BAXTER AND JONES. — ATOMIC WEIGHT OF PE^PHORUS. 149 



of silver phosphate or nitrate than the reverse method. The occlusion 

 of hydrobrornic acid can do no harm. The flask was shaken for twenty 

 minutes and was allowed to stand for several days until the precipitate 

 had completely settled. Then the precipitate was collected upon a 

 weighed Gooch crucible after many rinsings with pure water. In order 

 to protect the mat of the Gooch crucible from disintegration, it was 

 covered by a circular disk of thin platinum foil, perforated with many 

 small holes. The precipitate was dried in an electrically heated air 

 bath for several hours at 90°, then for seme time at 130°, and finally 

 for at least eight hours at 180°. After the crucible containing the 

 precipitate had been weighed, the silver bromide was transferred 

 to a porcelain crucible and the loss on fusion determined. The 

 presence of the platinum disk covering the mat makes it possible 

 to transfer very nearly all the silver bromide to the porcelain crucible 

 without contamination with asbestos and therefore it is unnecessary to 

 correct the loss on fusion for the small amount of silver bromide which 

 is not fused. The loss on fusion, which represents water remaining 

 in the silver bromide, was subtracted from the weight of the silver 

 bromide. The asbestos shreds carried away by the wash waters and 

 any silver bromide which may have escaped the Gooch crucible were 

 collected by passing the filtrate through a very small filter paper. 

 The paper was then burned and the residue, after treatment with a 

 drop of nitric and hydrobrornic acids to convert any reduced silver 

 into silver bromide, was again gently heated and finally was weighed. 

 The weight of the asbestos, corrected for the ash of the paper, was 

 added to the weight of the silver bromide. In order to determine the 

 soluble silver bromide, the filtrate was evaporated until most of the 

 excess of nitric acid was driven off. The precipitating flask and all 

 the flasks which had held the filtrate were rinsed with strong ammonia 

 and the rinsings added to the evaporated wash water. Enough 

 ammonia was added to make the solution alkaline and it was then 

 diluted to one hundred cubic centimeters in a graduated flask. The 

 amount of silver bromide present was determined by comparison in 

 the nephelometer with a very similar solution containing a known 

 amount of silver bromide. Both precipitates were dissolved in ammo- 

 nia and reprecipitated at the same time and under precisely similar 

 conditions 26 in the nephelometer tubes by a slight excess of nitric 

 acid. The amount found in this way was added to the weight of the 

 silver bromide. 



In order to determine whether silver phosphate is occluded by silver 



28 See Richards and Staehler, Pub. Carnegie Institute, No. 76, p. 20. 



