ATOMIC WEIGHTS 149 



RUBIDIUM. 



The atomic weight of rubidium has been determined by analyses of 

 the chloride and bromide. 



Bunsen/ employing ordinary gravimetric methods, estimated the ratio 

 between AgCl and EbCl. His rubidium chloride was purified by frac- 

 tional crystallization of the chloroplatinate. He obtained the following 

 results, to which, in a third column, I add the ratio between RbCl and 

 100 parts of AgCl: 



One grm. RbCl gave 1.1873 grm. AgCl. 84.225 



1.1873 " 84.225 



" 1.1850 " 84.388 



1.1880 " 84.175 



Mean, 84.253, ± .031 



Hence Eb = 85.309. 



The work of Piccard ' was similar to that of Bunsen. In weighing, 

 the crucible containing the silver chloride was balanced by a precisely 

 similar crucible, in order to avoid the correction for displacement of air. 

 The filter was burned separately from the AgCl, as usual; but the small 

 amount of material adhering to the ash was reckoned as metallic silver. 

 The rubidium chloride was purified by Bunsen's method. The results, 

 erpressed according to the foregoing standard, are as follows : 



1.1587 grm. RbCl = 1.372 AgCl + .0019 Ag. 84.300 



1.4055 " 1.6632 " .0030 " 84.303 



1.001 " 1.1850 " .0024 " 84.245 



1.5141 " 1.7934 " .0018 " 84.313 



Mean, 84.290, ± .0105 



Hence Eb = 85.362. 



Godeffroy,^ starting with material containing both rubidium and 

 caesium, separated the two metals by fractional 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 EbCl the AgCl was handled by the ordinary process of filtra- 

 tion. In the other two it was washed by decantation, dried and weighed 

 in a glass dish. The usual ratio is appended in the third column : 



iZeit. Anal. Chem., 1, 136. Poggend. Annal., 113, 339. 1861. 

 2Journ. prakt. Chem., 86, 454. 1862. Zeit. Anal. Chem., 1, 518. 

 3 Ann. Chem. Pharm., 181, 185. 1876. 



