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.

We have now examined those strata which may be considered as either
proper mineral coal, or as only a bituminous schistus; we are now to
class along with these another species of this kind of matter, which has
had a similar origin, although it may assume a different character.

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. It is the first of these
which we are now to consider more particularly, in order to see the
varieties which may be found in this species of mineral substance.

When that bituminous fossil, which is the common coal of this country,
is submitted to heat it is subject to melt more or less, and emits smoke
which is composed of water and oil. If it be thus completely distilled,
it becomes a perfect coal of a porous or spongy texture. Such a
substance as this is extremely rare among minerals; I have however found
it. It is in the harbour of Ayr, where a whinstone dyke traverses the
coal strata, and includes some of that substance in the state of coals
or cinder. I pointed this out many years ago to Dr Black; and lately I
showed it to Professor Playfair.

But the culm of S