108  Prof.  E.  Rutherford 
on  some 
and  Kleeman  to  explain  their  results  on  the  absorption  o£ 
the  a  rays  of  radium  is  untenable. 
On  the  contrary,  M.  Becquerel  considers  that  the  a  par- 
ticles from  radium  all  escape  into  the  air  with  the  same 
velocity,  and  that  this  velocity  is  not  altered  by  their  passage 
through  matter.  In  order  to  explain  the  decreasing  curvature 
of  the  path  of  the  rays  in  their  passage  through  air,  first 
shown  by  him,  M.  Becquerel  supposes  that  the  mass  of  the 
a  particle  in  some  way  increases  in  its  passage  through  air. 
There  is  thus  a  complete  difference  of  opinion  between 
M.  Becquerel  and  myself  on  several  very  important  points. 
Before. entering  on  a  detailed  discussion  of  these  points  of 
difference,  I  shall  first  briefly  describe  the  experiments  on 
which  M.  Becquerel*  has  largely  based  his  conclusions. 
After  I  had  shown  by  the  electric  method,  with  the  aid  of 
the  weak  radium  preparation  then  in  my  possession  (activity 
19,000) ,  that  the  a  particles  were  deflected  both  by  a  magnetic 
and  electric  field,  M.  Becquerel,  with  a  very  active  prepara- 
tion, repeated  the  magnetic  deflexion  by  the  photographic 
method.  By  a  simple  and  ingenious  arrangement,  he  was 
able  to  obtain  on  the  one  photographic  plate  the  trace  of  the 
rays  deflected  in  a  magnetic  field  over  a  considerable  dis- 
tance of  their  path.  A  thin  layer  of  radium  served  as  a 
source  of  a.  rays.  A  narrow  beam  of  rays,  after  passing 
through  a  parallel  slit,  fell  on  a  photographic  plate  placed  at 
right  angles  to  the  slit  and  inclined  at  a  small  angle  with  the 
vertical.  By  reversing  the  magnetic  field  at  intervals  two 
diverging  lines,  showing  the  trace  of  the  rays  in  a  magnetic 
field,  were  obtained  on  the  photographic  plate. 
M.  Becquerel  states  that  the  rays  showed  no  appreciable 
dispersion  in  a  magnetic  Held,  for  no  difference  in  the  width 
of  the  lines  was  observed  for  fields  of  10,000  and  20,000  units. 
He  consequently  concluded  that  he  was  dealing  with  a  homo- 
geneous beam  of  rays.  By  measurement  of  the  distance 
between  the  two  diverging  traces  obtained  on  the  plate,  he 
found  that  the  radius  of  curvature  of  the  path  of  the  rays 
increased  with  distance  from  the  source.  This  was  a  very 
remarkable  and  important  result.  As  an  explanation  M. 
Becquerel  suggested  that  the  mass  of  the  a  particle  pro- 
gressively increases  with  distance  by  accretions  from  the 
air. 
In  his  last  paper  (Joe.  cit.)  M.  Becquerel  describes  another 
interesting  experiment,  the  results  of  which,  in  his  opinion, 
are  directly  in  contradiction  to  my  own.     To  quote  his  own 
words : — "  Je  me  suis  alors  propose  de  reprendre  avec  mes 
*  Comptes  Rendus,  exxxvi.  pp.  199,  431,  977,  1517  (1903). 
