Beyond the pair-potential model of fluids at the liquid-vapor critical point

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.