What does the law of multiple proportions state?

The law of multiple proportions states that when two elements combine to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.1

Announced in 1803 by the British scientist John Dalton, this law provided critical evidence for his atomic theory, which posits that matter is composed of indivisible atoms. Dalton’s work demonstrated that different compounds are formed by combining atomic building blocks of varying masses, and the confirmation of this law across numerous compounds served as a powerful argument for the existence of atoms.12