Contact Theory of Galvanism. 209 



powerful than we are accustomed to obtain with zinc; this 

 is therefore well adapted to serve as an experiment of demon- 

 stration in lectures. The same was the case if I held the 

 bar of wood after slightly moistening it; and in the latter case, 

 it was quite indifferent for the production of the effect, whether 

 the ball was immersed in petroleum or not. I must therefore 

 suppose, that De la Rive, since he obtained a result out of 

 the petroleum, but none when the ball was in it, produced the 

 conducting connexion with the earth through wood or ivory 

 only within the petroleum, but when out of the petroleum by 

 a more humid conductor ; otherwise there would be a direct 

 contradiction between his and my observations. One may 

 easily believe, since this contradiction did not escape me, that 

 I frequently repeated and varied the form of this experiment, 

 in order to convince myself that on repeating it in various 

 ways, right would remain on my side. 



It remained now to observe, whether the absence of di- 

 vergence by the use of air-dried wood was owing to the want 

 of conducting power of the wood, or to the want of chemical 

 action : for it could not indeed be denied, that, if the po- 

 tassium ball was held with the hands or by means of a moist 

 bar of wood, their moisture must act chemically upon it, 

 which also, when the experiment is performed under petro- 

 leum, may easily be recognised by the bubbles of gas rising 

 from the point of insertion of the slightly moistened wooden 

 bar. The following experiment, more direct than all which 

 De la Rive cites in support of his views, proves quite decidedly 

 that the bad conducting power of the air-dried wood alone at 

 least suffices to explain the negative result. If I moistened 

 one half of the bar which stood in connexion with the po- 

 tassium, and also the point of insertion in the potassium, just 

 as in the former experiments in which I obtained a result, 

 but held the wood during the contact of the condenser with 

 the platinum by the half which remained air-dried, I obtained 

 the former negative result ; nay, this was even then the case, 

 if while thus manipulating I moistened the potassium during 

 the contact of the condenser with acidulated water, so that a 

 kind of explosive, chemical action took place. By this then 

 the insufficient conducting power of air-dried wood for such 

 experiments is sufficiently proved, and every one may easily, 

 and even without potassium, convince himself by substituting 

 zinc for that metal, when the phsenomena take place in quite 



ments concur in the view of Ohm and PfafF, that an isolated intermediate 

 layer of petroleum acted a part in De la Rive's experiments. 



Fhil, Mag. S. 3. Vol. 13. No. 81. Sept. 1838. P 



