M‘CreLtLanp— The Energy of Secondary Radvation. 25 
and remembering that 7*=0, when wz=d, we get 
(A — a) Re’ = A'{A4 ale + (h— ale}, 
and lie = liye ts A’ (@e@ a oh), 
Eliminating A’ we get 
Ry (Ataje’+(\-a)e™ 
But ; 
u 
a en Xe 
0) 
Ge = 5x (at ale e+ (A\—a)je*}, 
or 
‘ GY = 0 o(O= Iho, (3) 
where 
aos K 
A+ a _pevec THEE 
eo QE =P 2 =i 
C can be calculated for any substance from the value of « given in Table I. 
From equation (3) we see that Ns when d is | 1 
V = (20 —1)d when d is infinitely small. 
It follows, therefore, that when the coefficient of absorption i: 
the way described, different values should be found according to ¢ 
substance used even when the radiation is perfectly homogeneous. 
CO depends on the value of « for the substance used; substituting 
from Table I., we find that C varies from 1°32 for lead to 1:02 for carbon. 
The maximum value of )’ for lead is therefore 1:64, and for carbon 1:04). 
Tin is often used in determinations of the absorption coefficient, as it can 
easily be obtained in thin sheets. The value of )’ for tin for different values 
of d has been calculated from equation (3), giving the constant value 100. 
The result is shown in the following table, the maximum value of \’ being 144 :— 
Thickness of tin 
in mm. nr. 
‘O1 140 
05 126 
‘10 LING 
20 110 
30 106 
50 103 
It follows from these numbers that the 8 rays of radium are not so 
TRANS. ROY. DUB. SOC., N.S., VOL. IX., PART II. E 
