A physical visualization by Scottish physicist James Maxwell (left), constructed over the course of about seven months, from November 1874 to July 1875, based on the descriptions of thermodynamics surfaces described in two 1873 papers by American engineer Willard Gibbs. The molded shape depicts the geometry of the three-dimensional thermodynamic surface of the various states of existence of water: solid, liquid, orgas, shown on Cartesian coordinates of the entropy (x), volume (y), and energy (z) of the body. Maxwell constructed it in Cambridge and famously sent it to Gibbs in 1875 as a gift of appreciation for his graphical work.
Similarly, James Thomson constructed a plaster pressure-volume-temperature plot (right) in 1871, based on data for carbon dioxide collected by Thomas Andrews.
Source: Hmolpedia. Maxwell’s Thermodynamic Surface.