changed as to become a dense, black, glistening, brittle mass, showing
no obvious forms of leaves or texture of wood. But no one who has seen
how a quantity of damp hay closely pressed together will, after a time,
become heated and change in colour to black, can have any difficulty in
comprehending how chemical and mechanical actions may completely alter
the aspect of vegetable matter. We have, however, the most direct
evidence of the vegetable origin of coal in the numberless
unquestionable forms of trees and plants met with in all coal strata.
Sometimes the trunks of the trees fossilized into stony matter are found
upright in the very situation in which they grew. Thus in Fig. 341 is
represented the appearance exhibited by the trunks and roots of some
fossil trees, which were exposed to view in the formation of a railway
cutting between Manchester and Bolton. In every coal-field also
beautiful impressions of the stems and leaves of plants are met with—one
common form of which is shown in Fig. 342. Most of the plants so found
belong to the flowerless division of the vegetable kingdom. Some are
closely allied to the ferns of the present day—to the common
“mare’s-tail” (_Equisetum_), to the club-moss, and to other well-known
plants. The firs and pines of the coal age are scarcely distinguishable
from existing species. If a fragment of ordinary coal be ground to a
very thin slice—so thin as to be transparent—and placed under the
microscope, it will show a number of minute rounded bodies, which are,
there is good reason to believe, nothing else than the spores or seeds
of plants, closely resembling the existing club-mosses. The spores of
the club-moss (_Lycopodium_) are so full of resinous matter, that they
are used for making fireworks and the flashes of lightning at theatres.
It is, therefore, extremely probable that the bitumen of coal is due to
the resin of similar spores, altered by the effects of subterranean
heat. The immense abundance of these little spores in the coal is a
proof that they accumulated in the ancient forests as the mosses grew,
and therefore the matter of coal was not accumulated under water or
washed down into the sea; for these little spores are extremely light,
and they cannot be wetted by water, and therefore they would have
floated on the surface, and would not have been found so diffused
throughout the coal. Fig. 343 is a picture of the possible aspect of the
ancient forests of the coal age. In the humid atmosphere which probably
prevailed at that period, the large tree-ferns and gigantic club-mosses,
which are conspicuous in the picture, must have flourished luxuriantly.
[Illustration:
FIG. 343.—_Possible Aspect of the Forests of the Coal Age._
]
The immense importance of coal for domestic purposes will be obvious
from the fact that it is estimated that in the United Kingdom alone no
less than 30,000,000 tons are annually consumed in house fires. Another
great use of coal is in the smelting, puddling, and working of iron, and
this probably consumes as much as our domestic fireplaces. Then there is
the vast consumption by steam engines, by locomotives, and by
steamboats. Another purpose for which coal is largely used is the making
of illuminating gas; and to the foregoing must also be added the
quantity which goes to feed the furnaces necessary in so many of the
arts—such as in the manufacturing of glass, porcelain, salt, chemicals,
&c. The quantity of coal raised in Great Britain was not accurately
known until 1854, when it was ordered that a register should be kept,
and an annual return made. The following figures, in round numbers, are
the returns published up to 1873. The table is continued in Note A.
Year. Coal raised, in Tons.
1854 64,661,000
1855 64,453,000
1856 66,645,000
1857 65,395,000
1858 65,008,000
1859 71,979,000
1860 83,208,000
1861 85,635,000
1862 83,638,000
1863 88,292,000
1864 92,787,000
1865 98,150,000
1866 101,630,000
1867 104,500,000
1868 103,141,000
1869 107,427,000
1870 110,289,000
1871 117,352,000
1872 123,497,000
1873 127,017,000
The first return showed our annual produce to be 64,661,000 tons. The
amount did not greatly vary until 1859, when there was an increased
production of nearly seven millions of tons; in 1860 a further increase
of eleven millions of tons more. Since then the quantity annually raised
has been increasing. Comparing the quantity which has been raised in any
year after 1863 with that raised ten years before, we see that the
increase in ten years is nearly half as much again; or, that at the
present rate of increase the amount annually raised doubles itself at
least every twenty years. Now, the question arises, How long can this go
on? However great may be the store of coal, it must sooner or later come
to an end. Is it possible to calculate how long our coals will last? and
what are the results of such calculations? These calculations have been
made; but there are great discrepancies in the results, for the
estimates of the amount of available coal still remaining vary greatly,
and different views are held regarding the rate of consumption in the
future. A very liberal estimate, by an excellent authority, of the
quantity of coal remaining under British soil, makes it 147,000 millions
of tons. With a consumption stationary at the present rate, this will
last 1,200 years; with an increase of consumption of 3,000,000 tons a
year, 276 years; but if the consumption continues to increase in the
same geometrical ratio it has hitherto followed, the supply will
scarcely last 100 years. It cannot be conceived, however, that this last
will be the real case, for the increasing depth to which it will be
necessary to go will soon cause a great increase in the cost, and thus
effectually check the consumption. Great Britain will, however, be
compelled to retire from the coal trade altogether, by the cheaper
supplies which other countries will yield, long before the absolute
exhaustion of her own coal-fields. It is calculated that the coal-fields
of North America contain thirteen times as much as those of all Europe
put together. Coal is also found abundantly in India, China, Borneo,
Eastern Australia, and South Africa; and it is believed that these
stores will supply the world for many thousand years.
