Inactive Voltaic Circles

247

too small to be of any service in support of the contact theory;
while, on the other hand, it affords delicate, and, therefore,
strong indications in favour of the chemical theory.

827. A change was made in the form and arrangement of
the cup D, fig. 65, so as to allow of experiments with other bodies
than the metals. The solution of

sulphuret of potassium was placed PJ^^ /~mzrz.

in a shallow vessel, the platinum
plate was bent so that the immersed
extremity corresponded to the bot-
tom of the vessel; on this a piece
of loosely folded cloth was laid in
the solution, and on that again the
mineral or other substance to be Fig. 55.

compared with the platinum; the

fluid being of such depth that only part of that substance was
in it, the rest being clean and dry; on this portion the platinum
wire, which completed the circuit, rested. The arrangement
of this part of the circuit is given in section at fig. 66, where H
represents a piece of galena to be compared with the platinum P.

828. In this way galena, compact yellow copper pyrites,
yellow iron pyrites, and globules of oxide of burnt iron, were
compared with platinum (the solution of sulphuret of potassium
being the electrolyte used in the circuit), and with the same
results as were before obtained with metals (817, 821).

829. Experiments hereafter to be described gave arrange-
ments in which, with the same electrolyte, sulphuret of lead was
compared with gold, palladium, iron, nickel, and bismuth (873,
874); also sulphuret of bismuth with platinum, gold, palladium,
iron, nickel, lead, and sulphuret of lead (882), and always with
the same result. Where no chemical action occurred there
no current was formed; although the circuit remained an
excellent conductor, and the contact existed by which, it is
assumed in the contact theory, such a current should be
produced.

830. Instead of the strong solution, a dilute solution of the
yellow sulphuret of potassium, consisting of one volume of strong
solution (800) and ten volumes of water, was used. Plates
of platinum and iron were arranged in this fluid as before
(812): at first the iron was negative (1037), but in ten minutes
it was neutral, and the needle at o. Then a weak chemical
current excited at x (819) easily passed: and even a thermo
current (818) was able to show its effects at the needle. Thus to platinum for