432  Mr.  E.  F.  Burton  on  the  Properties  of 
such  a  generalization  was  inexact.     They  give  the  following- 
results  : — 
Suspensions  of  aniline  blue,  arsenic  sulphide,,  indigo, 
iodine,  shellac,  silicic  acid,  starch,  and  sulphur  in  water, 
and  of  bromine  in  alcohol  show  attraction  to  the  positive 
electrode. 
The  following  materials  suspended  in  water  move  in  the 
opposite  direction  :  ferric  hydrate,  haemoglobin,  Hoffmann's 
violet,  Magdala  red,  methyl  violet,  and  rosaniline  hydro- 
chloride. 
As  a  conclusion  to  their  work,  Linder  and  Picton  make 
the  significant  statement  that  experiment  seems  to  showT  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,  i.  e.  the 
particle  is  positively  charged  ;  if  the  solution  is  acidic,  motion 
is  to  the  positive  pole,  and  consequently  the  particles  are 
negatively  charged. 
To  the  above  lists  may  now  be  added  from  the  results  of 
Bredig  for  electrically  prepared  solutions  :  suspensions  of 
gold,  platinum,  and  silver  show  convection  to  the  positive 
pole. 
The  theory  of  this  motion  of  finely  divided  particles  in 
suspension  in  liquids  was  long  since  propounded  by  von 
Helmholtz*  and  later  amplified  by  Lambf.  "Without 
assuming,  for  the  moment,  anything  writh  regard  to  the 
cause  of  the  formation  of  the  colloidal  solutions,  we  may 
apply  the  same  theoretical  considerations  to  the  movement 
of  these  particles. 
The  fundamental  assumption  is  that  when  a  particle  sus- 
pended in  a  liquid  becomes  charged,  there  exists  about  it  a 
double  electric  layer;  when  the  particle  is  negatively  charged 
there  is  a  layer  of  negative  electricity  on  the  surface  of  the 
solid  particle,  while  in  the  liquid  immediately  surrounding  it 
there  is  a  corresponding  layer  of  positive  electricity.  "  On 
the  whole  the  algebraic  sum  of  the  two  equals  zero,  and  the 
centre  of  gravity  of  the  complete  system,  solid  particle  and 
surrounding  positively  charged  fluid  layer  taken  together, 
cannot  be  moved  by  the  electric  forces  which  arise  from  the 
potential  fall  in  the  liquid  through  which  the  current  passes. 
HowTever,  the  electric  force  will  tend  to  bring  about  a  dis- 
placement, relatively  to  each  other,  of  the  positively  charged 
fluid  layer  and  the  negatively  charged  particle,  whereby  the 
*  Wied.  Annalen,  vii.  p.  337  (1879),  and  Memoirs  Loud.  Plrvs.  Soc. 
1888. 
f  Brit.  Assoc.  Rep.  1887,  p.  495. 
