Manchester Memoirs, Vol. xvli. (1902), No.W. 3 



The mistake which both Rose and Henry made 

 was in the use of free hydrochloric acid before the 

 addition of chlorine (or bromine) and the carbonate. 

 Rose mentions having tried a neutral solution, but with 

 unsatisfactory results. On the other hand, I find that if 

 there is no free acid present, barium or calcium carbonate, 

 in presence of chlorine or bromine, will precipitate cobalt 

 completely in three or four minutes. Perhaps barium 

 carbonate acts a little more rapidly than calcium car- 

 bonate, but, in a great many experiments I have made, 

 neither of them has ever failed to precipitate the cobalt 

 in five minutes. 



With a neutral solution of nickel, on the other hand, 

 I have never found any precipitation of the black oxide 

 within an hour. After a longer time, a black deposit 

 begins to form on the side of the vessel at the edge of the 

 liquid, where the air has access to it. If air is excluded 

 nickel scarcely appears to be precipitated at all. I have 

 had solutions of nickel in presence of both barium and 

 calcium carbonate and bromine water in stoppered bottles 

 for some weeks without any appearance of precipitation 

 of the black oxide. When the liquid is heated to a tem- 

 perature of 70° or 80° C, the nickel is rapidly precipitated 

 as the black sesquioxide. 



If, however, much free acid is present when the car- 

 bonate and bromine water are added, as was always the 

 case in following Rose's original method, the precipitation 

 of the cobalt is very considerably retarded. It may take 

 hours, and sometimes does not even begin for over half- 

 an-hour. This explains why Rose found it necessary to 

 allow his solutions to stand so many hours. 



The retarding effect of the free acid is rather sur- 

 prising, because all free acid naturally becomes neutralised 

 by the carbonate employed, which must always be in 



