURES OF COAL TAR 
 
 sively introduced, are obtained by heating various 
 organic materials with sulphur and sodium sulphide, 
 and are now manufactured in large quantity and 
 in various shades of red, yellow, brown, green, 
 violet, blue, and black. These dyes are, at the 
 present time, in much favour with dyers, for most 
 of them are substantive or direct-dyeing colours, 
 and possess a fastness to light and to washing at 
 least equal to that of such a fast dye as indigo. For 
 the production of these sulphide dyes aromatic 
 amino-compounds and nitro-derivatives of the 
 phenols are most suitable for use, but a number 
 have also been obtained from anthraquinone. 
 Three of these sulphur dyes, Khaki yellow C, Khaki 
 Brown C, and Cross Dye Black F.N.G., are largely 
 used at the present time for the production of khaki 
 colour on cotton. 
 
 The series of dyes to which reference has just 
 been made, the sulphur dyes as also the indanthrene 
 dyes derived from anthraquinone, belong to what 
 are called by dyers, vat-dyes. Owing to the insolu- 
 bility of these dyes in water, it is not possible to 
 prepare dye-baths in the ordinary manner ; and 
 for the purpose of dyeing a less direct method must 
 be employed. In using these dyes advantage is 
 taken of the fact that they are comparatively readily 
 reduced to compounds (so-called leuco-compounds), 
 which are soluble in alkalies. The material to be 
 
ANTHRACENE DYES AND VAT DYES 89 
 
 dyed is therefore dipped in the alkaline solution 
 of the leuco-compound now generally produced 
 from the dye by reduction with sodium hydro- 
 sulphite which is readily taken up both by animal 
 and vegetable fibres. On exposing the material 
 to the air, the original dye-stuff is produced in the 
 fibre in an exceedingly fast form, owing to the 
 oxidation of the leuco-compound by the atmo- 
 spheric oxygen. Sometimes the leuco-compound 
 of the dye is colourless or very faintly coloured, 
 but in other cases, e.g. in the case of the indanthrene 
 dyes, the leuco-compound may have a very marked 
 colour which is generally different from that of the 
 original dye-stuff. Thus, indanthrene yellow or 
 flavanthrene yields a blue leuco-compound which, 
 on exposure to the air, changes into a fast yellow 
 dye. Similarly, indanthrene red and indanthrene 
 green yield purple and blue reduction compounds 
 respectively, which are taken up by the fabric 
 from the bath and which then, on exposure to the 
 air, change to red and green. Although, formerly, 
 vat-dyeing was a somewhat difficult and uncertain 
 process, it has now been rendered as easy and as 
 simple as dyeing from the ordinary dye-bath. 
 That this is so is due largely to the careful investiga- 
 tion of the process consequent on the successful 
 artificial production of by far the most important 
 of the vat-dyes, indigo, a discussion of which is 
 reserved for the following chapter. 
 
CHAPTER VIII 
 
 INDIGO AND ITS DERIVATIVES 
 
 OF all the dyes now in use none equals in commercial 
 importance or has aroused such general interest as 
 indigo, not only owing to the fact that its production 
 from coal-tar hydrocarbons constitutes one of the 
 greatest achievements of pure and applied chemistry, 
 but also by reason cf the enormous economic con- 
 sequences of that success. Known from a very 
 remote period, indigo was, until about twenty years 
 ago, obtained solely from certain species of plants, 
 the Indigoferce, cultivated more especially in India, 
 China, and Egypt. Even in Europe, as late as the 
 seventeenth or eighteenth century, an indigo-bear- 
 ing plant, the woad (I satis tinctoria), was cultivated 
 to no small extent, and its cultivation still lingers 
 on in the eastern counties of England. In the 
 sixteenth century, with the opening up of trade 
 with the East, the superior Indian indigo began 
 to make its appearance in Europe, but for many 
 years, owing to the influence of those interested in 
 the growing of woad, its introduction met with a 
 powerful opposition, and the use of the " devilish 
 drug," as it was called, was prohibited by law. In 
 
 90 
 
INDIGO AND ITS DERIVATIVES 91 
 
 the eighteenth century, however, this ban was 
 removed and the use of Indian indigo gradually 
 extended over the whole of Europe. As a con- 
 sequence, the Indian indigo plantations came to 
 control the markets of the world. 
 
 Indigo does not occur as such in the plant, but as 
 a glucoside, called indican, which is found almost 
 exclusively in the leaves. To obtain the indigo, 
 the cut plant is placed in steeping vats and covered 
 with water. Owing to the presence of an enzyme 
 (or ferment) in the leaves, fermentation takes place 
 and the indican undergoes decomposition into 
 glucose and the leuco-compound (p. 88) of indigo. 
 On agitating the resulting solution with air, this 
 leuco-compound is oxidised with production of 
 indigo, which separates out as an insoluble powder. 
 Although indigo-blue or indigotin is the main con- 
 stituent, the natural indigo also contains varying 
 amounts of other compounds, indirubin or indigo 
 red, indigo brown, indigo yellow, and indigo gluten. 
 
 The industry was a large and lucrative one. In 
 1896-7 the area under cultivation amounted to 
 1,583,808 acres, and the weight of indigo produced 
 was 8433 tons, the value of which amounted to 
 about 4,000,000. It was a rich prize, therefore, 
 which the large German chemical manufacturers 
 saw before them when they set themselves earnestly 
 to capture the indigo market by producing the 
 dye artificially. The fight was a long one, for 
 
92 THE TREASURES OF COAL TAR 
 
 seventeen years the struggle went on, and close 
 on 1,000,000 was spent on the campaign, but in 
 the end the genius and resourcefulness of the 
 chemist, the persistence and enterprise of the direc- 
 tors of German chemical industry themselves expert 
 chemists won the day ; and in October 1897 syn- 
 thetic indigo was placed on the market in com- 
 petition with the product from the Indian planta- 
 tions. And what, to-day, is the result of the com- 
 petition ? Since 1896-7 the area under cultivation 
 for indigo fell from 1,583,808 acres to 214,500 acres 
 in 1912-13 ; and whereas, in 1896, India exported 
 indigo to the value of over 3,500,000, in 1913 her 
 export was only worth about 60,000. In 1913, 
 on the other hand, Germany exported nearly 6700 
 tons of pure synthetic indigo (indigotin or indigo- 
 blue), valued at about 2,750,000. In the above 
 period, moreover, the price of pure indigo was about 
 halved. 
 
 Whether the decline of the Indian indigo plan- 
 tations will continue cannot be foreseen. Until 
 recently, the processes employed in recovering the 
 indigo were crude and unscientific, but in recent 
 years many improvements have been effected, and 
 new species of plants, producing a larger proportion 
 of indigo, have been introduced. Further improve- 
 ments in this direction may still be possible, and as 
 many dyers still feel a preference for the natural 
 product, for securing certain effects at least, it is 
 
INDIGO AND ITS DERIVATIVES 93 
 
 possible that the Indian production of natural 
 indigo may still be maintained. A considerable 
 change has, however, already been produced in 
 Indian agriculture, and many acres of land formerly 
 under cultivation for indigo have been made avail- 
 able for the growth of cotton or of food-stuffs. 
 
 As far back as 1880 the artificial production of 
 indigo was first achieved by the German chemist 
 Adolf von Baeyer, who used as his raw material 
 the coal-tar hydrocarbon toluene. The patent 
 of this process was acquired jointly by the two 
 largest dye-manufacturing firms in Germany, the 
 Badische Anilin- und Soda-Fabrik of Ludwigshaven, 
 and Meister, Lucius & Briining of Hoechst. But 
 although the laboratory production of indigo con- 
 stituted an achievement of the highest scientific 
 importance, its commercial development proved 
 to be impracticable. Indigo could, of course, be 
 manufactured, and manufactured in quantity, but 
 not at a price which would allow the artificial to 
 compete with the natural dye. Moreover, the raw 
 material, toluene, was not at that time procurable 
 in sufficient amount to make the complete displace- 
 ment of the natural indigo possible. 
 
