CRYSTALLOGRAPHICAL RELATIONSHIP 251 



of Jlllj and jlOlj are well developed ; and |110j occurs as fairly 

 broad faces. The reflections are very good. 



Pratt (8) described this salt as crystallising in the tetragonal 

 system, but with only one molecule of water of crystallisation. 



Rb 3 TlBr 6 2H 2 

 FIG. 5 



The results of his investigations are given in a separate column 

 in the following statement of the crystallography of the salt. 



System : Ditetragonal bipyramidal. 



Axial constants : a : c=l : '8038. 



Forms present : JlOOj, jllOj, J001|, }101j, {lllj. 



S. G. -4-077 at 20. 



The following are the results of analysis : 



Rb. Tl. Br. 



25-69 



Calculated for Rb,TlBr.2H,O 26-25 

 Rb,TlBr, H S 



26-76 



20-46 

 20-90 

 21-30 



48-85 

 49-15 

 50-06 



H a O 

 3-60 

 3-69 

 1-88 



The analysis shows that the salt crystallises with two 

 molecules of water. All the water of crystallisation is driven 

 off by heating the salt to 120 C., and it is rather difficult to 



