Excitement and Chemical Action 303

oxide which is formed upon it by the heat employed in the
process of breaking it up. Then two platinum wires, connected
by a galvanometer, may be put into the acid, and one of them
pressed against the piece of tin, yet without producing an
electric current. If, whilst matters are in this position, the tin
be scraped under the acid by a glass rod, or other non-conduct-
ing substance capable of breaking the surface, the acid acts on
the metal newly exposed, and produces a current; but the
action ceases in a moment or two from the formation of oxide of
tin and an exhausted investing solution (906), and the current
ceases with it. Each scratch upon the surface of the tin re-
produces the series of phenomena.

1021. The case of iron in strong nitric acid, which acts and
produces a current at the first moment (831, 939, 989), but is
by that action deprived of so much of its activity, both chemical
and electrical, is also a case in point.

1022. If lead and tin be associated in muriatic acid, the
lead is positive at the first moment to the tin. The tin then
becomes positive, and continues so. This change I attribute
to the circumstance that the chloride of lead formed partly
invests that metal, and prevents the continuance of the action
there; but the chloride of tin, being far more soluble than that
of lead, passes more readily into the solution; so that action
goes on there, and the metal exhibits a permanent positive
state.

1023. The effect of the investing fluid already referred to in
the cases of tin (907) and cadmium (906), some of the results
with two metals in hot and cold acid (954), and those cases
where metal in a heated acid became negative to the same
metal in cold acid (941, etc.), are of the same kind. The latter
can be beautifully illustrated by two'pieces of lead in dilute
nitric acid: if left a short time, the needle stands nearly at o°,
but on heating either side, the metal there becomes negative
20° or more, and continues so as long as the heat is continued.
On cooling that side and heating the other, that piece of lead
which before was positive now becomes negative in turn, and
so on for any number of times.

1024. When the chemical action changes the current changes
also.—This is shown by the cases of two pieces of the same
active metal in the same fluid. Thus if two pieces of silver be
associated in strong muriatic acid, first the one will be positive
and then the other; and the changes in the direction of the
current will not be slow as if by a gradual action, but exceedinglyiron or platinum is ineffectual in producing ae Chimie, 1828, xxxvii. p. 240.ith nitric acid and iron or silver