364 EXPERIMENTAL INVESTIGATIONS. 



a small spot of rust was formed, which I could not reduce. 

 With all the metals a slight whitish film, like the mercurialised 

 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 electrisation 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. 



loth. 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 iodised 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. 



nth. I now substituted for the coil apparatus a very good 

 electrical machine, the cylinder of which was 16 inches dia- 

 meter, 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 whether I should have detected the 

 phenomenon with the latter, had I not become previously well 

 acquainted with it by the former apparatus. Probably an ex- 

 tensive series of the water battery or a steam hydro-electric 

 machine would succeed equally well or better than the coil 

 machine. 



1 2th. A solution of hyposulphite of soda removed the 

 spots formed by electrisation from the silver plate, just as it 

 removes the iodine from an iodised plate. 



1 3th. In some of the above experiments I remarked a ten- 

 dency in the spots produced by the discharge to show circles 

 or zones of oxidation in different degrees, and in a more 



