so high as to cause oxidation of the zinc far more rapidly than
if the pair of plates were plunged into dilute sulphuric acid; for the
oxygen is not part of an electrolyte, and cannot therefore conduct the
forces onwards by decomposition, or even as metals do by itself. Or if its
gaseous state embarrass the minds of some, then liquid chlorine may be
taken. It does not excite a current of electricity through the two plates
by combining with the zinc, for its particles cannot transfer the
electricity active at the point of combination across to the platina. It is
not a conductor of itself, like the metals; nor is it an electrolyte, so as
to be capable of conduction during decomposition, and hence there is simple
chemical action at the spot, and no electric current[A].

[A] I do not mean to affirm that no traces of electricity ever appear
in such cases. What I mean is, that no electricity is evolved in any
way, due or related to the causes which excite voltaic electricity, or
proportionate to them. That which does appear occasionally is the
smallest possible fraction of that which the acting matter could
produce if arranged so as to act voltaically, probably not the one
hundred thousandth, or even the millionth part, and is very probably
altogether different in its source.

922. It might at first be supposed that a conducting body not electrolytic,
might answer as the third substance between the zinc and the platina; and
it is true that we have some such capable of exerting chemical action upon
the metals. They must, however, be chosen from the metals themselves, for
there are no bodies of this kind except those substances and charcoal. To
decide the matter by experiment, I made the following arrangement. Melted
tin was put into a glass tube bent into the form of the letter V, fig. 78,
so as to fill the half of each limb, and two pieces of thick platina wire,
_p_, _w_, inserted, so as to have their ends immersed some depth in the
tin: the whole was then allowed to cool, and the ends _p_ and _w_ connected
with a delicate galvanometer. The part of the tube at _x_ was now reheated,
whilst the portion _y_ was retained cool. The galvanometer was immediately
influenced by the thermo-electric current produced. The heat was steadily
increased at _x_, until at last the tin and platina combined there; an
effect which is known to take place with strong chemical action and high
ignition; but not the slightest additiona