Rays emitted by Substances exposed to y Rays. 641 



zinc, or tin, and the supposition made at the beginning of this 

 paragraph therefore true. 



It will be seen that the differences (N m — Al) are least 

 affected when the thin screen of lead is replaced by a screen 

 of tin. This is probably due to the fact that the atomic 

 weight of tin lies between that of lead and zinc, and there- 

 fore partakes to a greater extent of the properties of lead 

 than the substances zinc, copper, and iron. The effect of a 

 screen of lead, it will be remembered, is to increase these 

 differences. 



It will also be seen that the difference in the radiating 

 power of aluminium and a substance, when the thin screen 

 of lead is replaced by a screen of this substance, is in nearly 

 all cases decreased to a greater extent than any of the other 

 differences. The magnitude of the decrease of the differences 

 becomes smaller as we pass progressively from this difference 

 to the neighbouring differences. Now, if the radiation from 

 a substance A is decreased in a greater proportion than that 

 from any one of a number of other substances B m , when the 

 thin lead screen is replaced by a screen of the substance A, 

 the value of (k a — 1) of (A— Al) will decrease more than its 

 value for any of the differences (B m — Al). Therefore we 

 conclude that the rays that are most efficient in producing 

 secondary radiation from a substance are most easily absorbed 

 by a screen of the same substance. 



A further examination of the differences obtained with 

 the screens of copper, iron, zinc, and tin, shows that with 

 these screens the differences (Ni — Al) and (Cu~ Al) become 

 approximately equal to one another. Now, if the radiation 

 from copper decreased in a greater proportion than that from 

 nickel, when the thin lead screen is replaced by one of these 

 screens, this would decrease the value of (& a — 1) for the 

 difference (Cu— Al) more than its value for the difference 

 (Ni— Al). And since the value of (Cu — Al) is larger than 

 that of (Ni — Al) with the thin lead screen, this would have 

 the tendency of making the values of (/c a — 1) of these differ- 

 ences more nearly equal. Thus a screen of iron, copper, 

 zinc, or tin, absorbs to a slightly greater extent the rays that 

 are most efficient in producing secondary radiation from 

 copper, than the rays that are most efficient in producing 

 secondary radiation from nickel. 



The large change in the value of the difference (C — Al), 

 when the thin lead screen is replaced by a screen of iron, copper, 

 zinc, or tin, remains to be examined. We have seen that 

 the decrease of the differences (N m — Al), when the thin lead 

 screen is replaced by one of these screens, can be explained 



