Wells and Penfield — Series of Caesium, Trihalides. 19 



Melts Melts Becomes white 



in open tube. in sealed tube. in open tube. 



(uncorr.) (uncorr.) (approximate.) 



Csl 3 210° 201°-208° 330° 



CsBr„l 246° 243°-248° 320° 



CsClBrI 238° 225°-235° 290° 



CsClJ 238° 225°-230° 290° 



CsBrI 2 208° 155°-190° 260° 



CsBr 3 whitens 180° 160° 



CsCLBr ... whitens 205° 150° 



CsClBr 2 ... whitens 191° 150° 



. Behavior with solvents. 



All these bodies except CsBrI 2 , which is almost completely 

 decomposed by water, can be recrystallized by treating with 

 warm water and cooling the solution. There is usually some 

 decomposition during this operation, accompanied by the sepa- 

 ration of iodine or the volatilization of this or the other 

 halogens. 



All the trihalides containing iodine can be dissolved in 

 alcohol and recrystallized from it. There is usually a slight 

 deposition of normal halide at the same time, which can be 

 avoided by adding a little water to the alcohol. Csl 3 is much 

 more soluble in alcohol than in water. The other iodine com- 

 pounds, with the exception of CsBrI 2 , which decomposes with 

 water, are apparently more soluble in water. Those bodies 

 containing no iodine are all decomposed by alcohol, leaving a 

 white residue. Mixtures of alcohol and water are good sol- 

 vents for all the trihalides. 



Ether has no immediate action on the more stable com- 

 pounds, Csl 3 , CsBr 2 I, CsClBrI and CsCl 2 I, but it decomposes 

 all the others with separation of normal halides. "When CsBrI 2 

 is thus decomposed, pure CsBr is left. 



Crystallography. 



The crystallization of the caesium trihalides is orthorhom- 

 bic. The salts form an isomorphous group, the chief features 

 of which will first be given, followed by a brief description of 

 the different individuals. 



The forms which have been observed are : 



a, 100, i-i g, 012, \-i 



b, 010, i-i d, 011, 14 



c, 001, O f, 021, 2-1 

 m, 110, I e, 102, t-i 



Of these m, d and e are the most prominent and usually 

 determine the habit of the crystals. Either m or d usually 

 predominates to such an extent that the crystals are prismatic 

 in the direction of the vertical or the brachy-axes. The dome 



