19G 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Sample A Na 2 NH 4 As0 4 



Sample B Na 2 HAs0 4 



Sample C Na.,HAs0 4 



Sample D (NH 4 ) 3 As0 4 



Sample E Na 2 NH 4 As0 4 

 Sample F Na 3 As0 4 

 Sample G Na 3 As0 4 



It is to be expected that the basicity due to hydrolysis would be most 

 marked with Samples F and G, less in the case of Samples A and E, 

 still less with Sample D, and least in the case of Samples B and C. In 

 the case of Samples B and C, acid accumulates in the solution during 

 the precipitation of the silver arsenate. In comparing the results from 

 the different samples of silver arsenate it must be noted that occluded 

 basic salt would increase the apparent percentage of silver in the 

 arsenate. In the case of Samples F and G the conditions are most 

 favorable for the occlusion of basic salts, and these two samples actually 

 yield a higher percentage of silver than the other samples. On the other 

 hand accumulation of acid in the solution in which the precipitation of 

 the silver arsenate is taking place is found to have no tendency to pro- 

 mote occlusion of acid salts, since Samples B and C give results agree- 

 ing closely with those of Samples A, D, and E. These two facts lead 

 to the conclusion that Samples A to E represent normal trisilver 

 arsenate, and that Samples F and G contain basic impurities. 



In order to calculate the atomic weight of arsenic from the per cent 

 of silver in silver arsenate a knowledge of the ratio of the atomic 

 weights of silver and oxygen is necessary. Some uncertainty exists as 

 to this ratio, hence calculations have been made upon the basis of 

 several possible values for silver, oxygen being assumed to have the 

 value 16.000. This has been done only with the results of Series I, II, 

 and III, since, as has been pointed out, they are probably nearer the 

 truth than those of Series IV, V, and VI. The difference between the 

 two sets of results amounts only to six one hundredths of a per cent in 

 the atomic weight of arsenic. 



VThcn the results of Series I and II are averaged with those of 

 Series III, it is found that 



