Page:The American Cyclopædia (1879) Volume IV.djvu/738

726 house, and of slugs and snails in the garden; it lives in the same region as the other species.  COAL, a black, opaque, inflammable substance, generally hard and compact, though laminated and stratified in beds between layers forming the crust of our earth. Coal has become one of the essential elements of modern civilization; in fact, the progress of the civilization of a country is now recorded by the amount of coal obtainable and employed by the inhabitants in a given time.—Mineral coal is a compound especially of carbon or of decomposed woody matter, with inflammable substances and hydrogen and oxygen gases. According to the different proportions of the volatile matter, in common language inaccurately called bitumen, the coal has a somewhat different aspect, flames more or less rapidly and actively, and develops heat in different degrees. These differences have served as a basis for a kind of classification of the coals, which, though scarcely limitable in its divisions, is generally admitted for common use. The more essential of these divisions are the following: 1. Anthracite or glance coal, a very hard, compact, lustrous, grayish black mineral, breaking in conchoidal fracture, though still bearing evidence of its original lamination. It burns slowly, with little or scarcely any flame, producing a high degree of heat. On account of the minute proportion of volatile matter in its composition, the coal is also called non-bituminous. When this coal is somewhat less dense, and has an increasing amount of volatile matter, it burns with more flame, and is then semi-anthracite. 2. Bituminous coal, though still hard, breaks more easily and more irregularly, often dividing into large cubic pieces in the plane of stratification and by cleavage. It is generally quite black, still with some lustre, contains less carbon with a larger proportion of inflammable substances than anthracite, and therefore takes fire more easily and rapidly, and burns with a bright yellow flame, developing less heat. The amount of volatile combustible matter in its composition is extremely variable, and therefore its appreciable characters, either in its value as a combustible material or in its appearance, vary in the same degree, considerably blending the classification and multiplying its names. With a moderate proportion of inflammable gases it is dry coal; with more bitumen it becomes fat coal, which passes to caking coal when in burning the matter softens and coalesces like paste. Of the moderately bituminous coals, the best known in America is called semi-bituminous, of which very large quantities are produced from the Cumberland district of Maryland, and the Broad Top, Clearfield, and Blossburg districts of Pennsylvania, along the S. E. margin of the Alleghany coal field. Of the more highly bituminous coals the most valuable is the splint or block coal of N. W. Pennsylvania, Ohio, and Indiana, which owing to its peculiar structure can be used in its raw state in the blast furnace. 3. Cannel coal is also a kind of bituminous coal. It differs much from the numerous other varieties by its fine, equal, compact, homogeneous texture, resembling a dusky black paste hardened to a mineral substance or to stone. It breaks therefore with a conchoidal fracture, and is at once distinguished from the other kinds of bituminous coal by its equal, non-laminated structure, or the absence of those horizontal thin layers which in the common kinds of bituminous coal are seen alternating in different degrees of lustre and apparent density. By distillation it yields a larger proportion of mineral oil than any other coal. Sometimes it is so highly bituminous, as in the case of the Breckinridge coal in Kentucky, that it is dangerous to use it in steamboats, or in grates through which the oil percolates when inflamed. It burns like candles, and hence its name.

—If the more marked characters which indicate the several species of mineral coal are easily recognized at first sight, and if everybody knows the bituminous coal from the anthracite or the cannel, it is not the less certain that, considering the matter in itself and in its compounds, coal is an indivisible whole. Not only have all the kinds of coal the same constituent chemical elements, merely varying in proportion in a slight degree, but all the varieties of coal, of bituminous especially, are found in some localities in the same vein. Anthracite passes to semi-anthracite, and this to bituminous coal, by inappreciable degrees. The coal beds of Shamokin and Trevorton in Pennsylvania give anthracite and semi-anthracite. The Spadra coal of Arkansas is semi-anthracite at one place and bituminous at another. In Kentucky some veins have one half of their thickness bituminous, the other half cannel; or at other localities, as for example on the Louisa river, the miners work bituminous laminated coal at one