The pressures of gaseous mixtures. Part III

Part II of this series of papers dealt with the compressibilities of hydrogen, helium, and mixtures of the two. The empirical result of that paper was to describe the effects of composition and of temperature upon the pressure- volume relationships in these mixtures. The chief theoretical results were (i) to show that the partial pressure law of Lennard-Jones quantitatively covers the facts; Dalton’s partial pressure law being inapplicable, as we had already proved in other cases in Part If; (ii) to measure the coefficients of the fields of attractive and repulsive force involved in an encounter between a pair of these molecules, by applying Lennard-Jones’ equations; (iii) to deter­mine the kinetic diameters of the molecules of helium and of hydrogen. The kinetic diameter of a molecule depends upon the temperature, being less at a high temperature than at a low. It appeared that at a given temperature, and especially when this is not a low one, a molecule of one of this pair of gases preserves the same diameter whether it is colliding with another of its own kind or with a molecule of the other kind. If this rule were generally applicable, the isotherms of a mixture could be predicted from those of the pure com­ponents. The case of helium and hydrogen was rather a special one, in that both gases deviate positively from Boyle’s law at all ordinary temperatures, and present no great range in the properties measured. Both kinds of molecule are light, small, and "hard," their behaviour in the mass is not necessarily typical, and the theoretical deductions from it might not be generally true. Accordingly, we have extended the work so as to study mixtures of each of those two gases in turn withargon .