140 Messrs. Gee and Holden on 



The method of placing the anode in the acid has an 

 important influence upon the value of A, as the following 

 experiments show : — 



Exp. I. — The end of a Pt wire used as an anode was im- 

 mersed in the acid, and then the film was formed by the 

 lowest value of the current-density. The anode was then 

 gradually lowered into the acid, when it was noticed that the 

 film was formed over the whole wire. When the wire was 

 lowered rapidly the film remained on the lower part for a 

 time, but the upper portion was conducting, as evidenced by 

 the escape of gas from it. 



Exp. II. — The reverse of the previous experiment is not 

 easy to perform. If we have at first the whole length of the 

 wire immersed and use a current-density just less than A, 

 and then try to form the film by diminishing the length of 

 the wire immersed — the strength of the current meanwhile 

 being kept constant — it is difficult to do so. 



d. Persistence of Film. — The disappearance of the film was 

 generally tested for by means of a weak current as follows : — 

 Knowing what deflection would be obtained, when the film 

 was not formed, with a certain large resistance in circuit, 

 then after the film had been formed the resistance was in- 

 creased to tin's value : if the current corresponding to this 

 resistance without the film was able to pass, it showed that 

 the film had been totally removed, and generally, from the 

 amount of current which did pass, could be estimated the 

 degree of disappearance of the film. 



After the formation of the film there are no signs of its dis- 

 appearance as long as the battery is left in circuit with the 

 voltameter, but if the circuit be broken the film tends to dis- 

 appear, the rate of disappearance depending on the complete- 

 ness with which the film had previously been formed. It ' 

 disappears very quickly if the voltameter be short-circuited, 

 and instantaneously if the current be reversed. 



The film is removed at once if the anode be taken out of 

 the liquid and wiped, but if the anode be left in the liquid — 

 the electrodes being still in connexion with the battery — the 

 anode may be moved about in the liquid or rubbed with a 

 glass rod without destroying the film. If the rubbing be 

 vigorous the film will be partially destroyed,, but on ceasing 

 the friction the film immediately reforms. The anode may 

 be removed from the liquid and held in the air some time 

 without thoroughly destroying the film. When in the liquid 

 a stream of air-bubbles may be blown violently against the 

 anode without destroying the film, but if the anode is removed 

 from the liquid a moderate current of air blown against it 

 removes the film. 



