THE VELOCITY OF POLYMORPHIC CHANGES BETWEEN 



SOLIDS. 



By P. W. Bridgman. 



Received, October 29, 1915. 



In preceding papers ^ I have presented complete data for a number 

 of polymorphic transitions between solids under high pressures. It is 

 well kno-\\Ti, however, that data which may be complete from the point 

 of view of thermodynamics may not be at all complete from other 

 points of view, and therefore cannot be complete enough to determine 

 the mechanism of a process. For example, a complete description 

 of the thermodynamic behavior of a perfect gas gives no hold on the 

 viscosity or the thermal conductivity. The kinetic theory of gases, 

 however, which describes the mechanism, does account for viscosity 

 and thermal conductivity as well as for the thermodynamic prop)erties. 

 A knowledge of other properties than those of thermodynamics is 

 important, therefore, because of the additional light it may throw on 

 the complete mechanism. In this paper additional data of this kind, 

 data for the reaction ^•elocity from one phase to another, are given 

 for many of the substances for which the thermodynamic data have 

 been given in preceding papers. 



The plan of presentation is as follows. First, is given such descrip- 

 tion as may be necessary of experimental methods, the method of 

 computation from the data, and the abbreviated graphical method by 

 which an entire curve is represented by a single point. This will 

 involve emphasis of two important facts, namely that there is a 

 distinct region on both sides of the equilibrium point within which the 

 reaction will not run, and that the reaction velocity varies according 

 to the direction in which it is running, being almost always slower in 

 that direction which is accompanied by rising pressure. The data for 



1 P. W. Bridgman. (A) Proc. Amer. Acad., 439-558 (1912). 



(B) Phys. Rev., 126-141 and 153-202 (1914). 



(C) Jour. Amer. Chem. Soc, 36, 1344-1363 (1914). 



(D) Phys. Rev., 6, 1-33 and 94-112 (1915). 



(E) Proc. Amer. Acad., 51, 55-124 (1915). 



(F) Proc. Nat. Acad., 1, 513-516 (1915). 



(G) Proc. Amer. Acad., 51, 576-625 (1916). 

 Throughout the rest of this paper reference will be made to these papers by 



letter. 