 Ten years later, in 1890, a new method of syn- 
 thesising indigo was discovered by Heumann, and 
 this method was subsequently developed along 
 two different lines into commercially successful 
 
94 THE TREASURES OF COAL TAR 
 
 processes. Both methods involve a considerable 
 number of distinct reactions and require the use of 
 a number of different substances, of which sulphuric 
 acid, ammonia, chlorine, acetic acid, and sodium 
 are the chief ; and the success of the synthesis as 
 a whole depends on the success with which each 
 step of the process can be carried out, and on the 
 cost of the substances employed. We shall now 
 see how the great difficulties involved were overcome. 
 In the process worked at Ludwigshaven the start- 
 ing-point in the synthesis is naphthalene, one of 
 the most abundant constituents of coal-tar ; and 
 the various steps of the process can be represented, 
 graphically, thus : 
 
 A /c \ 
 
 COOH 
 
 / co \ 
 
 Naphtha- 
 lene 
 
 COOH 
 Phthalic 
 acid 
 
 NH 2 
 
 OOH 
 Anthranilic 
 acid 
 
 H 2 COOH 
 
 XOOH 
 
 Phenyl-glycine-ortho- 
 carboxylic acid 
 
 NH NH 
 
 Indoxyl 
 
 (7) 
 
 CO 
 
 Indigotin 
 
INDIGO AND ITS DERIVATIVES 95 
 
 (i) Naphthalene is converted into phthalic acid 
 by heating with fuming sulphuric acid ; (2) phthalic 
 acid, on being heated, passes into phthalic anhy- 
 dride ; (3) phthalic anhydride, on being heated with 
 ammonia, yields phthalimide which (4) on being 
 treated with bleaching powder or with sodium 
 hypochlorite, forms anthranilic acid. By the action 
 of monochloracetic acid on the latter, (5), phenyl- 
 glycine-ortho-carboxylic acid is produced, and (6) 
 this compound, on being fused with caustic soda, 
 passes into indoxyl ; (7) and on oxidising this 
 substance with atmospheric oxygen, indigo-blue 
 or indigotin is formed. On attempting to carry 
 out this series of operations on a large scale, it was 
 found that all the steps, except the first, could be 
 carried out in a commercially successful manner. 
 In the case of the first stage of the process, however, 
 it was found that the conversion of naphthalene 
 into phthalic acid did not proceed sufficiently readily, 
 and the cost involved was so great that it rendered 
 the industrial production of indigo unremunerative. 
 A small obstacle, apparently, but a very effective 
 one ! While engaged in an endeavour to overcome 
 this difficulty, a fortunate mischance came to the 
 assistance of the manufacturer, for, through the 
 accidental breaking of a thermometer immersed 
 in the heated mixture of naphthalene and sulphuric 
 acid, it was discovered that mercury acts as an 
 efficient catalyst in the conversion of naphthalene 
 
96 THE TREASURES OF COAL TAR 
 
 into phthalic acid, and facilitates the process to 
 such a degree as to allow it to be carried out with 
 commercial success. It was, in fact, this fortunate 
 discovery that first ensured the success of the 
 synthetic production of indigo. 
 
 Another process, likewise based on the work of 
 Heumann, has also been successfully developed, 
 and has been employed for many years by the firm 
 of Meister, Lucius & Briining. In this case benzene 
 forms the starting-point. In the manner already 
 described (p. 54), benzene is converted first into 
 nitro-benzene and then into aniline. By the action 
 of monochloracetic acid on aniline, phenyl-glycine 
 is produced, and when this is fused with sodamide, 
 indoxyl is formed and can then be converted into 
 indigo tin by oxidation, as in the previous process. 1 
 The steps of this process can be represented thus : 
 
 C 6 H 6 > C 6 H 5 'N0 2 > C 6 H 5 -NH 2 --> C 6 H 5 -NH'CH 2 -COOH 
 Benzene Nitro- Aniline Phenyl-glycine 
 
 benzene 
 
 /NHv / NH \ / NH \ 
 
 ->C 6 H/ >CH 2 -> C 6 H/ >C = C< >C 6 H 4 
 \CO / \CO / \CO / 
 
 Indoxyl Indigotin 
 
 1 In August 1916 the indigo factory at Ellesmere Port, formerly 
 belonging to the German firm, Meister, Lucius & Briining, was 
 transferred to Messrs Levinstein, Ltd., of Blackley, Manchester, 
 and by the end of that year British-made synthetic indigo was 
 placed on the market. Although, owing to the exigencies of 
 the war, chloracetic acid was commandeered by the British 
 Government, a method of producing phenyl-glycine without the 
 use of that compound was successfully worked out by the 
 chemists of the British company, and an adequate supply of 
 British-made indigo (known as Indigo LL) is now available. 
 
INDIGO AND ITS DERIVATIVES 97 
 
 The commercial success of the production of 
 indigo depended, however, not only on the success 
 with which the different steps in the process could 
 be carried out, but also on the production of the 
 necessary reagents at a sufficiently low cost. In 
 the first process the conversion of naphthalene 
 requires a fuming sulphuric acid of a much greater 
 concentration than the acid produced by the old 
 leaden-chamber process ; and, moreover, large 
 quantities of sulphur dioxide were formed during 
 the reaction, the recovery of which in an advan- 
 tageous manner was an essential condition of suc- 
 cess. These requirements, therefore, led to the 
 development of the so-called " contact process," 
 in which a mixture of sulphur dioxide and air is 
 passed over heated platinised asbestos. The sulphur 
 dioxide combines with the oxygen of the air to form 
 sulphur trioxide which unites with water to form 
 sulphuric acid and the trioxide is passed into 
 concentrated sulphuric acid. In this way fuming 
 sulphuric acid, or " oleum " as it is technically 
 called, is produced. From this description the pro- 
 cess doubtless appears to be a very simple one, 
 but on attempting to employ it for the industrial 
 production of fuming sulphuric acid, a difficulty 
 was met with which seemed at first to be insur- 
 mountable. On passing the mixture of sulphur 
 dioxide and air over the platinised asbestos all went 
 well for a time ; but soon the reaction stopped and 
 G 
 
98 THE TREASURES OF COAL TAR 
 
 no more sulphur trioxide was formed. After a 
 considerable amount of investigation the cause of 
 this behaviour was traced to the presence of minute 
 quantities of arsenic in the sulphur dioxide, but it 
 still required some years' further work before a 
 successful method of removing this arsenic was 
 discovered. For the production of chlorine, also, 
 of which enormous quantities were required for the 
 manufacture of hypochlorite and of monochlor- 
 acetic acid, the old method of obtaining the gas 
 from hydrochloric acid was useless, and a new 
 method had to be introduced, namely, by passing 
 a current of electricity through a solution of common 
 salt or of potassium chloride, the chlorine being 
 then obtained in a pure state by liquefaction. In 
 this process, also, caustic soda and hydrogen are 
 produced ; the former of these is required for the 
 conversion of phenyl-glycine-ortho-carboxylic acid 
 into indoxyl, and the latter is now available for the 
 production of ammonia (also used in the indigo 
 synthesis), by direct combination with the nitrogen 
 of the air. The acetic acid, of which 3000 tons 
 are used annually in the manufacture of indigo, is 
 obtained by the distillation of 150,000 cubic yards 
 of wood. The whole most impressive story of 
 the development of the manufacture of synthetic 
 indigo is one of unshakable faith in science, of 
 chemical and engineering ability and resourceful- 
 ness, and of untiring perseverance. 
 
INDIGO AND ITS DERIVATIVES 99 
 
 Closely related, chemically, with indigo is that 
 other ancient dye, Tyrian purple. Some years ago 
 the nature of this dye was investigated by a German 
 chemist, Friedlander, who extracted it from the 
 glands of two species of marine snail, the Murex 
 brandaris and the Murex trunculus, and ascertained 
 that this most valuable of all the ancient dyes 
 is a derivative of indigo in which two atoms of 
 hydrogen are replaced by bromine ; and this dye, 
 for which, however, there is now no demand, can 
 be prepared, artificially, with comparative ease. 
 Other chlorine and bromine derivatives of indigo 
 are also known, and some are used as dyes under 
 the name of Ciba dyes. 
 
CHAPTER IX 
 
 DRUGS, PERFUMES, AND PHOTOGRAPHIC DEVELOPERS 
 
 IN the sixteenth and seventeenth centuries, following 
 on the long period of alchemistic activity and the 
 somewhat sterile search for the " philosopher's 
 stone," chemistry, under the influence of Para- 
 celsus, found its main glory in acting as the hand- 
 maid of medicine, and its chief task in the prepara- 
 tion of drugs and in the study of their action on 
 the human organism. But the efforts of those 
 early medico-chemists, or iatro-chemists as they 
 have been called, have been completely eclipsed 
 by the brilliant discoveries of the modern organic 
 chemist, who has made available for use a large 
 array of new drugs and medicinal preparations. 
 Since many of the most important of these sub- 
 stances are prepared from the constituents of coal 
 tar, it will readily be understood that this branch 
 of chemical industry as indeed the whole domain 
 of the so-called " fine " (organic) chemicals has 
 been developed mainly in Germany, and this largely 
 as an offshoot or companion industry of the manu- 
 facture of artificial colouring matters. This is 
 accounted for partly by the fact that in many cases 
 100 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 101 
 
 the raw materials of manufacture are the same, and 
 that many of the reagents and coal-tar " inter- 
 mediates," required for the manufacture of dyes, 
 serve also for the manufacture of drugs, perfumes, 
 and other organic chemicals. But a further reason 
 is to be found in the greater encouragement given 
 to the study of chemistry and to chemical research, 
 which has made possible the extraordinary achieve- 
 ments in the domain of synthetic dyes and drugs. 
 
