Coal

The great mineral fuel of the world, consisting of the remains of extinct, chiefly cryptogamic plants, often of large size. See SIGIL LARI A, LEP I DO DENDRON, CALAMITES. The remains of these and of many other plants have been ma ed together, covered up and compre ed; much of their volatile matter has escaped, and the residue forms our now existing coal-measures. Some of these have evidently been formed where the plants grew, but in other cases the material seems to have been drifted together. Fireclay, containing roots, is the usual base of a coal-seam, and shale, often crowded with plant remains, frequently forms the roof. Coal-seams vary in size from the Mammoth Vein of e. Pa., which is in some places 50 ft. and more in thickne , down to mere films of coaly matter. In America at present few beds le than 30 in. are considered worth working, but in Europe beds 12 in. thick and even le are taken out. An acre of coal in the ground, 3 ft. thick, contains rather more than 5,000 tons of coal, of which in most collieries in America one-third to one-half has usually been mined, the rest having been left in the ground or destroyed in the mining. Greater economy is now being introduced and a higher percentage of coal saved in the better cla of collieries, but the waste is still enormous. This waste is largely increased by the lo in its employment as fuel. The best steam engines utilize only about one-tenth or one-eighth of the energy of the coal. A pound of coal is capable of evaporating 15 lbs. of boiling water, but in practice more than 9 lbs. of steam is seldom obtained. The rapidly diminishing store of fuel has in some countries drawn attention to this waste, and great and succe ful efforts are being made to reduce it.