In order to determine the phase diagram of the symmetric van der Waals
model for gas-liquid binary systems we employ a method developed by K
incaid and Cohen for the molecular held model of the metamagnet. The m
ethod is based on an expansion in the dominant order parameter. The si
milarities and difference of the magnetic system and the van der Waals
equation are discussed. It is shown how their method can be simplifie
d and adapted to more general cases. The result is compared with the c
alculations of Korteweg and found to be in accordance with his results
.