356 PBOCEEDINGS OF THE AMEEICAN ACADEMY. 



A slight sublimation of the cobaltous bromide took place during the 

 reduction in almost every case. The amount of this sublimed material 

 was determined by washing out the tube with a few cubic centimeters of 

 nitric acid, and evaporating this liquid to dryness. After the solution of 

 the residue in water and the addition of an excess of ammonia, a very 

 dilute standard solution of potassic permanganate was run in until a pink 

 color appeared. This method of Winkler's is applicable only when ex- 

 tremely small amounts of cobalt are present, because the brown color of 

 the cobaltic salt interferes seriously with the end point in the presence 

 of large amounts of cobalt. The weight of the cobaltous bromide thus 

 sublimed never amounted to more than three tenths of a milligram, and 

 seldom exceeded one tenth of a milligram. 



The platinum boat used in the earlier work served to contain the 

 bromide in these experiments also. Although the metallic cobalt alloyed 

 itself with the surface of the boat to a slight extent, we were able to 

 remove completely the alloy by treating the inside of the boat with aqua 

 regia. After this treatment and scrubbing with round sand, the boat 

 showed no trace of darkening upon ignition. Evidently, then, the cobalt 

 had been completely removed. Of course a gradual loss of weight took 

 place, owing- to solution of small amounts of platinum, but this loss 

 amounted to only half a gram in the course of the work. 



In the first two determinations the hydrogen was generated from hydro- 

 chloric acid by means of pure zinc. It was purified by passing through 

 bulbs containing silver nitrate, potash, silver nitrate again, then through 

 a hard glass tube heated to redness. From this point the gas came in 

 contact only with glass, being conducted through three towers containing 

 glass beads moistened with silver nitrate, then by means of T-tubes, 

 either directly in a moist state, or through a long drying tube containing 

 stick potash, into the reduction tube. The reduction tube was connected 

 with the rest of the apparatus by means of a ground glass joint. 



A small amount of white sublimate, which appeared beyond the boat 

 during each of the preliminary ignitions, proved to be ammonic bromide. 

 The source of the ammonia was not at first apparent, as it was hard to 

 believe that the cobaltous bromide could retain ammonic bromide at a 

 temperature between 400° and 500°. Upon examination of the silver 

 nitrate columns it was found that reduction*had taken place there, metal- 

 lic silver being precipitated upon the beads.* Of course the reduction of 



* The fact that silver nitrate is reduced by molecular hydrogen has already 

 been noted by other experimenters : Russell, Jour. Chem. See, [2], XII. 3, (1874) ; 

 Pellet, Compt. Rend., LXXVIII. 1132, 



