51 



bally polarized. When this elcelrode was now covered with a skin 

 of great resistance, an immersion of one extremity of the aluminium 

 electrode in the mercurj' should not exei-t any influence on the 

 difference of tension l^etween the aluminium anode and the auxiliarj^ 

 electrode. If, however, this skin does not exist, the aluminium elec- 

 trode will get into contact with the mercury during the just described 

 manipulation, and the said difference of tension will be modified. 



The result was that when during the anodic polarisation the 

 aluminium anode was immei-sed in the mercury, and the current 

 was then broken, the difference of tension with the auxiliary elec- 

 trode was absolutely unciianged, which pi-oved therefore that the 

 aluminium electrode did not get in contact with the mercury, but 

 was surrounded with a coat of electrolyte. This appeared to l)e no 

 specific property of the anode, for the same thing was obser\'ed after 

 cathodic polarisation. An unpolarized Al-wire," immersed from the 

 electrolyte in the mercnry layer, immediately assumed the potential 

 of the mercury, from which therefore follows that the gas layer on 

 the aluminium retains tiie electrolyte with great force. 



In this way the question of the skin could therefore not be solved. 

 What is remarkable is this that the skin formed during anodic 

 polarisation, immediately seems to disappear again by cathodic 

 polarisation. The assumption of a film of Al^Og is attended with 

 great difficulties, in the first place this oxide cannot be reduced 

 under these circumstances b}^ H in status nascens, and in the second 

 place it appears, tliat nothing is to be perceived of this skin, at 

 least with the naked eye, as no annealing colours are to be observed, 

 and the metal remains clearly reflecting. It seems therefore not too 

 hazardous to us to conclude in virtue of this that the skin cannot 

 be an oxide layer, and the only thing left to us is to assume, as 

 we did for iron, that the oxygen dissolves in the aluminium during 

 anodic polarisation, and that this solution possesses a great electric 

 resistance for aluminium. In this way we come to the assumption 

 of a layer with great resistance, of which it is, however, to be 

 understood, that it entirely disappears on cathodic polarisation to 

 make room for a solution of hydi'ogen and aluminium. Accordingly 

 this layer is metallic, and can amalgamate in course of time when 

 in contact with mercury, through which the resistance disap[)ears. 

 The result at which we arrive is therefore this that the anodically 

 measured tension is so extraordinarily great for aluminium, much 

 greater than the liberation tension of O^ can be here, because the 

 dissohed oxygen not only I'etards the setting in of the internal 

 equilibrium, but iilso a layer of great electric resistance is formed. 



4* 



