l82 



RESEARCHES UPON ATOMIC WEIGHTS. 



The amount of water evolved is hardly greater than the probable error in 

 weighing the phosphorus pentoxide tubes, and is less than the probable error 

 in determining the amount of silver in the salt. We are therefore justified in 

 concluding that the material which was used for the determination of silver was 

 essentially free from water and that no correction need be applied to the results 

 for inefficient drying. 



This result also furnishes evidence that the samples are free from acid phos- 

 phates, which, owing to conversion into pyrophosphate or metaphosphate, would 

 evolve water when fused, although it is possible that occluded acid phosphates 

 might have been converted into pyrophosphate or metaphosphate during the 

 drying. Sample O, which was prepared under conditions most favorable for the 

 formation of the acid silver phosphate, does not appear to contain more water 

 than Sample P, which was prepared under conditions which were unfavorable 

 to the formation of acid phosphate. Since these two samples, which differed 

 most widely in their method of preparation, showed no difference in the amount 

 of water retained, it seemed unnecessary to test the other samples also. Unfor- 

 tunately this method of detecting acid phosphate is not very sensitive, owing 

 to the unfavorable relation of the atomic weights involved — one molecule of 

 water corresponding to a deficiency of two atoms of silver. 



THE SPECIFIC GRAVITY OF SILVER PHOSPHATE. 



In order that the apparent weight of the silver phosphate might be corrected 

 to the vacuum standard, the specific gravity of this salt was found by deter- 

 mining the weight of toluol displaced by a known quantity of salt. The spe- 

 cific gravity of the toluol at 25° referred to water at 4° was 0.8633. Great care 

 was taken to remove air from the salt when covered with the toluol by warming 

 the pycnometer, then placing it in a vacuum desiccator and boiling the toluol 

 under reduced pressure. The salt and toluol were mechanically stirred to assist 

 the escape of air bubbles. This process was repeated several times. 



The Specific Gravity of Silver Phosphate. 



Weight of 



silver phosphate 



in vacuum. 



gm. 



22.955 

 16.942 



Weight of 



displaced toluol 



in vacuum. 



gm. 



3-113 



2.29s 



Volume of 

 silver phosphate. 



e.c. 



3.606 

 2.658 



Density of 

 silver phosphate. 



6.366 

 6.374 



Mean 6.37 



Therefore the apparent weight of silver phosphate was corrected to the vacuum 

 standard by adding 0.000044 gm. per gram of salt. Similarly 0.000041 gm. 

 was added for every gram of silver bromide. 



