320 
ON  PERCOLATION  OR  DISPLACEMENT. 
condition  of  dryness  or  moisture  at  the  time  it  is  packed,  and 
the  form  of  the  apparatus  used.  Boullay  almost  invariably  re- 
commended the  powders  to  be  packed  dry — others  dampen  the 
powder  or  even  saturate  it  by  previous  maceration.  Our  Phar- 
macopoeia, based  on  the  experience  of  many  operators,  almost 
invariably  directs  the  powders  to  be  previously  macerated  in  a 
portion  of  the  solvent.  My  own  practice  has  been  in  favor  of 
macerating  the  powders  in  all  cases  where  the  menstruum  is 
water  or  very  weak  alcohol,  where  the  powders  swell  by  absorp- 
tion ;  whilst  with  alcohol  or  ether  to  proceed  at  once  with  the 
percolation,  wTith  or  without  previously  moistening  the  powders 
at  the  time  of  packing,  according  to  circumstances.  Prof. 
Grahame  invariably  uses  fine  powders,  and  previously  dampens 
them  with  menstruum,  just  sufficiently  to  facilitate  (or  invite,  so 
to  speak,)  the  entrance  and  descent  of  the  fluid  through  the 
the  powders  without  destroying  their  pulverulent  condition  and 
interfering  with  their  regular  packing. 
The  degree  of  pressure  to  be  exerted  in  compacting  the  pow- 
ders must  vary  with  their  nature  ;  hard  ligneous  substances 
will  require  more  compaction  than  such  as  tend  to  expand  by 
contact  with  fluid.  There  is  a  class  of  drugs,  of  which  rhubarb 
and  columbo  are  a  type,  which  become  adhesive  by  contact  with 
water  or  weak  alcohol,  and  constantly  give  trouble  to  the  opera- 
tor in  the  ordinary  cylinder,  owing  to  their  tendency  to  swell 
up  and  form  an  impervious  mass.  It  will  be  seen  by  the  sequel 
that  the  employment  of  a  conical  percolator  in  great  measure 
remedies  this  difficulty.  There  is  another  class  that  contain  in- 
soluble gum  or  mucus  analogous  to  bassorin,  like  senna  and 
althaea  root,  which  swells  up  and  forms  a  gelatinous  mass  with 
water,  wholly  unfit  for  percolation  in  an  ordinary  percolator. 
With  the  conical  displacer  even  these  may  be  treated  with  an 
aqueous  menstruum ;  but  unless  the  mucilage  is  desirable  in  the 
preparation,  as  in  syrup  of  althaea,  it  is  better  to  use  a  partially 
alcoholic  menstruum.  In  those  cases  where  the  swelling 
is  due  more  to  the  expansion  of  the  cellular  tissue  of  the 
particles  by  contact  with  water  than  to  mucilage,  even  though 
a  large  amount  of  soluble  extractive  matter  be  present, 
as  in  the  case  of  gentian,  it  will  be  found  advantageous 
to  use  a  fine  powder,  50  or  60  to  the  inch,  moistened  with 
