It  is  at  present  not  certain  whether  thorium  D  is  formed  as
a  result  of  each  of  the  two  modes  of  transformation  of  thorium  C
or  whether  it  arises  from  only  one  of  them,  although  we  have  seen
in  the  last  section  that  the  latter  is  more  probable.  There
appears  to  be  no  doubt  that  the  separation  of  thorium  D  by  recoil
is  a  consequence  of  the  expulsion  of  an  a  particle.  Some  light
may  be  thrown  on  the  origin  of  thorium  D  by  an  examination  of
the  range  of  the  recoil  atoms  of  thorium  D  in  air  at  reduced
pressure.
Hahn  and  MeitnerJ  found  that  the  ^  rays  from  thorium  D
were  absorbed  exponentially  with  a  coefficient  of  absorption  in
aluminium  of  about  /i  =  15  (cms.)""^  The  velocities  of  the  com-
ponent groups  of  rays  emitted  from  this  product  have  already
been  given  in  Section  95.  The  7  rays  from  thorium  D  are
slightly  more  penetrating  than  those  emitted  from  radium  C.
No  residual  activity  has  been  observed  after  sufficient  time

*  Hahn  and  Meitner,  Verh.  d.  D,  Phys.  Qes.  11,  p.  55,  1909.
t  V.  Lerch  and  v.  Wartburg,  Wien  Ber,  118,  p.  1576, 1909.
X  Hahn  and  Meitner,  Phyt,  Zeit.  9,  p.  821, 190a


XVl]  THORIUM    AND   ITS   PRODUCTS  543

lias  elapsed  for  the  complete  decay  of  the  ordinary  active  deposit.
Consequently  thorium  D  is  to  be  regarded  as  the  last  active
product  of  the  thorium  series.  The  question  as  to  the  nature  of
tile  end  product  will  be  discussed  later  in  Chapter  XVIII.
