438 Mr. E. Rutherford on the Velocity and Rate 



ion in conduction under the Rontgen rays (10*4 cm. per sec.) 

 is over 36,000 times as great. 



From considerations based on the kinetic theory of gases 

 the velocity of a small charged body moving through the 

 gas under the influence of an electric field may be determined. 



Let e be the charge on the positive ion, 

 — e on the negative ; 

 mi m 2 the masses of the positive and negative ions ; 

 % electromotive intensity ; 

 k x k 2 the quotient of pressure by density for the positive 

 and negative ions. 



Then, assuming that the partial pressure of the dissociated 

 gas is small, the velocity u x of the positive ion is given by 



h^-^-Di, 



m x Ki 



where D x is the coefficient of interdiffusion of the positive 

 ions and the undissociated gas. (J. J. Thomson, Brit. Assoc. 

 Report, 1894, and Art. " Diffusion," Encyclopedia Brittanica.) 

 The velocity u 2 of the negative ion is given by 



i«s=-^- D 2 , 



where D 2 is the coefficient of interdiffusion of the negative 

 ions and the gas. 



The sum of the velocities of the ions is thus given by 



u = u l + u 2 



nil Ki m 2 k 2 



When the ions are of equal mass 



u— -^D. 



inK 



We have no means of determining D, the coefficient of 

 interdiffusion of the ions into the gas, nor the mass and 

 charge of the carrier. If, however, we assume that the ion 

 carries the same charge that it does in the electrolysis of 

 liquids, we can theoretically deduce the velocity with which 

 a molecule carrying the atomic charge would move through 

 the gas. D then becomes the coefficient of interdiffusion of 

 a gas into itself, and is given by the relation 



D= 1-5435^, 

 P 



