Use of Metallic Contact in the Voltaic Apparatus, 39 



action of that body, also in the act of decomposition, which 

 produces the voltaic current. 



893. ThQ use oi metallic contact in a single pair of plates, 

 and the cause of its great superiority above contact made by 

 other kinds of matter, become now very evident. When an 

 amalgamated zinc plate is dipped into dilute sulphuric acid, 

 the force of chemical affinity exerted between the metal and 

 the fluid is not sufficiently powerful to cause sensible action at 

 the surfaces of contact, and occasion the decomposition of 

 water by the oxidation of the metal, although it is sufficient 

 to produce such a condition of the electricity (or the power 

 upon which chemical affinity depends) as would produce a 

 current if there were a path open for it (916. 956.); and that 

 current would complete the conditions necessary, under the 

 circumstances, for the decomposition of the water, 



894. Now the presence of a piece of platina touching both 

 the zinc and the fluid to be decomposed, opens the path re- 

 quired for the electricity. Its direct communication with the 

 zinc is effectual, far beyond any communication made between 

 it and that metal, {i. e. between the platina and zinc,) bv 

 means of decomposable conducting bodies, or, in other words, 

 electrolytes, as in the experiment already described (891.); 

 because, when they are used, the chemical affinities between 

 them and the zinc produce a contrary and opposing action to 

 that which is influential in the dilute sulphuric acid ; or if that 

 action be but small, still the affinity of their component parts 

 for each other has to be overcome, for they cannot conduct 

 without suffering decomposition : and this decomposition is 

 found experimentally to react back upon the forces which in 

 the acid tend to produce the current (904. 910. &c.), and in 

 numerous cases entirely to neutralize them. Where direct 

 contact of the zinc and platina occurs, these obstructing forces 

 are not brought into action, and therefore the production and 

 the circulation of the electric current and the concomitant ac- 

 tion of decomposition are then highly favoured. 



895. It is evident, however, that one of these opposing ac- 

 tions may be dismissed, and yet an electrolyte be used for the 

 purpose of completing the circuit between the zinc and platina 

 immersed separately into the dilute acid; for if, in fig. 1, the 

 platina wire be retained in metallic contact with the zinc plate 

 «, at X, and a division of the platina be made elsewhere, as 

 at 5, then the solution of iodide placed there, being in contact 

 with platina at both surfaces, exerts no chemical affinities for 

 that metal ; or if it does, they are equal on both sides. Its 

 power, therefore, of forming a current in opposition to that 

 dependent upon the action of the acid in the vessel r, is re- 

 moved, and only its resistance to decomposition remains as 



