"0 
550      Mr.  H.  E.  Hurst :  Genesis  of  Ions  by  Collision  and 
5  per  cent,  of  oxygen  gives  a  minimum  spark-potential  of 
about  312  volts. 
In   the    tables    before    given  it  will    be  noticed  that  the 
theoretical  values  of  n,  calculated  from  the  formula 
(a-ff)6(«-fl* 
agree  with  the  values  of  the  current  q  determined  experi- 
mentally, so  that  the  theory  furnishes  an  explanation  of  the 
currents  which  take  place  in  a  gas  before  sparking  occurs. 
In    preceding    papers    on    this    subject    it   was    shown   that 
—  and  —  are  functions  of  — ,  and  the  accompanying  curves, 
figs.  4  and  5,  show  the  relations  between  these  variables.     In 
15 
_ 
9 
8 
: 
Fig 
4. 
^ 
*<* 
-~~~~} 
> 
& 
/i1"1 
I 
j 
/ 
I 
/ 
/ 
/ 
zRe 
'ertc 
first 
mitogen. 
' 
/ 
,<m 
'tree 
secer 
d  nUroger 
'- 
/ 
f 
/ 
f 
, 
/ 
3 
I 
w\ 
1k- 
/ 
i 
r 
n  1      i 
i00    ZOO    300    400 
X-^ 
I000   IIOO    1200   1300  1400  1500    I600    )700  1S00    (900    2C00 
those  for  nitrogen,  the  curves  relate  to  the  first  set  of  experi- 
ments, and  the  values  for  the  second  experiments  were  put 
in  after  the  curves  were  drawn.  It  will  be  seen  that  the 
discrepancy  between  the  two  sets  is  comparable  with  the 
errors  of  experiment,  so  that  we  shall  give  the  following 
conclusions  drawn  from  the  results  of  the  first  experiments, 
since  it  seems  that  the  presence  of  small  quantities  of 
impurities  only  produces  small  errors.      The  larger  values 
a  -1SVP 
of  a    can    be    represented    by    the    formula    -  =  Ne      x 
(J.  S.  Townsend,  Phil.  Mag.  April  1903),  obtained  on  the 
assumption  that  new  ions  are  formed  by  collision  when  the 
colliding  ion  has  a  velocity  due  to  a  fall  of  potential  <fc  V. 
