Am.  Jour.  Pb arm. ) 
July,  1883.  J 
Fads  Concerning  Filtration. 
349 
The  subject  seems  naturally  to  divide  itself  under  three  heads,  viz. : 
The  liquid  to  be  filtered,  the  medium  through  which  it  is  to  pass,  and 
the  form  in  which  that  medium  is  presented.  As  it  is  not  intended, 
however,  to  make  these  notes  exhaustive,  it  will  be  unner^essary  to  deal 
with  it  in  precisely  this  order.  Let  us  rather  take  some  facts  as  they 
occur  and  see  if  we  can  by  their  consideration  render  available  any- 
thing of  practical  value  for  every-day  use. 
The  most  simple  operation  of  this  kind  is  to  filter  through  paper  in 
small  quantity^a  liquid  containing  a  solid  body  for  which  it  has  no 
affinity. 
Text-books  tell  us  at  the  outset  that  it  is  very  necessary  to  use  a 
funnel,  the  sides  of  which  form  an  angle  of  60°,  this  being  the  angle 
formed  by  the  folded  paper.  Now  I  take  exception  to  this  very 
exacting  requirement.  We  do  not  get  our  straining  bags  or  per- 
colators made  of  such  a  shape,  and  that  because  our  experience 
teaches  us  how  much  more  suitable  is  a  form  in  which  the  angle  is  de- 
cidedly more  acute  ;  the  same  volume  of  liquid  in  this  latter  form  pro- 
ducing a  longer  column,  and  consequently  a  greater  downward  pres- 
sure. Then,  as  to  the  paper  fitting  the  funnel ;  we  know  quite  well 
that  all  else  being  equal,  the  less  perfectly  it  fits,  the  more  rapidly 
filtration  proceeds,  so  that,  for  any  useful  purpose  it  is  quite  unneces- 
sary to  insist  on  this  very  orthodox  shape.  One  has,  say,  a  pint  of 
fluid  to  filter,  and  for  this  purpose  a  funnel  of  about  8  ozs.  or  10  ozs. 
capacity  is  taken.  I  would  use  one  of  the  long  French  pattern,  fold 
the  filter  in  plaits  and  before  opening  it  out,  place  it  fairly  well  down 
in  its  position  in  the  funnel ;  or  if  there  were  reasons  for  not  plaiting 
the  filter,  then  it  should  be  folded  first  in  half,  and  then  the  two  outer 
portions,  representing  rather  more  than  one-eighth  each  of  the  entire 
paper,  should  be  turned  back  so  as  to  overlap  each  other  slightly  at  the 
top,  and  not  to  form  a  very  acute  point.  In  either  case,  the  paper 
whilst  being  fairly  well  supported  would  have  comparatively  little 
surface  adhesion,  and  but  small  resistance  would  be  offered  to  the  pas- 
sage of  the  fluid  in  any  part.  Funnels  of  this  shape  in  much  larger 
sizes  can  be  used  with  advantage,  but  it  is  then  desirable  to  have  them 
ribbed.  The  ribs  of  funnels  (especially  of  large  ones)  to  be  of  any  real 
value  should  be  much  deeper  than  they  usually  are,  and  should  not 
run  vertically,  but  spirally.  A  piece  of  muslin  placed  between  the 
paper  and  funnel  not  only  strengthens  and  supports  the  paper,  but 
assists  filtration  by  preventing  adhesion ;  a  cone  formed  of  coarse  hair 
