202 
EXPERIMENTS   WITH  PAPER  FILTERS. 
retarded  in  its  passage  by  the  attraction  of  the  glass  ;  therefore, 
those  funnels  having  the  greater  portion  of  the  paper  free  from 
the  glass  would  be  the  best ;  that  is,  a  broad-throated  funnel,  other 
things  being  equal,  will  filter  faster  than  a  narrow -throated  funnel. 
To  test  this  point  I  selected  two  large  funnels  ;  No.  1  had 
three  times  as  broad  a  thrpat  as  No.  2.  With  the  first  filters 
they  ran  : 
117  :  100       123  :  100       133  :  100       118  :  100. 
The  reason  for  this  low  difference  was  found  in  a  thin  spot 
near  the  point  of  No.  2. 
Other  sets  of  filters  gave  : 
2d  set,       292  :  100       318  :  100 
3d  set,       288  :  100       335  :  100 
4th  set,      300  :  100  burst. 
5th  set,      384  :  100       407  :  100       482  :  100 
6th  set,      242  :  100 
In  the  last  set  a  porous  filter,  though  off  the  same  sheet  as 
No.  1,  was  given  to  No.  2.    Throughout  the  whole  series  of  ex- 
periments every  fair  advantage  was  given  to  the  weaker  party, 
it  being  the  first  filled  and  the  last  emptied. 
To  make  assurance  doubly  sure,  I  tried  filters  in  like  funnels, 
stopping  the  pores  of  the  paper  at  various  points.  Paraffin 
applied  whilst  liquid  was  the  substance  first  used  to  prevent 
filtration. 
Two  filters  were  chosen  from  the  same  sheet  and  of  as  uniform 
a  texture  as  possible.  No.  1  was  stopped  over  one- third  the 
radius  from  the  point.  No.  2,  all  but  one-third  the  radius  at  the 
point. 
They  filtered  at  nearly  the  same  rate,  No.  1  slightly  the 
faster.  The  paraffin  made  the  paper  stiff,  and  as  water  does  not 
adhere  to  it,  free  passage  was  allowed  between  it  and  the  funnel 
to  the  water  of  the  upper  part  of  No.  2. 
Here  we  see  that  one-ninth  of  the  surface  of  the  filter,  when 
free,  did  as  much  work  as  eight-ninths  adherent  to  the  glass. 
The  experiment  was  repeated  with  glycerin  instead  of  paraffin. 
No.  1  ran  28  per  cent,  the  faster.  It  might  be  objected  that 
glycerin  would  wash  out  ;  so  a  pap  of  paraffin  and  spirits  of  tur- 
pentine was  used  to  repeat  the  experiments.    Each  filter,  after 
