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



ments succeeded; better, I should say, when there was rather 

 an excess of hydi'Ogen as compai-ed with the equivalent of oxygen 

 in the atmospheric air ; about one volume of hydrogen to one 

 and a haK of air succeeded well ; when excess of air was present, 

 oxidation took place whether the plate was positive or negative, 

 and when excess of hydi'ogen was present no oxidation took place. 



2nd. I experimented \\4th an air vacuum (to borrow an ex- 

 pression of Dr. Faraday), and found that oxidation took place 

 whether the plates were positive or negative, but in diiferent de- 

 grees; when the plate was positive, a small circular spot was 

 rapidly formed, quickly deepening in colour, and apparently 

 eating into the plate ; when the plate was negative, a large dif- 

 fuse spot was formed, the oxidation was more slow, and the plate 

 not so rapidly corroded. 



3rd. I now operated with a hydrogen vacuum ; when the plate 

 was clean no discoloration took place, the plate retained its polish, 

 though after a long continuance of the discharge a molecular 

 change was perceptible, producing a frosted appearance similar 

 to the mercurialized portions of a Daguerreotype. 



AYhen the plate had been previously oxidated by the discharge 

 in an air vacuum, the oxidation was rapidly and beautifully 

 cleared off by the discharge in the hydiogen vacuum, and this 

 whether the plate was positive or negative, the effect being, how- 

 ever, better and more rapidly produced in the latter case. 



4th. I substituted respectively for the steel needle, wu'es of 

 coppei', silver and platinum, and found the effect produced by all 

 and with nearly equal facility ; if there were any difference, the 

 platinum point was the least eflScient ; this may be due to the 

 peculiar effect of platinum in itself combining the gases, or to 

 its inoxidable character, the oxygen being thrown off from its 

 surface, and not uniting with it as with the more oxidable metals ; 

 the flame or luminous appearance which surrounded the wire 

 when the platinum was negative, was larger and more diffuse 

 than with the other metals. 



5th. As air, notwithstanding its containing a great excess of 

 nitrogen, gave an effect of oxidation at both electrodes, though 

 different in degree, I increased the proportion of nitrogen by 

 passing into the receiver nitrogen which had been formed by the 

 slow combustion of phosphorus, the phos])horous acid having 

 been well washed away, and potash being always in the receiver ; 

 no more air was allowed to be present than the very small quan- 

 tity contained in the apertures of the stopcock ; with this mix- 

 ture, viz. a maximum of nitrogen and a minimum of oxygen, 

 and rarefied as before, a similar effect was produced to that shown 

 in the mixture of air and hydrogen, the positive plate being oxi- 

 dated by the discharge, and the spot when made negative being 



