Relation of Metals Inverted by Heat 283

mutual relation of the metals in some cases, and even invert
their order: on making circuits with two metals and electro-
lytes, I found the following cases.

949. In the solution of sulphuret of potassium, hot tin is well
positive to cold silver: cold tin is very slightly positive to hot
silver, and the silver then rapidly tarnishes.

950. In the solution of potassa, cold tin is fairly positive to
hot lead, but hot tin is much more positive to cold lead. Also
cold cadmium is positive to hot lead, but hot cadmium is far
more positive to cold lead. In these cases, therefore, there
are great differences produced by heat, but the metals still
keep their order.

951. In dilute sulphuric add, hot iron is well positive to cold
tin, but hot tin is still more positive to cold iron. Hot iron is
a little positive to cold lead, and hot lead is very positive to cold
iron. These are cases of the actual inversion of order; and
tin and lead may have their states reversed exactly in the same
manner.

952. In dilute nitric acid} tin and iron, and iron and lead
may have their states reversed, whichever is the hot metal being
rendered positive to the other. If, when the iron is to be
plunged into the heated side (918) the acid is only moderately
warm, it seems at first as if the tin would almost overpower the
iron, so beautifully can the forces be either balanced or rendered
predominant on either side at pleasure. Lead is positive to tin
in both cases; but far more so when hot than when cold.

953. These effects show beautifully that in many cases,
when two different metals are taken, either can be made positive
to the other at pleasure, by acting on their chemical affinities;
though the contacts of the metals with each other (supposed
to be an electromotive cause) remain entirely unchanged. They
show the effect of heat in reversing or strengthening the natural
differences of the metals, according as its action is made to
oppose or combine with their natural chemical forces, and thus
add further confirmation to the mass of evidence already
adduced.

954. There are here, as in the cases of one metal, some
instances where the heat renders the metal more negative than
it would be if cold. They occur, principally, in the solution of
sulphuret of potassium. Thus, with zinc and cadmium, or zinc
and tin, the coldest metal is positive. With lead and tin, the
hot tin is a little positive, cold tin very positive. With lead ander times we are sure, from the chemical