Radiation  from  Ordinary  Materials.  223 
no  difference  in  the  behaviour  of  lead  when  shielded  with  an 
iron  or  a  lead  screen.  The  large  secondary  radiation  from 
platinum  is  remarkable ;  it  may  throw  some  light  on  the 
large  variations  in  the  activity  of  that  metal  which  were 
found  by  Strutt*. 
(1)  Value  of  V  and  V. — Not  much  reliance  can  be  placed 
on  these  values,  for  a  change  in  the  intensity  of  the  external 
radiation  (such  as  is  known  to  arise  from  the  variation  in  the 
quantity  of  radium  emanation  present  in  the  air),  during  the 
nine  or  ten  days  that  were  required  to  complete  the  observa- 
tions for  any  one  metal,  would  invalidate  the  calculation. 
Moreover,  the  plates  of  metal  used  to  cover  the  parallel  sides 
were  not  thick  enough  to  cut  out  all  rays  from  the  underlying 
wood,  if  these  were  of  a  penetration  equal  to  that  of  /3  rays. 
In  fact,  it  is  clear  that  there  are  inconsistencies  in  the  table  ; 
tin-plate  is  only  iron  covered  with  a  thin  layer  of  tin,  not 
nearly  thick  enough  to  absorb  /3  rays,  and  yet  the  value  of  v 
is  less  for  tin-plate  than  for  iron.  But  it  does  not  seem  that 
any  information  of  importance  could  accrue  from  a  more 
accurate  estimation  of  these  quantities. 
(5)  Value  of  a. — The  variation  of  Bragg's  constant  for 
the  different  metals  is  the  one  point  of  importance  in  the 
whole  paper.  This  variation  was  suggested  in  the  first  paper, 
and  has  been  proved  beyond  donbt  in  the  second.  It  shows 
that  the  emission  of  ionizing  rays  is  a  property  inherent  in 
the  material  itself,  and  cannot  be  attributed  to  the  presence 
of  a  radioactive  impurity  common  to  all.  This  is  my  main 
contention. 
It  is  noticeable  that  in  every  case  except  aluminium,  the 
penetration  of  the  rays  from  ordinary  metals  is  greater  than 
that  of  the  most  penetrating  rays  from  radium  (a  =  7,  see 
Bragg,  Phil.  Mag.  Sept.  1905).  But  in  view  of  the  con- 
siderations urged  by  Rutherford  f,  there  seems  to  be  no 
reason  why  this  should  not  be  the  case.  An  interesting 
problem  arises,  whether  the  rays  from  ordinary  metals  are 
homogeneous.  In  some  cases,  a.  g.  lead  and  platinum,  the 
experimental  curve  agrees  so  well  with  that  calculated  for 
the  corresponding  value  of  a,  that  the  rays  appear  to  be  all 
of  one  type.  But  in  others,  especially  tin  and  copper,  the 
experimental  curve  is  steeper  at  first  than  the  calculated  :  it 
is  quite  possible  that  we  are  dealing  with  two  varieties  of 
rays,  of  which  one  is  more  easily  absorbed  than  the  other. 
But  at  present,  the  accuracy  of  experiments  on  such  extremely 
small  ionizations  is  not  sufficient  to  enable  the  question  to  be 
investigated  directly. 
*  '  Nature,'  Feb.  19,  1903. 
t  Phil.  Mag.  July  1905. 
