POLYMORPHISM AT HIGH PRESSURES. 



143 



In view of the known chenncal behavior of such double salts, there 

 seems httle question as to the proper interpretation. We have here 

 not a polymorphic transition in the proper sense of the word, but a 

 reversible decomposition of the double salt into two simple salts. 

 The decomposition is further complicated by the formation of mixed 

 crystals between the various products. The correctness of this 

 surmise would be at once verified if the extrapolated transition at 

 atmospheric pressure agreed with the known temperature of decom- 

 position. I have not been able to find, however, that this point has 

 ever been determined; the decomposition point of the hydrous salt 

 is well known, but no work seems to have been done on the anhydrous 

 salt. 



T 



E 



O 

 > 



2000 4000 

 Pressure, kgm./cm.^ 



K^Zn (504 



Pressure 



Fig. 28. Fig. 29. 



Figure 28. Zinc Potassium Double Sulfate. The observed equilibrium 

 pressures and temperatures. This transition is probably a decomposition of 

 the double salt into two simple salts. 



Figure 29. Zinc Potassium Double Sulfate. Shows the general nature 

 of the change of volume. 



If this explanation is correct, it removes this substance at once from 

 the range of this work, and I did not try for further data. It is ob- 

 vious, however, that there is here an immense field, as yet untouched. 

 For instance, it would be of great interest to examine the other double 

 sulfates of this series. The careful w^ork of Tutton shows that the 

 hydrous salts of the series present remarkable similarities. I am not 

 aware that any measurements have ever been made on the decomposi- 

 tion of double salts, or on mixed crystals, under pressure. The work 

 above showed that accurate measurements on mixed crystal equilibria 

 under pressure is going to involve very tedious manipulation, because 

 diffusion takes place so slowly. 



Thallium. — This substance was procured for the purpose of making 

 the nitrate from it; the measurements on the pure metal were under- 



