A Peculiar Condition of Iron        327
upon the peculiarities of this oxide and of nitric acid of the
strength required for these experiments; but as a matter of
fact it is well known that the oxide produced upon the surface of
iron by heat, and showing itself by thin films of various colours,
is scarcely touched by nitric acid of the given strength though
left in contact with it for days together. That this does not
depend upon the film having any great thickness, but upon its
peculiar condition, is rendered probable from the fact that
iron oxidised by heat, only in that slight degree as to offer no
difference to the eye, has been left in nitric acid of the given
strength for weeks together without any change. And that
this mode of superficial oxidation, or this kind of oxide, may
occur in the voltaic cases, is rendered probable by the results
of the oxidation of iron in nitrate of silver. When nitrate of
silver is fused and common iron dipped into it, so as to be
thoroughly wetted, being either alone or in association with
platina, the iron does not commence a violent action on the
nitrate and throw down silver, but it is gradually oxidised on
the surface with exactly the same appearances of colour, uni-
formity of surface, etc., as if it were slowly oxidised by heat
in the air.
Professor Schoenbein has stated the case of iron when acting
as the positive electrode of a couronne des tosses. If that
instrument be in strong action, or if an ordinary battery be
used containing from two to ten or more plates, the positive
iron instantly becomes covered in the nitric acid with a coat
of oxide, which though it does not adhere closely still is not.
readily dissolved by the acid when the connection with the
battery is broken, but remains for many hours on the iron,
which itself is in the peculiar inactive state. If the power of
the voltaic apparatus be very weak, the coat of oxide on the
iron in the nitric acid often assumes a blue tint like that of the
oxide formed by heat. A part of the iron is however always
dissolved in these cases.
If it be allowed that the surface particles of the iron are
associated with oxygen, are in fact oxidised, then all the other
actions of it in combination with common iron and other metals
will be consistent; and the cause of its platina-like action, of
its forming a strong voltaic current with common iron in the
first instance, and then being thrown into action by it, will be
explained by considering it as having the power of determining
and disposing of a certain portion of hydrogen from the elec-
trolyte at the first moment and being at the same time brought