prOttiote the setting in of the internal equilibrium in the at first 

 passive part, and this will be the explanation of the fact that the 

 iron becomes active after the current has been broken. 



Also after the nse of the iron electrodes as cathode the current 

 was broken, and as was to be expected, the much smaller cathodic 

 polarisation tension of ±0,15 appeared to run very rapidly back to 0. 



Aluminium. 



7. As far as its electromotive behaviour is concerned, aluminium 

 is undoubtedly one of the most interesting metals. For anodic pola- 

 risation the current density decreased regulai-ly, and the tension 

 increased, as is shown in the following table. 



TABLE 9. » 



Aluminium electrode in '/2 NAI2 (S04)3-solution. 



Accordingly we find anodic polarisation tensions of about 4 Volts 

 for this metaK already at \ery small current densities, which points 

 to the fact that here a layer of great resistance must have been formed. 



Up to now it has been tried to explain this strong anodic polari- 

 sation for aluminium by the formation of an insulating skin of AljO,. 

 With greater densities of current the anode is really covered with 

 an oxide skin, and it is therefore natural to assume the formation 

 of this skin also for smaller densities of current, and attribute the 

 observed phenomenon to this skin of Al^Oj with great resistance. 

 There are however objections to adopting this explanation, for in 

 our experiments no trace of annealing colours was to be observed, 

 and the metal remained beautifully refiecting. 



To ascertain whether in our experiments a skin of great resistance 

 had, formed round the anode, we made the following experiment. 



The bottom of the vessel with the AP(S()J^ solution was covered 

 with a layer of mei'cury, and the aluminium electrode was anodi- 



