Preliminary.
It was pointed out by Lord Rayleigh and one of the authors that it is a legitimate conclusion to draw, from the found ratio between its specific heat at constant pressure and that at constant volume, that argon is a monatomic element (‘ Phil. Trans.,’ 1895, A, p. 235). A similar deduction can be drawn regarding helium (‘ Chem. Soc. Trans.,’ 1895, p. 699). And as the molecular weight of hydrogen is accepted as twice its atomic weight, and as the density of helium is approximately^ 2, and that of argon approximately 20, the molecular weights of these elements are approximately 4 and 40 respectively. If, however, the molecule is identical with the atom, then the atomic weights must also necessarily be 4 and 40.
But argon, with an atomic weight of 40, finds no place in the periodic table of the elements, if, as is usual, it is contended that the elements must necessarily follow each other in the numerical order of their atomic weights.
Certain suppositions may be made which would obviate this difficulty. First, the evidence from the ratio of the specific heats may lead to a false conclusion. But it is inconceivable that any structure, except one of the simplest kind, should transform all energy communicated to it as heat, into kinetic energy of translation. Still, before a final decision on this point is arrived at, it would be well to actually determine the specific heat of argon, and this will shortly be done. It may, however, be mentioned, that preliminary experiments have shown it to be much lower than that of hydrogen, air, or carbon dioxide, volume for volume.
Second, helium and argon may consist of a mixture of monatomic' with diatomic molecules. The perfectly normal expansion of these gases appears to negative this supposition (‘ Phil. Trans.,’ loc. p. 239, and ‘ Roy. Soc. Proc.,’ vol. 59, p. 60). Even at a temperature of —88° there appears to be no marked tendency towards association. It is true that the ratios of the specific heats do not quite reach the theoretical number 1'667. That found for helium was 1’652, and that for argon P659, with the most carefully purified samples. Assuming (what there seems good ground to doubt) that the last decimal place may be trusted, helium can be calculated to contain nearly 7 per cent, of diatomic molecules, and argon rather more than 3 per cent. If this calculation be permitted, the atomic weight of helium would become 4'02, taking its found density at
