How did John Dalton discover the law of multiple proportions?

John Dalton discovered the law of multiple proportions through his pioneering work on atomic theory in the early 19th century.12

His discovery was rooted in his efforts to determine the relative masses of different atoms. Dalton hypothesized that elements consist of atoms with fixed, characteristic masses and that they react in definite proportions to form compounds because their constituent atoms combine in specific numerical ratios.32

To test this, he measured the masses of various elements—such as hydrogen, oxygen, carbon, and nitrogen—as they combined with fixed masses of other elements. By observing these combinations, he realized that when two elements form more than one compound, the weights of one element that combine with a fixed weight of the other are always in a ratio of small whole numbers.31

Dalton announced this law in 1803, using it as a foundational argument to support his theory that matter is composed of indivisible atoms. Although his specific assumptions about the number of atoms in certain molecules were sometimes incorrect, the underlying principle of multiple proportions became a cornerstone of modern chemistry.132