786  The  Theory  of  Percolation.  { 
extracted  with  a  solvent  immiscible  with  water  the  common  prac- 
tice is  to  mix  thoroughly  with  a  solution  of  sodium  or  potassium  car- 
bonate, bicarbonate,  or  hydroxide,  or  ammonia  and  dry  the  mixture. 
Calcium  hydrate,  barium  hydrate,  magnesia  or  zinc  oxide  may  be 
similarly  handled. 
Lloyd  ^  has  suggested  bringing  drugs  to  their  natural  moisture 
content  by  adding  water  to  them  and  allowing  them  to  swell  before 
treating  them  with  the  menstruum  designed  to  extract  them.  This 
has  not  been  adopted.  Indeed  it  would  necessitate  a  thorough  re- 
vision of  all  our  menstrua  except  those  which  are  strictly  aqueous  and 
would  give  us  different  products  in  many  cases  than  those  produced 
by  the  processes  in  present  use.  The  resinous  and  terpene  drugs 
would,  of  course,  be  most  affected  and  while  it  is  possible  to  extract 
many  if  not  all  of  these  drugs  when  they  are  fresh  with  dilute  alcohols, 
it  is  doubtful  whether  the  addition  of  water  to  the  dried  specimen, 
where  the  plant  fluids  have  lost  their  solvent,  would  effect  a  re-solu- 
tion of  the  resinous  or  terpene  derivatives  originally  held  in  solu- 
tion in  the  plant  fluids.  Probably  the  alcoholic  menstrua  used  on 
such  drugs  would  not  dissolve  the  active  principles  until  the  added 
water  had  been  nearly  completely  washed  out. 
An  ingenious  method  of  moistening  small  amounts  of  drug  has 
been  brought  forward  by  Eberle,^  who  places  the  drug  in  a  covered 
can,  adds  the  menstruum  and  stirs.  A  few  glass  stoppers  are  then 
mixed  with  the  drug  and  the  whole  is  shaken.  The  stoppers  prevent 
lumping  of  the  powder  and  the  drug  is  thoroughly  moistened  without 
evaporation  of  the  solvent. 
When  the  drug  has  been  moistened  it  is  customary  to  allow  the 
mass  to  remain  in  a  covered  container  until  it  has  finished  swelling. 
The  extent  to  which  drugs  will  swell  as  well  as  the  time  consumed 
in  the  process  varies  considerably  and  the  variation  is  due  some- 
what to  the  menstrua  employed  in  moistening  them.  In  general  drugs 
will  swell  more  according  to  the  density  of  their  fibre,  that  is  the  more 
compact  their  cellulose  structure  is,  the  presence  of  mucilaginous  or 
carbohydrate  constituents,  and  the  more  aqueous  the  menstruum. 
The  swelling  is  due  to  an  imbibition  of  one  or  more  of  the  constituents 
of  the  menstruum  and  drug  particles  appear  to  show  much  discrim- 
ination between  the  various  solvents  in  use.  Water  is  most  readily 
and  most  largely  absorbed  by  them,  glycerin,  acetone,  dilute  acetic 
1  Pharm.  Rund.  1889,  165.    Proc.  A.  Ph.  A.  1890. 
2  Drtig.  Circ.  1900,  11.    Proc.  A.  Ph.  A.  1900,  398. 
