106 



BRIDGMAN. 



the red form changing to yellow at this temperature, and that presum- 

 ably the yellow had the larger volume, so that the transition line would 

 rise with increasing pressure. It will be easier to follow the descrip- 

 tion of the many runs made with this substance to turn at once to the 

 phase diagram of Figure 24. The first run was made at room tempera- 

 ture, and no transition was found to 12000. Temperature was then 

 raised to 150°, and transition found at about 8000 with very wide limits 

 of indifference. Other points were then found at 108°, 141°, 159°; 

 the pressure limits are indicated in the diagram. Temperature was 

 then raised to 200°, and no trace whatever of the transition could be 

 found between 12000 and 700. Pressure was then raised to 6800, and 

 temperature reduced to 160°, but no transition could be found down 



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 Pressure, kgm./cm.^ x 10* 

 Mercuric '"dide 



10 



Figure 26. Mercuric Iodide. The computed values of the heat of transi- 

 tion and the change of internal energy 



to 700. This was difficult to explain. What had become of the 

 transition found at lower temperature and also of that at atmospheric 

 pressure at 127°? On starting again at room temperature, raising 

 pressure to 8400 and temperature to 162°, the transition was found 

 again where expected, but on raising temperature to 180°, the transi- 

 tion disappeared. Another run at 185°, however, indicated a transi- 

 tion at 9600, but this was later traced to leak. Several other curious 

 effects between 160° and 200°, and 8000 to 12000 kgm. were also later 

 traced to leak, but were at first thought to be due to another transition. 

 At this stage the most plausible explanation seemed to be a rising 

 transition curve from 1 kgm. and 127° to about 7500 and 160°, and 

 here a triple point with the curve found below 160°, and the supposed 

 curve above 160°. To test this, temperature was raised from 126° 



