BAXTER AND JONES. — ATOMIC WEIGHT OF PHOSPHORUS. 153 



0.01 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 applied. 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. 



The Determination of Water in the Dried Silver Phosphate. 



In order to find out how efficient the drying of the silver phosphate 

 had been, experiments were made to determine the amount of water 

 retained by silver phosphate which had been dried for analysis as 

 described above. (See page 147.) The water was determined by 

 fusing the dried phosphate in a current of dry air and collecting the 

 moisture set free in a weighed phosphorus pentoxide tube. Since the 

 melting point of pure silver phosphate is considerably above the soft- 

 ening point of hard glass, it was found advantageous to lower the 

 melting point of the phosphate by the use of silver chloride as a flux. 



About fifteen grams of silver phosphate were placed in one end of a 

 large silver boat and in the other end about twelve grams of previously 

 fused silver chloride. The boat was then inserted in a hard glass tube 

 and dried under the same conditions as prevailed in preparing the 

 samples for the determination of the silver content. After the silver 

 phosphate had been heated for seven hours in a current of purified air 

 dried by phosphorus pentoxide, the air passing over the boat in the 

 furnace was conducted through a weighed U-tube containing resub- 

 limed phosphorus pentoxide for one half hour. This was done to make 

 sure that all the water which had been liberated from the silver phos- 

 phate without fusion had been swept out of the apparatus. In no case 

 was there a gain in weight during this process of more than 0.05 nig., 

 which is about the limit of error in weighing the phosphorus pentoxide 

 tubes. The backward diffusion of moisture was prevented by a second 

 tube containing pentoxide. 



The carefully weighed phosphorus pentoxide tube was again attached 

 to the tube containing the silver boat with its charge of silver phosphate 

 and silver chloride. The latter tube was then heated hot enough to 

 fuse the silver chloride, which flowed down to the silver phosphate and 

 readily caused the entire charge to fuse completely. The liberated 

 water was swept into the phosphorus pentoxide tube by a current of 

 dry air for about thirty minutes. The tube was then reweighed to 

 determine the water evolved by the fusion of silver phosphate. The 

 pentoxide tube was weighed by substitution for a very similar counter- 



