EFFECTS OF PRESSURE ON NITRATES. 605 



Several direct determinations of the difTerence of compressibility 

 were made, which were not, however, very accurate. At the triple 

 point I-II-III, there are no data for the computation, and at the 

 triple point II-III-IV the values are not accurate enough. This 

 much may, however, be stated with some confidence. Ill is more 

 compressible than I; the differe ice is of the order of O.O5I, and proba- 

 bly the diflference decreases with increasing pressure: III is also more 

 expansible than I, and the difference is of the order of O.O46. The 

 specific heat of I is less than that of III, the difference being about 

 1 kgm. cm. per gm. II is probably less compressible than III, and it is 

 probable that the difference of expansion between II and III changes 

 sign along the transition curve; at the higher temperature II is less 

 expansible, and at the lower temperatures more expansible. Ill is 

 more compressible than IV and the difference is of the order of O.O5I. 

 There is also a direct experimental determination of the difference of 

 specific heats of I and II by Bellati and Romanese,^° at 0.014 cal., 

 but the authors themselves say that the difference of specific heat 

 cannot be expected to be accurate. 



Search was made for other modifications to 12000 kgm. at 20° and 

 200°, but without success. 



Ammonium Nitrate, The material used was Kahlbaum's purest, 

 "zur Analyse," obtained from Eimer and Amend. Two series of 

 runs were made, separated by an interval of a year. For each of 

 these series of runs the material was hammered dry into a perforated 

 steel shell, and the pressure transmitted directly to it by kerosene. 

 For the second run, the salt was subjected to a preliminary drying in 

 vacuum at 100°. No difference in behavior between the two runs to 

 be attributed to this cause was to be detected. The second series, 

 repeating the first, was made necessary by the fact that the measure- 

 ments of Av given by the first series were irregular. The reason for 

 this is that ammonium nitrate is a substance for which it is particu- 

 larly difficult to force the reaction from one phase to another to run 

 to completion. It is very curious that a reaction which will start 

 to run with only a little subcooling or superheating will run to 

 completion only with a much greater subcooling or superheating. I 

 have found this same behavior several times with other substances. 

 It has also been observed for ammonium nitrate by Behn; ^^ he found, 

 for example, that for the transition IV-V it was necessary to lower 

 the temperature fully 30° beyond the transition temperature before 



12 U. Behn, Proc. Roy. Soc. 80, 444-457 (1907-08). 



