Aciion of Iron upon Solutions of some Met dlic Salts. 271 



speak in favour of the idea already suggested that the che- 

 mical affinity of iron for oxygen is destroyed by one kind of 

 current, and called forth again by the other. It is true, one 

 of the most sagacious philosophers of the age, Mr. Faraday, 

 has started an idea which seems to account very satisfactorily 

 for the phiienomena in question. According to his view the 

 peculiar condition of iron depends either upon a film of oxide 

 covering the metal, or upon a relation of oxygen to iron equi- 

 valent to oxidation, so that the particles forming the surface 

 of the inactive iron have satisfied in one way or other their affi- 

 nity for oxygen. Applying the same hypothesis to account 

 for the bearing of iron in the solution of blue vitriol which 

 Mr. Faraday has made use of for explaining the singular ac- 

 tion of iron upon nitric acid, we must s^y, an inactive iron 

 wire does not act upon the solution of the copper salt, because 

 there is no immediate contact between the truly metallic par- 

 ticles of the wire and the said solution, on account of the in- 

 terposing film of oxide, or something similar to it. But if 

 now another metal be put into the solution, which is chemically 

 acted upon by the latter, there is a current produced, pro- 

 ceeding from the active metal, and passing through the solu- 

 tion into the inactive iron. By this current water is decom- 

 posed, hydrogen evolved at the iron ; the film, or what is equi- 

 valent to it, deprived of its oxygen; and by this means a truly 

 metallic surface of the iron wire produced. Though this 

 way of accounting for the facts in question recommends itself 

 by its beautiful simplicity, and what is still more valuable by 

 the great advantage of bringing back an apparently anomalous 

 case to a general law, still there are weighty reasons stated 

 by me elsewhere, which will hardly allow the adoption of 

 Mr. Faraday's sagacious hypothesis. 



After having examined the action of iron upon the common 

 sulphate of copper, I was curious to see how the same metal 

 acts under similar circumstances upon the solutions of the 

 nitrates of mercury. Before entering into details upon the 

 subject, I must not omit to state, that I did not observe any 

 essential difference of action between the protonitrate and 

 pernitrate of mercury. A common iron wire, cleaned or not, 

 when put into a solution of either of the neutral nitrates of 

 mercury, does not act in the least upon the salt, that is to say, 

 no mercury is precipitated on the iron ; but what is still more 

 surprising, the iron wire, after having been immersed only for 

 a few seconds in such a solution, shows all the properties of 

 inactive iron ; it will, for instance, not be acted upon by com- 

 mon nitric acid, nor by a solution of blue vitriol. Even when 

 a strong solution of the mercurial salt is diluted with 1000 



