Probably the best known addition to Dalton’s theory, to enrich it in the direction of testability, was Avogadro’s hypothesis: “The first hypothesis to present itself in this connection, and apparently even the only admissible one, is the supposition that the number of integral molecules in any gases is always the same for equal volumes, or always proportional to the volumes” ( 1811, p. 58 ).
Source: wiktionary