Although in theoretical calculations of Cv we are concerned only with perfect gases equations applying to reversible adiabatic expansion can be derived that are not limited to perfect gases. We shall here derive a general expression for (aplav)s which we will apply to a perfect gas to obtain Eq. (9); we shall also apply it to a van der Waals gas. The adiabatic expansion method may be incapable of sufficient precision to justify a van der Waals treatment, but the expressions obtained may be used with the more precise sound velocity method.