It is shown here that if all degrees of freedom in a vapor are nearly classical and if there is no association, the vapor pressure of the lighter isotope is always higher. If only the external degrees of freedom are nearly classical and the internal ones are in the lowest state, the coupling of purely harmonic vibrations has either no influence or tends probably to a further increase of the excess vapor pressure of the light isotope. Anharmonicity and the change in the van der Waals forces probably account for those cases in which the heavier isotope has the higher vapor pressure.

• Received 10 October 1938

DOI:https://doi.org/10.1103/PhysRev.54.912