158 Sir E. Rutherford on the Penetrating Power of 



obtained for the absorption by aluminium of the end 

 radiation after passing through iron : — 



Volts. [x. fi/p. 



92,000 



•38 



•14 



144,000 



•30 



•11 



183,000 



•23 



•085 



The corresponding values of fi were found to be higher if 

 lead were used as initial absorber instead of iron. 



The absorption was measured by placing the aluminium 

 plates close to the electroscope between the latter and the 

 iron plate. Under such conditions the greater part of the 

 forward scattered radiation enters the electroscope, and 

 consequently the absorption coefficient as measured is inter- 

 mediate between ju, and fju + a (where //, is the true absorption 

 coefficient and a the scattering coefficient), and probably 

 closer to the former. The value of /x as given by Hull and 

 Miss Rice corresponds to fi + cr in the above notation. 



In a recent paper, S. J. Allen and Alexander* have 

 examined the absorption of X rays from a Cooliclge tube 

 when different metals are used as filters for the rays. With 

 a tin filter, they found that the absorption coefficient in 

 aluminium for the issuing rays was lower than for any 

 other metal. The value, fi/p — 0'12, for aluminium was 

 observed with a steady voltage of about 120,000 volts ; 

 with an iron filter ^/p = 0*134 under the same conditions. 

 These numbers are in good agreement with those found by 

 the writer. 



Application to the icave-lengths of gamma rays. 



The observations on the absorption of X rays in aluminium 

 and lead throw important light on the difficult question of 

 the probable wave-lengths of the penetrating gamma rays 

 from radioactive substances. For convenience, the approxi- 

 mate results so far obtained are collected in the following- 

 table. The minimum wave-length is deduced from the 

 voltage or vice versa on the assumption that the quantum 

 relation, E = 7ii/, holds. 



The rows with an asterisk give values of fi/p obtained by 

 Hull and Miss Rice (loc. cit.). In their case, the values of 

 /nip include the effect of scattering as well as absorption, and 

 are consequently not strictly comparable with the values 

 found bv the author for aluminium, in which the correction 

 for scattering is less important. The values of fi/p for the 



* Allen and Alexander, Phys. Eev. ix. p. 198 (1917). 



