ELECTRO-CHEMICAL POLARITY OF GASES. 361 



on the silver, presenting in succession yellow, orange, and blue 

 tints, very similar to the successive tints given by iodising in 

 the ordinary manner a Daguerreotype plate. Upon the poles 

 being reversed and the plate made negative, this spot was 

 entirely removed, and the plate became perfectly clean, leav- 

 ing, however, a dark, polished spot occasioned by molecular 

 disintegration, and therefore distinguishable from the re- 

 mainder of the plate. 



The experiment was repeated a great many times, and 

 with varying proportions of gas, and I found that with pro- 

 portions varying from equal volumes of hydrogen and air to 

 those of one volume of the former to two and a half of the 

 latter, the experiments succeeded ; better, I should say, when 

 there was rather an excess of hydrogen as compared with the 

 equivalent of oxygen in the atmospheric air ; about one 

 volume of hydrogen to one and a half of air succeeded well ; 

 when excess of air was present, oxidation took place, whether 

 the plate was positive or negative, and when excess of 

 hydrogen was present no oxidation took place. 



2nd. I experimented with an air vacuum (to borrow an 

 expression of Dr. Faraday), and found that oxidation took 

 place whether the plates were positive or negative, but in dif- 

 ferent degrees ; 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 diffuse 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 discolouration took place, the plate retained 

 its polish, though after a long continuance of the discharge a 

 molecular change was perceptible, producing a frosted appear- 

 ance similar to the mercurialised portions of a Daguerreotype. 



When the plate had been previously oxidated by the dis- 

 charge in an air vacuum the oxidation was rapidly and beau- 

 tifully cleared off by the discharge in the hydrogen vacuum, 

 and this whether the plate was positive or negative, the effect 

 being, however, better and more rapidly produced in the 

 latter case. 



4th. I substituted respectively for the steel needle wires 



