430  Mr.  E.  F.  Burton  on  the  Properties  of 
struction  of  the  slide.  At  the  centre  of  a  circular  piece  of 
glass  A  an  area  of  1  sq.  mm.  is  divided  into  small  squares 
of  gV  mm-  side  by  means  of  fine  lines  ruled  with  a  diamond 
point.  The  plate  B  surrounds  A  so  as  to  leave  an  annular 
trough  about  the  central  disk.  The  upper  surface  of  B  is 
exactly  T  mm.  above  that  of  A,  so  that  when  the  cover- glass 
C  is  placed  on  B,  a  layer  ^  mm.  thick  exists  between  A  and 
C.  The  surfaces  of  A,  B,  and  C  are  of  course  ground  per- 
fectly plane.  When  a  drop  of  a  sol  is  placed  on  A  and  covered 
with  C,  a  volume  of  4^0,7  cu-  mm-  can  De  discerned  through 
the  microscope.  By  raising  and  lowering  the  objective  very 
slightly,  it  is  possible  to  bring  all  the  particles  in  a  layer 
-j1^  mm.  thick  into  view,  and  so,  for  very  dilute  solutions,  the 
number  of  particles  per  cu.  mm.  can  be  very  approximately 
determined.  A  microscope  which  magnified  about  350  times 
was  used,  and  the  slide  was  moved  slightly  so  that  the  par- 
ticles corresponding  to  400  of  the  unit  volumes  (each  4^0 
cu.  mm.)  were  counted.  The  Brownian  movement,  at  any 
rate  in  such  a  thin  layer  of  liquid,  did  not  show  any  constant 
drift  of  the  particles,  and  was  not  sufficient  to  sensibly  alter 
the  disposition  of  the  particles  during  the  time  (about  one  hour) 
taken  to  count  the  number  in  the  volume  (T\j  cu.  mm.). 
If  the  amount  of  metal  in  1  c.c.  of  a  given  colloidal  solution 
can  be  ascertained,  then,  assuming  that  the  specific  gravity 
of  the  metal  in  this  state  of  fine  subdivision  retains  its  ordinary 
value,  an  idea  of  the  size  of  each  particle  can  be  obtained 
from  knowing  the  number  of  particles  in  unit  volume.  Such 
determinations  have  been  made  for  the  colloidal  solutions  of 
platinum,  gold,  and  silver.  The  weight  of  metal  in  a  given 
volume  of  the  solution  was  obtained  by  evaporating  the  liquid 
in  a  previously  weighed  porcelain  crucible,  and  finding  the 
wreight  of  the  residue.  A  reference  to  the  specific  conduc- 
tivities of  the  various  solutions  (Table  II.)  will  show  that  the 
weight  of  dissolved  impurity  remaining  after  evaporation 
would  be  practically  infinitesimal.  The  following  is  a  sample 
of  such  determination,  typical  as  regards  method  and  the 
magnitude  of  the  quantities  involved  : — 
A  silver  solution  containing  6*8  milligrams  of  metal 
per  100  c.cs.  was  diluted  with  distilled  water  to  one 
hundred  times  its  original  volume.  A  drop  of  the  dilute 
liquid  showed  the  presence  of  300  particles  in  the  volume, 
cu.  mm. 
So  that  in  the  original  solution,  per  c.c,  there  are  3  x  108 
particles  of  total  weight  0*8  x  10~6  grms.  If  the  specific 
gravity  be  taken  as  10' 5,  the  mean  volume  of  the  particles 
