Book: The Story of a Piece of Coal

E >> Edward A. Martin >> The Story of a Piece of Coal




[Illustration: FIG 30.--_Lepidodendron_. Portion of Sandstone stem after
removal of bark of a giant club-moss]

There is but one more form of carbon with which we have to deal in
running through the series. We have seen that coal is not the _summum
bonum_ of the series. Other transformations take place after the stage of
coal is reached, which, by the continued disentanglement of gases,
finally bring about the plumbago stage.

What the action is which transforms plumbago or some other form of carbon
into the condition of a diamond cannot be stated. Diamond is the purest
form of carbon found in nature. It is a beautiful object, alike from the
results of its powers of refraction, as also from the form into which its
carbon has been crystallised. How Nature, in her wonderful laboratory,
has precipitated the diamond, with its wonderful powers of spectrum
analysis, we cannot say with certainty. Certain chemists have, at a great
expense, produced crystals which, in every respect, stand the tests of
true diamonds; but the process of their production at a great expense has
in no way diminished the value of the natural product.

The process by which artificial diamonds have been produced is so
interesting, and the subject may prove to be of so great importance, that
a few remarks upon the process may not be unacceptable.

The experiments of the great French chemist, Dumas, and others,
satisfactorily proved the fact, which has ever since been considered
thoroughly established, that the diamond is nothing but carbon
crystallised in nearly a pure state, and many chemists have since been
engaged in the hitherto futile endeavour to turn ordinary carbon into the
true diamond.

Despretz at one time considered that he had discovered the process, which
consisted in his case of submitting a piece of charcoal to the action of
an electric battery, having in his mind the similar process of
electrolysis, by which water is divided up into the two gases, hydrogen
and oxygen. He obtained a microscopic deposit on the poles of the
battery, which he pronounced to be diamond dust, but which, a long time
after, was proved to be nothing but graphite in a crystallised state.
This was, however, certainly a step in the right direction.

The honour of first accomplishing the task fell to Mr Hannay, of Glasgow,
who succeeded in producing very small but comparatively soft diamonds, by
heating lampblack under great pressure, in company with one or two other
ingredients. The process was a costly one, and beyond being a great
scientific feat, the discovery led to little result.

A young French chemist, M. Henri Moissau, has since come to the front,
and the diamonds which he has produced have stood every test for the true
diamond to which they could be subjected; above all, the density of the
product is 3.5, _i.e._, that of the diamond, that of graphite reaching 2
only.

He recognised that in all diamonds which he had consumed--and he consumed
some L150 worth in order to assure himself of the fact--there were always
traces of iron in their composition. He saw that iron in fusion, like
other metals, always dissolves a certain quantity of carbon. Might it not
be that molten iron, cooling in the presence of carbon, deep in volcanic
depths where there was little scope for the iron to expand in assuming
the solid form, would exert such tremendous pressure upon the particles
of carbon which it absorbed, that these would assume the crystalline
state?

He packed a cylinder of soft iron with the carbon of sugar, and placed
the whole in a crucible filled with molten iron, which was raised to a
temperature of 3000 deg. by means of an electric furnace. The soft cylinder
melted, and dissolved a large portion of the carbon. The crucible was
thrown into water, and a mass of solid iron was formed. It was allowed
further to cool in the open air, but the expansion which the iron would
have undergone on cooling, was checked by the crucible which contained
it. The result was a tremendous pressure, during which the carbon, which
was still dissolved, was crystallised into minute diamonds.

These showed themselves as minute points which were easily separable from
the mass by the action of acids. Thus the wonderful transformation from
sugar to the diamond was accomplished.

It should be mentioned that iron, silver, and water, alone possess the
peculiar property of expanding when passing from the liquid to the solid
state.

