The molecular fields of carbon dioxide and nitrous oxide

Abstract
Experiments show that the thermal separation produced in a mixture of two gases by a temperature gradient may generally be regarded as proportional to log ( T 1 / T 2 ) over a considerable range of ordinary temperature, T 1 being the absolute temperature of the hot side and T 2 that of the cold side. Thus the total difference in concentration in the mixture may be written Δλ = k t log ( T 1 / T 2 ), where the constant k t is the coefficient of thermal separation. The theoretical meaning of the constancy of k t (Chapman 1918) is that the unlike molecules may be regarded as point centres of repulsive force obeying an inverse power law. At lower temperatures there is a general tendency for k t to decrease gradually with temperature. The general expression for the experimental value of k t may be written k t = - dλ 1 / d log T = dλ 2 / d log T' where λ 1 is the proportion of the heavier gas, λ 2 the proportion of lighter gas, and λ 1 + λ 2 = 1. A peculiarity has been found in the measurements of thermal separation in mixtures containing carbon dioxide, in that k t changes from one definite value below about 145° C to a higher definite value above that temperature (Ibbs and Wakeman 1932). This led to the conclusion that there is a change in the nature of the molecular field of carbon dioxide from a “soft” molecule to a “harder” molecule, i.e. the behaviour above 145° C becomes more like that of a rigid elastic sphere. Measurements of the viscosity of carbon dioxide support this conclusion.

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