38 ROYAL SOCIETY OF CANADA 



In Table 111 there is collected and given in row 1, the maximum 

 saturation currents in the chamber due to the (:i, /i secondary, y and y 

 secondary radiations and that due to natural causes, for the screens 

 of different thicknesses of tinfoil; in row II, the saturation currents 

 due to the y, and y secondary radiations and that due to natural 

 causes, and in row III, the deduced maximum ionisation due to the 

 fi and /^ secondary radiations for the same screens of tinfoil sheets. 



The ordinates of the curves in Fig. 6, are the values of the maxi- 

 mum fi and fi secondary ionisations taken from row III of the above 

 table. The curve, as will be seen, is drawn with an initial rise, although 

 no values were obtained from which the position of the highest point 

 could be determined. Some observations to be given later, however, 

 on the determination of the maximum thickness of aluminium 

 necessary to absorb all the /i and /Ï secondary radiations, shew clearly 

 that for aluminium the ionisation in the chamber due to the fi rays rose 

 and fell as the thickness of aluminium was increased. The inference was 

 therefore draw-n that for all metals this rise and fall in the conduc- 

 tivity due to (i rays striking a wall of the chamber would occur, and 

 would be made manifest if thin enough sheets of the metal were used. 



From the regular manner in which the curve in Fig. 6 falls away, 

 it is clear that in the experiments with tinfoil the thickest screen used 

 was amply sufficient to absorb all the f^ rays and the secondary 

 rays excited by them. An examination of the curve, moreover, makes 

 it evident that even a thickness of 2.5 millimetres of tinfoil was amply 

 sufficient for that purpose. 



B. Measurements on reflected rays. 



A series of measurements was also made on the secondar,y radia- 

 tion produced at the front side of sheets of tinfoil when /? and y ^'^ys 

 were allowed to fall on them. One layer of aluminium foil .0065 mms. 

 in thickness was placed over the opening at the top of the chamber, 

 and layers of tinfoil of increasing thickness were placed over the open- 

 ing at the bottom. In these experiments the capsule containing 

 the radium bromide was placed vertically above the ionisation cham- 

 ber, so that the rays after passing between the poles of the magnet 

 im])inged directly on the thin sheet of aluminium foil forming the 

 upper wall of the ionisation chamber, and after passing through it, 

 traversed the air in the chamber and then impinged on the tinfoil at 

 the bottom. As the magnet was excited by increasing currents the 

 fi rays were more and more deflected until all were swept aside by the 

 field and y rays alone entered the chamber. 



In column II of Table IV is given a tyi)icalset of these measure- 

 ments, and a typical curve plotted from them is shewn in Fig. 7. 

 The values obtained with the complete set of reflectors used are 



