494 PROCEEDINGS OF THE AMERICAN ACADEMY. 



noted, that it is impossible to superheat II with respect to III. Evi- 

 dently if it had been necessary to superheat II even a single degi'ee in 

 order to start the reaction, the change of volume would not have been 

 sufficient to carry the pressure automatically back to the equilibrium 

 value, and the reaction would have run to completion. 



The procedure in getting the points was to start with II at some 

 pressure and temperature below the equilibrium line, and to raise the 

 temperature by small intervals. Arrival at the equilibrium line was 

 shown by an abnormally large rise of pressure. This pressure and 

 the corresponding temperature gave one point on the equilibrium line. 

 The reaction was not allowed to run to completion, because of the 

 difficulty of recovering II after it has once been changed to III, but the 

 pressure was immediately raised from the equilibrium pressure, bring- 

 ing the material back into the region of stability of II, and causing the 

 reverse reaction III-II to run to completion. Another equilibrium 

 point was then found starting with this higher pressure. 



The job was a rather fussy one because of the narrowness of the 

 critical temperature interval The temperature had to be maintained 

 by dropping solid CO2 into the DcAvar flask, and some little practice 

 was necessary before satisfactory results were obtained. Determina- 

 tions of the curve were made on three occasions. The first two sets 

 confirm the last set, but sufficient adeptness had not yet been attained 

 with the method, and the points were discarded. 



The final points, four in number, are shown in Table XVI. and Fig- 

 ure 9. The curve rises with increasing pressure from lower to higher 

 temperature and is convex upward ; it is terminated at either end by 

 triple points, at the lower end by the point I-II-III, and at the upper 

 end by the point II-III-V. This curve is new ; there are no previous 

 results with which to compare it. 



The change of volume determinations were made also with a slightly 

 modified method. The usual method of measuring piston displace- 

 ment at constant temperature would have been available if the meas- 

 urements had been made with decreasing pressure from the region of 

 II to that of III, but the method was undesirable practically for two 

 reasons. There was the difficulty of maintaining the temperature in 

 the Dewar flask constant for a sufficiently long interval of time by 

 hand, and there was the necessity of running down to the veiy lowest 

 temperatures in order to recover II for the next determination after 

 the reaction to III had been allowed to run to completion. This 

 would have required much time and would have been very wasteful of 

 solid CO2. 



The method used was a modification of that used in finding the I-II 



