VARIOUS FORMS OF COAL AND CARBON.
In considering the various forms and combinations into which coal enters,
it is necessary that we should obtain a clear conception of what the
substance called "carbon" is, and its nature and properties generally,
since this it is which forms such a large percentage of all kinds of
coal, and which indeed forms the actual basis of it. In the shape of
coke, of course, we have a fairly pure form of carbon, and this being
produced, as we shall see presently, by the driving off of the volatile
or vaporous constituents of coal, we are able to perceive by the residue
how great a proportion of coal consists of carbon. In fact, the two have
almost an identical meaning in the popular mind, and the fact that the
great masses of strata, in which are contained our principal and most
valuable seams of coal, are termed "carboniferous," from the Latin
_carbo_, coal, and _fero_, I bear, tends to perpetuate the existence of
the idea.
There is always a certain, though slight, quantity of carbon in the air,
and this remains fairly constant in the open country. Small though it may
be in proportion to the quantity of pure air in which it is found, it is
yet sufficient to provide the carbon which is necessary to the growth of
vegetable life. Just as some of the animals known popularly as the
_zoophytes_, which are attached during life to rocks beneath the sea, are
fed by means of currents of water which bring their food to them, so the
leaves, which inhale carbon-food during the day through their
under-surfaces, are provided with it by means of the currents of air
which are always circulating around them; and while the fuel is being
taken in beneath, the heat and light are being received from above, and
the sun supplies the motive power to digestion.
It is assumed that it is, within the knowledge of all that, for the
origin of the various seams and beds of coaly combinations which exist in
the earth's crust, we must look to the vegetable world. If, however, we
could go so far back in the world's history as the period when our
incandescent orb had only just severed connection with a
gradually-diminishing sun, we should probably find the carbon there, but
locked up in the bonds of chemical affinities with other elements, and
existing therewith in a gaseous condition. But, as the solidifying
process went on, and as the vegetable world afterwards made its
appearance, the carbon became, so to speak, wrenched from its
combinations, and being absorbed by trees and plants, finally became
deposited amongst the ruins of a former vegetable world, and is now
presented to us in the form of coal.
We are able to trace the gradual changes through which the pasty mass of
decaying vegetation passed, in consequence of the fact that we have this
material locked up in various stages of carbonisation, in the strata
beneath our feet. These we propose to deal with individually, in as
unscientific and untechnical a manner as possible.
First of all, when a mass of vegetable matter commences to decay, it soon
loses its colour. There is no more noticeable proof of this, than that
when vitality is withdrawn from the leaves of autumn, they at once
commence to assume a rusty or an ashen colour. Let the leaves but fall to
the ground, and be exposed to the early frosts of October, the damp mists
and rains of November, and the rapid change of colour is at once
apparent. Trodden under foot, they soon assume a dirty blackish hue, and
even when removed they leave a carbonaceous trace of themselves behind
them, where they had rested. Another proof of the rapid acquisition of
their coaly hue is noticeable in the spring of the year. When the trees
have burst forth and the buds are rapidly opening, the cases in which the
buds of such trees as the horse-chestnut have been enclosed will be found
cast off, and strewing the path beneath. Moistened by the rains and the
damp night-mists, and trodden under foot, these cases assume a jet black
hue, and are to all appearance like coal in the very first stages of
formation.
But of course coal is not made up wholly and only of leaves. The branches
of trees, twigs of all sizes, and sometimes whole trunks of trees are
found, the last often remaining in their upright position, and piercing
the strata which have been formed above them. At other times they lie
horizontally on the bed of coal, having been thrown down previously to
the formation of the shale or sandstone, which now rests upon them. They
are often petrified into solid sandstone themselves, whilst leaving a
rind of coal where formerly was the bark. Although the trunk of a tree
looks so very different to the leaves which it bears upon its branches,
it is only naturally to be supposed that, as they are both built up after
the same manner from the juices of the earth and the nourishment in the
atmosphere, they would have a similar chemical composition. One very
palpable proof of the carbonaceous character of tree-trunks suggests
itself. Take in your hand a few dead twigs or sticks from which the
leaves have long since dropped; pull away the dead parts of the ivy which
has been creeping over the summer-house; or clasp a gnarled old monster
of the forest in your arms, and you will quickly find your hand covered
with a black smut, which is nothing but the result of the first stage
which the living plant has made, in its progress towards its condition as
dead coal. But an easy, though rough, chemical proof of the constituents
of wood, can be made by placing a few pieces of wood in a medium-sized
test-tube, and holding it over a flame. In a short time a certain
quantity of steam will be driven off, next the gaseous constituents of
wood, and finally nothing will be left but a few pieces of black brittle
charcoal. The process is of course the same in a fire-grate, only that
here more complete combustion of the wood takes place, owing to its being
intimately exposed to the action of the flames. If we adopt the same
experiment with some pieces of coal, the action is similar, only that in
this case the quantity of gases given off is not so great, coal
containing a greater proportion of carbon than wood, owing to the fact
that, during its long burial in the bowels of the earth, it has been
acted upon in such a way as to lose a great part of its volatile
constituents.
