878 
formed aluminium-surface is immediately covered with a solid, not 
porous layer of oxide of molecular thickness. This layer, indeed, 
insulates, but according to him it can be pierced by the anions of 
the salt-solutions or by the O”-ion on anodic polarisation, and the 
oxygen formed then combines with the metal to Al,O,. The porous 
oxide layer offers an ever increasing resistance with increasing 
thickness, and at last the anions are almost exclusively discharged 
at the layer of oxide, and only very few succeed in traversing this 
layer, and reaching the metal, which he tries to prove by the fact 
that the quantity of generated oxygen is 96°/, of the quantity of 
electricity transmitted. 
When with a certain thickness of layer a definite potential gradient 
has been reached, sparking commences, which puts a stop to the 
increase of tension and the thickening of the oxide layer. This 
maximum tension is greatly dependent on the nature and the con- 
centration of the anions; when this concentration increases, the 
maximum tension diminishes. It is remarkable that, when the current 
is reversed, no current passes below a certain potential, and that 
this minimum potential of the cathode is then many times smaller 
than the anodic-minimum-potential. Also the cathodic minimum 
potential depends greatly on the nature of the ions. 
SCHULZE, and before him Taytor and INeus ') and Gurue ®), thought 
that they could find the explanation of this peculiar phenomenon 
by assigning to the gas layer that is formed in the pores of the 
Al, (OH),, the property of allowing the anions to pass less easily 
than the cations. 
It is clear that this explanation is not entirely satisfactory, the 
more so because there are still a great many other exceedingly 
remarkable phenomena on which it does not throw any light. Two 
of them may be mentioned here, first the phenomenon that amal- 
gamated aluminium does not show valve-action, and secondly that 
a chlor-ion concentration in the electrolyte of 0.2°/, renders the 
valve-action quite impossible. 
2. When aluminium is considered from the point of view of the 
theory of the electromotive equilibria, the conclusion is readily reached 
that this theory is able to account for the above-mentioned remarkable 
behaviour of aluminium by means of the same principles as the 
polarisation-phenomena in the other metals. 
In the first place it may be pointed out that it can easily be 
3) Phil. Rev. 15, 327, (1902). 
