432 Sir Humphry Davy on the magnetic phcenomena 
redness, and let the flame of a spirit lamp be applied to any 
part of it, so as to heat that part to whiteness, the rest of the 
wire will instantly become cooled below the point of visible 
ignition. For the converse of the experiment, let a piece of 
ice or a stream of cold air be applied to a part of the wire ; the 
other parts will immediately become much hotter ; and from 
a red, will rise to a white heat. The quantity of electricity that 
can pass through that part of the wire submitted to the chan- 
ges of temperature, is so much smaller when it is hot than 
when it is cold, that the absolute temperature of the whole 
wire is diminished by heating a part of it, and, vice versa , 
increased by cooling a part of it. 
In comparing the conducting powers of different metals, I 
found much greater differences than I had expected. Thus 
six inches of silver wire of -J- discharged the whole of the 
electricity of sixty-five pair of plates of zinc and double cop- 
per made active by a mixture of about one part of nitric acid 
of commerce, and fifteen parts of water. Six inches of copper 
wire of the same diameter discharged the electricity of fifty- 
six pairs of the same combination, six inches of tin of the 
same diameter carried off that of twelve only, the same quan- 
tity of wire of platinum that of eleven, and of iron that of 
nine. Six inches of wire of lead of seemed equal in their 
conducting powers to the same length of copper wire of ~o- 
Ail the wires were kept as cool as possible by immersion in 
a basin of water.* 
I made a number of experiments of the same kind, but the 
results were never precisely alike, though they sometimes 
* Water is so bad a conductor, that in experiments of this kind its effects may be 
neglected altogether ; and these effects were equal in all the experiments. 
