A study of the vapour pressures of magnesium, thallium, and zinc, and the determination of their chemical constants

Abstract

The thermodynamical vapour pressure equation log p = - ^ + Cp/nT - j* J^ 5 j* cp <rr + c , where y 0 is the latent heat of vaporization at absolute zero, C p the specific heat of the vapour at constant pressure and c p that of the condensate, involves knowledge of many thermal constants, and accurate determination of the vapour pressure of an element is therefore of considerable importance. The constant of integration C, known as the “chemical constant,” can be calculated, for statistical theory indicates that C = C0 + f log M + log g /g ', where C 0 is a universal constant ( = ^ ■, iS -j..'), r N ' M is the molecular weight, and g/g' is the ratio of the statistical weights in the vapour and condensed state respectively (monatomicity of the vapour is assumed), g' is taken to be unity, and g is obtained from spectroscopic data where g = 2 j + 1, the value being supported by the Stern and Gerlach magnetic experiments. Although it is possible to estimate the value of the constant in this way, and therefore from a knowledge of the vapour pressure to determine the entropy and other thermal constants of the metal, it has been necessary to check the validity of the statistical theory by careful measurement of the vapour pressure and energy terms of the thermodynamical equation, and so to evaluate the constant and compare it with the theoretical value.