USE OP METALLIC CONTACT IN THE VOLTAIC APPARATUS. 429 



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 con- 

 dition 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 decompo- 

 sition 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 required 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,) by 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 pro- 

 duce 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 ob- 

 structing forces are not brought into action, and therefore the production and the 

 circulation of the electric current and the concomitant action of decomposition are 

 then highly favoured. 



895. It is evident, however, that one of these opposing actions 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 a, at x, and a division of the 

 platina be made elsewhere, as at s, then the solution of iodide placed there, being in 

 contact with platina at both surfaces, exerts no chemical affinities for tliat 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 c, is removed, 

 and only its resistance to decomposition remains as the obstacle to be overcome by 

 the affinities exerted in the dilute sulphuric acid. 



896. This becomes the condition of a single pair of plates where metallic contact is 

 allowed. In such cases, only one set of opposing affinities are to be overcome by those 

 which are dominant in the vessel c ; whereas, when metallic contact is not allowed, 

 two sets of opposing affinities must be conquered (894.). 



897. It has been considered a difficult, and by some an impossible, thing to decom- 

 pose bodies by the current from a single pair of plates, even when it was so powerful 

 as to heat bars of metal red hot, as in the case of Hare's calorimeter, arranged as a 



MDCCCXXXIV. 3 K 



