POLYMORPHISM AT HIGH PRESSURES. 



115 



Tabic VIL Concluded. 



In determining points on the V-VI line there is the same difficulty 

 as on the IV-V line; the difficulty increases toward the triple point 

 at the upper end. 



On the VI-IV line the points could not be determined with any 

 approach to accuracy because this is a reaction that will run in only 

 one direction. If pressure is released, starting with IV, the reaction 

 to VI will run with a discontinuous change of volume, but on increas- 

 ing the pressure again within reasonable limits the transition cannot 

 be made to run backwards. This does not mean at all that IV is 

 an unstable form. The behavior here is much like that on the rising 

 branch of the curve for mercuric iodide. The amount by which the 

 phase VI may be carried into the region of IV without the transition 

 running is very large. At 110°, the transition from VI to IV did not 

 run when pressure was increased to 9000 kgm. The points on this 



