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, Maxwell's 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 Maxwell's own statement. Related models Maxwell's model was not the first plaster model of a thermodynamic surface: in 1871, even before Gibbs' papers, James Thomson had constructed a plaster pressure-volume-temperature plot, based on data for carbon dioxide collected by Thomas Andrews. Around 1900, the Dutch scientist Heike Kamerlingh Onnes, together with his student Johannes Petrus Kuenen and his assistant Zaalberg van Zelst, continued Maxwell's work by constructing their own plaster thermodynamic surface models. These models were based on accurate experimental data obtained in their laboratory, and were accompanied by specialised tools for drawing the lines of equal pressure. See also History of thermodynamics Process leading from Gibbs' drawings to Maxwell's thermodynamic surface References External links Photograph of one of the two Cambridge copies in the Museum at the Cavendish Laboratory; for better readable legends to go with the axes, see here Thermodynamic Case Study: Gibbs' Thermodynamic Graphical Method at Virginia Tech's Laboratory for Scientific Visual Analysis Maxwell’s thermodynamic surface at the "Encyclopedia of Human Thermodynamics" 1874 in science 1874 sculptures Historical scientific instruments Sculptures in England Sculptures in the United States Scottish sculpture Numerical function drawing Scientific models Thermodynamics Works by James Clerk Maxwell