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Title: Coal - and What We Get from It
Author: Meldola, Raphael
Language: English
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  A Romance of Applied Science.






  PH.D., F.R.S.,


This is neither a technical manual, nor a treatise dealing with the
history of a particular branch of applied science, but it partakes
somewhat of the character of both. It is an attempt--perhaps somewhat
bold--to present in a popular form an account of the great industry which
has arisen out of the waste from the gas-works. In the strictest sense it
is a romance of dirt. To render intelligible the various stages in the
evolution of the industry, without assuming any knowledge of chemical
science on the part of the general reader, has by no means been an easy
task, and I have great misgivings as to the success of my effort. But
there is so much misapprehension concerning the history and the mode of
production of colouring-matters from coal-tar, that any attempt to strip
the industry of its mystery in this, the land of its birth, cannot but
find justification. Although the theme is a favourite one with popular
lecturers, it is generally treated in a superficial way, leaving the
audience only in possession of the bare fact that dyestuffs, &c., have by
some means or other been obtained from coal-tar. I have endeavoured to go
somewhat beyond this, and to give some notion of the scientific principles
underlying the subject. If the reader can follow these pages, in which not
a chemical formula appears, with the same interest and with the same
desire to know more about the subject that was manifested by the audience
at the London Institution, before whom the lecture was delivered, my
object will have been accomplished. To the Board of Managers of that
Institution my thanks are due for the opportunity which they have afforded
me of attempting to extend that popular knowledge of applied science for
which there is such a healthy craving in the public mind at the present

  R. M.
  _6 Brunswick Square, W.C._



Origin of Coal, 9. Coal of various ages, 11. Graphite, 12. Recent
Vegetable Deposits, 13. Mode of occurrence of Coal, 13. Structure of Coal,
15. Uses of Coal, 16. Coal a source of Energy, 17. Mechanical Equivalent
of Heat, 19. Value of Coal as a Fuel, 20. Small efficiency of
Steam-engines, 21. Mechanical value of Coal, 22. Whence Coal derives its
Energy, 22. Chemical Composition of Coal, 23. Growth of Plants, 26. Solar
Energy, 28. Transformation of Wood into Coal, 30. Destructive Distillation
of Coal, 33. Experiments of Becher, 34; of Dean Clayton, 35; of Stephen
Hales, 37; of Bishop Watson, 37; of the Earl of Dundonald, 39. Coal-gas
introduced by Murdoch, 40. Spread of the new Illuminant, 41. Manufacture
of Coal-gas, 42. Quantitative results, 45. Uses of Coke, 47. Goethe's
visit to Stauf, 48. Bishop Watson on waste from Coke-ovens, 50. Shale-oil
Industry, 50. History of Coal-mining, 57. Introduction of Coal into
London, 58. The Coal resources of the United Kingdom, 60. Competition
between Electricity and Coal-gas, 62.


Ammoniacal Liquor of Gas-works, 64. Origin of the Ammonia, 65. Ammonia as
a Fertilizer, 65. Other uses of Ammonia, 67. Annual production of Ammonia,
68. Utilization of Coal-tar, 69. The Creosoting of Timber, 70. Early uses
of the Light Tar Oils, 71. Discovery of Benzene by Faraday; isolation from
Tar Oil by Hofmann and Mansfield, 73. Discovery of Mauve by Perkin, 74.
History of Aniline, 75. The Distillation of Coal-tar, 77. Separation of
the Hydrocarbons of the Benzene Series, 82. Manufacture of Aniline and
Toluidine, 87. History and Manufacture of Magenta, 89. Blue, Violet, and
Green Dyes from Magenta, 92. The Triphenylmethane Group, 97. The Azines,
108. Lauth's Violet and Methylene Blue, 111. Aniline Black, 114.
Introduction of Azo-dyes, 115. Aniline Yellow, Manchester Brown, and
Chrysoïdine, 118. The Indulines, 121. Chronological Summary, 122.


Natural Sources of Indigo, 124. Syntheses of the Colouring-matter, 126.
Carbolic Oil, its treatment and its constituents, 129. Phenol Dyes, 132.
Salicylic Acid and its uses, 134. Picric Acid, 136. Naphthalene and its
applications, 139. The Albo-carbon Light, 140. Phthalic Acid and the
Phthaleïns, 145. Magdala Red, 149. Azo-dyes from the Naphthols,
Naphthylamines, and their Sulpho-acids, 150. Naphthol Green, the Oxazines,
and the Indophenols, 161. Creosote Oil, 163. The Lucigen Burner, 163.
Anthracene Oil, 167. The Discovery of Artificial Alizarin, and its effects
on Madder growing, 167. The industrial isolation of Anthracene and its
conversion into Colouring-matters, 171. Pitch, and its uses, 176. Patent
Fuel, or Briquettes, 178. Coal-tar products in Pharmacy, 178. Aromatic
Perfumes, 185. Coal-tar Saccharin, 186. Coal-tar Products in Photography,
188. Coal-tar Products in Biology, 192. Value of the Coal-tar Industry,
194. The Coal-tar Industry in relation to pure Science, 196. Permanence of
the Artificial Colouring-matters, 198. Chronological Summary, 200.
Addendum, 202.



     "Hier [1771] fand sich eine zusammenhängende Ofenreihe, wo
     Steinkohlen abgeschwefelt und zum Gebrauch bei Eisenwerken tauglich
     gemacht werden sollten; allein zu gleicher Zeit wollte man Oel und
     Harz auch zu Gute machen, ja sogar den Russ nicht missen, und so
     unterlag den vielfachen Absichten alles zusammen."--Goethe, _Wahrheit
     und Dichtung_, Book X.

To get at the origin of the familiar fuel which blazes in our grates with
such lavish waste of heat, and pollutes the atmosphere of our towns with
its unconsumed particles, we must in imagination travel backwards through
the course of time to a very remote period of the world's history. Ages
before man, or the species of animals and plants which are contemporaneous
with him, had appeared upon the globe, there flourished a vegetation not
only remarkable for its luxuriance, but also for the circumstance that it
consisted to a preponderating extent of non-flowering or cryptogamic
plants. In swampy areas, such as the deltas at the mouths of great rivers,
or in shallow lagoons bordering a coast margin, the jungles of ferns and
tree-ferns, club-mosses and horse-tails, sedges, grasses, &c., grew and
died down year by year, forming a consolidated mass of vegetable matter
much in the same way that a peat bed or a mangrove swamp is accumulating
organic deposits at the present time. In the course of geological change
these beds of compressed vegetation became gradually depressed, so that
marine or fresh-water sediment was deposited over them, and then once more
the vegetation spread and flourished to furnish another accumulation of
vegetable matter, which in its turn became submerged and buried under
sediment, and so on in successive alternations of organic and sedimentary

But these conditions of climate, and the distribution of land and water
favourable to the accumulation of large deposits of vegetable matter,
gradually gave way to a new order of things. The animals and plants
adapted to the particular conditions of existence described above gave
rise to descendants modified to meet the new conditions of life. Enormous
thicknesses of other deposits were laid down over the beds of vegetable
remains and their intercalated strata of clay, shale, sandstone, and
limestone. The chapter of the earth's history thus sealed up and stowed
away among her geological records relates to a period now known as the
Carboniferous, because of the prevalence of seams or beds of coal
throughout the formation at certain levels. By the slow process of
chemical decomposition without access of air, modified also by the
mechanical pressure of superincumbent formations, the vegetable deposits
accumulated in the manner described have, in the lapse of ages, become
transformed into the substance now familiar to us as coal.

Although coal is thus essentially a product of Carboniferous age, it must
not be concluded that this mineral is found in no other geological
formation. The conditions favourable for the deposition of beds of
vegetable matter have prevailed again and again, at various periods of
geological time and on different parts of the earth, although there is at
present no distinct evidence that such a luxuriant growth of vegetation,
combined with the other necessary conditions, has ever existed at any
other period in the history of the globe. Thus in the very oldest rocks of
Canada and the northern States of America, in strata which take us back to
the dawn of geological history, there is found abundance of the mineral
graphite, the substance from which black-lead pencils are made, which is
almost pure carbon. Now most geologists admit that graphite represents the
carbon which formed part of the woody tissue of plants that lived during
those remote times, so that this mineral represents coal in the ultimate
stage of carbonization. In some few instances true coal has been found
converted into graphite _in situ_ by the intrusion of veins of volcanic
rock (basalt), so that the connection between the two minerals is more
than a mere matter of surmise.

Then again we have coal of pre-Carboniferous age in the Old Red Sandstone
of Scotland, this being of course younger in point of time than the
graphite of the Archæan rocks. Coal of post-Carboniferous date is found in
beds of Permian age in Bavaria, of Triassic age in Germany, in the
Inferior Oolite of Yorkshire belonging to the Jurassic period, and in the
Lower Cretaceous deposits of north-western Germany. Coming down to more
recent geological periods, we have a coal seam of over thirty feet in
thickness in the northern Tyrol of Eocene age; we have brown coal deposits
of Oligocene age in Belgium and Austria, and, most remarkable of all, coal
has been found of Miocene, that is, mid-Tertiary age, in the Arctic
regions of Greenland within a few degrees of the North Pole. Thus the
formation of coal appears to have been going on in one area or another
ever since vegetable life appeared on the globe, and in the peat bogs,
delta jungles, and mangrove swamps of the present time we may be said to
have the deposition of potential coal deposits for future ages now going

Although in some parts of the world coal seams of pre-Carboniferous age
often reach the dignity of workable thickness, the coal worked in this
country is entirely of Carboniferous date. After the explanation of the
mode of formation of coal which has been given, the phenomena presented by
a section through any of our coal measures will be readily intelligible
(see Fig. 1). We find seams of coal separated by beds of sandstone,
limestone, or shale representing the encroachment of the sea and the
deposition of marine or estuarine sediment over the beds of vegetable
remains. The seams of coal, varying in thickness from a few inches to
three or four feet, always rest on a bed of clay, known technically as the
"underclay," which represents the soil on which the plants originally
grew. In some instances the seams of coal with their thin "partings" of
clay reach an aggregate thickness of twenty to thirty feet. In many cases
the very roots of the trees are found upright in a fossilized condition in
the underclay, and can be traced upwards into the overlying coal beds; or
the completely carbonized trunk is found erect in the position in which
the tree lived and died (see Fig. 2).

[Illustration: FIG. 1.--Section through Carboniferous strata showing seams
of coal. Dislocations, or "faults," so common in the Coal Measures, are
shown at H, T, and F. Intrusions of igneous rock are shown at D. At B is
shown the coalescence of two seams, and at N the local thinning of the
seam. The vertical lines indicate the shafts of coal mines.]

[Illustration: FIG. 2.--Section showing coal seams and upright trunks
attached to roots _in situ_. A', A'', A''', beds of shale. B, coal seams.
C, underclay. D, sandstone.]

Owing to the chemical and mechanical forces to which the original
vegetable deposit has been subjected, the organic structure of coal has
for the most part been lost. Occasionally, however, portions of leaves,
stems, and the structure of woody fibre can be detected, and thin sections
often show the presence of spore-cases of club-mosses in such numbers that
certain kinds of coal appear to be entirely composed of such remains. But
although coal itself now furnishes but little direct evidence of its
vegetable origin, the interstratified clays, shales, and other deposits
often abound with fossilized plant remains in every state of
preservation, from the most delicate fern frond to the prostrate tree
trunk many yards in length. It is from such evidence that our knowledge of
the Carboniferous flora has been chiefly derived.

Now this carbonized vegetation of a past age, the history of which has
been briefly sketched in the foregoing pages, is one of the chief sources
of our industrial supremacy as a nation. We use it as fuel for generating
the steam which drives our engines, or for the production of heat wherever
heat is wanted. In metallurgical operations we consume enormous quantities
of coal for extracting metals from their ores, this consumption being
especially great in the case of iron smelting. For this last operation
some kinds of raw coal are unsuitable, and such coal is converted into
coke before being used in the blast furnace. The fact that the iron ore
and the coal occur in the same district is another cause of our high rank
as a manufacturing nation.

It has often been a matter of wonder that iron ore and the material
essential for extracting the metal from it should be found associated
together, but it is most likely that this combination of circumstances,
which has been so fortunate for our industrial prosperity, is not a mere
matter of accident, but the result of cause and effect. It is, in fact,
probable that the iron ore owes its origin to the reduction and
precipitation of iron compounds by the decomposing vegetation of the
Carboniferous period, and this would account for the occurrence of the
bands of ironstone in the same deposits with the coal. In former times,
when the area in the south-east of England known as the Weald was thickly
wooded, the towns and villages of this district were the chief centres of
the iron manufacture. The ore, which was of a different kind to that found
in the coal-fields, was smelted by means of the charcoal obtained from the
wood of the Wealden forests, and the manufacture lingered on in Kent,
Sussex, and Surrey till late in the last century, the railings round St.
Paul's, London, being made from the last of the Sussex iron. When the
northern coal-fields came to be extensively worked, and ironstone was
found so conveniently at hand, the Wealden iron manufacture declined, and
in many places in the district we now find disused furnaces and heaps of
buried slag as the last witnesses of an extinct industry.

From coal we not only get mechanical work when we burn it to generate heat
under a steam boiler, but we also get chemical work out of it when we
employ it to reduce a metallic ore, or when we make use of it as a source
of carbon in the manufacture of certain chemical products, such as the
alkalies. We have therefore in coal a substance which supplies us with the
power of doing work, either mechanical, chemical, or some other form, and
anything which does this is said to be a source of energy. It is a
familiar doctrine of modern science that energy, like matter, is
indestructible. The different forms of energy can be converted into one
another, such, for example, as chemical energy into heat or electricity,
heat into mechanical work or electricity, electricity into heat, and so
forth, but the relationship between these convertible forms is fixed and
invariable. From a given quantity of chemical energy represented, let us
say, by a certain weight of coal, we can get a certain fixed amount of
heat and no more. We can employ that heat to work a steam-engine, which we
can in turn use as a source of electricity by causing it to drive a
dynamo-machine. Then this doctrine of science teaches us that our given
weight of coal in burning evolves a quantity of heat which is the
equivalent of the chemical energy which it contains, and that this
quantity of heat has also its equivalent in mechanical work or in
electricity. This great principle--known as the Conservation of
Energy--has been gradually established by the joint labours of many
philosophers from the time of Newton downwards, and foremost among these
must be ranked the late James Prescott Joule, who was the first to measure
accurately the exact amount of work corresponding to a given quantity of

In measuring heat (as distinguished from temperature) it is customary to
take as a unit the quantity necessary to raise a given weight of water
from one specified temperature to another. In measuring work, it is
customary to take as a unit the amount necessary to raise a certain weight
at a specified place to a certain height against the force of gravity at
that place. Joule's unit of heat is the quantity necessary to raise one
pound of water from 60° to 61° F., and his unit of work is the foot-pound,
_i.e._ the quantity necessary to raise a weight of one pound to a height
of one foot. Now the quantitative relationship between heat and work
measured by Joule is expressed by saying that the mechanical equivalent of
heat is about 772 foot-pounds, which means that the quantity of heat that
would raise one pound of water 1° F. would, if converted into work, be
capable of raising a one-pound weight to a height of 772 feet, or a weight
of 772 lbs. to a height of one foot.

This mechanical equivalent ought to tell us exactly how much power is
obtainable from a certain weight of coal if we measure the quantity of
heat given out when it is completely burnt. Thus an average Lancashire
coal is said to have a calorific power of 13,890, which means that 1 lb.
of such coal on complete combustion would raise 13,890 lbs. of water
through a temperature of 1° F., if we could collect all the heat generated
and apply it to this purpose. But if we express this quantity of heat in
its mechanical equivalent, and suppose that we could get the corresponding
quantity of work out of our pound of coal, we should be grievously
mistaken. For in the first place, we could not collect all the heat given
out, because a great deal is communicated to the products of combustion by
which it is absorbed, and locked up in a form that renders it incapable of
measurement by our thermometers. In the next place, if we make an
allowance for the quantity of heat which thus disappears, even then the
corrected calorific power converted into its mechanical equivalent would
not express the quantity of work practically obtainable from the coal.

In the most perfectly constructed engine the whole amount of heat
generated by the combustion of the coal is not available for heating the
boiler--a certain quantity is lost by radiation, by heating the material
of the furnace, &c., by being carried away by the products of combustion
and in other ways. Moreover, some of the coal escapes combustion by being
allowed to go away as smoke, or by remaining as cinders. Then again, in
the engine itself a good deal of heat is lost through various channels,
and much of the working power is frittered away through friction, which
reconverts the mechanical power into its equivalent in heat, only this
heat is not available for further work, and is thus lost so far as the
efficiency of the engine is concerned. These sources of loss are for the
most part unavoidable, and are incidental to the necessary imperfections
of our mechanism. But even with the most perfectly conceivable constructed
engine it has been proved that we can only expect one-sixth of the total
energy of the fuel to appear in the form of work, and in a very good
steam-engine of the present time we only realize in the form of useful
work about one-tenth of the whole quantity of energy contained in the
coal. Although steam power is one of the most useful agencies that science
has placed at the disposal of man, it is not generally recognized by the
uninitiated how wasteful we are of Nature's resources. One of the greatest
problems of applied science yet to be solved is the conversion of the
energy latent in coal or other fuel into a quantity of useful work
approximating to the mechanical equivalent much more closely than has
hitherto been accomplished.

But although we only get this small fraction of the whole working
capability out of coal, the actual amount of energy dormant in this
substance cannot but strike us as being prodigious. It has already been
said that a pound of coal on complete combustion gives out 13,890 heat
units. This quantity of heat corresponds to over 10,000,000 foot-pounds of
work. A horse-power may be considered as corresponding to 550 foot-pounds
of work per second, or 1,980,000 foot-pounds per hour. Thus our pound of
coal contains a store of energy which, if capable of being completely
converted into work without loss, would in one hour do the work of about
five and a half horses. The strangest tales of necromancy can hardly be so
startling as these sober figures when introduced for the first time to
those unaccustomed to consider the stupendous powers of Nature.

If energy is indestructible, we have a right to inquire in the next place
from whence the coal has derived this enormous store. A consideration of
the origin of coal, and of its chemical composition, will enable this
question to be answered. The origin of coal has already been discussed.
Chemically considered, it consists chiefly of carbon together with smaller
quantities of hydrogen, oxygen, and nitrogen, and a certain amount of
mineral matter which is left as ash when the coal is burnt. The following
average analyses of different varieties will give an idea of its chemical

    Variety of Coal. | Carbon. | Hydrogen. | Oxygen. | Nitrogen. | Ash.
  S. Staffordshire   |  73·4   |    5·0    |  11·7   |    1·7    | 2·3
  Newcastle (Caking) |  80·0   |    5·3    |  10·7   |    2·2    | 1·7
  Cannel (Wigan)     |  81·2   |    5·6    |   7·9   |    2·1    | 2·5
  Anthracite (Welsh) |  90·1   |    3·2    |   2·5   |    0·8    | 1·6

There are in addition to these constituents small quantities of sulphur
and a certain variable amount of water (5 to 10 per cent.) in all coals,
but the elements which most concern us are those heading the respective

From the foregoing analyses, which express the percentage composition, it
will be seen that carbon is by far the most important constituent of coal.
Carbon is a chemical element which is found in a crystalline form in
nature as the diamond, and which forms a most important constituent of all
living matter, whether animal or vegetable. Woody fibre contains a large
quantity of this element, and the carbon of coal is thus accounted for; it
was accumulated during the growth of the plants of the Carboniferous

Now carbon is one of those elementary substances which are said to be
_combustible_, which means that if we heat it in atmospheric air it gives
out heat and light, and gradually disappears, or, as we say, burns away.
The heat which is given out during combustion represents the chemical
energy stored up in the combustible, for combustion is in fact the
chemical union of one substance with another with the development of heat
and light. When carbon burns in air, therefore, a chemical combination
takes place, the air supplying the other substance with which the carbon
combines. That other substance is also an element--it is the invisible gas
which chemists call oxygen, and which forms one-fifth of the bulk of
atmospheric air, the remainder consisting of the gas nitrogen and small
quantities of other gases with which we shall have more to do
subsequently. When oxygen and carbon unite under the conditions described,
the product is an invisible gas known as carbon dioxide, and it is because
this gas is invisible that the carbon seems to disappear altogether on
combustion. In reality, however, the carbon is not lost, for matter is as
indestructible as energy, but it is converted into the dioxide which
escapes as gas under ordinary circumstances. If, however, we burn a given
weight of carbon with free access of air, and collect the product of
combustion and weigh it, we shall find that the product weighs more than
the carbon, by an amount which represents the weight of oxygen with which
the element has combined. By careful experiment it would be found that
one part by weight of carbon would give three and two-third parts by
weight of carbon dioxide. If, moreover, we could measure the quantity of
heat given out by the complete combustion of one pound of carbon, it would
be found that this quantity would raise 14,544 lbs. of water through 1°
F., a quantity of heat corresponding to over eleven million foot-pounds of
work, or about seven and three-quarters horse-power per hour.

Here then is the chief source of the energy of coal--the carbon of the
plants which lived on this earth long ages ago has lain buried in the
earth, and when we ignite a coal fire this carbon combines with
atmospheric oxygen, and restores some of the energy that was stored up at
that remote period. But the whole of the energy dormant in coal is not due
to the carbon, for this fuel contains another combustible element,
hydrogen, which is also a gas when in the free state, and which is one of
the constituents of water, the other constituent being oxygen. In fact,
there is more latent energy in hydrogen, weight for weight, than there is
in carbon, for one pound of hydrogen on complete combustion would give
enough heat to raise 62,032 lbs. of water through 1° F. Hydrogen in
burning combines with oxygen to form water, so that the products of the
complete combustion of coal are carbon dioxide and water. The amount of
heat contributed by the hydrogen of coal is, however, comparatively
insignificant, because there is only a small percentage of this element
present, and we thus come to the conclusion that nearly all the work that
is done by our steam-engines of the present time is drawn from the latent
energy of the carbon of the fossilized vegetation of the Carboniferous

The conclusion to which we have now been led leaves us with the question
as to the _origin_ of the energy of coal still unanswered. We shall have
to go a step further before this part of our story is complete, and we
must form some kind of idea of the way in which a plant grows. Carbon
being the chief source of energy in coal, we may for the present confine
ourselves to this element, of which woody fibre contains about 50 per
cent. Consider the enormous gain in weight during the growth of a plant;
compare the acorn, weighing a few grains, with the oak, weighing many
tons, which arises from it after centuries of growth. If matter is
indestructible, and never comes into existence spontaneously, where does
all this carbon come from? It is a matter of common knowledge that the
carbon of plants is supplied by the atmosphere in the form of carbon
dioxide--the gas which has already been referred to as resulting from the
combustion of carbon. This gas exists in the atmosphere in small
quantity--about four volumes in 10,000 volumes of air; but insignificant
as this may appear, it is all important for the life of plants, since it
is from this source that they derive their carbon. The origin of the
carbon dioxide, which is present as a normal constituent of the
atmosphere, does not directly concern us at present, but it is important
to bear in mind that this gas is one of the products of the respiration of
animals, so that the animal kingdom is one of the sources of plant carbon.

The transition from carbon dioxide to woody fibre is brought about in the
plant by a series of chemical processes, and through the formation of a
number of intermediate products in a manner which is not yet thoroughly
understood; but since carbon dioxide consists of carbon and oxygen, and
since plants feed upon carbon dioxide, appropriating the carbon and giving
off the oxygen as a waste product, it is certain that work of some kind
must be performed. This is evident, because it has been explained that
when carbon combines with oxygen a great deal of heat is given out, and as
this heat is the equivalent of the energy stored in the carbon, it follows
from the doctrine of the Conservation of Energy, that in order to separate
the carbon from the oxygen again, just the same amount of energy must be
supplied as is evolved during the combustion of the carbon. If a pound of
carbon in burning to carbon dioxide gives out heat equivalent to eleven
million foot-pounds of work, we must apply the same amount of work to the
carbon dioxide produced to separate it into its constituents. Neither a
plant nor any living thing can create energy any more than it can create
matter, and just as the matter composing a living organism is assimilated
from external sources, so must we look to an external source for the
energy which enables the plant to do this large amount of chemical work.

The separation of carbon from oxygen in the plant is effected by means of
energy supplied by the sun. The great white hot globe which is the centre
of our system, and round which this earth and the planets are moving, is a
reservoir from which there is constantly pouring forth into space a
prodigious quantity of energy. It must be remembered that the sun is more
than a million times greater in bulk than our earth. It has been
calculated by Sir William Thomson that every square foot of the sun's
surface is radiating energy equivalent to 7000 horse-power in work. On a
clear summer day the earth receives from the sun in our latitude energy
equal to about 1450 horse-power per acre. To keep up this supply by the
combustion of coal, we should have to burn for every square foot of the
sun's surface between three and four pounds per second. A small fraction
of this solar energy reaches our earth in the form of radiant heat and
light, and it is the latter which enables the plant to perform the work of
separating the carbon from the oxygen with which it is chemically
combined. It is, in fact, well known that the growth of plants--that is,
the assimilation of carbon and the liberation of oxygen--only takes place
under the influence of light. This function is performed by the leaves
which contain the green colouring-matter known as chlorophyll, the
presence of which is essential to the course of the chemical changes.

If we now sum up the results to which we have been led, it will be seen--

(1) That the chief source of the energy contained in coal is the carbon.

(2) That this carbon formed part of the plants which grew during the
Carboniferous period.

(3) That the carbon thus accumulated was supplied to the plants by the
carbon dioxide existing in the atmosphere at that time.

(4) That the separation of the carbon from the oxygen was effected in the
presence of chlorophyll, by means of the solar energy transmitted to the
earth during the Carboniferous period.

We thus arrive at the interesting conclusion, that the heat which we get
from coal is sunlight in another form. For every pound of coal that we now
burn, and for every unit of heat or work that we get from it, an
equivalent quantity of sunlight was converted into the latent energy of
chemical separation during the time that the coal plant grew. This energy
has remained stored up in the earth ever since, and reappears in the form
of heat when we cause the coal to undergo combustion. It is related that
George Stephenson when asked what force drove his locomotive, replied that
it was "bottled-up sunshine," and we now see that he was much nearer the
truth in making this answer than he could have been aware of at the time.

Before passing on to the consideration of the different products which we
get from coal, it will be desirable to discuss a little more fully the
nature of the change which occurs during the transformation of wood into
coal. Pure woody fibre consists of a substance known to chemists as
cellulose, which contains fifty per cent. of carbon, the remainder of the
compound being made up of hydrogen and oxygen. It is thus obvious that
during the fossilization of the wood some of the other constituents are
lost, and the percentage of carbon by this means raised. We can trace this
change from wood, through peat, lignite, and the different varieties of
coal up to graphite, which is nearly pure carbon. It is in fact possible
to construct a series showing the conversion of wood into coal, this
series comprising the varieties given in the table on p. 23, as well as
younger and older vegetable deposits. The series will be--

    I. Woody fibre (cellulose).
   II. Peat from Dartmoor.
  III. Lignite, or brown coal, an imperfectly carbonized vegetable
       deposit of more recent geological age than true coal.
   IV. Average bituminous coal.
    V. Cannel coal from Wigan.
   VI. Anthracite from Wales.
  VII. Graphite, the oldest carbonaceous mineral.

The percentage of the chief elements in the members of this series is--

         Carbon. Hydrogen. Oxygen.

    I.      50·0    6·0      44·0
   II.      54·0    5·2      28·2
  III.      66·3    5·6      22·8
   IV.      77·0    5·0      11·2
    V.      81·2    5·6       7·9
   VI.      90·1    3·2       2·5
  VII.   94-99·5, the remainder being ash.

