536     Mr.  H.  E.  Hurst:  Genesis  of  Ions  hij  Collision  and 
is  given  by 
where  a.  is  the  number  of  new  ions  of  one  kind,  either 
positive  or  negative,  produced  by  a  negative  ion  in  travelling- 
through  1  centimetre  of  the  gas,  (3  is  the  number  produced 
by  a  positive  ion  in  travelling  through  1  centimetre,  and  d 
is  the  distance  between  the  parallel  plate  electrodes.  The 
values  of  a.  and  /3  depend  upon  the  electric  intensity  X 
between  the  plates,  and  also  on  the  pressure  p  of  the  gas. 
They  may  be  determined  for  any  values  of  X  and  _p,  by 
measuring  the  currents  for  different  distances  between  the 
electrodes,  X  and  p  being  kept  constant. 
It  was  also  shown  that  at  a  distance  d,  given  by 
a— ySe(a~^  =  0?  sparking  takes  place  when  the  difference 
of  potential  between  the  plates  is  X<^,  when  ultra-violet  light 
falls  on  the  negative  electrode.  If  a  is  the  value  of  d 
satisfying  this  equation 
_  log  a.  —  log  j3 
a~~      m-P      ~' 
The  carbon  dioxide  used  in  these  experiments  was  prepared 
by  the  action  of  pure  hydrochloric  acid  on  marble.  Before 
entering  the  drying  apparatus  it  passed  through  glass  wool 
and  over  calcium  chloride.  It  was  dried  by  passing  through 
sulphuric  acid  and  standing  over  phosphorus  pentoxide. 
In  the  following  tables  q  is  proportional  to  the  currents 
determined  experimentally,  for  fixed  values  of  X  and  p,  and 
different  distances  between  the  plates.  The  distance  d 
between  the  plates  is  given  in  centimetres,  and  X  the  electric 
intensity  in  volts  per  centimetre,  a  and  /3  are  calculated 
so  that  the  formula 
n  =  n0 / Sw^ 
a-(3e{a-m 
shall  be  in  agreement  with  the  currents  at  three  different 
distances.  n0  is  taken  as  unity.  Below  the  tables  are  given 
the  values  of  a  determined  from  the  equation 
log  ol  —  loo-  R 
Xa,  and  the  sparking-potential  V  determined  experimentally. 
It  will  be  seen  that  the  theoretical  values  of  n  agree  with 
the  values  of  the  current  q  determined  experimentally.  In 
some  of  the  tables  fi  is  too  small  to  be  determined  with  the 
