RICHARDS AND ARCHIBALD. — ATOMIC WEIGHT OF CAESIUM. 469 



AgCI (143.385) : KCl gives K = 39.137 ± 0.001 



Ag (107.93) : KCl gives K = 39.141 ± 0.001 



N2O5 (108.08) : K2O gives K = 3.9.141 ± 0.002 



Average 39.140 



This result confirms the results of Stas, but disagrees with Clarke's 

 value (39.11) by nearly one tenth of a per cent. The reason for the 

 flifference is to be traced chiefly to the influence of sonae relatively inexact 

 analyses of potassic iodide by Marignac, which have seriously reduced 

 Clarke's averase value ; moreover Clarke's estimate of chlorine is some- 

 what lower than ours. The value given above, 39.14, seems to us to 

 represent more nearly the true atomic weight of potassium. 



On the other hand, assuming from the experiments with the halides, 

 that the atomic weights of caesium and potassium are respectively 132.879 

 and 39.139, it becomes possible to calculate the atomic weight of nitrogen 

 from the analysis of the nitrate. Since the atomic weight sought is not 

 included in both of the substances weighed, an error is not magnified by 

 the calculation. 



CS2O (281.758) : N2O5, N = 14.040 



K2O (94.278) : N2O5, N = 14.037 



Average . . . N = 14.039 



This value affords another confirmation of the remarkable work of 

 Stas. 



Many other combinations of the preceding data with those of other 

 experimenters might have been made, but the preceding are enough to 

 show the most important features of the new investigation. 



It is a pleasure to acknowledge our indebtedness to the Cyrus M. 

 "Warren Fund for Research in Harvard University for some of our 

 apparatus, and to Dr. Wolcott Gibbs for invaluable platinum utensils, as 

 well as to Professor Wells for his great generosity in providing the 

 caesium material. 



X. Summary. 



1. As an outcome of forty-two analyses involving seven diflerent ratios 

 and three compounds of caesium (the chloride, bromide, and nitrate), the 

 atomic weight of caesium is found to be 132.879, if oxygen is IG.OOO. 



2. Incidentally it is found that the atomic weights of potassium and 

 nitrogen most consistent with the new data are nearly 39.14 and 14.04. 



3. In confirmation of Wells, caesium dichloriodide is found to afford 



