POLYMORPHIC TRANSFORMATIONS OF SOLIDS. 



75 



Table III. The high temperature phase, that is, the phase with the 

 smaller volume is less compressible, more expansible, and has the 

 smaller specific heat. The differences between the two phases become 

 smaller at the higher pressures. The sign of the difference of compres- 

 sibility seems natural, but one might perhaps expect the high tempera- 

 ture phase to have the liigher specific heat. It is remarkable that the 

 behavior of the two phases here is the exact opposite of water and ice I. 

 Water at low pressures is more compressible, less expansible, and has a 

 higher specific heat than ice. 



No other modifications were found to 12000 kgm. at 0°, or to HOOD 

 at 100°. The decomposition at high temperatures prevented search 

 for other modifications at higher temperatures, and also made it use - 



TABLE III. 



Ammonium Sulfocyanide. 



less to try for points on the melting curve. It is a little surprising 

 that no other modifications were found. The abnormality of the high 

 temperature modification, since it is formed with decrease of volume, 

 might lead one to expect that at high pressures it would be replaced 

 by another modification giving a phase diagram something like that 

 of Agl. It may be, of course, that it is not the high temperature 

 form that is abnormal, but that instead it is the low temperature 

 form that has an abnormally large volume. In this case, the phase 

 diagram is the natural one. 



The possible relationship of the several modifications of KSCN and 

 NH4SCN is suggested by the crystalline forms. At first sight it is 

 surprising to find different types of phase diagrams for these two sub- 

 stances, because K and NHi usually replace each other isomorphously. 

 Gossner * has shown, however, by a comparison of the crystalline forms, 



