  Precise calculations of the coexistence-curve diameters of a hard-core
square-we ll (HCSW) fluid and the restricted primitive model (RPM) electrolyte
exhibit mar ked deviations from rectilinear behavior. The HCSW diameter
displays a $|t|^{1- alpha}$ singularity that sets in sharply for $|t|\equiv
|T-T_c|/T_c\lesssim 10^{-3}$; this compares favorably with extensive data for
${SF}_6$, also reflec ted in C$_2$H$_6$, N$_2$, etc. By contrast, the curvature
of the RPM diameter va ries slowly over a wide range $|t|\lesssim 0.1$; this
behavior mirrors observati ons for liquid alkali metals, specifically Rb and
Cs. Amplitudes for the leading singular terms can be estimated numerically but
their values cannot be taken li terally.
