I 
224  COMMERCIAL  VALUE  OF  INDIGO* 
unequal.  He  considers  that  these  discrepancies  are  dependent 
upon  circumstances  with  which  we  are  not  sufficiently  acquainted  ; 
for  instance,  the  duration  of  the  process,  a  small  quantity  of  salt 
producing  as  great  an  effect  in  a  certain  time,  as  a  larger  quan- 
tity in  a  less  time  ;  but  when  the  reaction  occupies  too  long  a 
time,  the  liquid  becomes  concentrated  and  the  result  impaired. 
The  decolorization  of  weak  solutions  cannot  be  effected  at  the  or- 
dinary temperature  only  after  the  lapse  of  some  time  ;  if  they 
are  employed  at  an  elevated  temperature  the  discrepancy  in  the 
results  is  increased  still  more. 
Wittstein,  therefore,  comes  to  the  conclusion,  that  the  old  re- 
duction test  should  be  again  adopted,  since  it  furnishes  not 
merely  relative,  but  absolute  and  tangible  results,  which,  when  a 
few  precautions  are  taken,  are  very  trustworthy.  It  is,  more- 
over, easy  to  carry  out,  and  does  not  occupy  more  than  two  days. 
He  gives  the  following  directions  for  its  application  : 
Ten  grains  of  indigo  are  rubbed  with  water  in  a  porcelain 
mortar,  to  this  twenty  grains  of  hydrate  of  potash  are  added, 
together  with  a  few  drops  of  water,  the  rubbing  being  continued, 
and  then  forty  grains  of  sulphate  of  iron,  the  mass  mixed  with 
water,  so  as  to  form  a  thick  paste,  and  introduced  into  a  six 
ounce  stoppered  bottle,  which  has  been  previously  weighed  ;  the 
mortar  should  then  be  rinsed  and  the  bottle  nearly  filled  up  with 
water.    After  inserting  the  stopper  the  whole  is  weighed  and- 
well  shaken  for  some  time.    After  the  lapse  of  a  few  hours  the 
indigo  is  decolorized  and  dissolved.  The  mixture  is  then  allowed 
to  settle,  the  clear  liquid  is  poured  into  a  glass  cylinder  as  com- 
pletely as  possible,  and  the  bottle  again  weighed.    The  clear 
liquid  is  then  to  be  treated  with  hydrochloric  acid,  the  blue  pre- 
cipitate collected  upon  a  weighed  filter,  washed  with  water,  dried 
and  weighed.    By  means  of  a  proportion  it  may  be  ascertained 
how  much  indigo  blue  is  contained  in  the  ten  grains  of  indigo ; 
if,  for  instance,  the  contents  of  the  bottle  weighed  3140  grains, 
the  residue  left  on  decantation,  535  grains,  and  the  2605  grains 
of  clear  liquor  yielded  2.5  grains  of  indigo  blue,  then  : — 
2605:  3140=2.5:  x 
x  =  3.01. 
Consequently,  the  indigo  examined  would  contain  30.1  per  cent, 
of  pure  coloring  matter —  Vierteljahresschrift  fur  praJtt  Phar- 
macie,  from  London  Pharm.  Journ.  March,  1853. 
