so PROCEEDINGS OF THE AMERICAN ACADEMY 



octahedral salt was rather difficult to obtain perfectly pure by this 

 method, I had recourse to the decomposition of sulphate of roseoco- 

 balt by baric nitrite. A solution of the last-named salt is to be added 

 to one of the sulphate as long as a precipitate is formed. The sherry- 

 wine-colored filtrate is then to be cautiously evaporated, when fine dark 

 wine-colored octahedral crystals form. Of these crystals 



0-4750 gr. gave 0-230.3 gr. SO^Co= 18-46 per cent cobalt. 

 0-1220 gr. gave 0-0594 gr. SO^Co= 18-54 per cent cobalt. 

 0-3129 gr. gave 0-0403 gr. water, when heated to 140° C. = 12-87 per 



cent. 

 0-4289 gr. gave 0-1141 gr. ammonia = 26-60 per cent. 



The formula Co2(NH3)jo(N02)6 -f 40 Hg requires 



Calculated. Found. 



Cobalt, 2 18-55 18-46 18-54 



Ammonia, 10 26-72 26-60 



Water, 4 11-32 12-87 



The percentage of water in the analysis is too high, and would seem to 

 show that a slight decomposition of the salt had taken place. I at- 

 tempted to determine the percentage of NO^ in this salt by titrition 

 with potassic hypermanganate, but though the analyses were made with 

 the greatest care, I obtained as a mean of three determinations, agree- 

 ing well together, only 11-24 per cent, which would correspond to less 

 than two atoms. In other cases also I found that the method could 

 not be employed. 



So far as the empirical formula is concerned, the salt may be regarded 

 as a nitrite of purpureocobalt or roseocobalt, Co2(NH3)jq(N02)5 -|- 

 4OH2. Its solution gives, however, the reactions of salts of xanthocobalt 

 with the greatest distinctness, and I regard it, therefore, as the normal 

 nitrite of this series, with the formula Co2(NH3)jo(N02)2(N02)4-f- 

 4OH2. Its formation from sulphate of roseocobalt and baric nitrite 

 is expressed by the equation : — 



Co2(NH3),„(SO,)3 + 3Ba(N02)2 = Co2(NH3),„(N02)s + 3S0,Ba, 



and from chloride of purpureocobalt and argentic nitrite, by the equation, 



Co2(NH3),oCl«-f 6AgN02= Co2(NH3)j„(N02)«+ 6AgCl. 



The formation of the silver salt of Erdmann's series, Co2(NH3)4(N02)g 

 Ag.-,, is probably due to a secondary action, and may, perhaps, be 

 expressed by the equation 



Co2(NH3),o(N02)„+ 2AgN02 = Co2(NH3),(N02)3Ag2 + 6NH,. 



