lONIZATION BY NEGATIVE IONS 37 



much greater than the velocity of agitation that it would 

 have in a gas at 15 C. 



The velocity acquired by a negative ion under the 

 action of the potential difference V may be also compared 

 with the velocity of the (3 particles emitted by radio- 

 active bodies, or with that of the cathode rays. In the 

 former case some of the particles travel with a velocity 

 of 2*85 X 10 10 centimetres per second, and in the latter 

 case the velocity is much smaller, and depends on the 

 circumstances under which they are generated. The ratio 

 of the charge e to the mass i*. of the particle has been 

 found by Simon 1 to be 1*86 XlO 7 for cathode particles 

 travelling with an average speed of 7 X 10 9 centimetres 

 per second, e being expressed in electro-magnetic units. 



The potential required to impart a velocity of 7 X 10 9 

 centimetres per second to a particle for which the ratio 



p 



'- has this value is 13,000 volts, which is large compared 



with the voltages required to generate ions by collisions. 

 Taking the latter to be of the order of 20 volts and the 



p 



ratio -- to have the above value for negative ions 



generated by collisions in gases, the velocity they require 

 to generate others by collisions is of the order of 

 2'7 X 10 8 centimetres per second. 



1 Simon, Wied. Annal., p. 589, 1899. 



