But, in distinguishing fossil coal by this species of chemical analysis,
not only is there to be found a perfect or indefinite gradation from a
body which is perfectly combustible to one that is hardly combustible in
any sensible degree, we should also fall into an inconveniency similar
to that already mentioned, of confounding two things extremely different
in their nature, a bituminous body, and a perfect charcoal. Thus, if we
shall found our distinction upon the fusibility and different degree of
having been charred, we shall confound fossil coals of very different
degrees of value in burning, or of very different compositions as
strata; if, again, we found it upon the purity of composition, in
judging from the ashes, we shall confound fossil bodies of very
different qualities, the one burning with much smoke and flame, the
other without any; the one fusible almost like wax, the other fixed and
infusible as charcoal.
It will now appear, that what cannot be done in either the one or other
of those two methods, may in a great degree, or with considerable
propriety, be performed in employing both.
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