24 Wells and Penfield — Series of Caesium Trihalides. 



Where two halogens were present they were determined 

 in the usual way by weighing their silver salts and determin- 

 ing the loss in weight of these when heated in chlorine. In 

 the cases where all three halogens were present, use was made 

 of the extremely satisfactory method described by Gooch and 

 Ensign.* 



Csl s . 



This can be made by dissolving about one-fourth the theo- 

 retical amount of iodine in a solution of one part of caesium 

 iodide in ten parts of water. It generally gives a crop of 

 brilliant, slender crystals. If a larger proportion of iodine is 

 used, the substance generally separates in the form of crystal- 

 line plates without distinct faces. They are possibly a dimor- 

 phous form of the substance. If weak alcohol is used as a 

 solvent instead of water, the theoretical amount of iodine can 

 be taken and a well-crystallized product is obtained. 



The following numbers show the composition : 





Found 



Calculated for 





Slender Crystals. Plates. 



Csl s . 



Caesium .. 



25-41 23-71 



25-88 



Iodine 



12-61 



74-12 



When iodine is being dissolved in a warm aqueous solution 

 of Csl, or when an attempt is made to dissolve Csl 3 in warm 

 water, a heavy black liquid is formed at about 73° which 

 solidifies on cooling to a crystalline mass. It is much richer in 

 iodine than Csl 3 and probably contains a higher polyiodide. 

 Analyses of the substance gave varying results, and although 

 most of these approached the composition Csl 7 , it is still un- 

 certain what body this is. The low melting point of the sub- 

 stance is remarkable since Csl 3 melts at 210° and iodine at 

 114°. 



To find the solubility of Csl 3 in an aqueous solution of Csl, 

 the mother-liquor from a crop of crystals deposited at about 

 20° was analyzed. It gave : 



Csl 16-99 per cent. 



I (free) 0-416 per cent or Csl 3 0-842 per cent. 



The specific gravity of this mother-liquor was 1*154, hence 

 1 c.c. contained O0097gCsI 3 . The body is so insoluble that, 

 were we to use Csl in the place of KI in making a volumetric 

 iodine solution, we could only obtain, at ordinary tempera- 

 tures, a solution which would be about -£% normal. 



*This Journal. Ill, xl, 145, 1890. 



