188 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Insoluble Residue. 



All the specimens of silver arsenate, after being heated at 250° C, 

 when dissolved in dilute nitric acid, were found to contain a small 

 amount of insoluble residue, which would dissolve only in rather con- 

 centrated nitric acid. Although the proportion of this residue was 

 apparently increased by exposure to light, specimens of the arsenate 

 which had been prepared wholly in the dark room were not free from 

 it. No process of purification to which the soluble arsenate used in 

 the preparation of the silver arsenate w r as subjected seemed to have the 

 slightest effect upon the proportion of insoluble matter. A similar phe- 

 nomenon was met by Dr. Grinnell Jones in the preparation of silver 

 phosphate. 



Although the amount of this residue in one gram of silver arsenate 

 which had been treated as in the analyses for silver was not over 

 0.00005 gram, it w T as important to determine its silver content. This 

 was done in three cases in which the proportion of residue had been 

 purposely increased as much as possible by exposure to light. The 

 arsenate was dissolved in dilute nitric acid, and the residue was col- 

 lected upon a weighed platinum Gooch crucible, the detached asbestos 

 shreds being carefully determined by filtration upon a filter paper. 

 The weight of residue w^as found by rew 7 eighing the crucible. After 

 the residue had been dissolved in concentrated nitric acid and the 

 solution had been diluted to definite volume, the silver content 

 of the solution was ascertained by comparison w T ith standard silver 

 solutions in a nephelometer. 



The first of the above determinations w r as made with a sample of 



silver arsenate which had been exposed to bright light inside a desic- 

 cator for a month. During this time the quartz tube containing the 



