86 



BRIDGMAN. 



I to the phase II, it must pass over the intervening maximum of energy. 

 This passage may take place if the range of irregular thermal agitation 

 is so great that some molecules occasionally possess an amount of 

 energy greater than the average equal to the height of the hill, but 

 otherwise the transition will not take place, even when the phases are 

 in contact. In this way we get a region of indifference. 



If there is a region of indifference, its width is fixed by the position 

 of the first curve beyond the equilibrium curve on which the height of 

 the hill can be surmounted by the haphazard temperature agitation, 

 aided by the orienting forces of the more stable phase, which away 

 from the equilibrium point prevail over those of the unstable phase. 

 According to the specific effect of pressure and temperature on the 

 shape of the potential energy curves and on the random distribution 



1 



L 



p~ 2p 



Figure 23. Shows at constant temperature the potential energy of posi- 

 tion against position coordinate in the neighborhood of a transition point for 

 a substance which has two polymorphic modifications. 



of temperature agitation, it is easy to see that the band may become 

 broader or narrower at higher temperatures on the transition line. 

 It is conceivable, although unlikely, that the height of the hill to be 

 surmounted should so increase on both sides of the equilibrivun point 

 that the reaction will never run, no matter how far one goes from the 

 equilibrium point. Or the other extreme is more probable, that at 

 points sufficiently far from equilibrium, the hill, together with one of 

 the minima, should totally disappear, resulting in the absolute in- 

 stability of the corresponding phase. This is Ostwald's labile state, 

 or the unstable portion of James Thomson's isothermal. 



The pressure acceleration of the transition velocity is evidently 

 intimately connected with the sharpness of the region of random 

 temperature agitation. If all the. molecules once in every few oscilla- 



