Let's say I have a general phase transition S (phase 1) ::equil:: S (phase 2). At a certain point in pressure and temperature for each phase (they can have different pressures and temperatures, we might say P1 and T1 for phase 1 and P2 and T2 for phase 2), how might we express the forward and reverse rates of this phase transition? No doubt the forward rate will be a function of P1, T1 and the activity of S (phase 1); the backward rate will presumably be an analogous function of P2, T2 and the activity of S (phase 2). But what function? And how should we work out how much the temperature of phase 1 changes on the basis of how many moles/what activity of S (phase 1) there is, and temperature of phase 2 changes on the basis of how many moles/what activity of S (phase 2) there is.