In the above table the increase of carbon and the decrease of oxygen is
well brought out; the hydrogen also on the whole decreases, although with
some irregularity. The exact course of the chemical change which occurs
during the passage of wood into coal is at present involved in obscurity.
The oxygen may be eliminated in the form of water or of carbon dioxide or
both; some of the carbon is got rid of in the form of marsh gas, a
compound of carbon and hydrogen, which forms the chief constituent of the
dangerous "fire-damp" of coal mines.

Marsh gas is an inflammable gas which becomes explosive when mixed with
air and ignited; it often escapes with great violence during the working
of coal seams, the jets blowing out from the coal or underclay with a
rushing noise, indicative of the high pressure under which the hydrocarbon
gas has accumulated. These jets of escaping gas are known amongst miners
as "blowers." If the air of a mine contains a sufficient quantity of the
gas, and a flame accidentally fires the mixture, there results one of
those disastrous explosions with which the history of coal mining has
unfortunately only made us too familiar.

From the account of coal which has thus far been rendered, it will be seen
that as a source of mechanical power, we are far from using it as
economically as could be desired; and when we look at our open grates with
clouds of unburnt carbon particles escaping up the chimney, and so
constructed that only a small fraction of the total heat warms our rooms,
it will be seen that the tale of waste is still more deplorable. But we
are at present rather concerned with what we actually do get from coal
than with what we ought to get from it, and here, when we come to deal
with the various material products, we shall have a better account to

If instead of heating coal in contact with air and allowing it to burn, we
heat it in a closed vessel, such as a retort, it undergoes decomposition
with the formation of various gaseous, liquid, and solid products. This
process of heating an organic compound in a closed vessel without access
of air and collecting the products, is called destructive distillation.
The tobacco-pipe experiment of our boyhood is our first practical
introduction to the destructive distillation of coal. We put some powdered
coal into the bowl of the pipe, plaster up the opening with clay and then
insert the bowl in a fire, allowing the stem to project from between the
bars of the grate. In a few minutes a stream of gas issues from the
orifice of the stem; on applying a light it burns with a luminous flame,
and we have made coal-gas on a small scale.

In the destructive distillation of organic substances, such as wood or
coal, there are always produced four things--gas, watery liquid, and
viscous products known as tar, while a residue of coke or charcoal is left
in the retort. This is a very old observation, and was made so long ago
that it becomes interesting as a point in the history of applied science
to know who first submitted coal to destructive distillation. According to
Dr. Gustav Schultz, we must credit a German with this observation, which
was made towards the end of the seventeenth century (about 1680) by a
chemist named Johann Joachim Becher. The experiment is described in such a
quaint manner that the exact words of the author are worthy of being
reproduced, and the passage is here given as translated by Dr. Lunge in
his work on _Coal Tar and Ammonia_--

"In Holland they have peat, and in England pit-coals; neither of them is
very good for burning, be it in rooms or for smelting. But I have found a
way, not merely to burn both kinds into good coal (coke) which not any
more smokes nor stinks, but with their flame to smelt equally well as with
wood, so that a foot of such coal makes flames 10 feet long. That I have
demonstrated with pit-coal at the Hague, and here in England at Mr.
Boyles', also at Windsor on the large scale. In this connection it is also
noteworthy that, equally as the Swedes make their tar from firwood, I have
here in England made from pit-coal a sort of tar which is equal to the
Swedish in every way, and for some operations is even superior to it. I
have made proof of it on wood and on ropes, and the proof has been found
right, so that even the king has seen a specimen of it, which is a great
thing in England, and the coal from which the tar has been taken out is
better for use than before."

This enterprising chemist, moreover, brought his results to a practical
issue, for he secured a patent, in conjunction with Henry Serle, in 1681,
for "a new way of making pitch and tarre out of pit-coale, never before
found out or used by any other."

No less interesting is the work of our own clergy during the last century,
when many eminent divines appear to have devoted their leisure to
experimental science. Thus, about the year 1688 the Rev. John Clayton,
D.D., Dean of Kildare, went to examine a ditch two miles from Wigan in
Lancashire, the water in which had been stated to "burn like brandy" when
a flame was applied to it. The Dean ultimately traced the phenomenon to an
escape of inflammable gas from an underlying coal seam, and he followed up
the matter experimentally by studying the destructive distillation of
Wigan coal in retorts. The results were communicated to the Hon. Robert
Boyle, but were not published till after the death of the latter, and long
after the death of the author. The following account is taken from the
abridged edition of the _Philosophical Transactions_ (1739):--

"At first there came over only phlegm, afterwards a black oil, and then
also a spirit arose, which he could noways condense, but it forced the
luting, or broke the glasses. Once, when it had forced the lute, coming
close to it to try to repair it, he observed that the spirit which issued
out caught fire at the flame of the candle, and continued burning with
violence as it issued out in a stream, which he blew out and lighted again
alternately for several times. He then tried to save some of this spirit.
Taking a turbinated receiver, and putting a candle to the pipe of the
receiver while the spirit rose, he observed that it caught flame, and
continued burning at the end of the pipe, though you could not discern
what fed the flame. He then blew it out, and lighted it again several
times; after which he fixed a bladder, flatted and void of air, to the
pipe of the receiver. The oil and phlegm descended into the receiver, but
the spirit, still ascending, blew up the bladder. He then filled a good
many bladders with it, and might have filled an inconceivable number more;
for the spirit continued to rise for several hours, and filled the
bladders almost as fast as a man could have blown them with his mouth; and
yet the quantity of coals he distilled was inconsiderable.

"He kept this spirit in the bladders a considerable time, and endeavoured
several ways to condense it, but in vain. And when he wished to amuse his
friends, he would take one of the bladders, and pricking a hole with a
pin, and compressing gently the bladder near the flame of a candle till it
once took fire, it would then continue flaming till all the spirit was
compressed out of the bladder."[1]

The Rev. Stephen Hales, D.D., Rector of Farringdon, Hants, was the author
of a book entitled _Statical Essays, containing Vegetable Staticks_,
printed in 1726-27, and of which the third edition bears the date 1738. At
p. 182 of this work, after a previous description of the destructive
distillation of all kinds of substances in iron or other retorts, he

"By the same means also I found plenty of air [gas] might be obtained from
minerals. Half a cubick inch, or 158 grains of Newcastle coal, yielded in
distillation 180 cubick inches of air [gas], which arose very fast from
the coal, especially while the yellowish fumes ascended."

Still later, viz. about 1767, we have the Rev. R. Watson, D.D., Regius
Professor of Divinity in the University of Cambridge, and Bishop of
Llandaff, interesting himself in chemistry. He wrote a series of
_Chemical Essays_, one of which is entitled, _Of Pit Coal_, and in this he
describes the production from coal (by destructive distillation) of
illuminating gas, ammonia-water, tar, and coke. He further compares the
relative quantities of the different products from various kinds of coal,
but he appears to have been chiefly interested in the tar, and disregarded
the gas and other products. Not the least interesting part of his book is
the preface, in which he apologizes for his pursuits in the following

"Divines, I hope, will forgive me if I have stolen a few hours, not, I
trust, from the duties of my office, but certainly from the studies of my
profession, and employed them in the cultivation of natural philosophy. I
could plead in my defence, the example of some of the greatest characters
that ever adorned either this University or the Church of England."

This is quoted from the 5th edition, dated 1789, the essay on coal being
in the second of five volumes. As the learned bishop published other works
on chemistry, we may suppose that the forgiveness which he asks from his
brother divines was duly accorded.

None of these preliminary experiments, however, led to any immediate
practical result so far as concerns the use of coal-gas as an illuminating
agent. Towards the end of the last century the lighting of individual
establishments commenced, and the way was thus prepared for the
manufacture of the gas on a large scale. One of the earliest pioneers was
the ninth Earl of Dundonald, an inventive genius, who in 1782 at Culross
Abbey became one of the first practical tar distillers. He secured letters
patent in 1781 for making tar, pitch, essential oils, volatile alkali,
mineral acids, salts, and cinders from coal. The gas was only a waste
product, and, strange as it may appear, the Earl, whose operations were
financial failures, did not realize the importance of the gas, the tar and
coke being considered the only products of value. Here is the account of
the experiments by his son, Admiral Dundonald, the Sailor Earl, quoted
from his _Autobiography of a Seaman_:--

"In prosecution of his coal-tar patent, my father went to reside at the
family estate of Culross Abbey, the better to superintend the works on his
own collieries, as well as others on the adjoining estates of Valleyfield
and Kincardine. In addition to these works, an experimental tar-kiln was
erected near the Abbey, and here coal-gas became accidentally employed in
illumination. Having noticed the inflammable nature of a vapour arising
during the distillation of tar, the Earl, by way of experiment, fitted a
gun-barrel to the eduction pipe leading from the condenser. On applying
fire to the muzzle, a vivid light blazed forth across the waters of the
Frith, becoming, as was afterwards ascertained, distinctly visible on the
opposite shore."

A few years later the foundation of the coal-gas manufacture was laid by
William Murdoch, a Scotchman, who must be credited with the practical
introduction of this illuminating agent. The idea had about the same time
occurred to a Frenchman, Lebon, but in his hands the suggestion did not
take a practical form. Murdoch was overseer of some mines in Cornwall, and
in 1792 he first lighted his own house at Redruth. He then transferred his
services to the great engineering firm of Boulton and Watt at Soho, near
Birmingham, where he erected apparatus in 1798, and in the course of a few
years the whole of this factory was permanently lighted by gas. From this
time the introduction of gas into other factories at Manchester and
Halifax was effected by Murdoch and his pupil, Samuel Clegg. From single
factories coal-gas at length came into use as a street illuminant,
although somewhat tardily. Experiments were made in London at the Lyceum
Theatre in 1803, in Golden Lane in 1807, and in Pall Mall two years later.

It was fifteen years from the time of Murdoch's first installation at Soho
before the streets of London were lighted by gas on a commercial scale.
Our grandfathers seem to have had a great dread of gas, and public
opposition no doubt had much to do with its exclusion from the metropolis.
There were even at that time eminent literary and scientific men who did
not hesitate to cast ridicule upon the proposal, and to declare the scheme
to be only visionary. But about 1806 there came into this country an
energetic German who passed by the name of Winsor, and who is described as
an ignorant adventurer, whose real name was Winzler. Whatever his origin,
he certainly helped to rouse the public interest in gas lighting. He took
out a patent, he gave public lectures, and collected large sums of money
for the establishment of gas companies. Most of the capital was, however,
squandered in futile experiments, but at length in 1813, Westminster
Bridge, and a year later St. Margaret's parish, was successfully lighted.
From that period the use of gas extended, but it was some time before the
public fears were allayed, for it is related that Samuel Clegg, who
undertook the lighting of London Bridge, had at first to light his own
lamps, as nobody could be found to undertake this perilous office. Even
after gas had come into general use as a street illuminant, it must have
found its way but slowly into private houses. In an old play-bill of the
Haymarket Theatre, dated 1843--thirty years after the first introduction
into the streets--it is announced--

"Among the most important Improvements, is the introduction (for the first
time) of Gas as the Medium of Light!"

The manufacture of coal-gas, first rendered practicable by the energy and
skill of the Scotch engineer Murdoch, is now carried on all over the
country on a colossal scale. It is not the province of the present volume
to deal with the details of manufacture, but a short description of the
process is necessary for the proper understanding of the subsequent
portions of the subject (see Fig. 3). The coal is heated in clay
cylinders, called retorts, provided with upright exit pipes through which
the volatile products escape, and are conducted into water contained in a
horizontal pipe termed the "hydraulic main." In the latter the gas is
partially cooled, and deposits most of the tar and watery liquor which
distil over at the high temperature to which the retorts are heated. The
tar and watery liquor are allowed to flow from the hydraulic main into a
pit called the "tar well," and the gas then passes through a series of
curved pipes exposed to the air, in which it is further cooled, and
deposits more of the tar. From this "atmospheric condenser" the gas passes
into a series of vessels filled with coke, down which a fine spray of
water is constantly being blown. These vessels, known as "scrubbers,"
serve to remove the last traces of tar, and some of the volatile sulphur
compounds which are formed from the small quantity of sulphur present in
most coals. The removal of sulphur compounds is a matter of importance,
because when gas is burnt these compounds give rise to acid vapours, which
are deleterious to health and destructive to property.

[Illustration: FIG. 3.--Sectional diagram of gas plant. The retorts and
furnace are on the right; the gas rises through the upright pipe T into
the hydraulic main B; from there it passes into the atmospheric condensers
D, from the lower cistern of which the condensed tar flows into the
tar-well, H. Passing up through K, the gas is conducted into the scrubber,
O, and from there into the purifier, M. From there it emerges through K'
into the purifier, M, and then into the gas-holder for distribution. (From
Schultz's _Chemie des Steinkohlentheers_.)]

From the scrubbers the gas is sent through another series of vessels
packed with trays of lime or oxide of iron, in order to remove
sulphuretted hydrogen and other sulphur compounds as completely as
possible. A small quantity of carbon dioxide is also removed by these
"purifiers," as the presence of this gas impairs the illuminating power of
coal-gas. From the purifiers the gas passes into the gas-holders, where it
is stored for distribution. It remains only to be stated that the
distillation of coal is effected under a pressure somewhat less than that
of the atmosphere, the products of distillation being pumped out of the
retorts by means of a kind of air-pump, called an exhauster, which is
interpolated between the hydraulic main and the condenser, or at some
other part of the purifying system. The coke left in the retort is used as
fuel for burning under the retorts or for other purposes. The oxide of
iron used in the purifiers can be used over and over again for a certain
number of times by exposing it to the air, and when it is finally
exhausted, the sulphur can be burnt out of it and used for making that
most important of all chemical products, sulphuric acid. Thus the small
quantity of sulphur present in the original coal (probably in the form of
iron pyrites) is rendered available for the manufacture of a useful

The necessarily brief description of this important industry will suffice
for the general reader. Those who desire further information on points of
detail will refer to special works. We are here rather concerned with the
subsequent fate of the different products, four of which have to be dealt
with, viz. the gas, watery liquor, tar, and coke. The first and last of
these having already been accounted for--the one as an illuminating agent
and the other as fuel--may now be dismissed.

No story of applied science is complete unless we can form some idea of
the quantities of material used, and the amount of the products obtained.
From one ton of Newcastle coal we get about 10,000 cubic feet of gas, 110
to 120 lbs. of tar, 20 to 25 gallons of watery liquor, and about 1500 lbs.
of coke. Different coals of course give different quantities, and the
latter vary also according to the heat of distillation; but the above
estimate will furnish a good basis for forming our ideas with some
approach to precision. It has been estimated also that we are now
distilling coal at the rate of about ten million tons per annum, so that
there is annually produced 100,000 million cubic feet of gas, and about
500,000 tons of tar, besides proportionate quantities of the other
products. The great metropolitan companies alone are consuming nearly
three million tons annually for the production of gas, a consumption
corresponding to about 6000 cubic feet per head of the population. This of
course takes no account of the coal used for other manufactures or for
domestic purposes, but it is interesting to compare these estimates with
the consumption of coal in London about a century ago, before the
introduction of gas, when, as Bishop Watson tells us in his work already
referred to, the annual consumption was 922,394 tons.

The enormous quantity of tar resulting from our gas manufacture furnishes
the raw material for the production of a multitude of valuable
substances--colouring-matters, medicines, perfumes, flavouring-matters,
burning and lubricating oils, &c. Out of this unsavoury waste material of
the gas-works, the researches of chemists have enabled a great industry to
spring up which is of continually growing importance. It will be the
object of the remaining portion of the present volume to set forth the
achievements of science in this branch of its application. The foundation
of the coal-tar industry was laid in this country--the country where coal
was first distilled on a large scale for the production of gas, and where
the first of the coal-tar colouring-matters was sent forth into commerce.
We are at the present time the largest tar producers in Europe; it has
been stated that we produce more than double the whole quantity of tar
made in the gas-using countries of Europe; but in spite of this, our
manufacture of finished products is by no means in that flourishing
condition which might be expected from our natural resources in the way of
coal, and the facilities which we possess for manufacturing the raw
materials out of it.

But we must now take a glance at some of the other uses to which coal is
put in order to realize more completely the truth of the statement made
some pages back, viz. that this mineral has been the chief source of our
industrial prosperity. Great as is the consumption of coal by the gas
manufacturer, there is an equal or even a greater demand for the
carbonaceous residue left when the coal has been decomposed by destructive
distillation or by partial combustion. This residue is coke--the substance
left in the retorts after the gas manufacture. There is a great demand for
coke for many purposes; it is used in most cases where a cheap smokeless
fuel is required; it is burnt in the furnaces of locomotives and other
engines, and is very largely consumed by the iron smelter in the blast

To meet these demands a large quantity of coal is converted into coke by
being burnt in ovens with an insufficient supply of air for complete
combustion, or in suitably constructed close furnaces. The tar and other
products have in this country until recently been allowed to escape as
waste, but the time is approaching when these must be utilized. It will
give an idea of the industrial importance of coke when it is stated, that
about twelve million tons of our coal annually undergo conversion into
this form of fuel. Chemically considered, coke consists of carbon together
with all the mineral constituents of the coal, and small quantities of
hydrogen, oxygen, and nitrogen. The amount of carbon varies from 85 to 97,
and the ash from 3 to 14 per cent.

The conversion of coal into coke is a very venerable branch of
manufacture, which was first carried out on a large scale in this country
about the middle of the seventeenth century. As an operation it may appear
utterly devoid of romance, but as Goethe has described his visit to the
earliest of coke-burners, this fragment of history is worth narrating.
When the great German philosophical poet was a student at Strassburg
(1771), he rode over with some friends to visit the neighbourhood of
Saarbrücken where he met an old "coal philosopher" named Stauf, who was
there carrying on the industry. This "philosophus per ignem" was manager
of some alum works, and the ruling spirit of the "burning hill" of
Duttweiler. The hill no doubt owed its designation to the coke ovens at
work upon it, and which had been in operation there for some six or seven
years before Goethe's visit, _i.e._ since 1764. The coke was wanted for
iron smelting, and even at that early period Stauf had the wisdom to
condense his volatile products, for we are told that he showed his
visitors bitumen, burning-oil, lampblack, and even a cake of sal ammoniac
resulting from his operations. Goethe has put upon record his visit to the
little haggard old coke-burner, living in his lonely cottage in the forest
(_Aus meinem Leben: Wahrheit und Dichtung_, Book X). It is probably
Stauf's ovens which are described by the French metallurgist, De Gensanne,
in his _Traité de la fonte des Mines par le feu du Charbon de Terre_,
published in Paris in 1770. After long years of coke-making, without any
regard to the value of the volatile products, we are now beginning to
consider the advisability of doing that which has long been done on the

It is not unlikely that Bishop Watson in the last century had heard of
the attempt to recover the products from coke ovens, for he gives the
following very sound advice in his _Chemical Essays_:--

"Those who are interested in the preparation of coak would do well to
remember that every 96 ounces of coal would furnish four ounces at the
least of oil, probably six ounces might be obtained; but if we put the
product so low as five ounces from 100, and suppose a coak oven to work
off only 100 tons of coal in a year, there would be a saving of five tons
of oil, which would yield above four tons of tar; the requisite alteration
in the structure of the coak ovens, so as to make them a kind of
distilling vessels, might be made at a very trifling expense."--5th ed.,
1789, vol. ii. p. 351.

We have yet to chronicle another chapter in the history of coal philosophy
before finishing with this part of the subject. There is a branch of
manufacture carried on, especially in Scotland, which results in the
production of burning and lubricating oils, and solid paraffin, a wax-like
substance which is used for candle-making. The manufacture of candles out
of coal will perhaps be a new revelation to many readers of this book. It
must be admitted, however, that the term "coal" is here being extended to
only partially fossilized vegetation of younger geological age than true
coal, and to bituminous shales of various ages. Shale, geologically
considered, is hardened mud; it may be looked upon as clay altered by time
and pressure. Now if the mud, at the period of its deposition, was much
mixed up with vegetable matter, we should have in course of time a mixture
of more or less carbonized woody fibre with mineral matter, and this would
be called a carbonaceous or bituminous shale. Shales of this kind often
contain as much as 80 to 90 per cent. of mineral matter, and seldom more
than 20 per cent. of volatile matter, _i.e._ the portion lost on ignition,
and consisting chiefly of the carbonaceous constituents.

The story of the shale-oil industry is soon told. About the year 1847 oil
was "struck" in a coal mine at Alfreton in Derbyshire, and in the hands of
Mr. James Young this supply furnished the market with burning-oil for
nearly three years. Then the spring became exhausted, and Mr. Young and
his associates had to look out for another source of oil. Be it remembered
that this happened some nine years before the utilization of the great
American petroleum deposits. Many kinds of vegetable matter were submitted
to destructive distillation before a substance was found which could be
profitably worked, but at length Mr. Young tried a kind of cannel coal
which had about that time been introduced for gas making. This substance
was called Boghead gas coal or Torbane Hill mineral, from the place where
it occurred, which is at Bathgate in Linlithgow. This mineral was found to
yield a large amount of paraffin oil and solid paraffin on destructive
distillation, and from that time (1850) to this, the industry has been
carried on at Bathgate and other parts of Scotland, where similar
carbonaceous deposits occur.

It may seem a matter of unimportance at the present time whether this
Torbane Hill mineral is a true coal or not. About forty years ago,
however, the decision of this question involved a costly law-suit in
Edinburgh. The proprietor of the estate had granted a lease to a firm,
conveying to the latter the right to work coal, limestone, ironstone, and
certain other minerals found thereon, but excluding copper and all other
minerals not mentioned in the contract. The lessees then found that this
particular carbonaceous mineral was of very great value, both on account
of the high quality of the gas, and afterwards on account of the paraffin
which it furnished by Young's process of distillation. Thereupon the
lessor brought an action against the lessees, claiming £10,000 damages, on
the ground that the latter had broken the contract by removing a mineral
which was not coal. Experts gave evidence on both sides; some declared in
favour of the substance being coal, others said it was a bituminous shale,
while others called it bituminated clay, or refused to give it a name at
all. Judgment was finally given for the defendants, so that in the eye of
the law the mineral was considered a true coal. As a matter of fact, it is
impossible to draw a hard and fast line between coal and bituminous shale,
as the one is connected with the other by a series of intermediate
minerals, and the Torbane Hill mineral happens to form one of the links.
It contains about 69 per cent. of volatile matter, and leaves 31 per cent.
of residue, consisting of 12 parts of carbon and 19 of ash.

The manufacture started by Young has developed into an important industry,
in spite of the fact that the original Torbane Hill coal has become
exhausted, and that enormous natural deposits of petroleum are worked in
America, Russia, and elsewhere. There are now some fifteen companies at
work in Scotland, representing an aggregate capital of about two and a
half million pounds sterling. Bituminous shales of different kinds are
distilled at a low red heat in iron retorts, and from the volatile
portions there are separated those valuable products which have already
been alluded to, viz. burning and lubricating oils, solvent mineral oil,
paraffin wax for candles, and ammonia. We may fairly claim these as coal
products, although the shales used contain much mineral matter, the carbon
averaging about 20 per cent., the hydrogen three per cent., the nitrogen
0·7, and the ash about 67 per cent. The shales worked are approximately of
the same age as true coal, _i.e._ Carboniferous. The Scotch companies are
distilling about two million tons of shale per annum, this quantity
producing about sixty million gallons of crude oil, and giving employment
to over 10,000 hands.

It is not the province of the present work to enter into the chemical
nature of the products of destructive distillation in any greater detail
than is necessary to enable the general reader to know something of the
recent discoveries in the utilization of these products. We shall,
however, have occasion later on to make ourselves acquainted with the
names of some of the more important raw materials which are derived from
this source, and certain preliminary explanations are indispensable. In
the first place then, let us start from the fact that coal--including
carbonaceous shale and lignite--when heated in a closed vessel gives gas,
tar, coke, and a watery liquor. A clear understanding must be arrived at
concerning the manner in which these products arise.

There is a widely-spread notion that the substances derived from coal and
utilized for industrial purposes are present in the mineral itself, and
that the art of the chemist has been exercised in separating the said
substances by various processes. This idea must be at once dispelled. It
is true that there is a small quantity of water and a certain amount of
gas already present in most coals, but these are quite insignificant as
compared with the total yield of gas and watery liquor. So also with
respect to the tar; it is possible that in some highly bituminous minerals
we might dissolve out a small quantity of tarry matter by the use of
appropriate solvents, but in the coals mostly used for gas-making not a
trace of tar exists ready formed, and still less can it be said that the
coal contains coke. All these products are formed _by the chemical
decomposition of the coal_ under the influence of heat, and their nature
and quantity can be made to vary within certain limits by modifying the
temperature of distillation.

Having once realized this principle with respect to coal itself, it is
easy to extend it to the products of its destructive distillation. The
tar, for instance, is a complicated mixture of various substances, among
which hydrocarbons--_i.e._ compounds of carbon with hydrogen--largely
predominate. The different components of coal-tar can be separated by
processes which we shall have to consider subsequently. Of the compounds
thus isolated some few are immediately applicable for industrial purposes,
but the majority only form the raw materials for the manufacture of other
products, such as colouring-matters and medicines. Now these
colouring-matters and other finished products no more exist in the tar
than the latter exists in the coal. They are produced from the
hydrocarbons, &c., present in the tar _by chemical processes_, and bear
much about the same relationship to their parent substances that a
steam-engine bears to the iron ore out of which its metallic parts are
primarily constructed. Just as the mechanical skill of the engineer
enables him to construct an engine out of the raw material iron, which is
extracted from its ore, and converted into steel by chemical processes, so
the skill of the chemist enables him to build up complex
colouring-matters, &c., out of the raw materials furnished by tar, which
is obtained from coal by chemical decomposition.

The illuminating gas which is obtained from coal by destructive
distillation consists chiefly of hydrogen and gaseous hydrocarbons, the
most abundant of the latter being marsh gas. There are also present in
smaller quantities the two oxides of carbon, the monoxide and the dioxide,
which are gaseous at ordinary temperatures, together with other
impurities. Coal-gas is burnt just as it is delivered from the mains--it
is not at present utilized as a source of raw material in the sense that
the tar is thus made use of. In some cases gas is used as fuel, as in
gas-stoves and gas-engines, and in the so-called "gas-producers," in which
the coal, instead of being used as a direct source of heat, is partially
burnt in suitable furnaces, and the combustible gas thus arising,
consisting chiefly of carbon monoxide, is conveyed to the place where it
undergoes complete combustion, and is thus utilized as a source of heat.

Summing up the uses of coal thus far considered, we see that this mineral
is being consumed as fuel, for the production of coke, for the manufacture
of gas, and in many other ways. Lavishly as Nature has provided us with
this source of power and wealth, the idea naturally suggests itself
whether we are not drawing too liberally upon our capital. The question of
coal supply crops up from time to time, and the public mind is
periodically agitated about the prospects of its continuance. How long we
have been draining our coal resources it is difficult to ascertain. There
is some evidence that coal-mining was carried on during the Roman
occupation. In the reign of Richard I. there is distinct evidence of coal
having been dug in the diocese of Durham. The oldest charters take us back
to the early part of the thirteenth century for Scotland, and to the year
1239 for England, when King Henry III. granted a right of sale to the
townsmen of Newcastle. With respect to the metropolis, Bishop Watson, on
the authority of Anderson's _History of Commerce_, states that coal was
introduced as fuel at the beginning of the fourteenth century. In these
early days, when it was brought from the north by ships, it was known as

     "Go; and we'll have a posset for 't soon at night, in faith, at the
     latter end of a sea-coal fire."--_Merry Wives of Windsor_, Act I.,
     Sc. iv.

