Another method of specifying the vdW constants pioneered by W.B. Kay, and known as Kay's rule. specifies the effective critical temperature and pressure of the fluid mixture by T c x = ∑ ∑ i = 1 n T c i x i and p c x = ∑ ∑ i = 1 n p c i x i In terms of these quantities the vdW mixture constants are then, a x = 27 64 ( R T c x ) 2 p c x b x = 1 8 R T c x p c x and Kay used these specifications of the mixture critical constants as the basis for calculations of the thermodynamic properties of mixtures.