ATOMIC WEIGHTS 309 



show that the quantities of bromide given in the third column are pro- 

 portional to 100 parts of silver : 



8.3246 grm. AsBr3 = 8.58 grm. Ag. 97.023 



4.4368 " 4.573 " 97.022 



5.098 " 5.257 ". 96.970 



Mean, 97.005, ± .012 



Hence As =74.19. Why this value should be so much lower than 

 that from the chloride is unexplained. 



The volumetric work done by Kessler/ for the purpose of establishing the 

 atomic weights of chromium and of arsenic, is described in the chromium 

 chapter. In that investigation the amount of potassium dichromate 

 required to oxidize 100 parts of ASjOg to AS2O5 was determined and 

 compared with the quantity of potassium chlorate necessary to produce 

 the same effect. From the molecular w^eight of KCIO3, that of Kr,CT^O^ 

 was then calculable. 



From the same fignres, the molecular weights of KCIO3 and of KjCrgOy 

 being both known, that of As^O., may be easily determined. The quan- 

 tities of the other compounds proportional to 100 parts of ASoO^ are as 



follows : 



K.Cr,0,. KCIO,. 



98.95 41.156 



98.94 41.116 



99.17 41.200 



98.98 41.255 



99.08 41.201 



99.15 41.086 



41.199 



Mean, 99.045, ± .028 41.224 



41.161 

 41.193 

 41.149 

 41.126 



Mean, 41.172, ± .009 



Another series with the dichromate gave the following figures 



99.08 

 99.06 

 99.10 

 98.97 

 98.97 



Mean, 99.036, ± .019 

 Previous series, 99.045, ± .028 



General mean, 99.039, ± .016 



1 Poggend. Annal., 95, 204. 185.5. Also 113, 134. 1861. 



