Avogadro had complete sets of the scientific journals of his time as printed in Italian, French, German, and English. He had read in those journals that two volumes of hydrogen gas had been found to combine with one volume of oxygen gas in the formation of water. He had read, too, that some chemists argued that this proved that two atoms of hydrogen combine with one atom of oxygen in forming water particles. Then, reading on, Avogadro came to the report of Gay-Lussac in which he had elaborated upon the experiment of the combination of hydrogen and oxygen gases. To be sure, all gases obey the law that Boyle found about pressure and volume.
All gases expand at the same rate when heated.
It might be all right to suppose that such things are true because every gas has the same number of atoms in a pint space. He picked up a pencil and drew some boxes, all of the same size. Two will be for the hydrogen gas, one for the oxygen gas. On the right will be two for the water vapor formed. Each box contains the same number of atoms.