PROUST, JOSEPH LOUIS (1754-1826), French chemist, was born on Sept. 26, 1754, at Angers, where his father was an apothecary. After beginning the study of chemistry in his father's shop he came to Paris and became apothecary in chief to the Salpetriere, also lecturing on chemistry at the musee of the aero naut J. F. PiI5,tre de Rozier, whom he accompanied in a balloon ascent in 1784. Next, he went to Spain, where he taught chemistry first at the artillery school of Segovia, and then at Salamanca, finally becoming in 1789 director of the royal laboratory at Madrid. In 18o8 he lost both his position and his money by the fall of his patron (Charles IV.), and retired first to Craon in Mayenne and then to Angers, where he died on July 5, 1826. Proust's great contribution to chemistry was his establishment of the fundamental principle of the constant composition of many compounds. On this subject he maintained a long controversy with C. L. Berthollet, who was led by his doctrine of mass-action to deny that substances always combine in constant and definite proportions. Proust, on the other hand, maintained that com pounds always contain definite quantities of their constituent elements, and that in cases where two or more elements unite to form more than one compound, the proportions in which they are present vary per saltum, not gradually. In 1799 he proved
that copper carbonate, whether natural or artificial, always has the same composition, and later he showed that the two oxides of tin and the two sulphides of iron always contain the same rela tive weights of their components and that no intermediate in determinate compounds exist. His analytical skill enabled him to demonstrate the inaccuracy of the researches by which Ber thollet attempted to support the opposite view, and to show among other things that some of the compounds which Berthollet treated as oxides were in reality hydrates containing chemically combined water, and the upshot was that by 1808 he had fully vindicated his position. Proust also investigated the varieties of sugar that occur in sweet vegetable juices, distinguishing three kinds, and he showed that the sugar in grapes, of which he an nounced the existence to his classes at Madrid in 1799, is identical with that obtained from honey by the Russian chemist 3. T. Lowitz (1757-1804).