Contact of Metals Perfectly Passive  245
a certain current. No such current is produced, and I conceive,
with the admission above, that such a result proves that the
contacts iron-zinc and platinum-zinc are entirely without
electromotive force.
 822.	Gold, silver, potassium, and copper were introduced
at x with the like negative effect; and so no doubt might every
other metal, even according to the relation admitted amongst
the metals by the supporters of the contact theory (797).   The
same negative result followed upon the introduction of many
other conducting bodies at the same place;   as, for instance,
those  already  mentioned  as   easily  conducting  the  thermo
current (808);  and the effect proves, I think, that the contact
of any of these with either iron or platinum is utterly ineffective
as a source of electromotive force.
 823.	The only answer which, as it appears to me, the contact
theory can set up in opposition to the foregoing facts and
conclusions is to say that the solution of sulphuret of potassium
in the cup D, fig. 65, acts as a metal would do (797), and so the
effects of all the contacts in the circuit are exactly balanced.
I will not stop at this moment to show that the departure with
respect to electrolytes, or the fluid bodies in the voltaic pile,
from the law which is supposed to hold good with the metals
and solid conductors, though only an assumption, is still essential
to the contact theory of the voltaic pile (798, 849);1 nor to
prove that the electrolyte is no otherwise like the metals than
in having no contact electromotive force whatever.   But be-
lieving that this will be very evident shortly, I will go on with
the experimental results, and resume these points hereafter
(847; 877).
824.	The experiment was now repeated with the substitution
of a bar of nickel for that of iron, fig. 65 (812), all other things
remaining the same.2   The circuit was again found to be a good
conductor of a feeble thermo current, but utterly inefficient as
a voltaic circuit when all was at the same temperature, and due
precautions taken (1039).   The introduction of metals at the
1	See Fechner's words.—Philosophical Magazine, 1838, xiii. 377.
2	There is another form of this experiment which I sometimes adopted,
in which the cup E, fig. 65, with its contents, was dismissed, and the
platinum plates in it connected together.    The arrangement may then be
considered as presenting three contacts of iron and platinum, two acting
in one direction, and one in the other.    The arrangement and the results
are virtually the same as those already given.    A still simpler but equally
conclusive arrangement for many of the arguments, is to dismiss the iron
between a and b altogether, and so have but one contact, that at x, to
consider.