200 PROCEEDINGS OP THE AMERICAN ACADEMY 



would wholly remove the difficulty. Some of the copper films obtained 

 in the course of the work were almost free from the minute spots, 

 while others of the same series showed a larger number of them ; but 

 the phenomenon did not appear to affect the results. From this it 

 would seem that the supposed impurity might have consisted merely 

 of finely divided copper, but even on the worst supposition the amount 

 was so infinitesimal that it could have had no influence upon the 

 observed atomic weight. 



The brilliant metallic film thus prepared was carefully separated 

 from the platinum and thoroughly washed with hot water. The sepa- 

 ration of the copper from the dish is greatly facilitated by previously 

 rubbing the surface of this electrode with an exceedingly small quan- 

 tity of pure semi-liquid parafllin, which seems to fill the microscopic 

 cavities of the platinum without interfering with the continuity of the 

 copper film. The pure washed copper was returned to the clean dish, 

 dissolved in very pure nitric acid, and the product slowly converted 

 into basic nitrate, and finally into oxide, by gradually increasing heat. 

 The last part of the operation was conducted at a low red heat in 

 porcelain, and of the product only the central portions were used in 

 the work. The resulting cupric oxide was a fine black powder, which 

 dissolved completely in acids, and gave every evidence of being chemi- 

 cally pure. 



A large quantity of this copper oxide was dissolved in a slight 

 excess of pure hydrobromic acid, and the solution evaporated to dry- 

 ness over a steam bath. Of course, every possible precaution against 

 dust and other impurity was taken with regard to this as well as to 

 every other operation of the research, neither hydrochloric acid nor 

 ammonia being used in the room devoted to it. During the process 

 of evaporation a very faint odor of bromine was j^erceptible ; and upon 

 the subsequent solution of the brilliant black scales of cupric bromide 

 in water, the clear blue liquid slowly deposited a few microscopic crys- 

 tals of the basic bromide, which will be described in another place. 

 After standing a few days the crystals ceased to be deposited, and the 

 filtered clear odorless solution remained for about four weeks without 

 further change, serving as the basis of a number of experiments, and 

 furnishing material for the first series of preliminary analyses. 



Since it was evidently impossible to obtain the normal salt by the 

 evaporation of the solution at 100° in the air, many attempts were 

 made to prepare it by concentration of the neutral solution over sul- 

 phuric acid in air and in nitrogen gas of low tension. The first trials 

 all ended in failure, and it was not until much later that the problem 



