504 Mr. Grove on the Electro -chemical Polarity of Gases. 



Lead oxidated easily, but the spot of oxide could with difficulty 

 be reduced. Tiu, zinc, and copper required the admission of a 

 great quantity of air to produce oxidation ; and I could not suc- 

 ceed 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 

 electrization 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. Pla- 

 tinum 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 colom-, and then made it nega- 

 tive 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 pro- 

 perly 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 quanti- 

 tative eflfects over the ordinary electrical machine ; and I question 

 whether I should have detected the pha;nomenon with the latter, 

 had I not become pre\'iously well acquainted with it by the former 

 apparatus. Probably an extensi\'e series of the water battery or 

 a steam hydi'O-electric machine would succeed equally well, or 

 better than the coil machine. 



12th. A solution of hyposulphite of soda removed the spots 

 formed by electrization from the silver plate just as it removes 

 the iodine from an iodized plate. 



13th. 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 markea 

 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, after some experiments, an atmosphere of 



