136 



BRIDGMAN. 



shown in Figure 24, the computed values for the latent heat and the 

 change of internal energy in Figure 25, and the numerical values are 

 collected in Table XII. Both sets of pressure-temperature point's 

 are shown, but only the Av values of the second set of determinations. 

 The direction of curvature of the transition line, convex toward the 

 pressure axis, is noteworthy. It is in the same direction as for NH4I. 

 It was known previously that NH4Br has a second modification. 

 Its existence was first suspected from the beha\'ior of the mixed cry- 

 stals with NH4I, and was afterward established by Wallace ^^ by 



TABLE XL 



Ammonium Iodide. 



microscopic examination. No satisfactory measurements of the 

 physical constants are extant, however. Wallace was able to state 

 only that the high temperature modification has the larger volume. 

 Not only is this true, but the change of volume is one of the highest 

 known for solids. That Wallace did not comment on this shows the 

 difficulty of making quantitative estimates of the change of volume 

 from any alteration of appearance. I can verify this from my own 

 experience. The transition temperature found by Wallace was L59° 

 against L37.8° above. There would seem to be no question l)ut that 



Extrapolated. 



