1886.] 



A Theory of Voltaic Action. 



295 



or by the bi -metallic ring or quadrants method of Sir William 

 Thomson, is due to the chemical action of a film of condensed 

 vapour or gas on the surfaces of the metals.* 

 That the two metals with their liquid or quasi-liquid films are 

 quite similar to a galvanic cell composed of the same metals as 

 elements, and a liquid similar to that of the films as electrolyte ; 

 the said electrolyte being (in the ordinary static " contact " ex- 

 periment) divided by the intervening insulating diaphragm of 

 air or other gas. 



2. Therefore in these experiments it is the difference of potential 

 at the outer surfaces of the two films that is measured. 



In the case of a single metal, e.g., zinc covered by its chemically 

 active electrolytic film, there is, according to the view here advocated, 

 at the surface of contact between the film and the metal, an electro- 

 motive force due to the chemical action between them, charging the 

 zinc negatively and the film positively. The two charges being bound 

 have no outer manifestation of any kind. 



Copper again, for example, is affected similarly, but less strongly, 

 than zinc. If it be connected metallically to the zinc, the potentials 

 of the metals will be equalised, a portion of the negative charge 

 going through the connexion from the zinc to the copper and thence 

 to the film on the copper. 



3. This theory as applied to the Volta condenser experiment is 

 referred to somewhat in detail by Wiedemann (' Die Lehre von der 

 Elektricitat,' vol. 2, pp. 986—987) ; and the view held by De la Hive 

 (' Traite de l'filectricite,' vol. 2, 1856, p. 776) was very similar, differ- 

 ing mainly in the supposition that the positive charge separated off in 

 the film was maintained there by the insulating nature of the film, 

 whereas it is evident that an electromotive force at the surface of 

 contact of the film with the metal sufficient to cause the separation 

 of the two charges will also be sufficient to keep them separate. We 

 are, therefore, in so far not debarred from considering the film an 

 electrolytic conductor. De la Rive's view seems, however, to have 

 received little general attention, perhaps less than it deserved. 



4. It seems advisable to discuss these views experimentally while 

 trenching as little as possible on the uncertain domain of speculative 

 molecular physics, and in describing the experiments I shall refer in 

 detail to those only which gave reliable results. Many preliminary 

 trials were made before disturbing causes were eliminated, and many 

 negative results were obtained. It has, however, been gratifying to 

 find that no reason has appeared requiring the alteration of anything 

 in the two short papers previously published on this subject (' Phil. 

 Mag.,' Aug., 1878, Feb., 1879). 



* [No account is here taken of any infinitesimal effects that may be due to 

 thermo-electric action, &c. — December 3, 1886.] 



VOL. XLI. X 



