o the circuit was sufficient almost entirely to destroy the
action on the galvanometer. Fused and cooled chloride of lead produced the
same effect. The conducting power of these bodies, _when fluid_, is very
great (395. 402.).

431. 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 (394.); and also with the opinion I have supported, in the Third
Series of these Researches, of the identity of electricity derived from
different sources (360.).

432. 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[A].

[A] Philosophical Transactions, 1821, p. 131.

433. 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,
pulverizing 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.

434. When a piece of this sulphuret, half an inch in thickness, was put
between surfaces of platina, terminating the poles of a voltaic battery of
twenty pairs of four-inch plates, a galvanometer 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.

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