The diamonds so obtained were of both kinds. The particles of white
diamond resembled in every respect the true brilliant. But there was also
an appreciable quantity of the variety known as the "black diamond."
These diamonds seem to approximate more closely to carbon as we are most
familiar with it. They are not considered as of such value as the
transparent form, but they are still of considerable commercial value.
The _carbonado_, as this kind is called, possesses so great a degree of
hardness that by means of it it is possible to bore through the hardest
rocks. The diamond drill, used for boring purposes, is furnished around
the outer edge of the cylinder of the "boring bit," as it is called, with
perhaps a dozen black diamonds, together with another row of Brazilian
diamonds on the inside. By the rotation of the boring tool the sharp
edges of the diamonds cut their way through rocks of all degrees of
hardness, leaving a core of the rock cut through, in the centre of the
cylindrical drill. It is found that the durability of the natural edge of
the diamond is far greater than that of the edge caused by _artificial_
cutting and trimming. The cutting of a pane of glass by means of a ring
set with an artificially-cut diamond, cannot therefore be done without
injuring to a slight extent the edge of the stone.

The diamond is the hardest of all known substances, leaving a scratch on
any substance across which it may be drawn. Yet it is one whose form can
be changed, and whose hardness can be completely destroyed, by the simple
process of combustion. It can be deprived of its high lustre, and of its
power of breaking up by refraction the light of the sun into the various
tints of the solar spectrum, simply by heating it to a red heat, and then
plunging it into a jar of oxygen gas. It immediately expands, changes
into a coky mass, and burns away. The product left behind is a mixture of
carbon and oxygen, in the proportions in which it is met with in
carbonic-anhydride, or, carbonic acid gas deprived of its water. This is
indeed a strange transformation, from the most valuable of all our
precious stones to a compound which is the same in chemical constituents
as the poisonous gas which we and all animals exhale. But there is this
to be said. Probably in the far-away days when the diamond began to be
formed, the tree or other vegetable product which was its far-removed
ancestor abstracted carbonic acid gas from the atmosphere, just as do our
plants in the present day. By this means it obtained the carbon wherewith
to build up its tissues. Thus the combustion of the diamond into
carbonic-anhydride now is, after all, only a return to the same compound
out of which it was originally formed. How it was formed is a secret:
probably the time occupied in the formation of the diamond may be counted
by centuries, but the time of its re-transformation into a mass of coky
matter is but the work of seconds!

There is another form of carbon which was formerly of much greater
importance than it is now, and which, although not a natural product, is
yet deserving of some notice here. Charcoal is the substance referred to.

In early days the word "coal," or, as it was also spelt, "cole," was
applied to any substance which was used as fuel; hence we have a
reference in the Bible to a "fire of coals," so translated when the
meaning to be conveyed was probably not coal as we know it. Wood was
formerly known as coal, whilst charred wood received the name of
charred-coal, which was soon corrupted into charcoal. The
charcoal-burners of years gone by were a far more flourishing community
than they are now. When the old baronial halls and country-seats depended
on them for the basis of their fuel, and the log was a more frequent
occupant of the fire-grate than now, these occupiers of midforest were a
people of some importance.

We must not overlook the fact that there is another form of charcoal,
namely, animal charcoal or bone-black. This can be obtained by heating
bones to redness in closed iron vessels. In the refining of raw sugar the
discoloration of the syrup is brought about by filtering it through
animal-charcoal; by this means the syrup is rendered colourless.

When properly prepared, charcoal exhibits very distinctly the rings of
annual growth which may have characterised the wood from which it was
formed. It is very light in consequence of its porous nature, and it is
wonderfully indestructible.

But its greatest, because it is its most useful property, is undoubtedly
the power which it has of absorbing great quantities of gas into itself.
It is in fact what may be termed an all-round purifier. It is a
deodoriser, a disinfectant, and a decoloriser. It is an absorbent of bad
odours, and partially removes the smell from tainted meat. It has been
used when offensive manures have been spread over soils, with the same
object in view, and its use for the purification of water is well known
to all users of filters. Some idea of its power as a disinfectant may be
gained by the fact that one volume of wood-charcoal will absorb no less
than 90 volumes of ammonia, 35 volumes of carbonic anhydride, and 65
volumes of sulphurous anhydride.

Other forms of carbon which are well-known are (1) coke, the residue left
when coal has been subjected to a great heat in a closed retort, but from
which all the bye-products of coal have been allowed to escape; (2) soot
and lamp-black, the former of which is useful as a manure in consequence
of ammonia being present in it, whilst the latter is a specially prepared
soot, and is used in the manufacture of Indian ink and printers' ink.




