A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. 1 79 



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 had been devoted to the residue itself. The small 

 amount of material available rendered this part of the investigation difficult. 

 The silver phosphate, after precipitation and washing, but undried, dissolves 

 in dilute nitric acid, giving a solution which is perfectly clear to the naked eye, 

 although some samples gave a barely visible opalescence in the nephelometer. 

 The opalescence was much too small to have any effect on the analytical results. 

 The dried samples invariably gave an opalescence. 



Dry silver phosphate is very slowly darkened in color by the action of light. 

 This effect is even more pronounced when silver phosphate is exposed to the light 

 in the presence of water. These darkened samples gave a much greater residue 

 than the undarkened material. The residue was insoluble in ammonia, slowly 

 soluble in dilute nitric acid, especially when heated, and readily soluble in strong 

 nitric acid. The addition of hydrochloric acid to these nitric acid solutions gave 

 a precipitate of silver chloride, while ammonium molybdate indicated the pres- 

 ence of phosphate. 



In order to determine whether or not a loss of weight occurs during the 

 darkening by light, a sample of silver phosphate was dried and weighed as 

 usual and found to weigh 3. 01 901 gm. It was then exposed to the direct action 

 of bright sunlight for a day, while contained in a weighing-bottle which was 

 placed in a desiccator over sulphuric acid. It was found to have darkened 

 slightly in color and to weigh 3.01903. The gain of 0.02 mg. is within the limit 

 of error in the weighing. This sample, when treated with dilute nitric acid, 

 gave a much larger residue than usual, which weighed i . 8 mg. This is much more 

 residue than was usually found in samples containing from 4 to 8 gm. of silver 

 phosphate. It is estimated that the samples which had been protected from the 

 action of light as much as possible, except when unavoidably exposed to diffused 

 daylight while being weighed or transferred to the furnace and solution flask, 

 contained about o.oi per cent of this residue. 



Two analyses were made of the residue obtained by exposing silver phosphate 

 under water to the action of light for several days, then dissolving the excess of 

 silver phosphate in dilute nitric acid and thoroughly washing and drying the 

 residue. 0.02674 gm. of this residue yielded 0.03551 gm. of silver chloride, which 

 indicates that the residue contained 99.9 per cent of silver. In the case of an- 

 other sample of the residue prepared and analyzed in the same way, 0.04320 

 gm. of residue yielded 0.05747 gm. of silver chloride, which indicates that the 

 residue contained 100. i per cent of silver. The mean of the two analyses is loo.o 

 per cent of silver. These analyses prove conclusively that when silver phosphate 

 is acted on by light in the presence of water, it is so altered (perhaps by the for- 

 mation of a subphosphate similar to subchloride), that when treated with very 

 dilute nitric acid metallic silver remains. 



