20 



SCIENCE PROGRESS 



Then it ceases to grow and indeed begins to diminish as the 

 radium comes closer still : see fig. 3. 



Let us now try to understand what this experiment must 

 mean. In the first place a radiation which extends to a certain 

 distance from the parent source and then ceases with extreme 

 abruptness cannot be a wave motion. Wave radiation fades 

 away gradually as it proceeds from its source and spreads over 

 greater and greater surfaces ; it never ceases abruptly while 

 passing through a uniform medium. Only projectiles can have 

 a range which can be measured with, it is found, an accuracy of 



lorM&afiorx. 



Fig. 3. 



much less than 1 per cent. In the second place these projectiles 

 must be slowing down as they come to the end of their range ; 

 they could not keep up the same velocity all the way and then 

 all stop at the same moment. It cannot be a case of the 

 accidental and chance stopping of the particles in full career. 

 Since, therefore, the curve bends back from q to r, the effect of 

 the projectile measured by our apparatus is greater the slower 

 it goes. The slope of the upper part of the curve pq might be 

 explained by supposing that the projectiles are fired off with 

 slightly different speeds, the greater speed giving the longer 

 range ; but later experiments show this is not the case and that 



