EXPERIMENTS  WITH  PAPER  FILTERS. 
203 
being  painted,  was  wetted  to  prevent  the  spread  of  the  mixture 
by  capillary  action.  The  trials  were  not  sufficiently  numerous 
to  find  a  true  mean,  but  the  free  point  invariably  ran  the  most — 
from  4  or  5  per  cent,  excess  to  100  per  cent.  The  point  was 
assumed  to  be  a  circle  one-third  the  radius  of  the  filter. 
I  understood  the  idea  of  the  Fleitmann  filter  to  be  this,  that, 
likening  a  plain  filter  to  a  peat  bed  resting  upon  an  impermeable 
sub-soil,  it  might  be  compared  to  a  porous  substratum  interpo- 
lated between  the  swamp  and  the  clay  bottom. 
To  test  this  idea  a  Fleitmann  filter  was  made  and  wetted, 
carefully  patting  down  and  smoothing  out  any  irregularities.  It 
was  tried  against  a  plain  filter  which  was  placed  in  a  funnel  with 
but  two-thirds  as  wide  a  throat  as  that  of  the  Fleitmann.  It 
ran  114  :  100;  that  is,  the  passages  kept  open  by  the  elasticity 
of  the  paper,  the  creases  and  abutting  edges  liken  this  filter  to 
tile  drainage. 
To  increase  the  size  and  number  of  passages  I  tried  putting 
the  inner  filter  into  a  plaited  filter  of  coarse  paper.  Changing 
the  filters  after  each  trial,  I  found  this  form  gave  the  following 
results  as  compared  with  the  plain  filter,  calling  the  latter  one 
hundred. 
1st  trial,       184  :  100  4th  trial,       66  :  100 
2d    "  201  :  100  5th   "         170  :  100 
3d     "         250  :  100 
I  afterwards  found  a  thin  spot  in  the  plain  filter  of  the  fourth 
trial. 
Next  a  precipitate  of  sulphate  of  calcium  was  tried  ;  the  fil- 
trates were  as  131  :  100.  On  weighing  the  precipitates  collec- 
ted they  were  as  200  :  100. 
This  form  of  filter  was  abandoned,  since  it  does  not  filter  as 
fast,  is  not  as  strong,  takes  more  time  to  make  and  care  to-  use 
than  the  form  next  to  be  described.  It  is,  however,  better  than 
the  plain  filter  as  regards  speed  of  filtration,  equal  to  the 
plaited  in  this  respect  and  stronger  than  it. 
To  admit  the  use  of  very  broad-throated  funnels,  the  number 
of  outside  filters  was  increased  to  two  ;  these  over-coats  were 
pierced  with  long  narrow  apertures  running  from  the  point  to 
the  circumference. 
