Experiments on Positive Rays. 217 



rays are the only ones that can be relied on. The tests for 

 these primary lines are (1) that they must be parabolic, 

 and (2) that there should be a rapid increase in intensity in 

 all these lines at places all situated on a vertical line (if the 

 electrostatic deflexion is horizontal). The first condition is 

 theoretically sufficient except with very special distributions 

 of the electric and magnetic forces ; but I have found that in 

 practice the secondaries are not infrequently very approxi- 

 mately parabolic when they are near to their primaries, so 

 that unless a very high degree of accuracy were obtained in 

 the measurement of these lines, their secondary nature might 

 easily escape detection if this were the only test applied. 



The secondaries we are now considering arise from the 

 collision of neutral particles with corpuscles ; they cannot 

 be produced by collision of the particles with other particles 

 having a mass comparable with that of the neutral particles, 

 for such a collision Would deflect the path of the particles^ 

 and the lines in the photographs would become much more 

 diffuse than is actually the case. The collisions which ionize 

 the particles are between moving particles and corpuscles 

 which are approximately at rest ; the effects of the collision 

 will, however, only depend on the relative motion of the 

 particle and corpuscle, and will be the same as if the particle 

 were at rest and the corpuscle moving with the velocity of the 

 particle. Now in order that a moving corpuscle may ionize 

 an atom or molecule against which it strikes, the velocity of 

 the corpuscle must exceed a definite limit which probably 

 depends to some extent on the nature of the atom or molecule, 

 Let us call this limit for a particular kind of atom V ; then 

 in order that an uncharged atom of this kind should be 

 ionized when it strikes a corpuscle at rest, it must move 

 with a velocity greater than V : hence all the secondary 

 rays formed by this atom must move with a velocity greater 

 than V. There is thus an inferior limit to the velocity of the 

 secondary rays due to a particular kind of atom or molecule. 

 There is, however, a superior limit to the velocity of these 

 rays as well as an inferior one. For these rays are due to 

 particles which move with great velocity and which were 

 uncharged when they entered the electric and magnetic 

 fields ; they must, therefore, have been positively charged 

 before passing through the cathode so as to be able to 

 acquire this velocity, and then have become neutralized by 

 combining with a negative corpuscle. The rapidly moving 

 positively electrified particle must have attracted to itself 

 and held bound a negative corpuscle ; it cannot, however, do 



