hem, compressing the sheddings of a whole
forest into a thickness in some cases of a few inches of coal, and have
been acted upon by the internal heat of the earth, which has caused them
to part, to a varying degree, with some of their component gases. If we
reason from analogy, we are compelled to admit that the origin of coal is
due to the accumulation of vegetation, of which more scattered, but more
distinct, representative specimens occur in the shales and clays above
and below the coal-seams. But we are also able to examine the texture
itself of the various coals by submitting extremely thin slices to a
strong light under the microscope, and are thus enabled to decide whether
the particular coal we are examining is formed of conifers, horse-tails,
club-mosses, or ferns, or whether it consists simply of the accumulated
sheddings of all, or perhaps, as in some instances, of innumerable
spores.

In this way the structure of coal can be accurately determined. Were we
artificially to prepare a mass of vegetable substance, and covering it up
entirely, subject it to great pressure, so that but little of the
volatile gases which would be formed could escape, we might in the course
of time produce something approaching coal, but whether we obtained
lignite, jet, common bituminous coal, or anthracite, would depend upon
the possibilities of escape for the gases contained in the mass.

Everybody has doubtless noticed that, when a stagnant pool which contains
a good deal of decaying vegetation is stirred, bubbles of gas rise to the
surface from the mud below. This gas is known as marsh-gas, or light
carburetted hydrogen, and gives rise to the _ignis fatuus_ which hovers
about marshy land, and which is said to lure the weary traveller to his
doom. The vegetable mud is here undergoing rapid decomposition, as there
is nothing to stay its progress, and no superposed load of strata
confining its resulting products within itself. The gases therefore
escape, and the breaking-up of the tissues of the vegetation goes on
rapidly.

The chemical changes which have taken place in the beds of vegetation of
the carboniferous epoch, and which have transformed it into coal, are
even now but imperfectly understood. All we know is that, under certain
circumstances, one kind of coal is formed, whilst under other conditions,
other kinds have resulted; whilst in some cases the processes have
resulted in the preparation of large quantities of miner