CHAPTER IV.

THE COAL-MINE AND ITS DANGERS.


It is somewhat strange to think that where once existed the solitudes of
an ancient carboniferous forest now is the site of a busy underground
town. For a town it really is. The various roads and passages which are
cut through the solid coal as excavation of a coal-mine proceeds,
represent to a stranger all the intricacies of a well-planned town. Nor
is the extent of these underground towns a thing to be despised. There is
an old pit near Newcastle which contains not less than fifty miles of
passages. Other pits there are whose main thoroughfares in a direct line
are not less than four or five miles in length, and this, it must be
borne in mind, is the result of excavation wrought by human hands and
human labour.

So great an extent of passages necessarily requires some special means of
keeping the air within it in a pure state, such as will render it fit for
the workers to breathe. The further one would go from the main
thoroughfare in such a mine, the less likely one would be to find air of
sufficient purity for the purpose. It is as a consequence necessary to
take some special steps to provide an efficient system of ventilation
throughout the mine. This is effectually done by two shafts, called
respectively the downcast and the upcast shaft. A shaft is in reality a
very deep well, and may be circular, rectangular or oval in form. In
order to keep out water which may be struck in passing through the
various strata, it is protected by plank or wood tubbing, or the shaft is
bricked over, or sometimes even cast-iron segments are sunk. In many
shafts which, owing to their great depth, pass through strata of every
degree of looseness or viscosity, all three methods are utilised in turn.
In Westphalia, where coal is worked beneath strata of more recent
geological age, narrow shafts have been, in many cases, sunk by means of
boring apparatus, in preference to the usual process of excavation, and
the practice has since been adopted in South Wales. In England the usual
form of the pit is circular, but elliptical and rectangular pits are also
in use. On the Continent polygonal-shaped shafts are not uncommon, all of
them, of whatever shape, being constructed with a view to resist the
great pressure exerted by the rock around.

[Illustration: FIG. 31.--Engine-House and Buildings at head of a
Coal-Pit.]

If there be one of these shafts at one end of the mine, and another at a
remote distance from it, a movement of the air will at once begin, and a
rough kind of ventilation will ensue. This is, however, quite
insufficient to provide the necessary quantity of air for inhalation by
the army of workers in the coal-mine, for the current thus set up does
not even provide sufficient force to remove the effete air and impurities
which accumulate from hundreds of perspiring human bodies.

It is therefore necessary to introduce some artificial means, by which a
strong and regular current shall pass down one shaft, through the mine in
all its workings, and out at the other shaft. This is accomplished in
various ways. It took many years before those interested in mines came
thoroughly to understand how properly to secure ventilation, and in
bygone days the system was so thoroughly bad that a tremendous amount of
sickness prevailed amongst the miners, owing to the poisonous effects of
breathing the same air over and over again, charged, as it was, with more
or less of the gases given off by the coal itself. Now, those miners who
do so great a part in furnishing the means of warming our houses in
winter, have the best contrivances which can be devised to furnish them
with an ever-flowing current of fresh air.

Amongst the various mechanical appliances which have been used to ensure
ventilation may be mentioned pumps, fans, and pneumatic screws. There is,
as we have said, a certain, though slight, movement of the air in the two
columns which constitute the upcast and the downcast shafts, but in order
that a current may flow which shall be equal to the necessities of the
miners, some means are necessary, by which this condition of almost
equilibrium shall be considerably disturbed, and a current created which
shall sweep all foul gases before it. One plan was to force fresh air
into the downcast, which should in a sense push the foetid air away by
the upcast. Another was to exhaust the upcast, and so draw the gases in
the train of the exhausted air. In other cases the plan was adopted of
providing a continual falling of water down the downcast shaft.

These various plans have almost all given way to that which is the most
serviceable of all, namely, the plan of having an immense furnace
constantly burning in a specially-constructed chamber at the bottom of
the upcast. By this means the column of air above it becomes rarefied
under the heat, and ascends, whilst the cooler air from the downcast
rushes in and spreads itself in all directions whence the bad air has
already been drawn. On the other hand, to so great a state of perfection
have ventilating fans been brought, that one was recently erected which
would be capable of changing the air of Westminster Hall thirty times in
one hour.

