553 



HISTORY OP" SCIENCE. 



zinc, and regards the current itself as the necessary result of the chemical 

 actions. 



To follow the discussions and relate the experiments which the rival 

 theories of the pile have elicited would occupy no little space. A 

 fundamental experiment in favour of Volta's theory is represented in 

 Fig. 268, where E is a condensing electroscope, and c z a compound 

 bar, one moiety of copper, the other zinc. Holding the zinc in his 

 hand, the operator touches the copper plate of the condenser with the 



FIG. 268. 



copper end of the bar. The electroscope thereupon indicates that the. 

 copper possesses a feeble charge of positive electricity. The objec- 

 tion made by the supporters of the chemical theory to this experiment 

 was that the result was due to the action of the natural moisture of 

 the hand on the zinc. Other experiments were devised in refutation 

 or support of each theory : the most distinguished scientific men of the 

 time have contributed something to the discussion, as Wollaston, 

 Priestley, Biot, Davy, Gay-Lussac, Berzelius, De la Rive, Faraday, Sir 

 William Thomson, etc. How much may be said for either theory 

 may perhaps be inferred from the circumstance that, in twenty years 

 (1820 1840), the number of scientific papers which related to methods 

 of reconciling or blending the two rival theories amounted to more 

 than two thousand. A great number of ingenious and telling experi- 

 ments were brought forward by Faraday, and his arguments appeared 

 so convincing that for a time the chemical theory was triumphant. 



All the voltaic arrangements we have described have one incon- 

 venience ; their action, however powerful at first, rapidly diminishes. 

 The cause of this defect has been traced to the layer of hydrogen gas 



