of Radium, and Secondary Rays winch they produce. 653> 
is better absorbed by iron, a substance of Group II., than the 
radiation from zinc, a substance of the latter group, and so 
on. This shows, as before, that the substances of a group 
radiate those rays in least amount which they best absorb. 
It should be observed that we can only compare the coeffi- 
cients of absorption for the same substance. 
We have seen that the three groups of primary rays possess 
the same properties of selective absorption as the three 
groups of secondary rays. But it will be observed that 
it does not follow from this that a primary group of rays 
gives rise to the corresponding group of secondary rays, 
although this is very probable. Some further experiments 
were therefore made to demonstrate this. The experiments 
were carried out in the following manner. The coefficient 
of absorption of the secondary rays from a substance of one 
of the groups — say Group III. — was determined for a 
substance belonging to a different group — say Group II. 
A thick screen of a substance belonging to Group II. was 
then placed over the radium and the coefficient of absorption 
again determined ; the secondary rays being now produced 
by the primary rays which penetrated the thick screen 
placed over the radium. Now, since the radiator belongs to 
Group III., it radiates rays of Group II. and Group I. ; 
and since the substance whose coefficient of absorption is 
measured belongs to Group II., the part of the secondary 
radiation belonging to Group II. is absorbed to a greater 
extent by the substance than the remaining part. Therefore, 
if the secondary rays of Group II. are produced by the 
primary rays of Group II., the coefficient of absorption should 
decrease when the primary rays are passed through a screen 
of a substance of Group II. In the same manner, obviously, it 
may be investigated whether there is a relation between the 
primary and secondary group for other groups of rays. The 
magnitude of the decrease of the coefficient of absorption will 
be influenced by a large number of things. Thus it depends 
on the relative strength of the group of primary rays under 
investigation to that of the remaining groups. It also 
depends on the extent the screen over the radium absorbs 
the other groups besides the one which is best absorbed, and 
so on. 
In Table Y. some experimental results are given. The 
substances whose coefficient of absorption was determined 
under the different conditions, are given in the first vertical 
column, and their coefficients of absorption in adjacent hori- 
zontal columns. The horizontal column above the column 
containing the coefficients of absorption for a substance gives 
