Joseph Louis Proust

Compounds (Joseph Louis Proust)

Between 1798 and 1808, Joseph Louis Proust analyzed different sources of several compounds. He found that they always contained the same ratio by weight of their elements. Table salt, for example, always contained 1.5 times as much chlorine as sodium. He also found that water always contained eight times as much oxygen as hydrogen. Finally, he found that the mineral known as cinnabar always contained 6.25 times as much mercury as sulfur. These results led Proust to propose the law of constant composition, which states that the ratio by mass of the elements in a chemical compound is always the same, regardless of the source of the compound.