There is not perhaps a greater difference among the various qualities of bodies than that which may be observed to subsist between the burning of those two substances, that is, the inflammable bodies on the one hand, and those that are combustible on the other.  I have treated of that distinction in Dissertations upon subjects of Natural Philosophy, part 3d. where I have considered the different effects of those two kinds of bodies upon the incident light; and, in a Dissertation upon the Philosophy of Fire, etc.  I have distinguished those two kinds of substances in relation to their emitting, in burning, the fixed light which had constituted a part of those inflammable and combustible bodies.

All animals and vegetable bodies contain both those different chemical substances united; and this phlogistic composition is an essential part in every animal and vegetable substance.  There are to be found in those bodies particular substances, which abound more or less with one of those species of phlogistic matter, but never is the one species of those burning substances to be found naturally, in animal and vegetable bodies, without being associated with the other; and it is all that the chemical art can do to separate them in a great degree upon occasion.  Pure ardent spirit may perhaps be considered as containing the one, and the most perfect coal the other; the chemical principle of the one is proper carbonic matter; and of the other it is the hydrogeneous principle, or that of inflammable air.

Thus we so far understand the composition of animal and vegetable substances which burn or maintain our fires; we also understand the chemical analysis of those bodies, in separating the inflammable from the combustible substance, or the volatile from the fixed matter, the oil from what is the proper coal.  It is by distillation or evaporation, the effect of heat, that this separatory operation is performed; and we know no other means by which this may be done.  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.