48 ® Sir Humphry Davy on the magnetic phenomena 
sity is passed through conductors, must always interfere with 
the exact knowledge of the changes of their conducting 
powers, as is proved by the following experiment. A bat- 
tery of 20 pair of plates of zinc, and copper plates 10 inches 
by 6, was very highly charged with a mixture of nitric acid 
and water, so as to exhibit a considerable intensity of elec- 
trical action, and the relative conducting powers of silver and 
platinum in air and water ascertained by means of it. In air, 
6 inches of wire of platinum of discharged only 4 double 
plates, whilst 6 inches of silver wire of the same diameter, 
discharged the whole combination : the platinum was strongly 
ignited in this experiment, whilst the silver was scarcely 
warm to the touch. On cooling the platinum wire by plac- 
ing it in water, it was found to discharge 10 double plates. 
When the intensity of the electricity is very high, however, 
even the cooling powers of fluid media are of little avail : 
thus I found that fine wire of platinum was fused by the 
discharge of a common electrical battery under water ; so that 
the conducting power must always be diminished by the heat 
generated, in a greater proportion as the intensity of the elec- 
tricity is higher. 
It might at first view be supposed, that when a conductor 
placed in the circuit left a residuum of electricity in any bat- 
tery, increase of the power of the battery, or of its surface, 
would not enable it to carry through any additional quantity. 
This, however, is far from being the case. 
When saline solutions were placed in the circuit of a bat- 
tery of 20 plates, though they discharged a very small quan- 
tity only of the electricity, when the troughs were only £ full, 
yet their chemical decomposition exhibited the fact of a much 
