Time Period: 1754-1826

Background: French chemist who conducted many tests on chemical substances, Proust played a major role as a chemical annalist. Working with his pharmacist father, he taught Chemistry School at Sergovia and the University of Salamanca in Spain.

Belief: Based on his studies on copper carbonate reactions, Proust discovered that each pure compound has its own characteristic elemental composition. For example, the ratio of elements never changed even though its mass changed. H2O would always be a ratio of 2:1, whether it was 10 g or 1000g.

Contribution: In 1794, the Law of Definite Proportions stated that the proportion by the masses of two given elements would always remain the same. (With water, for example, it mattered not if it was water found in the lake or clear water - the elemental composition was determined by exact proportions of elements by mass, regardless of how the compound was created.) This discovery successfully extinguished Claude Berthollet's, which stated that the products of a reaction depended on the ratio of reactants.