EFFECTS OF PRESSURE ON NITRATES. 591 



of the acid; even heating to over 200° in vacuum for several hours 

 was not effective. The presence of free acid was shown by the corro- 

 sion of the steel shell and cylinder after the runs were finished. Cer- 

 tain very puzzling inconsistencies in the behavior under pressure are 

 probably to be explained by the presence of acid. Professor Richards 

 has therefore undertaken to prepare a second sample, this time 

 crystallized from neutral solution. In the preparation of this sample 

 several unexpected delays have arisen and the way does not yet seem 

 clear to a successful outcome. I have therefore thought it best to 

 publish tentatively the results already obtained, and to reserve for a 

 future note the data on the perfectly pure sample. None of the con- 

 clusions reached in this paper can be altered by a slight change in the 

 numerical values. My unwillingness to delay longer is because the 

 rest of the data of this paper have now been awaiting publication for 

 somewhat over a year, and a succeeding longer paper is entirely 

 finished, waiting only for the appearance of this paper. 



Runs were made at three different times. The first gave five 

 points from 1000 to COOO kgm. These results were fairly regular; 

 the Av point at the lowest pressure was lower than was to be expected, 

 however. After two months the points at approximately atmospheric 

 pressure were determined. The equilibrium temperature was nearly 

 1° low, judged by extrapolation from the previous high pressure 

 points, and Av was 10% low. There was no possibility of accidental 

 experimental error; two independent determinations at 78 kgm. 

 gave very concordant results. Again after two months, four more 

 points were redetermined at high pressures. The Av points of this 

 run were very much lower than the previous points and more irregular; 

 the greatest discrepancy being 30% at 4000 kgm. The equililjrium 

 temperatures also did not agree, at 1000 kgm. being 0.4° low, and at 

 4000 kgm. 1.5° high. The transition line as redetermined w^ould, 

 therefore, extrapolate to a lower transition temperature at atmospheric 

 pressure. It seems probable that there was some slow permanent 

 change taking place in the RbNOa due to the free acid. In the 

 Figure and the Table, I have therefore given only the results of the 

 first run, which seem more likely to be accurate. 



The equilibrium pressures and temperatures and the changes of 

 volume are shown in Figure 6, and the numerical values are collected 

 in Table IV. In view of the certainty of revision of these data, it 

 did not seem worth while to give a diagram as usual for AH and AE. 



RbNOs is known to have another transition point at atmospheric 

 pressure,^ near 219°, so that there is another modification not shown 



