Gases by the Motion of Negatively charged Ions. 223 



Taking a — 34*1 it will be found that the following numbers 

 .ire proportional to IS when the distances I given in the above 

 table are expressed in centimetres : — 



491, 117, 48-2, 31-9, 

 which agree with Stoletow's values of the current when the 

 pressure is 7*7 millims. 



/_ E\ . 



The electric force f X = — J used in these experiments was 



2186 volts per centimetre; and although no forces exceeding 

 800 volts per centimetre were used in my experiments, never- 

 theless the value of a calculated from the curve given in 

 Section 7 is in good agreement with the above number. 



Thus Xj = -=-=; = 284, the corresponding value of a, 



deduced from the curve is 4*8, so that a.—- 36*9. 



In order therefore to explain the variations in current 

 which occur for different distances between the plates when 

 X is constant, it is sufficient to assume that the number of 

 ions given off by the action of the light is independent of the 

 distance between the plates, and that other ions are produced 

 by collisions *. 



For pressures lower than 70 millims. it would appear from 

 Stoletow's experiments that the number of ions generated by 

 the action of the light is practically constant, but at higher 

 pressures the number of ions given off by the plates seems to 

 diminish. Thus the conductivity at 750 millims. is about 

 half the conductivity at 69 millims. when the force is 2186 

 volts per centimetre ; and since the current is practicallv 

 independent of the distance between the plates, this effect 

 cannot be explained by the collision theory. 



The following considerations, however, show that at a 

 pressure of 750 millimetres, the force of 2186 volts per centi- 

 metre is not sufficient to bring all the ions that are generated 

 at the surface into the gas away from the plate. When the 

 ions are set free by the light at the surface of the plate, they 

 tend to move in all directions with their velocities of agita- 

 tion; and unless the force acting on them is large enough to 

 give them a velocity away from the plate greater than the 

 velocity of agitation, some of them may. after colliding with 

 molecules, come into contact with the plate and be discharged. 



The mean free path of an ion in air at atmospheric pressure 



* These experiments cannot be explained by the theory of surface- 

 layers, since tne density of ionization is largest at the points furthest 

 from the plate. 



