Electrically  Prepared  Colloidal  Solutions.  431 
in  solution  is  2*2  X  10~u  c.cs.     Assuming  that  the  particles 
are  in  the  form  of  small  spheres,  the  mean  radius  being  <  a,1 
f  7ra?  =  22  x  10~15 
a=l-7xl0-r>. 
The  solutions  of  gold  and  platinum  gave  values  for  6  a '  of 
the  same  order  as  the  above,  but,  as  yet,  the  number  of  such 
determinations  made  permit  only  the  statement  that,  for 
colloidal  solutions  of  platinum,  gold,  and  silver  in  water,  the 
average  diameter  of  the  particles  lies  between  the  limits 
2-6  x  10-5.  Such  a  result  is  quite  in  keeping  with  the 
limits  assigned  bv  Bredig  from  theoretical  considerations 
(see  page  427). 
V.  Motion  of  Particles  in  an  Electric  Field. 
The  investigation  of  this  property  of  the  so-called  colloidal 
solutions  dates  from  the  work  of  Linder  and  Picton*.  Prior 
to  thatf,  experiments  on  the  motion  of  the  particles  of  ordinary 
suspensions  under  an  electric  field  had  been  carried  out  by 
various  workers,  and  briefly  the  results  obtained  were  as 
follows  : — 
The  particles  in  suspension  in  water,  of  starch,  platinum 
black,  finely  divided  gold,  copper,  iron,  graphite,  quartz, 
feldspar,  sulphur,  shellac,  silk,  cotton,  lycopodium,  paper, 
porcelain  earth,  asbestos,  move  towards  the  positive  pole. 
When  the  above  materials  are  suspended  in  a  similar 
manner  in  turpentine  oil,  they  all  move  towards  the  negative 
pole,  with  the  sole  exception  of  sulphur,  which  moves  in  the 
same  direction  in  turpentine  as  in  water. 
Fine  gas  bubbles  of  hydrogen,  oxygen,  air,  ethylene, 
carbon  dioxide,  and  small  liquid  globules  of  turpentine  and 
CS2  when  in  water  all  move  towards  the  positive  pole. 
Turpentine  globules  and  small  gas  bubbles  in  ordinary 
alcohol  move  to  the  positive  pole. 
Quartz  particles  and  air  bubbles  in  carbon  bisulphide 
move  to  the  negative  pole. 
These  results  led  to  the  conclusion  that  u  in  water  all  bodies 
appear,  through  contact,  to  become  negatively  charged,  while, 
through  rubbing  against  different  bodies,  the  water  becomes 
positively  charged.'"' 
When  Linder  and  Picton  J  tested  similar  properties  of  the 
particles  in   chemically  prepared  solutions,  they  found  that 
*  Loc.  cit. 
t  See  Wiedemann's  Eh-ldviciiat,  B.  i.  p.  1007  (1893). 
\  .Tour.  Chem.  Son.  lxxi.  p.  568  (1897). 
