VELOCITY OF POLYMORPHIC CHANGES BETWEEN SOLIDS. 83 



equilibrium point is in nearly every case greater with falling than with 

 rising pressure. There is no uniformity among different substances 

 as to the direction of variation of either the width of the band of indif- 

 ference or of the transition velocity with rising temperature along the 

 transition line. 



The first conclusion to be drawn is that the mechanism of a poly- 

 morphic transition cannot be the same in nature as that of a vapor- 

 ization. Equilibrium between a liquid and its vapor is a dynamic 

 equilibrium in which two independent processes are involved; mole- 

 cules of the liquid leave the surface of the liquid and enter the vapor 

 phase, and molecules of vapor impinge on the liquid surface and enter 

 the liquid phase. At those pressures and temperatures for which these 

 two streams of molecules in opposite directions each carry the same 



Figure 20. Curves showing at constant temperature the probable velocity 

 of vaporization of a liquid or condensation of its vapor as a function of pres- 

 sure. The equilibrium pressure is that at which the two curves cross. 



quantity of matter per unit time, there is equilibrium between liquid 

 and vapor. The velocity of these two streams of molecules may be 

 represented as a function of pressure at constant temperature as in 

 Figure 20. The equilibrium pressure is located at the point of 

 intersection of the two curves. It is immediately evident from the 

 diagram that at points near the equilibrium point the velocity of vapor- 

 ization or of condensation is equally great at points equidistant from 

 the equilibrium point on either side. 



Each of the two cardinal facts mentioned above shows that this 

 cannot be the mechanism of polymorphic transition. If such a 

 mechanism were consistent with the existence of a region of indif- 

 ference, the curves for velocity of transition in opposite directions 

 would have to be of the form in Figure 21. Further, if the mechanism 

 were consistent with unsymmetrical transition velocities on opposite 



