Excitement at the Cathode

3°7

sulphuret of potassium. White lead applied in the same
manner was also found to acquire the same state. Either of
these bodies when compared with platinum in dilute nitric acid
was,, on the contrary, very positive.

1035. The same effect is well shown by the action of oxidised
iron. If a plate of iron be oxidised by heat so as to give an oxide
of such aggregation and condition as to be acted on scarcely or
not at all by the solution of sulphuret, then there is little or no
current, such an oxide being as platinum in the solution (828).
But if it be oxidised by exposure to air, or by being wetted
and dried; or by being moistened by a little dilute nitric or
sulphuric acid and then washed, first in solution of ammonia
or potassa, and afterwards in distilled water and dried; or if it
be moistened in solution of potassa, heated in the air, and then
washed well in distilled water and dried; such iron associated
with platinum and put into a solution of the sulphuret will
produce a powerful current until all the oxide is reduced, the
iron during the whole time being negative.

1036. A piece of rusty iron in the same solution is power-
fully negative. So also is a platinum plate with a coat of pro-
toxide, or peroxide, or native carbonate of iron on it (1033).

1037. This result is one of those effects which has to be
guarded against in the experiments formerly described (814,
874). If what appears to be a clean plate of iron is put into
a dilute solution of the sulphuret of potassium, it is first negative
to platinum, then neutral, and at last generally feebly positive;
if it be put into a strong solution, it is first negative, and then
becomes neutral, continuing so. It cannot be cleansed so
perfectly with sand-paper but that when immersed it will
be negative, but the more recently and well the plate has been
cleansed, the shorter time does this state continue. This effect
is due to the instantaneous oxidation of the surface of the iron
during its momentary exposure to the atmosphere, and the
after reduction of this oxide by the solution. Nor can this
be considered an unnatural result to those who consider the
characters of iron. Pure iron in the form of a sponge takes
fire spontaneously in the air; and a plate recently cleansed, if
dipped into water, or breathed upon, or only exposed to the
atmosphere, produces an instant smell of hydrogen. The thin
film of oxide which can form during a momentary exposure
is, therefore, quite enough to account for the electric current
produced.

1038. As a further proof of the truth of these explanations,, 154 162.