VOLTAIC EXCITATION AFFECTED BY TEMPERATURE. 103 
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. 
1964. 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 (1930.) 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. 
1965. 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. 
1966. 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, princi- 
pally, 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 zinc, hot zinc is a little positive, cold 
zinc much more so. With silver and lead, the hot silver is a little positive to the 
lead, the cold silver is more, and well positive. In these cases the current is preceded 
by a moment of quiescence (1953.), during which the chemical action at the hot metal 
reduces the efficacy of the electrolyte against it more than at the cold metal, and the 
latter afterwards shows its advantage. 
1967. Before concluding these observations on the effects of heat, and in reference 
to the probable utility of the voltaic circuit in investigations of the intimate nature 
of chemical affinity (1959.), I will describe a result which, if confirmed, may lead to 
very important investigations. Tin and lead were conjoined and plunged into cold 
dilute sulphuric acid ; the tin was positive a little. The same acid was heated, 
and the tin and lead, having been perfectly cleaned, were reintroduced, then the 
lead was a little positive to the tin. So that a difference of temperature not limited 
to one contact, for the two electrolytic contacts were always at the same temperature, 
caused a difference in the relation of these metals the one to the other. Tin and iron 
in dilute sulphuric acid appeared to give a similar result; i. e. in the cold acid the 
