±28  Mr.  E.  F.  Burton  on  the  Properties  of 
of  light,  i.  e.  that  the  lower  limit  to  the  size  is  10 ~5  cms. 
Such  particles  were  first  rendered  visible  by  means  of  a 
special  apparatus  described  by  Zsigmondy  and  Siedentopf*, 
referred  to  later,  and  were  then  found  to  possess  the  Brownian 
movement  common  to  all  ordinary  suspensions  of  fine  par- 
ticles in  water.  Again,  in  perfect  analogy  with  the  cases  of 
ordinary  suspensions,  e.  g.  soap  solutions,  the  metallic  par- 
ticles are  completely  precipitated  by  the  addition  of  small 
quantities  of  any  aqueous  electrolytic  solution.  Of  great 
interest,  from  chemical  and  physiological  viewpoint,  is  the 
very  strong  catalytic  action  the  platinum  solution  shows  in 
bringing  about  the  decomposition  of  hydrogen  peroxide. 
Probably  the  most  interesting  of  the  properties  of  these 
solutions  is  the  movement  of  the  particles  under  the  influence 
of  an  electric  field  ;  in  all  the  solutions  which  he  prepared 
Bredig  found  that  if  two  electrodes  were  introduced  into  a 
tube  containing  the  solution,  the  particles  would  move  towards 
the  positive  electrode,  showing  that  they  possessed  a  negative 
charge. 
IY.  Determination  of  the  Size  of  the  Particles. 
The  first  method  devised  for  rendering  such  ultra-micro- 
scopic particles  visible  is  due  to  Zsigmondy  and  Siedentopf  f. 
An  intense,  thin  beam  of  light  is  focussed  in  the  solution  and 
viewed  at  right  angles  to  its  direction  by  a  high-power  micro- 
scope. The  particles  in  the  solution  appear  as  bright  points 
due  to  the  light  scattered  by  them  into  the  microscope. 
This  phenomenon  is  quite  analogous  to  the  very  common 
means  by  which  particles  of  dust  floating  in  the  atmosphere 
of  a  darkened  room  may  be  made  visible  by  casting  a  bright 
beam  of  light  into  the  room ;  viewed  at  right  angles  to  the 
direction  of  the  beam,  the  particles  of  dust  appear  as  bright 
moving  specks.  A  rather  simpler  application  of  this  prin- 
ciple, proposed  by  Mouton  and  Cotton  %,  was  used  in  the 
present  experiment.  The  apparatus  is  shown  diagram- 
matically  in  fig.  2,  which  somewhat  exaggerates  the  relative 
size  of  smaller  parts.  A  drop  of  the  solution  to  be  examined 
is  enclosed  beneath  a  thin  cover-glass  (C)  on  a  microscopic 
slide  (S).  The  plate  S  is  placed  on  a  special  block  of  glass 
A,  good  optical  contact  being  made  by  an  intervening  thin 
layer  of  liquid  whose  refractive  index  does  not  differ  much 
from  that  of  the  glass.  The  block  A  (for  which  a  Fresnel 
rhomb  serves  very  well)  allows  the  direction  of  a  beam 
of  light  to  be  easily  adjusted  so  that,  after  being  internally 
reflected  at  the  lower  surface  of  A,  the  succeeding  internal 
reflexion  at  the  upper  surface  may  be  made  to  take  place  at 
*  Drade's  Ann.  x.  p.  1  (1903).  t  Loc.  cit. 
%  Revue  Generate  des  Sciences,  xiv.  23,  p.  1184  (1903). 
