OF ARTS AND SCIENCES. 257 



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6. Tetrammon- Tncupriammonhcm Bromide, 

 3 Cu(NH3)2Br2 . 4 NHg. 



Long ago Rammelsberg* described two compounds of ammonia and 

 cupric bromide, to one of which, consisting of dark green crystals, he 

 ascribed the formula CuBrg . 3 NH3 ; and to the other, consisting of a 

 bright blue powder, he ascribed the formula CuBi'a . 5 NH3. Recent 

 investigation t has shown that the latter of the two substances must 

 have been in reality CuBrj . 6 NH3, which had lost some of its very 

 loosely combined ammonia by exposure to the air. The same investi- 

 gation brought to light an olive-green substance having the formula 

 Cu(NH3)2Br2. 



Repeated attempts were made at the same time to obtain Rammels- 

 berg's first substance. The product of these trials invariably consisted 

 of deep indigo, almost black crystals, which contained noticeably more 

 ammonia than the amount required by Rammelsberg's formula. These 

 deep blue crystals are best obtained by adding very cautiously strong 

 hydrobromic acid to a mixture of cupric bromide, alcohol, and just 

 enough aqua ammonia to keep all the copper in solution. Upon the 

 addition of enough acid to neutralize the ammonia, the crystals — 

 which are almost insoluble in alcohol — begin to form. 



Upon exposure to the air in a moist state these crystals lose am- 

 monia rather rapidly; but when dry they are much more stable. 

 Gentle heat (160°) readily converts them completely into the olive- 

 green Cu(NH3)2Br2, which still retains the crystalline form of the 

 more complex salt. It is not impossible that Rammelsberg's green 

 crystals consisted originally of the indigo-colored substance, which had 

 lost superficially a little of its ammonia. 



Tetrammon-tricupriammonium bromide is decomposed by water, a 

 noticeable amount of copper going into solution in the form of a sol- 



* Pogg. Annalen, LV. 240. 



t T. W. Richards, Ber. d. ch. Ges., 1890, p. 3790. 



VOL. XXVIII. (n. S. XX.) 17 



