Pentahalides, with their Crystallography. 43 



we have been unable to prepare them. It may be stated that 

 special efforts were made to obtain potassium and rubidium 

 pentaiodicles. 



Csl 6 . 



This is produced, in an impure state as a black liquid solidi- 

 fying at about 73°, by treating caesium triiodide with hot 

 water and also by treating solid iodine with a hot solution of 

 caesium iodide. Artificial mixtures of caesium triiodide and 

 iodine, representing compositions varying from Csl 4 to Csl 9 , 

 all melt at a uniform temperature of about 73°. It is evident 

 from this that the composition of the black liquid cannot be 

 determined from its melting-point. 



Caesium triiodide, which is readily soluble in alcohol, be- 

 comes much more soluble in that liquid in the presence of two 

 atoms of iodine to the molecule. A very concentrated solu- 

 tion of this kind gives crystals of the pentaiodide by cooling, 

 but a much better product is obtained by concentration over 

 sulphuric acid, using a slight excess of iodine to allow for loss 

 by volatilization. The crystals are well formed and have a 

 brilliant black color. They can be distinguished from crystals 

 of iodine, which may separate if too much of this substance 

 has been used, by their brittleness as well as their form. The 

 substance melts, not sharply, at 73°. It loses -iodine on expo- 

 sure about as rapidly as iodine itself volatilizes. It does not 

 contain water or alcohol. 



Samples of the crystals quickly dried with paper gave the 

 following results on analysis : 





Made by 



By evaporation, 



Calculated 





cooling. 



Separate products. 



for Csl 5 . 



Caesium 



15-20 



20-96 16-02 



17-32 



Iodine 









82-68 



CsBr b . 



When a concentrated solution of caesium bromide is shaken 

 up with a large excess of bromine there is no separation of 

 caesium tribromide, as is the case when the theoretical amount 

 of bromine is used. A large part of the caesium bromide 

 goes into solution in the liquid bromine, and on taking up a 

 sufficient quantity of caesium bromide this solution becomes 

 lignter in color than pure bromine. 



A solution of caesium bromide in bromine, made in the 

 manner above indicated, was allowed to evaporate spontane- 

 ously at a temperature below 0°. A dark red solid finally 

 separated and it was prepared for analysis by pressing with 

 papers at the same low temperature. After the adhering 

 bromine had been removed the substance gave off bromine- 

 vapor very rapidly, 



