250     PREPARATION  OF  ARTIFICIAL  COLORING  MATTERS,  ETC. 
reaction  has  terminated.  Oxalic  and  hydrochloric  acids  dis- 
solve the  nitrazophenylamine,  leaving  the  precipitated  sulphur, 
and  also  a  secondary  crystallized  product  of  a  dirty  green  color. 
The  base  may  be  obtained  pure  by  precipitating  a  boiling  oxalic 
or  hydrochloric  solution  with  ammonia  and  re-crystallizing  two 
or  three  times  from  a  solution  in  hot  alcohol.  In  this  way 
nitrazophenylamine  is  obtained  in  long  slender  needles,  united 
in  groups,  having  a  clear  red  color  when  dry,  and  showing  a 
golden  iridescence.  Water,  alcohol,  and  ether  easily  dissolve  it, 
and  the  concentrated  solutions  are  of  a  deep  red  color.  It 
melts  at  a  high  temperature,  and  the  greater  part  is  volatilized 
apparently  without  alteration.  Heated  suddenly,  it  slightly 
explodes,  leaving  a  carbonaceous  residue. 
The  salts  of  nitrazophenylamine  are  very  beautiful  compounds, 
characterized  by  a  dichroism  which  causes  a  peculiar  iridescence 
in  reflected  light.  All  the  salts  must  be  crystallized  in  the 
presence  of  an  excess  of  acid,  since  both  water  and  alcohol 
decompose  the  neutral  salts. 
Fuschine  prepared  by  chloride  of  tin,  according  to  the  process 
of  MM.  Renard  and  Franc,  appears  to  be  the  hydrochlorate  of 
a  base,  the  properties  of  which  greatly  resemble  those  of  nitra- 
zophenylamine. The  crystallized  salts  show  the  same  colored 
reflections,  and  are  decomposed  (in  part  at  least)  by  water  and 
alcohol,  the  solutions  being  of  a  deep  red  color. 
We  now  proceed  to  notice  the  processes  for  the  preparation 
of  coloring  matters  by  means  of  aniline,  and  the  method  of  era- 
ploying  them  for  dyeing  fabrics  and  yarns. 
The  first  on  the  list  is  that  of  Mr.  Perkin,  for  the  preparation 
of  aniline,  violet  or  mauve.  A  cold  solution  of  sulphate  of 
aniline  (rough  aniline  is  used)  and  a  cold  solution  of  bichromate 
of  potash  are  mixed  together  and  left  for  ten  or  twelve  hours. 
An  abundant  deposit  of  a  black  powder  is  thus  obtained,  which 
is  separated,  well  washed  with  water,  and  lastly,  dried  at  212^. 
The  dried  substance  is  then  digested  several  times  with  naptha 
or  commercial  benzole,  which  dissolves  a  brown  tarry  or  resinous 
substance,  contaminating  the  coloring  matter  in  the  deposit. 
The  residue  insoluble  in  the  naptha  is  dried  again,  and  then 
digested  with  wood  spirit  or  alcohol,  or  any  other  liquid  able  to 
dissolve  the  coloring  matter.    This  clear  solution  is  decanted 
