1 7* Oh the Causes of ike Neutrality of Iron in Nitric Acid. 



pears, that ihe most important cause of the chemical indiffer- 

 ence of tlie iron to the nitric acid is neither owing to its being 

 surrounded by a thin pellicle of oxide, nor to a certain quan- 

 tity of water in the acid, but directly to the electrical current 

 in "whatever manner it acts. Moreover, it is clear, that if the 

 indifference of the positive iron wire depended upon a thin 

 coat of oxide which surrounds it, the same wire ought to re- 

 main passive when it is separated from the pole, and put into 

 common nitric acid, which however is not the case. The 

 fact that the positive iron wire is acted upon similarly in other 

 dilute acids as in nitric acid is rather unfavourable to Faraday's 

 hypothesis. It is well known that iron becomes quite passive 

 by being merely once dipped into fuming nitric acid : how 

 then is the pellicle formed in this case? I suppose only by 

 the decomposition of the nitric acid, because nothing else is 

 possible. I doubt however very strongly whether this takes 

 place ; but if it does not take place, it is very difficult to ascer- 

 tain in what manner the iron does become oxidized. I must, 

 however, remark that the galvanometer during the dipping of 

 the iron in highly concentrated nitric acid indicates a weak 

 electric current, but it appears to me that this does not prove 

 the oxidation of the metal. To the above remarks I must 

 add one more, which 1 think is not unimportant as regards 

 this subject. In my last paper 1 spoke of an action of an 

 acid of 1 35 upon the iron which took place by starts, and 

 I showed that it was occasioned by the metal becoming 

 active and passive. Faraday would explain this appearance 

 by the supposition that at one instant a pellicle of oxide is 

 formed round the wire which protects it from the action of 

 the nitric acid, but that in the next the pellicle is dissolved 

 in the acid, and would expose by that means the clean me- 

 tallic surface of the wire to the acid fluid. But this man- 

 ner of explaining it contains a contradiction in itself; because 

 it first makes out the pellicle to be insoluble in nitric acid, 

 and then to be soluble ; it cannot therefore be correct. More- 

 over, one might put the unanswerable question, why iron by 

 frequent immersions in common nitric acid is rendered j)as- 

 sive; therefore, why an oxide is formed which in the first 

 place enters into combination with the acid, but afterwards 

 remains in the same acid an insoluble oxide? All the reasons 

 above given decide me to suppose that Faraday's views con- 

 cerning the passive state of the iron do not explain it satis- 

 factorily. 



Bale, October 2, 1836. 



