The pressures of gaseous mixtures. II.—Helium and hydrogen, and their intermolecular forces

Abstract

The work discussed in this paper was begun in 1924 in London, and was later on resumed in Durham. It forms part of an investigation which began eight years ago, of which the purpose was to gain information from a new quarter about intermolecular action; and the first part was published in 1923 by Masson and Dolley. In that part of the work, which dealt with the compressibilities of argon, of oxygen, of ethylene, and of a number of mixtures of each pair of these gases, a systematic study was made of deviations from Dalton’s law of partial pressures. It was possible to exhibit the cohesion of the molecules of one gas with those of another gas; and to prove, without using any special theory, that this type of cohesion is neither caused nor influenced by the possibility of chemical action between the atoms composing two colliding molecules. Thus this kind of cohesion differs from that which gives rise to unimolecular films, where the cohesion is chiefly traceable to the specific action of some particular atom within one molecule upon a particular atom in another, or in a surface. The bearing of this distinction has been discussed elsewhere, and thermodynamic aspects of the first paper have been recently dealt with by other authors. Meanwhile, it becomes clear from the results of that paper that any theory, proposed to explain the van der Waal’s cohesion between molecules of two different chemical kinds, need not invoke any types of force beyond those that could explain cohesion in a single pure gas. The present paper deals experimentally and theoretically with such forces. The first stage of the present experiments (1924) with helium and hydrogen indicated a new property in gases; for the data showed a certain range of mixtures of these two as being more incompressible than either constituent; a maximum being suggested at about 30 per cent, of helium in the mixture. Incidentally, this displaced hydrogen from its position as the least compressible gas. The effects were, however, slight; and, chiefly in order to test their genuineness, we resumed the work in 1926, and improved the methods in various ways. One result has been to confirm the new property, though the effect is very small; and meanwhile, Lennard Jones has independently noticed a similar effect in the experimental data of Holborn for a mixture of neon and helium; and in his recent theoretical work on gaseous mixtures he has been able to show it as a special case of the general properties of gases. The same general treatment proves to be applicable to the work we have done, as will be shown.