CESIUM. 91 



crystallization of their alums, and obtained salts of each 

 spectroscopically pure. The nitric acid employed was tested 

 for chlorine and found to be free from that impurity, and 

 the weights used were especially verified. In two of his 

 analyses of RbCl the AgCl was handled by the ordinary 

 process of filtration. In the other two it was washed b}' 

 decantation, dried, and weighed in a glass dish. The usual 

 ratio is appended in the third column : 



1.4055 grm. RbCl gave 1. 6665 gim. AgCl. 84.33S 



1.8096 " 2.1461 " 84.320 



2.2473 " 2.665 " 84.326 



2.273 " 2.6946 " 84.354 



Mean, 84.3345, ± .0051 



Combining the three series, we get the following result 



Bunsen 84. 253, 



Piccard 84.290, 



Godeffroy 84.3345, 



General mean 84.324, dr .0045 



Hence Rb = 85.251, ± .018. If = 10, Rb = 85.529. 



CESIUM. 



The atomic weight of caesium, like that of rubidium, has 

 been determined from the analysis of the chloride. The 

 earliest determination, by Bunsen,* was incorrect, because 

 of impurity in the material employed. 



In 18G3 Johnson and Allen published their results.f 

 Their material was extracted from the lepidolite of Hebron, 

 Maine, and the caesium was separated from the rubidium as 

 bitartrate. From the pure caesium bitartrate caesium chlo- 

 ride was prepared, and in this the chlorine was estimated as 



* Zeit. Anal. Cham., i, 137. 



f Amer. Journ. Sci. and Arts, (2,) 35, 94. 



