(chemistry) law stating that every pure substance always contains the same elements combined in the same proportions by weight WordNet 3.0 & Open English WordNet

In chemistry, the law of definite proportions, sometimes called Proust's law or the law of constant composition, states that a given chemical compound always contains its component elements in fixed ratio and does not depend on its source and method of preparation. Wikipedia

Statement that a given chemical compound always contains its elements in fixed ratio and does not depend on its source and method of preparation Wikidata