160 



WeUs, Wheeler and Penfield — Ccesium and 



Calculated 

 Found. for RbAuBr 4 



Rubidium. 1-4-1 S 



Gold 32-54 32-Y3 



Bromine 53-08 



Crystallography : — The crystallization of CsAuCl 4 , CsAuBr,, 

 RbAuCl 4 and RbAuBr 4 is monoclinic. The four salts form 

 an isomorphous group and are identical in crystalline habit. 

 The forms which have been observed on them are : 



c, 001, d, 021, 24 p, 111, 1 



to, 110. I e, 201, 2-1 



1. 2. 



The crystals are prismatic and are usually terminated by e, 

 fig. 1. When other faces are present they are always small, as 

 represented in fig. 2. The pyramid p, which is not shown in 

 the figure, frequently occurs as a small face, replacing the edge 

 between d and e. Among the crystals of CsAuBr 4 several 

 twins were observed, having p, 111 as the twinning plane, fig. 

 3, while fig. 4. represents a crystal of RbAuBr 4 twinned about e, 

 201. The letters belonging to the parts in twin position are 

 underlined. Both kinds of twins are abnormally developed as 

 represented in the figures. In all four compounds the cleav- 

 age is perfect parallel to the base. 



The rubidium salts, being the most soluble, form readily in 

 large crystals, several centimeters in length. The chloride, 

 especially, yielded magnificent crystals, which frequently were 

 only limited in length by the size of the vessel and volume of 

 the solution containing them. The caesium salts are less 

 soluble and were made in small prisms, seldom over 5 Tara in 

 length. The crystals were frequently hollow or cavernous at 

 the extremities, this was especially true of the two bromides. 

 The faces, for the most part, gave excellent reflections of the 

 signal on the o-oniometer. 



