John Dalton. Historical artwork of John Dalton (1766-1844), English chemist, collecting marsh gas to study. Dalton's main work was on his atomic theory, which he based on his observations of the behaviours of gases. Marsh gas (methane) is formed by rotting vegetation, and is trapped under a bed of degrading matter. Disturbing the bed with a pole (as here) releases some gas, which can be captured in a jar (lower right). Dalton proposed that gases were made up of combinations of atoms, indivisible spheres, in different quantities. He found that different elements' atoms weighed different amounts, but those of one element were the same. This forms the basis of chemistry today.