Thomas Midgley, Jr., (born May 18, 1889, Beaver Falls, Pennsylvania, U.S.—died November 2, 1944, Worthington, Ohio), American engineer and chemist who discovered the effectiveness of tetraethyl lead as an antiknock additive for gasoline. He also found that dichlorodifluoromethane (a type of fluorocarbon commercialized under the trade name Freon-12) could be used as a safe refrigerant.

The son of an immigrant inventor from London, Midgley grew up in Pennsylvania, New Jersey, and Columbus, Ohio. After graduating from Cornell University with a degree in mechanical engineering (1911), he worked in Dayton, Ohio, as a draftsman and designer at the National Cash Register Company (where only a few years earlier Charles F. Kettering had developed the first electric cash register) and then at his father’s automobile tire factory. In 1916 he went to work for Kettering, first in the research staff of the Dayton Engineering Laboratories Company (Delco) and then for the Dayton Metals Products Company, and he stayed with the latter company after it became a research arm of the General Motors Corporation in 1919.

Midgley’s research on the problem of engine knock made him a pioneer in the study of internal combustion. Starting his research in 1916, he quickly found that engine knock in the new high-compression gasoline engines for automobiles was caused not by the ignition system but by the fuel mixture, which did not burn evenly. Discovering by chance that a liquid compound called ethyl iodide reduced knock, he dedicated himself to finding an effective additive that would improve the combustion of gasoline and could be produced economically using the technology of the time. His efforts were interrupted by research for the U.S. war effort during World War I, when he worked on developing control systems for a propeller-driven “aerial torpedo” and on producing high-performance airplane fuel. At war’s end he resumed his search for a gasoline additive, systematically working his way through promising element groups in the periodic table, and in 1921 he and his team found that minute amounts of tetraethyl lead completely eliminated engine knock. Finding that the lead formed deposits in the engine, Midgley settled on using ethylene bromide as a compound that would cause the lead to be completely expelled in the engine’s exhaust. This led to the problem of finding an economical source of bromine, and here Midgley is credited with developing a method for extracting the element from seawater.

The toxic effects of lead were well known at the time, and at one point Midgley had to withdraw from his research for a short time to recover from lead poisoning. Still, he never wavered in his conviction that tetraethyl lead could be produced safely and that the small amounts of lead particulates expelled in engine exhaust would not pose a threat to public health. As vice president of the Ethyl Gasoline Corporation from 1923, he promoted the new leaded gasoline to a skeptical public and regulators. From 1933 he was also vice president of the Ethyl-Dow Chemical Company, which produced bromine from seawater.

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In 1930 Midgley was directed to find an odour-free, nontoxic, and nonflammable refrigerant gas that could be used in residential refrigerators and air conditioners. Within three days he settled on dichlorodifluoromethane, which was soon commercially produced as Freon-12 by Kinetic Chemicals, Inc., for which Midgley served as director. From 1940 to 1944 he was a director and vice president of the Ohio State University Research Foundation. For his chemical discoveries he received many honours, including four medals from the American Chemical Society and election to the National Academy of Sciences.

During World War II Midgley served as head of one branch of the National Defense Research Committee; he also conducted extensive research on the composition of natural and synthetic rubbers. In 1940 he was struck by polio. Though he lost the use of his legs, he continued to work. He died by strangulation in a hoist mechanism that he had invented to help him in and out of his bed.