﻿306 
  Dr. 
  Donnan 
  on 
  Condensation 
  of 
  the 
  Vapours 
  of 
  

  

  to 
  be 
  described 
  below 
  were 
  made 
  in 
  order 
  to 
  test 
  a 
  certain 
  

   view 
  of 
  the 
  matter, 
  which 
  may 
  be 
  briefly 
  sketched 
  as 
  follows. 
  

   In 
  a 
  solution 
  in 
  which 
  the 
  solute 
  is 
  more 
  or 
  less 
  ionized, 
  one 
  

   might 
  suppose 
  the 
  ions 
  to 
  be 
  surrounded 
  by 
  clusters 
  of 
  

   solvent-molecules 
  which 
  had, 
  so 
  to 
  speak, 
  " 
  condensed 
  " 
  round 
  

   them 
  and 
  opposed 
  an 
  obstacle 
  to 
  their 
  recombination. 
  Now 
  

   one 
  might 
  regard 
  this 
  state 
  of 
  things 
  as 
  being 
  caused 
  by 
  some 
  

   sort 
  of 
  " 
  specific 
  attraction 
  " 
  between 
  the 
  solvent- 
  molecules 
  

   and 
  electricity, 
  i. 
  e. 
  the 
  electrons 
  or 
  electrical 
  charges 
  which 
  

   are 
  associated 
  with 
  the 
  ions. 
  Were 
  this 
  the 
  case, 
  one 
  might 
  

   expect 
  this 
  specific 
  attraction 
  to 
  manifest 
  itself 
  in 
  other 
  wavs. 
  

   For 
  example, 
  if 
  electrical 
  nuclei 
  of 
  any 
  sort 
  were 
  present 
  in, 
  

   or 
  were 
  produced 
  by 
  any 
  means 
  in, 
  air 
  which 
  was 
  saturated 
  

   with 
  the 
  vapour 
  of 
  an 
  ionizing 
  liquid, 
  then 
  it 
  would 
  be 
  just 
  

   possible 
  that 
  the 
  specific 
  attraction 
  referred 
  to 
  above 
  might 
  

   help 
  to 
  produce 
  condensation 
  of 
  the 
  vapour 
  round 
  these 
  nuclei 
  

   under 
  suitable 
  conditions 
  ; 
  i. 
  e. 
  if 
  the 
  vapour 
  were 
  super- 
  

   saturated 
  by 
  a 
  sudden 
  adiabatic 
  expansion. 
  If 
  the 
  liquid 
  in 
  

   question 
  did 
  not 
  act 
  as 
  an 
  ionizing 
  solvent, 
  it 
  would 
  be 
  

   natural 
  to 
  expect 
  that 
  the 
  condensation 
  just 
  alluded 
  to 
  would 
  

   only 
  occur 
  when 
  the 
  vapour 
  entered 
  the 
  really 
  unstable 
  

   (labile) 
  region, 
  or 
  at 
  any 
  rate 
  would 
  only 
  be 
  produced 
  by 
  a 
  

   much 
  higher 
  degree 
  of 
  supersaturation. 
  

  

  The 
  very 
  interesting 
  experiments 
  of 
  C. 
  T. 
  R. 
  Wilson 
  * 
  on 
  

   the 
  condensation 
  of 
  water-vapour 
  in 
  the 
  presence 
  of 
  dust-free 
  

   air 
  and 
  other 
  gases, 
  have 
  rendered 
  it 
  extremely 
  probable 
  that 
  

   this 
  condensation 
  occurs 
  round 
  electrical 
  nuclei 
  of 
  some 
  sort. 
  

   It 
  was 
  found 
  that 
  an 
  adiabatic 
  expansion 
  produced 
  no 
  con- 
  

   densation 
  in 
  dust-free 
  air 
  saturated 
  with 
  aqueous 
  vapour 
  until 
  

   the 
  expansion 
  attained 
  to 
  a 
  certain 
  value, 
  viz., 
  1*25. 
  This 
  

   expansion 
  (ratio) 
  produces 
  a 
  small 
  shower 
  of 
  fine 
  rain, 
  and 
  

   expansions 
  ranging 
  from 
  the 
  above 
  value 
  up 
  to 
  1*38 
  produce 
  

   small 
  fine 
  showers 
  of 
  not 
  much 
  greater 
  density. 
  At 
  an 
  

   expansion 
  of 
  1*38, 
  however, 
  the 
  phenomenon 
  changes 
  in 
  

   character, 
  a 
  more 
  or 
  less 
  dense 
  fog 
  being 
  then 
  produced. 
  

   Now 
  air 
  or 
  any 
  other 
  gas 
  exposed 
  to 
  Rontgen 
  radiation 
  

   becomes 
  a 
  conductor 
  ; 
  and 
  the 
  experiments 
  of 
  Prof. 
  J. 
  J. 
  

   Thomson 
  and 
  his 
  fellow-investigators 
  have 
  made 
  it 
  practically 
  

   certain 
  that 
  this 
  conduction 
  is 
  of 
  a 
  convective 
  nature, 
  due 
  to 
  

   electrically-charged 
  nuclei 
  produced 
  in 
  the 
  gas. 
  The 
  im- 
  

   portance 
  of 
  this 
  in 
  the 
  present 
  connexion 
  lies 
  in 
  the 
  fact 
  that 
  

   Wilson's 
  experiments 
  showed 
  that 
  air 
  which 
  was 
  saturated 
  

   with 
  water-vapour 
  and 
  exposed 
  to 
  Rontgen 
  radiation 
  gives, 
  

  

  * 
  Phil. 
  Trans. 
  A. 
  1897, 
  vol. 
  clxxxix. 
  pp. 
  265-307 
  j 
  A. 
  1899, 
  vol. 
  cxciii. 
  

   pp. 
  289-308. 
  

  

  I 
  

  

  