246 Sir J. J. Thomson on 



Thus for the secondary radiation due to the dissociation of 

 the uncharged rays, 



V>wxnxl'7x 10 3 , 



V<2-3r Xl0\ 

 b 



Thus the velocity of each kind o£ secondary ray must be 

 between certain limits which do not depend on the potential 

 difference between the electrodes in the discharge-tube. If 

 these limits are very close together the velocity of the 

 secondary ray will be very nearly constant. I have found 

 that this is the case for the secondary rays corresponding 

 to the hydrogen atom. 



The curves on the photographic plate which pass through 

 the origin may arise either from the dissociation of the 

 uncharged Canalstrahlen- — the change from an uncharged 

 particle to a positive ray, or from the reverse process, the 

 change of a positive ray while in the electric and magnetic 

 fields into an uncharged particle by the coalescence with 

 it of a negatively charged corpuscle ; a method of distin- 

 guishing between these cases is given on p. 233. For such a 

 coalescence to take place, however, the velocity of the positive 

 ray must be below a certain value ; if the velocity is greater 

 than this the ray behaves like a primary one. When the 

 difference of potential between the electrodes in the dis- 

 charge-tube is much greater than is necessary to produce 

 this velocity any secondary ray must have fallen through 

 only a fraction of the potential difference in the tube, and must 

 therefore have been produced near the cathode. Now it is 

 just in this neighbourhood that the positive ions in the dark 

 space have their greatest velocity and are most likely to 

 produce fresh iong by collision. Thus it is probable that 

 among the ions in the secondary rays there are some which 

 have been produced by the collision of positive ions with the 

 molecules of the gas in the dark space, while the primary 

 rays which have fallen through the whole potential difference 

 have been produced by the collision of cathode particles 

 with these molecules. 



There are a few, but only a few, ions which occur both as 

 primary and secondary ; the positive atom of hydrogen with 

 one charge is the most conspicuous example of this class ; 

 others are the atoms of mercury and oxygen and the molecule 

 of hydrogen. In most cases, however, the ions are quite 

 distinct. Or looking at the list of ions due to secondary 

 radiation given on page 239 it will be seen that, with the 



