of Radium, and Secondary Rays which they produce. G59 
plate, are the same as the coefficients of absorption of the 
secondary and higher rays corresponding to group A in the 
uncompressed plate. The value of m, it will be observed, 
will not be altered by the compression. Therefore the 
amount of secondary radiation corresponding to group B 
radiated by the compressed plate is equal to the amount of 
secondary radiation corresponding to group A radiated by 
an uncompressed plate of the same thickness and area. The 
amount of secondary radiation of group B from an uncom- 
pressed plate must therefore be smaller than the amount of 
secondary radiation of group A from the same plate ; that is, 
the amount of secondary radiation corresponding to the more 
absorbable primary group of rays is greater than that corre- 
sponding to the other less absorbable group. But this result 
is exactly opposite to that obtained by experiment. Let us 
see what this means. According to the foregoing, the amount 
of secondary radiation generated by a group of primary rays 
in an element of volume of a substance is proportional to m\, 
where X is the coefficient of absorption of the rays. The 
amount of secondary radiation which is not absorbed by the 
element of volume, and which is therefore radiated away 
from the element, is approximately proportional to 
(mX — (mX)»X), or m\ 2 ( - — n ). 
Since X 2 increases more rapidly with an increase of X than 
I - — n) decreases, m^ 2 (r —n\ increases w T ith an increase of 
X, and we obtain as before that the amount of secondary 
radiation increases with the absorption of the primary rays. 
It follows, therefore, from the experiments that m and n 
cannot be the same for each group of primary rays. And 
since m 2 (- — -?i ) must decrease with X, either m must decrease 
or n increase with X, or both vary in this way with X. It 
seems improbable though that the decrease of the amount of 
secondary radiation with increase of absorption of the 
primary group is due to a decrease of m ; that is, to 
a decrease of the fraction of the absorbed energy which 
is converted into secondary rays. It is more probably 
due to an increase of n, that is, an increase in the 
relative absorption of a secondary group to a primary group 
with increase of absorption of the primary group. This may 
,also be expressed by saying that the selective absorption is 
more marked for the secondary rays than for the primary. 
If the transformation of primary rays into secondary is a 
