CYANIDE OP POTASSIUM. 
303 
been taken to saturate with sulphuretted hydrogen, (a few 
drops of hydrosulphuret of ammonia will answer,) so that the 
iron may be in the state of protoxide, be precipitated by an 
excess of cyanide of potassium, the iron immediately dis- 
solves as ferrocyanide, and the oxide of chromium is left be- 
hind. 
In certain cases cyanide of potassium may be employed 
with advantage to separate iron from alumina, (alumina being 
in great excess,) through the great solubility of the protoxide 
and sulphuret of this metal, and the insolubility of alumina in 
the cyanide of potassium. 
This latter body deserves study as a general means of se- 
paration; but unfortunately, we know of the great number of 
double combinations which it forms with other cyanides, 
only the composition, and not their modes of reaction with 
mineral and vegetable acids; this study, therefore, requires a 
review of the whole subject. A. G. V. 
Journ. de Chim. and de Pharm. 
