ELECTRO-CHEMICAL POLARITY OF GASES. 377 



a space of an inch is a convenient distance for exhibiting the 

 effect well. 



This dark discharge is elaborately described by Faraday 

 as produced by the ordinary electrical machine, Experimental 

 Researches, 1,544 ^ se< 2- 



Having in my mind the analogy of interference, it seemed 

 to me that this dark space might be due to the crossing 

 of the lines of discharge from the successive points of the 

 needle, the knob, or plate from which the negative discharge 

 issues. 



As the positive discharge appears to issue from a point, and 

 not to surround the wire, as does the negative, the position of 

 the dark space in close approximation to the negative termi- 

 nal was in favour of this view ; if correct, it should follow that 

 if the terminals were coated points instead of wires, knobs, or 

 plates, this dark space would not be observed, or its position 

 would be changed. Experiment verified this expectation : 

 when platinum wires sealed in glass were employed and a 

 good vacuum formed, the line of luminous discharge was con- 

 tinuous when the platinum points were brought to a distance 

 of half an inch. 



When these terminals are so far separated as not to give a 

 continuous line of discharge, a pencil appears on each termi- 

 nal, which gradually becomes fainter and fainter towards the 

 middle of the intervening space ; and if the distance be great, 

 the discharge ceases to be luminous towards the middle of 

 the intervening space, from excessive diffusion ; but this will 

 be seen to be a very different effect from the abrupt and well- 

 defined dark space which appears in close approximation to 

 the negative terminal when the coated wires are not em- 

 ployed. 



When the positive terminal is coated and the negative one 

 bare, the dark space appears on the point of the bare wire, the 

 wire itself being surrounded by a lambent flame ; but with the 

 converse arrangement there is no such dark space. All this is 

 much in favour of interference taking place, the coincidence of 

 positive and negative phases of the discharge producing at 

 certain points mutual neutralisation. 



