THE SMITHSONIAN INSTITUTION. 



413 



state can be corainnnicated to an ordinary iron wire in the same man- 

 ner as it is given by P to E. 



The ex[)eriment, of which the plan is sketched in Fip;. 38, is 



of special ira})()rtance in reference to the theory of passivity. At 



one of the ends of a galvanometer an iridized iron wire is fastened, 



Pj„ 3^ and at the other an ordinary iron wire. Now, first dipping 



/^--;x the passive and then the other wire into nitric acid of 1.35 



\X) spec, grav., the galvanometer indicates a transient current, 



' ^ nassing in the direction from the unchanged iron, through 



tiie liquid, to tlie iridized iron. 



These experiments afford us a deep insight into the nature 

 of the passivity of iron. In the first place, it is evident 



that by heating the wire the coating of oxide thus formed 



protects it from the action of the acid, and thus the idea is very 

 obvious, that passive iron, even in cases where such a coating is 

 not visible, as, for instance, an immersion in concentrated nitric 

 acid, owes this property to a thin film of oxide. But then the 

 circumstance, that the platinum wire, in Fig. 36, can be exchanged 

 for the heated iron wire, shows that the oxide of iron formed by heating 

 to redness performs the functions of platinum, that by such a coating 

 the iron, in a certain measure, suffers negative galvanic polarization. 

 All passive iron wires are changed into active in hot acid ; yet, in 

 the facility with which they change their state, there appears a con- 

 siderable difference between those which are made passive by a red 

 beat and such as are rendered passive by contact with a wire already 

 passive, on being immersed in the liquid. We will term the former 

 primary passive, the latter secondary passive. The first owes the 

 longer continuance of its passivity to a thicker coating of oxide. 



Everything which destroys the protecting coat, renders the wire 

 active again. 



§ 49. Action of iron electrodes. — In the experiment represented by 

 Fig. 36, E evidently forms the positive pole of a simple circuit ; there- 

 fore it might be supposed that iron would be made passive by dipping it, 

 as the positive pole of a voltaic pile, in an acid, which would attack it. 

 Schonbein has actually made this experiment, (Pog. Ann. XXXVII, 

 391.) To the positive pole of a circuit consisting of 15 inconstant 

 einc and copper elements, an iron wire was fastened, while the nega- 

 tive pole terminated in a platinum wire. The negative platinum wire 

 was first dipped in a vessel of nitric acid of 1.36 sp. gr., and the 

 circuit was then closed by the immersion of the positive pole, formed 



of the iron wire, in the same acid, 

 as shown in Fig. 39. The iron 

 wire appeared perfectly passive, 

 and after separation from the bat- 

 tery, possessed the same proper- 

 ties as a wire made passive by 

 being heated red-hot. 



If the passive iron wire, con- 

 tinuing as the -|- pole of the cir- 

 cuit, remain in the acid, a remarkable phenomenon is exhibited. The 



Fig. 39. 



