276 H. L. Wheeler— Double Ralides of 



obtained when a considerable quantity of antimony iodide is 

 present ; under these conditions large deep red lozenge shaped 

 crystals, separate. Analysis gave : 



Large excess Large excess Calculated for 



of RbT. of Sbl 8 . 3RbI . 2SbI 3 . 



Rb 16*28 14-82 15'64 



Sb 14-14 15-17 14-64 



I 69-76 69-55 6972 



On exposure to the air the crystals slowly lose their luster. 



Crystallography. 



The crystallization of the 3 : 2 double salts is hexagonal. In 

 general the habit is rhombohedral and they can all be referred 

 to axes of nearly equal length. The double bromide and 

 iodide have a perfect basal cleavage, like the salts of the 

 arsenic series, while the chloride gave only a conchoidal frac- 

 ture. The axial ratios of the salts is shown by the following 

 table, the ratios of the corresponding arsenic salts being given 

 for comparison. 



a : c a : c a ; c 



3RbCl . 2SbCl 3 1 : 1*125 3RbCl . 2As01 3 1 : 1-210 3CsCl . 2AsCl 3 1 : 1-209 

 3RbBr.2SbBr 3 1 : 1-207 3RbBr.2AsBr 3 1: 1-220 3CsBr. 2AsBr 3 1: 1-219 

 3RbI . 2SbI 3 1 : 1-230 3RbI . 2AsI 3 1 : 1*242 3CsI . 2AsI 3 1 : 1*244 



From this table it may be seen that the substitution of 

 arsenic by antimony produces little if any effect in the lengths 

 of the axes, and, in each series the vertical axes lengthen as 

 the atomic weight of the halogen increases. 



3mCL2SbCl 3 .— This salt un- 

 like the others of the series, shows 

 rhombohedral tetartohedrism. In 

 one crop, where the crystals 

 measured 5 to 7 mm in diameter, 

 the faces were developed on every 

 crystal as in fig. 1, the forms be- 

 ing 



Fig. 1. 



a, 1120, i-2 e, 0112, — | y, 2532, —pr 

 m, 1010, I v, 1322, -£, «r 



On a second crop only e and a were developed. 



Measured. Calculated. 

 e~e, 0112^1102 56° 18' 

 e*v, 0112 ~ 1322 29 53 29° 55' 



e~y, 1322^2532 11 29 11 26 



y*a, 2532^1210 20 29 20 29 



