Secondary  Roatgen  Radiation.  813 
The  results  o£  these  investigations  have  shown  that  a  sub- 
stance upon  which  a  beam  of  Rontgen  radiation  is  incident 
emits  two  kinds  of  radiation  :  an  easily  absorbed  radiation 
consisting  of  negatively  charged  corpuscles  or  electrons,  and 
a  heterogeneous  beam  of  X-radiation  differing  from  the 
primary  in  penetrating  power.  In  almost  all  cases  the 
secondary  X-radiation  has  been  found  to  be  more  easily 
absorbed  than  the  primary  producing  it  ;  and  never  has  it 
been  found  to  possess  greater  penetrating  power.  The 
absorbability  of  the  radiation  from  a  given  substance  varies 
with  that  of  the  primary;  and  -probably  as  a  consequence,  the 
results  obtained  by  different  experimenters  for  the  relative 
absorptions  of  the  radiations  froin  different  substances  do 
not  agree.  Recently  Bumstead  has  made  experiments  from 
which  he  concludes  that  the  absorption  of  a  given  X-ray  beam 
and  the  secondary  beam  arising  from  it,  results  in  the  gene- 
ration of  about  twice  as  much  heat  when  the  absorbing  and 
radiating  substance  is  lead  than  when  it  is  zinc. 
The  writer*  investigated  the  radiation  proceeding  from 
gases  and  light  substances  when  subject  to  X-rays,  and  found 
that  the  radiation  not  absorbed  by  a  few  centimetres  of  air 
under  ordinary  atmospheric  conditions  differed  exceedingly 
little  from  the  primary.  From  certain  gases  and  light  solids 
the  radiation  was  found  by  direct  experiment  not  to  differ 
appreciably  in  absorbability  from  the  primary ;  but  there  was 
indirect  evidence  of  greater  absorbability  in  air.  From  these 
substances  the  intensity  of  radiation  was  found  to  be  pro- 
portional to  the  quantity  of  matter  passed  tnrough  by  the 
primary  radiation  of  given  intensity. 
These  laws  were  accounted  for  by  the  theory  that  the 
corpuscles  or  electrons  constituting  the  atoms  scattered  the 
primary  radiation.  The  intensity  of  radiation  was  experi- 
mentally investigated,  and  a  close  agreement  found  between 
that  and  the  result  of  a  calculation  by  Prof.  J.  J.  Thomson  | 
when  applied  to  the  electrons.  The  theory  was  further 
verified  by  the  discovery  of  partial  polarization  in  a  primary 
beam  proceeding  from  an  X-ray  tube,  by  a  study  of  the 
intensity  of  secondary  radiation  in  different  directions,  and 
later  by  the  demonstration  of  much  more  complete  polari- 
zation of  secondary  radiation  proceeding  from  one  of  the 
substances  (carbon)  to  which  the  theory  was  supposed 
applicable  \. 
*  Barkla,  Phil.  Mag.  v.  pp.  685-698,  June  1903 ;  vii.  pp.  543-560, 
May  1904. 
t  J.  J.  Thomson,  '  Conduction  of  Electricity  through  Gases,'  p.  270. 
%  Barkla,  Phil.  Trans.  A,  vol.  204  (1905/  pp.  467-479 ;  Proc.  Hoy. 
Soc.  A.  vol.  lxxvii.  pp.  247-255  (1900). 
