A mixture of C4 to C12 hydrocarbons. Natural gasoline, obtained by fractional distillation of petroleum, contains mostly saturated hydrocarbons; commercial grades of motor gasoline contain paraffins, olefins, naphthenes, and aromatics, all in substantial concns. Motor gasolines are made chiefly by cracking processes, in which heavier petr fractions are converted into more volatile fractions by thermal or catalytic decompn; have also been made commercially by catalytic high-pressure hydrogenation of soft coal and by catalytic synthesis of hydrocarbons from carbon monoxide and hydrogen. Some gasolines sold in the U.S. contain a minor proportion of tetraethyllead added to motor gasoline to increase the octane numbers and thereby prevent “knock” in engines in which the gasoline is used as fuel. Knock is the audible manifestation of an excessive rate of pressure rise when the gasoline vapor is ignited under compression in an engine. The relative knocking tendencies of gasolines are measured in terms of “Octane Number,” which is defined as the percentage of iso-octane, having “100 Octane No.,” to be blended with n-heptane, having “0 Octane No.” by definition, in order to obtain the same degree of knock as is obtained with the gasoline being rated, under standard conditions in a standardized test engine. Additives such as ethanol, methanol, benzene, toluene, MTBE and MMT, q.q.v., are replacing tetraethyllead, and only a small percentage of leaded gasoline is sold in the U.S. Review: J. C. Lane in Kirk-Othmer Encyclopedia of Chemical Technology vol. 11 (Wiley-Interscience, New York, 3rd ed., 1984) pp 652-695. Review of toxicity: N. K. Weaver, Ann. N.Y. Acad. Sci. 534, 441-451 (1988); of carcinogenicity: M. A. Mehlman, Toxicol. Ind. Health 7, 143-152 (1990); of toxicology and human exposure: Toxicological Profile for Gasoline (PB95-264206, 1995) 224 pp. Symposium on toxicology, exposure and health effects: Environ. Health Perspect. 101, Suppl. 6, 1-212 (1993).