839.  Indigo. — This  important  coloring  substance  is  ob-
tained from  a  great  number  of  plants,  the  principal  of  which
are  the  Indigofera  tinctoria  and  I.  anil,  with  some  species
of  the  genera  Isatis,  Nerium,  and  Polygonum.  The  juices
of  these  contain  a  peculiar  colorless  principle  in  solution,
which,  when  exposed  to  the  air,  absorbs  oxygen,  and  is  con-
verted into  indigo.  In  the  manufacture  of  this  substance,
the  plants  are  steeped  in  water,  and  made  to  undergo  a  kind
of  fermentation ;  the  clear  liquid  is  then  exposed  to  the  air,
and  frequently  agitated  to  facilitate  the  absorption  of  oxy-
gen ',  by  this  process  it  gradually  becomes  blue,  and  deposits
the  insoluble  indigo.
840.  Indigo  is  obtained  in  strongly  cohering  masses  of  a
deep  blue,  which  assume,  when  rubbed,  a  coppery  metallic
lustre.  That  of  commerce  is  never  pure,  but  is  mixed  with
various  foreign  matters.  Indigo  is  insoluble  in  water,  alco-
hol, oils,  dilute  alkalies,  and  chlorohydric  acid  :  when  cau-
tiously heated  it  is  volatilized  as  a  purple  vapor,  which
condenses  in  delicate  crystals.  The  composition  of  indigo
is  expressed  by  C^gHgNOg.
