44 Faraday's Researches

chloride of lead produced the same effect. The conducting
power of these bodies, when fluid, is very great (131, 138).

167. These effects, produced by using the common machine
and the voltaic battery, agree therefore with each other, and
with the law laid down in this paper (130); and also with the
opinion I have supported, in the First Part of these Researches,
of the identity of electricity derived from different sources (96).

168. The effect of heat in increasing the conducting power
of many substances, especially for electricity of high tension, is
well known. I have lately met with an extraordinary case of
this kind, for electricity of low tension, or that of the voltaic
pile, and which is in direct contrast with the influence of heat
upon metallic bodies, as observed and described by Sir Humphry
Davy.1

169. The substance presenting this effect is sulphuret of
silver. It was made by fusing a mixture of precipitated silver
and sublimed sulphur, removing the film of silver by a file
from the exterior of the fused mass, pulverising the sulphuret,
mingling it with more sulphur, and fusing it again in a green glass
tube, so that no air should obtain access during the process.
The surface of the sulphuret being again removed by a file or
knife, it was considered quite free from uncombined silver.

170. When a piece of this sulphuret, half an inch in thick-
ness, was put between surfaces of platina, terminating the poles
of a voltaic battery of twenty pairs of four-inch plates, a gal-
vanometer being also included in the circuit, the needle was
slightly deflected, indicating a feeble conducting power. On
pressing the platina poles and sulphuret together with the
fingers, the conducting power increased as the whole became
warm. On applying a lamp under the sulphuret between the
poles, the conducting power rose rapidly with the heat, and at
last the galvanometer needle jumped into a fixed position, and
the sulphuret was found conducting in the manner of a metal.
On removing the lamp and allowing the heat to fall, the effects
were reversed, the needle at first began to vibrate a little, then
gradually left its transverse direction, and at last returned to a
position very nearly that which it would take when no current
was passing through the galvanometer.

171. Occasionally, when the contact of the sulphuret with the
platina poles was good, the battery freshly charged, and the
commencing temperature not too low, the mere current of elec-
tricity from the battery was sufficient to raise the temperature

1 Philosophical Transactions, 1821, p. 431.d and cooledted as perfectly as the former.