96 



H. L. Wheeler — Double Halides of 





Measured. 



Calculated 



c ~ c (twin) 0001 ^ 0001 



*71° 3' 





c a? 0001 ^ 1011 



54 21 



54° 28V 



m*r 1010^1011 



35 39 



35 3]| 



On examining this salt in convergent polarized light a uni- 

 axial cross is seen whose arms are not black but a deep and 

 brilliant blue, the character being negative. When examined 

 in monochromatic red light the crystals are nearly isotropic, 

 the double refraction being extremely weak and probably nega- 

 tive. In blue light, however, a distinct cross is seen accom- 

 panied by axial rings. This difference between red and blue 

 explains the colored cross seen in white light. Sections paral- 

 lel to the c axis show the deep peculiar blue characteristic of 

 uniaxial bodies with the above mentioned optical properties. 



SCsBr . %AsBi\. — This salt was made in crystals up to lj mm 

 in diameter. The forms observed are c, m, r and z This is 

 the only salt of the series that has a rhombohedral habit and 

 as the angle of the rhornbohedron is nearly 90° the crystals 

 look like cubes. Fig. 3 shows an ideal combination of r with 

 m, z and c. This form was not observed as the crystals are 



invariably twins. An ideal representation of the twinning is 

 given in fig. 4. The r faces were so curved and striated that 

 no exact measurements could be made from them. 







Measured. 



Calculated 



m a z 



0110^0111 



*35° 23' 





Z a 2 



1011 ~ 1101 





89° 50' 



SRbjBr . 2AsBr 3 . — This salt was made in small crystals, up to 

 2 mm in diameter. The forms observed were c, m and r. The 

 crystals were made in two habits. When prepared with an 

 excess of RbBr it separates in prismatic crystals which resem- 

 ble the form of caesium arsenious chloride, fig. 1. In one ex- 

 periment by using an excess of AsBr 3 contact twins were 

 obtained. Here the twinning plane is the unit rhornbohedron 

 as in fig. 2, but some of the faces are lengthened parallel to 

 the edge between the two basal planes (fig. 5). 



