140 PROCEEDINGS OF THE AMERICAN ACADEMY. 



silver phosphate in a perfectly definite and pure state. The greater 

 part of the following research was devoted to the solution of this prob- 

 lem which van der Platts found so difficult. 



The analysis of the halogen compounds of phosphorus offers certain 

 difficulties owing to the ease with which these substances are decom- 

 posed by water, and to the necessity for oxydizing the phosphorous acid 

 resulting from the decomposition of the halogen compounds with water 

 before the addition of silver nitrate. An investigation upon the tri- 

 bromide of phosphorus is now in progress in this laboratory. Phospho- 

 nium compounds were found utterly unsuited for exact analysis on 

 account of their instability. 



Purification of Materials. 



Water. All the water used in this research was made from the 

 laboratory supply of distilled water by distillation, first from an alka- 

 line permanganate solution, and then, after the addition of a trace of 

 sulphuric acid, through a block tin condenser. 



Ammonia. The best commercial ammonia was distilled into the 

 purest water. 



Nitric Acid. The best commercial concentrated acid was twice 

 fractionally distilled through a platinum condenser, with the rejection 

 of the first third of the distillate. Every sample was shown to be free 

 from chloride by careful nephelometric tests. 



Hydrochloric Arid. The best commercial C P. acid, diluted with an 

 equal volume of water, was distilled through a platinum condenser. 



Hydrobrornic Acid. This substance was prepared in conjunction 

 with Air. F.. B. Coffin, who was engaged in a parallel research upon the 

 atomic weight of arsenic 14 Commercial bromine was converted into 

 potassium bromide by addition to recrystallized potassium oxalate. 

 In a concentrated solution of this bromide, in a distilling flask cooled 

 with ice, bromine was dissolved, and distilled from the solution into a 

 flask cooled with ice. A portion of the purified bromine was then con- 

 verted into potassium bromide with pure potassium oxalate as before, 

 and the remainder of the bromine was distilled from solution in this 

 pure potassium bromide. The product obtained was thus twice dis- 

 tilled from a bromide, the bromide in the second distillation being 

 essentially free from chlorine. This treatment has already been proved 

 sufficient to free bromine from chlorine. 15 



14 Baxter and Coffin, These Proceedings, 1909, 44, 179. 



15 Baxter, These Proceedings, 1906, 42, 201. 



