It is dispersed through this stone in rounded masses
of all sizes from a nut to a pin's head; and many of these are mixed
with pyrites. There is therefore reason to believe that this plumbago
had been in fusion.
Now, if we consider that every species of coal and every species of
plumbago are equally, that is, perfectly combustible, and yield, in
burning, the same volatile principles, differing only perhaps a little
in the small quantity of fixed matter which remains, we shall be
inclined to believe, that they have all the same origin in a vegetable
substance; and that they are diversified by some very small composition
of other matter. This being allowed, one thing is certain, that it is by
the operation of mineral fire or heat that those combustible substances,
however composed, have been brought to their present state of coal,
although we are ignorant of the circumstances by which their differences
and their peculiar chemical and mechanical qualities have been produced.
Let us resume in a few words. There is not perhaps one substance in the
mineral kingdom by which the operation of subterraneous heat is, to
common understanding, better exemplified than that of mineral coal.
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