ON  THE  EXAMINATION  OF  INDIGO. 
571 
phate  of  iron  and  lime  with  indigo,  by  which  the  coloring  mat- 
ter is  deoxidized  and  dissolved  ;  if  now  precipitated  by  muria- 
tic acid,  the  pure  coloring  matter  may  be  dried  and  weighed. 
It  takes  about  two  days  to  finish  this  analysis,  but  the  whole  of 
the  coloring  matter  is  not  obtainable  in  this  way. 
Professor  Runge  has  lately  proposed  protoxide  of  tin  and 
potassa  for  deoxidation,  but  besides  the  necessity  of  preparing 
the  test  fresh  every  time  immediately  before  its  use,  the  reduc- 
tion takes  place  but  partially,  even  if  the  reaction  be  allowed  to 
continue  for  weeks. 
The  author  now  proposes  grape-sugar  and  potassa  as  reagents 
for  effecting  the  reduction,  and  thinks  that  they  will  realize  all 
just  expectations.  The  mode  of  proceeding  is  as  follows  :  10 
grains  of  finely  powdered  indigo,  30  grs.  dry  caustic  potassa, 
and  60  grs.  grape-sugar,  together  with  two  drachms  of  water, 
are  moderately  heated  in  a  sand  bath  ;  in  a  few  moments  a  re- 
action is  observed,  the  color  changes  ultimately  to  a  yellow 
green,  and  a  few  movements  with  the  pestle  are  now  sufficient 
to  rub  all  into  a  fine  powder.  This  is  now  washed  into  an 
ordinary  prescription  vial,  the  weight  of  which,  together  with  the 
well-fitting  cork,  has  been  previously  ascertained,  the  vial  is 
nearly  filled  with  water,  corked  and  weighed,  and  occasionally 
shaken  until  the  froth  has  lost  its  blue  or  green  color  and  be- 
come pale  yellow,  which  usually  takes  place  on  the  second  day. 
After  settling,  all  the  clear  liquid  is  decanted,  and  the  weight  of 
the  residue  ascertained,  which  contains  the  impurities  of  indigo, 
consisting  of  clay,  lime,  magnesia,  oxide  of  iron,  silica,  &c. 
The  decanted  liquid,  which  in  contact  with  the  air  turns  blue 
instantly,  is  over-saturated  with  diluted  sulphuric  or  muriatic 
acid,  the  precipitate  is  collected  upon  a  filter,  between  which 
and  the  funnel  another  filter  of  the  same  size  and  weight  is 
placed,  to  be  used  as  a  counterpoise,  and  is  afterwards  washed  un- 
til the  filtrate  ceases  to  redden  litmus  paper,  when  it  is  dried  in  a 
warm  place  and  weighed.  Now,  suppose  the  weight  of  the  whole 
contents  of  the  vial  to  have  been  1620  grains,  of  the  residue  after 
decantation  161-5  grs.  of  the  decanted  liquid  1620 — 161.5= 
1458.5,  and  of  the  blue  coloring  matter  5.59  grains  ;  the  10  grs, 
34 
