An extension of the theory of Brownian motion of heavy particles in a condensed medium of light particles is presented, which accounts for deviations of the state of the medium from equilibrium. The case of media under a thermal gradient is especially considered and the coefficients of diffusion and thermal diffusion of the heavy component are identified by solving the generalized kinetic equation for the Brownian particle distribution. The first terms of a systematic expansion of the thermal‐diffusion coefficient in powers of the mass ratio are explicitly given, and their sign is found by an approximate further evaluation. It is suggested that the thermal‐diffusion coefficient follows an asymptotic saturation law as far as the isotopic effect is concerned.

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© 1965 American Institute of Physics.
1965
American Institute of Physics
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