74 
Colloids  and  Crystals. 
Am.  Jour.  Pharm. 
February,  1914. 
So  much  for  calculation.  Now  what  are  the  facts  ?  As  a  matter 
of  fact,  the  dispersed  substance  in  a  colloidal  solution  does  not  settle 
at  all,  so  long  as  the  subdivision  is  maintained.  Colloidal  gold  solu- 
tions have  been  preserved  unchanged  for  years.  I  have  a  solution 
of  arsenious  sulphide  which  has  remained  apparently  unchanged  for 
three  years  and  whose  countless  particles  can  readily  be  seen,  engaged 
in  their  incessant  Brownian  movement,  with  an  ordinary  oil  immer- 
sion lens.  Whenever  settling  does  occur,  it  is  preceded  by  the 
aggregation  of  the  particles  into'  larger  particles,  which  finally  attain 
a  diameter  of  fx  or  over,  and  slowly  subside. 
Here,  then,  is  an  apparent  discrepancy  between  Stokes'  law  and 
the  facts.  The  law  informs  us  that  the  speed  of  subsidence  decreases 
rapidly  with  decreasing  radius  of  the  particles,  but  it  does  not  lead 
us  to  expect  the  total  absence  of  settling  which  presents  itself  when 
the  average  radius  is  10  f-  /*  or  thereabout. 
The  explanation,  of  course,  is  molecular  motion,  or,  in  other 
words,  heat.  The  particles  are  battered,  on  all  sides,  by  a  hail-storm 
of  molecular  impacts.  If  the  particle  is  large,  the  blows  of  the  mole- 
cules of  the  solvent  in  different  directions  neutralize  each  other. 
But  when  the  particle  is  not  so  very  much  larger  than  the  molecules 
themselves  a  molecule  striking,  say  on  the  left,  will  give  the  particle 
a  very  perceptible  push  toward  the  right,  "just  as  a  cork  follows  better 
than  a  large  ship  the  motion  of  the  waves  of  the  sea."  2  As  the 
dimensions  of  the  particle  approach  the  molecular  dimensions  it 
begins  to  behave  like  a  molecule  and  is  swept  along  in  the  endless 
molecular  movement.  The  cause  which  prevents  the  particles  in  a 
colloidal  solution  from  settling  is  in  no  way  different  from  the  cause 
which  prevents  the  earth's  atmosphere  from  subsiding  to  a  snowy 
layer  a  few  feet  deep  on  the  surface  of  the  planet. 
It  is  worth  remembering,  also,  that  the  particles  of  the  dispersed 
phase  ordinarily  possess  an  electric  charge,  which  is  usually  negative. 
The  effect  of  the  repulsion  of  these  similar  charges  would  be  to*  pre- 
serve the  distribution  of  the  particles  throughout  the  liquid.  It  is  a 
fact  that,  when  the  charges  are  removed,  the  system  becomes  instable 
and  subsidence — preceded  by  coalescence  of  the  small  particles — 
readily,  but  not  necessarily,  occurs. 
2  Perrin. 
