Shape of the Coexistence Curve near the Critical Temperature

Abstract

It is shown that diffusion in the gravitational field in binary liquid systems is so slow that the effect of gravity may be completely neglected in studies of the coexistence curve. The data arising from the direct measurement of the coexistence curve of the cyclohexane‐aniline system have been correlated with those from measurements of the densities of coexisting phases. A graph has been constructed which confirms the earlier conclusion that the coexistence curve is flatter at the top than can be accounted for by a cubic relation between temperature and density. Finally, a discussion is given of the liquid‐vapor equilibrium in a one‐component system. It is pointed out that the range of densities over which the meniscus has been reported to disappear within the tube in earlier work is so small that measurements of the coexistence curve need to be made with a temperature control of the order of 0.0001°. Only the recent work of Lorentzen on carbon dioxide meets this requirement. His data appear to be favorable to the idea that there is a short flat portion on the top of the coexistence curve, but are still not quite adequate to settle the question.