Law of Definite Proportions

The idea that compounds have defined chemical formulas was first proposed in the late 1700s by the French chemist Joseph Proust. Proust performed a number of experiments and observed that no matter how he caused different elements to react with oxygen, they always reacted in defined proportions. For example, two parts of hydrogen always reacts with one part oxygen when forming water; one part mercury always reacts with one part oxygen when forming mercury calx. Dalton used Proust's Law of Definite Proportions in developing his atomic theory.

The law also applies to multiples of the fundamental proportion, for example:

In both of these examples, the ratio of hydrogen to oxygen to water is 2 to 1 to 1. When reactants are present in excess of the fundamental proportions, some reactants will remain unchanged after thechemical reaction has occurred.

 

See the explosion

From:  http://www.visionlearning.com/en/library/Chemistry/1/Early-Ideas-about-Matter/49