250 MR. E. M. WELL1SCH ON THE MOBILITIES OF THE 



theoretical research and afforded rich material for the application of the kinetic theory 

 of gases. The measurement of the mobilities of the two kinds of ions formed by the 

 action of Rontgen rays in a series of vapours seemed, therefore, to form a fitting and 

 necessary continuation of the corresponding determinations in the case of gases ; with 

 this object in view the present research was undertaken. 



2. Experimental Method. 



The method employed throughout was that devised by LANGEVIN,* who measured 

 the ionic mobilities in air over a range of pressures varying from 7 '5 to 143 cm. of 

 mercury. The principle of the method is as follows : 



Suppose we have two parallel plates A and B at a distance d apart, and let there 

 be a imiform electric field X in the region between the plates, the force on a positive 

 charge being from B to A. Let the gas comprised between them be ionised uniformly 

 by a single flash of very short duration from a Rontgen-ray bulb. After the lapse of 

 a certain time t from the passage of the Rontgen-ray discharge, let the field between 

 A and B be suddenly reversed in direction ; from this time until all the ions have 

 been removed by the field A will receive only negative electricity. 



Neglecting effects due to the recombination and diffusion of ions, the total quantity 

 of electricity received by the plate A from the time of the Rontgen-ray discharge 

 until all the ions are removed is given by 



where Q = quantity of electricity of one sign liberated between the plates by the 



flash of Rontgen rays, 



&j = velocity of the positive ion under unit electric intensity, 

 l~ 2 = corresponding velocity for the negative ion. 



By varying the time interval t, a series of values of Q is obtained ; the relation 

 between Q and t as given by the above equation is representable by a straight line, 

 but this equation has necessarily to be modified by the conditions : 



(i) Each of the quantities kiX.t and & 2 X is to be regarded as zero for negative 

 values of t. This implies Q = Q for t < 0. 



(ii) Each of the qiiantities kiKt and & 2 X cannot numerically be greater than d. 

 This implies Q = Q for values of t greater than the larger of the two quantities 

 and 



With these conditions the relation between Q and t is expressed by a curve of the 

 character given in fig. 1. 



* ' Ann. de Chim. et de Phys.,' VII., 28, p. 495 (1903). 



