92 
MR. GROVE ON THE ELECTRO-CHEMICAL POLARITY OF GASES. 
9th. I now substituted for the silver plate, plates of the following metals : — 
bismuth, lead, tin, zinc, copper, iron and platinum, the former three metals being 
burnished, the latter polished. 
Bismuth showed the effect nearly, if not quite as well as silver ; it was oxidated in 
an air vacuum, reduced in a hydrogen vacuum, and oxidated or reduced in the mixed 
gas according as it formed the positive or negative terminal. 
Lead oxidated easily, but the spot of oxide could with difficulty be reduced. Tin, 
zinc and copper required the admission of a great quantity of air to produce oxida- 
tion ; and I could not succeed in reducing the oxide by the electrical discharge, at 
least so as to restore the polish of the plate ; a blackening effect was in some degree 
produced. Iron was not oxidated until the receiver was nearly filled with air, and 
then a small spot of rust was formed which I could not reduce. With all the metals 
a slight whitish film like the mercurialized portion of a Daguerreotype was visible 
beyond the circle marked by the discharge when the plate was rendered positive, 
which film was removed by negative electrolization in a hydrogen vacuum ; it seemed 
to me that this film, as well as others among those I have described, was affected 
by light, but I did not turn aside to examine this effect. Platinum showed no effect 
either of oxidation or reduction. 
10th. As it was impossible to operate with an atmosphere of chlorine with the 
apparatus which I possessed, and wishing to vary the electro-negative element, I 
iodized a silver plate by the vapour of iodine to a deep blue colour, and then made it 
negative in an atmosphere of hydrogen ; the iodine was beautifully removed in a 
circle or disc opposite the point which formed the positive terminal. 
11th. I now substituted for the coil apparatus a very good electrical machine, the 
cylinder of which was 16 inches diameter, and the prime conductor of which, when the 
machine was properly excited, gave a spark of 8 inches long. With this machine, and 
in an attenuated atmosphere of one volume hydrogen plus two of atmospheric air, I 
produced the effects of oxidation and reduction very distinctly, the plate being in 
turn connected with the conductor and with the ground ; but the comparative 
minuteness of the spot after many turns of the machine, showed the great superiority 
of the coil machine for producing quantitative effects over the ordinary electrical 
machine ; and I question Avhether I should have detected the phenomenon with 
the latter, had I not become previously well acquainted with it by the former appa- 
ratus. Probably an extensive series of the water battery or a steam hydro-electric 
machine would succeed equally well, or better than the coil machine. 
12th. A solution of hyposulphite of soda removed the spots formed by electriza- 
tion from the silver plate just as it removes the iodine from an iodized plate. 
13th. In some of the above experiments I remarked a tendency in the spots pro- 
duced by the discharge, to show circles or zones of oxidation in different degrees, 
and in a more marked manner than would be accounted for by the different colours 
of the thin films of oxide formed. I determined to examine this effect, and selected. 
