Princeton's WordNet

1. law of multiple proportions, Dalton's law(noun)

   (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

2. Dalton's law, Dalton's law of partial pressures, law of partial pressures(noun)

   (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