444  Mr.  E.  F.  Burton  on  the  Properties  of 
In  explaining  these  results  he  suggests  the  hypothesis  that 
a  positive  electrification  of  a  wall  bathed  by  an  acidic  liquid 
is  formed  by  H+  ions  situated  in  the  stationary  liquid  layer 
immediately  contiguous  to  the  wall.  Opposed  to  it  at  a  small 
distance  there  will  be  a  corresponding  excess  of  negative  ions 
forming  another  layer.  If  the  wall  assumes  a  negative  charge 
iit  is  on  account  of  similar  action  of  the  0H~  ions.  It  is 
found  that  H  and  OH  ions  move  much  more  quickly  than 
other  ions;  if,  then,  we  explain  this  high  velocity  by  assuming 
that  they  are  smaller  than  other  ions,  we  should  expect  them 
to  penetrate  nearer  to  the  boundary  of  the  liquid  and  so 
muster  at  the  limiting  layer  of  a  liquid  an  excess  of  electric 
charges  of  one  sign.  Although  the  analogies  between  this 
phenomenon  of  electric  osmose  and  that  of  the  coagulation  of 
colloidal  solutions  undoubtedly  help  in  explaining  the  latter, 
the  formation  of  these  solutions  can  hardly  be  credited  to 
merely  physical  diffusion  of  the  H+  and  0H~  ions.  As  we 
have  seen  from  the  sign  of  charges  borne  by  particles  in 
solution,  it  is  those  which  appear  to  depend  on  a  replaceable 
H  which  are  negative  while  those  depending  on  a  replaceable 
OH  are  positive. 
The  matter  is  summed  up  quite  clearly  in  a  recent  state- 
ment of  Noyes  *  : — 
"  In  regard  to  the  cause  and  character  of  the  electrification 
two  assumptions  deserve  consideration  :  one,  that  it  is  simply 
an  example  of  contact  electricity,  the  colloid  particle  assuming 
a  charge  of  one  sign  and  the  surrounding  water  one  of  the 
other.  This  correlates  the  phenomena  of  migration  with  that 
of  electric  endosmose.  It  does  not,  howxever,  give  an  obvious 
explanation  of  the  facts  that  the  basic  colloidal  particles 
become  positively  charged  and  the  acidic  and  neutral  ones 
negatively  charged.  The  other  assumption  accounts  for 
these  facts.  According  to  it  the  phenomenon  is  simply  one 
of  ionization.  Thus  each  aggregate  of  ferric  hydroxide 
molecules  may  dissociate  into  one  or  more  ordinary  hydroxyl 
ions  and  a  residual  positively-charged  colloidal  particle,  and 
each  aggregate  of  silicic  or  stannic  acid  molecules  into  one  or 
more  hydrogen  ions  and  a  residual  negatively-charged 
colloidal  particle.  To  explain  the  behaviour  of  neutral 
substances  like  gold  and  quartz  by  this  hypothesis,  it  is  neces- 
sary to  supplement  it  by  the  assumption  that  in  these  cases 
it  is  the  water  or  other  electrolyte  combined  with  or  adsorbed 
by  the  colloidal  particles  which  undergoes  ionization.  It 
seems  not  improbable  that  there  may  be  truth  in  each  of 
*  Journ.  Amer.  Cliem.  Soc.  vol.  xxyii.  Xo.  2,  p.  85. 
