RICHARDS AND BAXTER. — ATOMIC WEIGHT OP COBALT. 361 



The possibility of the presence of some other acid than hydrobromic 

 acid in the residues led us to make tests in this direction. Silicic acid 

 Avas of course the first one to suggest itself. Early in the work one test 

 liad been made by subliming about two grams of pure cobaltous bromide 

 from a platinum boat in a current of hydrobromic acid gas. After the 

 sublimation of the bromide the boat was perfectly bright, and gave not 

 the slightest evidence of the presence of any silica. One of the residues 

 was now treated with pure strong hydrochloric acid, heated to 130°, 

 again treated with hydrochloric acid, and filtered. The amount of silica 

 found, three one-hundredths of a milligram, is a negligible quantity. Al- 

 though the purity of the phosphoric anhydride used for drying our gases 

 had been proven by passing air through a tube filled with the pentoxide 

 into aqua regia, which upon evaporation gave no precipitate with ammo- 

 nic molybdate, nevertheless one of the soluble residues from the reduced 

 cobalt was tested with the same reagent with a negative result. 



Upon examining one of the earlier residues for sulphuric acid, how- 

 ever, a slight precipitate of baric sulphate was formed. The source of 

 this sulphuric acid was hard to discover, but finally it was found that the 

 strong sulphuric acid in some of the drying columns had become dis- 

 colored in places by organic matter. This must have led to a slight 

 decomposition of the acid and formation of sulphur dioxide, which was 

 subsequently oxidized to sulphuric acid by the bromine. The amount 

 of sulphuric acid present in the bromide was very small, 3.88 grams of 

 cobaltous bromide giving only 0.00036 gram of baric sulphate in one 

 case. As in the instance of nickel, however, this cause of error was 

 wholly eliminated in the later experiments, aod no trace of sulphuric 

 acid could be detected in the material used in the last series. 



The conclusions to be drawn from these experiments seem to be : — 



First, that our cobaltous bromide was almost if not completely reduced. 



Secondly, that the impurities, which consist of alkaline bromides (with, 

 in some cases, a minute trace of sulphates), can be completely removed 

 by leaching the cobalt. 



Thirdly, that the residue obtained by evaporating the water extract of 

 the cobalt after reduction represents within an exceedingly small amount 

 the weight of the impurities. 



In the fourth series, during which the truth of this third conclusion was 

 not realized, and a method similar to that used in the nickel research was 

 adopted, the weight of the residue had to be calculated. The basis of 

 calculation was the knowledge obtained from analysis 5, Series V. Un- 

 fortunately this is the only analysis of material similar to that used in 



