Arsenic with Caesium and Rubidium. 



95 



c 



0001 







a 



1120 



i-2 



m 



1010 



I 



1011 

 0111 

 2021 



The steep pyramid p was found only on the iodides. 

 The following table gives the lengths of the vertical axes. 



3CsC1.2AsC] 8 

 3RbCl . 2AsCl 3 

 3CsBr.2AsBr 3 

 3RbBr.2AsBr 3 

 3CsI.2AsIs 3 

 3RbI . 2AsT 



1-209 

 1-210 

 1-219 

 1-220 

 1-244 

 1-243 



A comparison of the above axial ratios shows the interesting 

 fact that the substitution of rubidium for caesium produces no 

 appreciable effect in the lengths of the axes, and that in this 

 series the vertical axes lengthen as the atomic weight of the 

 halogens increases. The crystals were 

 sufficiently stable to yield good meas- 

 urements, although on long exposure 

 they usually lose their luster. In the 

 lists of measurements the angles chosen 

 as fundamental are noted by an aster- 

 isk. 



SCsCl.2AsCl 3 .—ThhsaAt was made 

 in crystals about l-2 mm in diameter. 

 The forms observed are m, a, r, z and 

 c. A careful search was made for in- 

 dications of a rhombohedral development of the faces r and s, 

 but none was found. Apparently they are always holohedral 

 in their development, fig. 1. 



r=7 







Measured. 



Calculated 



m * c 



1010 ,s 0001 



90° 



90° 



c ~r 



oooi ~ ion 



*54 24' 





r a m 



1011 a 10.0 



35 39 



35 36' 



m ~ z 



1010*0111 



66 3 



66 1 



r * 2 



1011*0111 



47 53 



47 58 



There is a rather poor prismatic cleav- 

 age and plates parallel to this show par- 

 allel extinction. 



3fibCl.2AsCl 9 — This salt was made 

 in crystals up to about 5 mm in diameter. 

 The forms observed are c, m, r and z. 

 The faces r and z were seldom present, 

 but when these did occur it could not 

 be seen whether they exhibited rhombo- 

 hedral symmetry or not. Penetration 

 twins are common, the twjnning plane 

 being the rhombohedron 0111, tig. 2. 



