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 composition,
as well as that which is most impure or earthy; and we have shown that
there is a gradation, from the most bituminous state in which those
strata had been formed in being deposited at the bottom of the sea,
to the most perfect state of a chemical coal, to which they have been
brought by the operation of subterranean fire or heat.
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