Thus it will appear that, as this quality of being perfectly
charred is not originally in the constitution of the stratum, but an
accident to which some strata of every species may have been subjected,
we could not class them by this property without confounding together
strata which had differences in their composition or formation.
Therefore, we are led to inquire after some other distinction, which may
be general to strata of fossil coal, independent of those changes which
this substance may have undergone after it had been formed in a stratum.
Perfect mineral coal being a body of undistinguishable parts, it is only
in its resolution that we may analyse it, and this is done by burning.
Thus, in analysing coal by burning, we have, in the ashes alone, that by
which one species of coal may be distinguished from another; and, if we
should consider pure coal as having no ashes of itself, we should then,
in the weight of its ashes, have a measure of the purity of the coal,
this being inversely as the quantity of the ashes. Now, though this be
not accurately true, as the purest coal must have some ashes proper to
itself, yet, as this is a small matter compared with the quantity of
earthy matter that may be left in burning some species of coal, this
method of analysis may be considered as not far removed from the truth.
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