Electrically  Prepared  Colloidal  Solutions. 
437 
were  supported  at  a  convenient  level  in  the  two  limbs  o£  the  tube 
and  the  clear  water  allowed  to  rise  well  above  them.  The  elec- 
trodes were  attached  to  the  terminals  of  a  set  of  storage-cells 
of  constant  voltage,  and  when  the  circuit  was  completed,  the 
surface  of  separation  in  one  limb  would  at  once  begin  to  rise 
gradually,  while  that  in  the  other  sank.  In  practice  the 
connexions  were  made  through  a  reversing  key,  and  the 
voltage,  usually  fixed  at  about  110  volts,  was  left  on  one  way 
for  ten  minutes  and  then  reversed  for  twenty  minutes.  The 
velocity  was  reckoned  from  the  displacement  of  the  surfaces 
during  this  final  twenty  minutes ;  one  half  the  sum  of  the 
displacements  in  the  two  tubes  was  taken  as  the  distance 
travelled  by  a  particle  in  the  given  time.  A  typical  set  of 
observations  is  given  in  Table  I. 
Table  I. 
Time. 
Voltage. 
Sign  of  L. 
Temp. 
Height  of  Colloidal 
Surface. 
Observed 
Velocity 
Left, 
Eight. 
in  cm. ,sec. 
11-37 
+  118 
11°  C. 
54  mm. 
55  mm. 
11-47 
+-118 
61 
50 
Current  off 
11-48 
-118 
11°  c. 
61 
50 
I 
11-58 
-118 
55 
56 
t  96  X  10~° 
1208 
-118 
11°  c. 
50 
62 
1 
Elech^odes  at  15  mms.  in  each  limb. 
It  will  be  seen  from  the  table  that  there  has  been  an 
apparent  settling  of  the  colloid  in  the  tube  while  the  current 
was  running.  This  was  quite  usual,  but,  as  the  reckoning- 
was  made,  it  could  not  affect  the  rate,  since  this  slight  lowering 
of  the  surface  was  uniform  in  both  limbs,  so  that,  while  it  is 
added  to  the  velocity  in  one  limb,  it  is  subtracted  from  the 
velocity  in  the  other. 
In  order  to  find  the  value  of  the  electric  force  in  the  tube, 
it  is  of  course  necessary  to  know  the  effective  distance  between 
the  electrodes  A  and  B.  In  order  to  do  this,  the  tube  was 
filled  with  a  '01  normal  KC1  solution,  placed  in  the  water- 
bath,  and  the  resistances  taken  with  the  electrodes  placed  at 
the  successive  centimetre  marks  down  the  tube.  In  this  wav 
it  was  found  that  the   resistance   of   the  curved  parr  of    the 
