The surface in Fig. 1.8 is a plot of the pressure of a fixed amount of perfect gas
against its volume and thermodynamic temperature as given by eqn 1.8. The surface
depicts the only possible states of a perfect gas: the gas cannot exist in states that do not
correspond to points on the surface. The graphs in Figs. 1.4, 1.6, and 1.7 correspond
to the sections through the surface (Fig. 1.9).