646 Mr. R. D. Kleeman on Different Kinds of y Rays 
say that 'the substance A at least radiates rays which are well 
absorbed by the substance of the screen. In this manner, 
substances belonging to different groups were compared with 
one another, using successively screens belonging to different 
groups. It was found that when there is an indication that 
a radiator radiates more rays which are well absorbed by the 
screen, than some other radiator, the effect may be made 
more pronounced by increasing the thickness of either 
radiator or of both. The reason for this will appear when 
the results of this investigation are discussed. It is obvious 
that the effect can also be increased by increasing the ratio 
of the thick screen to the thin one. For our present purpose 
it is only necessary to make sure that there is a change in 
the ratios with increase of thickness of screen. 
Table III. gives the results of some experiments carried 
out in the manner described. Let us first consider the 
results obtained when the substance of the screen used 
belonged to Group I. The first and third vertical columns 
of this part of the table give the radiators and their thick- 
nesses. The column on the right of each of these columns 
gives the values of the secondary radiations corresponding to 
two different thicknesses of the screen, the nature of the 
screen being given at the top of the column. The radiation 
from lead has in each case been put equal to 100. It will 
be seen that there is a decrease in the ratio of the radiation 
from zinc to that from lead with an increase of thickness of 
the lead screen. Thus zinc radiates a greater proportion of 
rays which are well absorbed by lead than lead itself. The 
reason for this, as will afterwards be seen, is that the rays 
generated in a radiating plate, of the kind best absorbed by 
the plate, are almost entirely absorbed by the plate. Further, 
since the substances which we have denoted by Group II., 
of which zinc is one, radiate approximately the same kind of 
rays, and the substances of Group I., of which lead is one, 
also radiate the same kind of rays, it follows that the sub- 
stances of Group II. radiate a greater proportion of rays 
well absorbed by lead than the substances of Group I. If 
the substances of a group resemble one another in their 
absorption properties, this would be true for all the substances 
of Group I. besides lead. It will presently be shown in this 
connexion that the substances of Group II. possess the same 
absorption properties, and we may therefore take it for 
granted that this is true for all groups. Moreover, it will 
be seen, when the primary and secondary rays are compared, 
that the substances of each group possess the same absorption 
properties for the primary rays. 
