  We propose an extension of the law of corresponding states that can be
applied to systems - such as colloidal suspensions - that have widely different
ranges of attractive interactions. We argue that, for such systems, the
``reduced'' second virial coefficient is a convenient parameter to quantify the
effective range of attraction. This procedure allows us to give a simple
definition of the effective range of attraction of potentials with different
functional forms. The advantage of the present approach is that it allows us to
estimate the relative location of the liquid-vapor and solid-fluid coexistence
curves exclusively on basis of the knowledge of the pair-potential.
