Excitement and Chemical Action 305

action and no electric current, though the water was there, so
that contact with the water did nothing: the water and acid
were moved and mixed together by means of the end of the
wire i; in a few moments proper chemical action came on, the
iron evolving nitrous gas at the place of its action, and at
the same time acquiring a positive condition at that part,
and producing a powerful electric current.

1028. When the chemical action which either has or could
have produced a current in one direction is reversed or undone.,
the current is reversed (or undone) also.

1029. This is a principle or result which most strikingly
confirms the chemical theory of voltaic excitement, and is
illustrated by many important facts. Volta in the year 1802 1
showed that crystallised oxide of manganese was highly negative
to zinc and similar metals, giving, according to his theory,
electricity to the zinc at the point of contact. Becquerel
worked carefully at this subject in 1835^ and came to the con-
elusion, but reservedly expressed, that the facts were favourable
to the theory of contact. In the following year De la Rive
examined the subject,3 and shows, to my satisfaction at least,
that the peroxide is at the time undergoing chemical change
and losing oxygen, a change perfectly in accordance with the
direction of the current it produces.

1030. The peroxide associated with platinum in the green*
nitrous acid originates a current, and is negative to the platinum,.
at the same time giving up oxygen and converting the nitrous-
acid into nitric acid, a change easily shown by a common
chemical experiment. In nitric acid the oxide is negativel;.to-
platinum, but its negative state is much increased if a little-
alcohol be added to the acid, that body assisting in the reduc-
tion of the oxide. When associated with platinum in solution of
potash, the addition of a little alcohol singularly favours the-
increase of the current for the same reason. When the per-
oxide and platinum are associated with solution of sulphuret
of potassium, the peroxide, as might have been expected, is
strongly negative.

1031. In 1835 M. Muncke4 observed the striking power of
peroxide of lead to produce phenomena like those of the per-
oxide of manganese, and these M. de la Rive in 1836 immedi-
ately referred to corresponding chemical changes .5 M. Schoenbein

1 Annales de Chimie, 1802, xl. 224. 2 Ibid. 1835, Ix. 164, 171.

3 Ibid. 1836, Ixi. 40; and Bibliotheque Umverselle, 1836, i. 152, 158.
* Bibliothlque Umverselle, 1836, i. 160. 6 Ibid. 1836, i., 154 162.

U 1828, xxxvii. p. 240.ith nitric acid and iron or silver