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BRIDGMAN. 



transition was partly completed. When the transition once started 

 it ran rapidly to completion in less than fifteen minutes. The points 

 shown in Figure 14 are the pressures of spontaneous starting of the 

 transition, from IV to II with decreasing pressure, and from II to IV 

 with increasing pressure. The true equilibrium line lies somewhere 

 in the region limited by these points ; there is no reason why it should 

 lie in the center of the region. The actual position of the curve as 

 shown was determined from the conditions of compatibility at the 

 triple points. The curve I-II also has a small change of volume, but 

 it was large enough so that the equilibrium values of pressure and 

 temperature could be found when both phases were present simultane- 

 ously. On the line III-IV, which has the largest Av of all, equilibrium 

 points could be found in the regular way. The difficulty with this 

 line is in the rapidly increasing slowness of the reaction toward the 

 lower temperatures. At 50°, the equilibrium value could not be found 

 to better than 750 kgm., even when both phases were present together, 

 and it required more than one hour for the transition to run to appar- 

 ent completion more than 2500 kgm. below the equilibrium pressure. 

 This slowness doubtless has something to do with the bad A^ value at 

 50° on III-IV. It was quite out of the question to try for lower 

 points on this line; it has been mentioned that at 20° the transition 

 will not start at 12000, and at 50° it required a superpressure of 4000 

 kgm. to start the transition. The points on the II-III line presented 

 little difficulty. 



