l8o RESEARCHES UPON ATOMIC WEIGHTS. 



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

 mine 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. 

 Careful tests of the water used showed that this opalescence was not due to im- 

 purity in the water. It seems probable that the substance which caused this 

 opalescence was derived in part from the phosphate radical during the decom- 

 position 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 proportions. During the drying and 

 weighing a slight decomposition took place, undoubtedly owing in part at least 

 to the action of Hght. It seems probable that during this decomposition no 

 loss in weight took place, and therefore the sample contained the proper per- 

 centage of silver. When this slightly darkened silver phosphate is treated with 

 cold dilute nitric acid, the unchanged silver phosphate and perhaps also a por- 

 tion of the altered material dissolve, leaving a sUght opalescence, which in 

 some cases is deposited as a very slight residue on standing. This residue is 

 estimated to be about o.oi 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 

 can not influence the result, and no correction need be applied. Until the exact 

 nature of the decomposition products can be determined, there must remain 

 some vmcertainty 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 weigliing, the correction would be only 23 per cent of the weight of the 

 residue. If the residue amounts to o.oi per cent, as has been estimated, the 

 maximum correction would be 0.002 per cent. If part of the oxygen is lost, but 

 the phosphorus remains, the correction would of course be smaller. If there is 

 no loss in weight by the action of light on the dry silver phosphate, no correc- 

 tion need be appHed. From the evidence so far obtained the latter assumption 

 seems rather more probable than any of the others, and therefore no correction 

 has been applied. 



