Below we see the surface that was the subject of Gibbs' second paper: the energy (U) - entropy (S) - volume (V) function, here for the fluid phases of ethylene. Stable, metastable, and unstable states join at the fluid-phase critical point marked "C." A light-blue curve traces the critical isotherm, and a white constant- pressure line (an isobar) runs from the region near the critical point to the high temperatures in the foreground. The stable and metastable regions are biconvex and curve outward toward the viewer, but the unstable region is shaped like a saddle, with opposite curvatures in the entropy and volume directions.

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