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COAL; AND WHAT WE GET FROM IT.
THE ROMANCE OF SCIENCE.
COAL AND WHAT WE GET FROM IT.
A Romance of Applied Science.
EXPANDED FROM THE NOTES OF A LECTURE
DELIVERED IN THE THEATRE OF THE
LONDON INSTITUTION, JAN. 20th, 1890.
BY RAPHAEL MELDOLA, F.R.S., F.I.C., &C.,
PROFESSOR OF CHEMISTRY IN THE FINSBURY TECHNICAL COLLEGE,
CITY AND GUILDS OF LONDON INSTITUTE.
PUBLISHED UNDER THE DIRECTION OF THE COMMITTEE
OF GENERAL LITERATURE AND EDUCATION APPOINTED BY THE
SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE.
LONDON:
SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE,
NORTHUMBERLAND AVENUE, CHARING CROSS, W.C.;
43, QUEEN VICTORIA STREET, E.C.
BRIGHTON: 135, NORTH STREET. NEW YORK: E. & J. B. YOUNG & CO.
1891.
TO WILLIAM HENRY PERKIN,
PH.D., F.R.S.,
THE FOUNDER OF THE COAL-TAR COLOUR INDUSTRY,
THIS BOOK IS DEDICATED BY THE AUTHOR.
PREFACE.
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
time.
R. M.
_6 Brunswick Square, W.C._
CONTENTS.
CHAPTER I.
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.
CHAPTER II.
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
Chrysoidine, 118. The Indulines, 121. Chronological Summary, 122.
CHAPTER III.
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
Phthaleins, 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.
COAL; AND WHAT WE GET FROM IT.
CHAPTER I.
"Hier [1771] fand sich eine zusammenhaengende 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
deposits.
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 Archaean 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
on.
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
heat.
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 deg. to 61 deg. 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 deg. 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 deg. 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
composition:--
-------------------------------------------------------------------
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
columns.
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
period.
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 deg.
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 deg. 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
period.
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
present.
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
says--
"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
words--
"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
product.
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
furnace.
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
Saarbruecken 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 _Traite 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
Continent.
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 L10,000 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
"sea-coal":--
"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 L45,000,000 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
industries.
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.