314  Parchment  Paper  as  a  Filtering  Medium,  {^''ji^^i^gn''' 
In  making  the  parchment  paper  for  this  purpose,  the  following 
method  should  be  adopted.  It  differs  very  little  from  the  ordinary 
one,  except  as  regards  a  few  precautions  : — I  use  one  part  of  pure 
sulphuric  acid  and  one-half  part  of  distilled  water  well  mixed  in  a 
dish  or  shallow  vessel.  Where  practical,  this  mixture  should  be  ice 
cold,  and  under  no  circumstances  must  it  be  used  while  it  is  warm. 
Pieces  of  Swedish  filtering-paper  should  then  be  dexterously  floated 
upon  the  acid,  so  as  to  bring  every  particle  of  the  surface  in  contact 
with  it.  But  it  is  not  necessary  to  parchmentize  both  sides.  The 
next  point  of  importance  after  the  codling  of  the  acid  mixture  is  the 
quickness  used  in  the  washing,  which  must  be  thorough. 
This  paper,  which  has  proved  itself  so  useful  to  us  for  dialytic  pur- 
poses, forms  the  most  perfect  filtering  medium,  if  properly  managed,, 
with  which  I  am  acquainted.  Although,  under  ordinary  circumstan- 
ces, it  is  nearly  impervious  to  fluids,  they  pass  through  with  perfect 
facility  under  pressure.  The  structural  change  produced  by  sulphuric 
acid  upon  cellulose  is  the  converse  of  most  of  the  other  acids.  Thus 
in  paper  converted  into  pyroxylene  by  the  action  of  nitric  acid  the 
fibres  are  seen,  when  examined  with  the  microscope,  to  be  more  or 
less  contracted,  and  the  result  is  anon-contiguous,  or  friable  structure, 
covered  with  small  holes  ;  but  in  parchmentized  paper  the  fibres  are 
swelled  considerably  in  bulk,  and  are  converted  into  a  colloid  or  gelati- 
nous substance,  which,  although  slowly  pervious  to  fluids,  is  very 
homogeneous  in  texture,  and  hence  its  strength. 
In  Bunsen's  original  paper  he  speaks  of  the  difficulty  of  preventing 
filaments  of  the  paper  used  from  becoming  mixed  with  precipitates. 
Thus,"  he  says,  "  another  and  an  inestimable  advantage  springs- 
from  the  peculiar  condition  of  a  precipitate  filtered  by  this  method, — 
the  surface  of  the  filter  becomes  injured  and  torn,  so  that  the  precipi- 
tates becomes  mixed  with  filaments  of  paper.  Gelatinous  precipitates- 
(when  washed  under  pressure)  adhere  to  the  filter  in  a  thin  coherent 
layer,  and  m.ay  be  removed  piece  after  piece  so  completely  that  the 
paper  remains  perfectly  clean  and  white."  Now  parchment  paper  is 
of  that  nature  that  it  might  be  scraped  with  a  knife  or  brush,  without 
invalidating  a  quantitative  analysis. 
Parchment  paper  would  be  perfection  for  filtering  by  pressure ; 
but,  alas  !  it  has  one  drawback.  The  practical  difficulty  is  in  making 
the  filter  lie  close  to  the  funnel,  so  as  not  to  permit  atmospheric  air 
to  pass  down  by  the  side,  instead  of  exerting  its  pressure  upon  the 
