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XLVI.    The  Retardation  of  the  Velocity  of  the  a  Particles  in 
passing  through  Matter. 
To  the  Editors  of  the  Philosophical  Magazine. 
Gentlemen, 
IN  the  Philosophical  Magazine  of  July  1905,  I  gave  an 
account  of  some  preliminary  experiments  on  the  retar- 
dation of  the  velocity  of  the  a.  particles  from  radium  C  in 
passing  through  matter. 
Using  an  active  wire  coated  with  radium  G  as  a  homo- 
geneous  source  of  radiation,  the  velocity  of  the  ol  particles  was 
found  to  decrease  in  passing  through  aluminium,  and  the 
lowest  value  of  the  velocity  observed  was  *64Vo,  where  Vo  is 
the  initial  velocity  of  projection  of  the  a  particles  from  the 
hare  wire. 
I  have  recently  repeated  these  observations  under  much 
better  experimental  conditions,  using  more  active  wires,  and 
with  the  photographic  plate  closer  to  the  source  of  rays. 
I  have,  in  consequence,  been  able  to  measure  the  velocity  of 
the  a.  particles  after  their  passage  through  a  greater  thickness 
of  aluminium.  The  lowest  value  of  the  velocity  which  could 
be  measured  was  *43V0.  The  photographic  effect  observed  on 
the  plate  was  very  feeble,  and  certainly  less  than  4  per  cent, 
of  that  found  for  the  unscreened  wire.  The  thickness  of 
aluminium  in  this  case  was  equal  in  absorbing  power  to 
almost  7.  cms.  of  air — the  maximum  range  of  the  ionization  of 
the  a.  particle  from  radium  C. 
As  I  pointed  out  in  my  previous  paper,  such  a  result 
suggests  that  the  ionization  produced  by  the  a  particle  falls 
off  rapidly  when  its  velocity  falls  below  a  certain  critical 
value.  The  deduction  from  experiments  of  this  kind  are, 
however,  complicated  by  a  scattering  of  the  a  rays  in  their 
passage  through  matter.  This  is  initially  small,  but  increases 
with  the  diminution  of  the  velocity  of  the  a.  particle. 
I  have  determined  the  value  of  ejm  for  the  a.  particle  from 
radium  C  after  passing  through  a  screen  equal  in  absorbing- 
power  to  5*5  cms.  of  air,  and  found  it  the  same  as  for  the 
a.  particle  from  the  bare  wire.  This  experiment  shows  that 
the  a  particle  retains  its  charge  and  mass  unaltered  over  a 
greater  part  of  its  range  in  air. 
Further  experiments  are  in  progress  to  see  if  it  is  possible 
to  detect  by  special  methods  the  presence  of  the  a.  particles 
after  passing  through  an  absorbing  screen  equal  to  7  cms.  of 
air.     It  is  hoped  that,  by  such  methods,  it  may  be  possible  to 
Phil.  .1%.  S.  6.  Vol.  11.  No.  64.  April  H>06.        :?  0 