That the fuel was received at first with disfavour appears from the fact
that in the reign of Edward I. the nobility and gentry made a complaint to
the king objecting to its use, on the ground of its being a public
nuisance. By the middle of the seventeenth century the use of coal was
becoming more general in London, chiefly owing to the scarcity of wood;
and its effects upon the atmosphere of the town will be inferred from a
proclamation issued in the reign of Elizabeth, prohibiting its use during
the sitting of Parliament, for fear of injuring the health of the knights
of the shire. About 1649 the citizens again petitioned Parliament against
the use of this fuel on account of the stench; and about the beginning of
that century "the nice dames of London would not come into any house or
roome when sea-coales were burned, nor willingly eat of meat that was
either sod or roasted with sea-coale fire" (_Stow's Annals_).

For many centuries therefore we have been drawing upon our coal supplies,
and using up the mineral at an increasing rate. According to a recent
estimate by Professor Hull, from the beginning of the present century to
1875 the output has been more than doubled for each successive quarter
century. The actual amount of coal raised in the United Kingdom between
1882 and the present time averages annually about 170 million tons,
corresponding in money value to about £45,000,000 per annum. In 1860 the
amount of coal raised in Great Britain was a little more than 80 million
tons, and Professor Hull estimated that at that rate of consumption our
supplies of workable coal would hold out for a thousand years. Since then
the available stock has been diminished by some 3,650 million tons, and
even this deduction, we are told on the same authority, has not materially
affected our total supply. The possibility of a coal famine need,
therefore, cause no immediate anxiety; but we cannot "eat our loaf and
have it too," and sooner or later the continuous drain upon our coal
resources must make itself felt. The first effect will probably be an
increase in price owing to the greater depth at which the coal will have
to be worked. The whole question of our coal supply has, however,
recently assumed a new aspect by the discovery (February 1890) of coal at
a depth of 1,160 feet at Dover. To quote the words of Mr. W. Whitaker--"It
may be indeed that the coal supply of the future will be largely derived
from the South-East of England, and some day it may happen, from the
exhaustion of our northern coal-fields, that we in the south may be able
successfully to perform a task now proverbially unprofitable--_we may
carry coal to Newcastle_."

The coal-fields of Great Britain and Ireland occupy, in round numbers, an
area of 11,860 square miles, or about one-tenth of the whole area of the
land surface of the country. Within this area, and down to a depth of
4,000 feet, lie the main deposits of our available wealth. Some idea of
the amount of coal underlying this area will be gathered from the table[2]
on the next page.

This supply, amounting to over 90,000 million tons, refers to the exposed
coal-fields and to workable seams, _i.e._ those above one foot in
thickness. But in addition to this, we have a large amount of coal at
workable depths under formations of later geological age than the
Carboniferous, such as the Permian formation of northern and central
England. Adding the estimated quantity of coal from this source to that
contained in the exposed coal-fields as given above, we arrive at the
total available supply. This is estimated to be about 146,454 million
tons. To this we may one day have to add the coal under the south-eastern
part of England.

                                         Amount of coal in millions of
                                          tons to depths not exceeding
  Coal Fields of--                                         4,000 feet.

  South Wales                                                   32,456
  Forest of Dean                                                   265
  Bristol                                                        4,219
  Warwickshire                                                     459
  S. Staffordshire, Shropshire, Forest of Wyre and Clee Hills    1,906
  Leicestershire                                                   837
  North Wales                                                    2,005
  Anglesey                                                           5
  N. Staffordshire                                               3,825
  Lancashire and Cheshire                                        5,546
  Yorkshire, Derbyshire, and Northumberland                     18,172
  Black Burton                                                      71
  Northumberland and Durham                                     10,037
  Cumberland                                                       405
  Scotland                                                       9,844
  Ireland                                                          156

It is important to bear in mind, that out of the 170 million tons of coal
now being raised annually we only use a small proportion, viz. from 5 to
6 per cent. for gas-making. The largest amount (33 per cent.) is used for
iron-smelting,[3] and about 15 per cent. is exported; the remainder is
consumed in factories, dwelling-houses, for locomotion, and in the smaller

The enormous advancement which has taken place of late years in the
industrial applications of electricity has given rise to the belief that
coal-gas will in time become superseded as an illuminating agent, and that
the supply of tar may in consequence fall off. So far, however, the
introduction of electric-lighting has had no appreciable effect upon the
consumption of gas, and even when the time of general electric-lighting
arrives there will arise as a consequence an increased demand for gas as a
fuel in gas engines. Moreover, the use of gas for heating and cooking
purposes is likely to go on increasing. Nor must it be forgotten that the
quantity of tar produced in gas-works is now greater than is actually
required by the colour-manufacturer, and much of this by-product is burnt
as fuel, so that if the manufacture of gas were to suffer to any
considerable extent there would still be tar enough to meet our
requirements at the present rate of consumption of the tar-products. Then
again, the value of the tar, coke, and ammoniacal liquor is of such a
proportion as compared with the cost of the raw material, coal, that there
is a good margin for lowering the price of gas when the competition
between the latter and electricity actually comes about. It will not then
be only a struggle between the two illuminants, but it will be a question
of electricity _versus_ gas, _plus_ tar and ammonia.

While the electrician is pushing forward with rapid strides, the chemist
is also moving onwards, and every year witnesses the discovery of new tar
products, or the utilization of constituents which were formerly of little
or no value. Thus if the cost of generating and distributing electricity
is being lowered, on the other hand the value of coal tar is likely to go
on advancing, and it would be rash to predict which will come out
triumphant in the end. But even if electricity were to gain the day it
would be worth while to distil coal at the pit's mouth for the sake of the
by-products, and there is, moreover, the tar from the coke ovens to fall
back upon--a source which even before the use of coal-gas the wise Bishop
of Llandaff advised us not to neglect.


The nature of the products obtained by the destructive distillation of
coal varies according to the temperature of distillation, and the age or
degree of carbonization of the coal. The watery liquor obtained by the dry
distillation of wood is acid, and contains among other things acetic acid,
which is sometimes prepared in this way, and from its origin is
occasionally spoken of as "wood vinegar." The older the wood, the more
complete its degree of conversion into coal, and the smaller the quantity
of oxygen it contains, the more alkaline does the watery liquid become.
Thus the gas-liquor is distinctly alkaline, and contains a considerable
quantity of ammonia, besides other volatile bases. The uses of ammonia are
manifold, and nearly our whole supply of this valuable substance is now
derived from gas-liquor. The presence of ammonia in this liquor is
accounted for when it is known that this compound is a gas composed of
nitrogen and hydrogen. It has already been explained that coal contains
from one to two per cent. of nitrogen, and during the process of
distillation about one-fifth of this nitrogen is converted into ammonia,
the remainder being converted partly into other bases, while a small
quantity remains in the coke.

Ammonia, the "volatile alkali" of the old chemists, and its salts are of
importance in pharmacy, but the chief use of this compound is to supply
nitrogen for the growth of plants. Plants must have nitrogen in some form
or another, and as they cannot assimilate it _directly_ from the
atmosphere where it exists in the free state, some suitable nitrogen
compound must be supplied to the soil. It is possible that certain
leguminous plants may derive their nitrogen from the atmosphere through
the intervention of micro-organisms, which appear capable of fixing free
nitrogen and of supplying it to the plant upon whose roots they flourish.
But this is second-hand nitrogen so far as concerns the plant. It is true
also that the atmosphere contains small traces of ammonia and acid oxides
of nitrogen, which are dissolved by rain and snow, and thus get washed
down into the soil. These are the natural sources of plant nitrogen. But
in agricultural operations, where large crops have to be raised as rapidly
as possible, some additional source of nitrogen must be supplied, and
this is the object of manuring the soil.

A manure, chemically considered, is a mixture of substances capable of
supplying the necessary nitrogenous and mineral food for the nourishment
of the growing plant. The ordinary farm or stable manure contains
decomposing nitrogenous organic matter, in which the nitrogen is given off
as ammonia, and thus furnishes the soil with which it is mixed with the
necessary fertilizer. But the supply of this manure is limited, and we
have to fall back upon gas-liquor and native nitrates to meet the existing
wants of the agriculturist. Important as is ammonia for the growth of
vegetation, it is not in this form that the majority of plants take up
their nitrogen. Soluble nitrates are, in most cases, more efficient
fertilizers than the salts of ammonia, and the ammonia which is supplied
to the soil is converted into nitrates therein before the plant can
assimilate the nitrogen. The oxidation of ammonia into nitric acid takes
place by virtue of a process called "nitrification," and there is very
good reason for believing that this transformation is the work of a
micro-organism present in the soil. The gas liquor thus supplies food to a
minute organism which converts the ammonia into a form available for the
higher plants. Some branches of agriculture--such as the cultivation of
the beet for sugar manufacture--are so largely dependent upon an
artificial source of nitrogen, that their very existence is bound up with
the supply of ammonia salts or other nitrogenous manures. The relationship
between the manufacture of beet-sugar and the distillation of coal for the
production of gas is thus closer than many readers will have imagined; for
while the supply of native guano or nitrate is uncertain, and its freight
costly on account of the distance from which it has to be shipped, the
sulphate of ammonia from gas-liquor is always at hand, and available for
the purposes of fertilization.

Then again, there are other products of industrial value which are
associated with ammonia, such, for example, as ammonia-alum and caustic
soda. This last is one of the most important chemical compounds
manufactured on a large scale, and is consumed in enormous quantities for
the manufacture of paper and soap, and other purposes. Salts of this
alkali are also essential for glass making. Of late years a method for the
production of caustic soda has been introduced which depends upon the use
of ammonia, and as this process is proving a formidable rival to the older
method of alkali manufacture, it may be said that such indispensable
articles as paper, soap, and glass are now to some extent dependent upon
gas-liquor, and may in course of time become still more intimately
connected with the manufacture of coal-gas.

But quantitative statements must be given in order to bring home to
general readers the actual value of the small percentage of nitrogen
present in coal. Thus it has been estimated, that one ton of coal gives
enough ammonia to furnish about 30 lbs. of the crude sulphate. The present
value of this salt is roughly about £12 per ton. The ten million tons of
coal distilled annually for gas making would thus give 133,929 tons of
sulphate, equal in money value to £1,607,148, supposing the whole of the
ammonia to be sold in this form. To this may be added the ammonia obtained
during the distillation of shale and the carbonization of coal for coke,
the former source furnishing about 22,000 tons, and the latter about 2500
tons annually. Small as is the legacy of nitrogen bequeathed to us from
the Carboniferous period, we see that it sums up to a considerable annual
addition to our industrial resources.

The three products resulting from the distillation of coal--viz. the gas,
ammoniacal-liquor, and coke--having now been made to furnish their tale,
we have next to deal with the tar. In the early days of gas manufacture
this black, viscid, unsavoury substance was in every sense a waste
product. No use had been found for it, and it was burnt, or otherwise
disposed of. No demand for the tar existed which could enable the gas
manufacturers to get rid of their ever-increasing accumulation. Wood-tar
had previously been used as a cheap paint for wood and metal-work, and it
was but a natural suggestion that coal-tar should be applied to the same
purposes. It was found that the quality of the tar was improved by getting
rid of the more volatile portions by boiling it in open pans; but this
waste--to say nothing of the danger of fire--was checked by a suggestion
made by Accum in 1815, who showed that by boiling down the tar in a still
instead of in open pans the volatile portions could be condensed and
collected, thus furnishing an oil which could be used by the varnish maker
as a substitute for turpentine. A few years later, in 1822, the
distillation of tar was carried on at Leith by Drs. Longstaff and Dalston,
the "spirit" being used by Mackintosh of Glasgow for dissolving
india-rubber for the preparation of that waterproof fabric which to this
day bears the name of the original manufacturer. The residue in the still
was burnt for lamp-black. Of such little value was the tar at this time
that Dr. Longstaff tells us that the gas company gave them the tar on
condition that they removed it at their own expense. It appears also that
tar was distilled on a large scale near Manchester in 1834, the "spirit"
being used for dissolving the residual pitch so as to make a black

But the production of gas went on increasing at a greater rate than the
demand for tar for the above-mentioned purposes, and it was not till 1838
that a new branch of industry was inaugurated, which converted the
distillation of this material from an insignificant into an important
manufacture. In that year a patent was taken out by Bethell for preserving
timber by impregnating it with the heavy oil from coal-tar. The use of tar
for this purpose had been suggested by Lebon towards the end of the last
century, and a patent had been granted in this country in 1836 to Franz
Moll for this use of tar-products. But Bethell's process was put into a
working form by the great improvements in the apparatus introduced by
Bréant and Burt, and to the latter is due the credit of having founded an
industry which is still carried on by Messrs. Burt, Bolton and Haywood on
a colossal scale. The "pickling" or "creosoting" of timber is effected in
an iron cylindrical boiler, into which the timber is run; the cylinder
being then closed the air is pumped out, and the air contained in the
pores of the wood thus escapes. The creosoting oil, slightly warmed, is
then allowed to flow into the boiler, and thus penetrates into the pores
of the wood, the complete saturation of which is insured by afterwards
pumping air into the cylinder and leaving the timber in the oil for some
hours under a pressure of 8 to 10 atmospheres.

All timber which is buried underground, or submerged in water, is
impregnated with this antiseptic creosote in order to prevent decay. It
will be evident that this application of tar-products must from the very
commencement have had an enormous influence upon the distillation of tar
as a branch of industry. Consider the miles of wooden sleepers over which
our railways are laid, and the network of telegraph wires carried all over
the country by wooden poles, of which the ends are buried in the earth.
Consider also the many subaqueous works which necessitate the use of
timber, and we shall gain an idea of the demand for heavy coal-tar oil
created by the introduction of Bréant's process. Under the treatment
described a cubic foot of wood absorbs about a gallon of oil, and by far
the largest quantity of the tar oils is consumed in this way at the
present time. Now in the early days of timber-pickling the lighter oils of
the tar, which first come over on distillation, and which are too volatile
for the purpose of creosoting, were in much about the same industrial
position as the tar itself before its application as a timber
preservative. The light oil had a limited use as a solvent for
waterproofing and varnish making, and a certain quantity was burnt as
coal-tar naphtha in specially constructed lamps, the invention of the late
Read Holliday of Huddersfield, whose first patent was taken out in 1848
(see Fig. 4). Up to this time, be it remembered, that chemists had not
found out what this naphtha contained. But science soon laid hands on the
materials furnished by the tar-distiller, and the naphtha was one of the
first products which was made to reveal the secret of its hidden treasures
to the scientific investigator. From this period science and industry
became indissolubly united, and the researches of chemists were carried
on hand-in-hand with the technical developments of coal-tar products.

[Illustration: FIG. 4.--Read Holliday's lamp for burning light coal-tar
oils. The oil is contained in the cistern _c_, from whence it flows down
the pipe, when the stopcock is opened, into the burner _a_. Below the
burner is a little cup, in which some of the oil is kept burning, and the
heat from this flame volatilizes the oil as it flows down the pipe, the
vapour thus generated issuing from the jets in the burner and there
undergoing ignition. The burner and cup are shown on an enlarged scale at
_a_ in the lower figure.]

In 1825 Michael Faraday discovered a hydrocarbon in the oil produced by
the condensation of "oil gas"--an illuminating gas obtained by the
destructive distillation of oleaginous materials. This hydrocarbon was
analysed by its illustrious discoverer, and named in accordance with his
results "bicarburet of hydrogen." In 1834 the same hydrocarbon was
obtained by Mitscherlich by heating benzoic acid with lime, and by Péligot
by the dry distillation of calcium benzoate. For this reason the compound
was named "benzin" by Mitscherlich, which name was changed into "benzol"
by Liebig. In this country the hydrocarbon is known at the present time as
benzene. Twenty years after Faraday's discovery, viz. in 1845, Hofmann
proved the existence of benzene in the light oils from coal-tar, and in
1848 Hofmann's pupil, Mansfield, isolated considerable quantities of this
hydrocarbon from the said light oils by fractional distillation. At the
time of these investigations no great demand for benzene existed, but the
work of Hofmann and Mansfield prepared the way for its manufacture on a
large scale, when, a few years later, the first coal-tar colouring-matter
was discovered by our countryman, W. H. Perkin.

It is always of interest to trace the influence of scientific discovery
upon different branches of industry. As soon as it had been shown that
benzene could be obtained from coal-tar, the nitro-derivative of this
hydrocarbon--_i.e._ the oily compound produced by the action of nitric
acid upon benzene--was introduced as a substitute for bitter almond oil
under the name of "essence of mirbane." Nitrobenzene has an odour
resembling that of bitter almond oil, and it is still used for certain
purposes where the latter can be replaced by its cheaper substitute, such
as for the scenting of soap. Although the isolation of benzene from
coal-tar gave an impetus to the manufacture of nitrobenzene, no use
existed for the latter beyond its very limited application as "essence of
mirbane," and the production of this compound was at that time too
insignificant to take rank as an important branch of chemical industry.

The year 1856 marks an epoch in the history of the utilization of coal-tar
products with which the name of Perkin will ever be associated. In the
course of some experiments, having for their object the artificial
production of quinine, this investigator was led to try the action of
oxidizing agents upon a base known as aniline, and he thus obtained a
violet colouring matter--the first dye from coal-tar--which was
manufactured under a patent granted in 1858, and introduced into commerce
under the name of mauve. A brief sketch of the history of aniline will
serve to show how Perkin's discovery gave a new value to the light oils
from coal-tar and raised the manufacture of nitrobenzene into an important
branch of industry.

Thirty years before Perkin's experiments the Dutch chemist Unverdorben
obtained (1826) a liquid base by the distillation of indigo, which had the
property of forming beautifully crystalline salts, and which he named for
this reason "crystallin." In 1834 Runge discovered the same base in
coal-tar, although its identity was not known to him at the time, and
because it gave a bluish colour when acted upon by bleaching-powder, he
called it "kyanol." Again in 1840, by distilling a product obtained by the
action of caustic alkalies upon indigo, Fritzsche prepared the same base,
and gave it the name of aniline, from the Spanish designation of the
indigo plant, "anil," derived from the native Indian word, by which
name the base is known at the present time. That aniline could be obtained
by the reduction of nitrobenzene was shown by Zinin in 1842, who used
sulphide of ammonium for reducing the nitrobenzene, and named the
resulting base "benzidam." The following year Hofmann showed that
crystallin, kyanol, aniline, and benzidam were all one and the same base.
Thus when the discovery of mauve opened up a demand for aniline on the
large scale, the labours of chemists, from Unverdorben in 1826 to Hofmann
in 1843, had prepared the way for the manufacturer. It must be understood
that although Runge had discovered aniline in coal-tar, this is not the
source of our present supply, for the quantity is too small to make it
worth extracting. A mere trace of aniline is present in the tar ready
formed; from the time this base was wanted in large quantities it had to
be made by nitrating benzene, and then reducing the nitrobenzene.

The light oils of tar distillation rejected by the timber-pickling
industry now came to the front, imbued with new interest to the
technologist as a source of benzene for the manufacture of aniline. The
inauguration of this manufacture, like the introduction of steam
locomotion, is connected with a sad catastrophe. Mansfield, who first
showed manufacturers how to separate benzene and other hydrocarbons from
the light oils of coal-tar, and who devised for this purpose apparatus
similar in principle to that used on a large scale at the present time
(see Fig. 5), met with an accident which resulted in his death. In the
upper part of a house in Holborn in February 1856, this pioneer was
carrying on his experiments, when the contents of a still boiled over and
caught fire. In his endeavours to extinguish the flames he received the
injuries which terminated fatally. Applied science no less than pure
science has had its martyrs, and among these Mansfield must be ranked.

[Illustration: FIG. 5.--Mansfield's still. R the heating burner, A the
body of the still with stopcock, _i_, for running out the contents. B the
still-head kept in a cistern, C, of hot water or other liquid. The vapour
generated by the boiling of the liquid in A, partly condenses in B, from
whence the higher boiling-point portion flows back into the still. The
uncondensed vapour passes into the condensing-worm, D, which is kept cool
by a stream of water, and from thence flows into the receiver S. By
opening _m_ in the side-pipe any higher boiling-point oil condensing in
the delivery-pipe can be run back into the still.]

The operation of tar-distilling is about as unromantic a process as can be
imagined, but it must be briefly described before the subsequent
developments of the industry can be appreciated properly. It has already
been explained that the tar is a complex mixture of many different
substances. These various compounds boil at certain definite temperatures,
the boiling-point of a chemical compound being an inherent property. If a
mixture of substances boiling at different temperatures is heated in a
suitable vessel the compounds distil over, broadly speaking, in the order
of their boiling-points. The separation by this process is not absolute,
because compounds boiling at a certain temperature have a tendency to
bring over with them the vapours of other compounds which boil at a higher
temperature. But for practical purposes it will be sufficient to consider
that the general tendency is for the compounds of low boiling-point to
come over first, then the compounds of higher boiling-point, and finally
those of the highest boiling-point. This is the principle made use of by
the tar-distiller. The tar-still is a large iron pot provided with a
still-head from which the vapours boil out into a coil of iron pipe kept
cool in a vessel of water (see Fig. 6). The still is heated by a fire
beneath it, and the different portions which condense in the iron coil are
received in vessels which are changed as the different fractions of the
tar come over. The process is what chemists would call a rough fractional
distillation. The first fractions are liquid at ordinary temperatures, and
the water in the condenser is kept cold; then, as the boiling-point
rises, the fraction contains a hydrocarbon which solidifies on cooling,
and the water in the condenser is made hot to prevent the choking up of
the coil. Every one of these fractions of coal-tar, from the beginning to
the end of the process, has its story to tell--all the chief constituents
of the tar separated by this means have by chemical science been converted
into useful products.

[Illustration: FIG. 6.--Sectional diagram of tar-still with arched bottom.
The fireplace is at _i_; the hot gases pass over the bridge _k_ and
through _g_ into the flues _h_, _h_. The pipe at _c_ is to supply the
still with tar; _a_ is the exit pipe connected with the condenser, and _b_
a man-hole for cleaning out the still. The condenser and bottom pipe for
drawing off pitch have been omitted to avoid complication.]

It is customary at the present time to collect four distinct fractions
from the period when the tar begins to boil quietly, _i.e._ from the
point when the small quantity of watery liquor which is unavoidably
entangled with the tar has distilled over, by which time the temperature
in the still is about 110° C. The small fraction that comes over up to
this temperature constitutes what the tar-distiller calls "first
runnings." From 110° C. to 210° C. there comes over a limpid inflammable
liquid known as "light oil," and this is succeeded by a fraction which
shows a tendency to solidify on cooling, owing to the separation of a
solid crystalline hydrocarbon known as naphthalene. This last fraction,
boiling between 210° C. and 240° C., is known as "carbolic oil," because
it contains, in addition to the naphthalene, the chief portion of the
carbolic acid present in the tar. From 240° C. to 270° C. there comes over
another fraction which shows but little tendency to solidify in the
condensing coil, and which is known as "heavy oil," or "creosote oil."
From 270° C. up to the end of the distillation there distils a fraction
which is viscid in consistency, and has a tendency to solidify on cooling
owing to the separation of another crystalline hydrocarbon known as
anthracene, and which gives the name of "anthracene oil" to this last
fraction. When the latter has been collected there remains in the still
the black viscid substance known as pitch, which is obtained of any
desired consistency by leaving more or less of the anthracene oil mixed
with it, or by afterwards mixing it with the heavy oil from previous
distillations. The process carried out in the tar-still thus separates the
tar into--(1) First runnings, up to 110° C. (2) Light oil, from 110° to
210° C. (3) Carbolic oil, from 210° to 240° C. (4) Creosote oil, from 240°
to 270° C. (5) Anthracene oil, from 270° to pitch. (6) Pitch, left in

It has already been said that coal-tar is a complex mixture of various
distinct chemical compounds. Included among the gases, ammoniacal liquor,
and tar, the compounds which are known to be formed by the destructive
distillation of coal already reach to nearly one hundred and fifty in
number. Of the substances present in the tar, about a dozen are utilized
as raw materials by the manufacturer, and these are contained in the
fractions described above. The first runnings and light oil contain a
series of important hydrocarbons, of which the three first members are
known to chemists as benzene, toluene, and xylene, the latter being
present in three different modifications. The carbolic oil furnishes
carbolic acid and naphthalene, and the anthracene oil the hydrocarbon
which gives its name to that fraction. Here we have only half a dozen
distinct chemical compounds to deal with, and if we confine our attention
to these for the present, we shall be enabled to gain a good general idea
of what chemistry has done with these raw materials. The products
separated during the processes which have to be resorted to for the
isolation of these raw materials have also their uses, which will be
pointed out incidentally.

Beginning with the first runnings and the light oil, from which the
hydrocarbons of the benzene series are separated, we have to make
ourselves acquainted with the treatment to which these fractions are
submitted by the tar-distiller. The light oil is first distilled from an
iron still, similar to a tar-still, and the first portions which come over
are added to the oily fraction brought over by the water of the first
runnings. The separation of the oil from the water in this last fraction
is a simple matter, because the hydrocarbons float as a distinct layer on
the water, and do not mix with it. We have at this stage, therefore, four
products to consider, viz. 1st, the oil from the first runnings; 2nd, the
first portions of the light oil; 3rd, the later portions of the light oil;
and 4th, the residue in the still. The first and second are mixed
together, and the third is washed alternately with alkali and acid to
remove acid and basic impurities, and can then be mixed with the first and
second products. The total product is then ready for the next operation.
The last portion of the light oil which remains in the still is useless as
a source of benzene hydrocarbons, and goes into the heavy oil of the later
tar fractions.

The process of purification is thus far one of fractional distillation
combined with chemical washing. In fact, all the processes of purification
to which these oils are submitted are essentially of the same character.
The principle of fractional distillation has already been explained
sufficiently for our present purpose. The process of washing a liquid may
appear mysterious to the uninitiated, but in principle it is extremely
simple. If we pour some water into a bottle, and then add some liquid
which does not mix with the water--say paraffin oil--the two liquids form
distinct layers, the one floating on the other. On shaking the bottle so
as to mix the contents, the two liquids form a homogeneous mixture at
first, but on standing for a short time separation into two layers again
takes place. Now if there was present in the paraffin oil some substance
soluble in the oil, but more soluble in water, such as alcohol, we should
by the operation described wash the alcohol out of the oil, and when the
liquids separated into layers after agitation the watery layer would
contain the alcohol. By drawing off the oil or the water the former would
then be obtained free from alcohol--it would have been "washed." This
operation is precisely what the manufacturer does on a large scale with
the coal-tar oils. These oils contain certain impurities of which some are
of an acid character and dissolve in alkalies, while others are basic and
dissolve in acid. The oil is therefore agitated in a suitable vessel
provided with mechanical stirring gear with an aqueous solution of caustic
soda, and after separation into layers the alkaline solution retaining the
acid impurities is drawn off. Then the oil may be washed with water in the
same way to remove the lingering traces of alkali, and then with
acid--sulphuric acid or oil of vitriol--which dissolves out basic
impurities and certain hydrocarbons not belonging to the benzene series
which it is desirable to get rid of. A final washing with water removes
any acid that may be retained by the oil.

The total product containing the benzene hydrocarbons is put through such
a series of washing operations as above described, and is then ready for
separation into its constituents by another and more perfect process of
fractional distillation. This final separation is effected in a piece of
apparatus somewhat complicated in structure, but simple in principle. It
is a development on a large scale of the apparatus used by Mansfield in
his early experiments. The details of construction are not essential to
the present treatment of the subject, but it will suffice to say that the
vapours of the boiling hydrocarbons ascend through upright columns, in
which the compounds of high boiling-point first condense and run back into
the still, while the lower boiling-point compounds do not condense in the
columns, but pass on into a separate condenser, where they liquefy and are
collected. But even with this rectification we do not get a perfect
separation--the hydrocarbons are not perfectly pure from a chemical point
of view, although they are pure enough for manufacturing purposes. Thus
the first fraction consists of benzene containing a small percentage of
toluene, then comes over a mixture containing a larger proportion of
toluene, then comes a purer toluene mixed with a small percentage of
xylene. The boiling-points of the three hydrocarbons are 81° C., 111° C.,
and 140° C. respectively; but owing to the nature of fractional
distillation, a compound of a certain boiling-point always brings over
with it a certain quantity of the compound of higher boiling-point, and
that is why the rectifying column effects only a partial separation.

