AVaT'i8P78arra'}     Fluid  Extracts  by  Refer  eolation,  219 
was  subtracted  from  the  calculated  residue  of  the  fraction,  and  the 
remainder  was  assumed  as  dry  extract  of  cinchona.  Next,  as  these 
fractions  were  not  uniform  in  weight,  the  assumed  extract  did  not 
represent  a  uniform  proportion  and  rate  of  exhaustion.  To  show  this 
rate  of  exhaustion  better,  a  second  proportion  was  made  to  represent  it 
by  per  centage.  As  the  total  weight  of  the  fraction  is  to  the  weight 
of  extract  obtained  from  it,  so  is  100  to  the  percentage  of  extract  it 
contained.  For  example  :  The  first  fraction  of  the  table  weighed 
62*72  grams.  One  cc.  of  this  fraction  weighed  1*062  grams.  This, 
dried  on  a  watch  glass,  gave  a  residue  weighing  '488  gram.  Then> 
As  1*062  :  -488  :  :  62*72  :  28*82.  But  this  62*72  grams,  is  assumed 
to  give  16  per  cent,  of  its  weight  of  glycerin  which  does  not  go  off 
by  evaporation  or  (As  100  :  16  :  :  62*72  :)  10*04  grams,  glycerin  :  and 
28*82  residue  less  10*04  glycerin  leaves  18*78  as  the  approximate  dry 
extract  contained  in  the  fraction  of  62*72  grams,  of  percolate.  Then 
if  62*72  percolate  give  18*78  extract,  100  percolate  would  give  29*9 
extract,  equal  the  percentage  of  extract  contained  in  the  fraction  of 
percolate. 
The  remaining  column  of  the  table  gives  the  difference  in  specific 
gravity  at  similar  temperatures,  between  the  menstruum  and  the  various 
fractions  of  the  percolates,  and  this  also  illustrates  the  rate  of  exhaus- 
tion, but  in  a  way  much  easier  of  application  in  practice  than  that  of 
weighing  the  extract. 
