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 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.

Here is therefore a mineral operation in the preparation of those coals which we cannot imitate; and here is the clearest evidence of the operation of mineral fire or heat, although we are ignorant of the reason why some coal strata are charred, while others are not, and why, in some particular cases, the charred coal may be porous or spongy like our coals, while in general those blind coals (as they are called) are perfectly solid in their structure.