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 spungy like our coaks, while in general those blind coals (as they are called) are perfectly solid in their structure.
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.
M. Struve, in the Journal de Physique for January 1790, describes a mineral which he callsplombagine charbonneuse ou hexaëdre; and gives for reason,parce qu'elle ressemble extrêmement au charbon de pierre schisteux, ou d'hexaëdre. He says farther, "Il est très commun, dans une roche qui forme un passage entre les granits et les brèches, qu'on n'a trouvée jusqu'à présent qu'on masses roulées dans le pays de Vaud." He concludes his paper thus: "Ce fossile singulier ne paroît pas appartenir à la Suisse seule. J'ai dans ce moment devant les yeux une substance parfaitement semblable, si on excepte la couleur qui tient le milieu entre le gris de fer et le rouge modéré; elle vient du pays de Gotha de la Friedrischs-grube, proche d'Umneau. On le regarde comme un eisenrahm uni à du charbon de pierre."
The specimen which I have from Stair upon the water of Ayr, so far as I can understand, perfectly resembles thisplombagineof M. Struve. It consumes very slowly in the fire, and deflagrates like plumbago with nitre. Now this comes from a regular coal stratum; and what is more remarkable, in this stratum is contained a true plumbago, Farther up the country, the Earl of Dumfries has also a mine containing plumbago along with other coal strata; and though the plumbago of these two mines have not all the softness and beauty of the mineral of the same species from Cumberland, they are nevertheless perfect plumbago.
I have a specimen of steatetical whinstone or basaltes from some part of Cumberland, in which is contained many nodules of the most perfect and beautiful plumbago. It is dispersed through this stone in rounded masses of all sizes from a nut to a pin's head; and many of these are mixed with pyrites. There is therefore reason to believe that this plumbago had been in fusion.
Now, if we consider that every species of coal and every species of plumbago are equally, that is, perfectly combustible, and yield, in burning, the same volatile principles, differing only perhaps a little in the small quantity of fixed matter which remains, we shall be inclined to believe, that they have all the same origin in a vegetable substance; and that they are diversified by some very small composition of other matter. This being allowed, one thing is certain, that it is by the operation of mineral fire or heat that those combustible substances, however composed, have been brought to their present state of coal, although we are ignorant of the circumstances by which their differences and their peculiar chemical and mechanical qualities have been produced.
Let us resume in a few words. There is not perhaps one substance in the mineral kingdom by which the operation of subterraneous heat is, to common understanding, better exemplified than that of mineral coal. Those strata are evidently a deposit of inflammable substances which all come originally from vegetable bodies. In this state of their formation, those coal strata must all be oleagenous or bituminous. In many of them, however, these volatile parts are found wanting; and, the stratum is found in the state of the most perfect coal or caput mortuum. There, is, I presume, no other means to be found by which this eminent effect could be produced, except by distillation; and, this distillation perhaps proceeded under the restraining force of an immense compression.
To this theory it must not be objected, that all the strata of coal, which are found in the same place or neighbourhood, are not reduced to that caput mortuum or perfect coaly state. The change from a bituminous to a coaly substance can only take place in proportion as the distillation of the volatile parts is permitted. Now this distillation must be permitted, if any passage can be procured from the inflammable body submitted to the operation of subterraneous heat; and, one stratum of coal may find vent for the passage of those vapours, through some crevice which is not open to another. In this way, doubtless, some of those bodies have been inspissated or reduced to a fixed coal, while others, at a little distance, have retained most of their volatile parts.
We cannot doubt of this distilling operation in the mineral regions, when we consider that in most places of the earth we find the evident effects of such distillation of oily substances in the naphta and petroleum that are constantly emitted along with water in certain springs. These oily substances are no other than such as may be procured, in a similar manner, from the fusible or inflammable coal strata; we have therefore every proof of this mineral operation that the nature of things admit of. We have also sufficient evidence that those fusible and inflammable coals, which have not been distilled to a caput mortuum, had been subjected to the operation of subterraneous heat, because we find those fusible coals subject to be injected with pyrites, as well as the more perfect coal.
If we now consider those various appearances of mineral bodies which are thus explained by the theory of mineral fire, or exertion of subterraneous heat, appearances which it is impossible to reconcile by any supposition of aqueous solution, or that unintelligible language of mineral infiltration which has of late prevailed, we shall be fully satisfied, that there is a uniform system in nature of providing a power in the mineral regions, for consolidating the loose materials deposited at the bottom of the sea, and for erecting those masses of mineralized substances into the place of land; we shall thus be led to admire the wisdom of nature, providing for the continuation of this living world, and employing those very means by which, in a more partial view of things, this beautiful structure of an inhabited earth seems to be necessarily going into destruction.