From processes, therefore, which are to be seen going on around us, it is
easily possible to satisfy ourselves that vegetation will in the long run
undergo such changes as will result in the formation of coal.
There are certain parts in most countries, and particularly in Ireland,
where masses of vegetation have undergone a still further stage in
metamorphism, namely, in the well-known and famous peat-bogs. Ireland is
_par excellence_ the land of bogs, some three millions of acres being
said to be covered by them, and they yield an almost inexhaustible supply
of peat. One of the peat-bogs near the Shannon is between two and three
miles in breadth and no less than fifty in length, whilst its depth
varies from 13 feet to as much as 47 feet. Peat-bogs have in no way
ceased to be formed, for at their surfaces the peat-moss grows afresh
every year; and rushes, horse-tails, and reeds of all descriptions grow
and thrive each year upon the ruins of their ancestors. The formation of
such accumulations of decaying vegetation would only be possible where
the physical conditions of the country allowed of an abundant rainfall,
and depressions in the surface of the land to retain the moisture. Where
extensive deforesting operations have taken place, peat-bogs have often
been formed, and many of those in existence in Europe undoubtedly owe
their formation to that destruction of forests which went on under the
sway of the Romans. Natural drainage would soon be obstructed by fallen
trees, and the formation of marsh-land would follow; then with the growth
of marsh-plants and their successive annual decay, a peaty mass would
collect, which would quickly grow in thickness without let or hindrance.
In considering the existence of inland peat-bogs, we must not lose sight
of the fact that there are subterranean forest-beds on various parts of
our coasts, which also rest upon their own beds of peaty matter, and very
possibly, when in the future they are covered up by marine deposits, they
will have fairly started on their way towards becoming coal.
Peat-bogs do not wholly consist of peat, and nothing else. The trunks of
such trees as the oak, yew, and fir, are often found mingled with the
remains of mosses and reeds, and these often assume a decidedly coaly
aspect. From the famous Bog of Allen in Ireland, pieces of oak, generally
known as "bog-oak," which have been buried for generations in peat, have
been excavated. These are as black as any coal can well be, and are
sufficiently hard to allow of their being used in the manufacture of
brooches and other ornamental objects. Another use to which peat of some
kinds has been put is in the manufacture of yarn, the result being a
material which is said to resemble brown worsted. On digging a ditch to
drain a part of a bog in Maine, U.S., in which peat to a depth of twenty
feet had accumulated, a substance similar to cannel coal itself was
found. As we shall see presently, cannel coal is one of the earliest
stages of true coal, and the discovery proved that under certain
conditions as to heat and pressure, which in this case happened to be
present, the materials which form peat may also be metamorphosed into
true coal.
Darwin, in his well-known "Voyage in the _Beagle_" gives a peculiarly
interesting description of the condition of the peat-beds in the Chonos
Archipelago, off the Chilian coast, and of their mode of formation. "In
these islands," he says, "cryptogamic plants find a most congenial
climate, and within the forest the number of species and great abundance
of mosses, lichens, and small ferns, is quite extraordinary. In Tierra
del Fuego every level piece of land is invariably covered by a thick bed
of peat. In the Chonos Archipelago where the nature of the climate more
closely approaches that of Tierra del Fuego, every patch of level ground
is covered by two species of plants (_Astelia pumila_ and _Donatia
megellanica_), which by their joint decay compose a thick bed of elastic
peat.
"In Tierra del Fuego, above the region of wood-land, the former of these
eminently sociable plants is the chief agent in the production of peat.
Fresh leaves are always succeeding one to the other round the central
tap-root; the lower ones soon decay, and in tracing a root downwards in
the peat, the leaves, yet holding their places, can be observed passing
through every stage of decomposition, till the whole becomes blended in
one confused mass. The Astelia is assisted by a few other plants,--here
and there a small creeping Myrtus (_M. nummularia_), with a woody stem
like our cranberry and with a sweet berry,--an Empetrum (_E. rubrum_),
like our heath,--a rush (_Juncus grandiflorus_), are nearly the only ones
that grow on the swampy surface. These plants, though possessing a very
close general resemblance to the English species of the same genera, are
different. In the more level parts of the country the surface of the peat
is broken up into little pools of water, which stand at different
heights, and appear as if artificially excavated. Small streams of water,
flowing underground, complete the disorganisation of the vegetable
matter, and consolidate the whole.
