A Theory of the Aluminium Anode, 307 



substances. The tube is then placed in a solution of alum- 

 inium salt, and ammonia solution is put inside the tube. In 

 this way a film of Al(OH) 3 is formed where the two solutions 

 meet, i.e., in the interior of the gelatine. A solution of the 

 salt under examination is then added to the ammonia solution 

 in the inner tube, and from time to time the outer solution is 

 tested for the salt. From the fact that a film of aluminium 

 hydroxide can be formed in this way, one may conclude that 

 it is impermeable to Al"* ions, and to OH/ ions, as otherwise 

 diffusion would continue until one or other of the salts was 

 completely removed. 



Having set up a large number of cells we found that KC1, 

 KBr, KN0 3 , KClOo, and KCNS all diffuse through rapidly, 

 though not equally so ; NaC 2 H 3 2 diffuses slowly, and 

 K 2 S0 4 only to a very slight extent. To confirm this result, 

 more cells were set up, and mixtures of KC1 and K 2 S0 4 , 

 KBr and K 2 S0 4 , were added to the ammonia solution, so that 

 the rates of diffusion through the same film could be observed. 

 The same results were obtained. 



It now seemed very probable that the abnormal behaviour 

 of the aluminium anode in sulphuric acid was due to this 

 impermeability. According to Ditte *, the surface of alum- 

 inium is covered with a thin film of hydroxide which 

 preserves it from the further action of the air. If, there- 

 fore, a piece of aluminium be made the anode in dilute 

 sulphuric acid, the SO/' ions are unable to pass from the 

 solution through the film to the anode, and similarly Al"* 

 ions are unable to pass from the anode into the solution. 

 Hence there are no ions to carry the electricity through 

 the film, and no current can pass. A very slight current 

 does pass, and this may be due to Al"' ions being formed at 

 the anode, H* ions of the water passing at the same time 

 through the film and thus leaving OH' ions which form 

 Al(OH) 3 with the Al"" ions just formed. This aluminium 

 hydroxide replaces that which may be removed by solution in 

 the acid, and in this way the continuity of the film is main- 

 tained. 



If CI 7 , Br', or NO*/ ions are present, they can migrate 

 through the film, thus carrying electricity to the anode 

 where they unite with Al"' and form neutral salts; and 

 this formation of salt behind the film will break it loose, and 

 so enable the current to pass easily. In this wa} r the results 

 obtained admit of an easy and rational explanation. 



If this explanation is correct, it should be possible to 

 * Compt. Rend, cxxvii. p. 919 (1898). 



