54 RESEARCHES UPON ATOMIC WEIGHTS. 



pains to secure purity of the bromine. Stas removed iodine by shaking potas- 

 sium bromide several times with free bromine and carbon disulphide, and in 

 the course of the prolonged purification distilled the bromine twice from solu- 

 tion 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 involving the 

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

 danger of occlusion of metallic salts by the precipitated silver bromide. That 

 such an error actually exists to a slight extent is shown by the fact that the 

 average of the "indirect" determinations is slightly larger than the average of 

 the "direct" determinations. Obviously, if silver bromide is precipitated by 

 means of either ammonium bromide or hydrobromic acid, occluded ammonium 

 salts or free acids can be easily expelled by fusion of the bromide. This pre- 

 caution was observed in most of the determinations recorded on page 52, and is 

 absolutely essential for the complete elimination of water from the salt. Stas 

 and Marignac both fused the silver bromide in their syntheses, but this opera- 

 tion was omitted by Scott, who dried the bromide at 180°. Scott's statement 

 that the loss on fusion of silver bromide which had been dried at 180° was due 

 to the presence of asbestos is contradicted by the experiments recorded later in 

 this paper, in which the loss on fusion amounted to about o.oi per cent in the 

 case of silver bromide which had been dried in a similar fashion and which was 

 almost entirely free from asbestos. 



From this brief discussion of the more important errors which may have in- 

 fluenced previous determinations of the atomic weight of bromine, it is evident 

 that some uncertainty still exists as to the true value of this constant. In the 

 hope of throwing new light upon the subject, experiments were carried out by 

 two of the methods outlined above, with especial precautions to insure purity 

 of materials and to eliminate known possible errors in the experimental methods. 



Both the methods chosen — synthesis of silver bromide from a weighed 

 amount of silver, and conversion of silver bromide into silver chloride — have 

 already been recently tested in this laboratory ,1 and have been found to be at 

 least as satisfactory as any. 



PURIFICATION OF MATERIALS. 

 BROMINE. 



In purifying bromine for this research, the principles set forth on page 53 

 of this paper were applied; but in some cases the purifying processes were re- 

 peated after the product was apparently pure, in order to make certain that 

 further treatment had no effect. 



» Baxter: Proc. Amer. Acad., 40, 419; 41, 73; Jour. Amer. Chem. Soc, 26, 1577; 27, 876; 

 Zeit. anorg. Chem., 43, 14; 46, 36; Richards and Wells: Pui. Car. Inst., No. 28; Jour. Amer. 

 Chem. Sac, 27, 459; Zeit. anorg. Chem.,4T, 56. 



