RADIOACTIVITY 35 



electrons rebound from a plate on which they fall and that within 

 certain limits the percentage returned increases with the speed. 



It is interesting to observe that the treatment of ft particles 

 by atoms of various kinds through which they pass does not 

 appear to be quite the same. Heavy atoms cause far more 

 deflections in comparison with the amount of energy abstracted. 

 Perhaps the centres of force within them are more powerful or 

 more concentated. Rutherford has lately put forward this view 

 in consequence of certain experiments on the scattering of the 

 a particles. Thus the track of the ft particle through lead is 

 more broken than in aluminium and stretches to a smaller 

 distance from its origin. If we might take fig. 12 to represent 

 the track in aluminium, then in comparison fig. 13 would 

 represent a track in lead ; and this is why a lead screen is so 

 much more effective in stopping ft particles than an aluminium 

 screen of equal weight. 



There is one very interesting point in connection with the 



/ v ■ V 



Fig. 12. Fig. 13. 



ft ray which we may consider now. It may be put as a question : 

 Does the electron as it moves through an atom prompt the 

 emission of another electron by or from that atom with a speed 

 in considerable excess of the critical speed, so that there are two 

 rays where there was but one before ? And in particular, if it 

 does, whence comes the energy of the new ray ? 



If the atom can be stimulated to the expenditure of its own 

 energy on the emission of a new ft particle, we have a true 

 secondary radiation. It was very commonly held at one time 

 that such an effect existed but the trend of experiment has been 

 very much against it ; no one seems to maintain this now. The 

 ft rays which emerge from a substance on which a primary 

 stream of ft rays falls were at one time supposed to be a 

 radiation of this sort. But they are found to resemble the 

 primary rays closely, being in general just a little slower; 

 the nature of the reflecting material seems to be of little 

 influence except in indirect ways which would naturally be 

 expected. There is nothing to show that the atoms of the 



