766 Prof. J. 0. McLennan on the 



Assuming that this lining completely absorbed the secon- 

 dary radiation from the lead walls of the vessels, which is 

 probable as the secondary rays from lead are easily absorbed, 

 and neglecting the absorption of the gamma rays by the 

 aluminium lining, since from the numbers given in Table III. 

 it must necessarily have been less than one half of one per 

 cent., it follows that the difference between the readings 

 "c" and " a " represents the ionization produced in the 

 unlined lead cylinder by the radium; while the difference 

 between the readings " d" and " 6" represents the ionization 

 produced in the lined cylinder by the same cause. The 

 excess of this first difference over the second may then be 

 taken without appreciable error as a measure of the excess of 

 the ionization produced by the secondary rays in the respec- 

 tive cylinders when unlined over that produced by the 

 secondary rays with the lining inserted. In other words, it 

 may be taken as proportional to the difference between the 

 ionizing powers, in so far as the air in the cylinders is con- 

 cerned, of the secondary rays excited in lead and aluminium 

 by the penetrating rays which entered the cylinders. 



Or taking I& and I as as proportional to the ionizations 

 produced in one of the cylinders by the secondary rays 

 excited in lead and aluminium respectively by the gamma 

 rays which entered it, we have 



1^-1^= (Reading "c" -Reading "a") 



-(Reading u d "-Reading "6"). . (i.) 



Further, it is known from an investigation by Eve * that 

 the ionizing power of the secondary rays excited in aluminium 

 by a gamma radiation is 28*6 per cent, of that possessed by 

 the secondary rays excited in lead by the same rays. 



We have then this equation 



t 10 ° t r-\ 



Again, denoting by \i v the ionization produced in the lead 

 cylinder under examination by the gamma rays from the 

 radium alone, we have 



l lp +Ifc = The difference between readings u c" and u a" 

 with this cylinder. . . . (iii.) 



From equations (i.), (ii.), and (iii.) it is possible then to 



* Eve, Phil. Mag. Dec. 1904. 



