(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 WordNet 3.0 & Open English WordNet

In chemistry, the law of multiple proportions states that if two elements form more than one compound, then the ratios of the masses of the second element which combine with a fixed mass of the first element will always be ratios of small whole numbers. Wikipedia

Chemical law that, if two elements form more than one compound, then the ratios of the masses of the second element which combine with a fixed mass of the first element will always be ratios of small whole numbers Wikidata

When two elements can combine to form more than one compound then the amounts of one which combines with a fixed amount of the other will have a simple multiple relationship. Wiktionary