552  Genesis  of  Ions  by  Collision. 
values  of  — ,  one  could  be  obtained  on  the  hypothesis  that 
some   collisions  after  velocities  less  than  a  critical  velocity 
are  productive  of  new  ions.     This  view  obtains  some  support 
from  the  fact  that  the  curves  for  —  are  very  similar  to  those 
p  j 
portions  of  the  —  curves  corresponding  to  the  smaller  values 
of  -. 
p 
It  will  be  noticed  that  the  values  of  a  for  C02  and  nitrogen 
are  of  the  same  order  of  magnitude,  but  that  /3  is  much 
smaller  in  C02  than  in  nitrogen.  It  has  been  shown  that  all 
ions  have  the  same  charge.  Now,  if  we  assume  that  a  positive 
ion  is  approximately  of  the  same  size  as  the  molecules  of  the 
gas  in  uhich  it  is  formed,  and  a  negative  ion  very  small 
compared  with  these,  a  positive  ion  in  travelling  a  given 
distance  will  make  about  four  times  as  many  collisions  as  a 
negative  ion  in  travelling  the  same  distance.  Therefore  a 
negative  ion  in  a  gas  at  1  mm.  pressure  will  make  approxi- 
mately the  same  number  of  collisions  per  centimetre  as  a 
positive  ion  in  the  same  gas  at  "25  mm.  pressure.  In  C02  at 
1  mm.  pressure  the  number  of  collisions  made  by  a  negative 
ion  in  going  1  cm.  is  17*4,  and  when  the  electric  intensity  is 
175,  a  is  2*4.  Now  at  j  mm.  pressure  ft  is  '001  for  the  same 
force.  So  that  for  an  equal  number  of  collisions  under  the 
same  conditions,  a  negative  ion  makes  about  2400  times  as 
many  new  ions  in  C02  as  a  positive  ion.  In  nitrogen  a 
negative  ion  would  make  about  17 '4  collisions  per  centimetre 
at  a  pressure  of  1*28  mms.  For  the  same  electric  intensity 
175,  a  is  1*28.  At  '32  mm.  pressure  the  number  of  collisions 
of  a  positive  ion  in  nitrogen  would  be  approximately  the  same 
as  in  the  cases  already  considered,  and  for  intensity  175,  {3  is 
about  -058.  Under  these  conditions  then,  a  negative  ion  in 
nitrogen  produces  22  times  as  many  new  ions  as  a  positive 
ion  in  going  a  given  distance.  Hence  the  positive  ion  in 
nitrogen  differs  less  from  the  negative  ion  than  does  the 
positive  ion  in  C02.  Since  all  negative  ions  are  the  same, 
this  will  be  the  case  if  negative  ions  are  small  compared  with 
positive  ions,  and  positive  ions  in  nitrogen  smaller  than  those 
in  carbon  dioxide.  There  is  a  considerable  difference 
between  the  values  of  ft  for  nitrogen  and  those  for  air, 
ft  being  larger  in  nitrogen  than  in  air.  Fiom  this  it  would 
seem  that  ft  for  oxygen  is  small. 
In  conclusion,  1  wish  to  express  my  thanks  to  Professor 
Townsend  for  his  advice  and  assistance  during  the  course  of  the 
experiments,  and  also  to  Dr.  H.  B.  Baker  for  his  criticism  of 
some  points  connected  \vith  the  preparation  of  the  gases. 
