POLYMORPHISM AT HIGH PRESSURES. 



127 



and temperatures and the sometimes very uncertain values for the 

 changes of vohime. 



The phase diagram is shown in Figure 17, the measured changes of 

 volume in Figure 18, and the numerical results in Table IX. The 

 phase diagram itself may probably be accepted as substantially correct. 

 The results could be repeated, and there was no rounding of the 

 corners of the transition curves, making it unlikely that the impurity 

 or chemical dissociation affects the transition pressures and tempera- 

 tures. The approximate location of the melting curve may be esti- 

 mated from the fact that points at higher temperatures than those 

 shown on the lines I-III and III-IV were attempted, but were not 

 possible because of the closeness of the melting. 



6 7 



Pressure, kgm./cm.' x 10^ 

 Ammonium Acid Sulfate ' 



Figure 18. Ammonium Acid Sulfate. The observed differences of vol- 

 ume. 



In addition to the phases shown, it is almost certain that the II-IV 

 line should split at higher pressures with the appearance of a fifth 

 phase. It was practically impossible to determine the exact location of 

 the transition, because of the very small change of volume, but the 

 existence of the transition was made practically certain in the following 

 way. At room temperature pressure was raised to about 11000 and 

 then temperature was raised to 198.5°, the pressure rising to something 

 over 12000. Then the pressure was lowered at constant temperature, 

 but I could find no point in the expected place (11200) on the II-IV 



