Proust, Joseph-Louis

Joseph-Louis Proust, also known as Luis Proust, (born Sept. 26, 1754, Angers, France—died July 5, 1826, Angers), French chemist who proved that the relative quantities of any given pure chemical compound’s constituent elements remain invariant, regardless of the compound’s source. This is known as Proust’s law, or the law of definite proportions (1793), and it is the fundamental principle of analytical chemistry. Proust also carried out important applied research in metallurgy, explosives, and nutritional chemistry.