The density dependence of the refractivity of gases

A sensitive interferometer is described for measuring differentially the effects of molecular interactions on the refractivity of gases. The technique involves changing the density but not the amount of the gas in the optical path. Second refractivity virial coefficients, describing the change in molar refraction due to pairwise interactions, have been determined for neon, argon, nitrogen, carbon dioxide, methane, sulphur hexafluoride and fluoroform.