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  604 
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  LXIV. 
  On 
  the 
  Retardation 
  of 
  Alpha 
  Rays 
  hy 
  Metals 
  and 
  

   Gases, 
  % 
  T. 
  S. 
  Taylor 
  *. 
  

  

  Introduction, 
  

  

  IN 
  a 
  preliminary 
  paper 
  t 
  *' 
  On 
  the 
  Retardation 
  o£ 
  Alpha 
  

   Rays 
  by 
  Metal 
  Foils 
  and 
  its 
  Variation 
  with 
  the 
  Speed 
  

   o£ 
  the 
  Alpha 
  Particles," 
  the 
  writer 
  described 
  some 
  experi- 
  

   ments 
  which 
  showed 
  clearly 
  that 
  the 
  air-equivalents 
  o£ 
  metal 
  

   foils 
  decrease 
  with 
  the 
  range 
  o£ 
  the 
  alpha-particles 
  entering 
  

   the 
  foils 
  J. 
  By 
  "air-equivalent" 
  is 
  meant 
  the 
  amount 
  by 
  

   which 
  the 
  range 
  of 
  the 
  alpha-particles 
  in 
  air 
  is 
  cut 
  down 
  by 
  

   their 
  passage 
  through 
  the 
  foil. 
  It 
  was 
  shown 
  that 
  the 
  change 
  in 
  

   the 
  air-equivalents 
  is 
  small 
  for 
  thin 
  foils 
  of 
  the 
  lighter 
  metals 
  

   when 
  the 
  speed 
  of 
  the 
  alpha-particles 
  entering 
  the 
  sheets 
  is 
  

   high 
  : 
  but, 
  when 
  the 
  speed 
  of 
  the 
  particles 
  is 
  low 
  for 
  thin 
  

   sheets 
  or, 
  when 
  the 
  sheets 
  are 
  thicker, 
  the 
  change 
  becomes 
  

   quite 
  marked. 
  A 
  comparison 
  of 
  the 
  change 
  for 
  sheets 
  of 
  

   difEerent 
  metals 
  of 
  nearly 
  equal 
  air-equivalents 
  showed 
  the 
  

   rate 
  of 
  change 
  to 
  be 
  in 
  the 
  order 
  of 
  the 
  atomic 
  weights 
  of 
  

   the 
  metals. 
  The 
  results 
  obtained 
  in 
  these 
  experiments 
  were 
  

   not 
  sufficient 
  to 
  furnish 
  an 
  explanation 
  of 
  the 
  phenomenon 
  ; 
  

   but 
  the 
  continuation 
  of 
  the 
  experiments 
  during 
  the 
  last 
  year 
  

   under 
  somewhat 
  different 
  conditions 
  has 
  furnished 
  results 
  

   which 
  do 
  lead 
  to 
  conclusions 
  of 
  some 
  interest. 
  

  

  Scattering 
  of 
  the 
  Alpha 
  Rays. 
  

   In 
  the 
  determination 
  of 
  the 
  variation 
  in 
  the 
  air-equivalents 
  

   w4th 
  the 
  speed 
  of 
  the 
  alpha-particle, 
  as 
  described 
  in 
  the 
  

   paper 
  cited 
  above, 
  the 
  source 
  of 
  rays 
  (polonium) 
  with 
  the 
  

   metal 
  sheet 
  over 
  it 
  was 
  set 
  at 
  such 
  a 
  distance 
  from 
  the 
  

   ionization-chamber 
  that 
  some 
  part 
  of 
  the 
  top, 
  or 
  nearly 
  

   horizontal, 
  portion 
  of 
  the 
  Bragg 
  ionization-curve 
  fell 
  within 
  

   the 
  ionization-chamber. 
  A 
  slight 
  increase 
  in 
  the 
  range 
  of 
  

   the 
  particle 
  in 
  this 
  portion 
  of 
  curve 
  corresponds 
  to 
  a 
  con- 
  

   siderable 
  increase 
  in 
  the 
  ionization. 
  With 
  the 
  polonium 
  set 
  

  

  * 
  Communicated 
  by 
  Professor 
  H. 
  A. 
  Bumstead. 
  

  

  t 
  American 
  Journal 
  of 
  Science, 
  vol. 
  xxvi. 
  pp. 
  169-179, 
  Sept. 
  1908. 
  

  

  X 
  The 
  phenomenon 
  upon 
  which 
  this 
  work 
  was 
  based 
  was 
  first 
  observed 
  

   by 
  Mme 
  Curie 
  and 
  has 
  later 
  been 
  investigated 
  by 
  several 
  others. 
  Bragg 
  

   & 
  Kleeman 
  (Phil. 
  Mag. 
  Sept. 
  1905, 
  and 
  April 
  1907) 
  observed 
  that 
  the 
  

   stopping 
  power 
  of 
  a 
  metal 
  was 
  not 
  independent 
  of 
  the 
  speed. 
  Kucera 
  & 
  

   Masek 
  {Phys. 
  Zeitschrift, 
  xix. 
  pp. 
  630-40, 
  1906) 
  and 
  Meitner 
  {Phys. 
  

   Zeitschrift, 
  viii. 
  p. 
  489, 
  1907) 
  ascribe 
  the 
  effect 
  to 
  a 
  difference 
  in 
  the 
  

   amount 
  of 
  scattering. 
  McClung 
  (Phil. 
  Mag. 
  Jan. 
  1906), 
  Kutherford 
  

   (Phil. 
  Mag. 
  Aug. 
  1906), 
  and 
  Levin 
  {Phys. 
  Zeitschrift, 
  xv. 
  pp. 
  619-521, 
  

   1906) 
  obtained 
  results 
  which 
  indicate 
  that 
  each 
  successive 
  layer 
  of 
  

   aluminium 
  foil 
  diminishes 
  the 
  range 
  of 
  the 
  a-particle 
  by 
  the 
  same 
  

   amount. 
  

  

  