<: 



216 PROCEEDINGS OF THE AMERICAN ACADEMY 



inclination of about 94^° between the nor- 

 mals. Tbis value affords us the means of 

 calculating the axial ratio, x : z=l : 1.08,- 

 and the two figures suggest the angle 

 96° 38' and axial ratio 1 : 1.123 observed 

 in atacamite.* Indeed, the general resem- 

 blance in habit and in emerald green color 

 between the two substances is very marked, 

 and has already been noticed by Brun.f 

 Crystallized cupric oxybromide which has 

 . been dried in the air does not lose weight 



Magnified 250 diameters. ^^'^r sulphuric add. ^ It is of course Very 



soluble in mineral acids and in ammonia, as 

 well as in strong acetic acid. The mode of formation of the salt 

 shows that it must be slightly soluble also in very concentrated 

 solutions of the normal cupric bromide, and that dilution diminishes, 

 or perhaps wholly destroys, this solubility. The proof of its total 

 insolubility in water being of great importance in the preceding 

 research, the following experiment was made. 



Two hundred cubic centimeters of water were digested for four 

 days at 25° with a tenth of a gram of very fine crystals of copper 

 oxybromide, the mixture being occasionally shaken. The liquid was 

 then filtered, and thirty cubic centimeters contained in a long tube 

 were tested for copper with ammonia, giving an absolutely negative 

 result. One hundred and thirty cubic centimeters of the same liquid 

 were evaporated to dryness, and the apparently clean dish was rinsed 

 with a few drops of dilute nitric acid, to which was then added an 

 excess of ammonia. An exceedingly faint tinge only of blue was 

 apparent, much less than that produced by one tenth of a milligram 

 of copper under the same circumstances. 



Being thus essentially insoluble in water, the oxybromide could 

 hardly be present, even in traces, in the very dilute solution of cupric 

 bromide used for the determination of the atomic weight ; but for 

 certainty upon this point the normal state of this solution was tested 

 by experiments with methyl orange. These experiments, which have 

 already been described,! settled the question in a wholly satisfactory 



* Zepharovitch, Wien. Akad. Sitzungsber., 1871, vol. Ixiii. part i. p. 6, 



t Loc. cit. 



X This volume, page 201. 



