AND OP SALTS OF THE PEROXIDE OP IRON. 
255 
and tin, it is rapidly altered, and is colored blue with salts 
of the peroxide of iron. If the contact between metal and 
solution has lasted only for half a minute, and even at the 
ordinary temperature, there is nevertheless a perceptible 
blue coloring produced on the addition of the pernitrate of 
iron to the solution ; and if the metals in question be left 
for several days in the dissolved percyanide, this will af- 
ford considerable precipitates of prussian blue with persalts 
of iron. 
It is remarkable that cadmium acts very slowly on the 
percyanide, and this metal must have stood for several hours 
in contact with the solution before the latter exhibits a per- 
ceptibly blue tint on the addition of pernitrate of iron. Even 
copper, mercury and silver effect a change in the solution of 
the percyanide, and convert a portion of it into the proto- 
cyanide. But this reaction proceeds very slowly, and seve- 
ral days' contact is requisite for the solution to become blue 
on the addition of pernitrate of iron. 
A number of the above-mentioned metals lose their lustre 
(for instance lead) when placed in the solution of percyanide, 
and become coated with a film, the chemical nature of which 
the author has not examined more minutely. It is not im- 
probable that they are compounds of cyanogen, as with zinc 
and iron. A very easy and simple manner of demonstrating 
the chemical alteration which the solution of the percyanide 
undergoes during contact with the above metals, consists in 
conveying a drop of the liquid to the bright surface of one 
of the metals, and then adding to it a drop of dissolved ni- 
trate of iron. Immediately after the mixture of the two has 
taken place, the surface of the metal touched by them is 
coated with a layer of prussian blue, and the above-described 
reaction ensues almost instantaneously, even though the me- 
tals employed be copper, mercury or silver. 
As will appear from subsequent details, the latter metals 
induce the formation of prussian blue principally from their 
converting the dissolved persalt of iron partially into a pro- 
