18 Wells and Pmfield — Series of Caesium Trihalides. 



Color. 



In the following list the compounds are arranged in order, 

 from the darkest to the lightest. The colors given, unless 

 otherwise specified, are for crystals of considerable size, for 

 when the bodies are obtained as precipitates or when the 

 crystals are pulverized they are lighter in color. 



Csl 3 Brilliant black, nearly opaque ; powder brown. 

 CsBrI a Dark reddish-brown ; thin crystals transmit 



deep red light ; powder dark red. 

 CsBr 2 I Deep cherry-red. 



p t> . ( j Yellowish-red each having a somewhat 

 P n"R • \ ( yellower tint than the one preceding it. 



P PI T i Orthorhombic variety, deep orange. 

 2 | Rhombohedral variety, pale orange. 

 CsCl g Br Bright yellow. 



Stability on exposure. 



The five bodies containing iodine are much more stable than 

 the others, and will bear long exposure to the air at ordinary 

 temperatures with very slight superficial change. This ex- 

 posure in some cases may be continued for a week or more in 

 warm weather without producing any marked alteration of 

 color, but they constantly give off a slight odor and finally 

 begin to whiten. The three compounds containing no iodine 

 usually become white in a few hours on exposure, but even 

 these can be preserved indefinitely in tightly corked tubes. 

 Experiments showed that CsBrI 2 whitened more rapidly than 

 CsBr 2 I, also that CsClBr 2 decomposed more rapidly than 

 CsCl a Br. This indicates that their stability does not entirely 

 depend upon the volatility of the halogens contained in them, 

 a point which has a bearing on the constitution of this group 

 of bodies, and which will be considered subsequently. 



Behavior when heated. 



The following table shows the temperatures of complete 

 decomposition as determined by the change of color to white. 

 They are only approximate since they represent gradual 

 changes which vary somewhat with the rapidity of heating. 

 The melting-points are also given. In open tubes these are 

 usually sharp, but in sealed tubes often very gradual. 



