648 Mr. It. D. Kleeman on Different Kinds of <y Rays 
It will be further seen in the table that the ratio of the 
radiation from lead to that from carbon is also increased 
with an increase of the thickness of the lead screen. Thus 
carbon radiates a greater proportion of rays which are well" 
absorbed by lead than lead itself. It follows, therefore, 
reasoning in the same manner as before, that the substances 
of Group III. also radiate a greater proportion of rays which 
are well absorbed by a substance of Group I., than the 
substances of Group I. themselves. 
Let us next consider the results obtained when the substance 
of the screen used belonged to Group II. (The values for 
lead and zinc are put equal to 100 in each case.) It will be 
seen that the ratio of the radiation from lead to that from 
zinc is decreased with an increase in the thickness of the 
iron screen. Thus lead, a substance of Group I., radiates a 
greater proportion of rays well absorbed by iron, than zinc, 
a substance of Group II. It follows, therefore, that the 
substances of Group I. radiate a greater proportion of rays 
well absorbed by iron, a substance of Group II., than the 
substances of Group II. This was proved by experiment to 
be true for some other substances of Group II. besides iron. 
Thus, it was found (see table) that there is a decrease in the 
ratio of the radiation from lead to that from zinc, substances 
of Group I. and II., with an increase in the thickness of a 
screen of copper or aluminium, substances of Group II. 
From this it follows that the foregoing result is also true for 
aluminium and copper, and it may therefore be taken to be 
true for all the substances of Group II. 
Further, the ratio of the radiation from carbon to that 
from zinc is decreased, as will be seen in the table, with an 
increase in the thickness of an iron screen. It follows, 
therefore, that the substances of Group III. also radiate a 
greater proportion of rays well absorbed by a substance of 
Group II., than the substances of Group II. themselves. 
It remains to consider the results obtained when the sub- 
stance of the screen used belonged to Group III. (The 
values for lead and zinc are put equal to 100 in each case.) 
It will be seen that the ratio of the radiation from lead (a 
substance of Group I.) to the radiation from carbon (a sub- 
stance of Group III.) is decreased with an increase in the 
thickness of the carbon screen. The decrease, it will be 
observed, is greater for the thicker of the two lead radiators 
used. Further, the ratio of the radiation from zinc (a sub- 
stance of Group II.) to the radiation from carbon, is also 
decreased with an increase of the thickness of the carbon 
screen. It follows, therefore, in the same manner as before, 
