PROUST, JOSEPH LOUIS (1754–1826), French chemist, was born on the 26th of September 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 gained the appointment of apothecary in chief to the Salpetrière, also lecturing on chemistry at the musée of the aeronaut J. F. Pilâtre de Rozier, whom he accompanied in a balloon ascent in 1784. Next, at the instance of Charles IV., 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 1808 he lost both his job and his money by the fall of his patron, and retired first to Craon in Mayenne and then to Angers, where he died on 5th of July 1826. His name is best known in connexion with a long controversy with C. L. Berthollet. The latter chemist 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 compounds 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 carbonate of copper, whether natural or artificial, always has the same composition, and later showed that the two oxides of tin and the two sulphides of iron always contain the same relative weights of their components and that no intermediate indeterminate compounds exist. His analytical skills enabled him to demonstrate the inaccuracy of the researches by which Berthollet 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 announced the existence to his classes at Madrid, is identical with that obtained from honey by the Russian chemist J. T. Lowitz (1757–1804).
Besides papers in scientific periodicals he published Indagaciones sobre el estañada de cobre, la vajilla de estaño y el vidriado (1803); Mémoire sur le sucre de raisins (1808); Recueil des mémoires relatifs à la poudre à canon (1815); and Essai sur une des causes qui peuvent amener la formation du calcul (1824).