present some very interesting phenomena in
that respect. The sulphuret of antimony does not conduct the same current
sensibly either hot or cold, but is amongst those bodies acquiring
conducting power when fused (402.). The sulphuret of silver and perhaps
some others decompose whilst in the solid state; but the phenomena of this
decomposition will be reserved for its proper place in the next series of
these Researches.

441. Notwithstanding the extreme dissimilarity between sulphuret of silver
and gases or vapours, I cannot help suspecting the action of heat upon them
to be the same, bringing them all into the same class as conductors of
electricity, although with those great differences in degree, which are
found to exist under common circumstances. When gases are heated, they
increase in conducting power, both for common and voltaic electricity
(271.); and it is probable that if we could compress and condense them at
the same time, we should still further increase their conducting power.
Cagniard de la Tour has shown that a substance, for instance water, may be
so expanded by heat whilst in the liquid state, or condensed whilst in the
vaporous state, that the two states shall coincide at one point, and the
transition from one to the other be so gradual that no line of demarcation
can be pointed out[A]; that, in fact, the two states shall become
one;--which one state presents us at different times with differences in
degree as to certain properties and relations; and which differences are,
under ordinary circumstances, so great as to be equivalent to two different
states.

[A] Annales de Chimie, xxi. pp. 127, 178.

442. I cannot but suppose at present that at that point where the liquid
and the gaseous state coincide, the conducting properties are the same for
both; but that they diminish as the expansion of the matter into a rarer
form takes place by the removal of the necessary pressure; still, however,
retaining, as might be expected, the capability of having what feeble
conducting power remains, increased by the action of heat.

443. I venture to give the following summary of the conditions of electric
conduction in bodies, not however without fearing that I may have omitted
some important points[A].

[A] See now in relation to this subject, 1320--1242.--_Dec. 1838._

444. All bodies conduct electricity in the same manner from metals to lac
and gases, but in very different degrees.

445. Conducting powe