Therefore, wherever we
find peculiar effects of that separatory operation, we have a right to
infer the proper cause.
The subject, which we are to consider in this section, is not the
composition of strata in those of mineral coal, but the transformation
of those, which had been originally inflammable bodies, into bodies
which are only combustible, an end which is to be attained by the
separation of their volatile or inflammable substances. In the last
section, I have shown of what materials the strata of mineral coal had
been originally formed; these are substances containing abundance of
inflammable oil or bitumen, as well as carbonic matter which is properly
combustible; and this is confirmed by the generality of those strata,
which, though perfectly consolidated by fusion, retain still their
inflammable and fusible qualities. But now the object of investigation
is that mineral operation by which some of those strata, or some parts
of a fusible and inflammable stratum, have been so changed as to become
infusible and only combustible.
We have now examined those strata which may be considered as either
proper mineral coal, or as only a bituminous schistus; we are now to
class along with these another species of this kind of matter, which has
had a similar origin, although it may assume a different character.
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