Properties  of  the  a  Rays  from  Radium.  173 
First,  consider  the  experiments  with  an  uncovered  thick  layer 
o£  radium.  The  outside  edge  of  the  traces  obtained  on  the 
plate  by  reversal  of  the  field  will  be  due  to  the  most  deflected 
rays  which  are  just  able  to  produce  a  photographic  impression 
on  the  plate.  The  inner  edge  of  the  traces  will  correspond  to 
the  rays  which  are  least  deflectable,  i.  e.  which  are  expelled 
with  the  greatest  speed. 
Now  suppose  the  radium  is  covered  with  any  thickness  of 
foil,  provided  it  is  less  than  that  required  to  cutoff  completely 
the  photographic  effect  of  the  a.  rays.  All  the  a  particles 
will  be  reduced  in  speed,  and  the  more  slowly  moving  ones 
will  lose  their  photographic  effect.  The  edge  of  the  deflected 
trace  will,  however,  be  due  as  before  to  the  most  deflected 
rays  which  are  just  able  to  produce  an  appreciable  photo- 
graphic effect.  These  rays  will  have  exactly  the  same  velocity 
as  the  corresponding  rays  in  the  first  experiment,  and  conse- 
quently the  outside  edge  o£  the  photographic  traces  will  be 
continuations  of  that  observed  in  the  first  experiment.  This 
holds  equally  well  whether  the  experiments  are  made  in  air 
or  in  a  vacuum. 
Since,  however,  the  more  penetrating  rays  are  reduced  in 
velocity  by  their  passage  through  aluminium,  the  inside  edge 
of  the  photographic  traces  will  be  displaced  outwards.  This 
effect  will  be  extremely  difficult  to  detect  unless  a  very 
narrow  beam  of  rays  is  employed.  In  addition,  the  un- 
doubted scattering  o£  the  rays  in  air  (which  will  be  discussed 
later)  will  tend  to  obliterate  this  effect  in  experimenfs  similar 
to  those  of  M.  Becquerel,  which  were  made  in  air  at 
atmospheric  pressure. 
It  is  thus  seen  that  the  interesting  experiment  recorded  by 
M.  Becquerel  is  satisfactorily  explained  on  the  view  that  the 
particles  decrease  in  velocity  in  passing  through  matter; 
and,  taken  in  conjunction  with  my  own  experin  ents  on  the 
reduction  o£  speed  of  the  rays  from  radium  C  in  passing- 
through  matter,  affords  an  indirect  but  striking  proof  of  the 
general  correctness  of  the  theory  of  the  absorption  of  a  rays 
put  forward  by  Bragg.  It  is  not  possible  to  take  the 
position  that  the  rays,  emitted  from  an  active  wire  coated 
with  radium  0,  do  not  exist  in  radium  in  equilibrium ; 
for  apart  from  other  evidence,  McClung  (supra,  p.  131) 
has  shown  that  the  range  of  ionization  of  the  rays  from 
an  active  wire  coated  with  radium  C  is  identical  with  the 
range  of  the  corresponding  rays  in  radium  measured  by 
Bragg  and  Kleeman. 