 Although the production of synthetic drugs may 
 be said to date from the discovery of chloroform 
 and of chloral by Liebig in 1832 and who will be 
 so bold as to assess the value of these discoveries 
 to mankind? it was not till 1881 that the first 
 drug derived from the constituents of coal tar was 
 prepared. In that year were discovered kairine 
 and other antipyretic derivatives of quinoline, a 
 compound which is produced by heating aniline 
 with a mixture of sulphuric acid and glycerin, in 
 presence of nitrobenzene. These antipyretics had, 
 however, but small success. In 1883 Ludwig Knorr 
 prepared the important febrifuge, antipyrine or 
 phenazone, of which very large quantities were 
 at one time consumed. The commercial success, 
 indeed, of this drug was so great that, before the 
 expiration of the patent, the profits in one year are 
 stated to have amounted to no less than 60,000. 
 In the preparation of this compound there is used 
 a substance known as phenyl-hydrazine which is 
 
102 THE TREASURES OF COAL TAR 
 
 prepared from aniline, and this in turn from benzene. 
 The investigation of the physiological action of 
 antipyrine, undertaken on account of its supposed 
 chemical relationship with the alkaloid quinine, 
 led to the discovery of its valuable antipyretic 
 properties. It was the first of a series of synthetic 
 antipyretics which have, with a certain amount of 
 success, entered into competition with and partially 
 displaced the natural alkaloid quinine. It may, 
 however, be said that valuable as these drugs have 
 proved to be, they are drugs which combat the 
 symptoms of disease and not the disease itself, 
 and they do not possess the specific curative pro- 
 perties shown, for example, by quinine in relation 
 to malaria. 
 
 A derivative of antipyrine, known as pyramidone, 
 has also been introduced as an antipyretic. It is 
 more powerful than antipyrine, and has been found 
 to possess certain advantages over the latter, more 
 especially in not exercising an injurious influence 
 on the heart. 
 
 In 1887 antipyrine met with a powerful com- 
 petitor, antifebrine, and it is to the discovery 
 of this substance, more especially, that the great 
 development which has taken place in recent times 
 in the industrial production of synthetic drugs, is 
 due. The discovery of the antipyretic properties 
 of antifebrine was due to a mistake on the part of 
 a laboratory boy who supplied this substance in 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 103 
 
 place of naphthalene. During a pharmacological 
 investigation of the substance its strongly anti- 
 febrile action was detected, and from a chemical 
 analysis it was learned what the substance really 
 was. 
 
 Antifebrine is the trade name for the compound 
 known in chemistry as acetanilide, which, as is 
 shown by the formula, CH 3 -CO-NH-C 6 H 5 , is formed 
 by the combination of acetic acid, CH 3 -CO-OH, 
 with aniline, NH 2 -C 6 H 5 , with the elimination of 
 the OH-group from acetic acid and a hydrogen 
 atom from aniline. Mixed with bicarbonate of 
 soda, acetanilide has also been sold as a " head- 
 ache powder." 
 
 The discovery of the physiological action of anti- 
 febrine, and the circumstances, more especially, 
 under which that discovery was made, greatly 
 stimulated the investigation of the physiological 
 action of other substances. As a result of these 
 investigations, interesting relationships between 
 physiological action and chemical constitution 
 became known. Thus, the physiological action is 
 found, in many cases, to be due to the presence 
 of certain groupings of atoms in the molecule, and 
 can be modified or even entirely altered by the 
 introduction of different groups into the molecule. 
 Thus, aniline itself is a powerful febrifuge, but at 
 the same time it is highly poisonous, owing to its 
 destructive action on the red blood corpuscles. 
 
104 THE TREASURES OF COAL TAR 
 
 By introducing the group CH 3 -CO (aeetyl), the com- 
 pound is rendered more stable and the toxicity is, 
 in consequence, reduced. 
 
 Similar considerations led also to the production, 
 in 1887, of another antipyretic and antineuralgic 
 
 /0-C 2 H 6 
 drug, phenacetine, C 6 H 4 <^ , the derivation 
 
 X NH-CO'CH 3 
 
 /OH 
 
 of which from C 8 H 4 <^ (para-amino-phenol) is 
 
 X NH 2 
 
 obvious. This compound is prepared from phenol 
 in the same way as aniline is prepared from benzene 
 (p. 54), by converting phenol into a nitro-phenol, 
 
 /OH 
 
 CeH 4 \ , and then converting the nitro-group 
 
 X NO 2 
 
 into the amino-group by means of iron and hydro- 
 chloric acid. Phenacetine is the most important 
 and most largely used of all the synthetic antipyretic 
 and analgesic drugs, over eight tons of this com- 
 pound being imported into Great Britain in 1909. 
 
 Valuable medicinal preparations have also been 
 derived in recent years from salicylic acid. This 
 
 /OH 
 
 compound, C 6 H 4 <^ , prepared from phenol 
 XOOH 
 
 with the help of carbon dioxide or carbonic acid 
 gas, possesses anti-neuralgic and anti-rheumatic 
 properties, but its use gives rise to disorders of the 
 digestion. By the introduction of the acetyl-group 
 (CH 3 -CO), however, one obtains the compound, 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 105 
 
 acetyl-salicylic acid, C 6 H 4 < , which, under 
 
 the name of aspirin, has come to be recognised as 
 one of the most valuable of the anti-neuralgic and 
 anti-rheumatic drugs. Discovered in 1899, and 
 formerly manufactured only in Germany, it is 
 now produced by several firms in England and is 
 sold under the registered trade names of empirin 
 (Burroughs Wellcome & Co.), regepyrin (Boots), 
 etc. 
 
 Other derivatives of salicylic acid are employed 
 as intestinal antiseptics. Thus, by the introduc- 
 tion of the phenyl group into salicylic acid, there is 
 
 /OH 
 
 produced the compound salol, C 6 u/ , a 
 
 N coo-c 6 H 5 
 
 valuable intestinal antiseptic. It is readily prepared 
 from phenol and salicylic acid, or even by heating 
 salicylic acid alone. Other derivatives of a similar 
 nature can also be prepared and find a similar 
 application. These compounds, although not acted 
 on by the stomach juices, are broken up by the 
 alkaline secretions in the intestine, with produc- 
 tion of salicylic acid. This compound then produces 
 partial asepsis by restricting the development of 
 bacteria and undue fermentative action in the 
 alimentary canal. 
 
 With regard to general antiseptics, we have 
 already seen (p. 25) that phenol (carbolic acid) 
 
io6 THE TREASURES OF COAL TAR 
 
 and; in a still greater degree, the cresols, have a 
 powerful bactericidal action. The cresols, more- 
 over, have the advantage over phenol in being less 
 toxic to the organism. By the introduction of 
 bromine into the molecule of phenol or cresol, the 
 bactericidal action is greatly increased, so that 
 pentabrom-phenol (phenol with five bromine atoms), 
 for example, is about five hundred times as effective 
 as phenol. Similarly, tetrabrom-ortho-cresol (ortho- 
 cresol with four atoms of bromine) is a valuable 
 antiseptic which is almost non-toxic, but which, 
 even in a dilution of only i part in 200,000, will 
 destroy diphtheria bacilli. It is, in this respect, 
 250 times as effective as phenol. 
 
 In 1832, as we have already seen, Liebig dis- 
 covered the compound chloroform, the introduction 
 of which as an anaesthetic by Sir James Simpson, 
 in 1847, marked the beginning of a new era in 
 operative surgery. But neither this nor any of 
 the other general anaesthetics now employed are 
 derived from coal tar. In recent years, however, a 
 number of valuable local anaesthetics, derived from 
 the constituents of coal tar, have been prepared 
 and introduced as substitutes for the naturally 
 occurring alkaloid cocaine. Thus, anaesthesine, 
 
 NH 2 <^ \co-oc 8 H 8 , is an important local anaes- 
 thetic prepared from benzoic acid, which is, in turn, 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 107 
 
 prepared indirectly from toluene, and novocaine 
 is a similar compound of rather more complex 
 constitution, 
 
 NH 2 /~ ~\CO-OCH 2 -CH 2 'N(C 2 H 6 ) 2 ,HC1 
 
 Stovaine, alypine, and beta-eucaine are also 
 valuable local anaesthetics (the last-mentioned is 
 also used in the treatment of sciatica and neuralgia) 
 in the preparation of which coal-tar products play 
 a part. 
 
 Some of these anaesthetics are frequently used 
 along with another compound, adrenaline, which, 
 although not an anaesthetic, has powerful physio- 
 logical properties. When adrenaline, the active 
 principle of the supra-renal glands, is injected 
 subcutaneously or even applied externally to 
 the skin, it produces a violent contraction of the 
 arteries, with the result that the blood pressure 
 rapidly rises, the blood is driven away from the 
 injected tissues, and " bloodless " surgery becomes 
 a possibility. 
 
 Adrenaline was isolated for the first time by a 
 Japanese chemist, Takamine, in 1901, from the 
 supra-renal glands of sheep and oxen, close on 
 1000 Ibs. of tissue (representing the glands from 
 20,000 oxen) being required to yield I Ib. of adrena- 
 line. Within a few years, however, the chemical 
 nature of adrenaline had been ascertained, and a 
 process for preparing it on an industrial scale was 
 
io8 THE TREASURES OF COAL TAR 
 
 worked out in the laboratories of the great dye- 
 manufacturing firm of Meister, Lucius & Briining, 
 in Germany. It is now placed on the market under 
 the name of suprarenine. It is a derivative of the 
 OH 
 
 MOH 
 , which, although usually 
 
 prepared from guaiacol, a constituent of beech- 
 wood tar, may also be prepared from phenol or 
 carbolic acid. 
 
 It has already been pointed out how the industry 
 of synthetic drugs is closely related to that of 
 synthetic dyes ; and this relationship has become 
 a still closer one in recent years owing to the 
 important discovery of " dye drugs " which we 
 owe mainly to the brilliant investigations of Paul 
 Ehiiich in Germany. The synthetic substitutes for 
 quinine are, as we have seen, merely symptomatic 
 drugs, but the work of Ehrlich opened up a new 
 field and led to the discovery of drugs which 
 exercise specific curative properties. 
 
