88o  The  Theory  of  Percolation.         { ^  December^Y9''2'S; 
to  98  per  cent.,  Eriodictyon  from  77  per  cent,  to  86  per  cent.,  Grindelia, 
83  per  cent,  to  90  per  cent,  so  that  a  fluidextract  which  has  precip- 
itated will  show,  on  analysis,  a  larger  percentage  of  alcohol  than 
is  declared  on  the  label. 
CONCLUSION. 
We  have  now  considered  every  phase  of  the  process  of  percolation 
beginning  with  the  apparatus  and  continuing  with  a  discussion  of 
the  drug  and  its  preparation  for  percolation  and  an  analysis  of  the 
mechanism  of  extraction  to  an  account  of  the  various  improved 
methods  and  apparatus  suggested  during  the  past  hundred  years. 
Some  new  ideas  and  certain  novel  ways  of  regarding  different 
aspects  of  the  subject  have  been  presented  and  a  portion  of  these, 
at  least,  are  at  variance  with  current  notions. 
The  whole  survey  emphasizes  the  need  for  more  investigations 
of  the  process  of  percolation  and  particularly  investigations  in  which 
the  time-factor  is  not  ignored.  It  is  recommended  that  Colleges  of 
Pharmacy  might  well  include  some  of  this  work  in  their  courses 
on  the  practise  of  pharmacy  especially  with  advanced  students, 
many  of  whom  would  welcome  an  opportunity  to  contribute  to  our 
knowledge.  In  addition  to  its  considerable  educational  value,  such 
an  arrangement  would  furnish  us  with  a  great  mass  of  data  our 
supply  of  which  is  to-day  very  meager. 
One  of  the  chief  reasons  for  writing  this  paper  has  been  a  desire 
to  direct  attention  to  and  stimulate  investigation  of  the  many 
problems  of  percolation  which  still  await  solution. 
SUMMARY. 
A  general  survey  of  all  our  published  knowledge  on  the  subject 
of  percolation  has  been  made.  This  knowledge  has  been  classified, 
coordinated,  and  subjected  to  critical  analysis.  The  results  of  every 
important  contribution  are  presented  and  the  best  technique  under 
varying  conditions,  as  determined  by  the  experience  of  the  whole 
body  of  pharmacists,  stated. 
Certain  factors  which  bear  on  the  problems  of  percolation  and 
have  not  yet  been  investigated  are  discussed  and  attention  is  directed 
to  other  factors  which  require  reinvestigation  owing  to  the  inade- 
quateness  of  our  present  data. 
Explanations  for  a  number  of  phenomena  observed  during  per- 
colation are  offered  and  a  discussion  of  the  mechanism  of  extraction 
is  included. 
