152 PROCEEDINGS OF THE AMERICAN ACADEMY. 



similar to subchloride), that when treated with very dilute nitric acid 

 metallic silver remains. 



It does not follow, however, that it would be a correct procedure to 

 determine the per cent of this residue obtained from the samples used 

 for analysis and apply a correction on the assumption that the material 

 consisted of pure silver phosphate and a small amount of pure silver. 

 This procedure would assume that the other product of decomposition 

 is eliminated and not weighed. There are two facts which show that 

 this assumption would be incorrect. In nearly every analysis, when 

 the solution was diluted, after bringing the residue into solution by 

 heating on the steam bath, a slight opalescence was produced. Care- 

 ful tests of the water used showed that this opalescence was not due 

 to impurity in the water. It seems probable that the substance which 

 caused this opalescence was derived in part from the phosphate radical 

 during the decomposition which produced the residue. The other fact 

 is that dry silver phosphate does not lose weight when darkened by 

 exposure to sunlight, although this treatment increases the amount of 

 residue. The conclusion in regard to this residue may be summarized 

 as follows : The washed moist silver phosphate was free from residue 

 and contained silver and phosphoric acid combined in atomic propor- 

 tions. During the drying and weighing a slight decomposition took 

 place, undoubtedly owing in part at least to the action of light. It 

 seems probable that during this decomposition no loss in weight took 

 place, and therefore the sample contained the proper percentage of 

 silver. When this slightly darkened silver phosphate is treated with 

 cold dilute nitric acid, the unchanged silver phosphate and perhaps 

 also a portion of the altered material dissolve, leaving a slight opales- 

 cence, which in some cases is deposited as a very slight residue on 

 standing. This residue is estimated to be about 0.01 per cent of the 

 weight of the silver phosphate. When the solution is warmed until 

 perfectly clear, and then diluted, a very slight opalescence is usually 

 produced which could be again cleared up by warming the solution. 

 This opalescence is probably caused by the presence of the altered 

 phosphate anion. If this explanation is correct, the presence of the 

 residue cannot influence the result, and no correction need be applied. 

 Until the exact nature of the decomposition products can be deter- 

 mined, there must remain some uncertainty in regard to whether or 

 not any correction is necessary. 



The uncertainty from this cause is, however, not very great. Even 

 if all the phosphorus and oxygen corresponding to the residue of silver 

 is removed before the weighing, the correction would be only twenty- 

 three per cent of the weight of the residue. If the residue amounts to 