[Illustration:
FIG. 344.—_The Fireside._
]
Seeing, then, that our supply of coal has a limit, and that at the
present increasing rate of consumption, the chief source of the wealth
of Great Britain must necessarily be exhausted in a few more centuries,
it behoves us to turn our mineral treasures to the best account, and to
adopt every possible means of obtaining from our coal its whole
available heat and force. The amount of avoidable waste of which we are
guilty in the consumption of coal is enormous. This is especially the
case in its domestic use, where probably nineteen-twentieths of the heat
produced is absolutely thrown away—sent off from the earth to warm the
stars. In England people look upon the wide open fireplace as the image
and type of home comfort. No doubt there are, from long use and habit,
many pleasing associations which cluster round the domestic hearth; but
we, to whom it is given to “look before and after,” must think what it
takes to feed that wide-throated chimney. All but a very small fraction
of the heat thus escapes, and is lost to man and the world for ever; and
surely we shall deserve the curses of our descendants if we continue
recklessly to throw away a treasure which, unlike the oil in the widow’s
cruse, is never renewed—for there is no contemporaneous formation of
coal. Thanks to the enhanced price of coal during the last few years,
some attention has been directed to contrivances for the economical
consumption of coal in its domestic, as well as in its manufacturing,
applications.
A time, however, will sooner or later come, when the whole available
coal shall have been consumed. What will then be the fuel of the
engines, and steamboats, and locomotives of the future? The reader may
think that then it will only be necessary to burn wood. But wood is
already being consumed from the face of the earth much more rapidly than
it is produced. How, then, can it be available when coal fails? The
truth is, we are now consuming not merely the wood which the sun-rays
are building up in our own time, but in hewing down the forests we are
using the sun-work of a century, while in coal we have the forests of
untold ages at our disposal—the accumulated combustible capital stored
up during an immense period of the earth’s existence. Upon this
stored-up capital we are now living, our current receipts of sun-force
being wholly inadequate to meet our expenditure. The coal is the
sun-force of former ages; and it is from this we are now deriving the
energy which performs most of our work. George Stephenson long ago
declared that his locomotives were driven by sunshine—by the sunshine of
former ages bottled up in the coal. And he was right. The mechanical
energy of our steam engines, and the chemical energy of our blast
furnaces, are derived from the combustion of vegetable matter, in which
the heat and light of the sun—our present sun or that of the coal
ages—are in some way stored up. The burning of wood or coal is,
chemically, the reverse action to that performed by the sunlight: by the
former carbon and oxygen are united, by the latter they are separated.
We foresee, then, a future period—however distant may be that future—in
which the world’s capital shall have been exhausted, and the energies
which are now employed in doing the world’s work will no longer be
available. But the reader will perhaps think that by improvements in the
steam engine, and in other ways, means will be found of getting more and
more work out of coal. It is true that we obtain from coal only a
_fraction_ of its available energy; but the whole work which could, by
any possible process, be done by the combustion of coal is _definite and
limited_, although its amount is large. A pound of coal burnt in one
minute sets free an amount of energy which would, if it could all be
made available, do as much as 300 horses working in the same time. But,
again, the reader may think, even if at some distant future the supplies
of fuel for the steam engines of our remote posterity should fail, that
before that time some other form of force than steam or heat engines
will have been discovered—some application of electricity, for example.
