302 
CYANIDE OF POTASSIUM. 
amount, it is necessary to take care to use hydrochloric acid 
in great excess, to precipitate the metallic cyanides dissolved 
in the cyanide of potassium, and the mixture should, under 
these circumstances, be kept in ebullition for one hour. 
In fact, the precipitate formed in this case contains cyanide 
of nickel, which is decomposed with the potassa into cyanide 
of potassium and oxide of nickel; but this cyanide of potas- 
sium retains a portion of nickel in solution. 
By boiling the precipitate with hydrochloric acid, the 
cyanide of nickel is decomposed into chloride of nickel and 
hydrocyanic acid, which, by boiling, escapes, and thus no 
longer prevents complete precipitation. For the traces of 
hydrocyanic acid to disappear, it only remains to continue 
the boiling a sufficient length of time. 
The attempts to separate the two metallic cyanides by ebul- 
lition with deutoxide of mercury in cyanide of potassium, 
yield less certain results. 
It is necessary in this process to direct attention to the fol- 
lowing facts: 
As the cyanide of potassium contains a certain quantity of 
cyanate of potassa, there is formed during the decomposition 
by a mineral acid, a certain quantity of an ammoniacal salt, 
so that on ebullition, and the addition of caustic potassa, am- 
monia is set free, which retains a certain quantity of oxide of 
nickel in solution. Boiling for some minutes, or the addi- 
tion of caustic potassa, causes the complete deposit of 
this oxide of nickel. 
The same process is applicable to the separation of cobalt 
from manganese; only in this case a complete resolution of 
the precipitate, caused by the cyanide of potassium in the 
mixture of the salts of these metals, is not to be expected ; 
the greater part of the cyanide of manganese remaining un- 
dissolved. This residue is to be separated by the filter, and 
the liquor treated as in the separation of nickel from cobalt. 
The cyanide of potassium is not less advantageous in the 
separation of the oxide of chromium from oxide of iron. 
When a mixture of these two bodies, in which care has 
