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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