204 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the fusion. Silver prepared in this way was found by Richards and 

 Wells to contain traces of oxygen, derived from silver nitrate occluded 

 by the electrolytic crystals. In cases 8, 9, 10, 17, and 18, however, the 

 silver was fused in hydrogen. Richards and Wells showed also that 

 Stas's silver contained at least one one hundredth of a per cent of 

 impurity, since it yielded one one hundredth of a per cent less silver 

 chloride than their purest silver. ^^ Scott's silver in three cases was 

 merely heated, not fused, in hydrogen, and in two of the others was 

 fused before a blowpipe on calcic phosphate. In one experiment only 

 the metal was fused on lime. No details are given as to the purifica- 

 tion of the silver used by Marignac. 



Bromine also may be freed from impurities only with some difficulty. 

 Experience in this laboratory has shown that chlorine may be elimi- 

 nated most conveniently by distilling or precipitating the bromine 

 from solution in a bromide. One such distillation is sufficient to 

 remove chlorine completely only when the substance is initially com- 

 paratively pure. If, however, the process is repeated by converting a 

 portion of the partially purified product into a bromide, and dissolving 

 the remainder of the bromine in this comparatively pure bromide, the 

 chlorine is eliminated so completely that further repetitions of this 

 process have no apparent effect. ^"^ The removal of iodine may be 

 easily effected by converting the bromine into hydrobromic acid or a 

 soluble bromide, and boiling the solution with a small quantity of free 

 bromine. Here again it is well to repeat the process several times, 

 since the reaction between free bromine and the iodine ion, like 

 that between free chlorine and the bromine ion, is undoubtedly 

 incomplete. 



The greater part of the experiments cited on page 202 were made 

 with bromine which had been purified with due observance of these 

 precautions. Of the other investigators, Stas seems to have been the 

 only one to use sufficient pains to secure purity of the bromine. Stas 

 removed iodine by shaking potassium bromide several times with free 

 bromine and carbon disulphide, and in the course of the prolonged 

 purification distilled the bromine twice from solution in a bromide. 

 Marignac's purification consisted solely in crystallization of barium 

 bromate and Scott's in distillation of hydrobromic acid. 



Of the methods employed in these early determinations, that involv- 

 ing the analysis of metallic halides is least suited for the purpose, on 



" Loc. cit., page 62. 



13 Attention has already been called to these points by Richards and Wells, 

 These Proceedings, 41, 440 (1900). 



