BAXTER. — A REVISION OF THE ATOMIC WEIGHT OF BROMINE. 205 



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 am- 

 monium bromide or hydrobromic acid, occluded ammonium salts or 

 free acids could be easily expelled by fusion of the bromide. This 

 precaution was observed in most of the determinations recorded on 

 page 202, 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 operation 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 one one hundredth 

 of a 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 influenced previous determinations of the atomic weight of bro- 

 mine, 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,^* 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 204 of this paper were applied ; but in some cases the 

 purifying processes were repeated after the product was apparently 

 pure, in order to make certain that further treatment had no effect. 



Sample I was first completely dissolved in calcic bromide which 

 had been made from about one third of the original material by means 

 of lime and ammonia, and was then distilled from the solution. The 

 product was covered with several times its volume of water, and was 

 converted into hydrobromic acid by means of pure hydrogen sulphide 



1* Baxter, Tlieso Proceedings, 40, 419; 41, 73. Itkhards and Wells, Publica- 

 tions of the Carnegie Listitution, No. 28. 



