Electrically  Prepared  Colloidal  Solutions.  445 
these  hypotheses,  contact  electrification  occurring  in  the  case 
of  the  coarser  suspensions  and  ionization  in  the  case  of  those 
which  approximate  more  nearly  to  colloidal  solutions." 
Comparing  the  results  given  for  the  sign  of  the  charges 
borne  by  the  particles  in  different  solutions,  we  have  the 
following  : — 
1.  Water   (H+.OH~)    can   form   two  classes   of  colloids, 
whose  particles  are  respectively  positively  and  nega- 
tivelv  charged. 
2.  Replacing  the  mobile  H+  by  the  groups  C3H5  and  CH3 
so  as  to  form  the  alcohols,  seems  to  destroy  the  power 
of  forming  solutions  with  negatively-charged  particles; 
while 
3.  Ethyl  malonate,  CH2(COOC2H5)2  which  has  the  mobile 
H  readily  forms  those  solutions  containing  the  nega- 
tively-charged particles,  and  those  only. 
It  is  thus  evident  that  the  formation  of  the  solution  depends 
on  the  chemical  nature  of  the  solvent. 
This  leads  to  the  following  theory  of  the  constitution  of  the 
solution  : — 
1.  In  the  case  of  gold,  silver,  and  platinum   in  water  and 
ethyl  malonate,  we  have  an  incomplete  chemical  com- 
bination with  the  liquid  :  thus  for  platinum  and  water 
we  have  the  equation  : — 
n  .  Pt  +  Ef.  0H~=  (Pt~H)  +  OH. 
We  may  look  upon  the  platinum-hydrogen  aggregate 
as  dissociating  slightly  so  as  to  form  an  atmosphere  of 
positively-charged  hydrogen  ions  about  the  negatively- 
charged  colloidal  particle. 
2.  With  the  other  metals  in  water  and  the  alcohols,  we 
have  a   corresponding  formation   of    the  hydroxides, 
thus 
Pb  +  H+.  OH  =  (Pbl .  OH)  +  II 
and  by  slight  dissociation  of  the  aggregate  (Pb«OH), 
we    obtain    a    positively-charged    colloidal    particle, 
surrounded  by  a  layer  of  OH-  ions  in  the  liquid. 
In  accordance  with  Helmholtz^s  explanation,  we  may  look 
upon   the   motion   in  an  electric  field  as    primarily  due  to 
electric  endosmose.      On  this  view,  in  formula  (5)  (p.  4o4) 
I  will  equal  d  and  K  will  be  the  specific  inductive  capacity 
of  the  liquid  medium.     Values  of  V  for  different  cases  arc 
