DISRUPTIVE DISCHARGE — DARK DISCHARGE IN AIR. 139 



cause. Further, there is every likelihood that the dark parts which occur in feeble 

 sparks are also connected with these phenomena*. To understand them would be 

 very important, for it is quite clear that in many of the experiments, indeed in all that 

 I have quoted, discharge is taking place across the dark part of the dielectric to an 

 extent quite equal to what occurs in the luminous part. This difference in the result 

 would seem to imply a distinction in the modes by which the two electric forces are 

 brought into equilibrium in thie respective parts ; and looking upon all the pheno- 

 mena as giving additional proofs, that it is to the condition of the particles of the 

 dielectric we must refer for the principles of induction and discharge, so it would 

 be of great importance if we could know accurately in what the difference of action 

 in the dark and the luminous parts consisted. 



1548. The dark discharge through air (1552.), which in the case mentioned is very 

 evident (1544.), leads to the inquiry, whether the particles of air are generally ca- 

 pable of effecting discharge from one to another without becoming luminous ; and 

 the inquiry is important, because it is connected with that degree of tension which 

 is necessary to originate discharge (1368. 1370.). Discharge between air and con- 

 ductors without luminous appearances ai'e very common ; and non-luminous dis- 

 charges by carrying currents of air and other fluids (1562. 1595.) are also common 

 enough : but these are not cases in point, for they are not discharges between insu- 

 lating particles. 



1549. An arrangement was made for discharge between two balls (1485.) fig. 15., 

 but, in place of connecting the inducteous ball directly with the discharging train, 

 it was put in communication with the inside coating of a Leyden jar, and the dis- 

 charging train with the outside coating. Then working the machine, it was found 

 that whenever sonorous and luminous discharge occurred at the balls A B, the jar 

 became charged ; but that when these did not occur, the jar acquired no charge : 

 and such was the case when small rounded terminations were used in place of the 

 balls, and also in whatever manner they were arranged. Under these circumstances, 

 therefore, discharge even between the air and conductors was always luminous. 



1550. But in other cases, the phenomena are such as to make it almost certain, 

 that dark discharge can take place across air. If the rounded end of a metal rod, 

 0*15 of an inch in diameter, be made to give a good negative brush, the approach of a 

 smaller end or a blunt point opposite to it will, at a certain distance, cause a dimi- 

 nution of the brush, and a glow will appear on the positive inducteous wire, accom- 

 panied by a current of air passing from it. Now, as the air is being charged both at 

 the positive and negative surfaces, it seems a reasonable conclusion, that the charged 

 portions meet somewhere in the interval, and there discharge to each other, without 

 producing any luminous phenomena. It is possible, however, that the air electrified 

 positively at the glowing end may travel on towards the negative surface, and actually 

 form that atmosphere into which the visible negative brushes dart, in which case 



* See Professor Johnson's experiments. Silliman's Journal, xxv. p. 57. 



T 2 



