[     754     ] 
LXIX.  The  a.  Particles  of  Uranium  and  Thorium.  ByW.  H. 
Bragg,  ALA.,  Elder  Professor  of  Mathematics  and  Physics 
in  the  University  of  Adelaide  *. 
THIS  paper  is  divided  into  two  parts.  The  first  contains 
a  discussion  of  the  magnitude  of  the  ionization  current 
due  to  a  layer  of  radioactive  material  scattered  on  the  floor 
of  an  ionization-charnber,  and  covered  by  a  uniform  sheet  of 
metal  foil.  The  result  is  expressed  in  a  formula  which  is 
somewhat  complicated  in  its  general  form,  but  is  capable  of 
simplification  under  suitable  conditions.  Account  is  taken 
of  the  variation  of  ionization  with  velocity.  The  second 
contains  an  account  of  experiments  which  show: — 
(a)  That  the  values  of  the  current  in  various  cases,  calcu- 
lated from  the  formula,  agree  very  well  with  the  results  of 
observation. 
(6)  That  the  ranges  of  the  a  particles  of  uranium  and 
thorium  are  very  nearly,  perhaps  exactly,  equal  to  the  range 
of  the  a  particle  of  radium. 
(c)  That  the  rate  at  which  thorium  atoms  break  down  is 
nearly  *19  of  the  similar  rate  for  uranium. 
Part  I. 
The  method  which  was  used  by  Mr.  Kleeman  and  myself 
in  the  determination  of  the  ranges  of  the  a  particles  emitted 
by  radium  and  its  products  does  not  lend  itself  to  the  corre- 
sponding determinations  in  the  cases  of  uranium  and  thorium. 
It  is  a  necessary  feature  of  the  method  that  all  a  particles 
except  those  moving  normally  to  the  horizontal  layer  of 
radioactive  material  should  be  prevented  from  reaching  the 
ionization-chamber,  below  which  the  radium  is  placed.  This 
is  done  by  the  use  of  a  bundle  of  vertical  tubes  which 
stop  all  ex.  particles  other  than  those  moving  in  the  desired 
direction.  But  this  limitation  diminishes  very  greatly  the 
number  of  effective  a  particles,  and  in  the  cases  of  uranium 
and  thorium  the  effect  is  reduced  below  the  limits  of  con- 
venient measurement.  This  is  the  case  even  when  a  large 
surface  of  radioactive  material  is  employed.  In  order  to 
determine  the  ranges  of  uranium  and  thorium  another  method 
must  be  devised. 
I  have  therefore  calculated  the  ionization  due  to  a  radio- 
active layer  over  which  a  screen  has  been  placed.  The  result 
is  a  function  of  the  relation  of  the   stopping  power  of  the 
*  Communicated  by  the  Author. 
