74 



BRIDGMAN. 



melting. There are two reasons; the latent heat of transition is 

 usually very much less than that of melting, and the velocity of a 

 solid transition is usually less than that of melting or freezing. The 

 thermal effect is, however, entirely absent only when there is no heat 

 of transition; that is, when the transition curve is vertical. Several 

 substances approach rather close to this ideal. 



The unavoidable result of slow rate of dissipation of latent heat is 



7300 7900 

 Pressure 

 Mercury 



Fig. 18. 



4200 4800 

 Pressure 

 Benzophenone 



Fig. 19. 



FiGUEE 18. The time rate of change of pressure as a function of pressure 

 during the melting and freezing of mercury under pressure at 0°. The sym- 

 metry of the curves is due to slow thermal conduction. 



Figure 19. The rate of freezing or melting of benzophenone, expressed 

 as fractional parts of the total change per minute, as a function of pressure. 

 The steeper curve at the left shows that melting is more rapid than freezing. 



that only a relative significance can be attached to these results. 

 There are other factors also which rule out the possibility of an abso- 

 lute interpretation. The effect of the manner of the mechanical con- 

 straint of the specimen is to be considered. Most of the specimens 

 were originally in the form of dry powder or small crystals; these were 



