CRYSTALLOGRAPHICAL RELATIONSHIP 263 



elongation along the b axis : jlOOj is here very small, while 

 JlOlj is particularly well developed. JOllj has fairly good 

 faces, but J120J was found on only one of the crystals examined. 



The following are the crystal measurements : 



System : Rhombic bipyramidal. 



Axial constants : a : b : c='9734 : 1 : 1-4180. 



Forms present : jlOOi, J101|, J011| : J120J occurs only once. 



S. G.=3-776 at 20. 



Analyses give the following results : 



CB. In. Br. 



32-80 14-45 49-42 

 Calculated 33-30 14-38 50'07 



H 2 O 

 2-47 

 3-26. 



Ill : The Series RMx 4 xH 2 



From the point of view of the present investigation this 

 series is relatively unimportant, as each member of the series 

 crystallises in the cubic system. The crystallographical 

 values are therefore identical, and, as the values for the re- 

 fractive indices could not be obtained for the thallium salts, 

 there was nothing to indicate the change in crystalline structure 

 due to the replacement of one element by another. Conse- 

 quently no thorough investigation was made of the salts 

 belonging to this series. Nickles (12) stated thatthe two salts of 

 the composition KTlBr 4 2H 2 and NH 4 TlBr 4 2H 2 are iso- 

 morphous, crystallising in the rhombic system ; and Meyer (17) 

 also refers to the former of these two salts as rhombic. Pratt (8) 



