_Avogadro, who continued the researches of Dalton and Gay-Lussac, was born in Turin, Italy, June 9, 1776. In 1796, after receiving the doctor’s degree in law from the University of Turin, he was employed by the government for the following ten years. He began his work in science in 1806 and three years later was made professor of physics at Vercelli. In 1811 he announced his famous law. According to Merz, since the time of Boyle “it had been known that equal volumes of different gases under equal pressure change their volumes equally if the pressure is varied equally, and it was also known that equal volumes of different gases under equal pressure change their volumes equally with equal rise of temperature. These facts suggested to Avogadro, and almost simultaneously to Ampère, the very simple assumption that this is owing to the fact that equal volumes of different gases contain an equal number of the smallest independent particles of matter. This is Avogadro’s celebrated hypothesis. It was the first step in the direct physical verification of the atomic view of matter.”_ _In 1820 Avogadro became professor of physics at Turin University, where he remained for many years. He died July 9, 1856._ THE MOLECULES IN GASES PROPORTIONAL TO THE VOLUMES[27] I. M. Gay-Lussac has shown in an interesting Memoir (_Mémoires de la Société d’Arcueil_, Tome II.) that gases always unite in a very simple proportion by volume, and that when the result of the union is a gas, its volume also is very simply related to those of its components. But the quantitative proportions of substances in compounds seem only to depend on the relative number of molecules which combine, and on the number of composite molecules which result. It must then be admitted that very simple relations also exist between the volumes of gaseous substances and the numbers of simple or compound molecules which form them. 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. Indeed, if we were to suppose that the number of molecules contained in a given volume were different for different gases, it would scarcely be possible to conceive that the law regulating the distance of molecules could give in all cases relations so simple as those which the facts just detailed compel us to acknowledge between the volume and the number of molecules. On the other hand, it is very well conceivable that the molecules of gases being at such a distance that their mutual attraction cannot be exercised, their varying attraction for caloric may be limited to condensing a greater or smaller quantity around them, without the atmosphere formed by this fluid having any greater extent in the one case than in the other, and, consequently, without