398 TENTH ANNUAL REPORT OP 



in the presence of platinum, the electro-motive force of the plates will 

 not be destroyed. 



5. A platinum plate exposed for only a few seconds in an atmo- 

 sphere of hydrogen, is polarized positively. 



6. Gold and silver wire do not acquire electro-motive power by 

 immersion in hydrogen gas. 



7. A platinum wire placed in oxygen does not become negatively 

 polarized, nor do gold and silver. 



8. Platinum, gold and silver, exposed for a few seconds in chlorine 

 gas, become polarized negatively. Bromine gas produces the same 

 eflect on these metals. 



Before passing to a further elucidation of these facts, we will con- 

 sider the most advantageous way of showing the electrical polariza- 

 tion of a metallic plate. 



In a small cup of mercury a, Fi,<.\ 28, connected witl; the terminal 

 wire of a multiplier, the end of a wire of a platinum plate p is im- 

 j.j„ 2g_ mersed. The plate must be first perfectly 



cleaned, and then suspended in a glass of 

 acidified water. In the cup h the wire of 

 a second and exactly similar platinum plate 

 is placed — the plate being in like manner 

 cleaned and suspended in the acidified 

 water. The needle will, of course, remain 

 at rest, since both plates act exactly alike 

 electro-motively. But if the second plate, 

 which we will denote by^, should be po- 

 larized in any of the above ways, a deflection 

 of the galvanometer needle would follow, 

 from wliich the direction of the current 

 could be ascertained. 

 For example, if the platinum plate p' were immersed in hydrogen 

 gas, it would act electro-positively towards the other ; that is, the 

 galvanometer would indicate the current passing from^ through the 

 liquid io p. The plate y being immersed in chlorine gas, the deflec- 

 tion of the needle would show p' electro-negative top. 



If the platinum plate p' should have served as the negative pole in 

 the decomposition of Avater, it will act exactly as though it had been 

 plunged into a jar of hydrogen ; that is, if used for closing in the 

 apparatus of Fig. 28, it would generate a current passing from p' 

 through the liquid top. 



All the phenomena we have just considered, appear to indicate that 

 the stratum of gas which escapes at tlie electrodes during electrolysis 

 is really the cause of galvanic polarization. If such be the case, it is 

 perfectly evident that the stratum of gas will be destroyed by heating 

 the metal plates to redness. This circumstance alone, however, would 

 prove nothing, because such a heat must act destructively upon the 

 polarity, even if it should depend upon other causes than upon a stra- 

 tum of gas. The second experiment is decisive. The instantaneous 

 destruction of the positive polarity of a platinum wire, by chlorine, 

 can hardly take place otherwise than by the chemical action of the 

 chlorine on the oxygen,, by which every trace of hydrogen disappears 



