94 



H. L. Wheeler — Double Halides of 



double refraction. The rubidium compound was also obtained 

 in hexagonal crystals showing rhombohedral symmetry and 

 weak negative double refraction. 



CsT. As 2 0^ Rbl. As 2 3 and Kl. As 2 O r — The formation of 

 these compounds was observed when dilute hydriodic acid 

 solutions of the alkaline iodides were mixed with dilute acid 

 solutions of Asl 3 . If the solutions are mixed while hot these 

 double salts separated on cooling in the form of crystalline yel- 

 low crusts on the bottom and sides of the beaker. These crys- 

 tals are generally somewhat larger than those of the compounds 

 of As 2 O s with the chlorides and bromides. Under the micro- 

 scope they exhibited the form of six sided plates. These show 

 a strong negative double refraction. 



The potassium compound also appeared in the form of six 

 sided plates ; these remained dark when rotated between crossed 

 nicols. They were too small to afford an axial figure as the 

 largest plates did not exceed 00l mm in diameter. They are 

 probably hexagonal. Analysis gave : 



Cs _.. 2931 



As 32-01 



I 28-94 



O [9-74] 



Rb... 20-35 



As .._ 36-78 



I ._. 31-94 



O [10-93] 



K 10-75 



As 42-85 



I 34-13 



O [12-27] 



Calculated for 

 Csl . As 2 3 

 29-04 

 32-75 

 27-73 

 10-48 



Calculated for 

 Rbl. As 2 3 

 20-83 

 36-54 

 30'93 

 11-70 



Calculated for 

 KI . As 2 3 

 10-74 

 41-20 

 34-88 

 13-18 



Crystallography. 



The crystallization of the caesium and rubidium arsenious 

 halides is hexagonal. They were all measured and found to 

 be isomorphous. In general the habit was holohedral, although 

 in the case of caesium arsenious bromide it is rhombohedral. 

 All these salts show a pronounced basal cleavage, and plates 

 parallel to this, examined with the stauroscope, are uniaxial : 

 the double chlorides and bromides show a weak negative double 

 refraction while the double iodides are positive. The forms 

 observed are as follows : 



