438 Sir Humphry Davy on the magnetic phenomena 
duced in air, where there is reason to suppose the least resis- 
tance ; and as the presence of heat renders bodies worse con- 
ductors, another view may be taken, namely, that the exci- 
tation of heat occasions the imperfection of the conducting 
power. But till the causes of heat and of electricity are known , 
and of that peculiar constitution of matter which excites the 
one, and transmits or propagates the other, our reasoning on 
this subject must be inconclusive 
I found that when equal portions of wires of the same di- 
ameter, but of different metals, were connected together in 
the circuit of a powerful voltaic battery, acting as two sur- 
faces, the metals were heated in the following order : iron 
most, then palladium, then platinum, then tin, then zinc, then 
gold, then lead, then copper, and silver least of all. And 
from one experiment, in which similar wires of platinum and 
silver joined in the same circuit were placed in equal portions 
of oil, it appeared that the generation of heat was nearly 
inversely as their conducting power. Thus the silver raised 
the temperature of the oil only four degrees, whilst the pla- 
tinum raised it twenty-two. The same relations to heat seem 
to exist, whatever is the intensity of the electricity ; thus cir- 
cuits of wires placed under water, and acted on by the com- 
mon electrical discharge, were heated in the same order as by 
the voltaic battery, as was shown by their relative fusion ; 
thus, iron fusing before platinum, platinum before gold, and 
so on. 
If a chain be made of wire of platinum and silver, in alter- 
nate links soldered together, the silver wire being four or 
rive times the diameter of the platinum, and placed in a power- 
ful voltaic circuit, the silver links are not sensibly heated, 
