Thermodynamic Properties ofHe3andHe4Solutions

In the present paper the thermodynamics of mixing of ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ solutions has been investigated on the basis of the existing theories. De Boer and Gorter's theory is modified to take account of the non-ideal character of the solution above the $\lambda$ temperature. The theoretical vapor pressure results are compared with Sommers' measurements at the dew point. It is found that certain features can be explained only on the de Boer-Gorter-Taconis approach. However, the incompatibility of any theory based on Taconis' hypothesis with Sommers' extensive vapor pressure measurements at fixed liquid concentrations prevents us at present from drawing any definite conclusions. Valuable information about the proper choice of the model for ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ solutions might, however, be obtained by an experimental study of the variation of heat of mixing and specific heat of mixing with temperature and concentration.