But why should equal volumes of compounds form their comparable quantities? A plausible answer to this question is furnished by Avoga dro's hypothesis, according to which equal vol umes of all gases and vapors contain, under the same conditions of temperature and pressure, equal numbers of molecules. Comparing the com position of equal volumes is consequently tanta mount to comparing the composition of single molecules; and that a comparison of molecules should be expected to bring out the laws of chemical composition, is clear. The smallest weights of the elements found in equal volumes of compounds represent, evidently, the relative weights of single atoms of the several elements, and this is why those weights are termed 'atom ic weights.' It was the application of •Avo gadro's hypothesis that first led chemists to compare equal volumes of compounds. The first to demonstrate clearly the advantage of doing so was the French chemist Gerhardt, from whom the modern system of denoting the composition of substances is called Gerhardt's Notation.
The rule directing us to compare equal volumes of compounds in the gaseous state, and thus leading to a knowledge of atomic weights, has been extended also to the state of bodies when dissolved in some solvent. (See MOLECULES 1\JOLECULAR WEIGHTS.) In ease, however, com pounds can be neither vaporized without de composition nor dissolved in any suitable solvent, Avogadro's rule cannot be applied, and then some other principle has to be employed, if in the absence of better material the refractory compound must be used for determining the atomic weight of one of its elements.
lsomournism. One such principle is based on the general fact that iomorphous compounds, i. e. substances which have about the same crys talline form, usually resemble one another also in their chemical composition. An example may serve to explain how this general fact is made use of for the purpose of determining atomic weights. Suppose the oxide known as alumina were the only compound available for determin ing the atomic weight of its metal, aluminum. Alumina is neither volatile nor soluble, and hence its molecular weight could not be determined by Avogadro's rule. But it is isomorphous with the oxide of iron, which is known to contain 2 atomic weights of iron to 3 atomic weights of oxygen. Alumina is therefore supposed to have a similar composition, i. e. to be made up of 2 atomic weights of aluminum and 3 atomic weights of oxygen. A chemical analysis would show that alumina contains 53.03 per cent. of aluminum and 46.97 per cent. of oxygen. Calling the atom ic weight of aluminum Al, and remembering that the atomic weight of oxygen is 16, we therefore have: 2 Al : 3 X 10 : : 53.03 : 46.97.
This atomic weight is consequently found to be 3 X 16 X 53.03 Al = = 27.1—.
2 X 46.97