Voltaic Currents Determined by Heat  279
believe, which are able to produce thermo currents with some
of the metals, and not with others. Further, these metals,
copper, silver, etc., do not always show effects which can be
mistaken or pass for thermo-electric, for silver in hot dilute
nitric acid is scarcely different from silver in the same acid cold
(938); and in other cases, again, the hot metals become negative
instead of positive (941).
Cases of one Metal and one Electrolyte ; one Junction
being heated
 930.	The cases I have to adduce are far too numerous to
be given in detail;  I will therefore describe one or two, and
sum up the rest as briefly as possible.
 931.	Iron in diluted sulphuret of potassium.—The hot iron
is well positive to the cold metal.   The negative and cold wire
continues quite clean, but from the hot iron a dark sulphuret
separates, which becoming diffused through the solution dis-
colours it.    When the cold iron is taken out, washed and wiped,
it leaves the cloth clean; but that which has been heated leaves
a black sulphuret upon the cloth when similarly treated.
 932.	Copper and the sulphuretted solution.—The hot copper
is well positive to the cold on the first immersion, but the effect
quickly falls, from the general causes already referred to (906).
 933.	Tin and solution of potass a.—The hot tin is strongly and
constantly positive to the cold.
 934.	Iron and dilute sulphuric acid (923).—The hot iron was
constantly positive to the cold, 60° or more.    Iron and diluted
nitric acid gave even a still more striking result.
I must now enumerate merely, not that the cases to be
mentioned are less decided than those already given, but to
economise time.
 935.	Dilute solution of yellow sulphuret of potassium, consist-
ing of one volume of the strong solution (800), and eighteen
volumes of water.—Iron, silver, and copper, with this solution,
gave good results.   The hot metal was positive to the cold.
 936.	Dilute solution of caustic potassa (920).—Iron, copper,
tin, zinc, and cadmium gave striking results in this electrolyte.
The hot metal was always positive to the cold.   Lead produced
the same effect, but there was a momentary jerk at the galvano-
meter at the instant of immersion, as if the hot lead was negative
at that moment.   In the case of iron it was necessary to continue
the application of heat, and then the formation of oxide at it