POLYMORPHIC TRANSFORMATIONS OF SOLIDS. 57 



pressure isothermals preliminary to a reaction. Furthermore, in the 

 ease of those substances whose phase diagrams contain triple points, 

 it is possible to calculate completely the three differences from the 

 thermodynamic data of the transition curves alone. 



Among the data which do not enter into the thermodynamics of 

 phase change, but are nevertheless of significance for the mechanism 

 of the transition, may be mentioned the amount of superheating or 

 subcooling a phase will support, the velocity of transition, and the 

 width of the band of inrUfference. To fletermine these data so as to 

 have absolute significance is a matter of great experimental difficulty, 

 since slight impurities and the character of the apparatus produce 

 great changes in the measured effects. No such measurements have 

 been attemptefl in this work. Nevertheless a number of qualitative 

 and comparative measurements have been made incidentally in the 

 course of the measurements, which are of orienting value. Since 

 these measurements are so different in their character from the ther- 

 modynamic measurements which form the bulk of this paper, it has 

 seemed best to reserve most of them for anotlier paper. 



This paper is the first of several which are to deal with solid transi- 

 tions under pressure. In this first paper T will give the preliminary 

 thermodynamic discussion which will be necessary to all the work on 

 solid transitions, and also data for a number of such transitions. 

 Discussion of the data may be profitably left for later papers, after a 

 greater mass of data has been presented. 



Experimental Procedure. 



The procedure was, except for some minor details, like that of two 

 previous papers.^ The most important difference is in the receptacle 

 for holding the substance under investigation. Most of the sul)stances 

 were solid throughout the range of investigation. If the substance 

 was one not dissolved by kerosene it was usually rammed dry into a 

 thin shell made of a piece of steel tubing, open at both ends, the 

 tubing just fitting the inside of the pressure cylinder. The shell 

 was held in a heavy steel form during the filling, so that it should not 

 be bulged by the ramming into place of the material, which was accom- 

 plished with a steel piston and a heavy hammer. Frequently after 

 the shell had been filled, numerous holes were drilled laterally through 

 the walls, to procure as ready access as possible by the kerosene to all 



