254 INDIUM AND THALLIUM IN 



therefore decided to take this prism face as |110j, and the 

 shorter of the two almost equal axes as the axis a. This 

 prism had been formerly taken to be j 101} ; so that the new 

 position of the crystal simply involved an interchange of the 

 6 and c axes, the a axis remaining as before ; the change 

 necessitated, in other words, a rotation of the crystal through 

 90 round the a axis. In the various crystals of the series 

 there is a pronounced cleavage parallel to (Ollj, which has 

 the same indices for both positions of the crystal. There is 

 then no a priori reason, from cleavage considerations, why the 

 crystal should have been placed in a position which tends to 

 hide, rather than accentuate, its pronounced pseudo-tetragonal 

 character. 



Rb 2 TlCl 5 H 2 O 



From the solution containing the chlorides RbCl and 

 T1C1 3 in the proportion given in the salt formula, thin flakes of 

 Rb 3 TlCl 6 H 2 (monoclinic) are first precipitated. On further 

 evaporation these disappear, being replaced by large well- 

 formed crystals of Rb 2 TlCl 6 H 2 0. These crystals are much 

 distorted, being always tabular on jlOlj, the faces of which 

 are extremely well developed. Truncating the edges of the 

 pinacoid formed by two large faces of JlOlj are three smaller 

 faces on the upper side, and three on the lower side of the 

 crystal plate. These are the two remaining faces of JlOlj, 

 and the four faces of |001[. Were it not for the pronouncedly 

 tabular habit, the combination of the two forms JlOlj and 

 Ollj would strongly suggest the octahedron ; as it is, each 

 crystal appears as a triangular plate with the vertices cut 

 away. The characteristic habit of the crystal is shown in 

 Fig. 6. The faces of |100j occur very seldom, and are then 

 represented by narrow threadlike bands. The reflections are 

 very good. 



The interfacial angular measurements of Pratt are 

 appended to the following table of crystallographical data : 



