PREPARATION  OF  ARTIFICIAL  COLORING  MATTERS,  ETC.  131 
tion  of  oxalic  acid  is  now  added  to  the  impure  aniline,  which 
precipitates  oxalate  of  aniline,  as  a  mass  of  white  crystals, 
which  are  washed  with  alcohol,  and  then  pressed.  The  salt  is 
then  dissolved  in  a  small  quantity  of  water,  to  which  a  little 
alcohol  is  added.  From  this  solution  the  oxalate  crystalises  in 
stellated  groups  of  oblique  rhomboidal  prisms.  These  crystals 
are  decomposed  by  a  caustic  alkali,  to  set  free  the  aniline,  and 
when  this  is  distilled,  water  at  first  passes,  then  water  charged 
with  aniline,  and  lastly,  at  182°  C.  chemically  pure  aniline. 
Artificial  preparation  of  Aniline  by  the  reduction  of 
NiTRO-BENZOLE. — This  process,  which  constitute  sone  of  the  most 
curious  and  important  reactions  of  organic  chemistry,  allows  us  to 
obtain  aniline  in  any  quantity.  It  is  not  difficult  to  prepare,  but 
certain  precautions  are  necessary  when  operating  on  a  large 
scale. 
The  process  may  be  subdivided  into  three  distinct  operations. 
1.  Pre'paration  of  Beiizole. 
2.  Tramformatio7i  into  Nitro-henzole. 
8.  Reduction  of  Nitro-henzole  to  form  Aniline. 
1.  Pre'paration  of  Benzole. — The  only  process  we  have  space 
to  notice  is  that  by  which  benzole  is  obtained  on  a  large  scale, 
viz. — the  extraction  from  coal-tar,  or  from  the  first  products  of 
the  distillation  of  coal-tar,  light  oil,  or  crude  naphtha. 
The  manufacturer  who  wishes  to  distil  tar  in  order  to  procure 
the  largest  amount  of  benzole,  should  choose  a  light  fluid  tar, 
and  preferably  one  distilled  from  bog  head  or  cannel  coal.  To 
form  a  comparative  estimate  of  the  value  of  different  tars,  the 
following  experiment  may  be  performed  : — About  20  pints  of 
the  tar  are  distilled  until  the  vapors,  instead  of  condensing  into 
a  liquid,  furnish  a  product  which,  on  cooling,  becomes  solid,  or 
of  a  buttery  consistence.  By  carefully  observing  when  the 
condensed  oil  becomes  heavier  than  the  water,  and  measuring 
the  volume  of  the  lighter  oils  which  float  on  the  surface  of  the 
water,  and  then  comparing  the  volumes,  we  are  enabled  to  esti- 
mate with  tolerable  accuracy  the  value  of  the  tar.  Of  course 
that  which  yields  the  largest  amount  of  light  oil  is  the  best. 
Some  account  of  the  process  of  distillation  was  given  at  pages 
