612 BRIDGMAN. 



drawn, and considerations of the effect of incompleteness of the 

 reaction also indicate that the curve as drawn corresponds to the 

 actual facts. Thus, the change of volume between VI and IV found 

 at 173° is too small to lie on the curve through the points at higher 

 pressures by precisely the same amount that the change of volume 

 II-VI at the same temperature is too high to fall in with the values 

 demanded by the necessary conditions at the triple points. The 

 explanation is obviously that the reaction from IV to VI was not 

 complete because it was not possible to lower the pressure far enough, 

 and that the reaction was completed when the pressure was carried 

 across the line VI-II, the rest of the change from IV to VI appearing 

 as part of the change from VI to II. 



The data for the transition IV-V shown in the table and the diagram 

 were taken directly from the paper of Behn.-^^ Because of the great 

 stickiness of the reaction it would have been necessary to set up special 

 apparatus for reaching temperatures at least as low as — 50° if accu- 

 rate measurements of this transition were to be expected, and it did 

 not seem worth while to do this. The slope of the transition line 

 IV-V was not given by Behn, but has been computed by me from his 

 data. Behn states that he was not able to get values for the change 

 of volume which were self consistent to better than 20%, so the 

 value given for the slope may be in error by as much as this. 



The results for ammonium nitrate are different in one or two inter- 

 esting particulars from those found for most other solid transitions. 

 The change of volume II-IV increases with rising pressure instead of 

 decreasing, as it has for every other substance previously investigated. 

 The change of volume between III and IV also shows the same effect. 

 This is probably to be attributed to unusual properties of IV, as will be 

 brought out in a later discussion of the relations of the compressibili- 

 ties, thermal expansions, and specific heats. It is also very unusual, 

 although this is not the only example, that the III-IV line is concave 

 upwards. That this is true is indicated by several independent lines 

 of evidence. 



A good deal of work has been done on this substance by other 

 experimenters at atmospheric pressure, and there are at least two 

 pieces of work on the effects of pressure. The existence of four 

 modifications at atmospheric pressure has been known for some time, 

 but it is not so commonly mentioned that there is a fifth modification. 

 This seems to have been first discovered by Lehmann, was then 

 studied by Wallerant,^ and more lately by Behn^^; its existence 

 seems absolutely beyond question. The problems offered by this low 



