356      THE  PROCESS  OF  PERCOLATION  OR  DISPLACEMENT. 
Even  in  operating  with  volatile  fluids,  such  as  ether,  spt.  am- 
nion, arom.,  &c,  in  the  absence  of  a  better  instrument,  the  fun- 
nel may  be  made  to  answer  by  tying  over  the  top  a  piece  of  oiled 
silk  and  inserting  the  neck  tightly  in  a  cork  arranged  to  receive 
it. 
Having  procured  the  proper  form  of  apparatus,  the  next  con- 
sideration is  the  degree  of  comminution  to  which  to  reduce  the 
material  we  wish  to  operate  upon.  An  experience  of  many 
years  in  treatment  by  percolation,  induces  me  to  recommend,  in 
all  cases,  that  the  substance  be  brought  to  a  state  of  fine  divi- 
sion, of  uniform  character,  particularly  those  which  are  of  a 
firm  and  hard  texture  ;  but  in  all  instances,  the  material  should 
be  passed  through  a  sieve.  The  size  of  the  sieve  will  necessa- 
rily vary  ;  with  most  substances,  one  of  about  sixty  meshes 
to  the  inch,  especially  for  tinctures,  will  be  found  best  adapted, 
while  with  some  other  substances,  such  as  rhubarb,  gentian, 
&c,  one  of  about  forty  meshes  will  be  preferable. 
The  state  of  comminution  to  which  substances  are  brought  pre- 
vious to  treatment  by  percolation,  I  conceive  to  be  one  of  the 
most  important  conditions  of  a  successful  result.  Next  in  im- 
portance to  this,  is  the  amount  of  menstruum  added  to  the  pow- 
der previous  to  packing  it  in  the  apparatus. 
As  in  the  previous  case,  I  have  also  in  this,  to  depart  from 
the  rules  laid  down  in  the  standard  works  treating  of  this  sub- 
ject ;  for  this  condition  regulates,  in  a  great  measure,  the  next 
and  last  important  feature  to  be  observed,  that  is,  the  packing 
of  the  powder  in  the  apparatus. 
As  already  referred  to,  the  officinal  directions,  as  well  as  com- 
mon usage,  imply  the  necessity  of  sufficient  of  the  menstruum 
to  be  added,  to  form  a  soft,  pulp-like  mass  with  the  powder, 
and  in  some  instances  a  much  larger  quantity ;  and  suffer  the 
mixture  to  stand  some  time  before  transferring  to  the  percola- 
tor. It  is  almost  impossible  to  so  regulate  the  packing  of  the 
powder  when  in  this  condition,  as  to  produce  that  uniform  and 
steady  permeation  of  the  liquid  through  the  mass,  so  essential 
to  complete  success.  Hence  it  is,  that  instruments  furnished 
with  stopcocks  or  other  arrangements,  are  recommended  to  con- 
trol the  flow  of  liquid;  an  appendage  to  the  apparatus  wholly 
unnecessary,  as  the  process,  when  properly  begun,  is  its  own 
regulator  with  unvarying  certainty. 
