880 
as it is here, the potential of separation of the oxygen will soon 
be reached, and oxygen generation will set in, which under certain 
circumstances may lead to the formation of an adherent coating of 
oxide or hydroxide round the metal. Of course this coating gives 
rise to a certain resistance, which may rise to considerable amounts 
with increasing thickness. The assumption, however, that the resi- 
stance of such a coating should be different for different directions 
of current is not justifiable, so that there can be no doubt that the 
sudden decrease of the resistance on reversal of the current, must 
be owing to some other cause. 
So far there is no reason to doubt that oxygen and hydrogen are 
negative catalysts for the setting in of the internal metal equilibrium. 
Hence the slight quantities of oxygen absorbed by the metal on 
anodic polarisation have a greatly retarding effect (Fe, Co, Ni). 
Most probably this is likewise the case for aluminium, and to 
this it will have to be attributed that such a strong anodic polari- 
sation has been observed in aluminium. 
Accordingly this fact leads to the assumption that the metal is 
disturbed here to a great extent, i.e. that the metal surface becomes 
very poor in ions and electrons, or in other words, that the metal 
passes at its surface into a state which agrees with a metalloid in 
this that it possesses an exceedingly small electric conductivity. 
On this strong anodie disturbance the aluminium surface becomes, 
therefore, a metal coating of great resistance, and this coating is in 
its turn surrounded by another of Al, O,. 
As the study of the phenomenon of polarisation in other metals 
has taught, the disturbance that has arisen by anodic polarisation, 
stops immediately through reversal of the current. This behaviour 
must be explained by the fact that hydrogen, just as oxygen, though 
in a different degree, is a negative catalyst for the establishment of 
the internal metal equilibrium, and can yet apparently act positively 
catalytically, when it separates on a metal surface that has previ- 
ously absorbed oxygen, as both negative catalysts then disappear 
amidst formation of water. The small quantity of oxygen absorbed 
then enhances the disturbance of the aluminium on anodic polari- 
sation; hence the removal of this negative catalyst will immediately 
stop the disturbance, and the strongly metastable state of the alu- 
minium surface will be transformed with great velocity in the 
direction of the state of internal equilibrium. This transformation 
changes the metal coating of great resistance suddenly into another 
of smaller resistance, which must, therefore, be attributed to the 
great velocity of the reaction: 
