DULONG, PIERRE LOUIS (1785-1838), French chemist and physicist, was born at Rouen on Feb. 12 (or 13), 1785. After acting as assistant to Berthollet, he became successively professor of chemistry at the faculty of sciences and the normal and veterin ary schools at Alfort, and then (18 20) professor of physics at the cole Polytechnique, of which he was appointed director in 1830. He died in Paris on July 18 (or 19), 1838.
His earliest work was chemical in character. In 1811 he discov ered nitrogen trichloride ; during his experiments serious explosions occurred twice, and he lost an eye, besides sustaining severe in juries to his hand. He also investigated the oxygen compounds of phosphorus and nitrogen, and was one of the first to hold the hydrogen theory of acids.
Dulong's important research work in physics was on heat and was carried out in conjunction with Alexis Therese Petit (1791 182o), the professor of physics at the Ecole Polytechnique. In 1815 they made the first accurate comparisons between the mer cury and the air thermometer. The first published research (1816) dealt with the dilatation of solids, liquids and gases and with the exact measurement of temperature, and it was followed by one in 1817 in which they showed that Newton's law of cooling was only true for small differences in temperature, and one in 1818 on the measurement of temperature and the transference of heat, which was crowned by the French Academy. In another, "On some important points in the theory of heat" (1819), they stated the "law of Dulong and Petit" dealing with atomic heats. Subsequent papers by Dulong were concerned with "New deter minations of the proportions of water and the density of certain elastic fluids" (1820, with Berzelius) ; the property possessed by certain metals of facilitating the combination of gases (1823 with Thenard) ; the refracting powers of gases (1826) ; and the specific heats of gases (1829). In 1830 he published a research, under taken with Arago for the academy of sciences, on the elasticity of steam at high temperatures. In his last paper, published post humously in 1838, Dulong gave an account of experiments made to determine the heat developed in a chemical reaction, together with a description of the calorimeter he employed. He was so badly supplied with apparatus that he spent practically all his wealth in providing what was necessary for his researches.