Now, it will appear, from principles which will be discussed in a
subsequent article, that not only is there no probability of such a
discovery, but that now, when the relations of the whole available
energies of the globe have been traced and defined, Science can find no
ground for admitting such a possibility under the present condition of
the universe.
_PETROLEUM._
When coal is heated in closed vessels, there are given off, as we shall
presently see, a number of gaseous and volatile products—many being
compounds of carbon and hydrogen—which condense to liquids or solids at
ordinary temperatures. Carbon is by far the largest constituent of coal,
which commonly contains only about 10 per cent. of other substances,
although the proportions vary very widely. Another important constituent
of coal is its hydrogen, and the value of coal as a source of heat
depends almost entirely upon the carbon and hydrogen it contains. Carbon
is one of the most remarkable of all the elements of the globe for its
power of entering into an enormous number of compounds. Thus, for
example, the compounds of carbon with only hydrogen are innumerable; but
they are all definite, and their composition is expressible by the
admirable system of chemical symbols, of which the reader has now, it is
hoped, some definite notion. Perhaps these hydro-carbons are among the
best evidences which could be adduced that modern science has obtained a
grasp of certain conceptions which have a real correspondence with the
actual facts of nature, even as regards the intimate constitution of
matter. This is not the place to enter into a complete exposition of
this subject. We may, however, invite the reader’s attention to a few
simple facts. A very large number of compounds of carbon and hydrogen
are known. If the percentage compositions of these be compared together,
it is only the eye of a most expert arithmetician which can detect any
relation between the proportions of the constituents in the various
compounds. The chemist, however, by associating such of these compounds
as resemble each other in their general properties, finds that they can
be arranged in series, in which the composition is accurately expressed
by multiples of the proportions: C = 12, H = 1. And not only so, the
different series themselves form a series of series, having a simple
relation to each other. Thus, confining ourselves to some of the known
hydro-carbons, we have the following:
┌──────────────────────┬──────────────────────┬──────────────────────┐
│ A │ B │ C │
├──────────────────────┼──────────────────────┼──────────────────────┤
│ C H_{4} │ C H_{2} │ │
│ C_{2}H_{6} │ C_{2}H_{4} │ C_{2}H_{2} │
│ C_{3}H_{8} │ C_{3}H_{6} │ C_{3}H_{4} │
│ C_{4}H_{10} │ C_{4}H_{8} │ C_{4}H_{6} │
│ C_{5}H_{12} │ C_{5}H_{10} │ C_{5}H_{8} │
│ C_{6}H_{14} │ C_{6}H_{12} │ C_{6}H_{10} │
│ &c. │ &c. │ &c. │
│ C_{_n_}H_{2_n_ + 2} │ C_{_n_}H_{2_n_} │ C_{_n_}H_{2_n_ – 2} │
└──────────────────────┴──────────────────────┴──────────────────────┘
┌──────────────────────┬──────────────────────┬──────────────────────┐
│ D │ E │ F │
├──────────────────────┼──────────────────────┼──────────────────────┤
│ │ │ │
│ │ │ │
│ C_{3}H_{2} │ │ │
│ C_{4}H_{4} │ C_{4}H_{2} │ │
│ C_{5}H_{6} │ C_{5}H_{4} │ C_{5}H_{2} │
│ C_{6}H_{8} │ C_{6}H_{6} │ C_{6}H_{4} │
│ &c. │ &c. │ &c. │
│ C_{_n_}H_{2_n_ – 4} │ C_{_n_}H_{2_n_ – 6} │ C_{_n_}H_{2_n_ – 8} │
└──────────────────────┴──────────────────────┴──────────────────────┘
This table might be indefinitely extended, but enough is given to enable
the intelligent reader to discover the laws connecting these formulæ.
The series headed B, it will be observed, have all the same percentage
composition. Why, then, one formula rather than another? The answer to
this question is the statement of a theoretical law upon which the whole
science of modern chemistry is based; for it has the same relation to
that science as gravitation has to astronomy. It is a matter of fact
that all gases, whatever their chemical nature, expand alike with the
same application of heat, and all obey the same law, which connects
volumes and pressures. These are very remarkable uniformities, for gases
in this respect exhibit the most decided contrast to liquids and solids.
