The spin-1 lattice-gas model used in paper II to represent the behavior of a binary fluid is shown to represent as well the behavior of a ternary fluid kept at a fixed pressure. Tricritical points and quadruple points in binary and ternary fluids are described within the same model in the case of a zero crossed dipolar-quadrupolar interaction $L$. They are compared to those predicted or observed in simple fluids and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures.

• Received 19 February 1974

DOI:https://doi.org/10.1103/PhysRevA.11.2101