606 Mr. S. Oba on the 



(2) The material through which 7 rays were first passed 

 acts always to harden the rays. This is seen by the fact that 

 the values for /jl/D given in the first row are always greater 

 than those in the same column in any other row. The case 

 of aluminium hardener for thicknesses of iron between '2 

 and 2 cm. appears an exception, but the difference in this 

 case is almost within the experimental error. 



(3) The decrease of the value of /n/D becomes more pro- 

 nounced for larger thicknesses of the absorber, but the 

 decrease appears to tend to a limiting value. This result is 

 seen from the value of //,/D in the same column for different 

 thicknesses of lead as hardener. This effect becomes more 

 marked with increasing difference of atomic weights of the 

 hardener and the absorber. 



(4) The 7 rays which have been hardened become gradually 

 softer by passing through increased thickness of absorber. 

 Moreover, particularly in the case of iron, it is possible that 

 the hardened 7 rays may tend with large thickness of 

 absorber to the same penetrating power as that to which 

 primary 7 rays tend. 



As has already been stated, the hardening of 7 rays may 

 be ascribed to the heterogeneity of the primary 7 rays and a 

 greater amount of absorption and scattering of the softer 

 constituents. According to the analysis of Rutherford and 

 Richardson*, the 7 rays dealt with in these experiments will be 

 the easily absorbed rays from RaB with //, = '51 cm. -1 alumi- 

 nium, and the penetrating rays from RaC with yL6=*115 cm. -1 

 aluminium. After either 8*5 cm. aluminium, 1*5 cm. lead, 

 3'0 cm. iron, 2'7 cm. copper, or 3*3 cm. tin, the ionization 

 due to RaB 7 rays will be less than 1 per cent, of that due to 

 the rays from RaC. We may consequently take these thick- 

 nesses as a measure of the range over which we may expect 

 that hardening will be evident. 



It will be seen, however, that the various effects cannot be 

 completely explained as due to the soft radiations from RaB. 

 For in the first place this will not explain the softening effect 

 referred to in (4) above. Further, on such an hypothesis it 

 follows that when the soft RaB rays are removed by absorp- 

 tion, the absorption coefficient of the remaining radiation 

 should be the same and independent of the, so-called, 

 hardener. 



It must be remembered that the absorption coefficient /z. 

 being calculated from the equation 



Ij/Ig = ef* x *- x i\ 



* Rutherford & Richardson, Phil. Mag. xxv. p. 721 (1913). 



