48 



phenomenon, whioh has been alread}' often observed, and is called 

 the becoming passive of iron, has not been accounted for in a 

 satisfactory way. 



In the light of these new considerations the explanation, as was 

 already observed, is not difficult. '■) The iron, which shows this 

 sudden increase of the anodic polarisation, is entirely free from so- 

 called annealing colours and perfectly reflecting, so that an oxide 

 skin is out of the question. 



If we, however, assume that the metal dissolves a little oxygen, 

 and this oxygen retards in a high degree the setting in of the 

 internal equilibrium, the sudden considerable enobling of the metal 

 is explained in a simple way. 



Up to now it has been lost sight of too much that the pheno- 

 menon of passivity, arisen by an electrolytic way, and that called 

 into existence by a purely chemical way, must be explained from 

 one and the same point of view. By a purely chemical way iron 

 is made passive by being simply immersed in strong nitric acid for 

 a few moments. If then the iron is pat in a solution of copper 

 sulphate, the copper does not deposit. By a slight shake, the appli- 

 cation of a magnetic field etc. this passive state can, however, 

 at once be destroyed, and the iron is covered with a coat of 

 copper. 



If we consider the passive iron to be iron that is superficially^ 

 very far from the state of internal equilibrium, in which super- 

 ficially the easily reacting molecules are practically entirely wanting, 

 and assume that this state can be maintained for some time on 

 account of the negative catalytic action of oxygen under certain 

 circumstances, which state, however, outside the cell, can be destroyed 

 by vibrations, a magnetic field etc., the phenomenon of passivity 

 of iron becomes less unintelligible. '^) * 



Returning to the experiment, we will show in the first place 

 what was found when smaller current densities were worked with 

 after the iron had become "passive" at higher current density. 



This table exhibits therefore the great difference between the passive 

 and the active iron. As appears from the last table but one, the 

 active iron yields a difference of tension with the auxiliary electrode 

 of 0,026 Volts for a density of current of 50; the passive iron yields 

 a difference of tension of 2,18 Volts for the same current density. 



"0 Smits, These Proc January 25, 1913, XVI. p. 191. 



2) We have probably to do here with metal ions of difierent valency. (We shall 

 return to this later on.) 



