Theories of Electricity 147 



plate exposed to the action of ultraviolet light. In these 

 cases an external agent like the electric discharge, heat 

 or light is required to set free these electrons from mat- 

 ter, but a still further advance was made when it was 

 found that electrons of exactly the same mass were 

 emitted from radium and other radioactive substances 

 without any external stimulus. The radioactive bodies 

 are continuously engaged in the apparently congenial 

 task of spontaneously hurling electrons from their mass 

 with velocities much greater than can be impressed on 

 the electrons set free in a vacuum tube. The discovery 

 of Zeeman, coupled with the electronic theory of Lorentz, 

 still further broadened the field of application of elec- 

 tronic theories. The verification of Lorentz's predic- 

 tions by Zeeman showed that the light waves must arise 

 from a vibrating or rotating electrically charged par- 

 ticle and the comparison of theory with experiment 

 showed that the particle carried a negative charge and 

 was of the same small mass as the electron. The elec- 

 tron was thus shown to be a constituent of all matter, 

 and it was seen that the phenomenon of light arose from 

 the rapid movement of electrons within the atom. 



We have seen that, following the theories of Maxwell, 

 attention was directed from the current in the wire to 

 the medium surrounding it. In these developments, 

 however, scientific attention was again concentrated on 

 the actual nature of the carriers of electricity. The dis- 



