Secondary  Rontgen  Radiation.  823 
of  its  greater  absorbability,  and  hence  little  more  than  half 
the  full  variation  was  found,  the  rays  from  calcium  itself 
showing  only  about  half  the  variation  shown  by  those  from 
lighter  elements. 
In  the  radiation  from  copper  sulphate,  the  rays  from 
copper  were  so  easily  absorbed  that  the  effect  of  the  more 
penetrating  rays  from  sulphur  and  oxygen  was  swamped, 
and  no  evidence  of  variation  in  intensity  in  different  directions 
was  detected. 
Experiments  were  also  made  to  ascertain  if  the  more 
penetrating  portion  of  the  secondary  radiation  from  some  of 
the  heavier  substances  gave  evidence  of  polarization  in  the 
primary  ;  for  if  the  character  of  the  radiation  depended  on 
the  relation  between  the  pulse-thickness  and  the  distances 
separating  the  electrons,  we  might  expect  that  the  effect  of 
thin  pulses  passing  through  a  heavy  atom  would  be  similar 
to  thicker  pulses  passing  through  an  atom  in  which  the 
electrons  are  not  so  closely  packed. 
No  such  evidence  was  obtained  (see  paper  on  "  Polarized 
Rontgen  Radiation  "). 
[These  experiments  on  polarization  were  made  before  the 
absorbability  of  the  rays  from  many  substances  had  been 
determined.  It  will  be  interesting  to  learn  if  evidence  of 
polarization  reappears  when  such  a  substance  as  silver  is  used 
as  the  radiator  ^see  fig.  1) .  The  radiation  from  silver,  however, 
is  much  more  absorbable  than  the  primary  producing  it, 
though  from  a  thick  plate  its  ionizing  effect  is  diminished  bv 
a  smaller  fraction  than  that  of  the  primary  by  passage 
through  a  thin  plate  of  aluminium.  Enormous  differences 
are  observed  in  the  character  of  the  radiations  proceeding 
from  thin  and  thick  plates  of  the  same  substance.] 
Connexion  between  Atomic  Weight  of  Radiating  Substance 
and  Character  of  Secondary  Radiation. 
Though  it  is  impossible  to  determine  the  true  character  of 
the  radiation  as  it  proceeds  from  an  atom  of  the  radiating 
substance  except  by  using  very  thin  plates,  it  was  thought 
that  by  using  plates  of  sufficient  thickness  to  absorb  nearly 
all  the  incident  primary  radiation,  or  rather  that  portion  of  it 
which  produced  an  appreciable  effect  in  the  electroscope, 
some  law  might  be  found  to  exist  where  from  a  few  isolated 
results  there  appeared  to  be  great  irregularity. 
A  large  number  of  elements  were  therefore  in  turn  exposed 
to  the  primary  radiation,  and  the  penetrating  power  of  the 
rays  from  each  was  observed. 
