But this is also a perfect description of that species of coal which is called in England Kennel coal, and in Scotland Parrot coal. It is so uniform in its substance that it is capable of being formed on the turning loom; and it receives a certain degree of polish, resembling bodies of jet.
Thus, we have a species of coal in which we shall find but a small degree of fusibility, although it may not be charred in any degree. Such an infusible coal may therefore contain a great deal of aqueous substance, and volatile oily matter; consequently may burn with smoke and flame. But this same species of coal may also occasionally be charred more or less by the operation of subterranean heat; and, in that case, we should have a variety of coal which could only be distinguished, from a similar state of pure bituminous coal, by the ashes which they leave in burning. At least, this must be the case, when both species are, by sufficient distillation, reduced to the state of what may be properly termed a chemical coal.
But in the natural state of its composition, we find those strata of kennel or parrot coal, possessing a peculiar property, which deserves to be considered, as still throwing more light upon the subject.
We have been representing these strata of coal as homogeneous to appearance, and as breaking indifferently in all directions; this last, perhaps, is not so accurate; for they would seem to break chiefly into two directions, that is, either parallel or perpendicular to the bed. Thus we have this coal commonly in rectangular pieces, in which it is extremely difficult to distinguish the direction of the bed, or stratification of the mass. By an expert eye, however, this may be in general, or at least sometimes, distinguished, and then, by knowing the habit of the coal in burning, a person perfectly ignorant of the philosophy of the matter may exhibit a wonderful sagacity, or even of power over future events, in applying this body to fire; for, at his pleasure, and unknown to those who are not in the secret; he may apparently, in equal circumstances, make this coal either kindle quietly, or with violent cracking and explosions, throwing its splinters at a distance.
The explanation lies in this, that, though the rectangular mass of coal appears extremely uniform in its structure, it is truly a stratified mass; it is therefore affected, by the sudden approach of fire in a very different manner, according as the edge of the stratum, which is seen in four of the sides of this supposed cube, shall be applied to the fire, or the other two sides, which are in the line of the stratum, or parallel to the bed of coal. The reason of this phenomenon now remains to be considered.
When the edge of the coal is exposed to the fire, the stratification of the coal is opened gradually by the heat and expanding vapours, as a piece of wood, of a similar shape, would be by means of wedges placed in the end way of the timber. The coal then kindles quietly, and quickly flames, while the mass of this bituminous schistus is opening like the leaves of a book, and thus exhibits an appearance in burning extremely like wood. But let the fire be applied to the middle of the bed, instead of the edge of the leaves, and we shall see a very different appearance; for here the expanded aqueous vapours, confined between the laminae, form explosions, in throwing off splinters from the kindling mass; and this mass of coal takes fire with much noise and disturbance.
The ashes of this coal may be determined as to quality, being in general a subtile white earth; but, as to quantity, the measure of that earth produces an indefinite variety in this species of coal; for, from the kennel or parrot coal, which is valuable for its burning with much flame, to that black schistus which our masons use in drawing upon stone, and which, though combustible in some degree, is not thought to be a coal, there is a perfect gradation, in which coal may be found with every proportion of this earthy alloy.
Among the lowest species of this combustible schistus are those argillaceous strata in Yorkshire from whence they procure alum in burning great heaps of this stone, which also contains sulphur, to impregnate the aluminous earth with its acid. We have also, in this country, strata which differ from those aluminous schisti only in the nature of the earth, with which the bituminous sediment is mixed. In the strata now considered, the earth, precipitated with the bituminous matter, being calcareous, has produced a limestone, which, after burning especially, is perfectly fissile.
Therefore, with regard to the composition of mineral coal, the theory is this. That inflammable, vegetable, and animal substances, in a subtilised state, had subsided in the sea, being mixed more or less with argillaceous, calcareous, and other earthy substances in an impalpable state. Now, the chemical analysis of fossil coal justifies that theory; for, in the distillation of the inflammable or oily coal, we procure volatile alkali, as might be naturally expected.
Thus we have considered fossil coal as various, both in its state and composition; we have described coal which is of the purest composition, as well as that which is most impure or earthy; and we have shown that there is a gradation, from the most bituminous state in which those strata had been formed in being deposited at the bottom of the sea, to the most perfect state of a chemical coal, to which they have been brought by the operation of subterranean fire or heat.