CRYSTALLOGRAPHICAL RELATIONSHIP 247 



Double refraction very weak. For this and other thal- 

 lium salts, the refractive index is so high that no suitable 

 liquid can be found of sufficiently high refractive index to be 

 made use of in the total reflection method of measuring the 

 double refraction. Consequently no refractive index data 

 are given in this investigation. 



The following are the results of the analyses of the water of 

 crystallisation : 



Average of Analyses 

 H,0 6-43 % 6-31 % 4-82 % 



This salt undoubtedly crystallises with two molecules of 

 water of crystallisation. 



(NH 4 ) 3 T1C1 6 2H 2 



This salt crystallises tabular on |001| (Fig. 3). Large, 

 well-formed crystals were obtained, which in no case showed 



indications of prismatic growth parallel to the c axis. The 

 faces of jlOOj were well developed, while those of jllOj were 

 small, jlllj was always represented, and J113J occurred as 

 small faces giving quite good reflections. Faint indications of 

 J103J were also observed, but the reflections were not good 

 enough to give reliable measurements. The best reflections 

 were obtained from the faces of jlllj. 



Rammelsberg (10) mentions the forms J100J, jllOj, jlOlj, jlllj, 

 jOOlj, but he evidently found no indication of J113J or J103J. 



