[gray] rays of radium 149 



and Richardson* who found that 7 rays exponentially absorbed by 

 aluminium are ''hardened" by lead. We may generalize this result 

 by saying that rays which are absorbed exponentially by any material 

 are ''hardened" by substances of higher atomic weight. On the other 

 hand rays which are exponentially absorbed by lead would probably 

 be "softened" when passed through carbon or, 7 rays which are absorbed 

 exponentially by any substance are "softened" by substances of lower 

 atomic weight. Here the terms "softened" and "hardened" are used 

 to indicate a greater or smaller proportion of softer rays. These results 

 only apply to the cases where it is safe to say that as the 7 rays become 

 softer the absorption per unit mass increases with the atomic weight 

 of the absorber. 



Summary. 



1. When 7 rays are scattered, there is a change in quality, the 

 scattered rays being less penetrating the greater the angle of scattering. 

 The change is gradual and is small when the angle of scattering is small. 



2. A similar explanation must be given of the scattering of X 

 and 7 rays. There is probably very little direct scattering of X and 

 r rays. 



3. The quality and quantity of the scattered radiation is approx- 

 imately independent of the nature of the radiator. 



4. The softer the type of 7 rays, the greater the percentage 

 scattered per unit mass. 



5. A discussion is given of the bearing of the results on absorption 

 experiments and it is shown why an exponential law signifies heterogene- 

 ity of the rays. In the case of X rays, scattering is in most cases so small 

 compared with absorption that an exponential law probably signifies 

 homogeneity. 



♦Rutherford and Richardson, Phil. Mag., 25, p. 722, 1913. 



