<Tr> <Td> </Td> <Td> This article's tone or style may not reflect the encyclopedic tone used on Wikipedia . See Wikipedia's guide to writing better articles for suggestions . (August 2018) (Learn how and when to remove this template message) </Td> </Tr> <P> In chemistry, the law of definite proportion, sometimes called Proust's law or the law of definite composition, or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation . For example, oxygen makes up about / of the mass of any sample of pure water, while hydrogen makes up the remaining / of the mass . Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry . </P> <P> The law of constant proportion was given by Joseph Proust in 1794 . This observation was first made by the English theologian and chemist Joseph Priestley, and Antoine Lavoisier, a French nobleman and chemist centered on the process of combustion . </P> <P> I shall conclude by deducing from these experiments the principle I have established at the commencement of this memoir, viz. that iron like many other metals is subject to the law of nature which presides at every true combination, that is to say, that it unites with two constant proportions of oxygen . In this respect it does not differ from tin, mercury, and lead, and, in a word, almost every known combustible . </P>

When does the law of constant proportion fail