Proust, Joseph Louis (1754-1826) was a French chemist who became known for helping prove the idea that every pure chemical compound consists of elements in a definite proportion. Today, scientists accept this idea as the law of constant proportions. In Proust's day, chemists disagreed on whether the proportion of elements in a compound was definite. Claude-Louis Berthollet, an influential French chemist, debated Proust for years on the subject in the early 1800's. Chemists accepted Proust's evidence as correct by about 1808.

Proust was born on Sept. 26, 1754, in Angers, the capital of Anjou, in the Brittany-Normandy Hills region of France. He studied pharmacy and chemistry in Paris, becoming chief pharmacist at the Salpětrière Hospital in Paris. In his 30's, Proust moved to Spain, where he would spend most of his working life. There, he taught chemistry in several universities and experimented in many areas of chemistry. He was a professor of chemistry at the Royal Artillery College at Segovia and the director of the royal laboratories at Madrid from the last decade of the 1700's to the early 1800's. He then returned to France.

Proust is best known for two major advances in analytical chemistry. First, he developed the use of hydrogen sulfide as a reagent (a substance used to detect the presence of other substances by the chemical reactions it causes). Hydrogen sulfide is a colorless, extremely poisonous gas with a sweetish taste and a strong odor of rotten eggs. Chemical compounds containing sulfur produce hydrogen sulfide when they react with certain other chemical compounds. This is why the odor of hydrogen sulfide can be detected around decaying organic matter, when tarnish is removed from silver, or in the exhaust of some cars. Hydrogen sulfide is flammable and burns with a pale blue flame. It dissolves slightly in water, forming a weak acid called hydrosulfuric acid. Chemists make hydrogen sulfide in the laboratory by combining such strong acids as hydrochloric acid with such metal sulfides as iron sulfide. They use the gas to analyze the composition of mixtures and to produce other compounds.

The second major advance Proust made was to give results of his chemical analyses in terms of percentage weights. By doing this, he recognized that the proportions of the components of a chemical compound are always the same, no matter what method he used to prepare the compound. Proust announced this discovery in 1794, although many chemists of his day did not accept this finding. Berthollet became his main opponent in what became a famous controversy.

Although Proust was correct in his observations, the reason why reagents behave in the way he described did not become clear until English chemist John Dalton formulated his atomic theory in 1803. According to Dalton's theory, a fixed number of atoms of one substance always combined with a fixed number of atoms of another substance in forming a compound. Dalton realized that substances must combine in the same proportions by weight as the weight proportions of their atoms. Other chemists had already observed that pure substances do combine in fixed proportions. They called that finding the law of definite (or constant) proportions. Dalton's theory explained the law.

In the early 1780's, Proust conducted experiments in aerostatics with French scientist Jean F. Pilatre de Rozier and French chemist Jacques Alexandre Cesar Charles. Aerostatics is the branch of physics that deals with the static equilibrium of air and other gases, and of solid objects suspended or moving in them. In 1784, Proust became one of the first people to ascend in a hot-air balloon when he flew with Pilatre de Rozier in Versailles.

The year before, Pilatre de Rozier had made the first human flight in a hot-air balloon, which had been invented by the French Montgolfier brothers, Jacques Étienne and Joseph Michel. In 1785, Pilatre de Rozier attempted to cross the English Channel in a balloon. Within minutes of liftoff, the balloon caught fire, plunged to the earth, and took Pilatre de Rozier to his death. Pilatre de Rozier was both the first person to fly in a hot-air balloon and the first to die in one.

In other research, Proust developed ways of obtaining sugar from grapes (he was the first to identify the sugar that comes from grapes as glucose) and did other research on foods.