96 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
too much detail to be described fully here; but I will briefly point it out and illus- 
trate it by an example or two. 
1 923 . When two platinum wires were compared in hot and cold dilute sulphuric acid 
( 1935 .), they gave scarcely a sensible trace of any electric current. If any real effect 
of heat occurred, it was that the hot metal was the least degree positive. When silver 
and silver were compared, hot and cold, there was also no sensible effect. But when 
platinum and silver were compared in the same acid, different effects occurred. Both 
being cold, the silver in the A side fig. 7- ( 1915 .) was positive about 4 °, by the gal- 
vanometer ; moving the platina on the other side B did not alter this effect, but on 
heating the acid and platinum there, the current became very powerful, deflecting 
the needle 30 °, and the silver was positive. Whilst the heat continued, the effect con- 
tinued ; but on cooling the acid and platinum it went down to the first degree. No 
such effect took place at the silver; for on heating that side, instead of becoming ne- 
gative, it became more positive, but only to the degree of deflecting the needle 16 °. 
Then, motion of the platinum ( 1919 .) facilitated the passing of the current and the de- 
flection increased, but heating the platinum side did far more. 
1924 . Silver and copper in dilute sulphuric acid produced very little effect; the 
copper was positive about 1° by the galvanometer; moving the copper or the silver 
did nothing ; heating the copper side caused no change ; but on heating the silver 
side it became negative 20°. On cooling the silver side this effect went down, and 
then, either moving the silver or copper, or heating, the copper side, caused very 
little change ; but heating the silver side made it negative as before. 
1925 . All this resolves itself into an effect of the following kind ; that where two 
metals are in the relation of positive and negative to each other in such an electro- 
lyte as dilute acids (and perhaps others), heating the negative metal at its contact 
with the electrolyte enables the current, which tends to form, to pass with such faci- 
lity, as to give a result sometimes tenfold more powerful than would occur without 
it. It is not displacement of the investing fluid, for motion will in these cases do 
nothing : it is not chemical action, for the effect occurs at that electrode where the 
chemical action is not active ; it is not a thermo-electric phenomenon of the ordinary 
kind, because it depends upon a voltaic relation; i. e. the metal showing the effect 
must be negative to the other metal in the electrolyte ; so silver heated does nothing 
with silver cold, though it shows a great effect with copper either hot or cold ( 1924 .) ; 
and platinum hot is as nothing to platina cold, but much to silver either hot or cold. 
1926 . Whatever may be the intimate action of heat in these cases, there is no doubt 
that it is dependent on the current which tends to pass round the circuit. It is 
essential to remember that the increased effect on the galvanometer is not due to any 
increase in the electromotive force, but solely to the removal of obstruction to the 
current by an increase probably of discharge. M. de la Hive has described an effect 
of heat, on the passage of the electric current, through dilute acid placed in the cir- 
cuit, by platinum electrodes. Heat applied to the negative electrode increased the de- 
