CONDUCTION THROUGH GASES 149 



been observed with positive charges ; positive ions 

 are found to have masses equal to those of some 

 chemical atoms. The facts may provisionally be 

 explained by the hypothesis that the corpuscle con- 

 stitutes the isolated negative unit of electricity. 



Now the existence of electric units as a basis 

 of matter had been suggested already by Lorentz 

 and Larmor. The light and radiant heat emitted 

 by incandescent substances are electro-magnetic 

 waves, and must therefore arise from the vibration 

 of electric charges. The periods of vibration are 

 too quick to be due to the motion of atoms as 

 wholes, and we must therefore look within the 

 atom for the source of radiation. Hence it again 

 follows that atoms must be complex structures 

 with more minute internal parts containing 

 electric charges. Those parts themselves have 

 been pictured as electric units and given the 

 name of electrons — a word invented by Stoney. 

 They may be identifiedwith Thomson's corpuscles, 

 and these are indeed now generally called electrons. 



The ordinary phenomena of electrification may 

 be described in these new terms. 



An atom of ordinary matter, with one electron 

 beyond its proper number, is an atom negatively 

 electrified ; an atom with the electron detached 

 from it is an atom positively electrified. These 

 charged atoms act as ions, negative and positive 

 respectively, in accordance with the usual con- 

 vention about signs. 



A moving electrified body acts like an 

 electric current, and therefore must be associated 

 with electro-magnetic energy and electro-magnetic 

 momentum in the surrounding dielectric medium. 

 To change the velocity, therefore, requires the ex- 



