302 On the Theories of the 
chiefly, if not entirely, exerted in oxidizing, 
and dissolving the zinc plates; and why the 
evolution of hydrogen gas, or of nitrous gas, 
occurs chiefly at the copper surfaces. | 
An hypothesis, originally suggested by 
Fabroni, and reversing those which have been 
already stated, has been adopted sy several 
eminent philosophers in our own country. It 
assumes the oxidation of the metals composing 
galvanic arrangements to be the cause, and 
not the effect, of the evolution of electricity. 
In the solution of a metal (it bas been observ- 
ed by Dr. Wollaston) * it would appear that 
electricity is evolved by the action of the acid 
upon the metal ; and, in cases where hydro- 
gen is disengaged, that this evolution is re- 
quired to convert the hydrogen into gas. 
When a piece of zinc and another of silver are 
immersed in very dilute sulphuric acid, the 
zinc is dissolved and yields hydrogen gas; 
the silver, having no power of decomposing 
water, is not acted upon. But as soon as the 
‘two metaJs, placed under the diluted acid, are 
made to touch, hydrogen gas arises also from 
the surface of the silver. In this case, it is 
added, we have no reason to suppose that the 
contact of the silver imparts any new power; 
but merely that it serves as a conductor of 
® Phil, Trans. 
