5. 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 electricity from the battery was sufficient to
raise the temperature of the sulphuret; and then, without any application
of extraneous heat, it went on increasing conjointly in temperature and
conducting power, until the cooling influence of the air limited the
effects. In such cases it was generally necessary to cool the whole
purposely, to show the returning series of phenomena.

436. Occasionally, also, the effects would sink of themselves, and could
not be renewed until a fresh surface of the sulphuret had been applied to
the positive pole. This was in consequence of peculiar results of
decomposition, to which I shall have occasion to revert in the section on
Electro-chemical Decomposition, and was conveniently avoided by inserting
the ends of two pieces of platina wire into the opposite extremities of a
portion of sulphuret fused in a glass tube, and placing this arrangement
between the poles of the battery.

437. The hot sulphuret of silver conducts sufficiently well to give a
bright spark with charcoal, &c. &c., in the manner of a metal.

438. The native grey sulphuret of silver, and the ruby silver ore, both
presented the same phenomena. The native malleable sulphuret of silver
presented precisely the same appearances as the artificial sulphuret.

439. There is no other body with which I am acquainted, that, like
sulphuret of silver, can compare with metals in conducting power for
electricity of low tension when hot, but which, unlike them, during
cooling, loses in power, whilst they, on the contrary, gain. Probably,
however, many others may, when sought for, be found[A].

[A] See now on this subject, 1340, 1341.--_Dec. 1838._

440. The proto-sulphuret of iron, the native per-sulphuret of iron,
arsenical sulphuret of iron, native yellow sulphuret of copper and iron,
grey artificial sulphuret of copper, artificial sulphuret of bismuth, and
artificial grey sulphuret of tin, all conduct the voltaic battery current
when cold, more or less, some giving sparks like the metals, others not
being sufficient for that high effect. They did not seem to conduct better
when heated, than before; but I had not time to enter accurately into the
investigation of this point. Almost all of them became much heated by the
transmission of the current, and