100 BRIDGMAN. 



reactions on all three curves were fairly rapid. The total time re- 

 quired to shut the pressure within the limits above, both from above 

 and below, averaged about 20 minutes on the I-III curve, one hour 

 on the I-II curve, and 20 minutes on the II-III curve, except at 30°, 

 which required 50 minutes. In no case was much lag observed in 

 going across any transition line in either direction. No careful meas- 

 urements of this were made; at 107° the reaction from III to I ran 

 on crossing the transition line by only 45 kgm. 



The experimental results are shown in Figures 21 and 22, the com- 

 puted values of A// and AE in Figure 23, and the numerical results in 

 Table X. None of the experimental points have been discarded. It 

 should be noticed that although in the table AT is given as increasing 

 with increasing temperature on the II-III line, the increase is slight, 

 and perhaps within the limits of error. No theoretical deductions 

 should be based on this increase of AV. A notable feature of the 

 phase diagram is the downward convexity of the I-III curve; such 

 curvature is never shown by a melting curve, but there are several 

 other examples of it for a solid transition. 



There are several previous determinations of the constants of the 

 transition. For the temperature of transition there is 145° by Kohl- 

 rausch ^* by an electrical method, 142° by Rodwell ^^ from the dis- 

 continuity in length, Schwarz ^ gives 146.9° on heating and 145.4° 

 on cooling, by the sudden change of color, Bellati and Romanese ^^ 

 give the transition point as approximately 150°, Mallard and Le 

 Chatelier ^^ as 146°, and Steger ^^ 147°. Tammann ^^ extrapolates 

 to 144.2° from his measurements at higher pressures. The extrapo- 

 lated value from the above work is 144.6°. For the latent heat of 

 transformation Mallard and LeChatelier give 6.8 cal. and Bellati and 

 Romanese 6.25 cal. at 150° against 5.77 cal. (2.46 kgm. cm.) found by 

 calculation from Clapeyron's equation above. There seem to be no 

 direct measurements of the change of volume of the transition at 

 atmospheric pressure except the value 0.0028 cm^ per gm. which I have 

 deduced with rather questionable assumptions from data of Rodwell. 



The effect of pressure on the transition point was first investigated 

 by Mallard and Le Chatelier, and has later been made the subject 



14 W. Kohh-ausoh, Wied. Ann. 17, 642 (1882). 



15 G. F. Rodwell, Proc. Roy. See. 25, 280-291 (1876-77), and Trans. Roy. 

 Soc. 173, 1125-1168 (1882). 



16 M. Bellati and R. Romanese, R. Inst. Ven. 1, 1043-1069 (1882-83). 



17 E. Mallard and H. Le Chatelier, C. R. 97, 102-105 (1883). 



18 A. Steger, ZS. phys. Chem. 43, 595-628 (1903). 



