According to the common observations of mankind, the eminent quality by
which coal is to be distinguished, is the burning of that substance,
or its capacity for making a fire. Therefore, however similar in other
respects, a substance which had not that eminent quality of coal could
hardly be considered as being allied to it; far less could it be
supposed, as being in every other respect the same. We are however
to endeavour to show, that there are truly substances of this kind,
substances which to common observation, having none of the properties of
coal with respect to fire, consequently, no utility for the purpose of
burning, might be considered as another species of mineral, while at the
same time they are truly at bottom a composition very little different
from those which we have considered as the most perfect coal.
It must be recollected that we have distinguished coal in general as
of two different species, one perfect or proper coal, containing no
perceptible quantity of either oil or phlegm; the other as burning with
smoke and flame, consequently containing both aqueous and oleaginous
substances which it emits in distillation.
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