Along the $\lambda$ line in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures, the specific heat at constant chemical potential presumably becomes infinite. It is suggested that the $\lambda$ transition changes over to a first-order transition at a point at which the specific heat at constant composition becomes infinite. Possible reasons for such a change are discussed. The thermodynamic relationships are introduced through analogy with a system in which the variables are pressure and volume rather than chemical potential and composition.

• Received 5 July 1967

DOI:https://doi.org/10.1103/PhysRevLett.19.295