AND OP SALTS OP THE PEROXIDE OP IRON. 
253 
acquires a dark blue color. If into the solution of ferrid- 
cyanide air or pure oxygen be passed while it is in contact 
with the metallic iron, the formation of the blue substance 
proceeds more rapidly. When finely-divided iron is con- 
veyed into the solution instead of iron wire, and air blown 
in through a glass tube, the reaction takes place still more 
rapidly. 
However short a time the iron may have remained in 
contact with the solution of the ferridcyanide, the latter will 
have become so altered that it is colored blue when treated 
with a solution of a persalt of iron. 
If the solution be boiled until all the air is expelled from 
it, and an iron wire, from which likewise the adherent air 
has been removed by heating in boiling water, be placed 
in it, the formation of the blue substance around the wire 
does not occur so long as the liquid is kept boiling, at least 
not in any perceptible degree. As soon, however, as the 
solution is allowed to cool, the metal is seen to envelope it- 
self with a blue coating, i. e. if contact between the atmo- 
sphere and the solution of ferridcyanide be not prevented. 
The solution of ferridcyanide was allowed to boil in a 
glass flask for ten minutes, and a number of bright iron 
wires kept in boiling water for the same length of time ; 
the latter were then conveyed as quickly as possible into 
the boiling solution of ferridcyanide, and allowed to boil 
for a few minutes, upon which the flask was closed as air- 
tight as possible. During the first day the wires remained 
perfectly bright, but on the following day several blue spots 
were observed on them, the number and size of which be- 
came larger and larger, and from which subsequently pro- 
ceeded minute streaks of a somewhat dirty blue color. A 
similar flask was filled with unboiled solution of the ferrid- 
cyanide and bright iron wires, and left open. In a few 
hours not only had the wires become surrounded with a 
blue coating, but a large number of the above-mentioned 
blue streaks were seen to project from the wires into the 
vol. x. — NO. iv. 23 
