to the fire, the stratification of
the coal is opened gradually by the heat and expanding vapours, as a
piece of wood, of a similar shape, would be by means of wedges placed in
the end way of the timber. The coal then kindles quietly, and quickly
flames, while the mass of this bituminous schistus is opening like the
leaves of a book, and thus exhibits an appearance in burning extremely
like wood. But let the fire be applied to the middle of the bed, instead
of the edge of the leaves, and we shall see a very different appearance;
for here the expanded aqueous vapours, confined between the _laminae_,
form explosions, in throwing off splinters from the kindling mass; and
this mass of coal takes fire with much noise and disturbance.

The ashes of this coal may be determined as to quality, being in general
a subtile white earth; but, as to quantity, the measure of that earth
produces an indefinite variety in this species of coal; for, from the
kennel or parrot coal, which is valuable for its burning with much
flame, to that black schistus which our masons use in drawing upon
stone, and which, though combustible in some degree, is not thought to
be a coal, there is a perfect gradation, in which coal may be found with
every proportion of this earthy alloy.

Among the lowest species of this combustible schistus are those
argillaceous strata in Yorkshire from whence they procure alum in
burning great heaps of this stone, which also contains sulphur, to
impregnate the aluminous earth with its acid. We have also, in this
country, strata which differ from those aluminous schisti only in the
nature of the earth, with which the bituminous sediment is mixed. In
the strata now considered, the earth, precipitated with the bituminous
matter, being calcareous, has produced a limestone, which, after burning
especially, is perfectly fissile.

Therefore, with regard to the composition of mineral coal, the theory
is this. That inflammable, vegetable, and animal substances, in a
subtilised state, had subsided in the sea, being mixed more or less with
argillaceous, calcareous, and other earthy substances in an impalpable
state. Now, the chemical analysis of fossil coal justifies that theory;
for, in the distillation of the inflammable or oily coal, we procure
volatile alkali, as might be naturally expected.

Thus we have considered fossil coal as various, both in its state and
composition; we have described coal which is of the purest compos