Proust, Joseph Louis 1754-1826
French analytical chemist

Born in Angers, he studied pharmacy and chemistry in Paris, and spent most of his working life in Spain. In the early 1780s he conducted aerostatic experiments with Pilatre de Rozier and Jacques-Alexandre-C�sar Charles, and in 1784 was one of the first people to make an ascent in a balloon. He was appointed Professor of Chemistry at the Royal Artillery College at Segovia and director of the Royal Laboratories at Madrid (1789-1808), after which he returned to France. Proust made two significant advances in analytical chemistry: he developed the use of hydrogen sulphide as a reagent and he gave the results of his analyses in terms of percentage weights. By means of the percentages he realized that the proportions of the constituents in any chemical compound are always the same regardless of what method is used to prepare it. He announced this discovery, known as the 'law of definite proportions', in 1794. Not all his contemporaries accepted his findings, his principal adversary in a renowned controversy being Claude Louis Berthollet. Although Proust was correct in his observations, the reason why reagents behave in this way did not become clear until John Dalton formulated his atomic theory in 1803.