134 HUXLEY MEMORIAL LECTURES 



effects of a bundle of rays are being observed. 

 The explanation does not suggest that the ion- 

 ising power of any one ray is actually diminished 

 before it finally ceases to be an alpha ray. 



The full line in Fig. i gives the ionisation 

 curve which it may be expected would be struck 

 out by a single alpha ray. In it the ionisation 

 goes on increasing till it abruptly ceases alto- 

 gether, with the entire loss of the initial kinetic 

 energy of the particle. 



A highly remarkable fact was found out by 

 Bragg. The effect of the atom traversed by the 

 ray to check the velocity of the ray is independent 

 of the physical and chemical condition of the 

 atom. He measured the " stopping power " of 

 a medium by the distance the ray can penetrate 

 into it compared with the distance to which it can 

 penetrate in air. The less the ratio the greater 

 the stopping power. The stopping power of a 

 substance is proportional to the square root of its 

 atomic weight. The stopping power of an atom 

 is not altered if it is in chemical union with 

 another atom. The atomic weight is the one 

 quality of importance. The physical state, 

 whether the element is in the solid, liquid or 

 gaseous state, is unimportant. And when we 

 deal with molecules the stopping power is simply 

 proportional to the sum of the square roots of the 

 atomic weights of the atoms entering into the 

 molecule. This is the " additive law," and it 

 obviously enables us to calculate what the range 



