Gay Lussac first pointed out that a relation exists between the density of a gas and its atomic weight. AvOgadro greatly simplified the statement of these .relations by announcing the law of molecular volumes of gases, a law which prof. Clerk Maxwell: has since proved to be a necessary consequence.of the molecular theory,pf gases. This law is, that a given volume of gas at a given temperature and pressure contains the same number of molecules whatever be the nature of the gas.
From this law, to which we may give the name of "Avogadro's law," and from Boyle's law, and the law (often called Charles's law) that the volume of a as is directly proportional to the absolute temperature—that is, to its temperature reckoned from a point 273' centigrade below the freezing-point of water—it follows that the volume occupied by a given mass of a gas is a function of the pressure, the temperature, and the molecular weight of the gas; understanding by the "molecular weight" of a substance a number M, such that M : 2 :: the absolute weight of a molecule of the substance: the absolute weight'of a molecule of hydrogen. The number 2 appears in
this proportion because we assume the atom of hydrogen as our unit, both of atomic and of molecular weight, and it can be proved (see CHEMISTRY) that the molecule of hydrogen gas consists of two atoms. If, then, P be the pressure in millimeters of mer cury at 0' C; t, the temperature of the gas, as indicated by a centrigade thermometer; 31, the molecular weight of the substance; and V, the volume (in cubic centimeters) 760 t 273 2'200occupied by a gramme of the gas, V 273 x — — x In the gaseous state, •the average distance between the molecules, although extremely small, is great com pared with the size. of the molecules, so that the volume of the gas depends almost exclusively upon the distance between the molecules: it is not so in he case of solids and liquids, in which the molecules are so closely packed as to be almost always in contact. The volume occupied by solids and liquids depends, therefore, far more upon the atoms of which the substance is made up, than upon its molecular structure. For further recent modifications of the atomic theory, see CHEMISTRY.