Closely similar to coal in chemical matter–that is to say, consisting chiefly of definite chemical compounds, called hydrocarbons, built up of only two elements, carbon and hydrogen, and of no other–is a very remarkable class of mineral substances known to the ancients as "bitumen." In its widest sense, it includes "natural gas," the variously mixed liquids called "petroleum" and the solid "asphalts." In ancient times the more fluid kinds of petroleum issuing from the ground in South Russia and Persia were called "naphtha," and that name is still applied to the more volatile hydrocarbons obtained by the distillation of such substances as coal-tar (the residue of the extraction by heat of commercial gas from coal), bituminous shale, petroleum, wood and some other bodies which owe their existence to the activity either of living or of long-extinct and "fossilized" plants and animals. The bitumens, together with coal, present in their natural state a very large variety of inflammable constituents–gaseous, liquid, and solid hydrocarbons; but, when "distilled" at various temperatures and under conditions determined by the manufacturing chemist, they yield a still larger series of pure separable bodies, which have been minutely studied and classified according to their chemical constitution. They are produced in great chemical factories in large quantities for use in the most diverse ways invented by human ingenuity. Thus natural gas–superseded by distilled coal-gas–has served for fuel and for illumination: refined petroleum serves not only for those uses in general, but as the special source of power in the engines of motor-cars and aeroplanes. A wonderful solid crystalline wax-like substance, paraffin, as white as snow, is distilled in enormous quantities (nearly three million tons a year) from "bituminous shale" or "oil-shale" in this country alone. It can be obtained in soft (vaseline) and liquid forms, and in fact the "paraffin series" recognized by chemists starts from the gas "methane," or marsh-gas, and comprises some thirty kinds, leading from gas to volatile liquids, thence to viscid liquids, to butter-like solids, and up to hard crystalline substances which melt only at the temperature of boiling water. Endless chemical manufacturing industries–_e.g._, those of dye-stuffs and explosives–depend upon the chemical treatment of these paraffins and of various bodies obtained as secondary products in their preparation. Benzine and aniline are chiefly obtained from coal-tar. The oils and waxes of quasi-mineral origin have a great advantage over vegetable and animal oils in many uses, since they are not liable to become "rancid"; that is to say, to decompose owing to the action on them of bacteria. A marked difference between the paraffins (often distinguished, together with the "olefines," as "mineral" oils) and the oils and fats found in living plants and animals is that they do not