COAL, in mineralogy, a kind of solid inflammable substance, supposed to be of a bituminous nature, and commonly used for fuel. Of this substance there are various species.
1. Pit-coal (Lithanthrax), is a black, solid, compact,
past, brittle mass, of moderate hardness, lamellated structure, more or less shining, but seldom capable of a good polish; and does not melt when heated. According to Kirwan, it consists of petrol or asphaltum, intimately mixed with a small portion of earth chiefly argillaceous; seldom calcareous; and frequently mixed with pyrites. A red tincture is extracted from it by spirit of wine, but caustic alkali attacks the bituminous part. From some sorts of it a varnish may be made by means of fat oils. Fixed alkali has never been found in any kind of it, nor sulphur, unless when it happens to be mixed with pyrites.—None of the various kinds are found to be electrics per se (A).
The varieties of lithanthrax, enumerated by Cronstedt, are, 1. With a small quantity of argillaceous earth and vitriolic acid. It is of a black colour, and shining texture: it burns, and is mostly consumed in the fire, but leaves, however, a small quantity of ashes. 2. Slaty coal.
2. Culm coal, called kolm, by the Swedes, has a greater portion of argillaceous earth and vitriolic acid, with a moderate proportion of petrol. It has the same appearance with the foregoing, though its texture is more dull: it burns with a flame, without being consumed, but leaves behind it a flag of the same bulk with the original volume of the coal. The following is Mr Kirwan's description of it from the memoirs of the Stockholm academy. "Its fracture has a rougher section than the cannel coal; its specific gravity from 1300 to 1370. The best kind affords by distillation, at first fixed air, then an acid liquor, afterwards inflammable air, and a light oil of the nature of petrol; then a volatile alkali; and lastly pitch-oil. The residuum is nearly three quarters of the whole; and being slowly burnt, affords 13 per cent. of ashes, which consist mostly of argillaceous earth; and about three hundredth parts of them are magnetic. It is found in England, and among some aluminous ores in Sweden."
3. Slate-coal contains such a quantity of argillaceous earth, that it looks like common slate; however, it burns by itself with a flame. M. Magellan is of opinion that this is the bituminous substance already described (see CLAY, p. 51.) This schistus is of a dark bluish rusty colour; when thrown on the fire it burns with a lively flame, and almost as readily as the oily wood of dry olive tree, or lignum vitæ; emitting the very disagreeable smell of petrol. Such large quarries
of it are found near Purbeck in Dorsetshire, that the poorer part of the inhabitants are thence supplied with fuel. From the appearance of this slaty coal, Cronstedt has been induced to suppose that the earth of all kinds of coal is argillaceous, though it is not so easy to distinguish it after being burnt. The pit-coals, he says, contain more or less of the vitriolic acid; for which reason the smoke arising from them attacks silver in the same manner as sulphur does, let the coals be ever so free from marcasite, which, however, is often imbedded or mixed with them.
4. Cannel coal (Ampelites), is of a dull black colour; breaks easily in all directions; and, if broken transversely, presents a smooth conchoidal surface. It burns with a bright lively flame, but is very apt to fly in pieces in the fire; however it is said to be entirely deprived of this property by immersion in water for some hours previous to its being used. It contains a considerable quantity of petrol in a less condensed state than other coals. Its specific gravity is about 1.270. This kind of coal, being of an uniform hard texture, is easily turned on a lathe, and takes a good polish. Hence it is used for making various toys, which appear almost as well as if made of the finest jet.
5. Kilkenny coal is the lightest of any; its specific gravity being only about 1.400. It contains the largest quantity of asphaltum; burns with less smoke and flame, and more intensely, though more slowly, than the cannel-coal. The quantity of earth it contains does not exceed one twentieth part of its weight; but this kind of coal is frequently mixed with pyrites. It is found in the county of Kilkenny, belonging to the province of Leinster in Ireland. The quality of it as burning without smoke, is proverbially used as an encomium on the county.
6. Sulphureous coal consists of the former kinds mixed with a very considerable portion of pyrites; whence it is apt to moulder and break when exposed to the air, after which water will act upon it. It contains yellow spots that look like metal; burns with a sulphureous smell, leaving behind it either flag or sulphureous ashes, or both. Its specific gravity is 1.500 or more.
