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



the discharge might happen either at one or the other ; and that the least alteration 

 of either would immediately cause one to predominate constantly over the other. 

 But that under common circumstances is not the case. With air in the receiver, the 

 variation amounted to 0*2 of an inch nearly on the smaller interval of 06, and with 

 muriatic acid gas, the variation was above 0*4 on the smaller interval of 09. Why is 

 it that when a fixed interval (the one in the receiver) will pass a spark that cannot 

 go across 0*6 of air at one time, it will immediately after, and apparently under ex- 

 actly similar circumstances, not pass a spark that can go across 0*8 of air? 



1391. It is probable that part of this variation will be traced to particles of dust 

 in the air drawn into and about the circuit. I believe also that part depends upon a 

 variable charged condition of the surface of the glass vessel a. That the whole of the 

 effect is not traceable to the influence of circumstances in the vessel a, may be de- 

 duced from the fact, that when sparks occur between balls in free air they frequently 

 are not straight, and often pass otherwise than by the shortest distance. These va- 

 riations in air itself, and at different parts of the very same balls, show the presence 

 and influence of circumstances which are calculated to produce effects of the kind 

 now under consideration. 



] 392. When a spark had passed at either interval, then, generally, more tended to 

 appear at the same interval, as if a preparation had been made for the passing of the 

 latter sparks. So also on continuing to work the machine quickly the sparks gene- 

 rally followed at the same place. This effect is probably due in part to the warmth 

 of the air heated by the preceding spark, in part to dust, and I suspect in part to 

 something unperceived as yet in the circumstances of discharge. 



1393. A very remarkable difference, which is constant in its direction, occurs when 

 the electricity communicated to the balls s and S is changed from positive to nega- 

 tive, or in the contrary direction. It is that the range of variation is always greater 

 when the small balls are positive than when they are negative. This is exhibited in 

 the following Table, drawn from the former experiments. 



Pos. 

 . 019 



. 19 



. 013 



In Air the range was 

 Oxygen . . 

 Nitrogen 

 Hydrogen . 

 Carbonic acid 

 defiant gas 



Coal gas 



Muriatic acid 

 I have no doubt these numbers requ 



Neg. 

 009 



014 

 016 

 0-22 

 0-24 

 0-43 



002 

 Oil 

 005 

 002 

 008 

 012 

 0-08 



re considerable correction, but the general re- 

 sult is striking, and the differences in several cases very great. 



1394. Though, in consequence of the variation of the striking distance (1386.), the 

 interval in air fails to be a measure, as yet, of the insulating or resisting power of the 



