PROUST, JOSEPH-LOUIS

born Sept. 26, 1754, Angers, Fr. died July 5, 1826, Angers French chemist who proved (1808) that the relative quantities of any given pure chemical compound's constituent elements remain invariant, regardless of the compound's source, and thus provided crucial evidence in support of John Dalton's law of definite proportions, which holds that elements in any compound are present in fixed proportion by weight. Chief apothecary at the Salptrire Hospital, Paris, he became director of the Royal Laboratory at Madrid (17991806), where he determined quantitatively the elemental constitution of a great variety of chemical substances. His proof of the principle of the constant composition of chemical compounds resolved in his favour a long-standing dispute with the French chemist Claude-Louis Berthollet.