, 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 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
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 re-act 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
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. 73, 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
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 _c_, is removed,