Having procured a current of sufficient power, it will be at once
understood that, if left to its own will, it would take the nearest path
which might lie between its entrance and its exit, and, in this way,
ventilating the principal street only, would leave all the many
off-shoots from it undisturbed. It is consequently manipulated by means
of barriers and tight-fitting doors, in such a way that the current is
bound in turn to traverse every portion of the mine. A large number of
boys, known as trappers, are employed in opening the doors to all comers,
and in carefully closing the doors immediately after they have passed, in
order that the current may not circulate through passages along which it
is not intended that it should pass.

The greatest dangers which await the miners are those which result, in
the form of terrible explosions, from the presence of inflammable gases
in the mines. The great walls of coal which bound the passages in mines
are constantly exuding supplies of gas into the air. When a bank of coal
is brought down by an artificial explosion, by dynamite, by lime
cartridges, or by some other agency, large quantities of gas are
sometimes disengaged, and not only is this highly detrimental to the
health of the miners, if not carried away by proper ventilation, but it
constitutes a constant danger which may at any time cause an explosion
when a naked light is brought into contact with it. Fire-damp may be
sometimes heard issuing from fiery seams with a peculiar hissing sound.
If the volume be great, the gas forms what is called a _blower_, and this
often happens in the neighbourhood of a fault. When coal is brought down
in any large volume, the blowers which commence may be exhausted in a few
moments. Others, however, have been known to last for years, this being
the case at Wallsend, where the blower gave off 120 feet of gas per
minute. In such cases the gas is usually conveyed in pipes to a place
where it can be burned in safety.

In the early days of coal-mining the explosions caused by this gas soon
received the serious attention of the scientific men of the age. In the
_Philosophical Transactions of the Royal Society_ we find a record of a
gas explosion in 1677. The amusing part of such records was that the
explosions were ascribed by the miners to supernatural agencies. Little
attention seemed to have been paid to the fact, which has since so
thoroughly been established, that the explosions were caused by
accumulations of gas, mixed in certain proportions with air. As a
consequence, tallow candles with an exposed flame were freely used,
especially in Britain. These were placed in niches in the workings, where
they would give to the pitman the greatest amount of light. Previous to
the introduction of the safety-lamp, workings were tested before the men
entered them, by "trying the candle". Owing to the specific gravity of
fire-damp (.555) being less than that of air, it always finds a lodgement
at the roofs of the workings, so that, to test the condition of the air,
it was necessary to steadily raise the candle to the roof at certain
places in the passages, and watch carefully the action of the flame. The
presence of fire-damp would be shown by the flame assuming a blue colour,
and by its elongation; the presence of other gases could be detected by
an experienced man by certain peculiarities in the tint of the flame.
This testing with the open flame has almost entirely ceased since the
introduction of the perfected Davy lamp.

The use of candles for illumination soon gave place in most of the large
collieries to the introduction of small oil-lamps. In the less fiery
mines on the Continent, oil-lamps of the well-known Etruscan pattern are
still in use, whilst small metal lamps, which can conveniently be
attached to the cap of the worker, occasionally find favour in the
shallower Scotch mines. These lamps are very useful in getting the coal
from the thinner seams, where progress has to be made on the hands and
feet. At the close of the last century, as workings began to be carried
deeper, and coal was obtained from places more and more infested with
fire-damp, it soon came to be realised that the old methods of
illumination would have to be replaced by others of a safer nature.

It is noteworthy that mere red heat is insufficient in itself to ignite
fire-damp, actual contact with flame being necessary for this purpose.
Bearing this in mind, Spedding, the discoverer of the fact, invented what
is known as the "steel-mill" for illuminating purposes. In this a toothed
wheel was made to play upon a piece of steel, the sparks thus caused
being sufficient to give a moderate amount of illumination. It was found,
however, that this method was not always trustworthy, and lamps were
introduced by Humboldt in 1796, and by Clanny in 1806. In these lamps the
air which fed the flame was isolated from the air of the mine by having
to bubble through a liquid. Many miners were not, however, provided with
these lamps, and the risks attending naked lights went on as merrily as
ever.

