ELECTRO-CHEMICAL POLARITY OF GASES. 373 



When the plate is negative the effect is, as I have observed, 

 less marked and more uncertain ; but in this case it should be 

 recollected that the visible discharge issues from the point, 

 and does not extend, or extends to a very small degree, over 

 the surface of the needle. 



If the phenomena were such that the central portion were 

 always clear, while around it was one, and one only, circle of 

 oxide, it might be accounted for by the hypothesis that the 

 lines of polarisation and discharge between a point and flat 

 surface assume the form of a hollow cone ; but a cone of 

 negative bounded by cones of positive action still gives the 

 idea of some lateral fits or phases of undulation. 



The high rarefaction of the medium by the discharge, and 

 its intermitting character, might occasion pulsations by the 

 in-rushing of the surrounding gas, and thus vacua in circles 

 might be formed at the places where the action of oxidation 

 is rendered null ; but this view is, I think, inadmissible ; it 

 does not account for the effects obtaining only in certain mix- 

 tures, it does not account for the reducing action, the plate 

 being positive, and presents other difficulties. The point 

 involved in Experiments 13 and 14 presents apparently a 

 wide field of enquiry ; I therefore will not farther dilate on it 

 at present, and hope to make it the subject of future investi- 

 gation. 



Postscript, April 24. I may, I trust, be permitted to add 

 to this paper one or two experiments on the subject last dis- 

 cussed. Assuming that the alternations of oxidation and re- 

 duction were produced by interference in consequence of the 

 discharge proceeding from successive points of the terminal 

 or terminals, a difference of effect might be anticipated if the 

 electricity passed from a point only, and not from a line, as 

 was the case in Experiment 1 3. I therefore sealed a platinum 

 wire -g^th of an inch in diameter into a piece of glass tubing, 

 and then ground the extremity to a flat surface, so that the 

 section only of the wire was exposed ; this wire was placed 

 opposite, and at 0*07 of an inch distance from the polished 

 silver plate, in a mixture of one volume of oxygen with five 

 volumes of hydrogen attenuated until the barometer stood at 

 half an inch ; discharges from the secondary coil were then 