Of the hydrocarbons thus separated, benzene and toluene are by far the
most important; there is but a limited use for the xylenes at present, and
these and the hydrocarbons of higher boiling-point belonging to the same
series which distil over between 140° and 150° constitute what is known as
"solvent naphtha," because it is used for dissolving india-rubber for
waterproofing purposes. The hydrocarbons of still higher boiling-point
which remain in the still are used as burning naphtha for lamps. If
benzene of a higher degree of purity is required--as it is for the
manufacture of certain colouring-matters--the fraction containing this
hydrocarbon can be again distilled through the rectifying column, and a
large proportion of the toluene thus separated from it. Finally, pure
benzene can be obtained by submitting the rectified hydrocarbon to a
process of refrigeration in a mixture of ice and salt, when the benzene
solidifies to a white crystalline solid, while the toluene does not
solidify, and can be drained away from the benzene crystals which liquefy
at about 5° C.

The account rendered by the technologist with respect to the light oils of
the tar is thus a pretty good one. Already we see that benzene, toluene,
solvent naphtha, and burning naphtha are separated from them. Even the
alkaline and acid washings may be made to surrender their contained
products, for the first of these contains a certain quantity of carbolic
acid, and the acid contains a strongly smelling base called pyridine, for
which there is at present no great demand, but which may one day become of
importance. The actual quantity of benzene in tar is a little over one per
cent. by weight, and of toluene there is somewhat less. The naphthas are
present to the extent of about 35 per cent.

Now let us consider some of the transformations which benzene and toluene
undergo in the hands of the manufacturing chemist. The production of
aniline from benzene by acting upon this hydrocarbon with nitric acid, and
then reducing the nitrobenzene, has already been referred to. For this
purpose we now heat the nitrobenzene with iron dust and a little
hydrochloric (muriatic) acid, and then distil over the aniline by means of
a current of steam blown through the still. By a similar process toluene
is converted into nitrotoluene, and the latter into toluidine.

The large quantity of aniline and toluidine now made has opened up a
channel for the use of the waste borings from cast-iron. These are ground
to a fine powder under heavy mill-stones, and constitute a most valuable
reducing agent, known technically as "iron swarf." The metallic iron
introduced in this form into the aniline still is converted into an oxide
of iron by the action of the nitrobenzene, and this oxide of iron is used
by the gas-maker for purifying the gas from sulphur as already described.
When the oxide of iron is exhausted, _i.e._ when it has taken up as much
sulphur as it can, it goes to the vitriol-maker to be burnt as a source of
this acid. Here we have a waste product of the aniline manufacture
utilized for the purification of coal-gas, and finally being made to give
up the sulphur, which it obtained primarily from the coal, for the
production of sulphuric acid, which is consumed in nearly every branch of
chemical industry.

Nitrotoluene and toluidine each exist in three distinct modifications, so
that it is more correct to speak of the nitrotoluenes and the toluidines;
but the explanation of these differences belongs to pure chemical theory,
and cannot now be attempted in detail. It must suffice to say that many
compounds having the same chemical composition differ in their properties,
and are said to be "isomeric," the isomerism being regarded as the result
of the different order of arrangement of the atoms within the molecule.

Consider a homely illustration. A child's box of bricks contains a certain
number of wooden blocks, by means of which different structures can be
built up. Supposing all the bricks to be employed for every structure
erected, the latter must in every case contain all the blocks, and yet the
result is different, because in each structure the blocks are arranged in
a different way. The bricks represent atoms, and the whole structure
represents a molecule; the structures all have the same ultimate
composition, and are therefore isomeric. This will serve as a rough
analogy, only it must not be understood that the different atoms of the
elements composing a molecule are of different sizes and shapes; on this
point we are as yet profoundly ignorant.

Now as long ago as 1856, at the time when Perkin began making mauve by
oxidizing aniline with bichromate of potash, it was observed by Natanson,
that when aniline was heated with a certain oxidizing agent a red
colouring-matter was produced. The same fact was observed in 1858 by
Hofmann, who used the tetrachloride of carbon as an oxidizing agent. These
chemists obtained the red colouring-matter as a by-product; it was formed
only in small quantity, and was regarded as an impurity. In the same year,
1858, two French manufacturers patented the production of a red dye formed
by the action of chromic acid and other oxidizing agents on aniline, the
colouring matter thus made being used for dying artificial flowers. Then,
a year later, the French chemist Verguin found that the best oxidizing
agent was the tetrachloride of tin, and this with many other oxidizing
substances was patented by Renard Frères and Franc, and under their patent
the manufacture of the aniline red was commenced on a small scale in
France. Finally, in 1860, an oxidizing agent was made use of almost
simultaneously by two English chemists, Medlock and Nicholson, which gave
a far better yield of the red than any of the other materials previously
in use, and put the manufacture of the colouring-matter on quite a new
basis. The oxidizing material patented by Medlock and Nicholson is arsenic
acid, and their process is carried on at the present time on an enormous
scale in all the chief colour factories in Europe, the colouring-matter
produced by this means being generally known as fuchsine or magenta.

In four years the accidental observation of Natanson and Hofmann, made, be
it remembered, in the course of abstract scientific investigation, had
thus developed into an important branch of manufacture. A demand for
aniline on an increased scale sprung up, and the light oils of coal-tar
became of still greater importance. The operations of the tar-distiller
had to undergo a corresponding increase in magnitude and refinement; the
production of nitrobenzene and necessarily of nitric acid had to be
increased, and a new branch of manufacture, that of arsenic acid from
arsenious acid and nitric acid, was called into existence. Perkin's mauve
prepared the way for the manufacture of aniline, and the discovery of a
good process for the production of magenta increased this branch of
manufacture to a remarkable extent. Still later in the history of the
magenta manufacture, attempts were made, with more or less success, to use
nitrobenzene itself as an oxidizing agent, and a process was perfected in
1869 by Coupier, which is now in use in many factories.

The introduction of magenta into commerce marks an epoch in the history of
the coal-tar colour industry--pure chemistry and chemical technology both
profited by the discovery. The brilliant red of this colouring-matter is
objected to by modern æstheticism, but the dye is still made in large
quantities, its value having been greatly increased by a discovery made
about the same time by John Holliday and the Baden Aniline and Soda
Company, and patented by the latter in 1877. Magenta is the salt of a base
now known as rosaniline, and it belongs therefore to the class of basic
colouring-matters. The dyes of this kind are as a group less fast, and
have a more limited application than those colouring-matters which possess
an acid character, so that the discovery above referred to--that magenta
could be converted into an acid without destroying its colouring power by
acting upon it with very strong sulphuric acid--opened up a new field for
the employment of the dye, and greatly extended its usefulness. In this
form the colouring-matter is met with under the name of "acid magenta."

It must be understood that the production of magenta from aniline by the
oxidizing action of arsenic acid or nitrobenzene is the result of chemical
change; the colouring-matter is no more present in the aniline than the
latter is contained in the benzene. And just in the same way that the
colourless aniline oil by chemical transformation gives rise to the
intensely colorific magenta, so the latter by further chemical change can
be made to give rise to whole series of different colouring-matters, each
consisting of definite chemical compounds as distinct in individuality as
magenta itself. Thus in 1860, about the time when the arsenic acid process
was inaugurated, two French chemists, Messrs. Girard and De Laire,
observed that by heating rosaniline for some time with aniline and an
aniline salt, blue and violet colouring-matters were produced. This
observation formed the starting-point of a new manufacture proceeding from
magenta as a raw material. The production of the new colouring-matters was
perfected by various investigators, and a magnificent blue was the final
result. But here also the dye was of a basic character, and being
insoluble in water had only a limited application, as a spirit bath had to
be used for dissolving the substance. In 1862, however, an English
technologist, the late E. C. Nicholson, found that by the action of strong
sulphuric acid the aniline blue could be rendered soluble in water or
alkali, and the value of the colouring-matter was enormously increased by
this discovery. The basic and slightly soluble spirit blue was by this
means converted into acid blues, which are now made in large quantities,
and sold under the names of Nicholson's blue, alkali blue, soluble blue,
and other trade designations. There is at the present time hardly any
other blue which for fastness, facility of dyeing, and beauty can compete
with this colouring-matter introduced by Nicholson as the outcome of the
work of Girard and De Laire.

Other transformations of rosaniline have yet to be chronicled. In 1862
Hofmann found that by acting upon this base--the base of magenta--with the
iodide of methyl, violet colouring-matters were produced, and these were
for some years extensively employed under the name of Hofmann's violets.
And still more remarkable, by the prolonged action of an excess of methyl
iodide upon rosaniline, Keisser found that a green colouring-matter was
formed. The latter was patented in 1866, and the dye was for some time in
use under the name of "iodine green." The statement that technology
profited by the introduction of magenta has therefore been justified.

It remains to add, that the tar obtained from one ton of Lancashire coal
furnishes an amount of aniline capable of giving a little over half a
pound of magenta. The colouring power of the latter will be inferred from
the fact, that this quantity would dye 375 square yards of white flannel
of a full red colour, and if converted into Hofmann violet by methylation,
would give enough colour to dye double this surface of flannel of a deep
violet shade. It should be stated also, that during the formation of
magenta by the arsenic acid process, there are formed small quantities of
other colouring-matters which are utilized by the manufacturer. Among
these by-products is a basic orange dye, which was isolated by Nicholson,
and investigated by Hofmann in 1862. Under the name of "phosphine" this
colouring-matter is still used, especially for the dyeing of leather. Even
the spent arsenic acid of the magenta-still has its use. The arsenious
acid resulting from the reduction of this arsenic acid is generally
obtained in the form of a lime salt after the removal of the magenta by
the purifying processes to which the crude product is submitted. From the
arsenical waste arsenious acid can be recovered, and converted back into
arsenic acid by the action of nitric acid. Quite recently the arsenical
residue has been used with considerable success in America as an
insecticide for the destruction of pests injurious to agricultural crops.

Concurrently with these technical developments of coal-tar products, the
scientific chemist was carrying on his investigations. The compounds which
science had given to commerce were made on a scale that enabled the
investigator to obtain his materials in quantities that appeared fabulous
in the early days when aniline was regarded as a laboratory curiosity, and
magenta had been seen by only a few chemists.

The fundamental problem which the modern chemist seeks to solve is in the
first place the composition of a compound, _i.e._ the number of the atoms
of the different elements which form the molecule, and in the next place
the way in which these atoms are combined in the molecule. Reverting to
our former analogy, the first thing to be found is how many different
blocks enter into the composition of the structure, and the next thing is
to ascertain how the blocks are arranged. When this is done, we are said
to know the "constitution" or "structure" of the molecule, and in many
cases when this is known we can build up or synthesise the compound by
combining its different groups of atoms by suitable methods. The coal-tar
industry abounds with such triumphs of chemical synthesis; a few of these
achievements will be brought to light in the course of the remaining
portions of this work.

The chemical investigation of magenta was commenced by Hofmann, whose name
is inseparably connected with the scientific development of the coal-tar
colour industry. In 1862 he showed that magenta was the salt of a base
which he isolated, analysed, and named rosaniline. He established the
composition of this base and of the violet and blue colouring-matters
obtained from it by the processes already described. In 1864 he made the
interesting discovery that magenta is not formed by the oxidation of
_pure_ aniline, but that a mixture of aniline and toluidine is essential
for the production of this colouring-matter. In fact, the aniline oil used
by the manufacturer had from the beginning consisted of a mixture of
aniline and toluidine, and at the present time "aniline for red" is made
by nitrating a mixture of benzene and toluene and reducing the

From this work of Hofmann's suggestions naturally arose concerning the
"constitution" of rosaniline, and new and fruitful lines of work were
opened up. Large numbers of chemists of the greatest eminence pursued the
inquiry, but the details of their work, although of absorbing interest to
the chemist, cannot be discussed in the present volume. The final touch to
a long series of investigations was given by two German chemists, Emil and
Otto Fischer, who in 1878 proved the constitution of rosaniline by
obtaining from it a hydrocarbon, the parent hydrocarbon from which the
colouring-matter is derived. The purely scientific discovery of the
Fischers threw a flood of light on the chemistry of magenta, and enabled a
large number of colouring-matters related to the latter to be classed
under one group, having the parent hydrocarbon as a central type. This
hydrocarbon, it may be remarked, is known as triphenylmethane, as it is a
derivative of methane, or marsh gas. The blues and violets obtained from
rosaniline belong to this group, and so also do certain other
colouring-matters which had been manufactured before the Fischers'
discovery. In order to carry on the story of the utilisation of aniline,
it is necessary to know something about these other colouring-matters
which are obtained from it.

It has been explained that by the methylation of rosaniline Hofmann
obtained violet colouring-matters. Now as rosaniline is obtained by the
oxidation of a mixture of aniline and toluidine, it seems but natural that
if these bases were methylated first and then oxidized a violet dye would
be produced. The French chemist Lauth first obtained a violet
colouring-matter by this method in 1861. In 1866 this violet dye was
manufactured in France by Poirrier, and it is still made in large
quantities, being known under the name of "methyl violet." This
colouring-matter, and a bluer derivative of it discovered in 1868,
gradually displaced the Hofmann violets, chiefly owing to their greater
cheapness of production. We are thus introduced to methylated aniline as a
source of colouring-matters, and as the compound in question has many
different uses in the coal-tar industry, a few words must be devoted to
its technology.

Aniline, toluidine, and similar bases can be methylated by the action of
methyl iodide, but the cost of iodine is too great to enable this process
to be used by the manufacturer. Methyl chloride, however, answers equally
well, and this compound, which is a liquid of very low boiling point (-23°
C.), is prepared on a large scale from the waste material of another
industry, viz. the beet-sugar manufacture. It is interesting to see how
distinct industries by chemical skill are made to act and react upon one
another. Thus the cultivation of the beet, as already explained, is
largely dependent on the supply of ammonia from gas-liquor. During the
refining of the beet-sugar, a large quantity of uncrystallisable treacle
is separated, and this is fermented for the manufacture of alcohol. When
the latter is distilled off there remains a spent liquor containing among
other things potassium salts and nitrogenous compounds. This waste liquor,
called "vinasse," is evaporated down and ignited in order to recover the
potash, and during the ignition, ammonia, tar, gas, and other volatile
products are given off. Among the volatile products is a base called
trimethylamine, which is a derivative of ammonia; the salt formed by
combining trimethylamine with hydrochloric acid when heated gives off
methyl chloride as a gas which can be condensed by pressure.

Here we have a very pretty cycle of chemical transmigration. The nitrogen
of the coal plants, stored up in the earth for ages, is restored in the
form of ammonia to the crops of growing beet; the nitrogen is made to
enter into the composition of the latter plant by the chemico-physiological
process going on, and the nitrogenous compounds removed from the plant and
heated to the point of decomposition in presence of the potash (which also
entered into the composition of the plant), give back their nitrogen
partly in the form of a base from which methyl chloride can be obtained.
The latter is then made to methylate a product, aniline, derived indirectly
from coal-tar. The utilisation of the "vinasse" for this purpose was made
known by Camille Vincent of Paris in 1878.

The methylation of aniline can obviously be carried out by the foregoing
process only when beet-sugar residues are available. There is another
method which is more generally used, and which is interesting as bringing
in a distinct branch of industry. The same result can, in fact, be arrived
at by heating dry aniline hydrochloride, _i.e._ the hydrochloric acid salt
of aniline, with methyl alcohol or wood-spirit in strong metallic boilers
under great pressure. This is the process carried on in most factories,
and it involves the use of pure methyl alcohol, a branch of manufacture
which has been called into existence to meet the requirements of the
coal-tar colour maker.[4] This alcohol or wood-spirit is obtained by the
destructive distillation of wood, and is purified by a series of
operations which do not at present concern us. It must be mentioned that
the product of the methylation of aniline, which it is the object of the
manufacturer to obtain, is an oily liquid called dimethylaniline, which,
by virtue of the chemical transformation, is quite different in its
properties to the aniline from which it is derived. By a similar
operation, using ethyl alcohol, or spirit of wine, diethylaniline can be
obtained, and by heating dry aniline hydrochloride with aniline under
similar conditions a crystalline base called diphenylamine is also

Now these products--dimethylaniline, diethylaniline, and
diphenylamine--are derived from aniline, and they are all sources of
colouring-matters. Methyl-violet is obtained by the oxidation of
dimethylaniline by means of a gentle oxidizer; a mixture of bases is not
necessary as in the case of the magenta formation. Then in 1866
diphenylamine was shown by Girard and De Laire to be capable of yielding a
fine blue by heating it with oxalic acid, and this blue, on account of the
purity of its shade, is still an article of commerce. It can be made
soluble by the action of sulphuric acid in just the same way as the other
aniline blue. Furthermore, by acting with excess of methyl chloride on
methyl violet, a brilliant green colouring-matter was manufactured in
1878, which was obviously analogous to the iodine green already mentioned,
and which for some years held its own as the only good coal-tar green.
These are the dyes--methyl violet and green, and diphenylamine blue--which
were in commerce before the discovery of the Fischers, and which this
discovery enabled chemists to class with magenta, aniline blue, and
Hofmann violet in the triphenylmethane group.

Later developments bring us into contact with other dyes of the same
class, and with the industrial evolution of the purely scientific idea
concerning the constitution of the colouring-matters of this group.
Benzene and toluene again form the points of departure. By the action of
chlorine upon the vapour of boiling toluene there are obtained, according
to the extent of the action of the chlorine, three liquids of use to the
colour manufacturer. The first of these is benzyl chloride, the second
benzal chloride, and the third benzotrichloride or phenyl chloroform.
Benzyl chloride, it may be remarked in passing, plays the same part in
organic chemistry as methyl chloride, and enables certain compounds to be
benzylated, just in the same way that they can be methylated. The bluer
shade of methyl violet, introduced in 1868, and still manufactured, is a
benzylated derivative. By the action of benzotrichloride on
dimethylaniline in the presence of dry zinc chloride, Oscar Doebner
obtained in 1878 a brilliant green colouring-matter which was manufactured
under the name of "malachite green." It will be remembered that this was
about the time when the Fischers were engaged with their investigations.
These last chemists, by virtue of their scientific results, were enabled
to show that Doebner's green was a member of the triphenylmethane group,
and they prepared the same compound by another method which has enabled
the manufacturer to dispense with the use of the somewhat expensive and
disagreeable benzotrichloride. The Fischers' method consists in heating
dimethylaniline with bitter-almond oil and oxidizing the product thus
formed, when the green colouring-matter is at once produced. This method
brings the technologist into competition with Nature, and we shall see the

Benzoic aldehyde or bitter-almond oil is one of the oldest known products
of the vegetable kingdom, and has from time to time been made the subject
of investigation by chemists since the beginning of the century. It arises
from the fermentation of a nitrogenous compound found in the almond, and
known as amygdalin, the nature of the fermentative change undergone by
this substance having been brought to light by Wöhler and Liebig. The
discovery of a green dye, requiring for its preparation a vegetable
product which was very costly, compelled the manufacturer to seek another
source of the oil. Pure chemistry again steps in, and solves the problem.
In 1863 it was known to Cahours that benzal chloride, on being heated with
water or alkali, gave benzoic aldehyde, and in 1867 Lauth and Grimaux
showed that the same compound could be formed by oxidizing benzyl chloride
in the presence of water. It was but a step from the laboratory into the
factory in this case, and at the present time the aldehyde is made on a
large scale by chlorinating boiling toluene beyond the stage of benzyl
chloride, and heating the mixture of benzal chloride and benzotrichloride
with lime and water under pressure. By this means the first compound is
transformed into benzoic aldehyde, and the second into benzoic acid. This
last substance is also required by the colour-maker, as it is used in the
manufacture of blue by the action of aniline on rosaniline; without some
such organic acid the transformation of rosaniline into the blue is very

Benzoic acid, like the aldehyde, is a natural product which has long been
known. It was obtained from gum benzoïn at the beginning of the
seventeenth century, and its preparation from this source was described by
Scheele in 1755. The same chemist afterwards found it in urine, and from
these two sources, the one vegetable and the other animal, the acid was
formerly prepared. Its relationship to benzene has already been alluded to
in connection with the history of that hydrocarbon. It will be remembered
that by heating this acid with lime Mitscherlich obtained benzene in
1834. In one operation, therefore, setting out from toluene, we make these
two natural products, the aldehyde and acid, which are easily separable by
technical processes. The wants of the technologist have been met, and he
has been enabled to compete successfully with Nature, for he can
manufacture these products much more cheaply than when he had to depend
upon bitter almonds or gum benzoïn. The synthetical bitter-almond oil is
chemically identical with that from the plant. Besides its use for the
manufacture of colouring-matters, it is employed for flavouring purposes
and in perfumery, this being the first instance of a coal-tar perfume
which we have had occasion to mention. The odour in this case, it must be
remembered, is that of the actual compound which imparts the
characteristic taste and smell to the almond; it is not the result of
substituting a substance which has a particular odour for another having a
similar odour, as is the case with nitrobenzene, which, as already
mentioned, is used in large quantities under the name of "essence of
mirbane," for imparting an almond-like smell to soap.

The introduction of malachite green marks another epoch in the history of
the technology of the triphenylmethane colours. The action between benzoic
aldehyde and other bases analogous to dimethylaniline was found to be
quite general, and the principle was extended to diethylaniline and
similarly constituted bases. Various green dyes--some of them acids formed
by the action of sulphuric acid on the colour base--are now manufactured,
and many other colouring-matters of the same group are synthesised by the
benzoic aldehyde process.

One other development of this branch of manufacture has yet to be
recorded. The new departure was made in 1883 by Caro and Kern, who
patented a process for the synthesis of colouring-matters of this group.
In this synthesis a gas called phosgene is used, the said gas having been
discovered by John Davy in 1811, who gave it its name because it is formed
by the direct union of chlorine and carbon monoxide under the influence of
sunlight. Caro and Kern's process is the first technical application of
Davy's compound. By the action of phosgene on dimethylaniline and
analogous bases in the presence of certain compounds which promote the
chemical interaction, a number of basic colouring-matters of brilliant
shades of violet ("crystal violet") and blue ("Victoria blue," "night
blue") are produced, these being all members of the triphenylmethane
group. One of these dyes is a fine basic yellow known as "auramine," which
is a derivative of diphenylmethane.

                               (Light oil)
                                |       |
                                |       |
                ----------------        --------
                |                              |      / Benzyl chloride
             Benzene                         Toluene {  Benzal chloride }
                |                               |  |  \ Benzotrichloride}<-
                |                               |  ------------           |
             Aniline-> Magenta (and Phosphine)<-Toluidine     |           |
             /      \                |     \                  |           |
            /        \               |      \        ---Benzoic acid and<-
  Dimethylaniline    Diphenylamine   |       \       |  Benzoic aldehyde
        |                |           |        \      |              /
        |                |           |        Aniline Blues:       /
        |                |    Hofmann Violet    Nicholson and     /
        |                |   and Iodine Green   Soluble          /
        |            Diph. Blue                                 /
        |                                                      /
     ------------------------------------------------         /
     |                       |                       |       /
  Crystal Violet,      Methyl Violet                 |      /
   Auramine, &          and Green                    Malachite
  other Phosgene dyes                               Green series

From benzene and toluene alone about forty distinct colouring-matters of
the rosaniline group are sent into commerce. The relationship of those
compounds to each other and to their generating substances is not easy to
grasp by those to whom the facts are presented for the first time. The
scheme on page 107 shows these relationships at a glance.

The colouring-matters derived from these two hydrocarbons are far from
being exhausted. During the oxidation of aniline for the production of
mauve--which colouring-matter, it may be mentioned, is no longer made--a
red compound is formed as a by-product. This was isolated by Perkin in
1861, and studied scientifically by Hofmann and Geyger, who established
its composition in 1872, the dye being at that time manufactured under the
name of "saffranine." It appears to have been first introduced about 1868.
The conditions of formation of this dye were at first imperfectly
understood, but the problem was attacked by chemists and technologists,
and the first point of importance resulting from their work was that
saffranine was derived from one of the toluidines present in the
commercial aniline. To record the various steps in this chapter of
industrial chemistry would take us beyond the scope of the present work.
In addition to the chemists named, Caro, Bindschedler, and others
contributed to the technology, while the scientific side of the matter was
first taken up by Nietzki in 1877, by Otto Witt in 1878, and by Bernthsen
in 1886. It is to the work of these chemists, and especially to that of
Witt, that we owe our present knowledge of the constitution of this and
allied colouring-matters. Space will not admit of our traversing the
ground, although to chemists it is a line of investigation full of
interest; it will be sufficient to say that by 1886 these investigators
had accomplished for these colouring-matters what the Fischers had done
for the rosaniline group--they established their constitution, and showed
that they were derivatives of diphenylamine, containing two nitrogen atoms
joined together in a particular way. The parent-substance from which these
compounds are derived is known at the present time as "azine" (French,
azote = nitrogen), and the dyes belong accordingly to the azine group. The
first coal-tar colouring-matter, Perkin's mauve, is a member of this

The azine dyes are basic, and mostly of a red or pink shade; they are
somewhat fugitive when exposed to light, but possess a certain value on
account of their affinity for cotton, and the readiness with which they
can be used in admixture with other colouring-matters. Some of the best
known are made by oxidizing certain derivatives of aniline or toluidine,
in the presence of these or analogous bases. To make this intelligible a
little more chemistry is necessary. Aniline is a derivative of benzene in
which one atom of hydrogen is replaced by the residue of ammonia. Ammonia
is composed of one atom of nitrogen and three atoms of hydrogen; benzene
is composed of six atoms of carbon and six of hydrogen. If one atom of
hydrogen is supposed to be withdrawn from ammonia, there remains a residue
called the amido-group, and if we imagine this group to be substituted for
one of the hydrogen atoms in benzene, we have an amido-derivative, _i.e._
amidobenzene or aniline. Similarly, the toluidines are amidotoluenes. If
two hydrogen atoms in benzene or toluene are replaced by two amido-groups,
we have diamidobenzenes and diamidotoluenes, which are strongly basic
substances, capable of existing in several isomeric modifications. Certain
of these diamido-compounds when oxidized in the presence of a further
quantity of aniline, toluidine, and such amido-compounds, give rise to
unstable blue products, which readily become transformed into red dyes of
the azine group.

Some azine dyes are produced by another method, which is instructive
because it brings us into contact with a derivative of dimethylaniline
which figures largely in the coal-tar colour industry. By the action of
nitrous acid on this base, there is produced a compound known as
nitrosodimethylaniline, which was discovered by Baeyer and Caro in 1874,
and which contains the residue of nitrous acid in place of one atom of
hydrogen. The residue of nitric acid which replaces hydrogen in benzene is
the nitro-group, and the compound is nitrobenzene. The analogy with
nitrous acid will therefore be sufficiently understood--the residue of
this acid is the nitroso-group, and compounds containing this group are
nitroso-derivatives. In 1879, Otto Witt found that the nitroso-group in
nitrosodimethylaniline acted as an oxidizing group, and enabled this
compound to act upon certain diamido-derivatives of benzene and toluene,
with the formation of unstable blue compounds, which on heating the
solution changed into red colouring-matters of the azine group. This
process soon bore fruit industrially, and azines of a red, violet, and
blue shade were introduced under the names of neutral red, violet, and
blue, Basle blue, &c., some of these surviving at the present time.

