The law of multiple proportions states that when elements combine, they combine in the ratio of small whole numbers. For example, carbon and oxygen react and form CO or CO2 and not CO1.7.
The law of multiple proportions is a basic law of stoichiometry and is used to establish atomic theory. This law was discovered by British chemist John Dalton and is also referred to as Dalton's Law. It was published in 1808 in his book "New System of Chemical Philosophy" and stated that "if two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers." This law works well with simple compounds but does not apply to non-stoichiometric compounds.