BAXTER. — ATOMIC WEIGHT OF PHOSPHORUS. 597 



period of four months, but in no case was an appreciable change in the 

 end-point found after it had remained constant for a week. In most 

 cases the final end-point was reached within two weeks. 



This difficulty from occlusion by the silver bromide was unexpected 

 since it has not been met to the same extent in other similar cases in 

 this laboratory. 8 In fact, it was not discovered in this research until 

 Analyses 1, 2, 5, 9, and 10 of Series I had been completed. After its 

 discovery several unsuccessful attempts were made to eliminate the 

 occlusion. In Analyses 6 and 19 the solutions of ammonium bromide 

 and silver nitrate were each diluted to nearly four liters before precipi- 

 tation. In these two analyses, on account of the large size of the pre- 

 cipitating vessel no attempt was made to collect the silver bromide, 

 since the results of the comparison with silver were no different from 

 those obtained in more dilute solution. Although occlusion of bromide 

 seemed to be diminished in extent by this modification, the time 

 necessary for the establishment of equilibrium was not materially 

 lessened. Even in Analysis 6, where the bromide solution was added 

 to the silver solution, the occluded substance seemed still to be a 

 soluble bromide. Cooling the solutions to the temperature of ice water 

 before precipitation seemed to accentuate the occlusion. 



As soon as a permanent end-point had been reached, the precipitate 

 was thoroughly washed by decantation with pure w'ater and collected 

 upon a weighed Gooch-Munroe-Neubauer crucible. The crucible and 

 contents were heated gradually to nearly 200° and were kept at that 

 temperature for eighteen hours or more. Then they were cooled and 

 weighed by substitution for a similar counterpoise. In order to find 

 the moisture retained by the dried precipitate it was transferred as 

 rapidly and completely as possible to a small porcelain crucible which 

 was immediately weighed with its cover. Then the silver bromide 

 was fused by heating the small crucible contained inside a much larger 

 one. During the solidification of the bromide the system was carefully 

 rotated so that the fused bromide was stirred and caused to solidify in 

 a thin layer. The loss in weight on fusion was then determined. This 

 loss seldom amounted to more than 0.001 per cent of the weight of the 

 salt. The fused bromide was always clear and light yellow, whereas 

 an astonishingly small percentage of impurity is capable of producing 

 perceptible darkening of the salt. 



Since silver bromide possesses an appreciable solubility in water, ^ the 



' See, however, Richards and Rtaehler, Jour. Amer. Chem. Soc, 29, 632 

 (1907); Ber. d. d. chem. Gesell., 39, 3G18. 



» Bottger, Zeit. physik. Chem., 46, 602 (1903), 0.00008 gm. per Uter at 20°; 

 Kohlrausch, Ibid., 50, 536 (1905), 0.00011 gm. per liter at 21°. 



