Dalton’s work with the different elements allowed him to make generalizations that fit his findings. Dalton called these rules of an “atomic theory” which he proposed in the early 1800's based not only on his discoveries, but scientists before him.  The rules can be summed up as follows: 

1. Elements are composed of minute, indivisible particles called atoms.

2. Atoms of the same element are alike in mass and size.

3. Atoms cannot be created, destroyed, or subdivided.

4. Chemical compounds are formed by the union of two or more atoms of different elements.

5. When atoms combine they do so in simple numerical ratio.

6. Atoms of two elements may combine in different ratios to form molecules.

Today some of his statements must be modified: (1) atoms are composed of subatomic particles; (2) all the atoms of a specific element do not have the same mass; and (3) atoms under special circumstances can be decomposed.  Dalton also did not quite understand the concept of bonding.  He imagined that atoms simply were next to each other.  He did not theorize about a nucleus, protons, electrons, and neutrons, which were discovered later.

A discovery by two friends, French scientist Joseph Gay-Lussac (1778-1850) and German naturalist Alexander von Humboldt (1769-1859) provided the evidence that elements combined in a predictable way.  First, Gay-Lussac’s observed that the volume of gases, combined as a ratio of whole numbers.   Together with von Humboldt in 1805, they found that water combined in simple proportions of oxygen to hydrogen (8:1) by volume to form water.