594       Absorption  of  y  Rays  of  Radioactive  Substances. 
without  the  thorium  was  found  to  be  989.  Hence  the 
activity  of  the  radium  was  reduced  from  98*9  to  82*1,  by 
the  absorption  of  half  the  thickness  of  the  thorium  nitrate. 
But  \  for  thorium  and  radium  has  the  same  value,  and  in 
the  thorium  nitrate  the  central  layers  are  absorbed  to  an 
average  extent.  Hence  the  actual  reading  of  the  activity  of 
thorium  nitrate  being  100,  the  corrected  reading  would  be 
about  122. 
A  second  method  of  finding  the  self-absorption  was  by  the 
measurement  of  layer  after  layer,  superadded  to  one  another, 
in  a  circular  dish,  16  cms.  in  diameter,  adjusted  so  that  the 
centre  layer  was  at  a  constant  distance  from  the  electroscope. 
The  rise  of  activity  was  indicated  by  a  straight  line  when 
the  masses  were  taken  as  abscissae  and  the  activities  as 
ordinates.  The  absorption  of  a  thin  layer  of  200  grams 
could  not  be  large,  and  by  successive  approximations  the 
self-absorption  was  found  to  be  about  15  per  cent,  for 
1  kilogram  of  the  material  placed  beneath  *64  cm.  of  lead. 
These  two  results  are  in  rough  agreement.  Moreover,  the 
intensities  found  for  1  kilogram  of  thorium  nitrate,  1  kilo- 
gram of  pitchblende,  2  mg.  of  radium  bromide,  and  11  mg. 
of  radio-thorium,  followed  the  same  curve  for  various  thick- 
nesses of  lead.  It  is  fair  to  conclude  that  the  loss  of  activity 
of  a  kilogram  of  pitchblende,  due  to  self-absorption  and  to 
lack  of  concentration,  can  be  fully  corrected  by  an  addition 
of  twenty  per  cent,  to  the  observed  value^  when  *64  cm.  of 
lead  cut  off"  the  /3  rays. 
Summary. 
(1)  Radium,    uraninite,    thorium,   and   radio-thorium    emit 
y  rays  which  are  absorbed  at  the  same  rate  by  lead. 
(2)  For  thicknesses  of  lead  between  '64  and  3*0  cms.  the 
values  of  \  range  from  "57  to  *46  for  all  these  sub- 
stances. 
(3)  Uranium  nitrate  is  weak  in  y  rays,  and  these  are 
readily  absorbed  ;  X  =  1'4  between  2*8  and  12*1  mm.  of 
lead. 
(4)  Actinium  emits  four  types  of  rays  : — 
1.  a  rays. 
2.  /3  rays  which  are  homogeneous. 
\=163  (Godlewski). 
3.  More  penetrating  rays,  either  /3  or  y, 
A,  =  4*5  (G-odlewski). 
A  =  4*1  (Eve),  between  "45  and  2'8  mm. 
