214 THE ATOMIC WEIGHTS. 



Hence As = 73.668, .0436. 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 As. 2 O 3 to As 2 5 was determined 

 and compared with the quantity of potassium chlorate necessary to pro- 

 duce the same effect. From the molecular weight of KC10 3 , that of 

 K 2 Cr 2 O 7 was then calculable. 



From the same figures, the molecular weights of KC10 3 and of K 2 Cr 2 

 being both known, that of As 2 3 may be easily determined. The quan- 

 tities of the other compounds proportional to 100 parts of As 2 3 are as- 



follows : 



A- 2 2 <9 7 . KCIO* 



98.95 4i.i5 6 



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 



4M93 

 41.149 

 41.126 



Mean, 41.172, db .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, =b .028 



General mean, 99.039, =h .016 



Other defective series are given to illustrate the partial oxidation of 

 the As 2 3 by the action of the air. From Kessler's data we get two 

 values for the molecular weight of As 2 O 3 , thus : 



From KC1O 3 series As 2 O 3 = 196.951, .0445 



From K 2 Cr 2 O 7 series " = 196.726, db .0562 



General mean As 2 O 3 = 196.851, =b .0349 



And As = 74.607, .0175. 



* Poggend Annal., 95, 204. 1855. Also 113, 134. 1861. 