We have now to turn to another chapter in the history of dimethylaniline.
In 1876, Lauth discovered a new colour test for one of the
diamidobenzenes. By heating this base with sulphur, and oxidizing the
product, a violet colouring-matter was formed, and the same compound was
produced by oxidizing the base in an aqueous solution in the presence of
sulphuretted hydrogen. Lauth's violet was never manufactured in quantity
because the yield is small; but in the hands of Dr. Caro the work of Lauth
bore fruit in another direction. Instead of using the diamidobenzene, Caro
used its dimethyl-derivative, and by this means obtained a splendid blue
dye, which was introduced under the name of "methylene blue." Here again
we find scientific research reacting on technology. A few words of
chemical explanation will make this manufacture intelligible. By the
action of reducing agents on nitro and nitroso-compounds, the nitro and
nitroso-group become converted into the amido-group. Thus when
nitrobenzene is reduced by iron and an acid we get aniline; similarly when
nitrosodimethylaniline is reduced by zinc and an acid we get
amidodimethylaniline, and this is the base used in the preparation of
methylene blue. By oxidizing this base in the presence of sulphuretted
hydrogen, the colouring-matter is formed. Other methods of arriving at the
same result were discovered and patented in due course, but the various
processes cannot be discussed here.

Lauth's violet and methylene blue became the subjects of scientific
investigation in 1879 by Koch, and in 1883 a series of brilliant
researches were commenced by Bernthsen which extended over several years,
and which established the constitution of these compounds. It was shown
that they are derivatives of diphenylamine containing sulphur as an
essential constituent. The parent-compound is diphenylamine in which
sulphur replaces hydrogen, and is therefore known as thiodiphenylamine. It
can be prepared by heating diphenylamine with sulphur, and is sometimes
called thiazine, because it is somewhat analogous in type to azine. We
must therefore credit dimethylaniline with being the industrial generator
of the thiazines. The blue is largely used for cotton dyeing, producing on
this fibre when properly mordanted an indigo shade. By the action of
nitrous acid the blue is converted into a green known as "methylene

Although the scope of this work admits of our dealing with only a few of
the more important groups of colouring-matters, it will already be evident
that the chemist has turned benzene and toluene to good account. But great
as is the demand for these hydrocarbons for the foregoing purposes, there
are other branches of the coal-tar industry which are dependent upon them.
It will serve as an answer to those who are continually raising the cry of
brilliancy as an offence to æsthetic taste if we consider in the next
place a most valuable and important black obtained from aniline. All
chemists who studied the action of oxidizing agents, such as chromic acid,
on aniline, from Runge in 1834 to Perkin in 1856, observed the formation
of greenish or bluish-black compounds. After many attempts to utilize
these as colouring-matters, success was achieved by John Lightfoot of
Accrington near Manchester in 1863. By using as an oxidizing agent a
mixture of potassium chlorate and a copper salt, Lightfoot devised a
method for printing and dyeing cotton fabrics, the use of which spread
rapidly and created an increased demand for the hydrochloride of aniline,
this salt being now manufactured in enormous quantities under the
technical designation of "aniline salt." Lightfoot's process was improved
for printing purposes by Lauth in 1864, and many different oxidizing
mixtures have been subsequently introduced, notably the salts of vanadium,
which are far more effective than the salts of copper, and which were
first employed by Lightfoot in 1872. In 1875-76 Coquillion and
Goppelsröder showed that aniline black is produced when an electric
current is made to decompose a solution of an aniline salt, the oxidizing
agent here being the nascent oxygen resulting from the electrolysis. In
these days when the generation of electricity is so economically effected,
this process may become more generally used, and the coal-tar industry
may thus be brought into relationship with another branch of applied
science. Aniline black is seldom used as a direct colouring-matter; it is
generally produced in the fibre by printing on the mixture of aniline salt
and oxidizing compounds thickened with starch, &c., and then allowing the
oxidation to take place spontaneously in a moist and slightly heated
atmosphere. By a similar process, using a dye-bath containing the aniline
salt and oxidizing mixture, cotton fibre is easily dyed. The black cannot
be used for silk or wool, as the oxidizing materials attack these fibres,
but for cotton dyeing and calico printing this colouring-matter has come
seriously into competition with the black dyes obtained from logwood and
madder. The use of aniline for this purpose, first rendered practicable by
Lightfoot, is among the most important of the many wonderful applications
of coal-tar products in the tinctorial industry.

The year 1863 witnessed the introduction of the first of a new series of
colouring-matters which have had an enormous influence both on the art of
the dyer as well as in the utilization of tar-products which were formerly
of but little value. We can consider the history of some of these colours
now, because the earliest of them was produced from aniline. The formation
of a yellow compound when nitrous acid acts upon aniline was observed by
several chemists prior to the date mentioned. In 1863 the firm of Simpson,
Maule and Nicholson manufactured a yellow dye by passing nitrous gas into
a solution of aniline in alcohol, and this had a limited application under
the name of "aniline yellow." Soon afterwards, viz. in 1866, the firm of
Roberts, Dale & Co. of Manchester introduced a brown dye under the name of
"Manchester brown"--this compound, which was discovered by Dr. Martius in
1865, having been produced by the action of nitrous acid on one of the
diamidobenzenes. Ten years later Caro and Witt discovered an orange
colouring-matter belonging to the same class, and the latter introduced
the compound into commerce as "chrysoïdine." These three compounds are
basic, and the first of them is no longer used as a direct dye because it
is fugitive. Chrysoïdine is still used to a large extent, and the
brown--now known as "Bismarck brown"--is one of the staple products of the
colour manufacturer at the present time. From this fragment of
technological history let us now turn to chemical science.

The chemist whose name will always be associated with the compounds of
this group is the late Dr. Peter Griess of Burton-on-Trent. He commenced
his study of the action of nitrous acid on organic bases in 1858, and
from that time till the period of his death in 1888, he was constantly
contributing to our knowledge of the resulting compounds. In 1866, he and
Dr. Martius established the composition of aniline yellow, and the
following year Caro and Griess did the same thing for the Manchester
brown. In 1877 Hofmann and Witt established the constitution of
chrysoïdine, the final outcome of all this work being to show that the
three colouring-matters belonged to the same group. The further
development of these discoveries has been one of the most prolific sources
of new colouring-matters. A brief summary of our present position with
respect to this group must now be attempted.

When nitrous acid acts upon an amido-derivative of a benzenoid hydrocarbon
in the presence of a mineral acid, there is formed a compound in which the
amido-group is replaced by a pair of nitrogen atoms joined together in a
certain way, which is different to the mode of combination in the azines.
This pair of nitrogen atoms is combined on the one hand with the
hydrocarbon residue, and on the other with the residue of the mineral
acid. The resulting compound is very unstable; its solution decomposes
very readily, and generally has to be kept cool by ice. Freezing machines
turning out large quantities of ice are kept constantly at work in
factories where these produces are made. The latter are known as
"diazo-compounds"--Griess's compounds _par excellence_--and they are
prepared on a large scale by dissolving a salt of the amido-base,
generally the hydrochloride, in water with ice, and adding sodium nitrite.
The result is a diazo-salt; aniline, for example, giving diazobenzene
chloride, and toluidine diazotoluene chloride. Similarly all
amido-derivatives of a benzenoid character can be "diazotised." The
importance of this discovery will be seen more fully in the next chapter.
At present we are more especially concerned with aniline.

The extreme instability of the diazo-salts enables them to combine with
the greatest ease with amido-derivatives and with other compounds. The
very property which in the early days rendered their investigation so
difficult, and which taxed the ingenuity of chemists to the utmost, has
now placed these compounds in the front rank as colour generators. When a
diazo-salt acts on an amido-derivative there is formed a compound which is
more or less unstable, but which readily undergoes transformation under
suitable conditions into a stable substance in which two hydrocarbon
residues are joined together by the pair of nitrogen atoms. These products
are dye-stuffs, known as "azo-colours," and aniline yellow, Bismarck
brown, and chrysoïdine are the oldest known technical compounds belonging
to the group. The parent substance is "azobenzene," and these three
colouring-matters are mono-, di- and triamido-azobenzene respectively.

A new phase in the technology of tar-products was entered upon when Witt
caused a diazo-salt to act upon diamidobenzene. This was the first
industrial application of Griess's discovery. Azobenzene, which was
discovered by Mitscherlich in 1834, and azotoluene are now manufactured by
reducing nitrobenzene and nitrotoluene with mild reducing agents. These
parent compounds are not in themselves colouring-matters, but they are
transformed into bases which give rise to a splendid series of azo-dyes,
as will be described subsequently. Let it be recorded here that these two
compounds are to be added to the list of valuable products obtained from
benzene and toluene. And it must also be remembered that the introduction
of these azo-colours has necessitated the manufacture on a large scale of
sodium nitrite as a source of nitrous acid. Without entering into
unnecessary detail it may be stated broadly that this salt is made by
fusing Chili saltpetre, which is the nitrate of sodium, with metallic
lead, litharge or oxide of lead being obtained as a secondary product.
Then again, the manufacture of Bismarck brown requires dinitrobenzene,
this being made by the nitration of benzene beyond the stage of
nitrobenzene. The brown is made by reducing the dinitrobenzene to
diamidobenzene, and then treating a solution of the latter with sodium
nitrite and an acid. The azo-colour is formed at once, and no special
refrigeration is required in this particular case.

It has already been stated that the old aniline yellow of 1863 is no
longer used on account of its fugitive character. In 1878 Grässler found
that by the action of very strong sulphuric acid this azo-compound could
be converted into a sulpho-acid in just the same way that magenta can be
converted into acid magenta. Under the name of "acid yellow" this
sulpho-acid is now used, not only as a direct yellow colouring-matter, but
as a starting-point in the manufacture of other azo-dyes. The use of acid
yellow for this last purpose will be dealt with again in the next chapter.

There is one other use for aniline yellow which dates from the year of its
discovery, when Dale and Caro found that by adding sodium nitrite to
aniline hydrochloride and heating the mixture, a blue colouring-matter is
produced. The latter was introduced in 1864 under the name of "induline."
It was shown subsequently by the scientific researches of several chemists
that the blue produced by Dale and Caro's method results from the action
of the aniline salt on the aniline yellow, which is formed by the action
of the nitrous acid on the aniline and aniline salt. This explanation was
proved to be correct in 1872 by Hofmann and Geyger, who prepared the
colouring-matter by heating aniline yellow and aniline salt with alcohol
as a solvent. These chemists established the composition and gave it the
name of "azodiphenyl blue." Later, viz. in 1883, the manufacture was
improved by Otto Witt and E. Thomas, and the dye, under the old name of
"induline," is now largely manufactured by first preparing aniline yellow
and then heating this with aniline and aniline salt. The colouring-matter
as formed by this method is basic and insoluble in water; it is made acid
and soluble by treatment with sulphuric acid, which converts it into a
sulpho-acid. Induline belongs to the sober-tinted colours, and produces a
shade somewhat resembling indigo. Closely related thereto is a bluish-grey
called "nigrosine," obtained by heating nitrobenzene with aniline, as well
as a certain bluish by-product obtained during the formation of magenta,
and known as "violaniline."

It will be convenient here to pause and reflect upon the great industrial
importance of the two coal-tar hydrocarbons upon which we have thus far
concentrated our attention. Their uses are by no means exhausted as yet,
but they have already been made to account for such a number of valuable
products that the reader may find it useful to have the results presented
in a collected form. This is given below as a chronological summary--

  1856. Mauve discovered by Perkin; leading to manufacture of aniline.

  1860. Arsenic acid process for magenta discovered; leading to
        manufacture of arsenic acid.

  1860. Aniline blue discovered; leading in 1862 to soluble and Nicholson

  1861. Methyl violet discovered; manufactured in 1866; leading to a new
        use of copper salts as oxidizing agents, and to the manufacture of

  1862. Hofmann violets discovered; leading to manufacture of methyl
        iodide from iodine, phosphorus, and wood spirit.

  1862. Phosphine (chrysaniline) discovered in crude magenta.

  1863. Aniline black introduced; leading to a new use for potassium
        chlorate and copper salts, and to the manufacture of aniline salt.

  1863. Aniline yellow introduced, the first azo-colour.

  1864. Induline discovered; leading to new use for aniline yellow.

  1866. Manchester brown introduced, the second azo-colour; leading to the
        manufacture of sodium nitrite, and of dinitrobenzene.

  1866. Iodine green introduced; leading to further use for methyl iodide.

  1866. Diphenylamine blue introduced; leading to manufacture of

  1868. Blue shade of methyl violet introduced; leading to manufacture of
        benzyl chloride.

  1868. Saffranine introduced.

  1869. Nitrobenzene process for magenta discovered.

  1876. Chrysoïdine introduced, the third azo-colour.

  1876. Methylene blue introduced; leading to manufacture of

  1877. Acid magenta discovered.

  1878. Methyl green introduced; leading to utilization of waste from
        beet-sugar manufacture.

  1878. Malachite green discovered; leading to manufacture of benzoic

  1878. Acid yellow discovered; leading to new use for aniline yellow.

  1879. Neutral red and allied azines introduced; leading to a new use for

  1883. Phosgene colours of rosaniline group introduced; leading to
        manufacture of phosgene.


Among the most venerable of natural dye-stuffs is indigo, the substance
from which Unverdorben first obtained aniline in 1826. The colouring
matter is found in a number of leguminous (see Fig. 7), cruciferous, and
other plants, some of which are largely cultivated in India, China, the
Malay Archipelago, South America, and the West Indies; while others, such
as woad (see Fig. 8), are grown in more temperate European climates. The
tinctorial value of these plants was known in India and Egypt long before
the Christian era. Egyptian mummy-cloths have been found dyed with indigo.
The dye was known to the Greeks and Romans; its use is described by the
younger Pliny in his Natural History. Indigo was introduced into Europe
about the sixteenth century, but its use was strongly opposed by the woad
cultivators, with whose industry the dye came into competition. In France
the opposition was strong enough to secure the passing of an act in the
time of Henry IV. inflicting the penalty of death upon any person found
using the dye. The importance of indigo as an article of commerce is
sufficiently known at the present time; more than 8000 tons are produced
annually, corresponding in money value to about four million pounds. It is
of importance to us as rulers of India to remember that the cultivation
and manufacture of indigo is one of the staple industries of that country,
from which the European markets derive the greater part of their supply.

[Illustration: FIG. 7.--INDIGO PLANT (_Indigofera tinctoria_).]

Imagine the industrial revolution which would be caused by the discovery
of a process for obtaining indigo synthetically from a coal-tar
hydrocarbon, at a price which would compare favourably with that of the
natural product. This has not actually been done as yet, but chemists have
attempted to compete with Nature in this direction, and the present state
of the competition is that the natural product can be cultivated and made
more cheaply. Nevertheless the dye can be synthesised from a coal-tar
hydrocarbon, and this is one of the greatest achievements of modern
chemistry in connection with the tar-products. For more than half a
century indigo had been undergoing investigation by chemists, and at
length the work culminated in the discovery of a method for producing it
artificially. This discovery was the outcome of the labour of Adolf v.
Baeyer, who commenced his researches upon the derivatives of indigo in
1866, and who in 1880 secured the first patents for the manufacture of the
colouring-matter. It is to the laborious and brilliant investigations of
this chemist that we owe nearly all that is at present known about the
chemistry of indigo and allied compounds.

[Illustration: FIG. 8.--WOAD (_Isatis tinctoria_).]

Two methods have been used for the production of artificial indigo--benzal
chloride being the starting-point in one of these, and nitrobenzoic
aldehyde in the other. The generating hydrocarbon is therefore toluene. By
heating benzal chloride with dry sodium acetate there is formed an acid
known as cinnamic acid, a fragrant compound which derives its name from
cinnamon, because the acid was prepared by the oxidation of oil of
cinnamon by Dumas and Peligot in 1834. The acid and its ethers occur also
in many balsams, so that we have here another instance of the synthesis of
a natural vegetable product from a coal-tar hydrocarbon. The subsequent
steps are--(1) the nitration of the acid to produce nitrocinnamic acid;
(2) the addition of bromine to form a dibromide of the nitro-acid; (3) the
action of alkali on the dibromide to produce what is known as "propiolic
acid." The latter, under the influence of mild alkaline reducing agents,
is transformed into indigo-blue. The process depending on the use of
nitrobenzoic aldehyde is much simpler; but the particular nitro-derivative
of the aldehyde which is required is at present difficult to make, and
therefore expensive. If the production of this compound could be
cheapened, the competition between artificial and natural indigo would
assume a much more serious aspect.[5]

The light oil of the tar-distiller has now been sufficiently dealt with so
far as regards colouring-matters; let us pass on to the next fraction of
the tar, the carbolic oil. The important constituents of this portion are
carbolic acid and naphthalene. The carbolic oil is in the first place
separated into two distinct portions by washing with an alkaline solution.
Carbolic acid or phenol belongs to a class of compounds derived from
hydrocarbons of the benzene and related series by the substitution of the
residue of water for hydrogen. This water-residue is known to chemists as
"hydroxyl"--it is water less one atom of hydrogen. Carbolic acid or phenol
is hydroxybenzene; and all analogous compounds are spoken of as "phenols."
It will be understood in future that a phenol is a hydroxy-derivative of a
benzenoid hydrocarbon. Now these phenols are all more or less acid in
character by virtue of the hydroxyl-group which they contain. For this
reason they dissolve in aqueous alkaline solutions, and are precipitated
therefrom by acids. This will enable us to understand the purification of
the carbolic oil.

The two layers into which this oil separates after washing with alkali are
(1) the aqueous alkaline solution of the carbolic acid and other phenols,
and (2) the undissolved naphthalene contaminated with oily hydrocarbons
and other impurities. Each of these portions has its industrial history.
The alkaline solution, on being drawn off and made acid, yields its
mixture of phenols in the form of a dark oil from which carbolic acid is
separated by a laborious series of fractional distillations. The
undissolved hydrocarbon is similarly purified by fractional distillation,
and furnishes the solid crystalline naphthalene. The tar from one ton of
Lancashire coal yields about 1-1/2 lbs. of carbolic acid, equal to about 1
per cent. by weight of the tar, and about 6-1/4lbs. of naphthalene, so
that this last hydrocarbon is one of the chief constituents of the tar, of
which it forms from 8 to 10 per cent. by weight.

The crude carbolic acid as separated from the alkaline solution is a
mixture of several phenolic compounds, and all of these but the carbolic
acid itself are gradually removed during the process of purification.
Among the compounds associated with the carbolic acid are certain phenols
of higher boiling-point, which bear the same relationship to carbolic acid
that toluene bears to benzene. That is to say, that while phenol itself
is hydroxybenzene, these other compounds, which are called "cresols," are
hydroxytoluenes. The cresols form an oily liquid largely used for
disinfecting purposes under the designation of "liquid carbolic acid," or
"cresylic acid." Carbolic acid is a white crystalline solid possessing
strongly antiseptic properties, and is therefore of immense value in all
cases where putrefaction or decay has to be arrested. It was discovered in
coal-tar by Runge in 1834, and was obtained pure by Laurent in 1840.

The gradual establishment of the germ-theory of disease, chiefly due to
the labours of Pasteur, has led to a most important application of
carbolic acid. Once again we find the coal-tar industry brought into
contact with another department of science. Arguing from the view that
putrefactive change is brought about by the presence of the germs of
micro-organisms ever present in the atmosphere, Sir Joseph Lister proposed
that during surgical operations the incised part should be kept under a
spray of the germicidal carbolic acid to prevent subsequent mortification.
No operation upon portions of the body exposed to the air is at present
conducted without this precaution, and many a human life must have been
saved by Lister's treatment. To this result the chemist and technologist
have contributed, not only by the discovery of the carbolic acid in the
tar, but also by the development of the necessary processes for its
purification. It should be added that the phenol used must be of the
greatest possible purity, and the requirements of the surgeon have been
met by chemical and technological skill.

From surgery back to colouring-matters, and from these to pharmaceutical
preparations and perfumes, are we led in following up the cycles of
chemical transformation which these tar-products have undergone in the
hands of the technologist, guided by the researches of the chemist. It was
observed by Runge in 1834 that crude carbolic acid, on treatment with
lime, gave a red, acid colouring-matter which he separated and named
"rosolic acid." The observation was followed up, and many other chemists
obtained red colouring-matters by the oxidation of crude phenol. In 1859,
the colour-giving property of carbolic acid acquired industrial importance
from a discovery made by Kolbe and Schmitt in Germany, and by Persoz in
France. These chemists found that a good yield of the colouring-matter was
obtained by heating phenol with oxalic and sulphuric acids. Under the
names of "corallin" and "aurin" the dye-stuff was introduced into
commerce, and it is still used for certain purposes, especially for the
preparation of coloured lakes for paper-staining.

The scientific development of the history of this phenol dye is full of
interest, but we can only give it a passing glance. Its interest lies
chiefly in the circumstance that it is related to magenta, as was first
pointed out by Caro and Wanklyn in 1866. In fact they obtained rosolic
acid from magenta by the action of nitrous acid on the latter. We now know
that a diazo-salt is first formed under these circumstances, and that the
decomposition of this unstable compound in the presence of water gives
rise to the rosolic acid. Later researches have shown that by heating
rosolic acid with ammonia it is converted into rosaniline. It is also
known that the commercial corallin, like the commercial magenta, is a
mixture of closely related colouring-matters. The close analogy between
magenta and rosolic acid was further shown by Caro in 1866. In the same
way that Hofmann found that magenta could not be produced by the oxidation
of _pure_ aniline, Caro found that a mixture of phenol and cresol was
necessary for the production of rosolic acid when inorganic oxidizers were
used. It is indeed this series of investigations upon the phenol
dyes--investigations which have been taken part in not only by the
chemists named, but also by Graebe, Dale and Schorlemmer, and the
Fischers--which led up to the discovery of the constitution of the
colouring-matters of the rosaniline group, and, through this, to the
far-reaching industrial developments of the discovery as traced in the
last chapter. It is evident, from what has been said, that rosolic acid
and its related colouring-matters are members of the triphenylmethane
group. They are in fact the hydroxylic or acid analogues of the
amido-containing or basic dyes of the rosaniline series.

In the fragrant blossom of the meadowsweet (_Spiræa ulmaria_) there is
contained an acid which is found also as an ether in the oil of
wintergreen (_Gautheria procumbens_). This is salicylic acid, a white
crystalline compound which has been known to chemists since 1839. In 1860
Kolbe prepared the sodium salt of this acid by passing carbon dioxide gas
into phenol in which metallic sodium had been dissolved. It was found
subsequently that the same transformation was brought about by heating the
dry sodium salt of carbolic acid in an atmosphere of carbon dioxide. This
process of Kolbe's is now worked on a manufacturing scale for the
preparation of artificial salicylic acid. The acid and its salts and
ethers find numerous applications as antiseptics, for the preservation of
food, and in pharmacy.

Salicylic acid is employed also for the manufacture of certain azo-dyes in
a way that it will be very instructive to consider, because the process
used may be taken as typical of the general method of preparing such
compounds. Solutions of diazo-salts act not only upon amido- and
diamido-compounds, as we have seen in the case of aniline yellow and
chrysoïdine, but also upon phenols, forming acid azo-colours. This
important fact was made known in 1870 by the German chemists Kekulé and
Hidegh, but more than six years elapsed before this discovery was taken
advantage of by the technologist. Large numbers of these acid azo-dyes are
now made from various diazotised amido-compounds combined with different
phenols and phenolic acids. The mode of procedure is to diazotise the
amido-compound by sodium nitrite and hydrochloric acid in the manner
already described, and then add the diazo-salt solution to the phenolic
compound dissolved in alkali. The colouring-matter is at once formed.
Salicylic acid possesses the characters both of an acid and a phenol. It
combines readily with diazo-salts under the circumstances described, and
gives rise to azo-dyes, some of which are of technical value.

The manufacture of azo-dyes from salicylic acid brings us into contact
with certain amidic compounds which figure so largely in the tinctorial
industry that they may be conveniently dealt with here. These bases are
not azo-compounds themselves, but they are prepared from azo-compounds,
viz. from the azobenzene and azotoluene which were spoken about in the
last chapter. When these are reduced by acid reducing-agents, they become
converted into diamido-bases which are known as benzidine and tolidine
respectively. These bases can be diazotised, and as they contain two
amido-groups, they form double diazo-salts, _i.e._ tetrazo-salts, which
are capable of combining with amido-compounds, or phenols, in the usual
way. Thus diazotised benzidine and tolidine combine with salicylic acid to
form valuable yellow azo-dyes known as "chrysamines." The dyes of this
class obviously contain two azo-groups.

Some other uses of carbolic acid must next be considered. Of the
colouring-matters derived from coal-tar, none is more widely known than
the oldest artificial yellow dye, picric acid. This is a phenol
derivative, and was first obtained as long ago as 1771 by Woulfe, by
acting upon indigo with nitric acid. Laurent in 1842 was the first to
obtain this dye from carbolic acid, from which compound it is still
manufactured by acting upon the sulpho-acid with nitric acid. Chemically
considered, it is trinitrophenol. It has a very wide application as a dye,
and has been used as an explosive agent. A similar colouring-matter was
made from cresol in 1869, and introduced under the name of "Victoria
yellow," which is dinitro-cresol. Other dyes derived directly or
indirectly from phenol will take us back once again to toluene.

A new diazotisable diamido-compound was obtained from this last
hydrocarbon, and introduced in 1886 by Leonhardt & Co. One of the three
isomeric nitrotoluenes furnishes a sulpho-acid which, on treatment with
alkali, gives a compound derived from a hydrocarbon known as stilbene, and
this, on reduction, is converted into the diamido-compound referred to.
The latter, which is a disulpho-acid as well as a diamido-compound, can be
diazotised and combined with phenols, &c. The stilbene azo-dyes thus
prepared from phenol and salicylic acid, like the chrysamines, are yellow
colouring-matters, containing two azo-groups. It is a valuable
characteristic of these secondary azo-dyes that they all possess a special
affinity for vegetable fibre, and their introduction has exerted a great
influence upon the art of cotton-dyeing. We shall have to return to these
cotton-dyes again shortly.

Before leaving this branch of the subject, the following scheme is
presented to show the relationships and inter-relationships of the
products thus far dealt with in the present chapter--

         |                                                       |
     Light Oil                                              Carbolic Oil
      /       \           /-> Benzal Chloride                    |
     /         \         /          |         \                  |
    /           \       /           |          Benzaldehyde      |
   /             \     /       Cinnamic acid        |            |
  Benzene       Toluene             |        Nitrobenzaldehyde   |
     |              |      Nitrocinnamic acid    (+ acetone)     |
     |              |               |            /               |
     |              |               |           /    |-----------|
  Nitrobenzene  Nitrotoluene   Propiolic acid  /     |           |
     |              |     \         |         /    Phenols   Naphthalene
     |              |      \        |        /       |   \
     |              |       \       |       /        |    \
     |              |        \      |      /         |     ->Cresols
  Azobenzene    Azotoluene    \  Indigo<--/        Phenol        |
     |              |      Stilbene-                 |          Victoria
     |              |      derivative   <-------------------     yellow
     |              |      (Diazotised)\             |      \
  Benzidine    Tolidine                Salicylic  Picric   Corallin
        \       /                       acid      acid     & Aurin
      (Diazotised)                        \/

The existence of naphthalene in coal-tar was made known in 1820 by Garden,
who gave it this name because the oils obtained from the tar by
distillation went under the general designation of naphtha. The greater
portion of the hydrocarbon is contained in the carbolic oil, and is
separated and purified in the manner described. A further quantity of
impure naphthalene separates out from the next fraction--the creosote oil,
and this is similarly washed and purified by distillation. The large
quantity of naphthalene existing in tar has already been referred to, but
although it is such an important constituent, it was only late in the
history of the colour industry that it found any extensive application. In
early times it was regarded as a nuisance, and was burnt as fuel, or for
the production of a dense soot, which was condensed to form lampblack. It
will be remembered that the first of the coal-tar colours made required
only the light oils. There are at present only a few direct uses for
naphthalene, but one of its applications is sufficiently important to be

The hydrocarbon is a white crystalline solid melting at 80° C., and
boiling at 217° C. Although it has a high boiling-point, it passes readily
into vapour at lower temperatures, and the vapour on condensation forms
beautiful silvery crystalline scales. This product is "sublimed
naphthalene." The vapour of naphthalene burns with a highly luminous
flame, and if mixed with coal-gas, it considerably increases the
luminosity of the flame. Advantage is taken-of this in the so-called
"albo-carbon light," which is the flame of burning coal-gas saturated with
naphthalene vapour. The burner is constructed so that the gas passes
through a reservoir filled with melted naphthalene kept hot by the flame
itself (Fig. 9).

