820 
Dr.  C.  G.  Barkla 
on 
radiation  gave  much  more  constant  results    than    those   on 
substances  emitting  a  comparatively  penetrating  radiation. 
It  is,  however,  important  to  notice  the  effect  o£  hetero- 
geneity in  both  primary  and  secondary  beams,  assuming  the 
character  of  the  secondary  radiation  to  be  independent  of  that 
of  the  primary.  First  dealing  with  those  substances  which 
emit  a  radiation  of  fair  penetrating  power,  as  silver,  cadmium, 
and  tin,  the  layer  of  metal  from  which  the  secondary  r;svs 
proceed  has  a  thickness  comparable  to  that  penetrated  by  the 
primary  rays.  Consequently,  when  a  more  penetrating  pri- 
mary beam  is  used,  a  greater  proportion  of  the  secondary 
radiation  proceeds  from  the  deeper  layers,  and  in  its  passage 
to  the  surface  of  the  metal  is  robbed  of  its  more  absorbable 
constituents.  On  the  whole,  then,  the  emergent  secondary 
beam  consists  of  a  larger  proportion  of  penetrating  rays  than 
100 
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Atomic  Weight 
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ISO     200     £10 
when  set  up  by  a  more  easily  absorbed  radiation.  On  the 
other  hand,  when  the  secondary  radiation  is  very  easily 
absorbed,  as  in  the  case  of  the  radiation  from  iron,  the  layer 
from  which  secondary  rays  emerge  is  very  thin,  and  variation 
in  the  penetrating  power  of  the  primary  does  not  appreciably 
change  the  thickness,  all  the  primary  rays  getting  through 
with  little  diminution  in  intensity.  Hence,  whether  pene- 
trating or  comparatively  absorbable  rays  form  the  primary, 
the  emergent  secondary  is  from  the  same  thickness  of  metal, 
and  is  therefore  equally  penetrating. 
It  appears  possible  then  that  this  alone  would  account  for 
the  differences  observed  and  given  in  detail  above.  But  there 
were  found  to  be  enormous  changes  in  the  absorbability  of 
the  secondary  rays  from  antimony  and  iodine,  for  instance, 
