COALS AND COAL MINING. We shall here treat in succession of the formation of coal, the coal fields, coal mining, and the coal trade,—giving a few brief details under each heading.
Formation of Coal.—From the presence of tree-ferns, and palms, and other plants which now grow in tropical climates, in coal, it has been inferred that at the period at which the coal-beds were deposited the temperature of these parts of the world at least was much greater than it is at present. If the facts we actually possess arc soberly examined, they amount to these :— 1. That coal is of vegetable origin.
2. That at the period of its deposit the earth was covered with a rich vegetation, of which only a small portion has been preserved, and that of this portion all the species and several of the races are totally unknown at the present day.
3. That the climate may possibly have been something milder than it now is, but that there is no evidence in the vegetable king dom to show that it was materially different from that of the present day.
From these data it is assumed that coal is the result of vegetable substances which have been pressed down through unknown ages into a compact mass, which has passed through the intermediate stage of peat or bog, and become gradually consolidated into coal.
The report of Sir H. De la Beche and Dr. Lyon Playfair, on the quality of coal best suited for the steam navy, contains much valuable information on the different kinds of coal, and their heating effect. One kind will raise steam very quickly; another is slower in its action, but generates a greater abundance of steam. Two kinds may be equal in these respects, yet one would speedily crumble by stowage, while the other would remain un broken. Some kinds occupy much less bulk for a given weight than others. All these points being of importance, it has been the desire of the Admiralty to ascertain which kind of coal possesses the greatest number of advantages, and the fewest disadvantages, in respect to steam navigation. The commis sioners constructed apparatus, especiadly cal culated to further the enquiry; and experi mented on thirty varieties of coal. Among the data ascertained were—the number of pounds of water evaporated from an initial temperature of 212°, by one pound of coal; weight of one cubic foot of the coal, as used for fuel, and as calculated from the specific gravity ; space in cubic feet occupied by one ton; cohesive power of the coal; weight of water evaporated from by one cubic foot; rate of evaporation, or weight of water evapo rated in one hour; chemical constituents of the coal, in carbon, hydrogen, nitrogen, sul phur, oxygen, and ash ; per centage of coke after coking ; weight of water that can be raised from 32° to 212° by the heat of one pound of coal; per centage of the various pro ducts of combustion, in coke, tar, water, am monia, carbonic acid, sulphuretted hydrogen, olefiant gas, &c.; and the force generated or weight that could be raised to the height of one foot by the combustion of one pound of coal.
The commissioners show that fuel for war steamers ought to raise steam quickly ; to possess high evaporative powers ; to contain but little bitumen; to possess considerable cohesion among the particles ; to have density and closeness of structure; to contain but little sulphur ; and to have no liability to decay during storage. The kinds brought to the
test of experiment shewed, as might be ex pected, very different results under these seve ral headings ; insomuch that no one kind stood first on the list in respect to more than two or three of the above qualities. Three kinds of patent fuel, formed by admixtures of coal dust with bituminous and other sub stances, were included among the varieties examined; and it was found that Warlich's patent fuel stood higher than any kinds of coal in respect to many qualities. The com missioners expressed an opinion that it might , be practicable to produce a composite coal, or a mixture of many kinds of coal, which would possess a more useful aggregate of qualities than those possessed by any one sort of coal per se; and they recommend further experi ments as a means of putting this plan to the test. As an example of their mode of stating the evidence for and against any one species of coal, we may take Anthracite :—" It has a very high evaporative power ; but not being easily ignited, it is not suited for quick action. It has great cohesion in its particles, and is not easily broken up by attrition ; hut it is not a caking coal, and therefore would not cohere in the furnace when the ship rolled in a gale of wind. It emits no smoke ; but from the intensity of its combustion it causes the iron of the bars and boilers to oxidate or wear away rapidly." Coal F ields.—T hese are large accumulations of coal which exist in various parts of the world. Coal is found in these fields in strata of various thickness, alternating with slate clay and sandstone ; the alternations being frequently and indefinitely repeated. The coal beds, which are of various qualities, are principally distinguished by the proportion of bitumen in the coal. The coal-seams, toge ther with their alternating strata, called the coal-measures, usually lie on beds of millstone grit and shale (hard coarse-grained sandstone and slate clay), which sometimes exceed 120 fathoms in thickness. Under this series is the mountain or carboniferous limestone, an assemblage of calcareous strata, of variable thickness, sometimes exceeding 900 feet. The carboniferous limestone rests on a bed of old red sandstone, varying in thickness from 200 to 2000 feet. These four different series of strata are usually comprehended under the term ' coal-formation' The coal-fields of Great Britain are very numerous and deserve to he studied somewhat in detail. The Northumberland and Durham coal-field commences near the mouth of the river Coquet on the north, and extends nearly to the Tees on the south. Its greatest length is 58 miles, and its greatest breadth about 24. The beds of coal in some places appear at the surface, while in the opposite direction they are at great depths. The beds of these coal measures are about 80 in number, and con sist of alternating beds of coal, sandstone, and slate-clay; making an aggregate thick ness of 1820 feet, which varies however in different parts. The aggregate thickness of coal is supposed to be about 44 feet, of which about 30 feet are workable. This coal is shipped in immense quantities from the Tyne, the Wear, and the Tees. There is another coal-formation, which-extends through parts of Northumberland, and Cumberland, independent of the former.