62 Mr. Faraday on a peculiar Voltaic Co7idiiion of Iron. 



by the electric current produced at the first moment, and 

 which in such cases ahvays precedes the change of the iron 

 from the common to the peculiar state. That the coat of 

 oxide produced by common means might be so thin as not to 

 be sensible and yet be effectual, was shown by heating a piece 

 of iron an inch or two from the end, so that though blue at 

 the heated part, the end did not seem in the slightest degree 

 affected, and yet that end was in the pecuHar state. Again, 

 whether the iron be oxidized in the flame nmch or only to the 

 very slight degree just described, or be brought into the pe- 

 culiar state by voltaic association with other pieces or with 

 platina, &c., still if a part of its surface were removed even in 

 the smallest degree and then the new surface put into contact 

 with the nitric acid, that part was at the first moment as com- 

 mon iron; the state being abundantly evident by the electrical 

 current produced at the instant of immersion. 



Why the superficial film of oxide which I suppose to be 

 formed when the iron is brought into the peculiar state by 

 voltaic association, or occasionally by immersion alone into 

 nitric acid, is not dissolved by the acid, is I presume dependent 

 upon the peculiarities of this oxide and of nitric acid of the 

 strength required for these experiments; but as a matter of 

 fact it is well known that the oxide produced upon the surface 

 of iron by heat, and showing itself by thin films of various 

 colours, is scarcely touched by nitric acid of the given strength 

 though left in contact with it for days together. That this 

 does not depend upon the film having any great thickness, 

 but upon its peculiar condition, is rendered probable from the 

 fact that iron oxidized by heat, only in that slight degree as 

 to offer no difference to the eye, has been left in nitric acid of 

 the given strength for weeks together without any change. And 

 that this mode of superficial oxidation, or this kind of oxide, 

 may occur in the voltaic cases, is rendered probable by the 

 results of the oxidation of iron in nitrate of silver. When 

 nitrate of silver is fused and common iron dipped into it, so 

 as to be thoroughly wetted, being either alone or in associa- 

 tion with platina, the iron does not commence a violent action 

 on the nitrate and throw down silver, but it is gradually oxi- 

 dized on the surface with exactly the same appearances of 

 colour, uniformity of surface, &c., as if it were slowly oxi- 

 dized by heat in the air. 



Professor Schoenbein has stated the case of iron when acting 

 as the positive electrode of a couronne ties tasses. If that in- 

 strument be in strong action, or if an ordinary battery be used 

 containing from two to ten or more plates, the positive iron in- 

 stantly becomes covered in the nitric acid with a coat of oxide. 



