The thermodynamic functions of an ideal subetance represented by van der Waals equation are obtained with the help of the condition that these functions reduce to those of a perfect gas in the limiting case of vanisning prassure. The volumes of the liquid state and gas state in coexistence as determined by Maxwell's rule of equal areas are expressed in a parametrie form. The nature of the dependence of the constants a and b on the chemical composition of the gas is briefly considered.