As to the surface of the ground under which coal is found, it is much doubted if any peculiar character ex ists, (as has been advanced by some mineralogists) in Great Britain. It is found under every variety of sur face, excepting the wild and rocky mountains. It is, however, to be remarked, that a great part of the valua ble and extensive coal field of South Wales, is found in a mountain district, and the strata of the coal field form these mountains even to their summits, which are elevat ed not only very much above the level of the sea, but also above the adjoining country. These hills are smooth and covered with verdure, and present no rocky cliffs, on account of their comparatively soft strata. Hence the existence of coal in any district of country, not mountain ous, cannot be deduced from the appearance of the sur face, but rather from the relative situation of the par ticular district with respect to the rocks of the primitive or secondary class.
Of coals there are various species, viz.
1. Cubical Coal—Is black, shining, compact, and mo derately hard ; when wrought, it turns out in quadran gular masses, and when broken small, the fragments are cubical. The lamellated structure, or what is termed the reed of the coal, is always parallel to the bed or strata upon which it rests, as is the general case of all coals, with a very few exceptions. Of this coal there are two kinds, the caking coal, and the open burning coal. The caking coal of the best kinds, however small when kindled, undergoes a semi-fusion, and unites into a solid mass. The open burning coal burns more rapidly than the former, with much flame and heat ; the caking is very abundant in England, the other is abundant both in England and Scotland. The cubical coals, which unite when set on fire, are known by the name of caking' or smithy coals ; the open burning coals by the name of rough coal, cheery coal, and clod coal.
The \Vhitehaven caking coals, in the west of England, according to Kirwan, contain 2. Slate Coal—ls black, compact, and much harder than the first species ; dull in the colour ; in working turns out in large quadrangular masses, which can with ease be split into very thin pieces, similar to slates, hence the name. This coal is open burning, and produces much flame and smoke ; it contains a greater proportion of white ashes than the cubical coals. Some of the inferior kinds of this coal burn with difficulty, and produce a great quantity of white ashes. This coal is commonly known by the name of splint coal, and, according to Kirwan, contains 3. Glance Coal—Is black, with bright metallic lustre, and brittle ; it has the external characters of the preced ing species of coals, but the chemical properties of char coal, hence termed by some mineralogists native mineral carbon. It burns with considerable heat, with a blue
lambent flame, produces no smoke, and leaves a small proportion of ashes ; it appears to have a portion of sul phur in direct combination, so that in burning it gives out a most suffocating effluvia, it produces no soot, but on the contrary whitens the places where the fumes are condensed. This kind of coal abounds in Ireland, and is known by the name of Kilkenny coal ; it is the blind coal of Scotland, and the malting, or stone coal of Wales. This coal approximates to the graphite or plumbago species, and in some instances passes into it. From the trial of several specimens, it contains of carbon from 72.05 to 90.00. Specific gravity, 1.526.
4. Cannel Coal—Is black, with little lustre ; in work ing turns out four-sided coluinnar, breaks with a con choidal fracture, in any direction, like pitch, ignites very easily, and burns with a very bright flame like a candle, front which it derives its name. It is found very abundant in the coal field at Wigan, in Lancashire, where there is an entire stratum of it about four feet thick, without a mixture of any other coal. It produces levy little dust in working, and does not soil the lingers as the other coals do ; varieties of this coal arc found in Scotland, forming frequently part of the thickness of the common coals. What is termed pitch coal by some mineralogists, is only a variety of this species. The cannel coal, when first kindled, decrepitates very much, throws the burning splinters to a distance, and is on this account dangerous ; it is said that this fault is corrected by immersing the coals for some time in water, previous to their being used for fuel. In Scotland this coal is termed parrot coal, or bottle coal. . • 5. Sulphureous Coal is of a dull black colour, mixed with a great proportion of pyrites. Though pyrites-is found in distinct pieces in coal, this occurrence does not constitute it a sulphureous t oat, because the pyrites can easily be picked out ; but sulphureous coal, properly so called, has the pyrites so interwoven with its texture in small veins, or so intimately combined with the principles of the coal, that the sulphureous parts cannot be separat ed. It is very unsafe to work such coals, because, if any quantity of the small coals produced in working is left below ground, a decomposition ensues, heat is generated, and the mine is set on fire, of which there have been many instances. I‘lien laid in heaps above ground the same result ensues, hence it is very unsafe to be put on board a vessel for a voyage of any considerable length.