184 THE REALITIES OF MODERN SCIENCE 



of the ions of some liquids.) In the transfer of elec- 

 tricity, therefore, between two plates, say A and B, of 

 which B is positive and A negative, it makes no dif- 

 ference whether the transfer is an actual one of electrons 

 moving from A to B, or is accomplished by the motion 

 of certificates of electronic indebtedness from B to A, 

 or in part by each method. 



In the case of conduction through a vacuum, or 

 through any conductor if ionization does not occur, the 

 transfer is entirely the result of motions of the electrons. 

 In the case of gases the transfer is partly by individual 

 electrons, partly by the certificates or positive ions, 

 and partly by electrons which are combined with mo- 

 lecular masses. The latter are truly carriers in the 

 sense in which a horse is a carrier of his rider, and they 

 move in the direction in which the added electrons 

 would move individually. The word " carrier" is, 

 however, generally applied to both positive and neg- 

 ative ions. In the case of liquids, as we have noted, 

 the transfer of electricity is brought about entirely by 

 the motion of these carriers. 



Of course the carriers may not share the burden of 

 transfer equally, one kind being swifter than the other. 

 In other words, the positive and negative carriers of an 

 electrolyte may have what is called different " mobil- 

 ities." The same is true of the carriers of a gas. A 

 study of the mobility of the ions formed from various 

 gases and also of their rates of diffusion yielded some 

 of the earlier determinations of the value of the charge 

 carried by the ions and hence of the amount of elec- 

 tricity corresponding to an electron. 



So far we have considered only the mechanism 