 Guided by the principle that a drug acts only on 
 organisms by which it is absorbed, Ehrlich studied 
 the effect of various dyes on different tissues and 
 cells, and showed that certain dyes will " stain " 
 certain tissues but leave others unstained, just as 
 certain colouring matters will dye wool but not 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 109 
 
 cotton. Thus the dye, methylene blue, is absorbed 
 by and stains only the living nerve, so that when 
 the dye is injected into a living animal the nerve 
 tissues, but not the surrounding structures, are 
 stained. Similarly, different bacteria can be dis- 
 tinguished by their selective absorption of dyes. 
 This property of selective absorption has been 
 turned to use with especial success in the treatment 
 of diseases due to protozoal parasites, because it 
 becomes possible to introduce into an organism 
 substances which are poisons for the parasites but 
 are not absorbed by and are therefore not harm- 
 ful to the cells of the organism itself. Thus, 
 investigation showed that certain azo-dyes of the 
 type of Congo red are poisons to trypanosomes, 
 the trypanosome of the South American horse 
 disease, " mal de caderas," being destroyed by 
 the dye trypan red, and the trypanosome of the 
 cattle disease, " piroplasmosis," by another azo-dye, 
 trypan blue, derived from tolidine and naphthalene. 
 These dyes are therefore specific curative agents 
 for these diseases. Similarly, atoxyl (arsamin 
 or soamin), the sodium salt of a compound 
 obtained by heating arsenic acid with excess of 
 aniline, has the property, when injected into the 
 body, of killing the parasite Trypanosoma gambiense, 
 which causes the disease of " sleeping sickness." 
 
 Still more important is the success with which 
 the property of selective absorption has been utilised 
 
no THE TREASURES OF COAL TAR 
 
 in finding a cure for the disease syphilis, which is 
 due to a micro-organism, the Spirochcete pallida. 
 It has been known from the time of Paracelsus 
 that mercury is a specific against this disease, but 
 mercury is harmful also to the human organism. 
 The problem, therefore, which Ehrlich set himself 
 was to prepare a compound which would contain 
 a toxic material and which would be absorbed by 
 the germ of the disease but not by the human 
 organism. After many trials and many failures he 
 prepared the now well-known remedy salvarsan, or 
 as it is also called " 606," which is the serial number 
 of the compound in Ehrlich's record of preparations. 
 This compound, now manufactured in England by 
 Burroughs Wellcome & Co., under the name khar- 
 sivan, is a benzene derivative similar in structure to 
 an azo-dye but containing two arsenic (As) atoms in 
 place of the azo-group ( N : N ), thus : 
 
 HO / \ As = As /~ ~\OH 
 
 I I 
 
 NH 2 ,HC1 NH 2 ,HC1 
 
 As one of the most recent examples of " chemo- 
 therapy " based on selective absorption, there may 
 be mentioned the discovery by Dr Browning, of the 
 Bland- Sutton Institute of Pathology in London, 
 that the yellow coal-tar dye, trypa-flavine or 
 acri-flavine a derivative of a compound known 
 as acridine has the most valuable property that 
 while it destroys the germs of blood-poisoning it 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS in 
 
 does not interfere with the white " warrior cells " 
 of the blood, which are the natural defence of the 
 patient against the septic organisms. It is, there- 
 fore, an ideal antiseptic as compared with the 
 ordinary antiseptics which destroy with equal 
 impartiality the pathogenic organisms and their 
 natural foes, the white " warrior cells." 
 
 Owing to the practical monopoly enjoyed by 
 Germany in the manufacture of synthetic drugs, 
 this country was placed, by the outbreak of war, 
 in a position of great gravity as a result of the 
 cutting off of German supplies ; and the shortage 
 of drugs which was thereby produced was clearly 
 reflected in a great increase in price, as shown in 
 the following table : 
 
 EFFECT OF THE WAR ON THE PRICE OF 
 SYNTHETIC DRUGS 
 
 
 Price per pound. 
 
 
 Immedi- 
 
 
 
 
 
 ately 
 
 Jan. i, 
 
 Jan. i, 
 
 Jan. i, 
 
 
 before 
 
 1915- 
 
 1916. 
 
 1917- 
 
 
 war. 
 
 
 
 
 
 S. D. 
 
 S. D. 
 
 S. D. S. D. 
 
 S. D. 
 
 Acetanilide 
 
 o 10 
 
 2 O 
 
 6 9-7 o 
 
 2 IO 
 
 Acetylsalicylic acid . 
 
 2 O 
 
 6 6 
 
 48 0-50 o 
 
 18 6 
 
 Phenacetin . 
 
 2 9 
 
 6 6 
 
 60 o 
 
 92 6 
 
 Phenazone 
 
 6 6 
 
 9 6 
 
 75 o 
 
 33 o 
 
 Salol . 
 
 I 10 
 
 4 9 
 
 47 o 
 
 10 6 
 
 Salicylic acid 
 
 I O 
 
 5 o 
 
 20 o 
 
 4 9 
 
 Sodium salicylate . 
 
 i 3 
 
 *} o 
 
 22 
 
 5 9 
 
112 THE TREASURES OF COAL TAR 
 
 The situation, however, serious as it was, was 
 prevented from becoming disastrous through the 
 efforts of the academic chemists of the country. 
 The chemical laboratories of our Universities and 
 Technical Colleges were converted into miniature 
 factories, and a supply of the most necessary drugs 
 was ensured. In time the work of production 
 could be taken up by the regular manufacturers, 
 and supplies also began to be obtained from neutral 
 countries, more especially Switzerland and the 
 United States. That a vast improvement in the 
 situation has now taken place is amply shown by 
 the prices quoted in the last column of the above 
 table. 
 
 Not only has the chemist garnered from the 
 boundless treasures of coal tar colouring matters 
 which rival the manifold tints of flowers, but he 
 has also evolved from that same uninviting source 
 substances which surpass in sweetness the sweetest 
 of Nature's products. During the course of a purely 
 scientific investigation carried out in the laboratory 
 of Professor Ira Remsen in the Johns Hopkins 
 University in America, Dr C. Fahlberg, in 1879, 
 accidentally discovered that one of the compounds 
 which he had prepared possessed a remarkably 
 sweet taste ; and he afterwards (in 1887) manu- 
 factured the compound and placed it on the market 
 under the name of saccharine. The substance is 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 113 
 
 derived from toluene, which, by successive treat- 
 ment with concentrated sulphuric acid, chlorine, 
 ammonia, and oxidising agents such as perman- 
 ganate of potash, is transformed and built up into 
 the final product, benzoic sulphimide or saccharine, 
 
 NH. It is a white crystalline powder 
 
 and has a sweetness five hundred times greater than 
 that of cane sugar. How great was the disaster 
 which threatened to overtake the cane and beet- 
 root sugar industry as a result of this discovery can 
 readily be understood. It was as if the story of 
 the madder plantations (p. 81) was going to be 
 retold for the sugar plantations of the West Indies 
 and other parts of the world. The whole machinery 
 of Government intervention and supervision was 
 therefore set working, and the general use of sac- 
 charine as a sweetening agent in articles of human 
 consumption was prohibited, the manufacture of 
 the compound being put under licence and its sale 
 placed in the hands of the druggist. This step, 
 however, was taken not merely, perhaps not even 
 mainly, for the sake of upholding a threatened 
 industry, but from a recognition of the fact that 
 saccharine, unlike sugar, has no nutritive value at 
 all, and that, although it is of importance as an 
 
H4 THE TREASURES OF COAL TAR 
 
 edulcorant for use, more especially, by those to 
 whom sugar is forbidden (e.g. those suffering from 
 diabetes), its uncontrolled and unlimited consump- 
 tion is harmful and even poisonous to the human 
 organism. Saccharine is a medicament, and should 
 be treated as such. 
 
 Saccharine is a substance which dissolves in 
 water only, with difficulty. By treating it with 
 carbonate of soda, however, it is converted into a 
 sodium salt of saccharine which, although some- 
 what less sweet than saccharine itself, readily dis- 
 solves in water. Similarly, one can obtain the readily 
 soluble ammonium salt, which has the remarkable 
 property that it is even sweeter than saccharine 
 itself, its sweetness being six hundred times greater 
 than that of cane sugar. 
 
 Formerly manufactured almost exclusively in 
 Germany and Switzerland, preparations of sac- 
 charine are now made in England and sold in 
 tabloid form under the name of saxin (Burroughs 
 Wellcome & Co.). 
 
