250 Faraday's Researches

the clean iron (831). The oxide of iron was at first in the
least degree positive, and then immediately neutral. This
circuit, then, like the former, gave no current at common
temperatures; but it differed much from it in conducting power,
being a very excellent conductor of a thermo current, the oxide
of iron not offering that obstruction to the passage of the
current which the peculiar iron did (831, 832). Hence scale
'oxide of iron and platinum produce no current by contact, the
third substance in the proof circuit being nitrous acid; and so
the result agrees with that obtained in the former case, where
that third substance was solution of sulphuret of potassium.

836. In using nitrous acid it is necessary that certain pre-
cautions be taken, founded on the following effect. If a circuit
be made with the green nitrous acid, platinum wires, and a
galvanometer, in a few seconds all traces of a current due to
first disturbances will disappear; but if one wire be raised into
the air and instantly returned to its first position, a current is
formed, and that wire is negative, across the electrolyte, to the
other. If one wire be dipped only a small distance into the
acid, as for instance one-fourth of an inch, then the raising that
wire not more than one-eighth of an inch and instantly restoring
it, will produce the same effect as before. The effect is due
to the evaporation of the nitrous acid from the exposed wire
(925). I may perhaps return to it hereafter, but wish at present
only to give notice of the precaution that is required in con-
sequence, namely, to retain the immersed wires undisturbed
during the experiment.

837. Proceeding on the facts made known by Schcenbein
respecting the relation of iron and nitric acid, I used that acid
as the fluid in a voltaic current formed with iron and platinum.
Pure nitric acid is so deficient in conducting power (805) that
it may be supposed capable of stopping any current due to
the effect of contact between the platinum and iron; and it
is further objectionable in these experiments, because, acting
feebly on the iron, it produces a chemically excited current,
which may be considered as mingling its effect with that of
contact: whereas the object at present is, by excluding such
chemical action, to lay bare the influence of contact alone.
Still the results with it are consistent with the more perfect
ones already described; for in a circuit of iron, platinum, and
nitric acid, the joint effects of the chemical action on the iron
and the contact of iron and platinum, being to produce a current his investigations, and then used it instead ofher platinum, plate.and also perfectly compact "nickel; and if porous,