ELECTRO-CHEMICAL POLARITY OF GASES. 367 



and the oxide reduced when negative, just as with a vacuum 

 of air and hydrogen, but rather more slowly ; with a mixture 

 of five volumes of carbonic oxide and one volume of oxygen, 

 the rings were formed very distinctly, particularly if the plate 

 was made negative first, and then positive. The luminous 

 spot on the plate, when positive in this gas, was coloured 

 green. 



When the plate was negative in olefiant gas it darkened, 

 showing the rings of colour produced by thin plates, and very 

 distinct from the other rings of which I have spoken. After a 

 short time a pulverulent deposit was formed on the plate, 

 giving brilliant sparks or stars of light, which were not shown 

 by any other gas. 



This deposit was too minute for analysis, but I have no 

 doubt, from the gas used and the appearances presented, it 

 was carbon. 



I have given in the above experiments the conditions 

 under which they succeeded best ; but, upon repetition, al- 

 though the exact volumes of gases and other conditions were 

 carefully attended to, they sometimes required a slight altera- 

 tion to succeed, variations taking place from causes which I 

 could not detect ; thus it was sometimes necessary to add a 

 little more hydrogen, sometimes a little more oxygen or air, 

 to alter slightly the state of attenuation in the gas, &c. 



The necessarily varying condition of the battery, and the 

 state of the contact breaker, slight impurities in the gases or 

 on the surface of the plates, would be quite sufficient to account 

 for these irregularities. I mention them for the guidance of 

 anyone who may wish to repeat the experiments ; a very 

 little practice will enable any electrician to have the results at 

 his command. When there is too great a proportion of air or 

 oxygen, oxidation takes place at both poles ; when too much 

 hydrogen, reduction takes place at both ; and to effect oxida- 

 tion or reduction by reversing the direction of the discharge, 

 an intermediate condition is requisite ; so if the gas be not 

 sufficiently attenuated the oxidation is too rapid, and the 

 plate too much corroded to bring out the effects clearly ; if 

 too much attenuated too long a time is required, and the effect 

 is feeble and indistinct. 



