684 



It follows from this (able that these metals about which it was 

 derived from the plicnomenon of polarisation that they assume their 

 internal equilibrium much more slowlj- than Ac/ etc. really become 

 nobler when etched with acid, as was expected. 



This temporary change of the potential difference in the sense 

 noble must tind its origin in a change in the metal snrface, and so 

 it is perfectly clear that theories 3, 4, cannot be of any use here. 



At the same time they throw a peculiar light on Grave's theory. 

 Gravk thinks that the heterogeneous equilibrium metal-electrolyte 

 can be easily retarded, but that the hydrogen exerts a positively 

 catalytic influence on the heterogeneous equilibrium. According to 

 him iron would become passive by extraction of the dissolved hydrogen, 

 which wonld take place on anodic polarisation. Now specially in 

 ion-state hydrogen is certainly a catalyst for the transition passive 

 •^- active, but it is quite unpei'missible to derive from this that 

 hydrogen catalyses the heterogeneous equilibrium. It is of importance 

 to state emphatically that according to Grave's theory it could by 

 no means be expected that an enohUng of the metal surface would 

 be brought about when it was etched with HCI or H^SO^, in w^hich 

 the metal can absorb hydrogen. We ha\e to do here with a pheno- 

 menon that cannot be counteracted by the positively catalytic influence 

 of the hydrogen on the internal transformations in the metal surface. 

 ^ Further we made the following in our Opinion very important 

 experiment with iron. An iron electrode forged to a platinum wire 

 was made passive one time through anodic polarisation, another 

 time through immei-sion in strong nitric acid. Both times the potential 

 of these passive states of the iron electrode immerged in Vio N.FeSO^ 

 solution, was measured with respect to Vio norm, calomel, in which 

 the following appeared. 



The potential difference of the pa.^sive iron, both after anodic 

 polarisation and after etching with strong HNO3 at first decreases pretty 

 rapidly, then remains constant for a lime, and then descends again 

 rapidly. We now compared the temporarily constant potential differ- 

 ence of the quickly rotating electrode in the two cases, and then 

 found what foUow^s : 



Observations 



Tension of the constant part of 

 the potential of passive iron 



Passivity arisen by anodic polari- 

 sation 



Passivity arisen on attack of 

 strong HNO3 



+0,20 V with respect to "10 Norm. 

 Calomel electrode 



-f 0,205 V with respect to '/loNorm. 

 Calomel electrode 



