CHAPTER III.

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 beca