separation of iron from other metals. 295 
bonates of the above enumerated metals are readily soluble , to a 
certain extent, in the solutions in which they are formed ( though 
perfectly neutral). In the cases of cobalt and cerium, this 
re-dissolution of the recent precipitate formed by carbonate of 
ammonia is very considerable, and a solution of either of 
these metals, thus impregnated with the metallic carbonate, 
becomes a test of the presence of peroxide of iron, of a deli- 
cacy surpassing most of the re-agents used in chemistry, the 
minutest trace of it being instantly thrown down by them 
from a boiling solution, provided no marked excess of acid 
be present. To be certain however that we have not gone 
too far, it is advisable, after separating the ferruginous pre- 
cipitate, to test the clear liquid, while hot, with a drop of the 
alkaline carbonate. If the cloud which this produces be 
clearly re-dissolved on agitation, we may be sure that only 
iron has been separated. If otherwise, a little acid must 
be added, the liquor poured again through the filter, so as 
to wash the precipitate, and the neutralization performed 
anew. 
The precipitation of iron above described seems at first 
sight to result from a double decomposition. Were it so, 
the principle of the method would be merely a difference of 
solubility in the carbonates of iron and the other metals, and 
as such would have no claim to be regarded as rigorous. 
Such however is not the case. The iron is not separated in 
the state of a carbonate, but of a sub-salt, or a simple peroxide, 
the whole of the carbonic acid escaping with effervescence at 
each addition of the alkali. The phenomenon turns on a 
peculiarity in the peroxide of this metal, in virtue of which 
it is incapable of existing in a neutral solution at the boiling 
