242 Sir J. J. Thomson on 



conclude that the particles which suffer these large deflexions 

 must have been generated quite near to the cathode, so that 

 the production of these particles must be going on through 

 nearly the whole of the dark space. It is rather remarkable 

 that on some of the photographs all the lines are quite long, 

 and there are no indications of any particles which are pro- 

 duced exclusively at the junction of the dark space and the 

 negative glow. The very sharp line of demarcation, which 

 in general characterizes the junction of the dark space and 

 the negative glow, would lead us to expect that this region 

 would be associated with some form of chemical change 

 which does not exist in the rest of the dark space ; this, 

 however, seems not necessarily to be the case, as there are 

 several gases in which no short lines can be detected : one 

 of these is oxygen, and this is also one of the gases where 

 the contrast between the negative glow and the dark space is 

 specially well marked. 



With some few exceptions which will be considered later, 

 the brightest part of the parabolas corresponding to the 

 primary rays is the part which has experienced the least 

 electrostatic deflexion, these rays which produce this part of 

 the parabola are those which have the greatest kinetic energy 

 and would naturally produce a greater effect than the same 

 number of particles with less kinetic energy ; the decay in 

 the intensity as we recede from the head of the parabola 

 seems in some cases to be larger than can be accounted for 

 by the falling off in the kinetic energy, and seems to indicate 

 a considerable diminution in the number of the particles. 

 Now in the dark space ionization is produced (1) by the 

 cathode particles, these move aw T ay from the cathode, and 

 (2) by positively charged particles, these move towards the 

 cathode ; there are in addition other sources of ionization, 

 such as ultra-violet light and soft Rontgen radiation, which 

 we shall leave out of consideration for the present. The 

 positive particles starting from the negative glow will not at 

 its boundary have acquired any energy from the electric 

 field, and will therefore not be likely to produce ions in that 

 neighbourhood, while this is just the place where the cathode 

 particles are most numerous and most energetic. Now the 

 greater part of the particles forming what we have called 

 the primary rays come from the neighbourhood of the nega- 

 tive glow ; we conclude that they represent the ions produced 

 by the collision of rapidly moving cathode particles against 

 the molecules in the discharge-tube. On the other hand, 

 we should expect that the ions produced by the positive 

 particles would show the characteristics of what we have 



