132 HUXLEY MEMORIAL LECTURES 



The range is laid out horizontally in centimetres. 

 The numbers of ions are laid out vertically. The 

 remarkable nature of the results will be at once 

 apparent. We should have expected that the ray 

 at the beginning of its path, when its velocity and 

 kinetic energy were greatest, would have been 

 more effective than towards the end of its range 

 when its energy had almost run out. But the 

 curve shows that it is just the other way. The 

 lagging ray, about to resign its ionising proper- 

 ties, becomes a much more efficient ioniser than 

 it was at first. The maximum efficiency is, how- 

 ever, in the case of a bundle of parallel rays, not 

 quite at the end of the range, but about half a 

 centimetre from it. The increase to the maxi- 

 mum is rapid, the fall from the maximum to noth- 

 ing is much more rapid. 



It can be shown that the ionisation effected 

 anywhere along the path of the ray is inversely 

 proportional to the velocity of the ray at that 

 point. But this evidently does not apply to the 

 last 5 or 10 mms. of the range where the rate of 

 ionisation and of the speed of the ray change 

 most rapidly. To what are the changing proper- 

 ties of the rays near the end o~ r ,.eir path to be 

 ascribed? It is only recently that this matter 

 has been elucidated. 



When the alpha ray has sufficiently slowed 

 down, its power of passing right through atoms, 

 without appreciably experiencing any effects from 

 them, diminishes. The opposing atoms begin to 



