What did Joseph Louis Proust discover?

Joseph Louis Proust is a well-known chemist for discovering the law of definite proportions. At a time when it was not known if the composition of a compound was evenly distributed, Proust proved many wrong, including Berthollet, when he conclusively proved that any random sample of a compound would have approximately the same composition.

Proust showed, using various metal oxides, that there were no intermediates between the core elements and the final substance. However, since that time, there have been new compounds discovered that do not adhere to this law. In honor of Berthollet's original views, these compounds are called Berthollides.