POLYMORPHISM AT HIGH PRESSURES. 129 



line. Thinking tliat by some careless oversight I might have run 

 across the II-IV line while raising temperature, I lowered the tempera- 

 ture at 8000 to 140°, which is far enough to compel the transition from 

 IV to II. This procedure was made necessary because on the tran- 

 sition line II-IV the superpressure required to force IV to change to 

 II is so great that it cannot be reached in the limits of this work. The 

 reverse transition, II to IV, however, runs with little transgression 

 of the transition line. After lowering temperature to 140°, I then 

 raised the pressure to 12000 and the temperature to 198.5°, the pres- 

 sure rising to 12500. The rise of pressure accompanying this last 

 change of temperature was larger than usual. Furthermore, as 

 judged by the rise of pressure, the process of attaining temperature 

 equilibrium extended over an unusually long interval of time, and then 

 suddenly the supposed process of temperature equalization ceased. 

 The result of all this manipulation was that, provided the II-IV line 

 continues as it starts, the phase II must certainly have been in the 

 apparatus at 12500 kgm. and 198.5°. Now on lowering pressure the 

 transition to IV should have been found at about 11200. No such 

 transition was found, however, down to 7500. The explanation, of 

 course, that suggests itself is that the line II-IV does not continue as 

 it starts, but encounters a triple point and splits into two. The 

 apparent sluggishness in reaching temperature equilibrium was really 

 due to the new transition. The point 12500 and 198.5° is therefore 

 in the domain of the new phase, V, and the point at 7500 and 198.5° 

 is doubtless in the domain of IV, the transition from IV to V not being 

 noticed because of the smallness of the change of volume. This was 

 verified by lowering temperature at 7500 from 198.5° to 181.6°, and 

 then lowering pressure further. The transition IV-III was found 

 in the location expected. 



Figure 18, for Ay, requires some comment. The points on II-IV 

 lie smoothly and normally, and the results are probably near the truth. 

 The shape of the Av curve for II-III is unusual in the marked rise on 

 approaching the triple point II-III-IV. This rise, however, is con- 

 sistent with the marked drop of Av for III-IV with falling temperature. 

 The rise and the drop both mean the same thing, that near the triple 

 point the thermal expansion of III becomes unusually large. Further- 

 more, the curves for II-III, III-IV, and II-IV satisfy the additive 

 condition at the triple point without forcing. It is probable that 

 affairs are really as measured, and that III does increase markedly in 

 expansion near the triple point. At the triple point I-II-III, however, 

 the state of affairs was not nearly so satisfactory. The change of 



