Rays emitted by Substances exposed to 7 Rays. 643 



which increases (Pb— Al) to its previous value. This factor 

 gives rise to the increase of the other differences, since the 

 radiation from lead is decreased in a greater proportion than 

 that from aluminium and the other substances. 



From the foregoing we see why the difference (C— Al),. 

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

 copper, zinc, or tin, is more decreased than in the case when 

 the thin screen of lead is replaced by a screen of lead, mercury, 

 or bismuth. 



To sum up, the experiments as far as they have gone 

 indicate that the rays from radium consist principally of two 

 groups of rays, the constituent rays of each group differing 

 not much from one another in their properties. The rays of 

 one of the groups are more efficient in producing secondary 

 cathode radiation from aluminium, sulphur, iron, nickel, zinc, 

 and tin, than from lead, and are all more or less easily 

 absorbed by each of these substances excepting lead, the 

 absorption by lead being much less. The rays of the other 

 group are more efficient in producing secondary cathode 

 radiation from lead than from the other substances, and are more 

 easily absorbed by lead, mercury, and bismuth, than by any 

 of the other substances. 



There is also a third — apparently weak group of rays 

 which is most efficient in producing secondary radiation 

 from carbon. This group of rays is less easily absorbed 

 by the above-mentioned substances than either of the other 

 groups. 



It may be pointed out in passing that according to the 

 foregoing, when it is required to shield a piece of apparatus 

 from the 7 rays of radium, it is better to use a combined 

 screen of lead and one of the metals iron, zinc, or copper, 

 than a screen composed of one of these metals only. 



The 7 rays of radium thus resemble X rays in the absorp- 

 tion by a substance depending on the nature of the rays and 

 that of the absorbing substance. Further, the amount of 

 secondary radiation from a substance exposed to 7 rays 

 depends on the nature of the rays, and this has been shown 

 to be also the case with X rays. These facts are additional 

 evidence that the general nature of the 7 and X rays is the 

 same. Both the y and X rays probably consist of electro- 

 magnetic pulses produced by the acceleration of electric 

 charges. Since the @ ray activity due to radium E is small 

 in comparison with that due to radium C, in the case of 

 radium only a few years old, the 7 rays from radium are 

 principally produced by the acceleration of the electrons 

 ejected by radium C. 



