64 ROYAL SOCIETY OF CANADA 



B. — Measurements on reflected rays from aluminium. 



A series of measurements was also made on the secondary 

 radiation produced at the front side of sheets of aluminium foil 

 when fi and fi rays were allowed to fall on them, and from these 

 the critical depth of the /i ray effect has been determined. The 

 arrangement of the apparatus was the same as for the measurements 

 on the reflected radiations from tin and lead. The values of the 

 saturation currents in the chamber found for the different thicknesses 

 of aluminium foil at the bottom are given in Table XIV, and 

 the maximum ionisations due to the (i rays have been deduced 

 from these tables and their corresponding curves. These maximum 

 currents are given in Table XV and the curve in Fig. 15 plotted 

 from them shews the manner in which the intensity of the secondary 

 radiation was increased. From this curve it is evident that the 

 maximum conductivity produced by the /? and the reflected (i 

 secondary rays attained a limiting value when the aluminium foil 

 sheets reached a thickness of .4 mms. 



It follows then from these results that a thickness of 7 mms. of 

 aluminium will completely absorb all the fi rays from radium and 

 the secondary rays which they excite in the metal. It follows too 

 that the secondary rays, emitted by the front side of a plate of the 

 metal when bombarded by the ft rays from radium do not come from 

 a depth in the metal greater than .4 mms. 



VI. — Experiments on the Secondary Rays Excited in Aluminium by y Rays. 



It has been stated in Section V that when a sheet of aluminium. 

 8.1 mms. in thickness which was sufficient to absorb all the ft rays and 

 the secondary rays excited by them was placed over the opening 

 at the top of the ionisation chamber the saturation currents were 

 not the same with a magnetic field applied in one direction as those 

 obtained with the same field reversed. This lack of symmetry in 

 the values of the saturation currents obtained when screens of alu- 

 minium were used is illustrated by curves E and E' in Fig. 13. In 

 the experiments' with lead and tin screens no effect of this kind was 

 observed, and in order to clear up the matter an additional series 

 of experiments was carried out to ascertain if possible the cause of it 

 in the case of aluminium. 



1, In the first experiment a thickness of 4.73 mms. of aluminium 

 was placed over the opening at the top of the chamber, and above 

 this a thickness of .964 mms. of lead. This thickness of lead, it will 

 be remembered was found in the earlier experiments sufficient to 

 absorb all the ft and the ft secondary radiations. It follows then, 

 that with this screen none but the y rays of radium could enter the 



