Inactive Conducting Circles 241

copper, sulphurets of bismuth, iron, and copper, globules of
oxide of burnt iron, oxide of iron by heat or scale oxide, con-
ducted the thermo current very well. Native peroxide of
manganese and peroxide of lead conducted it moderately well.

809. The following are bodies, in some respect analogous in
nature and composition, which did not sensibly conduct this
weak current when the contact surfaces were small:—artificial
grey sulphuret of tin, blende, cinnabar, haematite, Elba iron-
ore, native magnetic oxide of iron, native peroxide of tin or
tinstone, wolfram, fused and cooled protoxide of copper, per-
oxide of mercury.

810. Some of the foregoing substances are very remarkable
in their conducting power. This is the case with the solution
of sulphuret of potassium (801) and the nitrous acid (804), for
the great amount of this power. The peroxide of manganese
and lead are still more remarkable for possessing this power,
because the protoxides of these metals do not conduct either the
feeble thermo current or a far more powerful one from a voltaic
battery. This circumstance made me especially anxious to
verify the point with the peroxide of lead. I therefore prepared
some from red-lead by the action of successive portions of nitric
acid, then boiled the brown oxide, so obtained, in several por-
tions of distilled water, for days together, until every trace of
nitric acid and nitrate of lead had been removed; after which
it was well and perfectly dried. Still, when a heap of it in
powder, and consequently in very imperfect contact throughout
its own mass, was pressed between two plates of platinum and
so brought into the thermo-electric circuit (801), the current
was found to pass readily.

^f ii. Inactive Conducting Circles containing a Fluid or Electrolyte

811. De la Rive has already quoted the case of potash, iron
and platina,1 to show that where there was no chemical action
there was no current. My object is to increase the number of
such cases; to use other fluids than potash, and such as have
good conducting power for weak currents; to use also strong
and weak solutions; and thus to accumulate the conjoint
experimental and argumentative evidence by which the great
question must finally be decided.

812. I first used the sulphuret of potassium as an electrolyte
of good conducting power, but chemically inactive (799) when

1 Philosophical Magazine, 1837, xi. 275.

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