Joseph Louis 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 establish the importance of chemical formulas. Proust's work contributed significantly to the understanding of chemical composition and reactions. He conducted experiments that demonstrated how different substances combine to form compounds, influencing future scientists like John Dalton and Dmitri Mendeleev. Proust passed away on July 5, 1826, leaving a lasting legacy in the field of chemistry.