478     Prof.  W.  H.  Brags:  and  Mr.  R.  D.  Kleeman  on  tfi 
OO 
ejected  electron  does  not  get  far  enough  away  from  its  parent 
atom  before  it  is  stopped  by  encountering  another  atom,  then 
diminution  of  pressure  ought  to  make  it  much  easier  to 
saturate.  But  this  is  a  well-known  fact  (Rutherford, 
Philosophical  Magazine,  vol.  xlvii.  p.  100).  In  order  to 
obtain  results  comparable  with  those  we  had  already  obtained 
at  ordinary  pressures,  we  made  several  experiments  in  which 
all  the  conditions  were  the  same,  except  that  the  pressure 
was  less  than  that  of  the  atmosphere.  Curve  C  in  fig.  2 
shows  the  results  of  such  an  experiment.  It'  this  is  compared 
with  the  other  curves  in  the  same  figure,  it  will  be  clear  that 
alteration  has  taken  place  in  the  very  portion  of  the  curve 
where  we  should  have  expected  it,  and  where  change  in  the 
strength  of  the  current  has  small  influence,  viz.,  all  along  the 
upper  part  of  the  curve  up  to  the  high  potential  end.  The 
saturation  current  per  sq.  cm.  was  about  10-13  amp.  In 
further  support  of  our  hypothesis  it  may  be  pointed  out  that 
it  gives  a  ready  explanation  of  an  experiment  due  to 
Rutherford,  and  described  by  him  in  the  Philosophical 
Magazine,  vol.  xlvii.  p.  1,58*.  He  found  that  the  satu- 
ration value  of  the  current  through  a  gas  could  be  obtained  for 
a  much  lower  potential  gradient  when  the  gas  was  drawn 
away  from  the  uranium  which  ionized  it,  and  treated  in  a 
separate  vessel.  This  is  to  be  expected  when  it  is  considered 
that  under  the  circumstances  of  the  experiment  initial 
recombination  was  wholly  absent. 
It  is  now  convenient  to  consider  these  phenomena  as  they 
are  manifested  in  other  gases  than  air.  It  is  well  known 
that  the  relations  between  current  and  potential  in  carbon 
dioxide  are  in  some  way  abnormal.  But  the  peculiarities  of 
this  gas  are  even  intensified  in  ethyl  chloride  (C2H5C1).  The 
fact  is  that  this  effect,  which  makes  it  difficult  to  draw  all  the 
ions  to  the  electrodes  in  the  case  of  air,  is  far  greater  in  more 
complex  gases,  and  thus  it  is  extremely  difficult  to  obtain  the 
saturation  current  unless  very  high  potentials  are  employed. 
We  find  it  necessary  to  use  a  potential  gradient  of  two  to 
three  thousand  volts  per  cm.  in  the  case  of  ethyl  chloride  at 
60  cm.  pressure.  In  the  investigations  which  were  made  by 
us  (Philosophical  Magazine,  September  1905),  with  regard 
to  the  ionization  curves  in  different  gases,  we  found  the 
currents  to  be  unexpectedly  small  in  the  case  of  some  gases. 
We  suggested  that  possibly  some  of  the  ions  made  by  the  a 
particles  did  not  get  away  from  their  parent  atoms.  We 
proposed  to  make  a  special  investigation  of  the  point,  and  it 
*  This  statement  may  need  amendment.  The  matter  is  discussed  in  a 
later  paper. 
