Chemistry. — “The Electromotwe Behaviour of Aluminium.” 1. 
By Prof. A. Smrrs. (Communicated by Prof. H. A. Lorentz.). 
(Communicated at the meeting of February 28, 1920). 
1. Jntroduction. 
As early as 1914') we began to consider the behaviour of 
aluminium from the point of view offered by the new theory of 
the electromotive equilibria. 
As regards its electromotive behaviour aluminium is a most in- 
teresting metal. It has generally not been inserted in the electromotive 
series, because no certainty has been attained as yet about its place. 
In alkaline solutions aluminium precipitates the zinc, but it does 
not do so in neutral or acid solutions. To this is added the very 
remarkable fact that the amalgamated alumininm does precipitate 
the zine from neutral solutions, and acts with violent decom- 
position on water, that it further rapidly oxidizes when exposed to 
the air, and exhibits a character that indicates that aluminium in 
this condition must be placed directly after the metals of the alkaline 
earths, thus: Mg—Al—Mn—Zn. 
In connection with this the conclusion was obvious that commer- 
cial aluminium is in a noble, less active condition, or in other words 
that it is in a state of passivity. This was decidedly a step in the 
right direction, but an explanation of the behaviour of aluminium 
had not yet been given. 
Most handbooks and publications state that commercial alumi- 
nium is covered with a coat of oxide, and that its passivity is 
owing to this. 
Also the anodic polarisation of aluminium has made the peculiar 
character of this metal evident. It was found before, that when an 
Al,(SO,),-solution was used, the density of the current, i/o, on anodic 
polarisation continually decreased, whereas the electric potential rose, 
which may be seen from the following table, which has already 
been published before.*) Here the potential has been measured with 
respect to another aluminium rod as auxiliary electrode. 
1) Smits, Aten, These Proc. 22, 1133 (1914). 
8) SMITS, ATEN, Loc. cit. 
