*44  Fluid  Extracts  by  Rep er eolation.  {^^J^' 
thick,  with  a  hole  in  the  centre  for  the  well-tube,  is  then  put  on.  If 
made  of  sheet  rubber  this  cover  fits  so  closely  that  it  will  soon  save  its 
cost  by  preventing  loss  of  alcohol  by  evaporation.  Its  size  in  the 
larger  percolator  permits  one  side  to  be  bent  up  when  liquid  is  to  be 
poured  in.  But  in  the  smaller  percolator  it  is  very  convenient  to  have 
one  side  of  the  cover  cut  two-thirds  through  from  below,  as  shown  in 
the  cut,  the  undivided  portion  forming  a  good  spring  hinge  permitting 
this  part  of  the  cover  to  be  easily  raised  to  pour  on  liquid.  If  a  self- 
feeding  bottle  be  used,  as  is  generally  advisable  in  order  to  keep  the 
level  of  the  liquid  constant  and  thus  obtain  a  flow  at  a  uniform  rate,  it 
may  be  easily  applied  by  making  a  hole  of  suitable  size  in  the  cover. 
When  thus  supplied  and  covered  the  maceration  should  continue  for 
48  hours  at  least.  The  whole  arrangement  now  represents  a  well,  dug 
in  a  wet  soil  of  a  substance  to  be  percolated,  and  the  proposition  is,  to 
pump  out  this  well  at  so  slow  a  rate  that  the  liquid  from  outside  the 
well  coming  in  to  supply  that  rate,  through  the  disks  of  paper  and 
blanket,  which  represent  the  gravel  stratum  of  the  soil, — may  descend 
so  very  slowly  as  to  be  nearly  frictionless.  The  object  is,  to  give  the 
whole  liquid  up  as  nearly  as  possible  to  gravitation,  and  to  so  diminish 
the  rate  of  descent  that  the  particles  or  portions  of  liquid  which  pass 
between  and  around  the  particles  of  solid  matter  may  travel  downward 
no  faster  than  the  portions  of  liquid  which  pass  through  the  pores  or 
interstices  of  the  more  solid  substance  whose  soluble  portions  are  to 
be  washed  out.  Because,  if  the  whole  mass  of  liquid  travelled  down- 
ward at  absolutely  the  same  rate  through  a  perfectly  saturated  mass  of 
solid  permeable  substance  partially  soluble  in  the  liquid,  and  if  the 
molecules  of  liquid  passed  downward  in  absolutely  straight  lines, 
through  some  particles,  but  between  others,  and  always  at  a  uniform 
rate,  it  is  easy  to  see  that  all  the  first  portion  of  the  liquid  would  come 
through  saturated,  and  all  the  remainder  would  hold  nothing  in  solution 
because  the  substance  would  be  absolutely  exhausted  by  just  the  quan- 
tity of  liquid  which  it  was  capable  of  saturating.  The  writer  has  on 
one  or  two  occasions  made  percolations  so  slowly  as  to  somewhat 
approximate  this  theoretical  result.  The  dropping  in  one  case  was  not 
over  4  drops  in  each  24  hours,  or  about  a  cubic  centimetre  every  5 
days.  To  pump  out  this  well  at  a  uniform  rate  which  can  be  easily 
controlled,  is  therefore  the  most  important  element  in  a  successful  per- 
colation, and  the  only  one  which  has  offered  much  difficulty  in  the 
