96 ELECTRONS AND IONS [CH. ix. 



charge by making it concentrated enough. The 

 energy at a given speed of motion will be proportional 

 both to the quantity and the potential, and the latter 

 can be made as great as we please by making the size 

 of the body possessing the charge extremely small. 



It is the intense region of force close to the wire or 

 close to the charged particle which is the effective 

 region ; and so, as stated, a knowledge of the mass or 

 kinetic energy at a given speed suffices, on a purely 

 electric theory of matter, to determine the size of the 

 electron constituents of which it is hypothetically 

 composed. For whether there be any intrinsically 

 material inertia or not, there certainly is an electrical 

 inertia. The cause of it in the electrical case is 

 known : it is due to the reaction of the electric and 

 magnetic fields during acceleration periods, and is 

 denominated self-induction. 



Quite possibly there is no other kind. Quite 

 possibly that which we observe as the inertia of 

 ordinary matter is simply the electric inertia, or self- 

 induction, of an immense number of ionic charges, or 

 electric atoms, or electrons. 



This is by far the most interesting hypothesis, 

 because it enables us to progress, and is definite. 

 The admixture of properties partly explained, viz. 

 the electrical, partly unexplained, viz. the material 

 lands us nowhere ; unless, by some only partially 

 imagined means, we were able to estimate how much 

 of the total appertains to each ingredient. 



The mass of a corpuscle has been measured as 

 something akin to j^Vo f an atom of hydrogen, and 

 its charge asJ'lO" 10 electrostatic unit. Now this 

 amount of electricity will have that amount of inertia 

 if it exists on a sphere of radius 10~ 13 centimetre, 

 but not otherwise. Consequently we may assume 



