## Abstract

In continuation of earlier experiments (Harrison 1937) in which the thermal diffusion in radon-hydrogen and radon-helium mixtures was measured, the thermal diffusion of mixtures of radon-neon and radon-argon has now been studied. The mean value obtained for the ratio of the proportion by volume of radon on the cold side at 0 degrees C to that on the hot side at 100 degrees C, after thermal diffusion, was $1\cdot 074$ for radon-neon mixtures, and $1\cdot 008$ for radon-argon mixtures. In order to calculate the repulsive force field, F$_{12}$, between these two pairs of molecules, the present results were combined with measurements of ordinary diffusion of radon into neon and radon into argon (Hirst & Harrison 1939), and viscosity determinations at various temperatures of neon and argon (Trautz & Binkele 1930). The special theory, due to Chapman (1929), of thermal diffusion of a rare constituent in a binary mixture was used to derive F$_{12}$. The values obtained for the repulsive force field between the dissimilar molecules at collision were: $ \matrix F_{12}\, (\text{radon-neon})=1\cdot 9\times 10^{-51}\,d^{-6\cdot 1}=(d/d_{0})^{-6\cdot 1},\ \ d_{0}=4\cdot 8\times 10^{-9}, \\ F_{12}\, (\text{radon-argon})=2\cdot 1\times 10^{-43}\,d^{-5\cdot 1}=(d/d_{0})^{-5\cdot 1},\ \ d_{0}=4\cdot 3\times 10^{-9}, \endmatrix $ d being the distance between the point centres of repulsive force and d$_{0}$ the value of d at which F$_{12}$ is 1 dyne. A comparison of the values obtained for the repulsive force index for radon-neon and radon-argon molecules with those obtained by Atkins, Bastick & Ibbs (1939) for binary mixtures of the first five inert gases shows that radon is the 'softest' of the inert gas molecules. Radon-argon molecules are the closest approach to the Maxwellian case yet studied experimentally.