248 Faraday's Researches

a strong or weak solution of this electrolyte showed the same
phenomena.1 By diluting the solution still further, a fluid
could be obtained in which the iron was,, after the first effect,
permanently but fe bly positive. On allowing time, however,
it was found that in all such cases black sulphuret formed here
and there on the iron. Rusted iron was negative to platinum
(1037) in this very weak' solution, which by direct chemical
action could render metallic iron positive.

831, In all the preceding experiments the electrolyte used
has been the sulphuret of potassium solution; but I now
changed this for another, very different in its nature, namely,
the green nitrous acid (804), which has already been shown to
be an excellent conductor of electricity. Iron and platinum
were the metals employed, both being in the form of wires.
The vessel in which they were immersed was a tube like that
formerly described (803); in other respects the arrangement
was the same in principle as those already used (812, 824).
The first effect was the production of a current, the iron being
positive in the acid to the platina; but this quickly ceased, and
the galvanometer-needle came to o°. In this state, however,
the circuit could not in all things be compared with the one
having the solution of sulphuret of potassium for its electrolyte
(812); for although it could conduct the thermo current of
antimony and bismuth in a certain degree, yet that degree was
very small compared to the power possessed by the former
arrangement, or to that of a circle in which the nitrous acid was
between two platinum plates (804). This remarkable retarda-
tion is consequent upon the assumption by the iron of that
peculiar state which Schcenbein has so well described and illus-
trated by his numerous experiments and investigations. But
though it must be admitted that the iron in contact with the
acid is in a peculiar state (939, 989, 1021), yet it is also evident
that a circuit consisting of platinum, iron, peculiar iron, and
nitrous acid, does not cause a current though it have sufficient
conducting power to carry a thermo current.

832. But if the contact of platinum and iron has an electro-
motive force, why does it not produce a current ? The applica-
tion of heat (818), or of a little chemical action (819) at the
place of contact, does produce a current, and in the latter case

1 Care was taken in these and the former similar cases to discharge the
platinum surface of any reacting force it might acquire from the actio.i of
the previous current, by separating it from the other metals, and touching
it in the liquid for an instant with another platinum, plate.and also perfectly compact "nickel; and if porous,