 While we may regard the synthetic production 
 of colouring matters and drugs as being one of the 
 greatest achievements of organic chemistry, and 
 one which must take an important place in the 
 history of human endeavour and of human civilisa- 
 tion, notable success has also been obtained in the 
 artificial production of those sweet-smelling essences 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS] 115 
 
 and spices which in all ages and by all peoples have 
 been held in high esteem. In some cases the chemist 
 has succeeded in preparing substances which are 
 identical with those to which the odours of the 
 flowers are due ; in other cases the synthetic pro- 
 ducts merely imitate the naturally occurring per- 
 fumes and spices. In some cases the sweet-smelling 
 substance is built up, step by step, from the simple 
 compounds, benzene, toluene, etc., occurring in 
 coal tar ; in other cases these perfumes are obtained 
 by the transformation of naturally occurring, com- 
 plex compounds, as in the transformation of the 
 compound eugenol (occurring in oil of cloves) into 
 vanillin, the active principle occurring in the 
 vanilla bean, or of the compound citral (a constitu- 
 ent of oil of lemon-grass) into ionone or imitation 
 violet. Not even in the case of the purely synthetic 
 perfumes, however, are all the compounds derived 
 from coal tar. 
 
 The first of the naturally occurring perfumes to be 
 prepared by the chemist and first of all by W. H. 
 Perkin in 1868 from the products of coal tar 
 was coumarin, the fragrant principle of the Tonka 
 bean, of the sweet woodruff (Asperula odorata), 
 and of certain clovers, and used in the preparation 
 of the perfumes known as Jockey Club and New- 
 Mown Hay. 
 
 This compound can be prepared from phenol 
 or carbolic acid. Phenol is first converted into 
 
n6 THE TREASURES OF COAL TAR 
 
 >H 
 
 salicylic aldehyde, | , a substance which is 
 
 'COH 
 
 obviously closely related to salicylic acid (p. 104), 
 and the salicylic aldehyde can then, as W. H. 
 Perkin showed, readily be transformed, through 
 the agency of acetate of soda, into coumarin, 
 
 _o _ co 
 
 Vanillin, also, although generally 
 
 H:CH 
 
 obtained from oil of cloves, can also be prepared 
 from toluene. 
 
 To these earliest synthetic sweet-smelling sub- 
 stances numerous others have since been added, 
 so that from the constituents of coal tar the main 
 odoriferous principles of a considerable number 
 of naturally occurring essential oils and perfumes 
 have now been prepared. Among these one may 
 mention oil of winter-green (methyl salicylate, from 
 wood spirit and salicylic acid), oil of bitter almonds 
 (benzaldehyde, from toluene), hawthorn blossom 
 (anisic aldehyde, from phenol), oil of cinnamon 
 (cinnamic aldehyde, from benzaldehyde or from 
 toluene), Spircea ulmaria or meadowsweet (salicylic 
 aldehyde, from phenol). Imitation musk perfumes 
 can be prepared from toluene ; and nitrobenzene, 
 as we have already seen (p. 53), was prepared at 
 an early date and used, under the name of " essence 
 of mirbane," as a substitute for oil of bitter almonds. 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 117 
 
 Owing to the synthetic production of these and 
 many other odoriferous compounds at a cost very 
 much less than that of the natural perfumes, a 
 very great extension of the use of such substances 
 for the scenting of soaps, creams, and other toilet 
 preparations, has taken place. 
 
 But if the chemist by his transformation of the 
 constituents of coal tar has revolutionised the art 
 of dyeing and the science of therapeutics, and has 
 produced compounds which rival the perfumes 
 of the violet and the rose, he has exercised also 
 an important influence on that most practised of 
 all the arts, photography. The photographic dry 
 plate or film is coated with a layer of gelatin con- 
 taining a fine emulsion of the light-sensitive salt, 
 silver bromide. This salt is, however, not equally 
 sensitive to all the rays of light, but is mainly 
 affected by blue and violet rays, while red and 
 yellow light has practically no action. A photo- 
 graph taken with such a plate will, therefore, not 
 reproduce a multi-coloured object with the proper 
 colour-values the yellows, for example, will appear 
 darker than the blues. By dyeing the film with 
 different coal-tar dyes, however, the plate can be 
 made sensitive to light of different colours, and, in 
 this way, " orthochromatic " and " panchromatic " 
 plates have been prepared, the former specially 
 sensitised for green and yellow, the latter sensitised 
 for light of all colours. 
 
n8 THE TREASURES OF COAL TAR 
 
 But coal tar provides for the needs of the photog- 
 rapher not only by furnishing him with colour- 
 sensitive plates, but also by placing at his service 
 a considerable number of different " developers," 
 or substances by which the latent photographic 
 image can be made to appear. Since the character 
 of the image depends to some extent on the developer 
 employed, the intelligent worker is enabled readily 
 to obtain the special effect desired. 
 
 Substances suitable for use as developers belong 
 to the class known as " reducing " substances, and 
 must contain two or more hydroxyl (OH) groups, 
 or at least one hydroxyl group and one amino (NH 2 ) 
 group. Of such substances quite a number have 
 been prepared from the constituents of coal tar, 
 and find a more or less extensive use. One of the 
 most familiar and most widely used of these is 
 " pyro," or pyrogallic acid, or pyrogallol, as it is 
 known in chemistry. This is a derivative of benzene 
 
 OH 
 
 containing three hydroxyl groups, thus : 
 
 but although it can be prepared synthetically from 
 phenol (and therefore also from benzene, p. 20), it 
 is usually prepared from gallic acid, and is there- 
 fore not strictly to be included among the coal-tar 
 products. The first true coal-tar product to be 
 used as a photographic developer was hydroquinone, 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 119 
 
 a substance which has found much favour with 
 amateurs (especially when combined with other 
 developers), because of the fact that it does not, 
 as pyro does, stain the fingers. Although this 
 compound had long been known it was not till 
 1880 that its use as a photographic developer was 
 suggested by Sir William Abney. First obtained 
 from quinic acid, which is found in the medicinal 
 extract of Peruvian bark, it was later discovered 
 that it could very readily be prepared from aniline, 
 and the production of the compound was thus 
 established on a commercially successful basis. 
 By treating aniline with a cold solution of sulphuric 
 acid and bichromate of soda, it is converted into a 
 
 X 
 
 compound, I jl, known as quinone (C 6 H 4 O 2 ), and 
 
 this substance can then be readily converted into 
 
 OH 
 
 n 
 
 hydroquinone, , by treatment with sulphurous 
 
 OH 
 acid or a solution of sulphur dioxide in water. 
 
 Since hydroquinone yields strong and sometimes 
 rather harsh negatives it is very frequently com- 
 bined, for general use, with some other developer, 
 which gives softer effects. One of the commonest 
 
120 THE TREASURES OF COAL TAR 
 
 of these is " metol." When phenol is treated with 
 a mixture of nitric and sulphuric acids, it yields 
 OH 
 
 para-nitrophenol, , and when this is " reduced " 
 
 N0 2 
 
 with tin and hydrochloric acid, it is converted into 
 OH 
 
 para-aminophenol, , just as nitrobenzene is 
 
 NH 2 
 
 converted by similar treatment into aniline. The 
 salt of this para-aminophenol with hydrochloric 
 acid is the effective constituent of the developer 
 rodinal. If one replaces one of the hydrogen atoms 
 of the amino-group by the methyl-group (CH 3 ), 
 
 OH 
 
 one obtains the compound, , which is used 
 
 NH-CH 3 
 
 as a developer under the name of scalol. The salt 
 of scalol with sulphuric acid is the effective con- 
 stituent of the developer metol. The compound 
 OH 
 
 1NH'CH 3 
 
 , which is isomeric with " scalol," forms 
 
 the basis of the developer ortol. 
 
DRUGS & PHOTOGRAPHIC DEVELOPERS 121 
 
 Amidol is another developer also derived from 
 phenol, but containing two amino-groups, thus: 
 OH 
 
 )NH 2 
 
 ; and glycin, a somewhat more complex 
 
 NH 2 
 
 compound, having the formula, | , is 
 
 \f 
 
 NH'CH 2 -COOH 
 
 obtained by heating para-aminophenol (see above) 
 with monochloracetic acid, C1-CH 2 -COOH. 
 
 The above compounds are all derived from the 
 coal-tar hydrocarbon benzene, but similar de- 
 velopers have also been derived from naphthalene. 
 Of these the best known is eikonogen, a com- 
 pound discovered by the late Professor Meldola. 
 Its relation to naphthalene (p. 43) is clearly seen 
 from the formula, 
 
 NH 2 
 
 , 
 
 SOoONal 
 
 In view of the enormous development of the 
 practice of photography, it will readily be realised 
 how great is the wealth derived, in this particular 
 direction alone, from the invaluable coal tar. 
 
CHAPTER X 
 
 EXPLOSIVES 
 
 THE history of civilisation is, in large measure, 
 the history of man's ability to utilise, control and 
 direct energy, and in this respect the civilisation of 
 the nineteenth and twentieth centuries shows an 
 enormous advance on that of all previous times. 
 And it excels not only by the amount of energy 
 which it turns to useful account, but also by the 
 degree to which it can concentrate energy ; for 
 material progress may depend just as much, and 
 even more, on the concentration of energy as on 
 the actual amount of energy expended. Herein 
 lies the value of explosives, which represent highly 
 concentrated forms of potential energy, capable 
 of being set in motion at will, and of producing 
 stupendous results. In the peaceful progress of 
 civilisation, no less than in the devastation and ruin 
 of war, explosives have played an all-important 
 part, and have made possible the great engineering 
 works of the world, like the Suez and Panama 
 Canals, or the removal, in 1885, of the reefs, known 
 as Hell Gate, in the channel of the East River at 
 New York. On this occasion over one hundred 
 122 
 
EXPLOSIVES 123 
 
 tons of explosives, rackarock and dynamite, were 
 employed, and millions of tons of rock were dis- 
 lodged. But this " blast " was small compared 
 with the earth-shattering explosion of four hundred 
 and fifty tons of high explosive which preceded 
 the capture of the Messines Ridge by the British 
 Army on the morning of June 7th, 1917. 
 
