Rays emitted by Substances exposed to 7 Rays. 639 



differences (S-Al),(Fe-Al),(Ni-Al), (Cu-Al),(Zn-Al), 

 (Sn— Al), are increased with these screens. With a screen 

 o£ iron, copper, zinc, or tin, the difference (C— Al) is very 

 much decreased, while the other differences are also nearly 

 always more or less decreased. Thus it seems that the 

 7 rays from radium consist principally o£ two groups of 

 rays, the rays of one of the groups being much better 

 absorbed by a screen of lead, mercury, or bismuth than by 

 a screen of iron, copper, zinc, or tin, the opposite being the 

 case with the rays of the other group. 



Let us examine more closely the values given in the table. 

 It will be convenient to express the radiation from a substance 

 in terms of that from aluminium, so that lor (Pb— Al) we 

 may write (7^ — 1) Al, and in the case of any substance D write 

 (/fa— 1)A1 for (D— Al). It will also be convenient to denote 

 the group of substances sulphur, iron, nickel, copper, zinc, 

 and tin, by the symbol N, rt , so that (N OT — Al) stands for 

 (S-Al), (Fe-Al),"&c. 



Now, when the thin lead screen is replaced by the thick 

 screen of lead, we should expect, if we assume that the 

 7 rays are heterogeneous, that the radiation from lead would 

 decrease in a greater proportion than that from aluminium, 

 and the value of (&i~l) therefore decrease. Further, if we 

 assume that the radiation from each of the substances N,„ 

 is decreased in the same proportion as that from aluminium 

 each of the values of (k a — 1) of the differences (N»»— Al) will 

 remain unchanged. Therefore, if we multiply (k 1 — l)Al 

 and the values of (Jc a — 1)A1 by a factor which brings (^— 1)A1 

 to its previous value, each of the values of (k a — 1)A1 will 

 become greater than its previous value. This result agrees 

 with that obtained by experiment with the differences 

 (N TO — Al), when the thin lead screen was replaced by the 

 thick screen of lead (see table). The greater decrease of the 

 radiation from lead than that from aluminium and the 

 substances N" m , with increase of thickness of lead screen, 

 shows that the rays that are most efficient in producing 

 secondary radiation from lead are more easily absorbed by 

 lead, than the rays that are respectively most efficient in 

 producing secondary radiation from aluminium and the 

 substances N m . It appears also that the rays that are 

 respectively most efficient in producing secondary radiation 

 from aluminium and the substances N w are absorbed to an 

 approximately equal though small extent by lead. 



It will be seen in Table V. that the value of the difference 

 (C — Al) is decreased instead of increased, as the other 

 differences, when the thin lead screen is replaced by the 



2U2 



