This equation has the same form as the original, but the coefficients a and b, which
differ from gas to gas, have disappeared. It follows that, if the isotherms are plotted in
terms of the reduced variables (as we did in fact in Fig. 1.20 without drawing attention
to the fact), then the same curves are obtained whatever the gas. This is precisely the
content of the principle of corresponding states, so the van der Waals equation is
compatible with it.