Dr. Draper on the Electro-motwe Poisoer of Heat. 455 
increments of temperature throughout the range of the mer- 
curial thermometer. 
Let us now proceed to the second proposition, “ That the 
tension undergoes a slight increase with increase of tempera- 
ture, a phaenomenon due to the increased resistance to con- 
duction of metals when their temperature rises.” 
It will be seen, on consulting the following table, that pairs 
of different metals,^ at the same temperature, have tensions 
which are apparently very different. 
The currents, the tensions of which are here indicated, were 
generated by keeping one end of the thermal pair in boiling 
water, the other ends being maintained at a temperature of 
32° Fahr. 
Table III. 
A pair of 
Tension. 
A pair of 
Tension. 
Antimony and bismuth 
Copper and iron 
Silver and lead 
•137 
•183 
•307 
•313 
•380 
Platina and iron 
Copper and platina 
Platina and palladium 
Tin and iron 
Platina and tin 
•470 
•473 
•500 
•518 
•567 
Lead and palladium... 
Silver and platina 
We perceive, therefore, that there apparently exist specific 
differences in the qualities of electric currents derived from 
different sources. If, for example, we take a pair of platina 
and palladium, and expose it to a temperature which shall 
generate a current capable of deflecting the torsion balance 
through 1000 degrees, and then abstract it by a wire of such 
dimensions as to stop one half, or only allow 500 degrees 
to pass, and repeat the experiment with a current generated 
by bismuth and antimony, the temperature being still so ad- 
justed as to give a deflection of 1000 degrees, on making this 
pass through the same intercepting wire, perhaps not much 
more than one eighth of it will go through the galvanometer. 
It might be supposed that these characteristic differences of 
thermal currents, derived from different sources, were due to 
some modification of the electricity itself, similar to those of 
radiant heat, derived from different sources, or at different 
temperatures, which M. Melloni has attempted to show are 
analogous to the colours of light, being like them of different 
degrees of refrangibility, and permeating absorbent media 
with different degrees of facility. For in the same way that 
we regard glass as transparent to light, and rock salt as trans- 
parent to heat, so loo we might regard a copper wire or any 
conducting medium as transparent to electricity. 
