358       THE  PROCESS  OF  PERCOLATION  OR  DISPLACEMENT. 
mass,  but  merely  to  dampen  the  powder  without  causing  it  to 
lose  entirely  its  mobile  condition.  In  this  state  the  operator 
will  be  able  to  judge,  at  once,  by  the  feel  of  the  powder,  respect- 
ing the  next  step  of  the  process — that  of  packing — whether  it 
should  be  performed  with  little  or  much  pressure.  If  the  po\Vr 
der  should,  when  thus  moistened,  manifest  a  tendency  to  adhere 
much  among  its  particles,  the  packing  should  be  performed  with 
moderate  pressure  only,  and  in  some  instances,  scarcely  any, 
especially  with  an  aqueous  solvent ;  if  on  the  other  hand,  this 
adhesiveness  should  not  exist,  and  the  particles  of  the  powder 
move  freely  upon  each  other,  as  with  many  alcoholic  and  hydro - 
alcoholic  liquids,  the  pressure  should  be  considerable.  As  a 
general  rule,  considerable  pressure  may  be  used  with  most  sub- 
stances ;  but,  as  before  stated,  the  operator  will  soon  learn  to 
distinguish  the  requisite  condition  in  this  respect. 
The  dampened  powder  should  then  be  immediately  transferred 
to  the  percolator,  and  all  of  it  thus  transferred  before  any  pres- 
sure is  exerted  upon  it,  so  as  to  insure  the  powder  to  be  of  uni- 
form condition  throughout ;  which  cannot  be  the  case  if  one 
portion  of  it  is  packed  before  adding  another;  as  the  first  may 
not  only  be  packed  more  tightly  or  more  loosely,  but,  in  conse- 
quence of  evaporation  during  packing,  will  be  dryer  than  the 
portion  which  is  to  be  added,  and  all  combined  will  tend  to  di- 
rect the  permeation  of  the  fluid  unequally ;  and  consequently 
the  process  cannot  progress  successfully. 
If  it  is  desired,  as  in  some  cases  may  be  necessary,  as  with 
wild  cherry  bark,  to  permit  the  full  effect  of  the  water  or  other 
menstruum,  in  the  developement  of  the  active  principle,  it  may 
be  accomplished  by  adding  only  a  small  portion  of  the  menstruum 
to  the  packed  powder — a  quantity  that  will  be  absorbed  by  the 
powder,  but  insufficient  to  produce  dropping ;  and  at  the  proper 
time,  make  the  further  additions  of  liquid. 
When  it  is  not  thus  requisite  for  any  peculiar  action  of  the 
menstruum,  the  whole  quantity  of  liquid  (or  to  the  capacity  of 
the  instrument),  may  be  added  at  once  on  the  surface  of  the 
packed  powder,  arranged  in  the  manner  before  stated. 
Should  a  funnel  be  used,  it  will  be  necessary  to  lay  across  the 
top  a  piece  of  oiled  silk  to  prevent  evaporation. 
In  the  treatment  of  some  substances,  as  opium,  myrrh,  &c,  it 
is  necessary  to  mingle  with  them  some  suitable  material  for  the 
