The law that a given chemical compound always contains the same elements in the same fixed proportions by weight. Thus, whatever its source, silver chloride always contains 108 g (3.81 oz) of silver to every 35.45 g (1.251 oz) of chlorine. If a compound is formed by the union of m atoms of one element, each weighing a, with n atoms of another element, each weighing b, the composition by weight of one molecule of the compound is in the ratio ma:nb. This must be the composition of any mass of the compound, provided that all atoms of the same kind have the same weight. It is now known that this is not usually the case but that the atoms of an element may consist of a number of isotopes, having different masses. However, as long as any sample of the element always contains the same relative proportions of the isotopes, the law still holds.