in the Discharge of Electricity through Gases. 279 



in which there are no external sources available for the pro- 

 duction of ions, so that these must in some way or another 

 arise from the action of the field itself. There are many 

 difficulties in the way of supposing that single molecules are 

 torn asunder, by the electric force in the field pulling the 

 positive ion one way and the negative ion in the opposite 

 way. To mention only one of these difficulties, the electric force 

 necessary to produce discharge is almost vanishingly small 

 compared with the force due to an atomic charge acting at an 

 atomic distance. In my ' Recent Researches ' I suggested that 

 under the action of the electric field some of the molecules 

 formed long narrow aggregations, just as smoke and dust do 

 under similar circumstances : these aggregations, being bodies 

 of higher specific inductive capacity than their surroundings, 

 will act on the lines of force like long and pointed conductors, 

 and will therefore produce a great concentration of the lines 

 of force on the ends of the ao-oreoation, so that the maximum 

 electric force in the field will be enormously greater than the 

 average force, which is all we can measure. Since that was 

 written, many investigations have been made which have 

 proved that where electrified particles move through a gas 

 ions are produced under certain circumstances, at any rate if 

 the particle is negatively electrified. Thus Lenard showed 

 that his rays make a gas through which they pass a conductor 

 of electricity. Lenard rays are negatively electrified particles, 

 and on our theory the conductivity of the gas is due to the 

 production of ions. Again, I showed (Phil. Mag. Oct. 1897) 

 that the passage of cathode rays through a gas ionized it. 

 Thus we have evidence that in some cases at any rate the 

 motion of the corpuscles through a gas can ionize it. The 

 view that the motion of the ions through a gas can ionize a 

 gas is supported by the observation of Stoletow {Journal tie 

 Physique, ix. p. 468) that under the same electric force the 

 current between two plates, one of which is illuminated by 

 ultra-violet light, increases with the distance between the 

 plates, the increase being most marked when the pressure is 

 low. This is what we should expect if the negative ions, 

 starting from the illuminated plate, produced in the passage 

 through the gas fresh ions. The view which I wish to bring- 

 forward in this paper, is that the ionization in the ordinary 

 cases of discharge through gases is produced by the motion 

 through the gas of ions or corpuscles already present in the 

 gas. These ions or corpuscles under the action of the electric 

 field acquire velocity and kinetic energy ; and when this 

 velocity or energy reaches a definite value, which need not 

 be the same for the positive as the negative ion, these ions 

 or corpuscles are able, by their collision with the surrounding 



