In systems containing a single component, the change ΔT of the temperature of equilibrium between two phases resulting from a change of hydrostatic pressure ΔP upon the two phases is given by Clapeyron’s equation, ΔT/ΔP = TΔV/ΔH, where T is the absolute temperature, and ΔV and ΔH are the volume change and heat absorption, respectively, for a given mass of material in passing from one phase to the other. One phase may be considered the “high-temperature” phase, the other, the “low-temperature” phase. Heat is always absorbed in passing from the low-temperature phase to the high-temperature phase at constant pressure (the actual quantity may be negligibly small). In the same way, one of the phases will be the “low-pressure” phase, the other the “high-pressure” phase; in passing from the low-pressure phase to the high-pressure phase at constant temperature, the density always increases, or ΔV is negative. There is, however, no a priori way of knowing whether the high-temperature phase will be the high-pressure phase or the low-pressure phase. The liquid phase is “normally” the high-temperature, low-pressure phase with respect to solid phases; thus, in passing from a “normal” solid phase to the liquid phase, ΔV and ΔH are both positive, and ΔT/ΔP is positive (the melting temperature increases with . . .