CYANIDE OF POTASSIUM. 
299 
yields crystals on cooling. This crystallization is not neces- 
sary for the preparation of urea. 
Cyanide of Potassium as an J9gent for Reduction. 
It is not easy to convey an idea of the extreme facility 
with which cyanide of potassium abstracts oxygen or sulphur 
from certain metallic oxides or combinations of sulphur; a 
property in which it is the most nearly allied to pure potas- 
sium. 
The preparation of cyanide of potassium and of cyanate of 
potassa affords two examples of this reductive power, The 
oxides of iron fused with cyanide of potassium are reduced 
with great readiness ; the iron remains mixed with the fused 
cyanate of potassa in the form of powder, or rather as a spongy 
mass. 
On this reduction we may found a process to ascertain, in 
the dry way, at a single operation, the proportion of the metal 
in a mineral. If we expose to a strong red heat, in a porce- 
lain crucible, a weighed amount of the mineral with a mix- 
ture of cyanide of potassium and carbonate of potassa, the alu- 
mina and silica pass into the scoria, and the reduced iron may 
be separated by washing with cold water and weighed. Prot- 
oxide of manganese is not reduced by cyanide of potassium; 
hence, to determine its presence in a mineral of iron, a special 
operation becomes necessary. 
If the oxide of copper be thrown into fused cyanide of po- 
tassium, it is immediately reduced with development of light 
and heat; and after washing there is obtained a compact but- 
ton of pure copper. 
The most striking reductions are those of the oxides of tin 
and antimony; by a feeble red heat the oxide of tin is changed 
into a brilliant regulus, which can be separated from the 
scoria as a well fused button; and in the same manner the 
VOL. VIII. — NO. IV. 
38 
