Cathode Rays and certain Chemical Effects. 231 



N 

 a, as defined, is -=; and in these experiments I was 6*5 cm., 



whence a, for a velocity of 3*7 x 10 9 cm. per sec, =2*1. 

 That is, 1 cathode-ray corpuscle moving with a velocity of 

 3'7 x 10 9 cm. per sec. through nitrogen under a pressure of 

 1 mm. of mercury involves the absolute formation of 2*1 pairs 

 of ions. 



Other values of a, corresponding to other velocities of 

 rays, are collected in column 4 of Table IV. 



Conformity with theory of results. 



Sir J. J. Thomson * has recently considered from a theo- 

 retical point of view the problem of ionization in gases by 

 moving particles. 



Working on the assumptions : — 



(1) That the corpuscles in the gas atom are attracted or 

 repelled by cathode rays as these pass through the 

 atom and so acquire kinetic energy ; 



and (2) that if in any one instance this energy exceeds 

 a certain value — the value necessary to ionize the 

 atom — a corpuscle escapes, leaving us with a 

 positively charged atom and a free corpuscle ; 



and considering the special case of one cathode-ray corpuscle 

 approaching an atom, he arrives at the relation 



^"VQVT 2 "' 



where d is the perpendicular distance from a corpuscle — 

 relatively at rest in the atom — on the direction of propa- 

 gation of the first corpuscle. 



T is the kinetic energy of the first corpuscle, and Q is the 

 energy transferred to the second. Hence, if a corpuscle 

 requires an energy W to escape from the atom, then, for 

 ionization to occur, 



d must be > a / ( \\— . 



V VW /T 2 



If n is the number of corpuscles in the atoms in unit volume 

 of gas, the number of collisions for which d is not greater 



* " Ionization by moving particles," Phil. Mag. April 1912. 



