In 1803, John Dalton proposed the following postulates which underly modern chemistry:

  1. Elements are formed from small indivisible particles called atoms which are identical for a given element but different for any other element.
  2. Chemical compounds are formed by the combining of a definite number of atoms of each type of atom to make one molecule of the formed compound.

Confirmation of these postulates explained both the conservation of mass in chemical reactions and the law of definite proportions. Dalton's efforts to confirm his postulates led to the law of multiple proportions: whenever two elements combine to form more than one compound, there is a ratio of small whole numbers between the various masses of one element that combine with a constant mass of the other element in the compounds.