The volume of each solid and of each liquid has its own special
relations to temperature and pressure: here there is endless diversity.
The volumes of all gases have one and the same relation to temperature
and pressure: here there is absolute uniformity. As an explanation of
these and other facts relating to gases, Amedeo Avogadro, in 1811, put
forward this hypothesis—_Equal volumes of all gases, under like
circumstances of temperature and pressure, contain the same number of
molecules_. This hypothesis was revived by Ampère a few years later, and
sometimes is called his. A necessary consequence of this law is that the
weights of the molecules of gases are proportional to their densities or
specific gravities. Hence when the percentage composition of a
hydro-carbon has been determined, by burning or oxidizing it in such a
manner as to obtain and weigh the products, carbonic acid and water, the
next thing the chemist does is to obtain the weight of a volume of the
gas. The number of times this exceeds the weight of hydrogen gas, under
the same conditions, expresses how many times the molecule is heavier
than the hydrogen molecule. Now, the chemist’s unit of weight in these
inquiries is the weight of a single _atom_ of hydrogen; and, as there
are grounds for believing that the _molecule_ of hydrogen consists of
two atoms of that substance, its weight = 2. Now, if the molecule of
marsh gas, the first hydro-carbon in the above list, has the composition
assigned, it will be 12 + 4 = 16 times heavier than the _atom_ of
hydrogen, and 16/2 = 8 times heavier than the _molecule_ of hydrogen.
Hence, if Avogadro’s law be correct, marsh gas should be just eight
times heavier than hydrogen gas; which is really the fact. The formula
expressing the composition of the molecule of a hydro-carbon, or of any
chemical compound whatever, is always so fixed that the same relations
may hold; and almost the first thing a chemist does in examining a new
body is to endeavour to obtain it in the state of gas.
The first four members of the series headed A are gases at ordinary
temperatures, the fifth is a gas at temperatures above the
freezing-point, and a liquid at lower temperatures; the next following
members are liquids which boil (that is, are converted into gases) at
temperatures rising with each additional carbon atom about 20° F. The
specific gravities and boiling-points of these liquids augment as we
pass from one hydro-carbon to the next, and the lower members of the
series are solids, fusing at temperatures higher and higher as the
number of carbon atoms is greater. Similar gradations of properties are
exhibited by the other series of hydro-carbons. Petroleum or rock-oil is
the name given to liquid hydro-carbons found in nature, and consisting
chiefly of compounds belonging to the series marked A in the above list.
Some varieties of petroleum hold in solution other hydro-carbons, and in
some cases paraffin is extracted from the oils by exposing the liquid to
cold, when the solid crystallizes out. Paraffin is a solid belonging to
the B series, and it is for the most part obtained by heating certain
minerals.
Deposits of liquid hydro-carbons, perhaps formed by a kind of natural
subterranean distillation from coal or other fossil organic matter,
exist in various localities. These deposits have long been known and
utilized at Rangoon, in Burmah, and on the shores of the Caspian Sea. At
Rangoon the mineral oil is obtained by sinking wells about 60 ft. deep
in a kind of clay soil, which is saturated with it. The oily clay rests
upon a bed of slate also containing oil, and underneath this is coal. It
may be supposed that subterranean heat, acting upon the coal, has
distilled off the petroleum, which has condensed in the upper strata.
This petroleum, when distilled in a current of steam, leaves about 4 per
cent of residue, and the volatile portion contains about one-tenth of
its weight of a substance (paraffin) which is solid at ordinary
temperatures. After an agitation with oil of vitriol, and another
distillation, _rock oil_ or _naphtha_ is obtained, which, however, is
still a mixture of several distinct chemical compounds. Mineral oils
have also been found in China, Japan, Hindostan, Persia, the West India
Islands, France, Italy, Bavaria, and England. In one of the Ionian
Islands there are oil-springs which have flowed, it is said, over 2,000
years.
