]
We come now to the _second_ species of inflammable bodies called oily or
bituminous. These substances are also found variously mixed with mineral
bodies, as well as forming strata of themselves; they are, therefore, a
proper subject for a particular examination.
In the process of vegetation, there are produced oily and resinous
substances; and, from the collection of these substances at the bottom
of the ocean, there are formed strata, which have afterwards undergone
various degrees of beat, and have been variously changed, in consequence
of the effects of that heat, according as the distillation of the more
volatile parts of those bodies has been suffered to proceed.
In order to understand this, it must be considered, that, while immersed
in water, and under insuperable compression, the vegetable, oily, and
resinous substances, would appear to be unalterable by heat; and it is
only in proportion as certain chemical separations take place, that
these inflammable bodies are changed in their substance by the
application of heat. Now, the most general change of this kind is in
consequence of evaporation, or the distillation of their more volatile
parts, by which oily substances become bituminous, and bituminous
substances become coaly.
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