588 
Mr.  A.  S.  Eve  on  the  Absorption  of 
(7)  Actinium,  Debierne's  preparation,  activity  about  700. 
This  substance,  belonging  to  Sir  William  Ramsay, 
was  kindly  lent  to  me  by  Dr.  Hahn. 
Results. — 1£  I0  be  the  intensity  of  radiation  on  one  side 
of  a  lead  plate,  and  I  be  the  intensity  on  the  other  side  after 
absorption,  we  have  I  =  I0^~Az,  where  x  is  the  thickness  of  the 
plate,  and  X  is  the  coefficient  of  absorption.     This  formula, 
deduced  from  -=-  =  — \I,  assumes  that  the  absorption  by  a 
thin  layer  per  unit  thickness  is  proportioned  to  the  intensity. 
Jt  has  been  found  by  McClelland,  Wigger,  and  others,  that 
in  the  case  of  radium  X  is  not  constant  over  wide  ranges, 
but  the  less  penetrating  <y  rays  are  first  absorbed,  and  the 
subsequent  values  of  X  are  therefore  smaller.  In  the  present 
experiments,  the  range  of  the  thicknesses  between  which  the 
values  of  X  have  been  obtained  will  be  stated,  without 
including  the  minute  correction  for  the  air  traversed,  or  for 
the  *45  mm.  of  zinc  which  formed  the  base  of  the  electro- 
scope, or  for  the  glass  vessels,  such  as  test-tubes,  in  which 
some  substances  were  placed. 
Radium. — The  results  obtained  for  radium  are  in  fair 
agreement  with  those  found  by  McClelland,  using  the 
electrometer  method  (Phil.  Mag.  July  1904). 
7  rays. 
Thickness  of  lead 
in  cm. 
X  (cms.)    *. 
•64  to  1-21 
1-21  to  1-79 
1-79  to  2-36 
2-4    to  3-0 
•57 
•46 
•46 
McClelland  found  that  X  varied  from  *64  to  '44  through 
the  same  range.  The  result  is  shown  in  fig.  1,  where  the 
abscissae  denote  the  thickness  of  lead  traversed,  and  the 
ordinates  give  the  logarithms  of  the  intensities  as  measured 
by  the  gold-leaf  electroscope.  It  will  be  seen  that  trie  radium 
line  thus  plotted  is  not  straight,  and  that  X  has  a  gradual 
slight  decrease  in  value. 
Uraninite. — The  specimen  from  Joachimsthal  gave  values 
of  X  almost  identical  with  those  obtained  from  radium 
bromide,  although  in  one  case  the  radium  was  concentrated, 
and  in  the  other  was  distributed  through  a  kilogram  of 
pitchblende.     It  will  be  found  later  that  the  y  rays  from 
