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.