[Illustration: FIG. 9.--ALBO-CARBON BURNER.]

To appreciate properly the value of those discoveries which have enabled
manufacturers to utilize this hydrocarbon, it is only necessary to recall
to mind the actual quantity produced in this country. Supposing that ten
million tons of coal are used annually for gas-making, and that the
500,000 tons of tar resulting therefrom contain only eight per cent. of
naphthalene, there would be available about 40,000 tons of this
hydrocarbon annually. Great as have been the recent advancements in the
utilization of naphthalene derivatives, there is still a larger quantity
of this hydrocarbon produced than is necessary to supply the wants of the
colour-manufacturer. From this last statement it will be inferred that
naphthalene is now a source of colouring-matters. Let us consider how this
has been brought about.

The phenols of naphthalene are called naphthols--they bear the same
relationship to naphthalene that carbolic acid bears to benzene. Owing to
the structure of the naphthalene molecule there are two isomeric
naphthols, whereas there is only one phenol. The naphthols--known as
alpha- and beta-naphthol, respectively--are now made on a large scale from
naphthalene, by heating the latter with sulphuric acid; at a low
temperature the alpha sulpho-acid is produced, and at a higher temperature
the beta sulpho-acid, and these acids on fusion with caustic soda furnish
the corresponding naphthols. Similarly there are two amidonaphthalenes,
known as alpha- and beta-naphthylamine respectively. As aniline is to
benzene, so are the naphthylamines to naphthalene. The alpha-compound is
made in precisely the same way as aniline, viz. by acting upon naphthalene
with nitric acid so as to form nitronaphthalene, and then reducing the
latter with iron dust and acid. Beta-napthylamine cannot be made in this
way; it is prepared from beta-naphthol by heating the latter in presence
of ammonia, when the hydroxyl becomes replaced by the amido-group in
accordance with a process patented in 1880 by the Baden Aniline Company.
The principle thus utilized is the outcome of the scientific work of two
Austrian chemists, Merz and Weith. Setting out from the naphthols and
naphthylamines we shall be led into industrial developments of the
greatest importance.

The first naphthalene colour was a yellow dye, discovered by Martius in
1864, and manufactured under the name of "Manchester yellow." It is,
chemically speaking, dinitro-alpha-naphthol; but it was not at first made
from naphthol, as the latter was not at the time a technical product. It
was made from alpha-naphthylamine by the action of nitrous and nitric
acids. When a good method for making the naphthol was discovered in 1869,
the dye was made from this. The process is just the same as that employed
in making picric acid; the naphthol is converted into a sulpho-acid, and
this when acted upon by nitric acid, gives the colouring-matter.
Manchester yellow is now largely used for colouring soap, but as a
dye-stuff it has been improved upon in a manner that will be readily
understood. The original colouring-matter being somewhat fugitive, it was
found that its sulpho-acid was much faster. This sulpho-acid cannot be
made by the direct action of sulphuric acid upon the colouring-matter--as
in the case of acid yellow or acid magenta--but by acting upon the
naphthol with very strong sulphuric acid, three sulphuric acid residues or
sulpho-groups enter the molecule, and then on nitration only two of these
are replaced by nitro-groups, and there results a sulpho-acid of
dinitro-alpha-naphthol. This was discovered in 1879 by Caro, and
introduced as "acid naphthol yellow." It is now one of the standard yellow

The history of another important group of colouring-matters dependent on
naphthalene begins with A. v. Baeyer in 1871 and with Caro in 1874. Two
products formerly known only as laboratory preparations were called into
requisition by this discovery. One of these compounds, phthalic acid, is
obtained from naphthalene, and the other, resorcin or resorcinol, is
prepared from benzene. Phthalic acid, which was discovered in 1836 by
Laurent, is a product of the oxidation of many benzenoid compounds.
Chemically considered it is a di-derivative of benzene, _i.e._ two of the
hydrogen atoms of benzene are replaced by certain groups of carbon,
oxygen, and hydrogen atoms. We have seen how the replacement of hydrogen
by an ammonia-residue, amidogen, gives rise to bases such as amidobenzene
(aniline), or diamidobenzene. Similarly, the replacement of hydrogen by a
water-residue, hydroxyl, gives rise to a phenol. The group of carbon,
oxygen, and hydrogen atoms which confers the property of acidity upon an
organic compound is a half-molecule of oxalic acid--it is known as the
carboxyl group. Thus benzoic acid is the carboxyl-derivative of benzene,
and the phthalic acid with which we are now concerned is a
dicarboxyl-derivative of benzene. It is related to benzoic acid in the
same way that diamidobenzene is related to aniline. Three isomeric
phthalic acids are known, but only one of these is of use in the present
branch of manufacture. The acid in question, although a derivative of
benzene, is most economically prepared by the oxidation of certain
derivatives of naphthalene which, when completely broken down by energetic
oxidizing agents, furnish the acid. Thus the dinitronaphthol described as
Manchester yellow, if heated for some time with dilute nitric acid,
furnishes phthalic acid. The latter is made on a large scale by the
oxidation of a compound which naphthalene forms with chlorine, and known
as naphthalene tetrachloride, because it contains four atoms of chlorine.

The other compound, resorcinol, was known to chemistry ten years before it
was utilized as a source of colouring-matters. It was originally prepared
by fusing certain resins, such as galbanum, asafoetida, &c., with
caustic alkali. Soon after its discovery, viz. in 1866, it was shown by
Körner to be a derivative of benzene, and from this hint the technical
process for the preparation of the compound on a large scale has been
developed. Resorcinol is a phenolic derivative of benzene containing two
hydroxyl groups; it is therefore related to phenol in the same way that
diamidobenzene is related to aniline or phthalic acid to benzoic acid. The
relationships can be expressed in a tabular form thus--

  Amidobenzene or Aniline.    Benzoic acid.    Carbolic acid or Phenol.
  Diamidobenzene.             Phthalic acid.   Resorcinol.

Resorcinol is now made by heating benzene with very strong sulphuric acid
so as to convert it into a disulpho-acid, and the sodium salt of the
latter is then fused with alkali. As a technical operation it is one of
great delicacy and skill, and the manufacture is confined to a few
Continental factories.

When phthalic acid is heated it loses water, and is transformed into a
white, magnificently crystalline substance known as phthalic anhydride,
_i.e._ the acid deprived of water. In 1871, A. v. Baeyer, the eminent
chemist who subsequently synthesised indigo, published the first of a
series of investigations describing the compounds produced by heating
phthalic anhydride with phenols. To these compounds he gave the name of
"phthaleïns." Baeyer's work, like that of so many other chemists who have
contributed to the advancement of the coal-tar colour industry, was of a
purely scientific character at first, but it soon led to technological
developments. The phthaleïns are all acid compounds possessing more or
less tinctorial power. One of the first discovered was produced by heating
phthalic anhydride with an acid known as gallic acid, which occurs in
vegetable galls, and in the form of tannin in many vegetable extracts
which are used by the tanner. The acid is a phenolic derivative of benzoic
acid, viz. trihydroxybenzoic acid, and on heating it readily passes into
trihydroxybenzene, which is the "pyrogallic acid" or pyrogallol familiar
as a photographic developer. The phthaleïn formed from gallic acid and
phthalic anhydride really results from the union of the latter with
pyrogallol. It is now manufactured under the name of "galleïn," and is
largely used for imparting a bluish grey shade to cotton fabrics. By
heating galleïn with strong sulphuric acid, it is transformed into another
colouring-matter which gives remarkably fast olive-green shades when dyed
on cotton fibre with a suitable mordant. This derivative of galleïn is
used to a considerable extent under the name of "coeruleïn." These two
colouring-matters were the first practical outcome of v. Baeyer's
researches. There is another possible development in this direction which
chemistry may yet accomplish, and another natural colouring-matter may be
threatened, even as the indigo culture was threatened by the later work of
the same chemist. There is reason for believing that the colouring-matter
of logwood, known to chemists as hæmateïn, is related to or derived in
some way from the phthaleïns, and the synthesis of this compound may
ultimately be effected.

The dye introduced by Caro in 1874 is the brominated phthaleïn of
resorcinol. The phthaleïn itself is a yellow dye, and the solutions of its
salts show a splendid and most intense greenish yellow fluorescence, for
which reason it is called "fluoresceïn." When brominated, the latter
furnishes a beautiful red colouring-matter known as "eosin" (Gr. [Greek:
eôs], dawn), and the introduction of this gave an industrial impetus to
the phthaleïns which led to the discovery of many other related
colouring-matters now largely used under various trade designations.
About a dozen distinct compounds producing different shades of pink,
crimson and red, and all derived from fluoresceïn, are at present in the
market, and a few other phthaleïns formed by heating phthalic anhydride
with other phenolic compounds instead of resorcinol or pyrogallol (_e.g._
diethylamidophenol), are also of industrial importance. By converting
nitrobenzene into a sulpho-acid, reducing to an amido-sulpho-acid, and
then fusing with alkali, an amido-phenol is produced, the ethers of which,
when heated with phthalic anhydride, give rise to red phthaleïns of most
intense colouring power introduced by the Baden Aniline Company as

It remains to point out that the scientific spirit which prompted the
investigation of the phthaleïns in the first instance has followed these
compounds throughout their technological career. The researches started by
v. Baeyer were taken up by various chemists, whose work together with that
of the original discoverer has led to the elucidation of the constitution
of these colouring-matters. The phthaleïns are members of the
triphenylmethane group, and are therefore related to magenta, corallin,
malachite green, methyl violet, and the phosgene dyes.

It has been said that the phenolic and amidic derivatives of naphthalene,
_i.e._ the naphthols and naphthylamines, are of the greatest importance to
the colour industry. One of the first uses of alpha-naphthylamine has
already been mentioned, viz. for the production of the Manchester yellow,
which was afterwards made more advantageously from alpha-naphthol. A red
colouring-matter possessing a beautiful fluorescence was afterwards (1869)
made from this naphthylamine and introduced as "Magdala red." The latter
was discovered by Schiendl of Vienna in 1867. It was prepared in precisely
the same way as induline was prepared from aniline yellow. The latter,
which is amido-azobenzene, and which is prepared, broadly speaking, by the
action of nitrous acid on aniline, has its analogue in
amido-azonaphthalene, which is similarly prepared by the action of nitrous
acid on naphthylamine. Just as aniline yellow when heated with aniline and
an aniline salt gives induline, so amido-azonaphthalene when heated with
naphthylamine and a salt of this base gives Magdala red. The latter is,
therefore, a naphthalene analogue of induline, as was shown by Hofmann in
1869, and the knowledge of the constitution of the azines which has been
gained of late years, enables us to relegate the colouring-matter to this
group. This knowledge has also enabled the manufacture to be conducted on
more rational principles, viz. by the method employed for the production
of the saffranines, as previously sketched.

The introduction of azo-dyes, formed by the action of a diazotised
amido-compound on a phenol or another amido-compound, marks the period
from which the naphthols and naphthylamines rose to the first rank of
importance as raw materials for the colour manufacturer. The introduction
of chrysoïdine in 1876 was immediately followed by the manufacture of acid
azo-dyes obtained by combining diazotised amido-sulpho-acids with phenols
of various kinds, or with bases, such as dimethylaniline and
diphenylamine. From what has been said in the foregoing portion of this
volume, it is evident that all such azo-compounds result from the
combination of two things, viz. (1) a diazotised amido-compound, and (2) a
phenolic or amidic compound. Either (1) or (2) or both may be a
sulpho-acid, and the resulting dye will then also be a sulpho-acid.

The first of these colouring-matters derived from the naphthols, was
introduced in 1876-77 by Roussin and Poirrier, and by O. N. Witt. They
were prepared by converting aniline into a sulpho-acid (sulphanilic acid),
diazotising this and combining the diazo-compound with alpha- or
beta-naphthol. The compounds formed are brilliant orange dyes, the latter
being still largely consumed as "naphthol orange." Other dye-stuffs of a
similar nature were introduced by Caro about the same time, and were
prepared from the diazotised sulpho-acid of alpha-naphthylamine combined
with the naphthols. By this means alpha-naphthol gives what is known as
"acid brown," or "fast brown," and beta-naphthol a fine crimson, known as
"fast red," or "roccellin." Diazotised compounds combine also with this
same sulpho-acid of alpha-naphthylamine (known as naphthionic acid), the
first colouring-matter formed in this way having been introduced by
Roussin and Poirrier in 1878. It was prepared by diazotising a
nitro-derivative of aniline, and acting with the diazo-salt on napthionic
acid, and this dye is still used to some extent under the name of "archil
substitute." In 1878, the firm of Meister, Lucius & Brüning of
Höchst-on-the-Main gave a further impetus to the utilization of
naphthalene by discovering two isomeric disulpho-acids of beta-naphthol
formed by heating that phenol with sulphuric acid. By combining various
diazotised bases with these sulpho-acids, a splendid series of acid
azo-dyes ranging in shade from bright orange to claret-red, and to
scarlets rivalling cochineal in brilliancy were given to the tinctorial

The colouring-matters introduced in 1878 by the Höchst factory under the
names of "Ponceaux" of various brands, and "Bordeaux," although to some
extent superseded by later discoveries, still occupy an important
position. Their discovery not only increased the consumption of
beta-naphthol, but also that of the bases which were used for diazotising.
These bases are alpha-naphthylamine and those of the aniline series. The
intimate relationship which exists between chemical science and
technology--a relationship which appears so constantly in the foregoing
portions of this work--is well brought out by the discovery under
consideration. A little more chemistry will enable this statement to be

Going back for a moment to the hydrocarbons obtained from light oil, it
will be remembered that benzene and toluene have thus far been considered
as the only ones of importance to the colour-maker. Until the discovery
embodied in the patent specification of 1878, the portions of the light
oil boiling above toluene were of no value in the colour industry. Benzene
and toluene are related to each other in a way which chemists describe by
saying that they are "homologous." This means that they are members of a
regularly graduated series, the successive terms of which differ by the
same number of atoms of carbon and hydrogen. Thus toluene contains one
atom of carbon and two atoms of hydrogen more than benzene. Above toluene
are higher homologues, viz. xylene, cumene, &c., which occur in the light
oil, the former being related to toluene in the same way that toluene is
related to benzene, while cumene again contains one atom of carbon and two
atoms of hydrogen more than xylene. This relationship between the members
of homologous series is expressed in other terms by saying that the weight
of the molecule increases by a constant quantity as we ascend the series.

The homology existing among the hydrocarbons extends to all their
derivatives. Thus phenol is the lower homologue of the cresols. Also we
have the homologous series--

  Benzene.      Nitrobenzene.      Aniline.
  Toluene.      Nitrotoluene.      Toluidine.
  Xylene.       Nitroxylene.       Xylidine.
  Cumene.       Nitrocumene.       Cumidine.

The bases of the third column when diazotised and combined with the
disulpho-acids of beta-naphthol give a graduated series of dyes beginning
with orange and ending with bluish scarlet. Thus it was observed that the
toluidine colour was redder than the aniline colour, and it was a natural
inference that the xylidine colour would be still redder. At the time of
this discovery no azo-colour of a true scarlet shade had been manufactured
successfully. A demand for the higher homologues of benzene was thus
created, and the higher boiling-point fractions of the light oil, which
had been formerly used as solvent naphtha, became of value as sources of
colouring-matters. The isolation of coal-tar xylene (which is a mixture of
three isomeric hydrocarbons) is easily effected by fractional distillation
with a rectifying column, and by nitration and reduction, in the same way
as in the manufacture of aniline, xylidine is placed at the disposal of
the colour-maker.

Xylidine scarlet, although at the time of its introduction the only true
azo-scarlet likely to come into competition with cochineal, was still
somewhat on the orange side. The cumidine dye would obviously be nearer
the desired shade. To meet this want, cumidine had to be made on a large
scale, but here practical difficulties interposed themselves. The quantity
of cumene in the light oil is but small, and it is associated with other
hydrocarbons which are impurities from the present point of view, and from
which it is separated only with difficulty. A new source of the base had
therefore to be sought, and here again we find chemical science
ministering to the wants of the technologist.

It was explained in the last chapter that aniline and similar bases can be
methylated by heating their dry salts with methyl alcohol under pressure.
In this way dimethylaniline is made, and dimethyltoluidine or
dimethylxylidine can similarly be prepared. Now it was shown by Hofmann in
1871, that if this operation is conducted at a very high temperature, and
under very great pressure, the methyl-alcohol residue, _i.e._ the
methyl-group, does not replace the amidic hydrogen or the hydrogen of the
ammonia residue, but the methylation takes place in another way, resulting
in the formation of a higher homologue of the base started with. For
example, by heating aniline salt and pure wood-spirit to a temperature
considerably above that necessary for producing dimethylaniline, toluidine
is formed. In a similar way, by heating xylidine hydrochloride and methyl
alcohol for some time in a closed vessel at about 300° C. cumidine is
produced. Hofmann's discovery was thus utilized in 1882, and by its means
the base was manufactured, and cumidine scarlet, very similar in shade to
cochineal, became an article of commerce.

While the development of this branch of the colour industry was taking
place by means of the new naphthol disulpho-acids, the cultivation of the
fertile field of the azo-dyes was being carried on in other directions. It
came to be realized that the fundamental discovery of Griess was capable
of being extended to all kinds of amido-compounds. The azo-dyes hitherto
introduced had all been derived from amido-compounds containing only one
amido-group, and they accordingly contained only one azo-group; they were
_primary_ azo-compounds. It was soon found that aniline yellow, which
already contains one azo-group as well as an amido-group, could be again
diazotised and combined with phenols so as to produce compounds containing
two azo-groups, _i.e._ _secondary_ azo-compounds. The sulpho-acid of
aniline yellow--Grässler's "acid yellow"--was the first source of azo-dyes
of this class. By diazotising this amidoazo-sulpho-acid, and combining it
with beta-naphthol, a fine scarlet dye was discovered by Nietzki in 1879,
and introduced under the name of "Biebrich scarlet." Two years later a new
sulpho-acid of beta-naphthol was discovered by Bayer & Co. of Elberfeld,
and this gave rise, when combined with diazotised acid yellow and
analogous compounds, to another series of brilliant dyes introduced as
"Crocein scarlets."

From these beginnings the development of the azo-dyes has been steadily
carried on to the present time--year by year new diazotisable
amido-compounds or new sulpho-acids of the naphthols and naphthylamines
are being discovered, and this branch of the colour industry has already
assumed colossal dimensions. An important departure was made in 1884 by
Böttiger, who introduced the first secondary azo-colours derived from
benzidine. As already explained in connection with salicylic acid, this
base and its homologue tolidine form tetrazo-salts, which combine with
phenols and amines or their sulpho-acids. One of the first
colouring-matters of this group was obtained by combining diazotised
benzidine with the sulpho-acid of alpha-naphthylamine (naphthionic acid),
and was introduced under the name of "Congo red." Then came the discovery
(Pfaff, 1885), that the tetrazo-salts of benzidine and tolidine combine
with phenols, amines, &c., in two stages, one of the diazo-groups first
combining with one-half of the whole quantity of phenol to form an
intermediate compound, which then combines with the other half of the
phenol to form the secondary azo-dye. In the hands of the
"Actiengesellschaft für Anilinfabrikation" of Berlin this discovery has
been utilized for the production of a number of such azo-colours
containing two distinct phenols, or amines, or sulpho-acids. Tolidine has
been found to give better colouring-matters in most cases than benzidine,
and it is scarcely necessary to point out that an increased demand for the
nitrotoluene from which this base is made is the necessary consequence of
this discovery.

It is impossible to attempt to specify by name any of these recent
benzidine and tolidine dyes. Their introduction has been the means of
finding new uses for the naphthylamines and naphthols and their
sulpho-acids, and has thus contributed largely to the utilization of
naphthalene. An impetus has been given to the investigation of these
sulpho-acids, and chemical science has profited largely thereby. The
process by which beta-naphthylamine is prepared from beta-naphthol,
already referred to, viz. by heating with ammonia under pressure, has been
extended to the sulpho-acids of beta-naphthol, and by this means new
beta-naphthylamine sulpho-acids have been prepared, and figure largely in
the production of these secondary azo-colours. The latter, as previously
stated, possess the most valuable property of dyeing cotton fibre
directly, and by their means the art of cotton dyeing has been greatly
simplified. The shades given by these colours vary from yellow through
orange to bright scarlet, violet, or purple.

In addition to benzidine and tolidine, other diazotisable amido-compounds
have of late years been pressed into the service of the
colour-manufacturer. The derivative of stilbene, already mentioned as
being prepared from a sulpho-acid of one of the nitrotoluenes, forms
tetrazo-salts, which can be combined with similar or dissimilar phenols,
amines, or sulpho-acids, as in the case of benzidine and tolidine.
Various shades of red and purple are thus obtained from the diazotised
compound, when the latter is combined with the naphthylamines, naphthols,
or their sulpho-acids. These, again, are all cotton dyes. The
nitro-derivatives of the ethers of phenol and cresol, when reduced in the
same way that nitrobenzene and nitrotoluene are reduced to azobenzene and
azotoluene, also furnish azo-compounds which, on further reduction, give
bases analogous to benzidine and tolidine. Secondary azo-colours derived
from these bases and the usual naphthalene derivatives are also
manufactured. It is among the secondary azo-dyes that we meet with the
first direct dyeing blacks, the importance of which will be realized when
it is remembered that the ordinary aniline-black is not adapted for wool
dyeing. The azo-blacks are obtained by combining diazotised sulpho-acids
of amidoazo-compounds of the benzene or naphthalene series with naphthol
sulpho-acids or other naphthalene derivatives.

One other series of azo-compounds must be briefly referred to. It has long
been known that aniline and toluidine when heated with sulphur evolve
sulphuretted hydrogen and give rise to thio-bases, that is, aniline or
toluidine in which the hydrogen is partly replaced by sulphur. One of the
toluidines treated in this way is transformed into a thiotoluidine which,
when diazotised and combined with one of the disulpho-acids of
beta-naphthol, forms a red azo-dye, introduced by Dahl & Co. in 1885 as
"thiorubin." By modifying the conditions of reaction between the sulphur
and the base, it was found in 1887 by Arthur Green, that a complicated
thio-derivative of toluidine could be produced which possessed very
remarkable properties. The sulpho-acid of the thio-base is a yellow dye,
which was named by its discoverer "primuline." Not only is primuline a
dye, but it contains an amido-group which can be diazotised. If therefore
the fabric dyed with primuline is passed through a nitrite bath, a
diazo-salt is formed in the fibre, and on immersing the latter in a second
bath containing naphthol or other phenol or an amine, an azo-dye is
precipitated in the fibre. By this means there are produced valuable
"ingrain colours" of various shades of red, orange, purple, &c.

So much for the azo-dyes, one of the most prolific fields of industrial
enterprise connected with coal-tar technology. From the introduction of
aniline yellow in 1863 to the present time, about 150 distinct compounds
of this group have been given to the tinctorial industry. Of these over
thirty are cotton dyes containing two azo-groups. Sombre shades, rivalling
logwood black, bright yellows, orange-reds, browns, violets, and brilliant
scarlets equalling cochineal, have been evolved from the refuse of the
gas-works. The artificial colouring-matters have in this last case once
again threatened a natural product, and with greater success than the
indigo synthesis, for the introduction of the azo-scarlets has caused a
marked decline in the cochineal culture.

In addition to the azo-colours, there are certain other products which
claim naphthalene as a raw material. In 1879 it was found that one of the
sulpho-acids of beta-naphthol when treated with nitrous acid readily gave
a nitroso-sulpho-acid. A salt of this last acid, containing sodium and
iron as metallic bases, was introduced in 1884, under the name of
"naphthol green." It is used both as a dye for wool and as a pigment. It
may be mentioned here that other nitroso-derivatives of phenols, such as
those of resorcinol and the naphthols, under the name of "gambines," are
largely used for dyeing purposes, owing to the facility with which they
combine with metallic mordants to form coloured salts in the fibre. In
this same year, 1879, it was found that by heating nitrosodimethylaniline
with beta-naphthol in an appropriate solvent, a violet colouring-matter
was formed. This is now manufactured under the name of "new blue," or
other designations, and is largely used for producing an indigo-blue shade
on cotton prepared with a suitable mordant. The discovery of this
colouring-matter gave an impetus to further discoveries in the same
direction. It was found that nitrosodimethylaniline reacted in a similar
way with other phenolic or with amidic compounds. In 1881 Köchlin
introduced an analogous dye-stuff prepared by the action of the same
nitroso-compound on gallic acid. Gallocyanin, as it is called, imparts a
violet blue shade to mordanted cotton. Other colouring-matters of the same
group are in use; some of them, like "new blue," being derivatives of
naphthalene. These compounds all belong to a series of which the parent
substance is constructed on a type similar to azine; it contains a
nitrogen and oxygen atom linking together the hydrocarbon residues, and is
therefore known as "oxazine." The researches of Nietzki in 1888 first
established the true constitution of the oxazines.

Closely related to this group is a colouring-matter introduced by Köchlin
and Witt in 1881 under the name of "indophenol." It is prepared in the
same way as the azines of the "neutral red" group; viz. by the action of
nitrosodimethylaniline on alpha-naphthol, or by oxidizing
amidodimethylaniline in the presence of alpha-naphthol. Indophenol belongs
to that group of blue compounds formed as intermediate products in the
manufacture of azines, as mentioned in connection with "neutral red." But
while these intermediate blues resulting from the oxidation of a diamine
in the presence of another amine are unstable, and pass readily into red
azines, indophenol is stable, and can be used for dyeing and printing in
the same way as indigo. The shades which it produces are very similar to
this last dye, but for certain practical reasons it has not been able to
compete with the natural dye-stuff.

The story of naphthalene is summarized in the schemes on pp. 164, 165.

Light Oil:

       \-->Nitrobenzene}        } Sulphanilic acid.[7]
                       } Aniline} Amidoazobenzene and sulpho-acid.[7]
                       }        } Nitrosodimethylaniline[8] and
                       }        }    amidodimethylaniline.[8]
                       } Sulpho-acid-->Amidosulpho-acid-->Amidophenol
                       }                                  and ethers.[6]
                       } Azobenzene-->Benzidine.[7]

                         }Toluidines[7] Sulpho-acids.[7]
                         }              Amidoazotoluene[7]
                         }                 and sulpho-acid.[7]
                         }              Thiotoluidine and primuline.[7]
  Toluene-->Nitrotoluenes} Azotoluene-->Tolidine.[7]
                         } Sulpho-acid and Stilbene-derivative.[7]

                                    } Sulpho-acids.[7]
  Xylene->Nitroxylenes->Xylidines[7]} Amidoazoxylene[7] and sulpho-acid.[7]
                                    } Cumidine.[7]

Carbolic Oil:

  Phenol-->Nitrophenol and ethers } Azophenol ethers-->Diamidic bases[9]
                                  }     analogous to benzidine.
                                  } Amidophenol[9] and ethers.[9]

  Cresols-->Nitrocresols and ethers-->Amidocresols[9] and ethers.[9]

              |  |
              |  |>[Greek: a]-Naphthylamine[9][10]{Naphthionic acid.[9][10]
              |                                   {Amidoazonaphthalene
              |                                          |
              |                                         {Magdala red.
              |                                         {Sulpho-acid.[9]
  Naphthalene>|>Tetrachloride-->Phthalic acid and anhydride.
              |             }                     {Manchester yellow.
              |             }[Greek: a]-Naphthol  {Acid naphthol yellow.
              |             }                     {Sulpho-acids.[10]
              |             }                   {[Greek: b]-Naphthylamine
              |             }                   {  and sulpho-acids.[9][10]
              |             }[Greek: b]-Naphthol{Sulpho-acids &
              |             }                   {  [Greek: b]-naphthylamine
              |             }                   {  sulpho-acids.[9][10]
              |             }                   {Nitrososulpho-acid and
                                                {  naphthol green.

