Electrically  Prepared  Colloidal  Solutions.  443 
underneath  this  liquid,  very  stable  colloidal  solutions  were 
obtained ;  those  of  the  first  two  metals  named  are  apparently 
as  stable  as  the  corresponding  hydrosols,  while  the  gold 
solution  coagulates  at  the  end  of  a  month  or  so.  From  the 
velocities  which  are  given  in  Table  VII.,  it  will  be  seen  that 
these  particles  all  bear  a  negative  charge,  similar  to  those  of 
platinum,  silver,  and  gold  in  hydrosols.  When  the  other 
metals,  bismuth,  lead,  zinc,  and  iron  were  used,  a  colloidal 
solution  could  not  be  obtained. 
IX.    Theoretical  Considerations. 
There  is  practical  unanimity  in  the  opinion  that  these 
particles  in  colloidal  solutions  are  enclosed  by  a  double 
electric  layer,  the  electricity  of  one  sign  on  the  surface  of  the 
particle  being  in  equilibrium  with  an  equal  amount  of 
electricity  in  the  layer  of  liquid  immediately  surrounding  the 
particle.  It  is  a  matter  of  doubt  as  to  the  means  by  which 
this  double  layer  is  formed. 
Linder  and  Pieton  first  suggested  some  interaction  between 
the  liquid  and  the  particle: — "Experiment  seems  to  show 
that  if  the  solution  is  basic  or  tends  to  break  up  so  as  to 
leave  a  free  base  active,  the  motion  is  to  the  negative  pole. 
If  the  solution  is  acidic  (or  tends  to  break  up  so  as  to  leave 
a  free  '  H '  active)  motion  is  to  the  positive  pole." 
This  statement  is  remarkably  confirmed  by  the  deter- 
minations cited  in  the  present  paper  for  solutions  in  water, 
the  alcohols/ and  ethyl  malonate. 
Following  the  conclusion  given  by  Quincke  regarding 
suspensions  of  microscopic  particles  in  liquids,  many  writers 
have  been  contented  to  view  the  phenomenon  as  an  effect 
expressed  by  the  term  "  contact  electrification  3i ';  the  particles 
become  charged  by  the  rubbing  of  the  moving  particles  of 
the  liquid  itself  against  the  suspended  particles. 
The  recent  work  of  Perrin  *  has  produced  results  which 
throw  considerable  light  on  the  phenomena  of  electrification 
by  contact  between  liquids  and  solids.  By  measurements  of 
the  electric  osmose  of  liquids  through  diaphragms  of  various 
materials,  he  is  led  to  announce  these  two  laws : — 
1.  Electric  osmose  is  only  appreciable  with  ionizing  liquids, 
or,  in  other  words,  ionizing  liquids  are  the  only  ones 
which  give  strong  electrification  by  contact. 
2.  In  the  absence  of  polyvalent  radicals,   all   non-metallic 
substances  become  positive  in  liquids  which  are  acidic, 
and  negative  in  basic  liquids. 
*  Loc.  cit. 
2  G  2 
