146 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tion is maintained as nearly neutral as is possible, because the excess 

 of silver prevents the concentration of phosphate in solution from 

 exceeding a very small value, so that neither can the solution become 

 alkaline by hydrolysis nor can the concentration of hydrophosphate 

 attain an appreciable value. The absence of the hydrophosphate 

 ions would be expected to prevent the formation and occlusion of 

 acid silver phosphate in this sample, whereas in Sample N the same 

 result is probably brought about by the absence of the silver ion. 

 Unfortunately both of these favorable conditions cannot be combined 

 in one precipitation, as will be shown later. This precipitate settled 

 readily. The washing, testing, and drying were carried out as al- 

 ready described for Samples N and 0. This sample is designated 

 Sample P. 



Sample R. A 0.03 normal solution of sodium ammonium hydrogen 

 phosphate was slowly poured into a similar solution of an equivalent 

 amount of silver nitrate. Under these conditions the solution con- 

 tains an excess of silver, which tends to produce occlusion of acid 

 phosphates, since the solution becomes more and more acid as the pre- 

 cipitation proceeds, and as the precipitation is therefore far from 

 complete, the concentrations of the two hydrophosphate ions gradually 

 approach a very considerable value. At no stage could the solution 

 become alkaline by hydrolysis. It should be noticed that the pro- 

 cedure differs from that used in preparing Sample N in that the 

 precipitate is formed in the presence of an excess of silver nitrate 

 instead of an excess of phosphate, and that this difference in the 

 method of mixing greatly changes the conditions of precipitation. 



The precipitate, which was designated Sample R, coagulated and 

 settled quite readily. The washing and drying were completed as 

 usual. 



It will be shown that samples of silver phosphate prepared under 

 these various conditions have nearly, if not exactly, the same composi- 

 tion. Further proof of the absence of acid phosphate in these samples 

 is given by experiments to be described later which show that no 

 water is given off when this material is fused. 



An attempt to prepare a sample by pouring silver nitrate into di- 

 sodium ammonium phosphate yielded unsatisfactory results. Since 

 the disodium ammonium phosphate solution was alkaline, owing to 

 hydrolysis, it contained free ammonia, which prevented the precipita- 

 tion of silver phosphate at first. Nearly one-quarter of the silver 

 nitrate was added before a permanent precipitate was produced. At 

 the end of the precipitation the solution was of course essentially 

 neutral. Even after standing for four days the precipitate had not 



