236 PHYSICAL SCIENCE 



aether, perhaps partly owing to the great diffi- 

 culties of interpretation which we shall describe 

 presently. 



Nevertheless, from the theory of radiation, as 

 well as from Thomson's experiments, was reached 

 the conception of an electron theory of matter. 

 Within a few years, experimental confirmations 

 of the fundamental conceptions of that theory 

 gave it a firmer position than could have been 

 hoped at the time the theory was formulated. 



The property of mass, the most fundamental 

 property of matter for dynamical science, is 

 explained by the electron theory as an effect of 

 electricity in motion. Forasmuch as a moving 

 charge carries its lines of electric force with it, 

 it possesses something analogous to inertia in 

 virtue of its motion. The quantitative value of 

 this effect has been calculated by Thomson, 

 Heaviside, and Searle. Definite experimental 

 evidence was first given by Kaufmann, who found 

 that the ratio ejin of the charge to the mass for 

 the corpuscles ejected by radium diminishes as 

 their velocity increases. The charge is almost 

 certainly constant, and thus the mass must 

 increase with the velocity. Theory shows that, 

 for a slowly moving corpuscle, the electric inertia 

 outside a small sphere of radius a, surrounding 

 the electrified particle, does not depend on the 

 velocity, and is measured by 2^73^ where e is the 

 electric charge on the particle. But when the 

 velocity of light is approached, this electric mass 

 grows very rapidly ; and, on the assumption that 

 the whole of the mass is electrical, Thomson 

 calculated the ratio of the mass of a corpuscle 

 moving with different speeds to the mass of a 



