(1754-1826) French Chemist
Joseph-Louis Proust’s most important achievement was his formulation of the law of constant proportions, which held that the elements making up any compound—regardless of how that compound is prepared—are present in a fixed proportion of weight. The significance of this concept became apparent in 1808, when JOHN DALTON published his atomic theory that served as a foundation for modern physical science. Proust’s discovery was aided by his analytical skills and persistent adherence to quantitative measures.
Born on September 26, 1754, in Angers, France, Proust was the second son of Joseph and Rosalie Sartre Proust. His father was an apothecary, who encouraged young Joseph to pursue the same career. Proust received his early education from his godparents. He married Anne Rose Chatelain Daugbigne in 1798; the couple had no children.
Proust attended the local Oratorian college in Angers, then served as an apprentice to his father. In 1774, against his parents’ wishes, Proust opted to continue his education in Paris, where he studied chemistry with Hilaire-Martin Rouelle. While there he also absorbed the teaching of Clerembourg, an apothecary.
In 1776 Proust was named chief apothecary at the Salpetriere Hospital in Paris. He moved to Spain in 1778 to become a professor of chemistry at the recently founded Real Seminario Patriotico Vascongado in Vergara. But he remained there for only a short time before returning to France in 1780. He taught chemistry in France then returned to Spain, where he ultimately was appointed a professor of chemistry at the Royal Artillery College in Segovia in 1788. Eleven years later he was selected to head a new chemical laboratory in Madrid. That same year Proust discovered glucose, which he called “grape sugar” because he distilled it from that fruit. This finding was significant, as glucose proved to be a cheaper and more reliable source of sugar than cane, which had to be imported from the West Indies.
As head of the laboratory in Madrid, Proust had access to a first-rate scientific facility, which provided him the opportunity to refine his law of constant proportions. Whereas his counterpart Claude-Louis Berthollet maintained that chemical compounds could be formed from elements in a wide range of proportions, Proust firmly asserted that they contained elements only in certain proportions, for example, though he did not conceive of it in these terms, Proust’s law predicts that a molecule of water must always comprise two hydrogen and one oxygen atom, rather than some indeterminate or variable quantity of those two elements. Berthollet and Proust had a long and good-natured dispute about the issue of proportions.
After Proust returned to France in 1806, Napoleon’s army invaded Spain. In the ensuing political dislocation Proust’s laboratory was sacked. In 1810 the French government commissioned him to establish a “grape sugar” factory, and in 1820 Proust took over his brother’s pharmacy in Angers. He remained in France until his death.
The significance of Proust’s law of proportion was confirmed by JOHN DALTON’s atomic theory, which postulated that a definite number of atoms joined to form molecules. Late in his life Proust received official recognition. In 1816 he was elected to the Institut de France and three years later became a member of the Legion of Honor. He was granted a pension by Louis XVIII in 1820.