88 BRIDGMAN. 



measure of the speed of the transition we may use the pressure accelera- 

 tion of the speed at zero velocity. The acceleration of nearly every 

 transition is not symmetrical with respect to the direction of the 

 transition, but is greater in the direction accompanied by decreasing 

 pressure. Furthermore, there exists for most substances a distinct 

 region on both sides of the equilibrium point within which the transi- 

 tion will not run even when the two phases are in contact. This is the 

 "region of indifference." The acceleration of the transition and the 

 breadth of the band of indifference vary with pressure and tempera- 

 ture along the transition curve ; the manner of variation is different 

 for different substances. 



It results from these facts that an equilibrium between polymorphs 

 cannot be due to a balance between two transitions running in oppo- 

 site directions, as is the case for vaporization. This implies that in a 

 crystalline solid the velocities of temperature agitation of the mole- 

 cules cannot be distributed over a wide range of velocities, as is de- 

 manded by Maxwell's distribution law for gases or liquids, but must 

 be confined within a restricted range. The boundaries of this range 

 may be more or less sharp. The sharpness of the boundary is an 

 important factor in determining the acceleration of the transition 

 velocity. 



Acknowledgment. It is a pleasure to acknowledge generous 

 assistance from the Bache Fund of the National Academy of Sciences, 

 and from the Rumford Fund of the American Academy of Sciences, 

 with which materials and mechanical assistance have been obtained. 



The Jeffersox Physical Laboratory, 



Harvard Uxiversity, Cambridge, Mass. 



