But to what I would call more particularly the attention of mineral philosophers is this, that it is inconceivable to have this effect produced by means of water; we might as well say that heat were to be the cause of ice.  The production of coal from vegetable bodies, in which that phlogistic substance is originally produced, or from animal bodies which have it from that source, is made by heat, and by no other means, so far as we know.  But, even heat alone is not sufficient to effect that end, or make a perfect coal; the phlogistic body, which is naturally compound, consisting of both inflammable and combustible substances, must be separated chemically, and this must be the operation of heat under the proper circumstances for distillation or evaporation.

Here is the impossibility which in the last chapter I have alleged the aqueous theory has to struggle against; and here is one of the absolute proofs of the igneous theory.  Not only must the aqueous part of those natural phlogistic bodies be evaporated, in order to their becoming coal, but the oily parts must also, by a still increased degree of heat, be evaporated, or separated by distillation from the combustible part.  Here, therefore, is evidently the operation of heat, not simply that of fusion in contradiction to the fluidity of aqueous solution, but in opposition to any effect of water, as requiring the absence or separation of that aqueous substance.

But those natural appearances go still farther to confirm our theory, which, upon all occasions, considers the compression upon the bodies that are submitted to the operation of heat, in the mineral regions, as having the greatest efficacy in modifying that operation.  Coal strata, which are in the neighbourhood of each other, being of those two opposite species, the one fusible and inflammable, the other infusible and combustible, afford the clearest proof of the efficacy of compression; for, it is evident, that the coal, which was once bituminous or fusible, cannot be charred without the distillation of that substance; therefore, prevent the distillation by compression and the charring operation cannot proceed, whatever should be the intensity of the heat; and then, fusion alone must be the effect upon the bituminous body.  But now, as we have both those species of coal in the vicinity of each other, and even the same strata of coal part charred, while the rest is not, this natural appearance, so far from being a stumbling block, as it must be to the opposite theory, is most clearly explained by the partial escape of vapours from the mineral regions, and thus confirms the theory with regard to the efficacy of compression.

It is owing to the solidity of those natural charred coals, and the want of oil, that they are so very difficult of kindling; but, when once kindled in sufficient quantity, they make a fire which is very durable.  There are even some of them which, to common observation, seem to be altogether incombustible.  I have of this kind a specimen from a stratum at Stair, which shall be afterwards mentioned.