Joseph Proust was a French chemist born on September 26, 1754. He is best known for formulating the law of definite proportions, which states that a chemical compound always contains its component elements in fixed ratio by mass. This principle laid the groundwork for modern chemistry and helped scientists understand the composition of substances. Proust's work was significant during the late 18th and early 19th centuries, a time when chemistry was evolving rapidly. His contributions influenced later chemists, including John Dalton, who further developed atomic theory. Proust passed away on July 5, 1826, leaving a lasting legacy in the field of chemistry.