46

Faraday's Researches

phenomena of this decomposition will be reserved for its proper
place in the next series of these Researches.

177. Notwithstanding the extreme dissimilarity between
sulphuret of silver and gases or vapours, I cannot help suspect-
ing 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 (7); 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;1 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 circum-
stances, so great as to be equivalent to two different states.

178. I cannot but suppose at present that at that point where
the liquid and the gaseous state coincide, the conducting pro-
perties 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 con-
ducting power remains increased by the action of heat.

179. 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.

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

181. Conducting power is in some bodies powerfully in-
creased by heat, and in others diminished, yet without our per-
ceiving 'any accompanying essential electrical difference, either
in the bodies or in the changes occasioned by the electricity
conducted;

182. A numerous class of bodies, insulating electricity of low
intensity, when solid, conduct it very freely when fluid, and are
then decomposed by it.

183. But there are many fluid bodies which do not sensibly

1 Annales de Chimie, xxu. pp. 127, 178.se the temperature