Thermal separation in gaseous mixtures

It was pointed out by Chapman (‘ Phil. Trans.,’ A, vol. 216, p. 279 (1915); 'Roy. Soc. Proc.,’ A, vol. 93, p. 1 (1917)) that, according to his development of the kinetic theory of gases, the composition of a gaseous mixture should vary when the temperature is made to vary from point to point within it, the extent of separability depending upon the character of the intermolecular force-fields and upon the relative molecular weights, sizes and numerical concentrations. The prediction was verified, qualitatively, with mixtures of hydrogen with carbon dioxide (Chapman and Dootson, ‘Phil. Mag.’ (vi), vol. 33, p. 248 (1917)), in which a difference in composition of about 3 per cent, was found when an equimolecular mixture was kept in two communicating vessels at 10° and 230° respectively. The present experiments were at first subordinate to certain other work in which thermal diffusion would have affected the results, but they have yielded data which are of more general interest. Meanwhile, Ibbs ("Roy. Soc. Proc.,’ A, vol. 99, p. 385 (1921)) had further confirmed Chapman’s theory when he showed—again with hydrogen and carbon dioxide—that (a ) thermal separability reaches a maximum in an approximately equimolecular mixture as compared with mixtures in other proportions, and that (b ) the difference in composition attained with a given mixture is proportional to the logarithm of the temperature-ratio. That is, λc - λh =k T loge Th /Tc , in which λh , and λc are the molar fractions of the heavier constituent in regions whose absolute temperatures are respectively Th (hot) and Tc (cold); andk T is a constant, independent of temperature but characteristic of the components and proportions of the mixture.