In order to avoid explosions in mines which were known to give off large
quantities of gas, "fiery" pits as they are called, Sir Humphrey Davy in
1815 invented his safety lamp, the principle of which can be stated in a
few words.

If a piece of fine wire gauze be held over a gas-jet before it is lit,
and the gas be then turned on, it can be lit above the gauze, but the
flame will not pass downwards towards the source of the gas; at least,
not until the gauze has become over-heated. The metallic gauze so rapidly
conducts away the heat, that the temperature of the gas beneath the gauze
is unable to arrive at the point of ignition. In the safety-lamp the
little oil-lamp is placed in a circular funnel of fine gauze, which
prevents the flame from passing through it to any explosive gas that may
be floating about outside, but at the same time allows the rays of light
to pass through readily. Sir Humphrey Davy, in introducing his lamp,
cautioned the miners against exposing it to a rapid current of air, which
would operate in such a way as to force the flame through the gauze, and
also against allowing the gauze to become red-hot. In order to minimise,
as far as possible, the necessity of such caution the lamp has been
considerably modified since first invented, the speed of the ventilating
currents not now allowing of the use of the simple Davy lamp, but the
principle is the same.

During the progress of Sir Humphrey Davy's experiments, he found that
when fire-damp was diluted with 85 per cent. of air, and any less
proportion, it simply ignited without explosion. With between 85 per
cent. and 89 per cent. of air, fire-damp assumed its most explosive form,
but afterwards decreased in explosiveness, until with 94-1/4 per cent. of
air it again simply ignited without explosion. With between 11 and 12 per
cent. of fire-damp the mixture was most dangerous. Pure fire-damp itself,
therefore, is not dangerous, so that when a small quantity enters the
gauze which surrounds the Davy lamp, it simply burns with its
characteristic blue flame, but at the same time gives the miner due
notice of the danger which he was running.

[Illustration: FIG. 32.--Gas Jet and Davy Lamp.]

With the complicated improvements which have since been made in the Davy
lamp, a state of almost absolute safety can be guaranteed, but still from
time to time explosions are reported. Of the cause of many we are
absolutely ignorant, but occasionally a light is thrown upon their origin
by a paragraph appearing in a daily paper. Two men are charged before the
magistrates with being in the possession of keys used exclusively for
unlocking their miners' safety-lamps. There is no defence. These men know
that they carry their lives in their hands, yet will risk their own and
those of hundreds of others, in order that they may be able to light
their pipes by means of their safety-lamps. Sometimes in an unexpected
moment there is a great dislodgement of coal, and a tremendous quantity
of gas is set free, which may be sufficient to foul the passages for some
distance around. The introduction or exposure of a naked light for even
so much as a second is sufficient to cause explosion of the mass; doors
are blown down, props and tubbing are charred up, and the volume of
smoke, rushing up by the nearest shaft and overthrowing the engine-house
and other structures at the mouth, conveys its own sad message to those
at the surface, of the dreadful catastrophe that has happened below.
Perhaps all that remains of some of the workers consists of charred and
scorched bodies, scarcely recognisable as human beings. Others escape
with scorched arms or legs, and singed hair, to tell the terrible tale to
those who were more fortunately absent; to speak of their own sufferings
when, after having escaped the worst effects of the explosion, they
encountered the asphyxiating rush of the after-damp or choke-damp, which
had been caused by the combustion of the fire-damp. "Choke-damp" in very
truth it is, for it is principally composed of our old acquaintance
carbonic acid gas (carbon dioxide), which is well known as a
non-supporter of combustion and as an asphyxiator of animal life.

It seems a terrible thing that on occasions the workings and walls
themselves of a coal-mine catch fire and burn incessantly. Yet such is
the case. Years ago this happened in the case of an old colliery near
Dudley, at the surface of which, by means of the heat and steam thus
afforded, early potatoes for the London market, we are told, were grown;
and it was no unusual thing to see the smoke emerging from cracks and
crevices in the rocks in the vicinity of the town.

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