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The  Theory  of  Percolation. 
I  Am.  Jour.  Pharm  • 
\    Novamber,  1920. 
ingenious.  Arny  has  also  devoted  his  attention  to  this  subject  and 
has  proposed  several  practical  and  serviceable  forms,  ^  in  which  the 
menstruum  is  kept  out  of  contact  with  the  air.  That  it  is  not  neces- 
sary to  have  a  tubular  percolator  in  which  menstruum  enters  at  one 
end  and  percolate  issues  at  the  opposite  end  was  shown  by  Squibb^ 
with  his  tub  percolator.  Several  other  ideas^  in  percolator  design 
were  worked  out  by  this  indefatigable  pharmacist  and  are  worthy  of 
the  attention  of  every  worker  in  the  field  of  extraction. 
Finally,  a  word  may  be  said  about  the  materials  of  which  a  per- 
colator is  constructed.  Glass  is  the  best  material  at  hand  and  prac- 
tically all  small  percolators  are  made  of  it.  It  possesses  many  ad- 
vantages; transparency,  so  that  the  process  may  be  closely  watched 
and  inequalities  of  flow,  due  to  improper  packing  or  accident  detected, 
cleanliness;  and  minimum  solubility,  so  that  foreign  matter  is  kept 
out  of  the  product  and  undesirable  reactions  with  the  drug  avoided. 
Its  fragility,  however,  makes  it  impractical  for  apparatus  larger  than 
three  or  four  gallon  capacity  unless  it  is  reinforced  as  in  the  glass  lined 
percolators  of  v/ood  or  metal.  Lloyd's  experience  with  glass  per- 
colators of  ten  gallon  capacity  confirms  this  statement.* 
Large  size  percolators  are  commonly  made  of  galvanized  iron, 
some  are  made  of  tinned  copper  or  iron,  and  some  of  stoneware. 
The  galvanized  iron  percolator  is  strong,  substantial,  and  inexpensive. 
It  resists  the  corroding  action  of  acid  plant  solutions  very  well  but 
contaminates  the  percolate  with  small,  nearly  negligible  amounts  of 
zinc  and  iron.  It  is  somewhat  difficult  to  clean  thoroughly  and  sooner 
or  later  rusts  through  and  becomes  unserviceable.  Tinned  iron  per- 
colators offer  no  advantage  and  rust  through  more  quickly  than 
the  galvanized  iron  percolators.  Iron  percolators  with  enamelled  inside 
surface  are  extensively  used  and  are  serviceable  for  most  drugs. 
Tinned  copper  percolators  are  expensive  and  cumbrous  and  contami- 
nate the  percolate  with  small  quantities  of  tin.  Percolators  of  stone 
or  earthen  ware  are  frequently  used  for  large  batches  of  drugs  which 
are  to  be  extracted  by  strongly  acid  menstrua  which  would  rapidly 
destroy  metallic  apparatus.  A  five  or  ten  gallon  stoneware  jug  from 
which  the  bottom  has  been  removed  makes  a  very  useful  percolator 
of  this  type.    Plain  wooden  percolators  are  seldom  used.    They  are 
^  Proc.  A.  Ph.  A.  1892,  169. 
2  Proc.  A.  Ph.  A.  1872,  182. 
^  Proc.  A.  Ph.  A.  1878,  708. 
*West.  Drugg.  11,  159,  (1888). 
