36 



SCIENCE PROGRESS 



material have been stimulated to the exertion of some character- 

 istic effort. It is a matter of very great importance because it 

 makes all the difference in the world to our argument whether 

 we have to take into or may leave out of consideration the possi- 

 bility of any stores of energy belonging to the atom contributing 

 to one side or other of our energy equations. It seems to me to 

 be safe to deny the effect in this case because we should surely 

 find a real induced radioactivity, such as this would be, to 

 depend in quality on the nature of the atom, not on the nature 

 of the agent that merely pulls that trigger. The very term 



Fig. 14. 



This figure is intended to give an idea of the way in which /3 particles are deflected in going through 

 the atoms of the screen. A particle may go through numbers of atoms with hardly any result ; and then 

 in crossing some one atom it may go so close to a powerful centre of force within it that there is a violent 

 deflection. The circles stand for individual atoms. 



" induced radioactivity " was only introduced in the early 

 days of the new science under a false impression. It was 

 thought possible to make substances temporarily radioactive by 

 exposing them to the radium emanation ; it was soon discovered 

 that in reality a radioactive material was deposited on the 

 exposed substance and the latter was as idle as before. An 

 induced radioactivity must at least be characteristic of the sub- 

 stance in which it is induced. 



There is another reason against the existence of the effect. 

 If atoms were prompted to activity in this way we should expect 

 the new /3 rays they emitted to issue in all directions round 



