22S Sir J. J. Thomson : Furtli 



'tner 



atoms. Again, when the tube contained pure oxygen the 

 line on the plate due to the oxygen atom was brighter than 

 that due to the oxygen molecule ; the Faraday cylinder 

 showed, however, that there were many more molecules than 

 atoms carrying the discharge. In all cases, it is the lighter 

 atoms which produce an effect on the photographic plate 

 greater than is warranted by their number. 1 think the 

 explanation is that these particles only penetrate a very 

 short distance into the film, and so can affect only a very 

 small amount of silver, the faster particles penetrate more 

 deeply into the film than the slower ones ; there is thus 

 more silver at the disposal of the quick particles ; in other 

 words, the film is more sensitive to quick particles than to 

 slow ones. 



The discrepancy between the number of rays of a particular 

 kind and the intensity of the effects they produce is even 

 more marked in the phosphorescence they excite in a 

 willemite screen than the impression they produce on a 

 photographic plate. With a willemite screen I have found 

 that the brightness of the line due to the hydrogen atom 

 may be very much greater than that due to the oxygen 

 molecule when the Faraday cylinder method showed that 

 there were 300 times as many positively charged molecules 

 of oxygen as there were atoms of hydrogen. I think we may 

 get an explanation of this, if we consider the mechanism by 

 which ionization may be imagined to be produced. When a 

 molecule is ionized by a negative corpuscle, the corpuscle 

 has to possess a finite amount of energy. Let us suppose for 

 the sake of clearness that the energy required is that due to 

 a fall of the atomic charge through 10 volts; this corresponds 

 to a velocity of the corpuscle of 1*9 x 10 8 cm./sec, and we 

 may suppose that to enable a corpuscle to escape unattended 

 from a neutral atom, it must have a velocity of this order. 

 If this velocity is to be communicated to it by collision with 

 another body, that body must be moving with a velocity 

 comparable with that acquired by the corpuscle. 



Now in the case of the positive rays it is only in general the 

 lightest atoms and molecules, the atom and molecule of hydro- 

 gen and the atom of helium, which possess velocities as high 

 as this, and it is therefore only these rays which can produce 

 ionization of the type of that produced by cathode particles. 

 The heavier particles, though possessing far greater energy 

 than that due to a fall through 10 volts, are moving much 

 too slowly to impart by collision to a particle a velocity as 

 great as 1*9 x 10~ 8 cm./sec. By sending these heavy particles 



