produced  by  Rontgen  Rays  in  Different  Metals. 
313 
been  confirmed  by  Eve  *.  If  Sagnac's  results  were  true  in 
general,  it  would  of  course  dispose  of  all  possibility  of  ex- 
plaining the  present  experiments  by  means  of  the  energy  of 
the  secondary  rays ;  and  even  from  Townsend's  observations 
(when  allowance  is  made  for  the  difference  of  absorption  by 
air)  it  is  impossible  to  conclude  that  the  total  ionizing  power 
of  the  secondary  rays  from  zinc  is  much  greater  than,  or  even  as 
great  as,  that  of  the  rays  from  lead.  But  leaving  these  results 
out  of  account,  I  think  it  is  possible  to  show  that,  with  the 
rays  which  I  used  and  under  the  experimental  conditions,  it 
is  not  possible  to  attribute  the  results  obtained  to  this  cause. 
The  secondary  rays  from  metals  are  always  more  easily 
absorbed  than  the  primary  rays  which  generate  them. 
Barkla  f  has  shown  that  the  secondary  radiation  from  gases 
and  light  solids  is  practically  identical  in  character  with  the 
primary  radiation ;  and  this  had  previously  been  found  to  be 
the  case  for  such  substances  as  paraffin  by  Sagnac  (he.  cit.). 
But  all  experimenters,  so  far  as  I  am  aware,  are  agreed  that 
the  secondary  radiation  from  lead,  zinc,  and  other  similar 
metals  is  less  penetrating  than  the  primary  ;  and  no  evidence 
has  been  obtained  of  any  secondary  rays  more  penetrating 
than  the  rays  which  produce  them.  Thus,  if  \  and  \2  are 
the  coefficients  of  absorption  in  zinc  of  the  primary  and 
secondary  rays  respectively,  we  have 
Now  in  the  experiment  described  at  length  above,  the 
absorption  by  the  zinc  strip  was  about  0'8 ;  the  two-tenths 
observed  behind  the  zinc  included  not  only  the  primary  rays 
which  got  through,  but  also  some  of  the  more  penetrating 
secondary  rays  from  the  rear  surface.  Hence  e~Xll<0'2  ; 
and  as  Z=0*08  cm., 
\2>\>20. 
If  we  assume  as  before  that  one-half  of  the  secondary  rays 
generated  in  any  element  are  propagated  straight  back  toward 
the  front  surface  (which  will  exaggerate  the  intensity  of  the 
secondary  rays),  we  get  for  the  intensity  of  the  secondary  rays 
escaping  from  the  front  surface 
Writing  this 
*  Proc.  Roy.  Soc.  lxxiv.  p.  474  (1905). 
f  Phil.  Mag-,  viii.  p.  074  (1904). 
