146 Messrs. Gee and Holden on 



strengths of sodium benzoate giving the film without difficulty. 

 Careful observation showed that with weak currents the 

 oxygen produced at the anode was able to break through the 

 layer of benzoic acid, and thus prevent the film-formation ; 

 but with strong currents the solid acid obtained the mastery, 

 the oxygen then not being able to break through and so dis- 

 perse the acid. Hence, with strong currents a film, consisting 

 of solid benzoic acid in which oxygen bubbles were entangled, 

 was formed round the anode, thus stopping the current. 



6. Discussion of the possible Causes of the Film. 



The film in H 2 S0 4 with platinum electrodes is in many 

 respects very analogous to those obtained in the same liquid 

 with oxidizable electrodes, and also to that which causes the 

 passivity of iron, in which cases a solid oxide or sulphate 

 layer on the anode is acknowledged to be the cause of the 

 insulation. The non-formation of the film with platinum 

 electrodes in H 2 80 4 below a certain strength, the increase 

 of A with the temperature, the fact that the film disappears 

 gradually on breaking the circuit, and immediately on re- 

 versing the current, all point, it may be argued, to a solid 

 oxide or sulphate layer on the platinum anode ; the so- 

 lubility of this layer varying with the concentration and 

 temperature of the acid, and its disappearance being im- 

 mediately caused by a deposit of nascent H on it. For 

 although platinum is usually regarded as non-oxidizable, yet 

 under the condition of electrolysis, which is attended by the 

 production of ozone in quantity at the anode, it seems quite 

 possible for an oxide of platinum to be formed. This view 

 was strongly upheld by De La Rive. In his ' Electricity,' 

 vol. ii. p. 410, he says that " the part played by platinum is 

 very remarkable, for although it passes for being not oxidizable, 

 yet it comports itself like the metals that are so, but in a 

 feeble degree." The blackening of platinum electrodes in 

 dilute H 2 S0 4 caused by the passage of alternate or direct 

 intermittent currents (the former being more effective) is 

 urged as a proof that platinum is attackable by the electro- 

 lytic gases. De La Rive insists upon the view that the 

 blackening is due to a series of oxidations and reductions of 

 the platinum, the oxidation with the direct currents being 

 due to the dissolved in the acid. Another view, mentioned 

 by Wiedemann, is that the alteration of the surface is caused 

 by the mechanical action of the hydrogen, which is first 

 occluded and then evolved again from the electrode when the 

 current is diminished or reversed. De La Rive gives other 

 evidence in support of his theory of the successive oxidation 



