Contents Page Table 12 1. Melting and solid-solid transitions of elements 177 1.1. Melting temperatures of A, H2, He, N2, Ne, and O2 180 1.2. Formulas for melting curves 182 2. Melting and transitions of compounds 183 3. Liquid-vapor critical data 185
 Contents Page Table 12 1. Melting and solid-solid transitions of elements 177 1.1. Melting temperatures of A, H2, He, N2, Ne, and O2 180 1.2. Formulas for melting curves 182 2. Melting and transitions of compounds 183 3. Liquid-vapor critical data 185

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, ΔTP = TΔVH, 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 ΔTP is positive (the melting temperature increases with . . .

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