150 PROCEEDINGS OF THE AMERICAN ACADEMY. 



chloride, about six grams of silver phosphate were dissolved in nitric 

 acid and the solution was diluted and poured into an excess of hydro- 

 chloric acid. After standing until the supernatant liquid was clear, 

 the precipitate was washed very thoroughly with water and then dis- 

 solved in redistilled ammonia. The solution was diluted to one liter 

 and the silver chloride was reprecipitated with nitric acid. The 

 precipitate was filtered out and the filtrate evaporated in a platinum 

 dish until concentrated. A little sodium carbonate was added and 

 the dish was heated to expel all volatile ammonium salts. The residue 

 was dissolved in about three cubic centimeters of water and treated 

 with an excess of ammonium molybdate reagent with gentle warming. 

 After standing for three days, not the slightest precipitate or yellow 

 color had appeared, showing that no phosphate had been occluded by 

 the silver chloride. Although not tested experimentally, it is reason- 

 able to suppose that silver bromide also does not possess the property 

 of occluding appreciable quantities of silver phosphate or phosphoric 

 acid. 



Insoluble Residue. 



The presence of a slight residue or opalescence, after dissolving the 

 dried silver phosphate in dilute nitric acid, proved the most perplexing 

 difficulty which was encountered. The effort to discover the nature 

 of this insoluble matter and eliminate it consumed a large part of the 

 time devoted to this research. In an effort to make sure that it was 

 not due to some unknown impurity, nineteen different samples of 

 silver phosphate were prepared, the source of material, method of 

 purification, and precipitation being varied. Disodium phosphate, 

 trisodium phosphate, and sodium ammonium phosphate were carefully 

 purified and converted into silver phosphate under varying conditions 

 without appreciable effect upon the amount of the residue. Phospho- 

 rus oxychloride was twice fractionally distilled, converted into phos- 

 phoric acid, and then into disodium phosphate by means of sodium 

 hydroxide made from sodium amalgam. The product was crystallized 

 three times. Silver phosphate made from this material gave a slight 

 residue, very similar to that obtained from the best samples made in 

 other ways. Unfortunately, it was necessary to reject the analytical 

 results obtained with this specimen because it was found to contain a 

 small amount of metaphosphate. We did not succeed in preparing 

 a sample of silver phosphate entirely free from the residue. 



In the meantime attention bad been devoted to the residue itself. 

 The small amount of material available rendered this part of the inves- 



