622  Prof.  W.  H.  Bragg  on  the  Ionization  of 
which  has  not  previously  been  the  subject  o£  measurement 
so  far  as  I  am  aware. 
To  sum  up,  the  range  of  the  a  particle  in  a  given  gas  is, 
in  the  first  place  easily  measured,  and  in  the  second  place  is 
simply  related  to  the  constitution  o£  the  gas  and  independent 
o£  its  state.  It  is  a  delightful  contrast  to  some  other  radio- 
active quantities,,  and  often  gives  a  welcome  foothold  in 
difficult  places. 
The  quantity  I  is  in  quite  a  different  class.  It  is  much 
more  difficult  to  measure  accurately,  as  I  have  already 
described.  But  there  appears  to  be  a  more  important 
difference  in  that  the  total  ionization  of  a  gas  is  not  simply 
dependent  on  the  weights  of  the  atoms  of  which  it  is  composed. 
Molecular  structure  counts  for  something.  Perhaps,  also, 
the  various  atoms  do  not  yield  ions  in  simple  proportion  to 
the  energy  spent  on  them,  but  this  point  is  not  yet  sufficiently 
clear. 
An  example  of  this  want  of  uniformity  has  already  been 
given  in  the  paper  to  which  reference  has  been  made.  It 
was  shown  that  RI  in  ethyl  chloride  is  much  greater  than 
HI  in  air.  The  difference  must  be  yet  a  little  greater  than 
that  shown,  as  no  allowance  was  made  for  the  small  quantity 
of  air  mixed  with  the  heavy  gas.  Again,  PI  in  standard 
pentane  (mostly  C5Hl2)  is  nearly  half  as  much  again  as  in 
air,  and  the  same  is  almost  certainly  true  of  benzene  (C6H6), 
but  this  vapour  is  harder  to  treat  than  pentane,  since  a  high 
temperature  is  necessary.  Generally  speaking,  the  more 
complex  gases  yield  the  greater  number  of  ions.  But  the 
yield  does  not  depend  only  on  the  number  of  atoms  in 
the  molecule.  Acetylene  (C2H2)  yields  25  per  cent,  more 
than  air,  yet  COo  with  only  one  atom  less  yields  but  5  per 
cent,  more  ;  and  ethylene  (C2H4)  yields  the  same  as 
acetylene,  though  it  has  two  atoms  more.  Of  course  in  the 
last  case  the  atoms  added  are  very  light;  and  H2  itself  has, 
according  to  ray  measurements,  a  slightly  lower  value 
(for  RI)  than  air.  Rutherford  also  found  this  to  be  the 
case. 
On  the  other  hand,  the  influence  of  complexity  can  be 
illustrated  by  the  cases  of  acetylene  and  ethylene  as  compared 
with  benzene  and  pentane. 
In  order  to  bring  out  the  significance  of  these  comparisons, 
it  should  be  pointed  out  that  the  a  particle  spends  exactly 
the  same  amount  of  energy  in  every  gas  (Bragg,  Phil.  Mag. 
Nov.  1905). 
Thus    in    different    gases    different    numbers    of   ions    are 
