132 Prof. J. J. Thomson on the Velocity of 



the spark very much more rapidly than seems to be the case. 

 I am endeavouring by the aid of a rotating mirror to measure 

 the velocity of the discharge along a vacuum-tube 50 feet 

 long, and though the experiments are not yet completed, they 

 have gone far enough to show that the velocity through the 

 gas is comparable with that of light. The results of ex- 

 periments on the rate of propagation of electricity along 

 wires leads us to regard the conductor as merely guiding the 

 discharge, the correlation between the aether and the conductor 

 compelling the discharge to travel along the latter with the 

 velocity of light. 



The very rapid rate with which the electric discharge is 

 propagated through a rare gas compels us to admit that the 

 electricity is not carried by charged atoms moving with this 

 velocity. For if it were, then if the discharge were to take 

 place in air at atmospheric pressure between two parallel 

 plates one centimetre apart, charged to a potential difference 

 of approximately 30,000 volts, the kinetic energy which would 

 have to be communicated to the atoms to make them move 

 with this velocity would be greater than the original poten- 

 tial energy of the charged plates : assuming that the charge 

 on each atom is that deduced from electrolytic considerations. 



The evidence in favour of the view that the conduction of 

 electricity takes place by means of the atoms of the substance 

 through which the discharge passes appears to me to be so 

 strong, that I think it important to see whether this view of 

 the electric discharge cannot be reconciled with the very rapid 

 rate at which the electric discharge passes through a gas. 

 The following view of the behaviour of the gas seems to 

 explain both the rapid propagation of the discharge and some 

 of its most striking features, such as stratification. 



Before the electric field is intense enough to cause discharge, 

 the induction in the field polarizes the gas. We may regard 

 this polarization as being equivalent to the formation of chains 

 of molecules analogous to the " Grotthus chain" in electro- 

 lysis. As the intensity of the field increases, suppose the 

 molecules in one of these chains near an electrode, say the 

 negative, interchange their atoms ; and that it is not merely 

 those molecules which are next the electrode which split up, but 

 that the decomposition of the molecules extends along an appre- 

 ciable length of the chain. The positively electrified atoms 

 will cling to the negative electrode, and after a time, depend- 

 ing upon the number of free atoms, the distance between them, 

 and their mutual attractions, the chain will resume its original 

 molecular condition. This time may vary slightly from atom 

 to atom. Let us, however, for the sake of clearness, suppose 



