THEORY OF GALVANISM. 119 



contact, or this communication is preserved, they 

 must be in the same state of electricity. An elec- 

 trified body may communicate electricity either by 

 contact or by approximation ; in the first case the 

 electricity is the same with that of the communi- 

 cating body ; in the second, the communicated 

 electricity is of the opposite kind, but it is de- 

 stroyed as soon as the bodies actually come into 

 contact. The experiment of Volta, which comes 

 the most near to the circumstances of the galvanic 

 apparatus, is that mentioned above, where the 

 projecting parts of two metallic disks were found 

 to be in opposite electric states. But this is an 

 experiment of a most delicate nature, and Mr. 

 Cuthbertson, who attempted to repeat it, obtained 

 results contrary to those stated by Volta.* 



But, waving this objection, and admitting that chemical 



, , ., . , . . effects not 



the two metals, while they are in extensive con- accidental, 

 tact, may be in opposite electrical states, it may be 

 objected to Volta's hypothesis, in the second place, 

 that the chemical effects of the pile are not, as he 

 supposes, merely accidental. They seem, on the 

 contrary, to be absolutely essential to its action, 

 for when perfectly pure water is interposed be- 

 tween the metals, or when the apparatus is placed 

 in any situation where it is excluded from obtain- 

 ing a supply 'of oxygen, it ceases to act. The 

 same thing happens when the acid, or other oxi- 

 dating fluid, is all expended ; and in short it may 



* Nicholson's Journ. 8vo. H. 281. 



