The law of definite proportions was discovered by the French chemist Joseph- Louis Proust in the late 18th century, so it is often called Proust’s law.

This law states that a given chemical compound always contains the same elements in exactly the same proportion by mass, no matter how or where the compound is prepared. Proust’s careful analyses of compounds such as copper carbonate led him to show that natural and laboratory-prepared samples have identical fixed compositions, which helped lay the groundwork for later atomic theory.