Gay-Lussac stated in the Mémoires de la Société d'Arcueil, tome ii., p. 207, that "lorsque deux gaz se combinent, leurs volumes mesurés a la même température et à la même pression sont dans des rapports simples." It will be seen that Gay-Lussac anticipated the great law of Avogadro, although it was left to the Italian physicist to distinguish between the ultimate particles of compounds and elements—between molecule and atom. As Roscoe and Schorlemmer say: "The discovery by Gay-Lussac of the law of volume—combination, together with Avogadro's explanation of the law—served no doubt as most valuable supports of Dalton's atomic theory; but the truth of this latter theory was still further asserted by a discovery made by Dulong and Petit in 1819. These French chemists determined the specific heat of thirteen elementary bodies, and found that the numbers thus obtained, when compared with the atomic weights of the same bodies, showed that the specific heats of the several elements are inversely proportional to their atomic weights; or, in other words, the atom of each of these elements possesses the same capacity for heat. Although subsequent research has shown that this law does not apply in every case, it still remains a valuable means of controlling the atomic-weight determinations of many elements."
Gay-Lussac was the inventor of the tower, known by his name, in the manufacture of sulphuric acid. Formerly the nitrous fumes were allowed to escape into the air, but
