the solutions used are conductors, but the conduction in
them is essentially connected with decomposition (858.) in a certain
constant order, and therefore the appearance of the elements in certain
places _shows_ in what direction a current has passed when the solutions
are thus employed. Moreover, we find that when they are used at opposite
ends of the plates, as in the last two experiments (889. 890.), metallic
contact being allowed at the other extremities, the currents are in
opposite directions. We have evidently, therefore, the power of opposing
the actions of the two fluids simultaneously to each other at the opposite
ends of the plates, using each one as a conductor for the discharge of the
current of electricity, which the other tends to generate; in fact,
substituting them for metallic contact, and combining both experiments into
one (fig. 76.). Under these circumstances, there is an opposition of
forces: the fluid, which brings into play the stronger set of chemical
affinities for the zinc, (being the dilute acid,) overcomes the force of
the other, and determines the formation and direction of the electric
current; not merely making that current pass through the weaker liquid, but
actually reversing the tendency which the elements of the latter have in
relation to the zinc and platina if not thus counteracted, and forcing them
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 _x_ are strengthened, then the
latter gains the ascendency, and the decompositions are produced in a
contrary order.

892. Before drawing a _final_ conclusion from this mutual dependence and
state of the chemical affinities of two distant portions of acting fluids
(916.), I will proceed to examine more minutely the various circumstances
under which the re-action of the body suffering decomposition is rendered
evident upon the action of the body, also undergoing decomposition, which
produces the voltaic current.

893. 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 amalgamated zinc plate is dipped into
dilute sulphuric acid