214 The Motions of Ions in Gases [Jan. 25, 



It was also found that after the transition stage from the large to 

 the small mass, the velocity of agitation of the electrons is much 

 larger than the velocity corresponding to that of a particle of equal 

 mass in thermal equilibrium with the surrounding gas. The latter 

 velocity is approximately 10^ centimetres per second, whereas the 

 velocity of agitation of the electrons is l'6xlO^ centimetres per 

 second in dry air, when X/P = ' 2. This velocity increases with the 

 force and is 10^ centimetres per second, when X/P = 50. 



The velocity of agitation of the electrons is easily deducted from 

 observations on the lateral diffusion of a stream of particles moving 

 through the gas under an electric force. The spreading of the stream 

 is independent of the pressure of the gas, and has a definite normal 

 value depending on the force when the kinetic energy of agitation of 

 the ions or electrons is equal to that of the surrounding molecules. 

 When the pressure is reduced below a certain value, which is propor- 

 tional to the force, the lateral diffusion becomes abnormally large, 

 which can only be due to an increase in' the velocity of agitation of 

 the electrons. 



This result* shows that the electrons tend to retain the velocity 

 they acquire under the electric force, and the principal effect of a 

 collision with a molecule is not to reduce the velocity of an electron, 

 but only to alter its direction of motion. This action continues until 

 a steady state is attained in which the motion of agitation is so 

 large that the loss of energy by collisions is equal to the energy 

 acquired under the electric force. 



[J. S. T.] 



Proc. Roy. Soc, 1908, A, 81, p. 464. 



