Using these critical values to define reduced properties p r = p / p c , T r = T / T c , v r = v / v c renders the equation in the dimensionless form used to construct Fig. 1 p r = 8 T r 3 v r − − 1 − − 3 v r 2 This dimensionless form is a similarity relation; it indicates that all vdW fluids at the same T r will plot on the same curve. It expresses the law of corresponding states which Boltzmann described as follows: