120 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XII.) 



tions, and, by them, extends discharge to a greater distance than any other gas tried, 

 it is also that which constitutes four fifths of our atmosphere ; and as, in atmospheric 

 electrical phenomena, one, and sometimes both the inductive forces are resident 

 on the particles of the air, which, though probably affected as to conducting power 

 by the aqueous particles in it, cannot be considered as a good conductor, so the 

 peculiar power possessed by nitrogen, to originate and effect discharge in the form 

 of a brush or of ramifications, has, probably, an important relation to its electrical 

 service in nature, as it most seriously affects the character and condition of the dis- 

 charge when made. The whole subject of discharge from and through gases is a 

 most important one to science, and, if only in reference to atmospheric electricity, 

 deserves extensive and close experimental investigation. 



Difference of discharge at the positive and negative conducting surfaces. 



1465. I have avoided speaking of this well-known phenomenon more than was 

 quite necessary, that I might bring together here what I have to say on the subject. 

 When the brush discharge is observed in air at the positive and negative surfaces, 

 there is a very striking difference, the true and full comprehension of which would, 

 no doubt, be of the utmost importance to the physics of electricity ; it would throw 

 great light on our present subject, i. e. the molecular action of dielectrics under in- 

 duction, and its consequences, and seems very open to, and accessible by, experi- 

 mental inquiry. 



1466. The difference in question used to be expressed in former times by saying, 

 that a point charged positively gave brushes into the air, whilst the same point charged 

 negatively gave a star. This is true only of bad conductors, or of metallic conduct- 

 ors charged intermittingly, or otherwise controlled by collateral induction. If me- 

 tallic points project freely into the air, the positive and negative light upon them 

 differ very little in appearance, and the difference can be observed only upon close 

 examination. 



1467. The effect varies exceedingly under different circumstances, but, as we must 

 set out from some position, may perhaps be stated thus : if a metallic wire with a 

 rounded termination in free air be used to produce the brushy discharge, then the 

 brushes obtained when the wire is charged negatively are very poor and small, by 

 comparison with those produced when the charge is positive. Or if a large metal 

 ball connected with the electrical machine be charged positively, and a fine uninsu- 

 lated point be gradually brought towards it, a star appears on the point when at a 

 considerable distance, which, though it becomes brighter, does not change its form of 

 a star until it is close up to the ball : whereas, if the ball be charged negatively, the 

 point at a considerable distance has a star on it as before ; but when brought nearer, 

 (in my case to the distance of 1 J inches,) a brush formed on it, extending to the ne- 

 gative ball ; and when still nearer, (at ^ of an inch distance,) the brush ceased, and 

 bright sparks passed. These variations, I believe, include the whole series of differ- 



