Bituminous coal is the typical, the representative coal; and from a good specimen of this variety we may learn two important facts:—
(1) That true coals, no less than peat, are of vegetable origin. To see this we must look at the flat or charcoal surfaces of the coal. These soil the fingers like charcoal, and usually show the vegetable forms distinctly.
(2) That coals are stratified rocks. These dirty charcoal surfaces always coincide with the stratification, being merely the successive layers of vegetation deposited and pressed together to build up the coal; and when we look at the edge of the specimen the stratification shows plainly enough.
The bitumens form a similar though less perfect series, beginning with the organic tissues, and ending, in the opinion of some of the best chemists and mineralogists, with diamond. In fact the coals and bitumens form two distinct but parallel series. The coals are exclusively of vegetable origin, while the bitumens are largely of animal origin. The organic tissues in which the two series originate are chemically similar,—the animal tissues, which produce the lighter forms of bitumen, however, containing more hydrogen and less carbon and oxygen than vegetable tissues; while the final terms, as just shown, are probably chemically identical, being pure carbon,—graphite for the coals and diamond for the bitumens; so that the entire process of change in each series is essentially carbonization, a gradual elimination of the gaseous elements, oxygen and hydrogen, until pure solid carbon alone remains.
The principal differences between the coals and bitumens are the following:—
Coals are rich in carbon, with some oxygen and little hydrogen.
Bitumens are rich in hydrogen, with some carbon and little or no oxygen.
Bitumens are soluble in ether, benzole, turpentine, etc., and the solid forms are soluble in the more fluid, naphtha-like varieties.
Coals are never liquid, and cannot be melted or, with trifling exceptions, even softened by heat.
Many bitumens are naturally liquid, and all become so on the application of heat.
The coals partake of the characteristics of their chief constituent element, carbon, the most thoroughly solid substance known; while the bitumens similarly show the influence of hydrogen, the most perfectly fluid substance known.