Why did John Dalton believe atoms of different elements have different masses?

John Dalton, the British scientist who developed the modern atomic theory, proposed that all atoms of a given element possess the same fixed mass.1

His belief that atoms of different elements have different masses stemmed from his study of gases and his efforts to understand why different gases in the atmosphere do not separate by weight. Dalton concluded that atoms are limited to one specific kind for each element and that elements react in definite proportions to form compounds because their constituent atoms react in definite, fixed proportions. By assuming the simplest possible ratio for these combinations, he was able to calculate relative atomic weights.12

This framework allowed Dalton to explain the law of definite proportions and derive the law of multiple proportions, which provided a scientific basis for the existence of atoms.31