202 PROCEEDINGS OP THE AMERICAN ACADEMY 



Method of Analysis. 



The bromine and copper were determined in separate weighed 

 portions of the solution, according to usual methods. For the estima- 

 tion of the former, the diluted cupric bromide was slowly added to a 

 slight excess of pure argentic nitrate dissolved in one hundred and 

 fifty cubic centimeters of hot water. In some of the determinations 

 nitric acid was added to the silver nitrate before the precipitation, and 

 in the other cases the acid was added afterward ; but the difference 

 in procedure produced no difference in result. The silver bromide 

 settled readily, and was treated in the manner already described.* 



The copper was determined in another weighed portion of the solu- 

 tion by electrolysis, after the complete expulsion of the bromine by 

 means of nitric and sulphuric acids. One and a half to three grams 

 of cupric bromide were used in each determination, and usually one 

 part of nitric acid and two parts of sulphuric acid were taken to 

 decompose one part by weight of the salt ; but here again the rela- 

 tive quantities were varied without any perceptible effect upon the 

 result. When the bromine and nitric acid had been expelled over 

 the water bath, the residual copper sulphate and sulphuric acid were 

 dissolved in a small quantity of water, and the whole transferred to 

 the platinum crucible which was to serve as the negative electrode. 

 It was found that within reasonable limits the size of the crucible and 

 the dilution of its contents had no important effect upon the deposition 

 of the copper, provided that the strength of the current was properly 

 regulated. With six gravity cells and eighty ohms external resist- 

 ance it was possible to deposit perfectly half a gram of copper in a 

 coherent film from twenty-five cubic centimeters of solution. The 

 positive electrode consisted in every case of a strong platinum wire 

 fused into the centre of the watch-glass serving as a cover to the cru- 

 cible. Twice before the completion of each electrolysis the liquid 

 adhering to the lower surface of this lid was washed into the crucible, 

 and the current was always maintained for at least thirty-six hours. 

 Upon breaking the circuit, the sulphuric acid was at once wholly dis- 

 placed by a stream of distilled water, and after washing by decantation 

 with pure water, alcohol, and ether, the film was finally dried at 

 95° C. to perfectly constant weight. All the washings were collected 

 in a large porcelain dish, carefully examined in order to be sure that 

 no copper had been carried away, and after evaporation to small bulk 



* Page 198 of this paper. 



