What is "law of definite proportions"

Definite \Def"i*nite\, a. [L. definitis, p. p. of definire: cf. F. d['e]fini. See Define.]

1. Having certain or distinct; determinate in extent or greatness; limited; fixed; as, definite dimensions; a definite measure; a definite period or interval.

   Elements combine in definite proportions.
   --Whewell.

2. Having certain limits in signification; determinate; certain; precise; fixed; exact; clear; as, a definite word, term, or expression.

3. Determined; resolved. [Obs.]
   --Shak.

4. Serving to define or restrict; limiting; determining; as, the definite article.

   Definite article (Gram.), the article the, which is used to designate a particular person or thing, or a particular class of persons or things; -- also called a definitive. See Definitive, n. -

   Definite inflorescence. (Bot.) See Determinate inflorescence, under Determinate.

   Law of definite proportions (Chem.), the essential law of chemical combination that every definite compound always contains the same elements in the same proportions by weight; and, if two or more elements form more than one compound with each other, the relative proportions of each are fixed. Compare Law of multiple proportions, under Multiple.

n. (chemistry) law stating that every pure substance always contains the same elements combined in the same proportions by weight [syn: law of constant proportion]

In chemistry, the law of definite proportions, 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 weight) and does not depend on its source and method of preparation. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have the same elemental composition by mass. 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.