This  early  led  to  the  view  that  the  ion  consists  of  a  charged
centre  surrounded  by  a  cluster  of  molecules  travelling  with  it,
which  are  kept  in  position  round  the  charged  nucleus  by  electrical
forces*.  A  rough  estimate  shows  that  this  cluster  consists  of  about
30  molecules  of  the  gas.  This  idea  is  supported  by  the  variation
in  velocity,  i.e.  the  variation  of  the  size  of  the  negative  ion,  in  the
presence  of  water  vapour;  for  the  negative  ion  undoubtedly
appears  to  have  in  most  cases  a  greater  mass  in  moist  that  in  dry
.  gases.  At  the  same  time  it  is  possible  that  the  apparently  large
size  of  the  ion,  as  determined  by  diffusion  methods,  may  be  in
part  a  result  of  the  charge  carried  by  the  ion.     The  presence  of

*  Rutherford,  Phil.  Mag.  M,  p.  422,  1897.
nj  lONISATION   OF  GASES  57

t  charge  on  a  moving  body  would  increase  the  frequency  of
collision  with  the  molecules  of  the  gas,  and  consequently  diminish
the  rate  of  diffusion.  The  ion  on  this  view  may  not  actually  be
of  greater  size  than  the  molecule  from  which  it  is  produced.
Theories  to  take  into  account  the  effect  of  the  charge  of  the
ion  on  its  rate  of  diffusion  have  been  advanced  by  Langevin*  and
WelUschf.  The  latter  concludes  that  the  experimental  results
can  be  explained  by  assuming  that  the  ions  have  the  same  mass
as  a  molecule  of  the  gas  in  which  they  are  formed.  From  physical
considerations,  however,  it  appears  very  probable  that  a  charged
ion  would  become  the  centre  of  an  aggregate  of  neutral  molecules
which  travel  with  it.  The  apparent  mass  of  the  ion  may  be  due
to  the  combined  effects  of  the  charge  and  of  the  attendant  cluster.
