WordNet sense 1 ((chemistry) law stating that when two elements can combine to form more than one compound the amounts of one of them that combines with a fixed amount of the other will exhibit a simple multiple relation):

law of multiple proportions

WordNet sense 2 ((chemistry and physics) law stating that the pressure exerted by a mixture of gases equals the sum of the partial pressures of the gases in the mixture; the pressure of a gas in a mixture equals the pressure it would exert if it occupied the same volume alone at the same temperature):

Dalton's law of partial pressures, law of partial pressures