S parking-Potentials  in  Carbon  dioxide  and  Nitrogen.       551 
N  is  the  average  number  of  collisions  of  a  negative  ion  in 
moving  through  a  centimetre  of  gas  at  1  mm.  pressure.     In 
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100     200    300    400    500    600     700    800    900     1000  1100    1200    1300  1400  1500 
1700  1800    1900   2000 
the  case  of  carbon  dioxide  when  N=17'4  and  Y=  18*8  volts, 
the  values  of  a.  calculated  from  the  above  formula  agree  with 
those  obtained  from  the  curve  for  values  of  —  between  2000 
P 
and  350.     For  nitrogen  the  numbers  ^=13-6  and  V  =  28'5 
give  values  of  a,  agreeing  with  experiment  for  values  of   — 
between   175  and   525.      For  the  smaller  values  of   -  it  is 
V 
necessary  to  suppose  that  some  new  ions  are  formed  when  the 
velocity  on  collision  is  less  than  that  due  to  the  critical  fall 
of  potential  V. 
The  values  of  N  and  V  for  nitrogen  do  not  differ  much 
from  those  for  air  given  in  the  paper  previously  mentioned, 
and  the  curve  there  given  falls  between  those  for  nitrogen 
8 
and   carbon    dioxide.      It   is   not    possible   to    represent    — 
within  the  range  of  experiments  performed  by  a  formula  of 
TSCVp 
the  type  Ns    "  x  .      Perhaps,  as  in  the  case  of  the  smaller 
