252	Faraday's Researches
former: this was solution of potassa, which has already been
employed by De la Rive (811) with iron and platina, and which
when strong has been found to be a substance conducting so
well; that even a thermo current could pass it (807), and there-
fore fully sufficient to show a contact current., if any such
exists.
 842.	Yet when a strong solution of this substance was ar-
ranged with silver and platinum (bodies differing sufficiently
from each other when connected by nitric or muriatic acid); as
in the former cases, a very feeble current was produced, and
the galvanometer-needle stood nearly at zero.   The contact of
these metals therefore did not appear to produce a sensible
current; and, as I fully believe, because no electromotive power
exists in such contact.   When that contact was exchanged for
a very feeble chemical action, namely, that produced by inter-
posing a little piece of paper moistened in dilute nitric acid
(819), a current was the result.    So here, as in the many former
cases, the arrangement with a little chemical action and no
metallic contact produces a current, but that without the
chemical action and with the metallic contact produces none.
 843.	Iron or nickel associated with platinum in this strong
solution of potassa was positive.    The force of the produced
current soon fell, and after an hour or so was very small.    Then
annulling the metallic contact at x, fig. 65, and substituting
a feeble chemical action there, as of dilute nitric acid, the
current established by the latter would pass and show itself.
Thus the cases are parallel to those before mentioned (837, etc.),
and show how little contact alone could do, since the effect of
the conjoint contact of iron and platinum and chemical action
of potash and iron were very small as compared with the con-
trasted chemical action of the dilute nitric acid.
 844.	Instead of a strong solution of potassa, a much weaker
one consisting of one volume of strong solution and six volumes
of water was used, but the results with the silver and platinum
were the same:  no current was produced by the metallic con-
tact as long as that only was left for exciting cause, but on
substituting a little chemical action in its place (819), the
current was immediately produced.
 845.	Iron and nickel with platinum in the weak solution also
produced similar results, except that the positive state of these
metals was rather more permanent than with the strong solu-
tion.    Still it was so small as to be out of all proportion to
what was to be expected according to the contact theory.