The fraction of coal-tar succeeding the carbolic oil, viz. the creosote
oil, does not at present supply the colour manufacturer with any raw
materials beyond the small proportion of naphthalene which separates from
it in a very impure condition as "creosote salts." This oil consists of a
mixture of the higher homologues of phenol with various hydrocarbons and
basic compounds. It is the oil used for creosoting timber in the manner
already described; and among its other applications may be mentioned its
use as an illuminating agent and as a source of lampblack. In order to
burn the oil effectively as a source of light, a specially-constructed
burner is used, which is fed by a stream of oil raised from a reservoir at
its foot by means of compressed air, which also aids the combustion of the
oil. There is produced by this means a great body of lurid flame, which
is very serviceable where building or other operations have to be carried
on at night (see Fig. 10). For lampblack the oil is simply burnt in iron
pans set in ovens, and the sooty smoke conducted into condensing chambers.
The creosote oil constitutes more than 30 per cent. by weight of the
tar--the time may come when this fraction, like the light oil and carbolic
oil, may be found to contain compounds of value to the colour-maker or to
other branches of chemical manufacture.


[Illustration: FIG. 11.--THE MADDER PLANT (_Rubia tinctoria_).]

The utilization of the next fraction, anthracene oil, is one of the
greatest triumphs which applied chemical science can lay claim to since
the foundation of the coal-tar colour industry. This discovery dates from
1868, when it was shown by two German chemists, Graebe and Liebermann,
that the colouring-matter of madder was derived from the hydrocarbon
anthracene. Like indigo, madder may be regarded as one of the most ancient
of natural dye-stuffs. It consists of the powdered roots of certain plants
of the genus _Rubia_, such as _R. tinctoria_ (see Fig. 11), _R.
peregrina_, and _R. munjista_, which were at one time cultivated on an
enormous scale in various parts of Europe and Asia. It is estimated that
at the time of Graebe and Liebermann's discovery, 70,000 tons of madder
were produced annually in the madder-growing countries of the world. At
that time we were importing madder into this country at the rate of 15,000
to 16,000 tons per annum, at a cost of £50 per ton. In ten years the
importation had fallen to about 1600 tons, and the price to £18 per ton.
At the present time the cultivation of madder is practically extinct.

There is no better gauge of the practical utility of a scientific
discovery than the financial effect. In addition to madder, a more
concentrated extract containing the colouring-matter itself was largely
used by dyers and cotton printers under the name of "garancin." In 1868 we
were importing, in addition to the 15,000 to 16,000 tons of madder, about
2000 tons of this extract annually, at a cost of £150 per ton. By 1878 the
importation of garancin had sunk to about 140 tons, and the price had
been lowered to £65 per ton. The total value of the imports of madder and
garancin in 1868 was over one million pounds sterling; in ten years the
value of these same imports had been reduced to about £38,000.
Concurrently with this falling off in the demand for the natural
colouring-matter, the cultivation of the madder plant had to be abandoned,
and the vast tracts of land devoted to this purpose became available for
other crops. A change amounting to a revolution was produced in an
agricultural industry by a discovery in chemistry.

In the persons of two Frenchmen, Messrs. Robiquet and Colin, science laid
hands on the colouring-matter of the _Rubia_ in 1826. These chemists
isolated two compounds which they named alizarin and purpurin. It is now
known that there are at least six distinct colouring-matters in the madder
root, all of these being anthracene derivatives. It is known also that the
colouring-matters do not exist in the free state in the plant, but in the
form of compounds known as glucosides, _i.e._ compounds consisting of the
colouring-matter combined with the sugar known as glucose. It may be
mentioned incidentally that the colouring-matter of the indigo plant also
exists as a glucoside in the plant. During a period of more than forty
years from the date of its isolation, alizarin was from time to time
submitted to examination by chemists, but its composition was not
completely established till 1868, when Graebe and Liebermann, by heating
it with zinc-dust, obtained anthracene. This was the discovery which gave
the death-blow to the madder culture, and converted the last fraction of
the tar-oil from a waste product into a material of the greatest value.

The large quantity of madder consumed for tinctorial purposes is
indicative of the value of this dye-stuff. It produces shades of red,
purple, violet, black, or deep brown, according to the mordant with which
the fabric is impregnated. The colours obtained by the use of madder are
among the fastest of dyes, the brilliant "Turkey red" being one of the
most familiar shades. The discovery of the parent hydrocarbon of this
colouring-matter which had been in use for so many ages--a
colouring-matter capable of furnishing both in dyeing and printing many
distinct shades, all possessed of great fastness--was obviously a step
towards the realization of an industrial triumph, viz. the chemical
synthesis of alizarin. Within a year of their original observation, this
had been accomplished by Graebe and Liebermann, and almost simultaneously
by W. H. Perkin in this country. From that time the anthracene, which had
previously been burnt or used as lubricating grease, rose in value to an
extraordinary extent. In two years a material which could have been bought
for a few shillings the ton, rose at the touch of chemical magic to more
than two hundred times its former value.

Anthracene is a white crystalline hydrocarbon, having a bluish
fluorescence, melting at 213° C. and boiling above 360° C. It was
discovered in coal-tar by Dumas and Laurent in 1832, and its composition
was determined by Fritzsche in 1857. It separates in the form of crystals
from the anthracene oil on cooling, and is removed by filtration. The
adhering oil is got rid of by submitting the crystals to great pressure in
hydraulic presses. Further purification is effected by powdering the crude
anthracene cake and washing with solvent naphtha, _i.e._ the mixture of
the higher homologues of benzene left after the rectification of the light
oil. Another coal-tar product, viz. the pyridine base referred to in the
last chapter, has been recently employed for washing anthracene with great
success. It is used either by itself or mixed with the solvent naphtha.
The anthracene by washing with these solvents is freed from more soluble
impurities, and may then contain from 30 to 80 per cent. of the pure
hydrocarbon. The washing liquid, which is recovered by distillation,
contains, among other impurities dissolved out of the crude anthracene, a
hydrocarbon isomeric with the latter, and known as phenanthrene, for which
there is at present but little use, but which may one day be turned to
good account. The actual amount of anthracene contained in coal-tar
corresponds to about 1/2 lb. per ton of coal distilled, _i.e._ from 1/4 to
1/2 per cent. by weight of the tar. Owing to the great value of alizarin
and the large quantity of this colouring-matter annually consumed,
anthracene is now by far the most important of the coal-tar hydrocarbons.

Alizarin, purpurin, and the other colouring-matters of madder are hydroxyl
derivatives of a compound derived from anthracene by the replacement of
two atoms of hydrogen by two atoms of oxygen. These oxygen derivatives of
benzenoid hydrocarbons form a special group of compounds known as
quinones. Thus there is quinone itself, or benzoquinone, which is benzene
with two atoms of oxygen replacing two atoms of hydrogen. There are also
isomeric quinones of the naphthalene series known as naphthaquinones. A
dihydroxyl derivative of one of the latter is in use under the somewhat
misappropriate name of "alizarin black." With this exception no other
quinone derivative is used in the colour industry till we come to the
hydrocarbons of the anthracene oil. Phenanthrene forms a quinone which
has been utilized as a source of colouring-matters, but these are
comparatively unimportant. The quinone with which we are at present
concerned is anthraquinone.

The latter is prepared by oxidizing the anthracene--previously reduced by
sublimation to the condition of a very finely-divided crystalline
powder--with sulphuric acid and potassium dichromate. The quinone is
purified, converted into a sulpho-acid, and the sodium salt of the latter
on fusion with alkali gives alizarin, which is dihydroxy-anthraquinone. It
is of interest to note that in this case a monosulpho-acid gives a
dihydroxy-derivative. During the process of fusion potassium chlorate is
added, by which means the yield of alizarin is considerably increased. In
the original process of Graebe and Liebermann, dibromanthraquinone was
fused with alkali; but this method was soon improved upon by the discovery
of the sulpho-acid by Caro and Perkin in 1869, and from this period the
manufacture of artificial alizarin became commercially successful.

In addition to alizarin, other anthracene derivatives are of industrial
importance. The purpurin, discovered among the colouring-matters of madder
in 1826, is a trihydroxy-anthraquinone; it can be prepared by the
oxidation of alizarin, as shown by De Lalande in 1874. Isomeric compounds
known as "flavopurpurin" and "anthrapurpurin" are also made from the
disulpho-acids of anthraquinone by fusion with alkali and potassium
chlorate. These two disulpho-acids are obtained simultaneously with the
monosulpho-acid by the action of fuming sulphuric acid on the quinone, and
are separated by the fractional crystallization of their sodium salts from
the monosulpho-acid (which gives alizarin) and from each other. The
purpurins give somewhat yellower shades than alizarin. Another
trihydroxy-anthraquinone, although not obtained directly from anthracene,
must be claimed as a tar-product. It is prepared by heating gallic acid
with benzoic and sulphuric acids, or with phthalic anhydride and zinc
chloride, and is a brown dye known as "anthragallol" or
"anthracene-brown." The anthracene derivative is in this process built up
synthetically. A sulpho-acid of alizarin has been introduced for wool
dyeing under the name of alizarin carmine, and a nitro-alizarin under the
name of alizarin orange. The latter on heating with glycerin and sulphuric
acid is transformed into a remarkably fast colouring-matter known as
alizarin blue, which is used for dyeing and printing. By heating alizarin
blue with strong sulphuric acid, it is converted into alizarin green.

The great industry arising out of the laboratory work of two German
chemists has influenced other branches of chemical manufacture, and has
reacted upon the coal-tar colour industry itself. A new application for
caustic soda and potassium chlorate necessitated an increased production
of these materials. The first demand for fuming sulphuric acid on a large
scale was created by the alizarin manufacture in 1873, when it was found
that an acid of this strength gave better results in the preparation of
sulpho-acids from anthraquinone. The introduction of this acid into
commerce no doubt exerted a marked influence on the production of other
valuable sulpho-acids, such as acid magenta in 1877, acid yellow in 1878,
and acid naphthol yellow in 1879. The introduction of artificial alizarin
has also simplified the art of colour printing on cotton fabrics to such
an extent that other colouring-matters, also derived from coal-tar, are
largely used in combination with the alizarin to produce parti-coloured
designs. The manufacture of one coal-tar colouring-matter has thus
assisted in the consumption of others.

Artificial alizarin is used in the form of a paste, which consists of the
colouring-matter precipitated from its alkaline solution by acid, and
mixed with water so as to form a mixture containing from 10 to 20 per
cent. of alizarin. The magnitude of the industry will be gathered from the
estimate that the whole quantity of anthracene annually made into
alizarin corresponds to a daily production of about 65 tons of 10 per
cent. paste, of which only about one-eighth is made in this country, the
remainder being manufactured on the Continent. The total production of
alizarin corresponds in money value to about £2,000,000 per annum. One
pound of dry alizarin has the tinctorial power of 90 pounds of madder.
Seeing therefore that the raw material anthracene was at one time a waste
product, and that the quantity of alizarin produced in the factory
corresponds to nearly five pounds of 20 per cent. paste for one pound of
anthracene, it is not surprising that the artificial has been enabled to
compete successfully with the natural product.

The industrial history of anthracene is thus summarized. (See opposite.)

    |                                      } Sulpho-acid (Alizarin carmine)
    |->Monosulpho-acid-->Alizarin----------} Purpurin
    |                                      } Nitro-alizarin-->Alizarin blue
    |->[Greek: a]-Disulpho-acid-->Flavopurpurin                   |
    |                                                             |
    |->[Greek: b]-Disulpho-acid-->Anthrapurpurin                  |
                                                            Alizarin green.

The black, viscid residue left in the tar-still after the removal of the
anthracene oil is the substance known familiarly as pitch. From the
latter, after removal of all the volatile constituents, there is prepared
asphalte, which is a solution of the pitchy residue in the heavy tar-oils
from which all the materials used in the colour industry have been
removed. Asphalte is used for varnish-making, in the construction of hard
pavements, and for other purposes. A considerable quantity of pitch is
used in an industry which originated in France in 1832, and which is
still carried out on a large scale in that country, and to a smaller
extent in this and other tar-producing countries. The industry in question
is the manufacture of fuel from coal-dust by moulding the latter in
suitable machines with pitch so as to form the cakes known as "briquettes"
or "patent fuel." By this means two waste materials are disposed of in a
useful way--the pitch and the finely-divided coal, which could not
conveniently be used as fuel by itself. From two to three million tons of
this artificial fuel are being made annually here and on the Continent.

The various constituents of coal-tar have now been made to tell their
story, so far as relates to the colouring-matters which they furnish. If
the descriptive details are devoid of romance to the general reader, the
results achieved in the short period of thirty-five years, dating from the
discovery of mauve by Perkin, will assuredly be regarded as falling but
little short of the marvellous. Although the most striking developments
are naturally connected with the colouring-matters, whose history has been
sketched in the foregoing pages, and whose introduction has revolutionized
the whole art of dyeing, there are other directions in which the coal-tar
industry has in recent times been undergoing extension. Certain
tar-products are now rendering good service in pharmacy. Salicylic acid
and its salts have long been used in medicine. By distilling a mixture of
the dry lime salts of benzoic and acetic acids there is obtained a
compound known to chemists as acetophenone, which is used for inducing
sleep under the name of hypnone. The acetyl-derivative of aniline and of
methylaniline are febrifuges known as "antifebrine" and "exalgine." Ethers
of salicylic acid and its homologues, prepared from these acids and
phenol, naphthols, &c., are in use as antiseptics under the general
designation of "salols."

In 1881 there was introduced into medicine the first of a group of
antipyretics derived from coal-tar bases of the pyridine series. It has
already been explained that this base is removed from the light oil by
washing with acid. Chemically considered, it is benzene containing one
atom of nitrogen in place of a group consisting of an atom of carbon and
an atom of hydrogen. The quantity of pyridine present in coal-tar is very
small, and no use has as yet been found for it excepting as a solvent for
washing anthracene or for rendering the alcohol used for manufacturing
purposes undrinkable, as is done in this country by mixing in crude
wood-spirit so as to form methylated spirit. The salts of pyridine were
shown by McKendrick and Dewar to act as febrifuges in 1881, but they have
not hitherto found their way into pharmacy. The chief interest of the base
for us centres in the fact that it is the type of a group of bases related
to each other in the same way as the coal-tar hydrocarbons. Thus in
coal-tar, in addition to pyridine, there is another base known as
quinoline, which is related to pyridine in the same way that naphthalene
is related to benzene. Similarly there is a coal-tar base known as
acridine, which is found associated with the anthracene, and which is
related to quinoline in the same way that anthracene is related to
naphthalene. The three hydrocarbons are comparable with the three bases,
which may be regarded as derived from them in the same manner that
pyridine is derived from benzene--

  Benzene    ...    ...    ...    Pyridine
  Naphthalene       ...    ...    Quinoline
  Anthracene ...    ...    ...    Acridine

Some of these bases and their homologues are found in the evil-smelling
oil produced by the destructive distillation of bones (Dippel's oil, or
bone oil), and the group is frequently spoken of as the pyridine group.
The colouring-matter described as phosphine, obtained as a by-product in
the manufacture of magenta (p. 94), is a derivative of acridine, and a
yellow colouring-matter discovered by Rudolph in 1881, and obtained by
heating the acetyl derivative of aniline with zinc chloride, is a
derivative of a homologue of quinoline. This dye-stuff, known as
"flavaniline," is no longer made; but it is interesting as having led to
the discovery of the constitution of phosphine by O. Fischer and Körner in

The antipyretic medicines which we have first to consider are derivatives
of quinoline. This base was discovered in coal-tar by Runge in 1834, and
was obtained by Gerhardt in 1842 by distilling cinchonine, one of the
cinchona alkaloïds, with alkali. Now it is of interest to note that the
quinoline of coal-tar is of no more use to the technologist than the
aniline; these bases are not contained in the tar in sufficient quantity
to enable them to be separated and purified with economical advantage. If
the colour industry had to depend upon this source of aniline only, its
development would have been impossible. But as chemistry enabled the
manufacturer to obtain aniline in quantity from benzene, so science has
placed quinoline at his disposal. This important discovery was made in
1880 by the Dutch chemist Skraup, who found that by heating aniline with
sulphuric acid and glycerin in the presence of nitrobenzene, quinoline is
produced. The nitrobenzene acts only as an oxidizing agent; the
amido-group of the aniline is converted into a group containing carbon,
hydrogen, and nitrogen, _i.e._ the pyridine group. The discovery of
Skraup's method formed the starting-point of a series of syntheses, which
resulted in the formation of many products of technical value. In all
these syntheses the fundamental change is the same, viz. the conversion
of an amidic into a pyridine group. We may speak of the amido-group as
being "pyridised" in such processes. Thus alizarin blue, which is formed
by heating nitro-alizarin with glycerin and sulphuric acid, results from
the pyridisation of the nitro-group. By an analogous method Doebner and v.
Miller prepared a homologue of quinoline (quinaldine) in 1881, by the
action of sulphuric acid and a certain modification of aldehyde known as
paraldehyde on aniline.

Quinoline and its homologue quinaldine have been utilized as sources of
colouring-matters. A green dye-stuff, known as quinoline green, was
formerly made by the same method as that employed for producing the
phosgene colours by Caro and Kern's process (p. 106). The phthaleïn of
quinaldine was introduced by E. Jacobsen in 1882 under the name of
quinoline yellow, a colouring-matter which forms a soluble sulpho-acid by
the action of sulphuric acid.

To return to coal-tar pharmaceutical preparations. At the present time
seven distinct derivatives of quinoline, all formed by pyridising the
amido-group in aniline, amido-phenols, &c., are known in medicine under
such names as kairine, kairoline, thalline, and thermifugine. The mode of
preparation of these compounds cannot be entered into here, Kairine, the
first of the artificial alkaloïds, is a derivative of hydroxy-quinoline,
which was discovered in 1881 by Otto Fischer. All these quinoline
derivatives have the property of lowering the temperature of the body in
certain kinds of fevers, and may therefore be considered as the first
artificial products coming into competition with the natural alkaloïd,
quinine. There is reason for believing that the latter alkaloïd, the most
valuable of all febrifuges, is related to the quinoline bases, so that if
its synthesis is accomplished--as may certainly be anticipated--we shall
have to look to coal-tar as a source of the raw materials.

Another valuable artificial alkaloïd, discovered in 1883 by Ludwig Knorr,
claims aniline as a point of departure. When aniline and analogous bases
are diazotised, and the diazo-salts reduced in the cold with a very gentle
reducing agent, such as stannous chloride, there are formed certain basic
compounds, containing one atom of nitrogen and one atom of hydrogen more
than the original base. These bases were discovered in 1876 by Emil
Fischer, and they are known as hydrazines, the particular compound thus
obtained from aniline being phenylhydrazine. By the action of this base on
a certain compound ether derived from acetic acid, which is known as
aceto-acetic ether, there is formed a product termed "pyrazole," and this
on methylation gives the alkaloïd in question, which is now well known in
pharmacy under the name of "antipyrine."

While dealing with this first industrial application of a hydrazine, it
must be mentioned that the original process by which Fischer prepared
these bases was improved upon by Victor Meyer and Lecco in 1883, who
discovered the use of a cold solution of stannous chloride for reducing
the diazo-chloride to the hydrazine. By this method the manufacture of
phenylhydrazine and other hydrazines is effected on a large scale--all
kinds of amido-compounds and their sulpho-acids can be diazotised and
reduced to their hydrazines. Out of this discovery has arisen the
manufacture of a new class of colouring-matters related to the azo-dyes.
The hydrazines combine with quinones and analogous compounds with the
elimination of water, the oxygen coming from the quinone, and the hydrogen
from the hydrazine. The resulting products are coloured compounds very
similar in properties to the azo-dyes, and one of these was introduced in
1885 by Ziegler, under the name of "tartrazine." The latter is obtained by
the action of a sulpho-acid of phenylhydrazine on dioxytartaric acid, and
is a yellow dye, which is of special interest on account of its
extraordinary fastness towards light.

Another direction in which coal-tar products have been utilized is in the
formation of certain aromatic compounds which occur in the vegetable
kingdom. Thus the artificial production of bitter-almond oil from toluene
has already been explained. By heating phenol with caustic alkali and
chloroform, the aldehyde of salicylic acid, _i.e._ salicylic aldehyde, is
formed, and this, on heating with dry sodium acetate and acetic anhydride,
passes into _coumarin_, the fragrant crystalline substance which is
contained in the Tonka bean and the sweet-scented woodruff. Furthermore,
the familiar flavour and scent of the vanilla bean, which is due to a
crystalline substance known as vanillin, can be obtained from coal-tar
without the use of the plant. The researches of Tiemann and Haarman having
shown that vanillin is a derivative of benzene containing the aldehyde
group, one hydroxyl- and one methoxy-group, the synthesis of this compound
soon followed (Ulrich, 1884). The starting-point in this synthesis is
nitrobenzoic aldehyde, so that here again we begin with toluene as a raw
material. A mixture of vanillin and benzoic aldehyde when attenuated to a
state of extreme dilution in a spirituous solvent, gives the perfume known
as "heliotrope."

Not the least romantic chapter of coal-tar chemistry is this production of
fragrant perfumes from the evil-smelling tar. Be it remembered that these
products--which Nature elaborates by obscure physiological processes in
the living plant--are no more contained in the tar than are the hundreds
of colouring-matters which have been prepared from this same source. It is
by chemical skill that these compounds have been built up from their
elemental groups; and the artificial products, as in the case of indigo
and alizarin, are chemically identical with those obtained from the plant.
Among the late achievements in the synthesis of vegetable products from
coal-tar compounds is that of juglone, a crystalline substance found in
walnut-shell. It was shown by Bernthsen in 1884 that this compound was
hydroxy-naphthaquinone, and in 1887 its synthesis from naphthalene was
accomplished by this same chemist in conjunction with Dr. Semper.

Another recent development in the present branch of chemistry brings a
coal-tar product into competition with sugar. In 1879 Dr. Fahlberg
discovered a certain derivative of toluene which possessed an intensely
sweet taste. By 1884 the manufacture of this product had been improved to
a sufficient extent to enable it to be introduced into commerce as a
flavouring material in cases where sweetness is wanted without the use of
sugar, such as in the food of diabetic patients. Under the name of
"saccharin," Fahlberg thus gave to commerce a substance having more than
three hundred times the sweetening power of cane-sugar--a substance not
only possessed of an intense taste, but not acted upon by ferments, and
possessing distinctly antiseptic properties. The future of coal-tar
saccharin has yet to be developed; but its advantages are so numerous that
it cannot fail to become sooner or later one of the most important of
coal-tar products. In cases where sweetening is required without the
possibility of the subsequent formation of alcohol by fermentation,
saccharin has been used with great success, especially in the manufacture
of aërated waters. Its value in medicine has been recognized by its recent
admission into the Pharmacopoeia.

The remarkable achievements of modern chemistry in connection with
coal-tar products do not end with the formation of colouring-matters,
medicines, and perfumes. The introduction of the beautiful dyes has had an
influence in other directions, and has led to results quite unsuspected
until the restless spirit of investigation opened out new fields for their
application. A few of these secondary uses are sufficiently important to
be chronicled here. In sanitary engineering, for example, the intense
colouring power of fluoresceïn is frequently made use of to test the
soundness of drains, or to find out whether a well receives drainage from
insanitary sources. In photography also coal-tar colouring-matters are
playing an important part by virtue of a certain property which some of
these compounds possess.

The ordinary photographic plate is, as is well known, much more sensitive
to blue and violet than to yellow or red, so that in photographing
coloured objects the picture gives a false impression of colour intensity,
the violets and blues impressing themselves too strongly, and the yellows
and reds too feebly. It was discovered by Dr. H. W. Vogel in 1873 that if
the sensitive film is slightly tinted with certain colouring-matters, the
sensitiveness for yellow and red can be much increased, so that the
picture is a more natural representation of the object. Plates thus dyed
are said to be "isochromatic" or "orthochromatic," and by their use
paintings or other coloured objects can be photographed with much better
results than by the use of ordinary plates. The boon thus conferred upon
photographic art is therefore to be attributed to coal-tar chemistry.
Among the numerous colouring-matters which have been experimented with,
the most effective special sensitizers are erythrosin, one of the
phthaleïns, quinoline red, a compound related to the same group, and
cyanin, a fugitive blue colouring-matter obtained from quinoline in 1860
by Greville Williams.

In yet another way has photography become indebted to the tar chemist. Two
important developers now in common use are coal-tar products, viz.
hydroquinone and eikonogen. The history of these compounds is worthy of
narration as showing how a product when once given by chemistry to the
world may become applicable in quite unexpected directions. Chloroform is
a case in point. This compound was discovered by Liebig in 1831, but its
use as an anæsthetic did not come about till seventeen years after its
discovery. It was Sir James Simpson who in 1848 first showed the value of
chloroform in surgical operations. A similar story can be told with
respect to these photographic developers.

Towards the middle of the last century a French chemist, the Count de la
Garaye, noticed a crystalline substance deposited from the extract of
Peruvian bark, then, as now, used in medicine. This substance was the lime
salt of an acid to which Vauquelin in 1806 gave the name of quinic acid
(_acide quinique_). In 1838 Woskresensky, by oxidizing quinic acid with
sulphuric acid and oxide of manganese, obtained a crystalline substance
which he called quinoyl. The name was changed to quinone by Wöhler, and,
as we have already seen (p. 172), the term has now become generic,
indicating a group of similarly constituted oxygen derivatives of
hydrocarbons. Hydroquinone was obtained by Caventou and Pelletier by
heating quinic acid, but these chemists did not recognize its true nature.
It was the illustrious Wöhler who in 1844 first prepared the compound in a
state of purity, and established its relationship to quinone. This
relationship, as the name given by Wöhler indicates, is that of the nature
of a hydrogenised quinone. The compound is readily prepared by the action
of sulphurous acid or any other reducing agent on the quinone.

It has long been known in photography, that a developer must be of the
nature of a reducing agent, either inorganic or organic, and many
hydroxylic and amidic derivatives of hydrocarbons come under this
category. Thus, pyrogallol, which has already been referred to as a
trihydroxybenzene (p. 146), when dissolved in alkali rapidly absorbs
oxygen--it is a strong reducing agent, and is thus of value as a
developer. But although pyrogallol is a benzene derivative, and could if
necessary be prepared synthetically, it can hardly be claimed as a tar
product, as it is generally made from gallic acid. Now hydroquinone when
dissolved in alkali also acts as a reducing agent, and in this we have the
first application of a true coal-tar product as a photographic developer.
Its use for this purpose was suggested by Captain Abney in 1880, and it
was found to possess certain advantages which caused it to become
generally adopted.

As soon as a practical use is found for a chemical product its manufacture
follows as a matter of course. In the case of hydroquinone, the original
source, quinic acid, was obviously out of question, for economical
reasons. In 1877, however, Nietzki worked out a very good process for the
preparation of quinone from aniline by oxidation with sulphuric acid and
bichromate of soda in the cold. This placed the production of quinone on a
manufacturing basis, so that when a demand for hydroquinone sprung up, the
wants of the photographer were met by the technologist. Eikonogen is
another organic reducing agent, discovered by the writer in 1880, and
introduced as a developer by Dr. Andresen in 1889. It is an
amido-derivative of a sulpho-acid of beta-naphthol, so that naphthalene is
the generating hydrocarbon of this substance.

