22 Wells and Penfield — Series of Caesium Trihalides. 



served. The crystals are thin tables somewhat lengthened 

 in the direction of the brachy-axis, fig. 4. On examining the 

 general table, it will be seen that this is the 

 only one of the first five salts, in which the 

 angle e^,e varies considerably from 78°. 

 Here, the variation amounts to a little over 

 one degree, but, in all probability, this is 

 not to be accounted for by imperfections 

 in the crystals or inaccuracy in the observa- 

 tion, for from two different crops of crys- 

 stals good reflections and almost identical 

 measurements were obtained. The crystals 

 were only a fraction of a millimeter in thickness and not over 

 10 mm long in the direction of the brachy-axis. With the polar- 

 izing microscope the tables show a decided pleochroism. For 

 rays vibrating parallel to the 6 axis the color is dark brown, 

 almost opaque, while for vibrations parallel to a it is a rich 

 reddish brown. A similar, though less marked, pleochroism 

 was observed in the remaining salts of the series, but owing 

 to the inability to obtain orientated pinacoid sections, it could 

 not be studied satisfactorily. In convergent polarized light, 

 the phenomena were not very distinct, but with the tables of 

 CsBrl„ apparently an obtuse bisectrix could be seen, the opti- 

 cal axis being in the macro-pi nacoid a. 



CsBr 2 I. — On this salt the forms «, c, m, d, f . and e were 

 observed. The habit is shown in fig 5. 

 The crystals were brilliant and gave ex- 

 cellent reflections. Those submitted for 

 measurement were about 3 mm in greatest 

 diameter. 



CsClBrl. — On this salt the forms b, e, 

 m, d, f and e were observed. The habit 

 is like fig. 5, but much longer, or pris- 

 matic, in the direction of the brachy-axis. 

 The crystals were about 2 mm in diameter 

 and 10 in length. 



CsGlJL. — This compound is dimorphous. 



On the orthorhombic modification the 

 forms «, c, m, d, f and e were observed, 

 but m and f are usually wanting. The 

 habit is shown in fig. 6. The crystals 

 were about 2 mm in diameter. 



The hexagonal, rhombohedral, variety 

 occurred in curious saddle-shaped scales, 

 with bright crystal faces only along the 

 edges. The forms which were ob- 

 served are r (1, 1011),/. (-2, 0221) and a 



