14S PROCEEDINGS OF THE AMERICAN ACADEMY. 



below by a large burner. The platinum boat was not attacked in the 

 least, as was shown by the fact that its weight remained constant. 



It was feared that in spite of this prolonged heating the silver 

 phosphate still retained a trace of water, but by making the conditions 

 in the different experiments as nearly uniform as possible it was hoped 

 that the amount of water retained would be constant. Proof will be 

 given later that the drying was highly efficient. 



The salt thus prepared for analysis was allowed to stand over night 

 in a desiccator covered with a black cloth in the balance room, and 

 was then weighed in its glass- stoppered bottle by substitution, with the 

 use of another weighing bottle of very similar surface and volume as a 

 counterpoise. 



The balance was a nearly new No. 10 Troemner balance. It was 

 easily sensitive to 0.02 mg. The weights had already been used 

 in an investigation of the atomic weight of sulphur, 25 and were re- 

 standardized with a very gratifying result. None of the corrections 

 found differed by as much as 0.02 mg. from those found a year before, 

 and only a few by 0.01 mg. The balance was provided with a few 

 milligrams of radium bromide of radioactivity 10000 to dispel electri- 

 cal charges generated during the handling of the weighing bottles 

 with cork-tipped pincers. 



The platinum boat containing the silver phosphate was transferred 

 to an Erlenmeyer flask of " non-sol " glass of one liter capacity and 

 treated with about 30 cubic centimeters of 5 normal nitric acid. 

 Solution took place rapidly. The solution was not perfectly clear, 

 however, owing to a very slight insoluble residue which sometimes 

 settled out on standing. The solution was then heated on a steam 

 bath until the residue dissolved completely. Upon the addition of 

 about one liter of cold water a very slight opalescence was produced, 

 which was visible only when the solution was carefully examined in a 

 very favorable light. The solution was again warmed until it became 

 perfectly clear. The water and nitric acid used in these processes did 

 not give an opalescence visible in the nephelomcter when treated 

 with silver nitrate. The nature of this residue will be discussed more 

 in detail after describing the remainder of the analytical process. 



About eight hundred cubic centimeters of water was placed in a 

 large glass-stoppered precipitating flask and a very slight excess of 

 hydrobromic acid was added from a burette. The silver phosphate solu- 

 tion was then very carefully poured into the hydrobromic acid solution. 

 This method of precipitation gives less opportunity for the occlusion 



25 Richards and Jones, Pub. Car. Inst., 1907, No. 69, 69. 



