RICHARDS AND BAXTER. ATOMIC WEIGHT OP COBALT. 359 



was treated with an excess of hydrobromic acid in order to remove the 

 silver, and the retiltered solution was evaporated to dryness. Upon a 

 careful qualitative analysis tests were obtained for nothing but sodium 

 and traces of cobalt. The sodium evidently came from the porcelain 

 tube used in the sublimation, and we were dealing with an impurity 

 precisely similar in source and nature to that described in the case of 

 nickelous bromide. As the two investigations progressed side by side, 

 the discovery was almost simultaneous in the two cases. We have 

 already said that this outcome was not an unexpected one. 



Since it would be impossible to calculate the weight of the soluble 

 bromides from the weight of the silver bromide without a quantitative 

 analysis of the two bases present, the effort was made to evaporate the 

 aqueous extract from the reduced cobalt and to weigh the dried residue 

 directly. In the work upon nickel this method was necessarily rejected 

 because the spongy nickel was oxidized and went into solution as nickel- 

 ous hydrate which could not be removed by filtration. Our spongy 

 cobalt oxidized much more rapidly than the nickel upon treatment with 

 water ; but the decantate, upon filtration and evaporation in the air, 

 deposited most of its cobalt as cobaltic hydroxide. The presence of salts 

 of the alkalis greatly increases this oxidation, cobalt which has once been 

 leached being oxidized but little. Heat also increases the oxidation, and 

 so probably does the galvanic action with the platinum dishes which were 

 used wherever possible through the whole course of this investigation. 



In view of the colloidal solubility of nickelous hydrate in water, it is 

 probable that cobaltous hydrate possesses the same property. When 

 cobalt is treated with water in the presence of air, the metal oxidizes and 

 goes into solution as cobaltous hydrate ; this is then further oxidized by 

 exposure of the solution to the air and thrown out of solution as cobaltic 

 hydrate, which can be filtered off. Since the dissolved cobalt is almost 

 completely removed by this process, it is obviously legitimate to weigh 

 the residue obtained by evaporating the aqueous extract of the reduced 

 cobalt, and thus to obtain directly the amount of impurity present in the 

 metal. This difference of procedure in the two cases is an interesting 

 example of the way in which subordinate side reactions may influence 

 two researches otherwise unusually analogous. 



In the next four analyses (5 to 8) the cobalt was leached with the purest 

 hot water in a platinum dish and the solution, after filtration, was evapo- 

 rated to dryness. The residue was taken up with water, filtered from 

 the deposited cobaltic hydroxide into a weighed platinum crucible, again 

 evaporated, heated to 130°, and weighed. That these residues contained 



