64) Mr. Faraday on a peculiar Voltaic Condition of Iron. 



and the relation of the metals was also reversed, the iron being 

 as platina, to the silver as zinc ; then another inversion took 

 place, and then another, and thus the changes went on some- 

 times eight or nine times together, ending at last generally in 

 a current constant in its direction, the iron being as zinc, to 

 the silver as platina : occasionally the reverse was the case, the 

 predominant current being as if" the silver acted as zinc. 



Tliis relation of iron to silver, which was before referred to 

 page 58, produces some curious results as to the precipitation 

 of one metal by another. If a piece of clean iron is put into 

 an aqueous solution of nitrate of silver, there is no immediate 

 apparent change of any kind. After several days the iron will 

 become slightly discoloured, and small irregular crystals of sil- 

 ver will appear; but the action is so slow as to require time and 

 care for its observation. When a solution of nitrate of silver 

 to which a little nitric acid had been added was used, there 

 was still no sensible immediate action on the iron. When the 

 solution was rendered very acid, then there was direct imme- 

 diate action on the iron; it became covered with a coat of pi'e- 

 cipitated silver: the action then suddenly ceased, the silver was 

 immediately redissolved, and the iron left perfectly clear, in 

 the peculiar condition, and unable to cause any further preci- 

 pitation of the silver from the solution. It is a remarkable 

 thino- in this experiment to see the silver rapidly dissolve away 

 in a solution which cannot touch the iron, and to see the iron 

 in a clean metallic state unable to precipitate the silver. 



Iron and platina in an aqueous solution of nitrate of silver 

 produce no electric current; both act as platina. When the 

 solution is rendered a little acid by nitric acid, there is a very 

 feeble current for a moment, the iron being as zinc. When 

 still more acid is added so as to cause the iron to precipitate 

 silver, there is a strong current whilst that action lasts, but 

 when it ceases the current ceases, and then it is that the 

 silver is redissolved. The association of the platina with the 

 iron evidently helps much to stop the action. 



When iron is associated with mei'cury, copper, lead, tin, 

 zinc, and some other metals, in an aqueous solution of nitrate 

 of silver, it produces a constant electric current, but always 

 acts the part oi platinum. This is perhaps most striking with 

 mercury and copper, because of the marked contrast it affords 

 to the effects produced in dilute sulphuric acid and most or- 

 dinary solutions. The constancy of the current even causes 

 crystals of silver to form on the iron as the negative electrode. 

 It might at first seem surprising that the power which tends 

 to reduce silver on the iron negative electrode did not also 

 bring back the iron from its peculiar state, whether that be a 



