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 South Wales, the Kilkenny coal of Ireland, and the blind
coal of Scotland, notwithstanding that these are a perfect coal, or
charred to a coal, have nothing of the porous construction of the
specimen which I have just now mentioned; they are perfectly solid, and
break with a smooth shining surface like those which emit smoke and
flame.
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