Contact Theory of Galvanism. 213 



I soldered to each a brass knob {bout de laiton) ; I covered 

 their inner surface with a thin layer of lac varnish, so that 

 they might perform the office of the plates of a condenser ; 

 I further entirely covered the exterior surface of one of the 

 zinc plates with a layer of the same varnish, so that this plate 

 was not in immediate contact with the air at any of its points. 

 Various experiments were made, forming the condenser some- 

 times with the two zinc plates, sometimes with one of them 

 only and with a brass plate. When the zinc plate which I 

 employed was that the entire surface of which was covered 

 with varnish, I constantly obtained electrical signs much less 

 strong than in employing that one of which the exterior sur- 

 face was entirely uncovered and exposed to the immediate 

 contact of the air. Presuming that the electricity, very feeble 

 it is true, which was developed with the varnished zinc plate, 

 arose from the circumstance that the layer with which it was 

 covered was too thin to intercept completely all chemical action 

 of the air and of the humidity, I successively increased the 

 thickness of this layer, and I succeeded in rendering it such 

 that the plate ceased to give any electrical signs. What 

 moreover proves that it is to the action, even through the 

 varnish, which the moist air could exercise upon the surface 

 of the zinc, that the production of the electricity was owing, 

 was that I observed after a short time a commencement of 

 oxidation take place on this surface. We see, therefore, that 

 when a zinc plate, by means of a layer of varnish, is entirely 

 protected from the action of the air or of those agents which 

 might exert a chemical action upon it, it does not become 

 electric in its contact with a brass knob. Still more ; this in- 

 active apparatus conducts itself as a homogeneous plate of 

 brass. Thus on touching the brass knob which was soldered 

 to the surface of the zinc, with the copper of a heterogeneous 

 plate, of which I held the zinc in my hand, I succeeded in 

 charging it with negative electricity. Or, according to the 

 contact theory, the two coppers being in immediate contact, 

 and placed between the two zincs soldered to them, viz. that 

 of the condenser and that which I held in my hand, no re- 

 sult ought to have taken place. We might obtain a result of 

 the same kind, perhaps, still more striking, by uniting for the 

 purpose of forming the conducter the two plates of zinc and 

 making the two brass knobs soldered to them communicate 

 with one another. In the theory of contact, the opposition of 

 these two pairs, perfectly similar, ought to have neutralized 

 all kind of action ; however, the experiment showed that the 

 zinc plate, the naked surface of which was exposed to the air, 

 became charged with positive electricity, while that of which the 



