The characteristics of thermal diffusion

Abstract
When a gas mixture is contained in a vessel in which a steady temperature gradient is maintained, a concentration gradient is in general set up, whose amount is determined by the logarithm of the temperature ratio, and by k T , the thermal diffusion ratio; the general theory of non-uniform gases gives successive approximations to k T , and the first of these, [k T ]1, is accurate within a few per cent. The paper discusses the dependence of [k T ]1 on ( a ) the ratio of the molecular masses; ( b ) their concentration ratio ( c 1 or c 2 ); ( c ) the two ratios of the molecular diameters, inferred from the coefficient of viscosity, to their joint diameter, inferred from the coefficient of diffusion; and ( d ) three parameters depending on the mode of interaction between the unlike molecules. When this interaction is according to the inverse-power law, the three parameters ( d ) are all expressible in terms of the mutual force index, and [k T ]1, is a function of five independent variables. The general nature of its dependence on these variables is discussed, with particular reference to the end values (for c 1 or c 2 zero) of the thermal diffusion factor α, given by k T / c 1 c 2 ;these end values involve fewer variables (less by two) than the general values, and their functional character can be represented graphically. It is shown that k T may be zero not only when c 1 or c 2 is zero, but also for at most one intermediate mixture ratio. Formulae for [k T ]1 appropriate to various special cases are also given.