Electrically  Prepared  Colloidal  Solutions.  433 
fluid  layer  follows  the  flow  of  positive  electricity  while  the 
particle  moves  in  the  opposite  direction.  If  the  liquid  were 
a  perfect  insulator  the  new  position  would  still  be  a  condition 
of  equilibrium.  Since,  however,  through  the  displacement 
of  the  layers  the  equilibrium  of  the  galvanic  tension  between 
the  solid  particle  and  the  liquid  is  disturbed,  and,  on  account 
of  the  conductivity  of  the  liquid,  always  seeks  to  restore  itself, 
the  original  state  of  electrical  distribution  will  tend  to  be 
continually  reproduced,  and  so  new  displacements  of  the 
particle  with  respect  to  the  surrounding  liquid  will  continually 
occur""  *. 
This  theory  was  put  forward  by  Helmholtz  in  the  course 
of  his  mathematical  development  of  the  explanation  (sug- 
gested by  Quincke)  of  the  electric  transport  of  conducting 
liquids  through  the  walls  of  porous  vessels  or  along  capillary 
tubes  ;  Quincke  assumed  that  there  existed  a  contact  dif- 
ference of  potential  between  the  fluid  and  its  solid  boundaries. 
Throughout  his  treatment  of  the  phenomenon,  Helmholtz  con- 
siders that  there  is  no  slipping  of  the  fluid  over  the  surface 
of  the  solids  with  which  it  is  in  contact.  On  this  point  Lamb 
disagrees  with  Helmholtz,  holding  that  the  solid  offers  a  very 
great,  but  not  an  infinite,  resistance  to  the  sliding  of  the  fluid 
over  it,  and  that,  while  the  effect  of  this  slipping  would  be 
entirely  insensible  in  such  experiments  as  those  of  Poiseuille, 
it  leads  to  appreciable  results  in  the  present  case  in  con- 
sequence of  the  relatively  enormous  electrical  forces  acting 
on  the  superficial  film  of  the  liquid  and  dragging  the  fluid, 
as  it  were,  by  the  skin  through  the  tube.  The  practical 
difference  between  the  views  taken  by  Helmholtz  and  Lamb 
respectively  may  be  shoAvn  in  a  simple  case.  By  comparing 
with  the  numerical  results  found  by  Wiedemann,  Helmholtz 
infers  that  for  a  certain  solution  of  CuS04  in  contact  with 
the  material  of  a  porous  clay  vessel,  the  contact  difference  of 
potential  E  between  the  solution  and  the  solid  wall  is  given 
by 
where  D  is  the  E.M.F.  of  a  DanielFs  cell.  The  variation 
introduced  by  Lamb  would  change  this  equation  into 
E     I 
where  d=  the  distance  between  the  plates  of  an  air-condenser 
equivalent  to  that  virtually  formed  by  the  opposed  surfaces 
*  Helmholtz,  loc.  cit. 
