128 II I. Wells — Caesium- and Potassium- Lead llalides. 



Calculated 

 Found. for Cs 4 PbBr G . 



Caesium 43-61 43-42 43-64 



Lead 16-83 16-83 16-98 



Bromine 39-24 39-33 39-38 



9968 99-58 100-00 



GsPoBi\. — This compound is dimorphous. One modifica- 

 tion forms small prisms of a bright orange color, the other is 

 pure white and crystallizes in slender needles. The orange 

 salt is obtained when lead bromide is dissolved in somewhat 

 more dilute solutions of caesium bromide than those required 

 for the formation of Cs 4 PbBr„, and there is a narrow range of 

 conditions where it crystallizes upon the latter salt. There is, 

 therefore, no evidence of the existence of an intermediate 

 compound, Cs 2 PbBr 4 , corresponding to one of the potassium- 

 lead bromides. Whenever solid lead bromide is added to a 

 concentrated solution of caesium bromide, it instantly loses its 

 white color and takes on that of the orange salt. The white 

 needles are formed in solutions which are slightly more dilute 

 than those required for the orange modification. The limits 

 of the conditions, under which this white salt is formed, are 

 very narrow and a great many trials were necessary before 

 satisfactory crops were obtained. Two distinct samples of 

 each salt were analyzed. The white needles were not abso- 

 lutely free from the orange compound, but there is no doubt 

 that they were sufficiently pure to show their composition 

 accurately. 



Found. Calculated 



Orange salt White salt. for CsPbBtv 



Caesium... 23-19 23-13 23-02 22-49 22-93 



Lead 35-69 35-39 35'24 35-88 35-69 



Bromine.. 41*37 41-34 41-47 41-45 41-38 



100-25 99-86 99-73 99'82 10000 



On heating the white modification to about 140°, it gradu- 

 ally assumes the exact color of the orange salt, without chang- 

 ing its external form, and this color is permanent on cooling. 



CsPb„B/\. — This salt is produced in solutions which are 

 still more dilute than those from which the preceding com- 

 pounds are obtained. It was first noticed at a volume of 

 about 160 cc of a solution containing about 50 g of caesium 

 bromide. It continued to form, on further dilution and the 

 addition of lead bromide, until the volume reached 1250 cc , 

 when lead bromide began to be .deposited. The conditions 

 under which the salt is formed are, therefore, very wide. The 



