text: As explained by Gibbs and appreciated by Maxwell, the advantage of a U-V-S (energy-volume-entropy) surface over the usual P-V-T (pressure-volume-temperature) surface was that it allowed to geometrically explain sharp, discontinuous phase transitions as emerging from a purely continuous and smooth state function ; Maxwell's surface demonstrated the generic behaviour for a substance that can exist in solid, liquid, and gaseous phases. The basic geometrical operation involved simply placing a tangent plane (such as a flat sheet of glass) on the surface and rolling it around, observing where it touches the surface. Using this operation, it was possible to explain phase coexistence, the triple point, to identify the boundary between absolutely stable and metastable phases (e.g., superheating and supercooling), the spinodal boundary between metastable and unstable phases, and to illustrate the critical point.
  Maxwell drew lines of equal pressure (isopiestics) and of equal temperature (isothermals) on his plaster cast by placing it in the sunlight, and "tracing the curve when the rays just grazed the surface." He sent sketches of these lines to a number of colleagues. For example, his letter to Thomas Andrews of 15 July 1875 included sketches of these lines. Maxwell provided a more detailed explanation and a clearer drawing of the lines (pictured) in the revised version of his book Theory of Heat, and a version of this drawing appeared on a 2005 US postage stamp in honour of Gibbs.
  As well as being on display in two countries, Maxwells model lives on in the literature of thermodynamics, and books on the subject often mention it, though not always with complete historical accuracy. For example, the thermodynamic surface represented by the sculpture is often reported to be that of water, contrary to Maxwells own statement.