Quantitative description of departures from the law of rectilinear diameters in the liquid-vapour coexistence curves of Rb and Cs

Using the experimental data of Hensel and co-workers for the liquid-vapour coexistence curve of Rb, it is demonstrated that the average density (rho ( l) + rho (g))/2, with l for liquid and g for gas, measured in units of the critical density, is linear in the variable (1 - (T/T-c))(2/3), T-c being t he critical temperature. The long-range polarization interaction exposed by Blazej and March in liquid metals is proposed as the underlying reason for this marked departure from the law of rectilinear diameters.