Gases by the Motion of Negatively charged Ions. 209 



5. We may now proceed to investigate the relation connect- 

 in g «, p, aticf X, the temperature being constant. 



Let v be the velocity acquired by an ion in travelling 

 freely between two points differing in potential by P (volts). 

 Let e be the charge on an ion, and m its mass. 

 Then 



mv 2 <?xP /ox 



T~ = 1500 (3) 



Let n be the number of molecules in a c.c. of a gas at atmo- 

 spheric pressure (10 6 in C.G.S. units.), and temperature 20° C 

 (which was about the mean temperature at which the experi- 

 ments were made), and u the mean velocity of agitation of a 

 particle of mass m immersed in a gas at temperature 20°. 



The velocity u is given by the equation 



±mnu 2 = 10* (4) 



Hence from equations (3) and (4) we obtain 



J = 2nxpP 10 ^ =25p _ (5) 



since 



nxe=l'l 10 10 q.p* 



Hence the velocity acquired by an ion in travelling freely 

 between two points differing- in potential by 4 volts, is ten 

 times as great as its velocity of agitation at ordinary tem- 

 peratures. This result is independent of the mass of the 

 ion. 



Under the circumstances with which we are dealing, there 

 is a remarkable difference between the positive and negative 

 ions. The latter produce new ions when moving in a field 

 of force which is too small to maintain a continuous discharge. 

 It is therefore reasonable to suppose that the negative ions 

 with which we are dealing are the same as the negatively 

 charged particles which are given off when ultra-violet 

 light falls on a zinc plate. It has been shown by Professor 

 Thomson that the mass of these particles is ^-i-^ of the mass of 

 a molecule of hydrogen |. Becquerel and Curie have also 

 shown that the radiation emitted by radium is composed 

 of similar corpuscles. 



It seems probable in the present case also that the negative 

 ions are very small, and that the positive ions differ little 

 from ordinary molecules as far as their mass is concerned. 

 If we adopt this view, it is easy to see that the velocity of 



* John S. Townsend, Phil. Trans, vol. cxciii. 1899. 

 t J. J. Thomson, Phil. Mag. vol. xlviii. Dec. 1899. 