But it is the recently discovered and extremely copious springs and
wells in Pennsylvania and Canada which have given a vastly extended
importance to the trade in mineral oil. Rock oil is now used in enormous
quantities as the cheapest illuminating oil, and that which furnishes
the most intense light. Its consumption as a lubricating oil for
machines has also been very large. Mineral oil was occasionally found at
various places in the United States, and sometimes used by the
inhabitants of the locality before the recent discoveries; but it was
not until August, 1859, that it was met with in large quantities. About
this time a boring which was made at Oil Creek, Pennsylvania, reached an
abundant source, for 1,000 gallons a day were drawn from it for many
weeks. The news of the discovery of this copious oil-spring spread
rapidly: thousands of persons flocked to the neighbourhood in hopes of
easily making a fortune by “striking oil.” Before the end of 1860 more
than a thousand wells had been bored, and some of these had yielded
largely. The regions of North America in which petroleum has been found
cover a large part of the States, and comprise Pennsylvania, New York,
Ohio, Michigan, Kentucky, Tennessee, Kansas, Illinois, Texas, and
California. In the vicinity of Oil Creek the bore-holes are usually
about 3 in. or 4 in. in diameter, and are often 500 ft. deep, and even
800 ft. is not uncommon. To make a bore-hole 900 ft. deep, and procure
all the requisites—steam engines, barrels, &c., for pumping the
oil—costs about $5,000. In 1869 many of these wells still yielded
regularly 300 barrels a day, but the supply has not continued with the
same abundance. One of the luckiest wells flowed at its first opening at
the rate of about 25,000 barrels a day. The apparatus used for working
the oil-wells is very simple—a rude derrick, a small steam engine, a
pump, and some barrels and tubs being all that is necessary. Fig. 345
will give the reader an idea of the scene presented by a cluster of
oil-wells in the Oil Creek region. Oil Creek received its name before
the petroleum trade was established, from the oil found floating on the
surface of the water. It is on the Alleghany River, about 150 miles
above Pittsburg, and here at its mouth is situated Oil City. There is a
wharf in Pittsburg for the oil traffic, and the barrels are brought down
the river in flats, or the oil is poured into very large flat boxes,
divided into compartments, which are then closed, and the boxes floated
down in groups of twenty or more. The refining process consists in
placing the crude oil into a large iron retort, connected with a
condenser formed of a coil of iron pipes, surrounded by cold water. Heat
is applied, and the lighter hydro-carbons (naphtha) come over first.
After the naphtha, the oils which are used for illuminating purposes
distil off. A current of steam is then forced into the retort, and this
brings over the heavy oils which are used for greasing machinery. A
black tarry oil yet remains; and, finally, after the separation of this,
a quantity of coke. The products are subjected to certain processes of
purification, which need not here be described. The magnitude of the
American oil trade may be inferred from the fact that in the second year
of its existence, from January to June, 1862, more than 4,500,000
gallons were exported from four seaports. This can hardly be wondered
at, considering the extremely low price at which this excellent
illuminating and lubricating agent can be produced. Refined petroleum
can be bought at Pittsburg for 16 cents. per gallon. It is believed by
some that the supplies of petroleum which exist in various localities
are so abundant that they will furnish illuminating oils to the whole
world for centuries.
[Illustration:
FIG. 345.—_View on Hyde and Egbert’s Farm, Oil Creek._
]
_PARAFFIN._
In the course of some researches on the substances contained in the tar,
which is obtained by heating wood in close vessels, Reichenbach found a
white translucent substance, to which he gave the above name, because it
was not acted upon by any of the ordinary chemical reagents, such as
sulphuric acid, nitric acid, &c. This substance, which is composed of
carbon and hydrogen only, is not unlike spermaceti; it is colourless,
translucent, and without smell or taste. But when slightly warmed, it
becomes very plastic, and may then be moulded with the greatest ease—and
in this respect it differs from spermaceti. Paraffin melts at from 88°
to 150° C., to a colourless liquid, which is so fluid that it may be
filtered through paper like water, and at a higher temperature it can be
distilled unchanged. Paraffin does not dissolve in water, and is but
slightly soluble in alcohol. In ether, naphtha, turpentine, benzol, and
sulphide of carbon, it dissolves very readily. When heated with sulphur,
it is decomposed: the sulphur seizes upon its hydrogen, sulphuretted
hydrogen is given off, and the carbon is separated; and this action has
been proposed as a ready means of obtaining pure sulphuretted hydrogen
for laboratory use. It is probable that paraffin is a mixture of various
hydro-carbons, having a composition expressible by the formula,
C_{_n_}H_{2_n_}; for different specimens fuse at different temperatures,
according as the paraffin has been obtained from one or the other
source.