Contribution
Proust stated the Law of Definite Proportions. The law states that the ratio of elements in a compound is always the same/constant. He also hinted at the 'lego'ness of matter; he believed that matter could be put together in certain patterns to make bigger, different, unique matter
Significance of contribution
He set it up for Dalton to create the Law of Multiple Proportions and ultimately his Atomic Theory.
Fun Fact
When Napoleon invaded Spain, they burned down his laboratory.