VELOCITY OF POLYMORPHIC CHANGES BETWEEN SOLIDS. 85 



polymorphic phases; we must therefore find some other mechanism 

 wliich will carry the molecules from one phase to the other. As I have 

 mentioned in a previous paper/^ the fact that the conditions of equili- 

 brium involve the constants of both phases shows that a transition from 

 one phase to another does not take place because the first phase, at 

 the equilibrium pressure and temperature, suddenly becomes inher- 

 ently unstable, and falls apart into its elements, which then build 

 themselves up into some other arrangement which untler the conditions 

 does happen to be stable. The instability of one phase is only a 

 relative instability, into which the properties of the other phase enter. 

 The driving force from one phase to the other is doubtless to be found 

 in a definite orienting force exerted by the one phase on the molecules 

 of the other. The same orienting force comes into play when a crystal 

 separates from solution; there is a field of force like a skin over the 

 crystal which compels the molecules being freshly deposited to orient 

 themselves definitely with respect to the regular assemblage of mole- 

 cules already laid down. In the same way, when two polymorphs are 

 in contact, each phase reaches into the other and strives to orient the 

 molecules of the other into its own position. Above the equilibrium 

 point the orienting forces of one phase prevail, and below it those of 

 the other. This struggle for mastery between the orienting forces of 

 the two phases is a static rather than a dynamic struggle, like a tug of 

 war rather than a game of tennis. 



It is possible to represent graphically some of the counter-play of 

 forces on the molecules. We will go to the extreme of simplification 

 and suppose that at any constant temperature and pressure all possible 

 configurations, whether stable or unstable, of the molecules of a crystal 

 may be defined by a single position coordinate. Corresponding to 

 each arrangement there is a definite potential energy. We plot 

 potential energy against position coordinate. If the configuration is 

 a stable one, the potential energy is a minimum. If the substance has 

 two arrangements of possible stability (polymorphism) there will be 

 two minima, and the lowest one will correspond to the absolutely 

 stable form. At an equilibrium point the two phases are equally 

 stable and the two minima are at the same level. At pressures above 

 equilibrium pressure (at constant temperature) the minimum of one 

 phase becomes the absolute minimum, and vice versa. Such a state 

 of affairs is indicated in Figure 23. Let us now consider the curve 

 corresponding to equilibrium. If a molecule is to pass from the phase 



15 D, p. 108. 