 An explosive may be denned as a substance or 
 mixture, solid or liquid, capable of undergoing 
 extremely rapid combustion or decomposition, with 
 production of gaseous substances which occupy 
 a volume it may be ten or twelve thousand times 
 as great as that of the explosive itself. In the case 
 of gunpowder, cordite, and other propellants (low 
 explosives), there is a rapid combustion of the 
 explosive, but in the case of high explosives to 
 which class all the coal-tar explosives belong the 
 molecules of the compound are in a somewhat un- 
 stable condition, and, when subjected to a suit- 
 able shock, undergo decomposition into more stable 
 substances. This decomposition is generally initi- 
 ated by means of a " detonator," or substance 
 which is, comparatively, very sensitive to shock, 
 and the " explosive wave " which is set up is trans- 
 mitted with a very great velocity amounting in 
 some cases to more than four miles per second 
 and so causes an almost instantaneous decom- 
 position of the explosive. 
 
 Although, the first real explosive, black gun- 
 
124 THE TREASURES OF COAL TAR 
 
 powder, was discovered (by Roger Bacon) in the 
 thirteenth century, no further advance in the 
 chemistry of explosives was made until the nine- 
 teenth century. In that century a number of new 
 and very powerful explosives were introduced, and 
 although two of the most important of these gun- 
 cotton and nitroglycerin (dynamite) are not de- 
 rived from coal tar, derivatives of this have, 
 in recent times, begun to play a most important 
 part, especially in connection with naval and 
 military operations. 
 
 The first coal-tar explosive to be obtained was 
 picric acid. Although discovered in 1771, it was not 
 till 1843 that it was prepared from phenol, by the 
 action of a mixture of sulphuric and nitric acids. 
 The substance is now also prepared from benzene, 
 from which compound, also, phenol itself is now 
 largely produced owing to the greatly increased 
 demand for this substance. 
 
 Picric acid, or tri-nitro-phenol (to give it its 
 chemical name), is derived from phenol by the 
 replacement of three hydrogen atoms by three 
 nitro-groups (NO 2 ), as is represented by the formula 
 C 6 H 2 (N0 2 ) 3 -OH, or 
 
EXPLOSIVES 125 
 
 It is a lemon-yellow coloured crystalline substance 
 which found its first use, and still finds use to a 
 slight extent, as a dye for silk and wool ; and it 
 readily stains the skin also of a yellow colour. The 
 explosive decomposition of picric acid can be 
 effected by means of a suitable powerful detonator, 
 but, under ordinary conditions, it is a quite stable 
 substance which melts at a temperature of 122 C. 
 (252-6 F.), and, when strongly heated, burns with 
 production of a large amount of black smoke. This 
 stability and insensitiveness to ordinary shocks 
 and blows are clearly a great advantage from the 
 point of view of safety in handling ; and the sub- 
 stance picric acid was adopted in 1885 by the 
 French Government as a high explosive for filling 
 shells, under the name of melinite (from the honey- 
 like appearance of the molten compound), and some 
 years later by the British Government, under the 
 name of lyddite (from Lydd, in Kent, where its 
 explosive properties were tested). Other countries, 
 also, have adopted picric acid as a high explosive for 
 military purposes, and it forms the sole or main con- 
 stituent of the explosives pertite (Italy), shimosite 
 (Japan), and Dunnite (United States). Some idea 
 of the power of this explosive will be gained from 
 the statement that when one pound of picric acid 
 is exploded it liberates an amount of energy equal 
 to that required to raise a weight of over a ton to 
 a height of more than a hundred yards. 
 
126 THE TREASURES OF COAL TAR 
 
 From meta-cresol (p. 44), tri-nitro-cresol (known 
 in France as cresylite), similar to tri-nitro-phenol 
 (picric acid), has also been prepared. It is less 
 powerful than picric acid, but has sometimes been 
 used for mixing with the latter in order to lower 
 its melting-point, and so render it less inconvenient 
 to manipulate. Its ammonium salt was formerly 
 used as a high explosive by Austria under the name 
 of ecrasite. 
 
 Although picric acid itself is comparatively in- 
 sensitive to shock, it has the disadvantage that it 
 forms compounds (picrates) with metals, such as 
 lead, copper, iron, etc., which are much more 
 sensitive to shock and which may cause premature 
 explosion of the shell. Hence the necessity for 
 coating the interior of the shell with a varnish. 
 The priming composition known as Brugere powder 
 is a mixture of ammonium picrate and saltpetre 
 (potassium nitrate). 
 
 From phenol and methyl chloride there is pre- 
 pared the compound anisole, C 6 H 5 -OCH 3 ; and by 
 nitrating this one obtains tri-nitro-anisole, an 
 explosive which has recently been used by the 
 Germans for filling bombs. 
 
 From the hydrocarbons of coal-tar, also, powerful 
 explosives can be prepared. Of these the most im- 
 portant is undoubtedly tri-nitro-toluene, obtained 
 by nitrating toluene with a mixture of concentrated 
 
EXPLOSIVES 127 
 
 sulphuric and nitric acids. It forms a white crystal- 
 line substance which melts at a much lower tem- 
 perature (81 C. or 177-8 F.) than picric acid, is 
 even less sensitive to mechanical shock and rough 
 usage than this explosive, and does not form 
 dangerously explosive salts with metals. Although 
 it acts as a powerful explosive when exploded by 
 means of a suitable detonator, it is a comparatively 
 stable substance, so stable, indeed, and safe to 
 handle, that it does not come under the provisions 
 of the Explosives Act with respect to its manu- 
 facture, transport, and storage. The advantages 
 which tri-nitro-toluene thus possesses over picric 
 acid led to its adoption by Germany in 1902, and 
 by other Governments at a later date, as a high 
 explosive for filling shells ; and for this purpose 
 it has, although a less powerful explosive than 
 picric acid, largely displaced that compound. It 
 has also to a large extent taken the place of gun- 
 cotton as the explosive filling for torpedoes and 
 submarine mines. In the British Services it is 
 known as trotyl, or as T.N.T. When ignited, 
 trotyl, like picric acid, burns without explosion as 
 a rule, but disastrous explosions have also occurred 
 through the combustion of large quantities of the 
 compound, especially in presence of ammonium 
 nitrate. 
 
 The combustion of T.N.T. , as well as its decom- 
 position by detonation, are accompanied by the 
 
128 THE TREASURES OF COAL TAR 
 
 production of dense black clouds of carbonaceous 
 or sooty matter, owing to there being insufficient 
 oxygen in the compound to combine with all the 
 carbon present ; and this has led to the nicknames 
 of " Coal boxes " and " Jack Johnsons " being 
 applied to the shells filled with this explosive. In 
 order to secure more perfect combustion and, at 
 the same time, to reduce the amount of trotyl 
 required, ammonium nitrate (NH 4 NO 3 ), a substance 
 containing an excess of oxygen, is frequently added. 
 In this way the British service high explosive 
 amatol, a mixture of trotyl and ammonium nitrate, 
 is obtained. 
 
 Not only is trotyl used as an explosive by itself, 
 but it also forms a constituent of a number of com- 
 posite explosives. Thus, the Austrian explosive 
 ammonal is a mixture of trotyl (30 per cent.), 
 ammonium nitrate (47 per cent.), aluminium powder 
 (22 per cent.), and charcoal (i per cent.). By the 
 combustion of the aluminium powder the tempera- 
 ture of the explosion is considerably raised and 
 the explosive force consequently increased. Trotyl 
 also forms a constituent of the Belgian high ex- 
 plosive macarite (trotyl and lead nitrate), and 
 of the blasting explosives rexite and Withnell 
 powder. 
 
 Di-nitro-toluene, in which only two NO 2 -groups 
 are present, is also used to some extent in the pre- 
 paration of composite explosives for blasting pur- 
 
EXPLOSIVES 129 
 
 poses. Of these the most important are the various 
 cheddites (so called from Chedde, in France, where 
 they are manufactured), consisting, for example, of 
 ammonium perchlorate and di-nitro-toluene, mixed 
 with a small amount of castor oil, to diminish 
 the sensitiveness of the mixture to friction. Other 
 similar mixtures are also prepared under the name 
 cheddite. 
 
 Although of less importance than tri-nitro-toluene, 
 the nitro-derivatives of benzene are also used to 
 a considerable extent, more especially in the pro- 
 duction of composite blasting explosives. Even 
 nitrobenzene (C 6 H 5 -N0 2 ), itself, although not an 
 explosive, is used as a combustible material in such 
 explosives as rackarock (potassium chlorate and 
 nitrobenzene) and petrofracteur (potassium chlor- 
 ate, nitrobenzene, potassium nitrate, and antimony 
 sulphide). These explosives belong to the class 
 known as Sprengel explosives, in which the oxygen 
 producer (potassium chlorate, etc.) and the com- 
 bustible substance (nitrobenzene, etc.) are kept 
 separate and mixed just when and where the ex- 
 plosive is to be used. The employment of such 
 explosives is prohibited in Great Britain. 
 
