The problem of the self-diffusion of a gas has received considerable attention, and it is of special importance in the kinetic theory because of the similarity of the diffusing molecules. Chapman's exact classical kinetic theory (Chapman 1916, 1918) gives the expression: D 11 = kη /ρ, where D 11 = coefficient of self-diffusion, η = coefficient of viscosity, ρ = density, k = a constant. The value of k varies from 1⋅504 to 1⋅200 for Maxwellian molecules (index of repulsive force 5 ) and rigid elastic spheres respectively. Massey and Mohr (1933), however, applying the quantum theory to collision processes in gases have deduced that for self-diffusion the factor given by the classical theory must be reduced by nearly ½. From the point of view of this theory the molecules in true self-diffusion must be identical and indistinguishable, so that the prediction in that case cannot be experimentally verified. Although the conditions of these experiments do not correspond to the case of self-diffusion of the quantum theory, it is a matter of considerable interest to examine the behaviour in diffusion of these pairs of gases which have in many respects a striking physical similarity.

The diffusion of pairs of gases with molecules of equal mass. (1937). Proceedings of the Royal Society of London. Series A - Mathematical and Physical Sciences, 162(911), 511–520.