the  7  Rays  of  Radioactive  Substances,  589 
uranium  are  almost  absorbed  by  *64  cm.  of  lead,  so  that  the 
7  rays  from  uraninite  are  mainly  due  to  radium,  and  are  a 
measure  of  the  radium  present,  not  more  than  20  per  cent, 
being  lost  by  self-absorption. 
Fit?.  1. 
T^h ic n  ne ss    in   mm. 
Thorium. — One  kilogram  of  thorium  nitrate  was  sealed  in 
a  flat  glass  cylinder,  16  cm.  in  diameter.  The  values  of 
\  were  found  to  be  practically  the  same  as  those  for  radium 
and  for  uraninite. 
Radio-thorium. — It  is  important  to  note  that  radio-thorium 
gave  values  of  X  almost  identical  with  those  for  radium  and 
thorium,  but  initially  the  rays  from  radio-thorium  appear  to  be 
slightly  more  penetrating,  possibly  because  the  self-absorption 
is  less.  Dr.  Hahn  has  shown  that  radio-thorium  has  produced 
Th.X  and  Th.  Emanation,  and  since  radio-thorium  gives 
rays  similar  to  thorium  we  have  another  proof,  if  proof  were 
needed,  of  the  similarity  between  radio-thorium  and  thorium. 
Radio-thorium  has  not  yet  been  obtained  in  a  pure  state, 
but  the  eleven  milligrams  kindly  lent  me  by  Dr.  Hahn,  as 
measured  by  the  7  rays,  were  equivalent  to  1570  grams  of 
thorium  nitrate.  This  result  is  of  the  same  order  as  that 
found  by  Hahn,  when  using  ry  rays,  so  that  radio-thorium 
is  143,000  times  more  active  than  thorium  nitrate.  But  it 
will  be  seen  later  that  the  latter,  thorium  nitrate,  loses  about 
15  p^r  cent,  of  its  7  activity  by  self-absorption,  so  that 
radio-thorium  is  actually  about  J2xl04  more  active  than 
an  equal  weight  of  thorium  nitrate.  Now  radium  bromide 
was  found  to  be  about  15  x  106  times  as  active  as  thorium 
nitrate,  when  measured  by  the  7  rays;  hence  the  impure 
radio-thorium  had  an  activity  about  one-hundredth  part  of 
that  of  pure  radium  bromide. 
