| Oxygen | 1·402 |
| Nitrogen | 1·411 |
| Hydrogen | 1·412 |
| Carbon monoxide | 1·418 |
In all cases the numbers are too large, and this is a serious difficulty, because any tendency to rotate round the central line would cause the values to be less, not greater than 1·4. For triatomic molecules the calculated value of γ is 1⅓, but in actual fact the ratio in the case of triatomic molecules, such as H2O, CO2, N2O, etc., is always less than 1⅓. These speculations stand on a basis very different from the first conception, namely, that all heat must be employed in communicating translational motion to molecules of mercury gas, argon, and helium, and it appears that the atoms of these three elements must necessarily be regarded as having the properties of smooth elastic spheres. The atoms and the molecules must in their several cases be identical. And, inasmuch as the chemical evidence regarding mercury leads to the same conclusion, it appears legitimate to infer that argon and helium must also be monatomic elements.
[CHAPTER VII]
THE POSITION OF ARGON AMONG THE ELEMENTS
From what has been said in the preceding chapter there can be no doubt that the molecular weight of argon is 39·88. We have now to consider what this conclusion involves. Taken in conjunction with the fact that the ratio between its specific heat at constant volume and that at constant pressure is 1⅔, it follows that energy imparted to it is employed solely in communicating translational motion to its molecules. In the case of mercury gas such behaviour is taken as evidence that the conclusion following from the formulae of its compounds, from the density of its compounds in the gaseous state, and from its own vapour-density, as well as from its specific heat in the liquid state, namely, that its molecules are monatomic, is correct. Is it legitimate to conclude that because argon in the gaseous state has the same ratio of specific heats, therefore it also is a monatomic gas?
The conclusion will depend on our conception of an atom and a molecule, and in the present state of our ignorance regarding these abstract entities no positive answer can be given. It appears certain that, on raising the temperature of argon, very little, if any, energy is absorbed in imparting vibrational motion to its molecules; and our choice lies between our ability or inability to conceive of a molecule so constituted as to be incapable of internal motion. If there be any truth underlying Professor Boltzmann’s conception, a molecule of argon cannot consist of any complex structure of atoms, otherwise it would possess more than three degrees of freedom, and heat would be utilised in causing rotational motions. As we know for a fact that the ratio between the specific heats of gases diminishes with the increasing complexity of their molecules, perhaps the safest conclusion is the one adopted by the discoverers of argon, that the balance of evidence drawn from this source is in favour of its monatomic nature.
But this hypothesis raises difficulties which are not lightly to be met. These difficulties arise from a consideration of the position of argon when it is classified with other elements.
In 1863 Mr. John Newlands pointed out in a letter to the Chemical News that if the elements be arranged in the order of their atomic weights in a tabular form, they fall naturally into such groups that elements similar to each other in chemical behaviour occur in the same columns. This idea was elaborated farther in 1869 by Professor Mendeléeff of St. Petersburg and by the late Professor Lothar Meyer, and the table may be made to assume the subjoined form (the atomic weights are given with only approximate accuracy):—