Various  Gases  by  the  a.  Particles  of  Radium.  623 
produced  for  the  same  expenditure  o£  energy.  It  is  quite 
clear,  however,  that  this  does  not  imply  that  the  a.  particle 
finds  it  easier  to  produce  ions  in  some  gases  than  others. 
For,  if  so,  there  would  be  some  influence  on  the  stopping- 
power  of  atoms  dependent  on  the  number  of  ions  produced. 
But  the  stopping-power  is  connected  with  the  atomic  weight 
by  a  simple  law :  the  number  of  ions  produced  is  not. 
Plainly  the  energy  spent  by  an  a  particle  in  an  atom  and 
the  resulting  ionization  are  not  directly  connected :  there  is 
an  intervening  link. 
Either  the  ions  made  by  the  a  particle  produce  others  in 
some  cases,  or  some  of  the  ions  made  never  emerge  from  the 
atoms.  There  is  something  which  prevents  the  simplicity 
of  the  law  governing  the  expenditure  of  energy  by  the  ol 
particle  from  repeating  itself  in  the  amount  of  ionization 
produced.  I  think  it  is  increasingly  clear  from  our  experi- 
ments that  there  is  a  secondary  ionization  within  the  molecule 
itself.  The  ions  first  made,  or  possibly  X-ray  pulses 
accompanying  ionization,  have,  in  some  cases,  enough  energy 
to  make  fresh  ions  before  leaving  the  molecule.  Thus,  for 
example,  one  molecule  of  C6H6  is  found  to  rob  the  a,  particle 
of  just  as  much  energy  as  three  molecules  of  C2H2.  But 
more  ions  are  made  out  of  the  one  C6H6  than  out  of  the 
group  of  three  acetylene  molecules.  This  may  be  explained 
on  the  ground  that  the  twelve  atoms  are  crowded  together, 
so  that  an  ion  projected  under  ionization  from  one  of  the 
atoms  strikes  one  of  the  others  with  an  energy  undiminished 
by  motion  through  the  field  of  the  positive  from  which  it 
was  originally  separated,  and  so  capable  of  detaching  a 
second  electron.  In  further  consequence  the  ions  emerging 
from  a  C6H6  molecule  move  more  slowly  than  those  from  a 
C2H2,  and  are  more  liable  to  initial  recombination.  This  is 
in  agreement  with  experiment :  it  is  far  harder  to  saturate 
benzene  than  acetylene. 
The  secondary  ionization  would  appear  to  take  place 
within  rather  than  without  the  molecule,  because  the  amount 
of  it  does  not  depend  upon  the  distance  of  the  molecules 
from  one  another.  The  total  ionization  is  independent  of 
the  pressure.  It  is  certainly  not  due  to  the  electric  field, 
for  if  it  were  there  would  be  no  saturation  value  of  the 
current. 
I  subjoin  the  details  of  two  of  the  many  experiments 
which  Mr.  Madsen  and  I  have  made.  We  hope  to  give  a 
fuller  description  at  some  future  time. 