7. Bovey coal (Xylanthrax), is of a brown or brownish black colour, and of a yellow laminar texture. Its laminae are frequently flexible when first dug, though they generally harden when exposed to the
(A) "The varieties of this coal (says Mr Magellan) are very numerous according to the different substances with which it is mixed; but in regard to their economical uses, only two kinds are taken notice of by the British legislature, viz. culm and caking coals. The caking coals, in burning, show an incipient fusion, so that their smallest pieces unite in the fire into one mass; by which means the smallest pieces, and even the mere dust of this kind, are almost equally valuable with the largest pieces. The other sort called culm, does not fuse or unite in the fiercest fire; so that the small coal, being unfit for domestic purposes, can only be used in burning limestone.
"It should be an easy matter for any person to distinguish culm from small caking coal, either by trying to make fire with it in a common grate, without interposing any other fuel between it; when if it kindles, it is a caking coal; if not, it is culm: Or by putting some of these small fragments of coal on an ignited iron shovel; if they melt and run together, they belong to the caking kinds; if not, they are culm. But it seems that coal merchants are now in the custom of calling culm the powdery parts of pit-coal, of whatsoever kind they may happen to be. The reason of this is, that there is a difference in the duty payable by culm and by caking coals. There never was any difficulty, however, on the subject; nor would there be any difficulty in collecting the tax, were it not for the insufferable ignorance and love of despotic oppression which generally pervades the underling officers of the revenue."
Coal. the air. It consists of wood penetrated with petrol or bitumen, and frequently contains pyrites, alum, and vitriol. According to the German chemists its ashes contain a little fixed alkali; but Mr Mills differs from them on this subject. By distillation it yields a fetid liquor mixed with a volatile alkali and oil; part of which is soluble in spirit of wine, and part of a mineral nature, and insoluble. It is found in almost all the countries of Europe.
when made on a large scale. These ovens are so contrived, as to admit an under supply of air; and the coals, after being kindled, decompose themselves by a slow but incomplete combustion, which does not destroy the ingredients. The residuum left in the oven proves to be most excellent cinders or coaks; whilst the volatile parts, which otherwise would be dissipated in the air, are separated and condensed in reservoirs, or receptacles of capacious size, placed at proper distances beyond the reach of fire. Monf. Faujas de St Fond, who visited these works in a trip he made to Scotland, undertook to erect a similar kind of oven in France: and it is rather singular, that he endeavours to establish a claim of having discovered the same process before he saw them in Scotland; as if it did not reflect a greater honour on his industry, to carry back to his country some useful knowledge, than to return as ignorant as our English travellers," &c.
On subjecting pit-coal of any kind to distillation in close vessels, it first yields a phlegm or watery liquor; then an ethereal or volatile oil; afterwards a volatile alkali; and lastly, a thick and greasy oil: but it is remarkable, that, by rectifying this last oil, a transparent thin and light oil of a straw colour is produced, which being exposed to the air becomes black like animal oils. From this and other observations, the general opinion is, that all coals, bitumens, and other oily substances found in the mineral kingdom, derive their origin from vegetables buried in the earth; since it is well known that only organized bodies have the power of producing oily and fat substances. "The amazing irregularities, gaps, and breaks (says M. Magellan) of the strata of coals, and of other fossil substances, evince that this globe has undergone the most violent convulsions, by which its parts have been broken, detached and overturned in different ways, burying large tracts of their upper surfaces, with all the animal and vegetable productions there existing, at the time of those horrible catastrophes, whose epoch far precedes all human records. And it is easy to be conceived, that the various heaps and congeries of these vegetable and animal substances, remaining for ages and ages in the bowels of the earth, have obtained various consistencies, and still produce those oily and bituminous juices, which find way to gush out, leaving behind their thickest parts on the same places where they are found, and in many others where the industry of mankind never will be able to penetrate."
Coal-Mine. See COALERY.—Malleiciously setting fire to coal-mines is felony, by stat. 10. Geo. II. c. 32. § 6.
Small Coal. a sort of charcoal prepared from the spray and brushwood stripped off from the branches of coppice wood, sometimes bound in bairns for that purpose, and sometimes charred without binding, in which case it is called "coming it together."