﻿produced 
  in 
  Gases 
  by 
  Ultra- 
  Violet 
  Light. 
  575 
  

  

  mass 
  and 
  linear 
  dimensions 
  compared 
  with 
  a 
  molecule 
  of 
  

   hydrogen. 
  

  

  9. 
  The 
  size 
  of 
  the 
  negative 
  ion, 
  when 
  acted 
  on 
  by 
  small 
  

   forces, 
  may 
  be 
  estimated 
  from 
  the 
  determination 
  of 
  the 
  rate 
  

   of 
  diffusion 
  or 
  the 
  velocity 
  under 
  an 
  electromotive 
  force. 
  

   In 
  determining 
  the 
  latter 
  quality 
  in 
  air, 
  Rutherford* 
  made 
  

   experiments 
  over 
  a 
  range 
  of 
  pressures 
  extending 
  from 
  

   atmospheric 
  pressure 
  to 
  34 
  mms. 
  At 
  the 
  lowest 
  pressure 
  a 
  

   force 
  of 
  about 
  50 
  volts 
  a 
  centimetre 
  was 
  used, 
  so 
  that 
  the 
  

  

  v 
  

   value 
  of 
  — 
  was 
  less 
  than 
  2. 
  The 
  results 
  of 
  Rutherford's 
  

  

  V 
  

   investigations 
  showed 
  that 
  the 
  negative 
  ions 
  are 
  the 
  same 
  

   size 
  over 
  this 
  range 
  of 
  pressures, 
  and 
  are 
  large 
  compared 
  with 
  

   the 
  molecules. 
  

  

  On 
  the 
  other 
  hand, 
  it 
  can 
  be 
  seen 
  from 
  the 
  experiments 
  

   described 
  in 
  this 
  paper, 
  that 
  the 
  ions 
  in 
  hydrogen 
  at 
  67 
  

   millimetres 
  pressure 
  are 
  of 
  small 
  mass 
  while 
  under 
  a 
  force 
  

   of 
  1750 
  volts 
  per 
  centimetre, 
  since 
  they 
  produce 
  others 
  by 
  

   collisions. 
  From 
  the 
  theory 
  and 
  the 
  experiments 
  which 
  

  

  'X\ 
  

  

  showed 
  that 
  a=pf( 
  — 
  J, 
  it 
  is 
  to 
  be 
  supposed 
  that 
  the 
  negative 
  

  

  ions 
  would 
  remain 
  of 
  small 
  mass 
  and 
  produce 
  others 
  by 
  

   collision 
  in 
  a 
  gas 
  at 
  any 
  pressure 
  provided 
  that 
  the 
  electric 
  

   force 
  is 
  sufficiently 
  strong. 
  This 
  immediately 
  follows 
  from 
  

   the 
  fact 
  that 
  the 
  velocity 
  at 
  collision 
  depends 
  on 
  the 
  value 
  of 
  

  

  — 
  (see 
  section 
  6). 
  

  

  P 
  

  

  It 
  would 
  appear 
  from 
  these 
  investigations, 
  that 
  when 
  ions 
  

  

  are 
  generated 
  in 
  a 
  gas 
  the 
  negative 
  ions 
  are 
  of 
  a 
  definite 
  size 
  

   independent 
  of 
  the 
  gas 
  and 
  small 
  compared 
  with 
  the 
  mole- 
  

  

  cules 
  of 
  hydrogen. 
  When 
  the 
  ratio 
  — 
  is 
  small 
  the 
  positive 
  

  

  and 
  negative 
  ions 
  quickly 
  acquire 
  the 
  properties 
  of 
  masses 
  

   which 
  are 
  large 
  compared 
  with 
  molecules, 
  an 
  effect 
  which 
  has 
  

   usually 
  been 
  attributed 
  to 
  the 
  formation 
  of 
  groups 
  of 
  mole- 
  

   cules 
  round 
  the 
  ions. 
  If 
  the 
  force 
  acting 
  on 
  the 
  gas 
  is 
  

  

  X 
  

  

  increased, 
  and 
  the 
  ratio 
  - 
  exceeds 
  a 
  certain 
  limit, 
  some 
  of 
  

  

  V 
  

   the 
  collisions 
  between 
  the 
  ions 
  and 
  the 
  molecules 
  will 
  be 
  

   sufficiently 
  violent 
  to 
  prevent 
  the 
  formation 
  of 
  a 
  group 
  of 
  

   molecules 
  round 
  the 
  ions. 
  A 
  further 
  increase 
  in 
  the 
  force 
  

   causes 
  the 
  negative 
  ions 
  to 
  collide 
  with 
  sufficient 
  velocity 
  to 
  

  

  * 
  E. 
  Rutherford, 
  Cambridge 
  Phil. 
  Soc. 
  vol. 
  ix. 
  pt 
  viii. 
  (1898). 
  

  

  2Q2 
  

  

  