io8 PHYSICAL SCIENCE 



or chlorine constitutes a true natural unit of 

 electricity. No smaller quantity seems capable of 

 existing. As Helmholtz has insisted, electricity, 

 like matter, is not infinitely divisible ; it possesses 

 an atomic structure. 



In the year 1855 Hittorf examined the changes 

 In the concentration of a solution which occur on 

 the passage of an electric current, and explained 

 them by supposing that the two ions moved at 

 unequal rates. It is evident that more salt will 

 be taken from that end of the solution from which 

 comes the more mobile ion, and, on the assump- 

 tion that this is the only cause at work, Hittorf 

 calculated the ratio between the velocities of the 

 two ions in many cases. 



The next great step was made by Kohlrausch, 

 in 1873. The conductivity of a solution is 

 measured by the total quantity of electricity 

 which passes through the solution per second 

 under the action of a given electric force ; and, 

 since the current is carried by the motion of 

 charged ions, the conductivity must depend on 

 the number of the ions, that Is, on the con- 

 centration of the solution, and on the velocity 

 with which the opposite ions move through the 

 liquid. Thus, by measuring the conductivity, 

 the velocities of the ions under a given electric 

 force can be calculated. 



So far the movement of the Ions was visible 

 to the mind's eye only. Their passage through 

 a solution seemed necessary to explain the facts, 

 and, in an indirect way, their velocities could be 

 calculated, but no direct evidence of the reality 

 of these hypothetical phenomena was forthcoming. 

 However, In the year 1886 Sir Oliver Lodge, 