The thio-derivative of toluidine described as "primuline" (p. 160), has
recently been found by its discoverer to possess a most remarkable
property which enables this compound to be used for the photographic
reproduction of designs in azo-colours. Diazotised primuline, as already
explained, combines in the usual way with amines and phenols to form
azo-dyes. Under the influence of light, however, the diazotised primuline
is decomposed with the loss of nitrogen, and the formation of a product
which does not possess the properties of a diazo-compound. The product of
photochemical decomposition no longer forms azo-colours with amines or
phenols. If, therefore, a fabric is dyed with primuline, then diazotised
by immersion in a nitrite bath, and exposed under a photographic negative,
those portions of the surface to which the light penetrates lose the power
of giving a colour with amines or phenols. The design can thus be
developed by dipping the fabric into a solution of naphthol,
naphthylamine, &c. By this discovery another point of contact has been
established between photography and coal-tar products. Nor is this the
only instance of its kind, for it has also been observed that a
diazo-sulpho-acid of one of the xylenes does not combine with phenols to
form azo-dyes excepting under the influence of light. A fabric can
therefore be impregnated with the mixture of diazo-sulpho-acid and
naphthol, and exposed under a design, when the azo-colour is developed
only on those portions of the surface which are acted upon by light.

The last indirect application of coal-tar colouring-matters to which
attention must be called is one of great importance in biology. The use of
these dyes as stains for sections of animal and vegetable tissue has long
been familiar to microscopists. Owing to the different affinities of the
various components of the tissue for the different colouring-matters,
these components are capable of being differentiated and distinguished by
microscopical analysis. Furthermore, the almost invisible organisms which
in recent times have been shown to play such an important part in
diseases, have in many cases a special affinity for particular
colouring-matters, and their presence has been revealed by this means. The
micro-organism of tubercle, for example, was in this way found by Koch to
be readily stained by methylene blue, and its detection was thus rendered
possible with certainty. Many of the dyes referred to in the previous
pages have rendered service in a similar way. To the pure utilitarian such
an application of coal-tar products will no doubt compensate for any
defects which they may be supposed to possess from the æsthetic point of

From a small beginning there has thus developed in a period of
five-and-thirty years an enormous industry, the actual value of which at
the present time it is very difficult to estimate. We shall not be far out
if we put down the value of the coal-tar colouring-matters produced
annually in this country and on the Continent at £5,000,000 sterling. The
products which half a century or so ago were made in the laboratory with
great difficulty, and only in very small quantities, are now turned out by
the hundredweight and the ton.[12] To achieve these results the most
profound chemical knowledge has been combined with the highest
technological skill. The outcome has been to place at the service of man,
from the waste products of the gas-manufacturer, a series of
colouring-matters which can compete with the natural dyes, and which in
many cases have displaced the latter. From this source we have also been
provided with explosives such as picric acid; with perfumes and flavouring
materials like bitter-almond oil and vanillin; with a sweetening principle
like saccharin--compared with which the product of the sugar-cane is but
feeble; with dyes which tint the photographic film, and enable the most
delicate gradations of shade to be reproduced; with developers such as
hydroquinone and eikonogen; with disinfectants which contribute to the
healthiness of our towns; with potent medicines which rival the natural
alkaloïds; and with stains which reveal the innermost structure of the
tissues of living things, or which bring to light the hidden source of
disease. Surely if ever a romance was woven out of prosaic material it has
been this industrial development of modern chemistry.

But although the results are striking enough when thus summed up, and
although the industrial importance of all this work will be conceded by
those who have the welfare of the country in mind, the paths which the
pioneers have had to beat out can unfortunately be followed but by the
few. It is not given to our science to strike the public mind at once with
the magnitude of its achievements, as is the case with the great works of
the engineer. Nevertheless the scientific skill which enables a Forth
Bridge to be constructed for the use of the travelling public of this
age--marvellous as it may appear to the uninstructed--is equalled, if not
surpassed, by the mastery of the intricate atomic groupings which has
enabled the chemist to build up the colouring-matters of the madder and
indigo plants.

A great industry needs no excuse for its existence provided that it
supplies something of use to man, and finds employment for many hands. The
coal-tar industry fulfils these conditions, as will be gathered from the
foregoing pages. If any further justification is required from a more
exalted standpoint than that of pure utilitarianism it can be supplied. It
is well known to all who have traced the results of applying any
scientific discovery to industrial purposes, that the practical
application invariably reacts upon the pure science to the lasting benefit
of both. In no department of applied science is this truth more forcibly
illustrated than in the branch of technology of which I have here
attempted to give a popular account. The pure theory of chemical
structure--the guiding spirit of the modern science--has been advanced
enormously by means of the materials supplied by and resulting from the
coal-tar industry. The fundamental notion of the structure of the benzene
molecule marks an epoch in the history of chemical theory of which the
importance cannot be too highly estimated. This idea occurred, as by
inspiration, to August Kekulé of Bonn in the year 1865, and its
introduction has been marked by a quarter century of activity in research
such as the science of chemistry has never experienced at any previous
period of its history. The theory of the atomic structure of the benzene
molecule has been extended and applied to all analogous compounds, and it
is in coal-tar that we have the most prolific source of the compounds of
this class.

It was scarcely to be wondered at that an idea which has been so prolific
as a stimulator of original investigation should have exerted a marked
influence on the manufacture of tar-products. All the brilliant syntheses
of colouring-matters effected of late years are living witnesses of the
fertility of Kekulé's conception. In the spring of 1890 there was held in
Berlin a jubilee meeting commemorating the twenty-fifth anniversary of the
benzene theory. At that meeting the representative of the German coal-tar
colour industry publicly declared that the prosperity of Germany in this
branch of manufacture was primarily due to this theoretical notion. But if
the development of the industry has been thus advanced by the theory, it
is no less true that the latter has been helped forward by the industry.

The verification of a chemical theory necessitates investigations for
which supplies of the requisite materials must be forthcoming. Inasmuch as
the very materials wanted were separated from coal-tar and purified on a
large scale for manufacturing purposes, the science was not long kept
waiting. The laborious series of operations which the chemist working on a
laboratory scale had to go through in order to obtain raw materials, could
be dispensed with when products which were at one time regarded as rare
curiosities became available by the hundredweight. It is perhaps not too
much to say that the advancement of chemical theory in the direction
started by Kekulé has been accelerated by a century owing to the
circumstance that coal-tar products have become the property of the
technologist. In other words, we might have had to wait till 1965 to reach
our present state of knowledge concerning the theory of benzenoid
compounds if the coal-tar industry had not been in existence. And this is
not the only way in which the industry has helped the science, for in the
course of manufacture many new compounds and many new chemical
transformations have been incidentally discovered, which have thrown great
light on chemical theory. From the higher standpoint of pure science, the
industry has therefore deservedly won a most exalted position.

With respect to the value of the coal-tar dyes as tinctorial agents, there
is a certain amount of misconception which it is desirable to remove.
There is a widely-spread idea that these colours are fugitive--that they
rub off, that they fade on exposure to light, that they wash out, and, in
short, that they are in every way inferior to the old wood or vegetable
dyes. These charges are unfounded. One of the best refutations is, that
two of the oldest and fastest of natural colouring-matters, viz. alizarin
and indigo, are coal-tar products. There are some coal-tar dyes which are
not fast to light, and there are many vegetable dyes which are equally
fugitive. If there are natural colouring-matters which are fast and which
are æsthetically orthodox, these are rivalled by tar-products which fulfil
the same conditions. Such dyes as aniline black, alizarin blue, anthracene
brown, tartrazine, some of the azo-reds and naphthol green resist the
influence of light as well as, if not better than, any natural
colouring-matter. The artificial yellow dyes are as a whole faster than
the natural yellows. There are at the present time some three hundred
coal-tar colouring-matters made, and about one-tenth of that number of
natural dyes are in use. Of the latter only ten--let us say 33 per
cent.--are really fast. Of the artificial dyes, thirty are extremely fast,
and thirty fast enough for all practical requirements, so that the fast
natural colours have been largely outnumbered by the artificial ones. If
Nature has been beaten, however, this has been rendered possible only by
taking advantage of Nature's own resources--by studying the chemical
properties of atoms, and giving scope to the play of the internal forces
which they inherently possess--

  "Yet Nature is made better by no mean,
  But Nature makes that mean: so, o'er that art,
  Which, you say, adds to Nature, is an art
  That Nature makes."

The story told in this chapter is chronologically summarized below--

  1820. Naphthalene discovered in coal-tar by Garden.

  1832. Anthracene discovered in coal-tar by Dumas and Laurent.

  1834. Phenol discovered in coal-tar by Runge.

  1842. Picric acid prepared from phenol by Laurent; manufactured in
        Manchester in 1862.

  1845. Benzidine discovered by Zinin.

  1859. Corallin and aurin discovered by Kolbe and Schmitt and by Persoz;
        leading to manufacture from oxalic acid and phenol.

  1860. Synthesis of salicylic acid by Kolbe.

  1864. Manchester yellow discovered by Martius, leading to manufacture of
        alpha-naphthylamine and then to alpha-naphthol.

  1867. Magdala red discovered by Schiendl.

  1868. Synthesis of alizarin by Graebe and Liebermann, leading to the
        utilization of anthracene, caustic soda, potassium chlorate and
        bichromate, and calling into existence the manufacture of fuming
        sulphuric acid.

  1870. Galleïn, the first of the phthaleïns, discovered by A. v. Baeyer,
        followed in 1871 by coeruleïn, and in 1874 by the eosin dyes
        (Caro). These discoveries necessitated the manufacture of phthalic
        acid and resorcinol.

  1873. Orthochromatic photography discovered by Vogel.

  1876. Azo-dyes from the naphthols introduced by Roussin and Poirrier and
        Witt, leading to the manufacture of the naphthols, sulphanilic
        acid, &c.

  1877. Preparation of quinone from aniline by Nietzki, utilized in
        photography in 1880 for manufacture of hydroquinone.

  1878. Disulpho-acids of beta-naphthol introduced by Meister, Lucius, and
        Brüning, leading to azo-dyes from aniline, toluidine, xylidine,
        and cumidine.

  1879. Acid naphthol yellow introduced by Caro.

   "    Biebrich scarlet, the first secondary azo-colour, introduced by

   "    Nitroso-sulpho acid of beta-naphthol discovered by the writer;
        followed in 1883 by naphthol green (O. Hoffmann), and in 1889 by
        eikonogen (Andresen).

   "    Beta-naphthol violet, the first of the oxazines, discovered by the
        writer; followed in 1881 by gallocyanin.

   "    Coal-tar saccharin discovered by Fahlberg; manufacture made
        practicable in 1884.

  1880. Synthesis of indigo by A. v. Baeyer.

   "    Quinoline synthesised by Skraup's process.

  1881. Kairine introduced by O. Fischer, the first artificial febrifuge.

   "    Indophenol discovered by Köchlin and Witt.

   "    Azo-dyes from new sulpho-acid of beta-naphthol introduced by Bayer
        & Co.

  1883. Antipyrine introduced by L. Knorr, leading to manufacture of

  1884. Congo red, the first secondary azo-colour from benzidine,
        introduced by Böttiger. Beginning of manufacture of cotton
        azo-dyes, and leading to the production of benzidine and tolidine
        on a large scale.

  1885. Secondary azo-dyes from benzidine and tolidine containing two
        dissimilar amines, phenols, &c., introduced by Pfaff.

   "    Tartrazine discovered by Ziegler; manufacture of sulpho-acid of
        phenylhydrazine and of dioxytartaric acid.

  1885. Thiorubin introduced by Dahl & Co., leading to manufacture of
        thiotoluidine; followed by primuline, discovered by A. G. Green in

  1886. Secondary azo-dyes of stilbene series introduced by Leonhardt & Co.


By passing steam over red-hot carbon, a mixture of carbon monoxide and
hydrogen is formed. This mixture of inflammable gases is known as
"water-gas," and in the preparation of the gas on a large scale, coke is
used as a source of carbon. If, therefore, water-gas became generally
used, another use for coke would be added to those already referred to (p.

With reference to the consumption of coal in London (p. 46), it appears
from the Report of a Committee of the Corporation of London, issued at the
end of 1890, that the present rate of consumption in the Metropolis is
9,709,000 tons per annum. This corresponds to 26,600 tons per diem. It has
been proved by experiment, that when coal is burnt in an open grate, from
one to three per cent. of the coal escapes in the form of unburnt solid
particles, or "soot," and about 10 per cent. is lost in the form of
volatile compounds of carbon. It has been estimated that the total amount
of coal annually wasted by imperfect combustion in this country is
45,000,000 tons, corresponding to about £12,000,000, taking the value of
coal at the pit's mouth. Taking the unconsumed solid particles at the very
lowest estimate of 1 per cent., it will be seen that, in London alone, we
are sending forth carbonaceous and tarry matter into the atmosphere at the
rate of about 266 tons daily; and volatile carbon compounds at the daily
rate of 2660 tons (see p. 32). At the price of coal in London this means
that, in solid combustibles alone, we are absolutely squandering about
£10,000 annually, to say nothing of the damage caused by the presence of
this sooty pall. Such facts as these require no comment; they speak for
themselves in sombre gloom, and in the sickliness of our town
vegetation--they give a new meaning to the term "in darkest London," and
they plead eloquently for science and legislation to show us "the way


  Accum, condensation of tar-oils, 69

  Acetic acid, 64

  Acetophenone, 178

  Acid brown, 151

  Acid greens, 106

  Acid magenta, 92

  Acid naphthol yellow, 143

  Acid yellow, 120

  Acridine, 180

  Air, composition of, 24

  Albo-carbon light, 140

  Alizarin black, 172

  Alizarin blue, 174

  Alizarin carmine, 174

  Alizarin green, 174

  Alizarin orange, 174

  Alkali blue, 93

  Amidodimethylaniline, 112

  Ammonia in gas-liquor, 64

  Ammonia, origin in gas-liquor, 65

  Analyses of coal, 23

  Aniline black, 114

  Aniline, history, 75

  Aniline, manufacture, 87

  Aniline yellow, 116

  Annual production of ammonia, 68

  Anthracene, 171

  Anthracene brown, 174

  Anthracene oil, 81, 167

  Anthragallol, 174

  Anthrapurpurin, 174

  Anthraquinone, 173

  Antifebrine, 179

  Antipyrine, 183

  Archil substitute, 151

  Arctic coal, 12

  Arsenic acid process, 90

  Arsenic acid, recovery, 94

  Artificial alizarin, 173

  Artificial purpurin, 173

  Asphalte, 176

  Auramine, 106

  Aurin, 132

  Azines, 109

  Azo-blacks, 159

  Azo-colours, 118

  Azo-dyes for cotton, 158

  Azobenzene, 119

  Azo-dyes from salicylic acid, 135

  Azotoluene, 119

  Baeyer, A. v., indigo, 127

  Baeyer, A. v., phthaleïns, 146

  Basle blue, 111

  Becher, early experiments, 34

  Beet-sugar cultivation, 67

  Benzal chloride, 102

  Benzaldehyde, 103

  Benzene, discovery, 73

  Benzene, final purification, 86

  Benzene in tar-oil, 73

  Benzene theory, 196

  Benzidam, 75

  Benzidine, 136

  Benzoic acid, manufacture, 104

  Benzotrichloride, 102

  Benzyl chloride, 102

  Bernthsen, methylene blue, 113

  Bethell, timber preserving, 70

  Biebrich scarlet, 156

  Biology, dyes used in, 192

  Bismarck brown, 116

  Bitter-almond oil, 103

  Bone oil, 180

  Böttiger, Congo-red, 157

  Bréant, timber pickling, 70

  Briquettes, 178

  Burning naphtha, 86

  Calorific value of carbon, 25

  Calorific value of coal, 20

  Carbolic acid, 129

  Carbolic oil, 23, 80, 129

  Carbon dioxide, 24

  Carboniferous period, 11

  Caro and Kern, phosgene dyes, 106

  Caro and Wanklyn, rosolic acid, 133

  Caro, fluoresceïn and eosin, 147

  Caro, methylene blue, 112

  Chemical washing, 83

  Chlorophyll, 29

  Chrysamines, 136

  Chrysoïdine, 116

  Cinnamic acid, 128

  Clayton, Dean, distils coal, 35

  Clegg, Samuel, gas-engineer, 40

  Coal, amount raised, 59

  Coal-fields of United Kingdom, 60

  Coal-gas, composition, 56

  Coal-gas, manufacture, 42

  Coal-mining, history, 57

  Coal, origin, 9

  Coal, supply, 59

  Coeruleïn, 147

  Coke, composition of, 48

  Coke from gas-retorts, 45

  Coke-oven tar, 49

  Coke, uses of, 47

  Combustion, 23

  Composition of coal, 23

  Congo red, 157

  Conservation of energy, 18

  Constitution of molecules, 95

  Corallin, 132

  Cotton dyes, 137

  Coumarin, 185

  Coupier's process, 91

  Creosote oil, 163

  Creosoting of timber, 70

  Cresols, 131

  Cresylic acid, 131

  Cretaceous coal, 12

  Crocein scarlets, 156

  Crystal violet, 106

  Cumidine, 155

  Cyanin, 188

  Dale and Caro, induline, 120

  Destructive distillation, 33

  Diazo-compounds, 116

  Diazotype, 191

  Dimethylaniline, 100

  Dinitrobenzene, 120

  Diphenylamine, 101

  Diphenylamine blue, 101

  Distillation, fractional, 78

  Doebner, malachite green, 102

  Dover, coal under, 60

  Dumas and Laurent, anthracene, 171

  Dundonald, Earl, early experiments, 39

  Eikonogen, 189

  Electricity as an illuminating agent, 62

  Eocene coal, 12

  Eosin, 147

  Erythrosin, 188

  Essence of mirbane, 74

  Exalgine, 179

  Fahlberg, saccharin, 186

  Faraday, discovers benzene, 73

  Fast red, 151

  Fertilization by ammonia, 66

  Fire-damp, 32

  First runnings, 80

  Fischer, E. and O., rosaniline, 97

  Fischer, hydrazines, 183

  Fischer, malachite green, 103

  Flavaniline, 180

  Flavopurpurin, 174

  Fluoresceïn, 147

  Foot-pound, 19

  Fractional distillation, 78

  Fritzsche, aniline, 75

  Galleïn, 146

  Gallic acid, 146

  Gallocyanin, 162

  Gambines, 161

  Garancin, 168

  Garden, naphthalene, 139

  Gas producers, 57

  Gas, quantity obtained from coal, 45

  Girard and De Laire, rosaniline blues, 92

  Glucosides, 169

  Goethe, visit to coke-burner, 48

  Græbe and Liebermann, alizarin, 170

  Graphite, 12

  Grässler, acid yellow, 120

  Green, A. G., primuline, 160

  Griess, diazo-compounds, 116

  Hales, Rev. Stephen, distils coal, 37

  Hofmann, benzene in tar-oil, 73

  Hofmann, red from aniline, 89

  Hofmann's violets, 93

  Homologous series, 152

  Horse-power, 22

  Hull, Prof., coal supply, 59

  Hydrazines, 183

  Hydrocarbons of benzene series, 82

  Hydrogen, calorific value, 25

  Hydroquinone, 189

  Hypnone, 179

  Indigo plants, 124

  Indigo, syntheses, 126

  Indophenol, 162

  Indulines, 121

  Ingrain colours, 160

  Iodine green, 94

  Iron smelting, 16

  Iron swarf, 87

  Isochromatic plates, 188

  Isomerism, 88

  Joule, mechanical equivalent of heat, 19

  Juglone, 186

  Jurassic coal, 12

  Kairine, 182

  Kekulé, benzene theory, 196

  Kekulé and Hidegh, azo-dyes, 135

  Koch, tubercle, 193

  Köchlin, gallocyanin, 162

  Köchlin and Witt, indophenol, 162

  Kolbe, salicylic acid, 134

  Kolbe and Schmitt, phenol dye, 132

  Kyanol, 75

  Lampblack, 139, 166

  Laurent, phenol, 131

  Laurent, phthalic acid, 143

  Laurent, picric acid, 136

  Lauth, methyl violet, 98

  Lauth's violet, 111

  Leonhardt & Co., stilbene dyes, 137

  Lightfoot, aniline black, 114

  Light oil, 80

  Light oils, early uses, 71

  Lister, antiseptic surgery, 131

  London, coal introduced, 58

  London, illuminated by gas, 40

  Lucigen burner, 163

  Madder, 168

  Magdala red, 149

  Magenta, history, 89

  Malachite green, 102

  Manchester brown, 116

  Manchester yellow, 142

  Mansfield, isolation of benzene, 73

  Mansfield's still, 77

  Manures, 66

  Marsh gas, 32

  Mauve, discovery, 74

  Mechanical value of coal, 22

  Medlock, magenta process, 90

  Methyl chloride, 99

  Methylene green, 113

  Methyl green, 101

  Methyl violet, 98

  Mirbane, essence, 74

  Murdoch, introduces coal-gas, 40

  Naphthalene, annual production, 141

  Naphthalene in carbolic oil, 130

  Naphthionic acid, 151

  Naphthol green, 161

  Naphthol orange, 151

  Naphthols, 141

  Naphthylamines, 142

  Natanson, aniline red, 89

  Neutral red, 111

  Neutral violet, 111

  New blue, 161

  Nicholson blue, 93

  Nicholson, magenta process, 90

  Nietzki, azines, 109

  Nietzki, Biebrich scarlet, 156

  Nietzki, quinone, 191

  Night blue, 106

  Nigrosine, 121

  Nitrification, 66

  Nitrobenzene process, 91

  Nitrosodimethylaniline, 111

  Non-Carboniferous coal, 11

  Number of compounds in tar, 81

  Old Red Sandstone, coal in, 12

  Oligocene brown coal, 12

  Orthochromatic plates, 188

  Oxazines, 162

  Oxide of iron for gas purifying, 44

  Paraffin oil and wax, 50

  Patent fuel, 178

  Perfumes, 185

  Perkin, alizarin, 170

  Perkin, discovers mauve, 74

  Permanence of dyes, 198

  Permian coal, 12

  Persoz, phenol dye, 132

  Pharmaceutical preparations, 178

  Phenanthrene, 172

  Phenols, 129

  Phosgene dyes, 106

  Phosphine, 94, 180

  Photographic developers, 189

  Phthaleïns, 146

  Phthalic acid, 143

  Picric acid, 136

  Pitch, 81, 176

  Plants, growth of, 26

  Ponceaux, 151

  Primary azo-dyes, 156

  Primuline, 160

  Propiolic acid, 128

  Purpurin, 169

  Pyrazole, 183

  Pyridine, 87

  Pyridine bases, 179

  Pyrogallol, 146

  Quinaldine, 182

  Quinic acid, 189

  Quinoline, 180

  Quinoline green, 182

  Quinoline red, 188

  Quinoline yellow, 182

  Quinones, 172

  Read Holliday's lamp, 72

  Rectification of hydrocarbons, 84

  Resorcinol, 145

  Rhodamines, 148

  Roccellin, 151

  Roman coal-mining, 57

  Rosaniline blues, 92

  Rosaniline from rosolic acid, 133

  Rosolic acid, 132

  Runge, kyanol, 75

  Runge, phenol in tar, 131

  Saccharin, 186

  Saffranine, 108

  Salicylic acid, 134

  Salicylic aldehyde, 185

  Salols, 179

  Schiendl, Magdala red, 149

  Scotland, shale-oil industry, 53

  Sea coal, 58

  Secondary azo-dyes, 156

  Shale, nature of, 51

  Shale-oil industry, 50

  Skraup, quinoline, 181

  Sodium nitrite, 119

  Solar energy, 28

  Soluble blue, 93

  Solvent naphtha, 86

  Steam-engines, 20

  Stilbene azo-dyes, 137

  Structure of coal, 15

  Sulphanilic acid, 150

  Sulphuric acid, 45

  Sunlight, source of energy in coal, 30

  Surgery, antiseptic, 131

  Tar distilling, 77

  Tar, first utilization, 69

  Tar from coke-ovens, 49

  Tar, quantity obtained from coal, 45

  Tartrazine, 184

  Tertiary coal, 12

  Thalline, 182

  Thermifugine, 182

  Thiazines, 113

  Thiodiphenylamine, 113

  Thiorubin, 160

  Timber pickling, 70

  Tolidine, 136

  Toluene from tar, 81

  Torbane Hill coal, 52

  Transformation of wood into coal, 31

  Triassic coal, 12

  Trimethylamine, 99

  Trinitrophenol, 136

  Triphenylmethane, 97

  Tubercle bacillus, 193

  Turkey red, 170

  Underclay, 13

  Unverdorben, crystallin, 75

  Uses of coal, 16

  Vanillin, 185

  Vegetable deposits, recent, 13

  Verguin, red from aniline, 90

  Victoria blue, 106

  Victoria yellow, 137

  Vinasse, 99

  Vincent, methyl chloride, 100

  Violaniline, 121

  Wasteful use of coal, 32, 203

  Water, composition of, 25

  Water gas, 203

  Watson, Bishop, coke-oven tar, 50

  Watson, Bishop, distils coal, 37

  Wealden iron industry, 17

  Whitaker, W., coal in S.-E. England, 60

  Winsor, promotes introduction of gas, 41

  Witt, azines, 109

  Witt, chrysoïdine, 116

  Witt, naphthol orange, 150

  Wood vinegar, 64

  Woody fibre, 26

  Woulfe, picric acid, 136

  Xylenes from tar, 81

  Xylidine scarlet, 153

  Young, James, burning-oil, 51

  Ziegler, tartrazine, 184

  Zinin, benzidam, 75


_Richard Clay & Sons, Limited, London & Bungay._


[1] "An experiment concerning the Spirit of Coals," _Phil. Trans._
(abridged), vol. viii. p. 295.

[2] _Report of the Coal Commissioners_ (1866-71), vol. i.

[3] In a paper read before the Royal Statistical Society by Mr.
Price-Williams in 1889, this author points out that, owing to the
introduction of the Bessemer process and other economical improvements,
the amount of coal used in the iron and steel manufacture had fallen in
1867 to about sixteen and a half per cent. of the total quantity raised.

[4] This remark does not apply to Great Britain; our Excise regulations
have practically killed those branches of manufacture requiring the use of
pure wood-spirit.

[5] Since the above was written, new synthetical processes for the
production of indigo have been made known in Germany by Karl Heumann. Of
the commercial aspect of these discoveries it is of course impossible at
present to form an opinion.

[6] With phthalic anhydride gives fluoresceïn and dyes of eosin and
rhodamin series.

[7] Diazotised and combined with naphthylamines, naphthols, or their
sulpho-acids give azo-dyes.

[8] With naphthols give oxazines and indophenol.

[9] Diazotised and combined with phenols, amines, or sulpho-acids to form

[10] Combined with diazotised amido-compounds to form azo-dyes.

[11] Since the above was written the continuation of Koch's researches
upon the tubercle bacillus has culminated in the discovery of his now
world-renowned lymph for the inoculation of patients suffering from
tubercular disease.

[12] In one large factory in Yorkshire there is a set of stills kept
constantly at work making pure aniline at the rate of two hundred tons per
month. The monthly consumption of coal in this factory is two thousand
tons, equal to twenty-four thousand tons per annum.


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Transcriber's Notes:

Passages in italics are indicated by _italics_.

Punctuation has been corrected without note.

Spelling and hyphenation variants have been retained from the original.

The original text includes Greek characters. For this text version these
letters have been replaced with transliterations.

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