BRIDGMAN. — WATER UNDER PRESSURE. 463 



lated slope, latent heat, and then other quantities of thermodynamic 

 interest, such as the work done in passing from one form to the other, 

 and the change of internal energy. 



In the early part of this work up to 3500 kgm. constant reference will 

 be made to Tammann,* since his are practically the only previous 

 experiments on ice under pressure. It will be well, therefore, to briefly 

 outline his work and show where it needs verification or completion. 

 Tammann's work consisted essentially in following out the equilibrium 

 curves without systematic measurements of the changes of volume. He 

 discovered the existence of two new forms of ice (II and III), and 

 studied their relations to water and ordinary ice (ice I). He obtained 

 points on the equilibrium curves I-L, III-L, I-III, and I-II. The 

 change of volume I-III and I-II was measured at a few points without 

 any very great accuracy. The greatest possibility of question as to 

 the results is in regard to the equilibrium curves I-III and I-II. Tam- 

 mann found that these curves cross at —37° and 2200 kgm. Now it is 

 necessary thermodynamically that a third equilibrium curve should 

 start from the point of intersection of two equilibrium curves with a 

 common phase. In this case the predicted equilibrium curve would be 

 II-III. Tammann found no such curve and even maintained that no 

 such curve was necessary, claiming that the thermodynamic argument 

 was valid only when the two phases were present simultaneously, and 

 that he had never been able experimentally to produce II and III 

 together. The argument seems inadequate, however, and in this work 

 the missing curve had been actually found. The other points of differ- 

 ence between this work and Tammann's are more or less minor in char- 

 acter ; there seems to be an error in Tammann's pressure measurements 

 of 100 kgm. at 2000, and he found a curious point of inflection in the 

 I-L curve, which does not seem actually to exist. 



For convenience, the method used will be briefly outlined. It was 

 essentially the same as that of Tammann, and consists in plotting the 

 displacement of the piston by which pressure is produced against pres- 

 sure. Change from one form to the other is accompanied by change of 

 volume at constant pressure. This change of phase is shown, there- 

 fore, by a discontinuity in the curve piston-displacement vs. pressure. 

 The pressure at the point of discontinuity gives the equilibrium pres- 

 sure and the volume swept out by the piston, which is determined by 

 the amount of the discontinuity, gives the change of volume on passing 

 from one phase to the other. The water on which these experiments 



* Tammami, Kristallisieren imd Schmelzen, pp. 315-344 (Barth, Leipzig, 

 1903). 



