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THE  PROCESS  OF  PERCOLATION. 
charge  of  menstruum.  It  is  fitted  with  a  good  cork,  and  two 
syphon  tubes  that  pass  into  it  air  tight.  The  inner  syphon,  /, 
is  small,  and  passes  from  near  the  bottom  of  the  bottle  to  near 
the  surface  of  the  substance  in  the  percolator,  so  that  both  its 
ends  may  be  always  immersed  while  in  action.  It  conveys  the 
liquid  from  the  reservoir  to  the  percolator  upon  the  principle  of 
a  syphon,  but  can  only  act  as  air  is  admitted  to  the  reservoir. 
The  second  tube,  k,  is  of  much  larger  calibre.    If  less  then  4 
to  6  lines  diameter  it  works  badly,  while  if  less  than  3  lines  it 
does  not  do  at  all.    This  tube  should  be  about  half  inch  bore, 
no  matter  what  scale  the  apparatus  is  on.    It  passes  from  the 
neck  of  the  reservoir  bottle,  air  tight,  through  the  cork,  and 
down  into  the  percolator,  terminating  by  an  oblique  or  pen- 
shaped  end,  about  one  inch  above  the  end  of  the  syphon  for 
feeding.    This  large  tube  is  for  supplying  and  for  limiting  the 
supply  of  air  to  the  reservoir,  and  this  it  is  which  renders  the 
apparatus  automatic.    The  reservoir  being  filled  and  set  upon 
its  support,  and  the  percolator  ready,  the  reservoir  is  tilted 
backward  and  air  blown  through  the  large  tube.    This  immedi- 
ately starts  the  feeding  syphon,  when  the  reservoir  is  allowed 
to  take  its  proper  position,  and  the  apparatus  abandoned  to  it- 
self.   The  feeding  syphon  will  run  until  the  substance  is  fully 
saturated,  and  until  the  stratum  of  menstruum  above  it  rises 
high  enough  to  close  the  end  of  the  large  air  tube.    This  liquid 
then  of  course  shuts  off  the  air  from  the  reservoir,  and  as  soon 
as  the  columns  of  liquid  balance  each  other  the  feeding  syphon 
stops.    The  stop-cock,  /,  at  the  receiver,  is  now  carefully  ad- 
justed to  the  proper  rate  of  percolation,  and  the  apparatus  left 
to  take  care  of  itself.    As  soon  as,  by  percolation,  the  stratum 
of  liquid  above  the  substance  falls  so  as  to  expose  the  upper 
part  of  the  opening  at  the  end  of  the  large  air  tube,  a  bubble  of 
air  passes  up  into  the  reservoir,  and  a  corresponding  bulk  of  the 
menstruum  flows  out  through  the  syphon  tube,  and  this  action 
is  kept  up  so  long  as  there  may  be  any  menstruum  in  the 
reservoir. 
The  feeding  syphon,  j,  may  be  replaced  by  a  stop-cock,  i,  as 
shown  by  dotted  lines,  but  vessels  with  stop-cocks  are  very  ex- 
pensive, and  no  better  than  those  which  are  plain,  and  arranged 
with  a  syphon.    The  pendant  portions  of  both  the  tubes  from 