"The climate of the southern part of America appears particularly
favourable to the production of peat. In the Falkland Islands almost
every kind of plant, even the coarse grass which covers the whole surface
of the land, becomes converted into this substance: scarcely any
situation checks its growth; some of the beds are as much as twelve feet
thick, and the lower part becomes so solid when dry that it will hardly
burn. Although every plant lends its aid, yet in most parts the Astelia
is the most efficient.
"It is rather a singular circumstance, as being so very different from
what occurs in Europe, that I nowhere saw moss forming by its decay any
portion of the peat in South America. With respect to the northern limit
at which the climate allows of that peculiar kind of slow decomposition
which is necessary for its production, I believe that in Chiloe (lat. 41°
to 42°), although there is much swampy ground, no well characterised peat
occurs; but in the Chonos Islands, three degrees farther southward, we
have seen that it is abundant. On the eastern coast in La Plata (lat.
35°) I was told by a Spanish resident, who had visited Ireland, that he
had often sought for this substance, but had never been able to find any.
He showed me, as the nearest approach to it which he had discovered, a
black peaty soil, so penetrated with roots as to allow of an extremely
slow and imperfect combustion."
The next stage in the making of coal is one in which the change has
proceeded a long way from the starting-point. _Lignite_ is the name which
has been applied to a form of impure coal, which sometimes goes under the
name of "brown coal." It is not a true coal, and is a very long way from
that final stage to which it must attain ere it takes rank with the most
valuable of earth's products. From the very commencement, an action has
being going on which has caused the amount of the gaseous constituents to
become less and less, and which has consequently caused the carbon
remaining behind to occupy an increasingly large proportion of the whole
mass. So, when we arrive at the lignite stage, we find that a
considerable quantity of volatile matter has already been parted with,
and that the carbon, which in ordinary living wood is about 50 per cent.
of the whole, has already increased to about 67 per cent. In most
lignites there is, as a rule, a comparatively large proportion of
sulphur, and in such cases it is rendered useless as a domestic fuel. It
has been used as a fuel in various processes of manufacture, and the
lignite of the well-known Bovey Tracey beds has been utilised in this way
at the neighbouring potteries. As compared with true coal, it is
distinguished by the abundance of smoke which it produces and the choking
sulphurous fumes which also accompany its combustion, but it is largely
used in Germany as a useful source of paraffin and illuminating oils. In
Silesia, Saxony, and in the district about Bonn, large quantities of
lignite are mined, and used as fuel. Large stores of lignite are known to
exist in the Weald of the south-east of England, and although the mining
operations which were carried on at one time at Heathfield, Bexhill, and
other places, were failures so far as the actual discovery of true coal
was concerned, yet there can be no doubt as to the future value of the
lignite in these parts, when England's supplies of coal approach
exhaustion, and attention is turned to other directions for the future
source of her gas and paraffin oils.
Beside the Bovey Tracey lignitic beds to which we have above referred,
other tertiary clays are found to contain this early promise of coal. The
_eocene_ beds of Brighton are an important instance of a tertiary
lignite, the seam of _surturbrand_, as it is locally called, being a
somewhat extensive deposit.
We have now closely approached to true coal, and the next step which we
shall take will be to consider the varieties in which the black mineral
itself is found. The principal of these varieties are as follows, against
each being placed the average proportion of pure carbon which it
contains:--
Splint or Hard Coal, 83 per cent.;
Cannel, Candle or Parrott Coal, 84 per cent.;
Cherry or Soft Coal, 85 per cent.;
Common Bituminous, or Caking Coal, 88 per cent.;
Anthracite, Blind Coal, Culm, Glance, or Stone Coal, from South
Wales, 93 per cent.
As far as the gas-making properties of the first three are concerned, the
relative proportions of carbon and volatile products are much the same.
Everybody knows a piece of cannel coal when it is seen, how it appears
almost to have been once in a molten condition, and how it breaks with a
conchoidal fracture, as opposed to the cleavage of bituminous coal into
thin layers; and, most apparent and most noticeable of all, how it does
not soil the hands after the manner of ordinary coal. It is at times so
dense and compact that it has been fashioned into ornaments, and is
capable of receiving a polish like jet. From the large percentage of
volatile products which it contains, it is greatly used in gasworks.
