BITUMEN

any of various solid or semisolid mixtures of hydrocarbons that occur in nature or that are obtained as residues from the distillation of petroleum or coal. In Great Britain and continental Europe the terms bitumen and asphaltic bitumen are employed only with reference to the black or brown petroleum-like substances that are called asphalts in the United States. In its various forms, which include asphalt, petroleum, and tar, bitumen is one of the most widely distributed of substances. It occurs, in varying quantities, in nearly every part of the world and throughout the whole range of geological strata. In current terminology bitumen also may include synthetic hydrocarbon compounds. The different forms of bitumen are listed in the table. Liquid petroleum, or crude oil, is a mixture of many kinds of hydrocarbon compounds that were formed by the gradual decomposition of organic matter. The solid or very dense, highly viscous bitumens, such as asphalts, probably have all been derived from liquid petroleum, either by evaporation of the lighter, more volatile fraction under atmospheric conditions or by metamorphism occurring deep within the Earth's crust. Asphalts and other solid bitumens are fusible and soluble in carbon disulfide. They are related to but quite different from pyrobitumens, which are infusible and insoluble hydrocarbons that occur in oil shale, peat, and the various coals, including the subbituminous and bituminous forms. Asphalts are also different from asphaltites, which probably formed from sapropelic coals. The pyrobitumens, however, produce or become bitumen-like compounds when they are heated. Both bitumens and these bitumen-like compounds are employed as fuels, as roofing and paving materials, and in many other products. Compare pyrobitumens.