Equations which state that for the phase curve P(T ) of a second-order phase transition the derivative of pressure P with respect to temperature T is equal to (Cfp - Cip)/TV(γ^f - γⁱ) = (γ^f - γⁱ)/(K^f - Kⁱ), where i and f refer to the two phases, γ is the coefficient of volume expansion, K is the compressibility, C_p is the specific heat at constant pressure, and V is the volume.