A Theory of the Aluminium Anode. 311 



with an alternating current. There can be no migration of 

 the ions (with a high frequency and small current), and there- 

 fore a very small amount of certain salts in the film is 

 sufficient to enable a current to pass ; and will, in fact, largely 

 determine the conductivity. Now the formation of the film 

 by the interaction of the two salts necessarily produces such 

 a salt in the film ; and this, no doubt, is very difficult to 

 remove completely by mere washing*. 



We propose therefore to repeat the experiments with a 

 more suitable arrangement of electrodes, and with special 

 preparation of the film. We also hope to make measurements 

 of the resistances using continuous currents. 



The explanation of the peculiarities of aluminium when 

 used as an anode in solutions of sulphates, which has thus 

 been suggested, may be applied to the phenomena observed 

 in the reaction between aluminium and dilute acids. But it 

 seemed desirable to determine by experiment whether the 

 presence of those ions which have so marked an influence on 

 the anode has a similar influence on the rate of solution of 

 aluminium in sulphuric acid. Accordingly, the following 

 rough determinations were made of the rate of evolution of 

 hydrogen from the acid, both alone, and with addition of 

 certain ions. 



Preliminary trial showed that at temperatures of 80° C. to 

 85° C, and with a 2-molar solution of sulphuric acid, the 

 reaction proceeded at a rate which could be conveniently 

 measured. A thermostat was adjusted to 85° C, and all the 

 experiments were made at this temperature. Small flasks ot 

 about 80 c.c. capacity, and pieces of sheet aluminium of 

 uniform size and weight (25 mm. square, 0*45 gm.) were 

 used. Each flask contained one piece of the metal and 

 60 c.c. of a 2-molar sulphuric acid solution, and in addition 

 a known quantity of a concentrated solution of the salt 

 under investigation. The gas evolved was collected in a 

 burette. The salts employed were KC1, KBr, KNO :; , and 

 KCNS. The results are summarized below. 



Chloride. — Four experiments were made to determine the 

 effect of chloride, a 3*9 molar solution of potassium chloride 



* With a direct current this salt is removed from the lihu by the 

 action of the current. This is probably the reason why, in the series of 

 experiments last described, the current at first is s long- (20 hours) in 

 reaching- a minimum, although afterwards it reaches the same minimum 

 in much less time (about the same time that had elapsed between the 

 two previous commutations). 



Y2 



