Here is the impossibility which in the last chapter I have alleged the
aqueous theory has to struggle against; and here is one of the absolute
proofs of the igneous theory. Not only must the aqueous part of those
natural phlogistic bodies be evaporated, in order to their becoming
coal, but the oily parts must also, by a still increased degree of heat,
be evaporated, or separated by distillation from the combustible part.
Here, therefore, is evidently the operation of heat, not simply that
of fusion in contradiction to the fluidity of aqueous solution, but
in opposition to any effect of water, as requiring the absence or
separation of that aqueous substance.
But those natural appearances go still farther to confirm our theory,
which, upon all occasions, considers the compression upon the bodies
that are submitted to the operation of heat, in the mineral regions, as
having the greatest efficacy in modifying that operation. Coal strata,
which are in the neighbourhood of each other, being of those two
opposite species, the one fusible and inflammable, the other infusible
and combustible, afford the clearest proof of the efficacy of
compression; for, it is evident, that the coal, which was once
bituminous or fusible, cannot be charred without the distillation of
that substance; therefore, prevent the distillation by compression and
the charring operation cannot proceed, whatever should be the intensity
of the heat; and then, fusion alone must be the effect upon the
bituminous body.
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