A relation is derived by quantum-mechanical perturbation theory, using a center of gravity argument and the Kuhn-Reiche sum rule, between the polarizability and diamagnetic susceptibility of monatomic gases. On comparison with a similar paper by Kirkwood, who uses a variational method, a criterion is derived for the validity of a perturbed wave function of the form ${\psi }_0(1+\lambda v)$, where ${\psi }_0$ is the unperturbed wave function of the normal state and $v$ the perturbing potential energy of a uniform electric field. It is also pointed out that inclusion of an electronic interaction term $\sigma$ neglected by Kirkwood removes a difficulty appearing in his paper.

• Received 6 August 1932

DOI:https://doi.org/10.1103/PhysRev.41.813