The diameter of the coexistence curve,12lpar;rgr;lplus;rgr;grpar;, is calculated numerically for a model that is known to have an asymptotic singularity comparable with that inCvnear the critical point. The model is an extension of Mermin's decorated lattice gas to simple cubic, body centered cubic, and face centered cubic lattices. The model diameter has a gradual curvature at all temperatures instead of a sudden kink near the critical point. In the asymptotic critical region, the experimental data are not sufficiently precise to distinguish between a diameter predicted by the decorated lattice gas and a linear diameter, which is predicted by the ``law of rectilinear diameters.'' Away from the critical point, the model diameter gives good qualitative agreement with experimental data if the proper choice of one parameter is made.
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