Prof.  W.  H.  Bragg  on  the 
another    experiment    the    thorium 
766 
In  another  experiment  the  thorium  was  precipitated 
twice  at  intervals  of  two  days  and  then  five  times  at  intervals 
of  12  hours.     The  results  were  as  follows  : — 
I. 
II. 
III. 
IV. 
pd  of  A\  foil 
X  10*. 
i/L 
From  Curve  A. 
pd  of  full  range 
X  105. 
534 
•655 
•114 
470 
1046 
•425 
•221 
473 
1633 
•248 
•347 
471 
2133 
•154 
•438 
486 
In  this  case  the  mean  of  the  figures  in  the  last  column  is 
•00475.  As  in  the  case  of  radium,  this  result  is  probably  a 
little  too  high  as  it  is  impossible  to  get  rid  of  all  the  radioactive 
products  of  thorium,  and  all  these  have  ranges  higher  than 
thorium  itself.  For  Rutherford  has  shown  that  the  a  particle  of 
the  induced  activity  of  thorium  has  the  same  penetrating  power 
as  the  a  particle  of  the  induced  activity  of  radium ;  and  some 
rough  experiments  which  I  have  made  with  ThX  go  to  show 
that,  as  in  the  case  of  Ra,  the  second  and  third  active  products 
have  ranges  intermediate  between  the  first  and  fourth*. 
It  may  also  be  calculated  from  an  experiment  of  Ruther- 
ford's ('  Radioactivity ;  2nd  ed.  p.  263)  that  the  range  of  the 
emanation  a  particle  is  about  6  cm.  ;  but  there  is  some 
uncertainty  as  to  the  stopping  power  of  the  mica  sheet  which 
he  used. 
The  general  conclusion  is  therefore  that  uranium,  thorium, 
and  radium  eject  a  particles  of  nearly,  if  not  exactly,  the  same 
speed.  Considering  the  many  parallelisms  already  known  to 
exist  between  the  processes  of  disintegration  of  these  substances 
and  their  products,  this  new  fact  is  certainly  suggestive.  It 
would  be  very  interesting  to  know  the  ranges  of  Th  X  and  Th 
emanation. 
An  expression  is  found  in  Part  I.  of  this  paper  for  the 
total   ionization    over  an   uncovered   deep    layer    of   active 
*  An  experiment  by  Schmidt  (Fhys.  Zeit.  No.  25,  p.  897)  has  shown 
that  Ea  A  has  two-thirds  of  the  penetrating  power  of  Ka  O.  Hence  its 
range  must  be  the  longer  of  the  two  intermediate  ranges,  determined  by 
Kleeman  and  myself,  viz.  4-83  ;  and  the  range  of  the  emanation  must  be 
4-23.  Thus  in  the  radioactive  sequence  each  explosion  is  more  violent 
than  the  last. 
