CAESIUM. 89 



For the molecular weight of RbCl, two values are calculable : 



From (i) RbCl= 119.981, + .0109 



From (2) " 119.881, .0218 



General mean RbCl = 119.961, =b .0097 



To the value from ratio (2) I have arbitrarily assigned a weight rep- 

 resented by the probable error as written above. The data for system- 

 atic weighting are deficient, and no other course of procedure seemed 

 advisable. 



From RbCl Rb = 84.782, .0109 



From RbBr, ratio (3) " 84.786, .0329 



General mean Rb =r 84.783, .0103 



If = 16 Rb 85.429. 



CESIUM. 



The atomic weight of caesium, like that of rubidium, has been deter- 

 mined from the analysis of the chloride. The earliest determination, 

 by Bunsen,* was incorrect, because of impurity in the material employed. 



In 1863 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 chloride was prepared, and in this the chlorine was 

 estimated as silver chloride by the usual gravimetric method. Reducing 

 their results to the convenient standard adopted in preceding chapters, 

 we have, in a third column, the quantities of CsCl equivalent to 100 

 parts of AgCl : 



I-837 1 g rm - CsCl gave 1.5634 grm. AgCl. ' 117.507 



2.1295 " i. Si n " n7-5 8 



2.7018 " 2.2992 " 117-511 



1.56165 " 1.3302 " ( "7-399 



Mean, 117.499, .025 



Shortly after the results of Johnson and Allen appeared a new series 

 of estimations was published by Bunsen. J His caesium chloride was 

 purified by repeated crystallizations of the chloroplatinate, and the ordi- 



*Zeit. Anal. Chem., i, 137. 



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



J Poggend. Annalen, 119, i. 1863. 



