REGNAULT, HENRI VICTOR (1810–1878), French chemist and physicist, was born on the 21st of July 1810 at Aix-la-Chapelle. His early life was a struggle with poverty. When a boy he went to Paris and obtained a situation in a large drapery establishment, where he remained, occupying every spare hour in study, until he was in his twentieth year. Then he entered the École Polytechnique, and passed in 1832 to the École des Mines, where he developed an aptitude for experimental chemistry. A few years later he was appointed to a professorship of chemistry at Lyons. His most important contribution to organic chemistry was a series of researches, begun in 1835, on the haloid and other derivatives of unsaturated hydrocarbons. He also studied the alkaloids and organic acids, introduced a classification of the metals according to the facility with which they or their sulphides are oxidized by steam at high temperatures, and effected a comparison of the chemical composition of atmospheric air from all parts of the world. In 1840 he was recalled to Paris by his appointment to the chair of chemistry in the École Polytechnique; at the same time he was elected a member of the Académie des Sciences, in the chemical section, in room of P. J. Robiquet (1780–1840); and in the following year he became professor of physics in the Collège de France, there succeeding P. L. Dulong, his old master, and in many respects his model. From this time Regnault devoted almost all his attention to practical physics; but in 1847 he published a four-volume treatise on Chemistry which has been translated into many languages.
Regnault executed a careful redetermination of the specific heats of all the elements obtainable, and of many compounds—solids, liquids and gases. He investigated the expansibility of gases by heat, determining the coefficient for air as 0·003665, and showed that, contrary to previous opinion, no two gases had precisely the same rate of expansion. By numerous delicate experiments he proved that Boyle's law is only approximately true, and that those gases which are most readily liquefied diverge most widely from obedience to it. He studied the whole subject of thermometry critically; he introduced the use of an accurate air-thermometer, and compared its indications with those of a mercurial thermometer, determining the absolute dilatation of mercury by heat as a step in the process. He also paid attention to hygrometry and devised a hygrometer in which a cooled metal surface is used for the deposition of moisture.
In 1854 he was appointed to succeed J. J. Ebelmen (1814–1852) as director of the porcelain manufactory at Sèvres. He carried on his great research on the expansion of gases in the laboratory at Sèvres, but all the results of his latest work were destroyed during the Franco-German War, in which also his son Henri (noticed above) was killed. Regnault never recovered from the double blow, and, although he lived until the 19th of January 1878, his scientific labours ended in 1872. He wrote more than eighty papers on scientific subjects, and he made important researches in conjunction with other workers. His greatest work, bearing on the practical treatment of steam-engines, forms vol. xxi. of the Memoires de l'Académie des Sciences.