306 Dr. Taylor and Mr. Iuglis : 



Similar experiments were made with other salts, and the 

 results may be very briefly described. 



Nitrate. — A 2*6 molar solution o£ potassium nitrate was 

 used. After addition of 3*0 c.c. of the solution to 25 c.c. of 

 acid, the current at once rose steadily ; considerable irregu- 

 larity was shown after addition of 2*0 c.c. of the nitrate 

 solution. 



Acetate. — Addition of sodium acetate to sulphuric acid had 

 no effect. In order to attain a considerable concentration of 

 acetions, a saturated solution of sodium sulphate was then 

 used instead of sulphuric acid, but this made no difference. 



Thiocyanate. — A 2*0 molar solution of potassium thio- 

 cyanate was used. Addition of 2*0 to 3'0 c.c. of the solution 

 was found necessary to enable the current to pass readily. 



Chlorate. — Addition of potassium chlorate was also found 

 to enable the current to pass readily. 



As the presence of aluminium salt might, conceivably, 

 influence the results, several of the above experiments were 

 repeated after 1*0 c.c. of 0*5 molar aluminium sulphate solution 

 had been added to the acid, but no differences were found. 



These experiments show that the presence of certain ions, 

 even in small concentration, enables a large current to pass 

 through the cell ; and it seemed to us probable that the reason 

 is that the film of aluminium hydroxide with which the anode 

 is covered is permeable to certain ions, but impermeable to 

 others *. If this is so, any anion which can readily pass 

 through the film will enable a current to pass, whilst anions 

 which cannot readily pass through will not enable it to do 

 so. The anomalous behaviour in sulphuric acid would then 

 be due to the impermeability of the film to SO/' ions, and 

 also to Al • * ' ions. This explanation is also in accord with the 

 fact that reversal of the current immediately causes a current 

 to pass through the cell, this being due to the permeability 

 of the film to H ions, for it is difficult to suppose that reversal 

 of the current immediately removes the film and subsequent 

 reversal immediately restores it. 



We next made a series of experiments to determine the 

 relative rates of diffusion of these ions through a film of 

 aluminium hydroxide. The method adopted is one devised 

 by Walden | and consists in forming a film of gelatine 

 containing ammonium chromate over one end of a glass tube, 

 exposing it to daylight, and then washing out all soluble 



* Cf. Ostwald, Zeit.f. Phys. Chem. vi. p 71 (1890). 

 t Zeit.f. Physik Chem. x. p. 699 (1892). 



