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Class 1. Mining and Mineral Products
Btructed as to accumulate at its base, and to keep it in a rotten fluid Rtate, the surface of the bog might be ultimately so raised, and its continuity below so totally destroyed, as to cause it to flow over the retaining obstacle, and flood the adjacent country.
In mountain districts, the progress of the phenomenon is similar. Pools, indeed, cannot in so many instances be formed, the steep slopes facilitating drainage, but the clouds and mists resting on the summits and sides of mountains, amply supply their surface with moisture, which comes, too, in the most favourable form for vegetation, not in a sudden torrent, but unceasingly and gently, drop by drop. The extent of such bags is also affected by the nature of the rock below them. On quartz they are shallow and small; on any rock yielding by its decomposition a clayey coating, they are considerable; the thickness of the bog (for example in Knocklaid, in the county of Antrim, which is 1,685 feet high) being nearly 12 feet. The summit bogs of high mountains are distinguishable from those of lower levels, by the total absence of large trees.
As turf includes a mass of plants in different stages of decomposition, its aspect and constitution vary very much. Near the surface it is light-coloured, spongy, and contains the vegetable matter but little altered; deeper it is brown, denser, and more decomposed; and finally, at the base of the greater bogs, some of which present a depth of 40 feet, the mass of turf assumes the black colour, and nearly the density of coal, to which also it approximates very much in chemical composition. The amount of ash contained in turf is also variable, and appears to increase in proportion as we descend. Thus, in the section of a bog 40 feet deep, at Timahoe, those portions near the surface contained 1 k per cent, of ashes, the centre portions 3| per cent., whilst the lowest four feet of turf, contained 19 per cent, of ashes. In the superficial layers, it may also be remarked, that the composition is nearly the same as that of wood, the vegetable material being but little altered, and in the lower we find the change into coal nearly complete. Notwithstanding these extreme variations, we may yet establish the ordinary constitution of turf with certainty enough for practical use, and, on the average specimens of turf selected from various localities, the following results have been obtained :—
The calorific power of dry turf is about half that of coal; it yields, when ignited with litharge, about fourteen times its weight of lead. This power is, however, immensely diminished in ordinary use, by the water which is allowed to remain in its texture, and of which the Hpongy character of its mass renders it very difficult to get rid. There is nothing which requires more alteration than the collection and preparation of turf; indeed, for practical purposes, this valuable fuel is absolutely spoiled as it is now prepared in Ireland. It is cut in a wet season of the year; whilst drying it is exposed to the weather; it hence is in reality not dried at all. It is very usual to find the turf of commerce containing one-fourth of its weight of water, although it then feels dry to the hand. But let us examine how that affects its calorific power. One pound of pure dry turf will evaporate 6 lbs. of water; now in 1 lb. of turf, as usually found, there are J lb. of dry turf, and 11 lb. of water. The j lb. can only evaporate 4J lbs. of water; but out of this it must first evaporate the J lb. contained in its mass, and hence the water boiled away by 1 lb. of such turf is reduced to 4| lbs. The loss is hero 30 per cent., a proportion which makes all the difference between a good fuel and one almost unfit for use. When turf is dried in the air, under cover, it still retains one-tenth of its weight of water, which reduces its calorific power 12 per cent., 1 lb. of such turf evaporating 5J lbs. of water. This effect is sufficient, however, for the great majority of objects; the further desiccation is too expensive and too troublesome to be used, except in some special cases.
The characteristic fault of turf as a fuel is its want of density, which renders it difficult to concentrate within a limited space the quantity of heat necessary for many operations. The manner of heating turf is, indeed, just
the opposite to that of anthracite. The turf yields a vast body of volatile inflammable ingredients, which pass into the flues and chimney, and thus distribute the heat of combustion over a great space, whilst in no one point is the heat intense. Hence for all flaming fires turf ia applicable, and in its application to boilers it is peculiarly useful, as there is no liability to that burning away of the metal, which may arise from the local intensity of the heat of coke or coal. If it be required, it is quite possible, however, to obtain a very intense heat with turf.
The removal of the porosity and elasticity of turf, so that it may assume the solidity of coal, has been the object of many who have proposed mechanical and other processes for the purpose. It has been found that the elasticity of the turf fibre presents great obstacles to compression, and the black turf, which is not fibrous, is, of itself, sufficiently dense.
Not merely may we utilize turf in its natural condition, or compressed, or impregnated with pitchy matter, but we may carbonize it, as we do wood, and prepare turf charcoal, the properties of which it is important to establish. The methods of carbonization are of two kinds :—1. By heating turf in close vessels; by this mode loss is avoided, but it is expensive, and there is no compensation in the distilled liquors, which do not contain acetic acid in any quantity. The tar is often small in quantity, and the gases are deficient in illuminating power, hence the charcoal is the only valuable product. Its quantity varies from 30 to 40 per cent, by weight of the dry turf. The products of the distillation of 1,157 lbs. of turf were found by Blavier to be—charcoal, 474 lbs., or 41 percent. ; watery liquid, 226 lbs., or 19'3 per cent.; gaseous matter, 450 lbs., or 39 per cent.; and tar, 7 lbs., or <> per cent.; but the proportion of tar is variable, sometimes reaching to 24-5 per cent, when coked in close vessels.
The economical carbonization of turf is best carried on in heaps, in the same manner as that of wood. The gods must be regularly arranged, and laid as close as possible; they are the better for being large—15 inches long, by 6 broad, and 5 deep. The heaps, built hetuispherically, should be smaller in size than the heaps of wood usually are. In general 5,000 or fi,0i>0 large soda may go to a heap, which will thus contain 1,500 cubic feet. The mass must be allowed to heat more than is necessary for wood, and the process requires to be very carefully attended to, from the extreme combustibility of the charcoal. The quantity of charcoal obtained in this mode of carbonization is from 25 to 30 per cent, of the weight of dry turf.
The charcoal so obtained ia very light and very inflammable; it possesses nearly the volume of the turf. It usually burns with a light flame, as the volatile matters are not totally expelled. This is shown by the composition of a specimen analyzed with the following result :—
Oxygen and nitrogen . . 4-20
For many industrial uses the charcoal so prepared is too light, as, generally speaking, it is only with fuels of considerable density that the most intense heat can be produced; but by coking compressed turf, it has been already shown, that the resulting charcoal may attain a density of 1,040, which is far superior to that of wood charcoal, and even equal to that of the best coke made from coal. As to calorific effect, turf charcoal is about the same as coal cokes, and little inferior to wood charcoal.
It is peculiarly important, in the preparation of the charcoal from turf, that the material should be selected as free as possible from earthy impurities, for all such are concentrated in the coke, which may be thereby rendered of ,:Hle comparative value. Hence the coke from