 Di-nitro-benzene, also, although it can be de- 
 tonated only with difficulty and is not used as an 
 explosive by itself, forms a constituent of certain 
 composite explosives, such as securite (di-nitro- 
 benzene and ammonium nitrate) ; and chlor- 
 i 
 
130 THE TREASURES OF COAL TAR 
 
 di-nitro-benzene (or di-nitro-benzene in which a 
 hydrogen atom has been replaced by chlorine), 
 when mixed with ammonium nitrate, yields the 
 powerful blasting explosive roburite. Tri-nitro- 
 benzene, on the other hand, is an explosive which 
 is more powerful than either picric acid or tri-nitro- 
 toluene, but owing to the greater difficulty and 
 expense of its manufacture has not so far come 
 into general use. 1 
 
 Other nitro-derivatives of coal-tar hydrocarbons, 
 although of less importance than those already 
 mentioned, have also been proposed for use as 
 explosives, and have even been adopted to some 
 extent. Of these one may mention di-nitro-naph- 
 
 1 The nitro-derivatives of benzene, and to a somewhat less 
 extent tri-nitro-toluene, exercise a very marked toxic action, 
 to which some individuals are more susceptible than others. 
 Absorption of these compounds into the system, which takes 
 place more especially through the skin, may give rise to derma- 
 titis, toxic gastritis, toxic jaundice, and finally death. It is, 
 therefore, of the highest importance not only that all factories 
 in which T.N.T. (the most important of the nitro-compounds 
 used at the present time) is made shall be efficiently ventilated, 
 but the greatest cleanliness also must be observed on the part 
 of the workers so as, more especially, to prevent continued 
 contact of T.N.T. with the skin. The handling with the un- 
 covered hands of T.N.T. or of articles which have been in con- 
 tact with T.N.T. should be as far as possible avoided. As a 
 measure of precaution it has been laid down as a rule by the 
 Minister of Munitions, in respect of workers in T.N.T. factories, 
 that " no person shall be employed for more than a fortnight 
 without an equal period of work at a process not involving 
 contact with T.N.T., or an equal period of absence from work 
 unless such employment has been approved by the Medical 
 Officer." 
 
EXPLOSIVES 131 
 
 thalene, which is employed as a constituent of the 
 blasting explosive known as favierite (ammonium 
 nitrate and di-nitro-naphthalene) and of schneiderite 
 (ammonium nitrate, 88 parts ; di-nitro-naphthalene, 
 ii parts ; resin, i part), used by the French for 
 filling high-explosive shells. 
 
 It may be mentioned that these ammonium 
 nitrate explosives, e.g. securite, roburite, favierite, 
 are of importance on account of the fact that they 
 are " safety explosives " ; that is to say, they do 
 not, on explosion, ignite mixtures of fire-damp 
 and air, and can therefore be used for blasting 
 purposes in coal-mines. 
 
 From aniline and other amino-compounds valu- 
 able explosives have also been prepared. Thus, in 
 recent years, there has been obtained, by the nitra- 
 tion of aniline, the compound which goes by the 
 common name of tetranyl (tetra-nitro-aniline), 
 which promises to find more extensive application, 
 especially as a primer ; and by the nitration of 
 
 diphenylamine, <^ y NH <^ \, there has 
 
 been obtained the compound hexa-nitro-diphenyl- 
 amine a substance first introduced as the dye 
 aurantia which has recently been used to some 
 extent by Germany for the filling of bombs. Di- 
 phenylamine is itself used as a stabiliser in 
 military smokeless powders. 
 The discharge of the various coal-tar explosives, 
 
132 THE TREASURES OF COAL TAR 
 
 now known in considerable numbers, is brought 
 about, as has already been mentioned, not by 
 ignition (as in the case of gunpowder and cordite), 
 but by the detonation of a more sensitive explosive. 
 Until recently, fulminate of mercury (from mer- 
 cury, nitric acid, and alcohol), alone or mixed with 
 potassium chlorate, was practically the only de- 
 tonator employed ; but this detonator is both 
 dangerous to handle and expensive to manufacture. 
 The discovery, therefore, that the amount of ful- 
 minate required could be very greatly diminished 
 if mixed with tri-nitro-toluene, or with picric acid, 
 was one which has had results of great importance. 
 Still better are the results obtained by means of 
 a mixture of " tetryl " (tri-nitro-phenyl-methyl- 
 
 CH 3 N0 2 
 
 N 
 
 nitramine, NO 2 / NNO 2 ), fulminate of mercury, and 
 
 N0 2 
 
 potassium chlorate. 
 
 Such are the main achievements of the chemist 
 in producing those powerful engines of civilisation, 
 explosives, from the constituents of coal tar. But 
 year by year new compounds are being added to 
 the armouries of the nations and the magazine of 
 the engineer ; and the successes of the past are but 
 an earnest of still greater triumphs in the future. 
 
INDEX 
 
 Abney, Sir William, 119 
 Acetanilide, 103 
 Acetyl-salicylic acid, 105 
 Acid yellow, 71 
 Acridine dyes, 84 
 Acri-flavine, 84, no 
 Adrenaline, 107 
 Alcohol, ethyl, 37 
 
 methyl, 37 
 Alcohols, 37 
 Algol red, 86 
 
 ,, yellow, 86 
 Aliphatic compounds, 38 
 Alizarin, 81 
 
 ,, annual production of, 
 
 83 
 
 ,, bordeaux, 84 
 ,, brown, 84 
 ,, orange, 84 
 ,, preparation of, 82 
 red S, 84 
 Alizarine delphinol, 86 
 Alypine, 107 
 Amatol, 128 
 Amidol, 121 
 Ammonal, 128 
 Ammonia, sulphate of, 5 
 Anaesthesine, 106 
 Aniline, 53, 54 
 black, 79 
 blue, 58 
 purple, 53 
 red, 55 
 ,, yellow, 70, 71 
 Anisic aldehyde, 116 
 Anthracene, 21, 43 
 
 blue, 84 
 ,, dyes, 80 
 
 oils, 17 
 
 Anthraquinone, 80 
 Antife brine, 102 
 Antipyretics, 101 
 Antipyrine, 101 
 Aromatic compounds, 40 
 Arsamin, 109 
 Asperula odorata, 115 
 Aspirin, 105 
 Atom, 32 
 Atoxyl, 109 
 Auxochromes, 65 
 Azo-dyes, 70 
 
 B 
 
 Bacon, Roger, 124 
 
 von Baeyer, Adolf, 93 
 
 Bechamp, 53 
 
 Becher, J. J., 2 
 
 Beehive ovens, 5 
 
 Benzaldehyde, 67, 116 
 
 Benzene, 18, 20 
 
 constitution of, 41 
 ,, poisonous action of 
 nitro-derivatives of, 
 130 
 
 Benzidine, 75 
 
 Benzoic acid, 68 
 
 Benzol, 18, 22 
 
 commercial, 19 
 ,, from coal gas, 18 
 ,, rectification of, 18 
 
 Berzelius, 32 
 
 Beta-eucaine, 107 
 
 Biebrich scarlet, 74 
 
 Bismarck brown, 72 
 
 Bitter almond oil, 67 
 
 Bohn, R., 85 
 
 Bottinger, 75 
 
 Brilliant green, 67 
 
 Browning, Dr, no 
 
 133 
 
134 THE TREASURES OF COAL TAR 
 
 Brugere powder, 126 
 By-product-recovery ovens, 7, 8 
 
 Carbolic acid, 19, 20, 21, 24, 42 
 
 oils, 17 
 Cheddites, 129 
 Chemical structure, 36 
 Chloranthrene blue, 86 
 Chlor-dinitro-benzene, 129 
 Chlorine, production of, 98 
 Chloroform, 106 
 Chromogen, 65 
 Chromophors, 65 
 Chrysoidine, 72 
 Ciba dyes, 99 
 Cinnamic aldehyde, 116 
 Clark, Sir James, 52 
 " Coal boxes," 128 
 Coal, destructive distillation 
 
 of, 3 
 
 products of, 3, ii 
 Coal tar, amounts of constitu- 
 ents, 20 
 ,, ,, annual consumption 
 
 of, 2 
 applications of , in raw 
 
 state, 22 
 
 distillation of, 13, 15 
 ,, ,, nature of, 10, 13, 15 
 production of, i, 3, 5, 
 
 12 
 
 ,, ,, solvents, use of, 24 
 Coke, production of, in Bee- 
 hive ovens, 5, 7 
 production of, in By- 
 product-recovery 
 ovens, 7, 8 
 Colin, 8 1 
 
 Colour and constitution, 64 
 Congo red, 76 
 Constant proportions, law of, 
 
 38 
 
 Constitution of molecules, 39 
 Contact process, 97 
 CoppSe oven, 8 
 Cordite, 123 
 
