Use of Metallic Contact	177
In the contrary direction to that they are inclined to follow,
that its own current may have free course. If the dominant
action at y be removed by making metallic contact there, then
the liquid at x resumes its power; or if the metals be not
brought into contact at y, but the affinities of the solution there
weakened, whilst those active at x are strengthened, then the
latter gains the ascendency, and the decompositions are produced
in a contrary order.
 627.	Before drawing a final  conclusion from this mutual
dependence and state of the chemical affinities of two distant
portions of acting fluids (651), I will proceed to examine more
minutely the various circumstances under which the reaction of
the body suffering decomposition is rendered evident upon the
action of the body, also undergoing decomposition, which pro-
duces the voltaic current.
 628.	The use of 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 amalga-
mated 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 (651, 691);  and that current would complete
the  conditions  necessary,  under the  circumstances,  for the
decomposition of the water.
629.	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, elec-
trolytes, as in the experiment already described (626); 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 experi-
mentally to react back upon the forces which in the acid tend
to produce the current (639, 645, etc.), and in numerous cases
m