BAXTER AND JONES. — ATOMIC WEIGHT OF PHOSPHORUS. 151 



tigation difficult. The silver phosphate, after its 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 sam- 

 ples 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 pre- 

 cipitate of silver chloride, while ammonium molybdate indicated the 

 presence 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 3.01901 grams. It was then 

 exposed to the direct action of bright sunlight for a day, while con- 

 tained in a weighing bottle which was placed in a desiccator over sul- 

 phuric acid. It was found to have darkened slightly in color and to 

 weigh 3.01903. The gain of 0.02 milligram 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 1.8 milligrams. 

 This is much more residue than was usually found in samples contain- 

 ing from four to eight grams 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 day- 

 light while being weighed or transferred to the furnace and solution 

 flask, contained about one one-hundredth of a 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 thor- 

 oughly washing and drying the residue. 0.02674 gram of this residue 

 yielded 0.03551 gram of silver chloride, which indicates that the res- 

 idue contained 99.9 per cent of silver. In the case of another sample 

 of the residue prepared and analyzed in the same way, 0.04320 gram 

 of residue yielded 0.05747 gram of silver chloride, which indicates that 

 the residue contained 100.1 per cent of silver. The mean of the two 

 analyses is 100.0 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 formation of a subphosphate 



