Proust proposed the law of definite Proportions Henry Cavendish isolated hydrogen and Lavoisier named it meaning water former' Lavoisier also realized that water is not an element, but a compound Joseph Proust found that hydrogen and oXygen always react in a particular mass ratio and proposed the Law of Definite Proportions: Elements combine in definite mass proportions to form compoundsHenry Cavendish isolated hydrogen and Lavoisier named it meaning “water former”. Lavoisier also realized that water is not an element, but a compound. Joseph Proust found that hydrogen and oxygen always react in a particular mass ratio and proposed the Law of Definite Proportions: Elements combine in definite mass proportions to form compounds. Proust proposed the Law of Definite Proportions