Contact Contradictions

255

and is carried off by the solution, and so leaves the case in a
very simple state; the fact, however, is as strong with any of
the"'other metals. Now if the contact theory be true, and if
the iron, platina, and solution of sulphuret of potassium give
contacts which are in perfect equilibrium as to their electro-
motive force, then why does changing the iron for zinc destroy
the equilibrium ? Changing one metal for another in a metallic
circuit causes no alteration of this kind: nor does changing one
substance for another among the great number of bodies which,
as solid conductors, may be used to form conducting (but
chemically inactive) circuits (855, etc.). If the solution of
sulphuret of potassium is to be classed with the metals as to its
action in the experiments I have quoted (813, etc.), then, how
comes it to act quite unlike them, and with a po*wr equal to
the best of the other class, in the new case; of zinc, copper,
silver, etc. (870, 873, etc.)?

853. This difficulty, as I conceive, must be met, on the part
of the contact theorists, by a new assumption, namely, that this
fluid sometimes acts as the best of the metals, or first class of
conductors, and sometimes as the best of the electrolytes or
second class. But surely this would be far too loose a method
of philosophising in an experimental science (857); and further,
it is most unfortunate for such an assumption, that this second
condition or relation of it never comes on by itself, so as to give
us a pure case of a current from contact alone; it never comes
on without that chemical action to which the chemist so simply
refers all the current which is then produced.

854. It is unnecessary for me to say that the same argument
applies with equal force to the cases where nitrous acid, nitric
acid, and solution of potash are used; and it is supported with
equal strength by the results which they have given (831,
837, 841).

855. It may be thought that it was quite unnecessary, but
in my desire to establish contact electromotive force, to do
which I was at one time very anxious, I made many circuits of
three substances, including a galvanometer, all being conductors,
with the hope of finding an arrangement, which, without chemi-
cal action, should produce a current. The number and variety
of these experiments may be understood from the following
summary; in which metals, plumbago, sulphurets and oxides,
all being conductors even of a thermo current, were thus com-
bined in various ways:. I might for