166 SUMMARY OF ELECTRON THEORY [CH. xvm. 



we are now dealing with the extraordinary variety 

 known as radio-activity. 



If radiation goes on at the expense of the 

 internal energy of an atom, as it must if the atom 

 contains revolving electrons, still more if, as on 

 the electric theory of matter, it is wholly composed 

 of electric charges, it becomes necessary to ask 

 further : Why are not all atoms temporary and 

 unstable ? Why are they not all liable to internal 

 catastrophe and disruption, akin to earthquakes and 

 volcanoes? Why do they not all exhibit the phe- 

 nomena of radio-activity ? 



The whole subject of radio-activity is a large 

 one, upon which I do not propose to enter at any 

 length here. Suffice it to realise that any difficulty 

 of explanation, in connection with it, is not the fact 

 itself, but rather the question why it is not more 

 notorious. 



However, so far as that most striking and interest- 

 ing phenomenon the excessive photographic and 

 electric radio-activity of certain rare substances is 

 concerned, it has been already hinted that the greater 

 part of that does not consist so much in the emission 

 of radiation proper whether in the form of pulses of 

 X-rays or any other form as in the flinging off of 

 particles ; sometimes negatively charged particles or 

 electrons, sometimes positive ions. And these ex- 

 pelled particles, when they strike a photographic plate, 

 appear to generate by the concussion electric waves 

 which affect the silver salt. The faint photographic 

 influence of ordinary substances, observed by Dr. W. 

 H. Russell, seemed to suggest that incipient power of 

 this kind is not limited to bodies with heavy atoms, 

 like Uranium, Radium, Polonium, etc., as described 

 by Becquerel and the Curies, though these substances 



