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Evidence is presented that many-body interactions in fluids have important consequences at the liquid-vapor critical point. In particular, three-body interactions in lattice-gas models are shown to lead to revised scaling variables and to a singularity in the coexistence-curve diameter with an amplitude proportional to the molecular polarizability. This is confirmed in experiments reported here. A companion van der Waals theory explains several other observed correlations between nonuniversal critical amplitudes.

Evidence is presented that the pair-potential model of fluids is insufficient in the critical region. In particular, data on the critical properties of Ne, N2, C~H4, C2H6, and SF6 are shown to exhibit welldefined trends in the variation of certain nonuniversal critical amplitudes with the critical temperature T, . Both the slope of the coexistence-curve diameter far from the critical point, and the deviations from linear behavior which appear closer to T" increase systematically with T"and are directly correlated with the molecular polarizability.These trends are explained on the basis of the increasing importance of three-body dispersion (Axilrod-Teller) forces in the more polarizable systems, and a simple mean-field theory is developed which accounts for the observed correlations. The possibility of incorporating the effects of three-body interactions into an effective pair potential is explored within the context of perturbation theory in the grand canonical ensemble, and it is shown that such an interaction is explicitly a function of fugacity. In the critical region, this is equivalent to a thermal scaling field which depends on the bare chemical potential of the system, and ultimately leads to a breakdown in the classical law of the rectilinear diameter. The magnitude of this field mixing, and hence of the diameter anomaly, scales with the product of the particle polarizability and the critical number density, in agreement with experiment.

The index of refraction of ethane at 6328 Å has been measured in the density range 0.02 to 0.36 g/cm3(1 to 200 amagats). The coefficient in the Lorentz–Lorenz expression [Formula: see text] is constant to within 0.5% for the temperature and density range studied. The coefficient [Formula: see text] increases with density reaching a maximum near the critical density and decreases with density for densities larger than the critical density. The critical density has been measured in the same experiment and is 0.2062 ± 0.0003 g/cm3.