The diffusion and thermal diffusion of hydrogen-deuterium, with a note on the thermal diffusion of hydrogen-helium

The diffusion coefficientD 12 has been determined for hydrogen-deuterium and is found to be in close agreement with the value predicted by the classical theory. The case is one of particular interest because the diffusing molecules have the same size and the same molecular field. The thermal separation in mixtures of this pair of gases has also been measured. As the molecules are of the same size, the thermal separation is due entirely to difference in mass. By using Chapman’s theory, the value of the repulsive force index (v 12 ) is deduced. By combining the values ofv 12 andD 12 , the repulsive force constantk 12can be determined, and so the complete law of force operating in collisions between unlike molecules can be obtained. On classical considerations this law of force would apply to either gas separately. Measurements of thermal separation in mixtures of hydrogen-helium have also been made and the value ofv 12 determined. It is shown that this pair of molecules is 'harder’ than hydrogen-deuterium. Comparison is made with earlier results of Elliott and Masson.