[     466     ] 
XLI.  On  the  Recombination  of  Ions  in  Air  and  other  Gases. 
By  W.  H.  Bragg,  M.  JL,  Elder  Professor  of  Mathematics 
and  Physics  in  the  University  of  Adelaide;  and  R.  D. 
Kleeman,  B.Sc,  Demonstrator  *, 
IT  is  well  known  that  when  positive  and  negative  ions  are 
distributed  through  a  given  space,  a  process  of  com- 
bination goes  on  until  ions  of  one  sign  only  are  left.  Let 
there  be  p  positive  ions,  and  n  negative  ions  in  each  cubic 
centimetre  at  any  instant,  and  suppose  that  the  relations  of 
any  ion  to  all  those  of  opposite  sign  are  of  the  same  character. 
Then  the  chance  that  an  ion,  say  a  positive  one,  will  enter 
into  combination  before  the  end  of  a  short  time  Bt  is  propor- 
tional to  n$t  ;  and  generally  the  number  of  combinations 
taking  place  in  that  time  may  be  denoted  by  apnSt,  where  a 
is  the  "  co-efficient  of  recombination/'  This  has  been  clearly 
established  by  the  experiments  of  Rutherford,  Townsend, 
McClung,  Langevin,  and  others. 
As  a  consequence,  the  current  passing  between  two  elec- 
trodes in  a  gas  in  which  ions  are  being  formed  by  external 
agents  depends  on  the  magnitude  of  the  potential  gradient 
or  electric  force.  The  relations  between  current  and  force 
have  been  carefully  studied  by  many  workers,  and  the 
observed  facts  have  been  compared  with  the  results  of  cal- 
culation based  on  theory.  The  comparison  is  partly,  but  not 
completely,  satisfactory. 
Certain  experimental  results  which  we  propose  to  describe 
in  this  paper  seem  to  throw  light  on  the  reason  of  the  dis- 
crepancy. They  point  to  the  existence  of  another  cause, 
distinct  from  that  represented  by  the  expression  anp,  which 
prevents  ions  from  reaching  the  electrodes  in  the  gas  in  which 
they  are  formed.  This  cause  appears  to  be  a  process  of  re- 
combination of  newly-formed  ions  with  the  atoms  from  which 
they  have  just  been  separated.  The  effects  of  it  are  propor- 
tional to  the  number  of  ions  formed  in  a  c.cm.  in  unit  time, 
not  to  the  product  of  the  existing  numbers  of  positives  and 
negatives.  They  are  independent  of  the  shape  of  the  ioni- 
zation chamber,  and  in  this  they  differ  from  those  of  general 
recombination.  They  depend  directly  on  pressure,  and  vary 
greatly  from  gas  to  gas. 
In  order  to  bring  these  effects  into  relief,  it  is  only  necessary 
to  reduce  the  number  of  ions  in  a  c.cm.  until  the  number  of 
those  that  are  lost  by  general  recombination  is  negligible 
compared  with  the  number  of  those  that  are  formed.     When 
*  Communicated  by  the  Authors.  From  the  '  Transactions  of  the  Royal 
Society  of  South  Australia,'  vol.  xxix.  1906". 
