ibed further on
(956.), consists in obtaining the spark upon making contact between a plate
of zinc and a plate of copper plunged into dilute sulphuric acid. In order
to make the arrangement as elementary as possible, mercurial surfaces were
dismissed, and the contact made by a copper wire connected with the copper
plate, and then brought to touch a clean part of the zinc plate. The
electric spark appeared, and it must of necessity have existed and passed
_before the zinc and the copper were in contact_.

916. In order to render more distinct the principles which I have been
endeavouring to establish, I will restate them in their simplest form,
according to my present belief. The electricity of the voltaic pile (856.
note) is not dependent either in its origin or its continuance upon the
contact of the metals with each other (880. 915.). It is entirely due to
chemical action (882.), and is proportionate in its intensity to the
intensity of the affinities concerned in its production (908.); and in its
quantity to the quantity of matter which has been chemically active during
its evolution (869.). This definite production is again one of the
strongest proofs that the electricity is of chemical origin.

917. As _volta-electro-generation_ is a case of mere chemical action, so
_volta-electro-decomposition_ is simply a case of the preponderance of one
set of chemical affinities more powerful in their nature, over another set
which are less powerful: and if the instance of two opposing sets of such
forces (891.) be considered, and their mutual relation and dependence borne
in mind, there appears no necessity for using, in respect to such cases,
any other term than chemical affinity, (though that of electricity may be
very convenient,) or supposing any new agent to be concerned in producing
the results; for we may consider that the powers at the two places of
action are in direct communion and balanced against each other through the
medium of the metals (891.), fig. 76, in a manner analogous to that in
which mechanical forces are balanced against each other by the intervention
of the lever (1031.).

918. All the facts show us that that power commonly called chemical
affinity, can be communicated to a distance through the metals and certain
forms of carbon; that the electric current is only another form of the
forces of chemical affinity; that its power is in proportion to the
chemical affinities producing it; that when it is d