874  The  Theory  of  Percolation.        { ^^eimber'^'S: 
tile  menstrua  in  percolation  and  has  described  three  forms  of  ap- 
paratus designed  to  minimize  this  defect.  In  1895  Forrest^  de- 
scribed an  apparatus  for  continuous  percolation  which  consists  of 
a  series  of  percolators  arranged  so  that  the  percolate  from  one 
becomes  the  menstruum  for  the  next  and  so  percolates  through 
the  series.  A  form  of  percolator  for  volatile  menstrua  was  designed 
by  Barnard."  The  whole  apparatus  is  air  tight  in  this  type  and  a 
small  tube  which  extends  from  the  receiver  up  through  the  drug 
in  the  percolator  to  the  inside  of  the  cover  serves  to  convey  the  air 
displaced  in  the  receiver  to  the  top  of  the  percolator.  Wood^  sug- 
gests a  modification  of  the  siphon  delivery  tube  in  the  well-tube 
percolator.  He  arranges  a  pair  of  corks  in  the  end  of  the  tube  in 
such  a  way  that  the  percolate  must  drop  slowly  and  its  flow  can  be 
controlled.  Phillips''  described  a  very  simple  apparatus  for  hot 
percolation. 
Barksdale''  suggested  a  percolator  fitted  with  a  stirring  device 
and  operated  as  follows:  the  drug  is  placed  in  the  percolator  and  a 
volume  of  menstruum  equal  to  half  the  quantity  of  fluidextract 
which  the  drug  should  yield  is  poured  on  the  drug.  The  whole  is 
stirred  for  at  least  thirty  minutes,  allowed  to  macerate  two  days, 
and  then  the  same  volume  of  menstruum  is  added  and  percolation 
started.  When  the  liquid  has  percolated  through  enough  more 
menstruum  is  added  to  furnish  a  75  per  cent,  reserve  (by  volume) 
and  then  the  drug  is  exhausted  by  simple  percolation  and  the  weak 
percolate  handled  according  to  the  official  process.  It  is  claimed 
that  the  drug  is  more  quickly  exhausted  by  this  process  than  by 
the  official  one. 
The  "double-tube"  percolator  is  an  ingenious  modification  of 
vSquibb's  well-tube  idea.  An  ordinary  percolator  is  fitted  with  a 
central  tube  of  rather  large  bore  around  which  the  drug  is  packed 
in  such  a  way  that  the  percolate  flows  into  the  tube.  Within  this 
central  tube  is  a  smaller  tube  which  extends  through  the  spout 
of  the  percolator  and  is  held  in  place  by  a  cork  or  rubber  nipple. 
This  inner  tube  may  be  raised  or  lowered  to  control  the  height 
of  the  menstruum  in  the  drug,  the  rate  of  flow  and  the  period  of 
1  Pharm.  Jour.  55,  538,  (1895). 
2  Merck's  Report,  1899,  220. 
3  Pharm.  Era,  1899,  359. 
^  West  Drug.  11,  210,  (1889) ;  from  the  Pharm.  Record. 
Ibid.  21,  116,  (1899). 
Remington,  "Practice  of  Pharmacy,''  Ed.  5,  1907,  265. 
