436 
Mr.  E.  F.  Barton  on  the  Properties  of 
Fiir.  5. 
y 
jump  in  the  value  of  the  electric  force,  just  at  the  surface. 
In  addition  to  this  the  method  consists  essentially  in  a  com- 
pression of  the  colloid  which  may  change  the  absolute  velocity 
of  the  particles.  Again,  as  the  authors 
do  not  give  the  temperatures  corre- 
sponding to  these  velocities  of  the  gold 
and  platinum  particles,  it  is  impossible 
to  apply  the  formula  (5),  which  in- 
volves the  coefficient  of  viscosity  of 
the  liquid. 
These  difficulties  are  obviated  if  one 
uses  a  velocity  tube  (fig.  5)  similar  to 
that  used  by  Hardy*  in  his  work  with 
colloidal  solutions.  That  used  in  the 
present  experiment  consisted  of  a 
U-tube  each  limb  of  which  was  about 
12  cms.  long  and  about  1'5  cms.  in 
diameter;  the  limbs  were  graduated 
mms.    throughout    their    length. 
X 
m 
Into  the  bottom  of  the  U-tube  is 
sealed  a  fine  delivery- tube  provided 
with  a  tap  (T)  and  funnel  (F) ;  this 
tube  is  bent  around  so  as  to  run  up 
behind  the  limbs  and  to  bring  the 
funnel  to  the  same  height  as  the  top 
of  the  U-tube. 
The  colloidal  solution  to  be  tried 
was  poured  into  the  funnel  so  as  to 
fill  the  small  tube  and  funnel  around 
to  the  tap  which  was  closed;  Avater 
having  a  specific  conductivity  equal 
to  that  of  the  colloid  was  then  poured 
into  the  U-tube  so  as  to  fill  it  to  a 
height  of  about  3  cms.  The  whole  tube 
was  then  placed  in  a  large  glass  water- 
bath  so  as  to  be  almost  submerged  : 
this  water  was  kept  at  a  constant 
temperature  during  the  course  of  any 
experiment.  At  the  end  of  a  few 
minutes  the  tap  (T)  was  very  slightly 
openedj  and  the  colloidal  solution  allowed  to  gently  force  the 
water  up  the  limbs  of  the  tube  to  any  required  height.  If  care- 
fully manipulated,  the  surface  of  separation  between  the  clear 
water  and  the  solution  was  very  distinct,  and  would  remain  so 
for  hours.     Two  electrodes  of  coiled  platinized  platinum  foil 
*  Journ.  of  Physiology,  vol.  xxix.  p.  26  (1903). 