Caking coal and the varieties of coal which exist between it and
anthracite, are familiar to every householder; the more it approaches the
composition of the latter the more difficult it is to get it to burn, but
when at last fairly alight it gives out great heat, and what is more
important, a less quantity of volatile constituents in the shape of gas,
smoke, ammonia, ash and sulphurous acid. For this reason it has been
proposed to compel consumers to adopt anthracite as _the_ domestic coal
by Act of Parliament. Certainly by this means the amount of impurities in
the air might be appreciably lessened, but as it would involve the
reconstruction of some millions of fire-places, and an increase in price
in consequence of the general demand for it, it is not likely that a
government would be so rash as to attempt to pass such a measure; even if
passed, it would probably soon become as dead and obsolete and impotent
as those many laws with which our ancestors attempted, first to arrest,
and then to curb the growth in the use of coal of any sort. Anthracite is
not a "homely" coal. If we use it alone it will not give us that bright
and cheerful blaze which English-speaking people like to obtain from
their fires.
It is a significant fact, and one which proves that the various kinds of
coal which are found are nothing but stages begotten by different degrees
of disentanglement of the contained gases, that where, as in some parts,
a mass of basalt has come into contact with ordinary bituminous coal, the
coal has assumed the character of anthracite, whilst the change has in
some instances gone so far as to convert the anthracite into graphite.
The basalt, which is one of the igneous rocks, has been erupted into the
coal-seam in a state of fusion, and the heat contained in it has been
sufficient to cause the disentanglement of the gases, the extraction of
which from the coal brings about the condition of anthracite and
graphite.
The mention of graphite brings us to the next stage. Graphite, plumbago,
or, as it is more commonly called, black-lead, which, we may say in
passing, has nothing of lead about it at all, is best known in the shape
of that very useful and cosmopolitan article, the black-lead pencil. This
is even purer carbon than anthracite, not more than 5 per cent. of ash
and other impurities being present. It is well-known by its grey metallic
lustre; the chemist uses it mixed with fire-clay to make his crucibles;
the engineer uses it, finely powdered, to lubricate his machinery; the
house-keeper uses it to "black-lead" her stoves to prevent them from
rusting. An imperfect graphite is found inside some of the hottest
retorts from which gas is distilled, and this is used as the negative
element in zinc and carbon electricity-making cells, whilst its use as
the electrodes or carbons of the arc-lamp is becoming more and more
widely adopted, as installations of electric light become more general.
One great source of true graphite for many years was the famous mine at
Borrowdale, in Cumberland, but this is now almost exhausted. The vein lay
between strata of slate, and was from eight to nine feet thick. As much
as £100,000 is said to have been realised from it in one year. Extensive
supplies of graphite are found in rocks of the Laurentian age in Canada.
In this formation nothing which can undoubtedly be classed as organic has
yet been discovered. Life at this early period must have found its home
in low and humble forms, and if the _eozoön_ of Dawson, which has been
thought to represent the earliest type of life, turns out after all not
to be organic, but only a deceptive appearance assumed by certain of the
strata, we at least know that it must have been in similarly humble forms
that life, if it existed at all, did then exist. We can scarcely,
therefore, expect that the vegetable world had made any great advance in
complexity of organism at this time, otherwise the supplies of graphite
or plumbago which are found in the formation, would be attributed to
dense forest growths, acted upon, after death, in a similar manner to
that which awaited the vegetation which, ages after, went to form beds of
coal. At present we know of no source of carbon except through the
intervention and the chemical action of plants. Like iron, carbon is
seldom found on the earth except in combination. If there were no growth
of vegetation at this far-away period to give rise to these deposits of
graphite, we are compelled to ask ourselves whether, perchance, there did
not then exist conditions of which we are not now cognisant on the earth,
and which allowed graphite to be formed without assistance from the
vegetable kingdom. At present, however, science is in the dark as to any
other process of its formation, and we are left to assume that the
vegetable growth of the time was enormous in quantity, although there is
nothing to show the kind of vegetation, whether humble mosses or tall
forest trees, which went to constitute the masses of graphite. Geologists
will agree that this is no small assumption to make, since, if true, it
may show that there was an abundance of vegetation at a time when animal
life was hidden in one or more very obscure forms, one only of which has
so far been detected, and whose very identity is strongly doubted by
nearly all competent judges. At the same time there _may_ have been an
abundance of both animal and vegetable life at the time. We must not
forget that it is a well-ascertained fact that in later ages, the minute
seed-spores of forest trees were in such abundance as to form important
seams of coal in the true carboniferous era, the trees which gave birth
to them being now classed amongst the humble _cryptogams_, the ferns, and
club-mosses, &c. The graphite of Laurentian age may not improbably have
been caused by deposits of minute portions of similar lowly specimens of
vegetable life, and if the _eozoön_ the "dawn-animalcule," does represent
the animal life of the time, life whose types were too minute to leave
undoubted traces of their existence, both animal life and vegetable life
may be looked upon as existing side by side in extremely humble forms,
neither as yet having taken an undoubted step forward in advance of the
other in respect to complexity of organism.
[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 £150 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° 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.