MORSE AND SHUDDEMAGEN. — AN ALUMINIUM ANODE. 371 



and the various theories which have been put forward to explain the 

 behavior of these metals are all connected with the nature of this film. 

 The theories may be summarized as follows : 



1. The anode becomes covered with a thin oxide film during elec- 

 trolysis. This oxide film may produce the effects mentioned : (a) By 

 opposing an actual ohmic resistance to the passage of the current in one 

 direction : (b) By acting as a dielectric pure and simple : (c) By act- 

 ing as a semi-permeable membrane which prevents the passage of the 

 anion and permits the cation to pass freely. 



2. The active film is a thin layer of oxygen gas. This acts as a 

 dielectric, and the entire system is a true condenser. According to 

 this theory the visible film on the aluminium plate, whatever its 

 chemical composition may be, plays only a secondary part in the 

 process. It serves merely as a support for the gas layer which is 

 produced between it and the plate. 



So far in the history of the subject no crucial tests have been found 

 which can decide definitely in favor of one theory or the other. A 

 resistance pure and simple seems insufficient to account for the facts. 

 The resistance in this case must be a variable quantity, decreasing as 

 the current increases, and it must furthermore be of a different order 

 of magnitude in two directions through the cell. Nor do we need a 

 " transition resistance " to explain the facts. There is evidence of the 

 most trustworthy kind that oxygen plays a considerable part in the 

 phenomenon, but it is just as evident that it is not necessarily 

 the only factor. The semi-permeable film theory has much to support 

 it. Membranes have been prepared by precipitating aluminium hy- 

 droxide on the surface of a platinum plate, and even in the pores of an 

 earthenware cup, and these membranes are capable of exhibiting all 

 the important peculiarities of an aluminium anode formed in the usual 

 way by electrolysis. It seems evident that neither chemical investiga- 

 tion alone nor the measurement of electrical properties alone can give 

 a satisfactory answer to all the questions which arise concerning the 

 nature of the film and its action in the cell. Chemical investigation 

 has shown that the film consists largely of aluminium oxide or 

 hydroxide, and that oxygen gas is also invariably present in it, and 

 this much we may certainly take as definitely determined. 



In the earlier period of research on electrolytic polarization some 

 measurements were made with galvanometers more or less ballistic in 

 nature. Streintz (71) called attention to the fact that the discharge 

 from an aluminium plate used as anode consists essentially of two 

 parts, one of which was of the nature of a condenser discharge prac- 

 tically complete within a fraction of a second. The other portion of 



