﻿Scattering 
  of 
  a 
  Particles 
  by 
  Gases. 
  

  

  703 
  

  

  variations, 
  a 
  very 
  large 
  number 
  would 
  have 
  to 
  be 
  counted 
  

   in 
  order 
  to 
  obtain 
  a 
  reliable 
  result. 
  This 
  would 
  involve 
  

   much 
  time 
  and 
  labour, 
  and 
  it 
  was 
  consequently 
  felt 
  desirable 
  

   to 
  use 
  a 
  more 
  indirect 
  but 
  rapid 
  method. 
  

  

  The 
  experimental 
  arrangement 
  finally 
  adopted 
  is 
  shown 
  in 
  

   fig. 
  1. 
  A 
  narrow 
  pencil 
  of 
  a 
  particles 
  was 
  obtained 
  by 
  placing 
  

  

  Fig. 
  1. 
  

   w 
  

  

  ■=M 
  

  

  a 
  thin 
  platinum 
  wire 
  W 
  (see 
  fig. 
  1), 
  coated 
  with 
  radium 
  C, 
  

   between 
  two 
  thick 
  glass 
  plates 
  A 
  and 
  B, 
  14*5 
  cm. 
  long 
  and 
  

   2 
  cm. 
  in 
  width, 
  and 
  kept 
  about 
  0*3 
  mm. 
  apart 
  by 
  mica 
  stops. 
  

   After 
  traversing 
  the 
  space 
  between 
  the 
  glass 
  plates, 
  the 
  beam 
  

   passed 
  through 
  a 
  mica 
  window 
  M, 
  of 
  stopping 
  power 
  equi- 
  

   valent 
  to 
  1*9 
  cm. 
  of 
  air, 
  and 
  then 
  into 
  an 
  ionization 
  chamber 
  

   D, 
  consisting 
  essentially 
  of 
  three 
  parallel 
  equidistant 
  brass 
  

   plates 
  5 
  cm. 
  long 
  and 
  3 
  cm. 
  wide, 
  7 
  mm. 
  apart. 
  The 
  outer 
  

   plates 
  were 
  earthed 
  and 
  the 
  central 
  one 
  insulated 
  and 
  con- 
  

   nected 
  to 
  the 
  gold-leaf 
  system 
  of 
  an 
  electroscope 
  E. 
  The 
  

   latter 
  was 
  kept 
  exhausted 
  in 
  order 
  to 
  diminish 
  the 
  ionization 
  

   due 
  to 
  /3 
  and 
  <y 
  rays. 
  The 
  upper 
  part 
  of 
  the 
  apparatus, 
  con- 
  

   sisting 
  of 
  a 
  cylindrical 
  glass 
  tube 
  (with 
  ground-glass 
  ends) 
  

   containing 
  the 
  plates 
  A 
  and 
  B 
  and 
  the 
  source 
  of 
  a 
  rays 
  W, 
  

   could 
  be 
  completely 
  exhausted 
  and 
  filled 
  with 
  any 
  gas 
  or 
  

   vapour 
  to 
  any 
  desired 
  pressure, 
  measured 
  in 
  the 
  usual 
  way 
  

   by 
  a 
  mercury 
  gauge. 
  It 
  was 
  necessary 
  to 
  employ 
  a 
  strong 
  

   source 
  of 
  « 
  particles, 
  and 
  this 
  was 
  obtained 
  by 
  exposing 
  

   about 
  one 
  centimetre 
  length 
  of 
  thin 
  platinum 
  wire 
  0*2 
  mm. 
  

   diameter 
  to 
  radium 
  emanation 
  for 
  three 
  hours 
  as 
  cathode 
  in 
  

   an 
  electric 
  field. 
  A 
  short 
  time 
  after 
  removal 
  from 
  the 
  

   emanation, 
  radium 
  C 
  is 
  the 
  only 
  a-ray 
  product 
  remaining 
  on 
  

   the 
  wire, 
  and 
  since 
  its 
  curve 
  of 
  decay 
  is 
  accurately 
  known, 
  

   the 
  amount 
  of 
  radium 
  C 
  present 
  at 
  any 
  subsequent 
  time 
  can 
  

   be 
  readily 
  calculated. 
  Care 
  was 
  taken 
  to 
  remove 
  any 
  traces 
  

  

  