136 Mr. Y. E. Pound on the Absorption of the 



is sufficient to absorb not only the transmitted secondary 

 rays excited by ft rays, but also the whole of the primary 

 radiation itself. 



This result, however, while giving definite information 

 regarding a lower limit to the thickness of tinfoil requisite 

 to absorb primary ft rays, gives only an upper limit to the 

 thickness necessary to absorb the transmitted secondary 

 radiations produced by such rays. In order to obtain a lower 

 limit to the thickness of tinfoil required to absorb the trans- 

 mitted ft secondary radiation alone which is excited by 

 ft rays, it would be necessary to modify considerably the 

 arrangement of the apparatus used in making these 

 measurements. 



IV. Experiments on the Absorption and Deflexion 

 or ft Rays by Lead. 



A. Measurements on the Transmitted Rays through Lead. 



Experiments were conducted with lead in an exactly 

 similar manner to those on the transmitted rays through 

 tinfoil, in order to find the minimum thickness of lead 

 necessary to prevent the emergence of any ft or ft secondary 

 radiations from the far side of a plate upon which the primary 

 ft rays of radium fell. The radium bromide was placed as in 

 fig. 1, and sets of readings were taken of the ionization in the 

 chamber, — 1st, with the top of the chamber open, and, 2nd, 

 with it covered by lead-foil of varying thicknesses, the bottom 

 of the chamber being always closed by a sheet of aluminium- 

 foil *0065 mm. in thickness. As before, these readings 

 were taken as the ft rays were deflected downwards into 

 the ionization-chamber, and upwards and away from it by 

 different magnetic fields. 



The sets of readings taken with the opening at the top of 

 the chamber uncovered, and also covered with the lead-foil 

 screens 0241 mm., 0*482 mm., r 723 mm., and 0'964 mm. in 

 thickness respectively, are given in columns II., III., IV., 

 and V. of Table VI. From the values of the ionizations given 

 in columns III., IV., and V. of this table the curves A, B, 

 and C (fig. 9) were drawn. The curves A', B x , and 0', also 

 shown in fig. 9, were plotted from the numbers in columns 

 III., IV., and V. of Table VII. Both sets of curves (it will 

 be seen) are of the same type as those for the tinfoil which 

 were fully discussed in Section If. A. 



