184 REPORTS OF INVESTIGATIONS AND PROJECTS. 



the precipitated silver lialide was collected and weighed. It was found that 

 at 225° all moisture was not eliminated from the silver chromate, while a 

 higher temperature produced incipient decomposition ; hence a correction for 

 the moisture retained by the salt at 225° was determined. Assuming the 

 following atomic weights, Ag= 107.93, CI = 35.473, Br = 79.953, the ratio 

 Ag2Cr04 : 2AgCl gives as the molecular weight of silver chromate the 

 value 331.915 and the ratio AgaCrOii 2AgBr the value 331.928, with an 

 average value 331.922. No regular differences in the results with material 

 prepared from potassium and ammonium chromate could be observed. If 

 silver is taken at 107.930 referred to oxygen 16.000, the atomic weight of 

 chromium appears to be 52.06, while if silver is assumed to be 107.88 the 

 atomic weight of chromium is 52.01. This investigation is being continued 

 by the analysis of silver dichromate. 



With Mr. Grinnell Jones a revision of the atomic weight of phosphorus 

 was begun by analysis of tri-silver phosphate. This salt was prepared by 

 precipitating a dilute solution of silver nitrate with solutions of di-metallic or 

 tri-metallic phosphates of sodium and ammonium. Portions of the carefully 

 washed salt were dried in a current of pure dry air at about 400° and were 

 weighed. Next they were dissolved in nitric acid and the silver was pre- 

 cipitated and determined either as silver bromide or as silver chloride. The 

 different samples of material all gave essentially the same result, with the 

 exception of the precipitate formed with tri-sodium phosphate which proved 

 to contain large amounts of sodium, showing that slightly different degrees of 

 alkalinity or acidity during the precipitation of the tri-silver phosphate pro- 

 duce no tendency for occlusion of other salts of silver and phosphoric acid. 

 Two corrections must be applied to the result — one for the small quantity of 

 moisture retained by the silver phosphate, and the other for slight decom- 

 position owing to the action of heat or light during the drying. These cor- 

 rections are probably very small and in an opposite direction. Until they 

 are accurately determined it is premature to state the results of the work. 



Parallel with the work upon phosphorus, Mr. F. B. Coffin carried on the 

 analysis of tri-silver arsenate by similar methods. The arsenate was formed 

 by adding dilute solutions of di-metallic or tri-metallic arsenates of sodium 

 and ammonium to dilute silver nitrate solution. The salt was washed, and 

 dried at 250° in a current of pure dry air. Then, after being weighed, it 

 was analyzed either by heating in a quartz tube in a current of pure hydro- 

 chloric acid gas until all arsenic was expelled, or by solution in nitric acid 

 and precipitation with either hydrochloric or hydrobromic acid. While all 

 three methods gave essentially identical results with the same sample, differ- 

 ent samples of material indicate slight differences in composition with 

 varying conditions of precipitation. These differences are still under inves- 

 tigation. The salt undoubtedly retains traces of water when dried at 250°, 

 and a correction for this moisture remains to be determined. Furthermore, 



