OP ARTS AND SCIENCES. 201 



was suocessfully solved. The purest salt obtained at this stage of the 

 work consisted of brilliant black crystalline scales, which, contrary to 

 all expectation and all literature * upon the subject, were not very 

 deliquescent unless moistened with strong hydrobromic acid. Since 

 the normal compound invariably seems to lose a trace of bromine ou 

 exposure to the air, the lack of deliquescence may be due to a very 

 thin superficial covering of the basic bromide. The normal salt is 

 soluble in a very small amount of water, forming a brownish black 

 solution ; this becomes deep purple after the addition of hydrobromic 

 acid, and upon gradual dilution goes through successive shades of 

 brown and dirty green to a most beautiful "robin's egg" blue. 



The basic salt above alluded to was found to be quite insoluble in 

 water, hence it seemed probable that the dilute solution of cupric 

 bromide from which it had separated was perfectly normal, and it 

 became an important problem to settle this point. As the normal 

 salts of copper are wholly without action upon a solution of methyl 

 orange, this indicator formed the most convenient test for the neu- 

 trality of the liquid in question. To about fifty cubic centimeters of 

 the clear solution of pure cupric bromide, which had remained standing 

 for three weeks, were added two drops of a dilute solution (1 : 400) 

 of methyl orange, and the greenish liquid was thoroughly shaken. 

 Upon equally dividing the solution between two test-tubes, the most 

 careful observation showed no difference in the color of the separate 

 portions. To one tube was then added 0.05 c.c. of a twentieth normal 

 solution of hydrobromic acid (= 0.2 milligram of bromine) and a 

 change from green to gray was perfectly evident when the portions 

 were again compared. A second equal addition of acid produced a 

 purplish hue. The gray was of course due to the simultaneous pres- 

 ence of the red color of the acidified methyl orange, the yellow of the 

 unchanged compound, and the blue of the copper bromide, and proved 

 that some of the indicator had been acted upon by the acid : hence 

 the solution could not have been basic in the first place. In a few 

 words, the solution, containing about one and a half grams of cupric 

 bromide, could not possibly have lacked more than 0.0002 gram of 

 its normal weight of bromine. This being the case, a determination 

 of the relative weights of copper and bromine in such a solution 

 would form a sufficiently accurate basis for the calculation of the 

 atomic weight of copper. 



* Berthemot, Ann. de Chim. et de Physique, 2d Series, xliv. 386 (1830); 

 Lowig, Liebig's Handwort. Ch., iv. 713; Rammelsberg, Pogg. Ann., Iv. 246: 

 and many others. 



