Who formulated the law of definite proportions around 1794?

Joseph Proust formulated the law of definite proportions, also called Proust’s law or the law of constant composition. It states that a pure chemical compound contains its constituent elements in a fixed mass ratio, regardless of its source or method of preparation.

For example, every sample of pure water contains hydrogen and oxygen in the same overall mass ratio: roughly one-ninth hydrogen and eight-ninths oxygen. The principle helped establish the distinction between a chemical compound and a physical mixture, an important issue in late-eighteenth-century chemistry.

The date in this question reflects a historical publishing wrinkle. Proust’s relevant article appeared in a volume dated 1794, but that volume was reportedly printed in 1797; references therefore commonly give either date. Proust’s view was opposed by Claude Louis Berthollet, who argued that elements could combine in continuously varying proportions. The law later helped support John Dalton’s atomic theory, although non-stoichiometric compounds show that the rule is not universal in modern materials science.