 Cotton dyes, direct, 75, 76 
 Coumarin, 115 
 
 Creosote, 26 
 
 annual production of, 
 
 28 
 ,, antiseptic properties 
 
 of, 29 
 applications of, 26/29 
 
 oils, 17, 20 
 Cresolin, 26 
 
 Cresols, 20, 25, 26, 44, 106 
 Cresylic acid, 25 
 Cr6sylite, 126 
 
 Cross Dye Black F.N.G., 88 
 Crystal violet, 64 
 
 D 
 
 Detonators, 132 
 Developers, photographic, 118 
 Diamine green, 76 
 Diesel engines, 29 
 Di-nitro-benzene, 129 
 Di-nitro-naphthalene, 130 
 Di-nitro-toluene, 128 
 Drugs, synthetic, 100 
 
 ,, price of, ill 
 
 Dunnite, 125 
 Dye drugs, 108 
 
 Dyes, coal tar, annual produc- 
 tion of, 51 
 
 ,, ' industrial pro- 
 duction of, 49 
 
 value of, 51 
 
 from coal tar, 48 
 
 imports of, into Great 
 Britain, 52 
 
 vat, 80, 88 
 
 Ecrasite, 126 
 
 Ehrlich, Paul, 108 
 
 Eikonogen, 121 
 
 Empirin, 105 
 
 Essence of Mir bane, 116 
 
 Ethane, 35 
 
 Ethylene, 35 
 
 series, 35 
 
 Explosives, 122 
 
 high, 123 
 low, 123 
 
INDEX 
 
 135 
 
 Fadeless fabrics, 85 
 Fahlberg, C., 112 
 Faraday, Michael, 52 
 Fatty compounds, 38 
 Favierite, 131 
 Fischer, Emil, 60 
 Otto, 60 
 
 Flavanthrene, 86 
 Formulae, 33 
 
 diagrammatic 
 (graphic), 34 
 
 Frankland, Sir Edward, 34 
 Friedlander, 99 
 Fuchsine, 55 
 
 Gas, coal, production of, 3, 4 
 
 ,, illuminating, 3 
 Gasoline, 35 
 Girard, 58 
 Glycin, 121 
 Graebe, 81 
 Green, A. G., 78, 79 
 Griess, Peter, 70 
 Gunpowder, 123 
 
 H 
 
 Hawthorn blossom, 116. 
 Heavy oils, 20 
 Helindon yellow, 86 
 Heumann, 93, 96 
 Hexa-nitro-diphenylamine, 131 
 Hofmann, A.W., 14, 52, 56 
 
 violets, 58 
 Hydrocarbon, saturated, 34 
 
 unsaturated, 35 
 
 Hydroquinone, 118 
 
 Ice colours, 79 
 
 Indanthrene blue, 85 
 dyes, 84 
 ,, green, 86 
 
 red, 86 
 
 ,, yellow, 86 
 
 Indican, 91 
 
 Indigo, 90 
 
 dye industry, 91 
 
 ,, synthetic, British, 96 
 
 Indigotin (indigo blue), 91 
 synthesis of, 94, 96 
 
 Ingrain dyes, 78 
 
 Intermediates, 49 
 
 Iodides, 37 
 
 lonone, 115 
 
 Isatis tinctoria, go 
 
 Isomerism, 38 
 
 " Jack Johnsons," 128 
 
 Jeyes' Fluid, 26 
 
 Jockey Club Perfume, 115 
 
 K 
 
 Kairine, 101 
 
 von Kekule, August, 36, 41 
 Khaki Brown C., 88 
 yellow C., 88 
 Khar si van, no 
 Knorr, Ludwig, 101 
 
 de Laire, 58 
 
 Lakes, 83 
 
 Law of Constant Proportions, 
 
 38 
 
 Leuco-compounds, 88, 89 
 Liebermann, 81 
 von Liebig, Justus, 52, 101, 
 
 106 
 
 Light oil, 17 
 Lucigen lamp, 29 
 Lyddite, 125 
 Lyons blue, 58 
 Lysol, 26 
 
 ^acarite, 128 
 Mackintosh, 14 
 ladder, 81 ^ 
 Magenta, 55 
 "Malachite green,[,67 
 Mai de caderas, 109 
 
136 THE TREASURES OF COAL TAR 
 
 Mansfield, Charles Blachford 
 
 Mauve, 53 
 
 Meadowsweet perfume, 116 
 
 Medlock, 56 
 
 Meldola, R., 121 
 
 Melinite, 125 
 
 Methane, 34 
 
 ,, series, 35 
 Methyl green, 64 
 
 salicylate, 116 
 
 violet, 64 
 Metol, 1 20 
 Middle oils, 19 
 Mir bane, essence of, 53 
 Mitscherlich, 52 
 Molecular architecture, 31 
 Molecule, 33 
 
 Molecules, constitution of 39 
 Murdoch, William, 3 
 Murex brandaris, 99 
 ,, trunculus, 99 
 Musk perfume, imitation, 116 
 
 N 
 
 Naphtha, 18, 22 
 Naphthalene, 19, 21, 26, 43 
 Naphthol, 73 
 Naphthylamine, 73 
 New Fuchsine process, 62 
 New Mown Hay perfume, 115 
 Nicholson E. C., 45, 56, 58 ' 
 Nicholson s blue, 58 
 Nitrobenzene, 53, 54 
 Novocaine, 107 
 
 Oil of bitter almonds, 116 
 cinnamon, 116 
 ,, wintergreen, 116 
 
 Oleum, 97 
 
 Oriol yellow, 78 
 
 Orthochromatic plates, 117 
 
 Ortho-nitrotoluene, 59 
 
 Ortho-toluidine, 60 
 
 Ortol, 120 
 
 Panchromatic plates, 117 
 Paracelsus, 100 
 Paraffin wax, 35 
 Para-nitraniline red, 78 
 Para-nitrotoluene 59 
 Para-red, 78 
 Para-rosaniline, 61 
 Para-toluidine, 60 
 Pentabrom-phenol, 106 
 Perfumes, synthetic, 114 
 Perkin Sir W. H., 45, 53, 71, 
 
 79,82,115 
 Pertite, 125 
 Petrofracteur. 129 
 Petrol, 35 
 Phenacetine, 104 
 Phenazone, 101 
 Phenol, 20, 21, 42, 105 
 Phenyl-hydrazine, 101 
 Photographic chemicals, 117 
 Picric acid, 124 
 Piroplasmosis. 109 
 Pitch, 29 
 Ponceaux, 74 
 Primuline, 78 
 
 brown, 78 
 
 orange, 78 
 
 red, 78 
 Prince Consort, 52 
 Propane, 35 
 Propylene, 35 
 Purpurin, 84 
 Pyramidone, 102 
 Pyrogallic acid, 118 
 Pyrogallol, 118 
 
 Q 
 
 Quinoline, 101 
 
 Rackarock, 129 
 Refined tars, 29 
 Regepyrin, 105 
 Remsen, Ira, 112 
 Rexite, 128 
 Rhodamines, 87 
 
INDEX 
 
 137 
 
 Robiquet, 81 
 
 Roburite, 130 
 
 Rodinal, 120 
 
 Roofing felt, 30 
 
 Royal College of Chemistry, 52 
 
 Rubia tinctoria, 81 
 
 Saccharine, 112 
 Safety explosives, 131 
 Saffranines, 87 
 Salicylic acid, 104 
 
 aldehyde, 116 
 Salol, 105 
 
 Salvarsan (" 606 "), no 
 Saxin, 114 
 Scalol, 120 
 Schneiderite, 131 
 Securite, 129 
 Serle, Henry, 2 
 Shimosite, 125 
 Simpson, Maule, and Nicholson, 
 
 .56 
 
 Simpson, Sir James, 106 
 
 Sleeping sickness, 109 
 
 Soamin, 109 
 
 Spivosa ulmaria, 116 
 
 Spirits of wine, 37 
 
 Spirochcete palli da, no 
 
 Sprengel explosives, 129 
 
 Stovaine, 107 
 
 Sulphide dyes, 87 
 
 Sulphur dyes, 87 
 
 Sulphuric acid, manufacture of, 
 
 97 
 
 Sundour fabrics, 85 
 Suprarenine, 108 
 Symbols, 32 
 Syphilis, no 
 
 Takamine, 107 
 Tar, refined, 29 
 Tetrabrom-ortho-cresol, 106 
 
 Tetranyl, 131 
 Tetryl, 132 
 
 Timber, creosoting of, 14, 27 
 preservation (" pick- 
 
 , , poisonous action of , 1 30 
 Tolidine, 75 
 Toluene, 18, 20, 21, 42 
 Tri-nitro-anisole, 126 
 Tri-nitro-benzene, 130 
 Tri-nitro-cresol, 126 
 Tri-nitro-phenol, 124 
 Tri-nitro-toluene, 126 
 Triphenyl-methane, 60 
 Tropaeoline O., 73 
 Tropaeolines, 72 
 Trotyl, 127 
 Trypa-flavine, 84 
 Trypan blue, 109 
 
 red, 109 
 
 Trypanosoma gambiense, log 
 Tyrian purple, 53, 99 
 
 Valency, doctrine of, 34 
 Vanillin, 115 
 Vaseline, 35 
 Vat dyes, 80, 88 
 Verguin, 55 
 Victoria green, 67 
 Violet, imitation, 115 
 
 W 
 
 Water blue, 58 
 Withnell powder, 128 
 Woad, 90 
 Wohler, 44 
 Wood spirit, 37 
 
 Xylene, 18, 19 
 
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