PREPARATION  OF  ARTIFICIAL  COLORING  MATTERS,  ETC.  255 
is  changed  into  the  coloring  matter,  and  care  must  be  taken  to 
keep  the  mixture  at  this  temperature  for  some  time.  The  heat 
may  then  be  increased,  but  it  must  never  pass  160^.  The  ope- 
ration lasts  four  or  five  hours. 
In  the  above  way  a  perfectly  homogeneous  mass  is  obtained, 
which  is  fluid  above  100^.  On  cooling  it  solidifies,  and  has  the 
appearance  of  a  hard,  brittle  substance,  with  a  bronze  lustre. 
It  is  very  soluble  in  water  to  which  it  communicates  a  pure  red 
color,  so  deep  that  concentrated  boiling  solutions  appear  black. 
The  solutions  may  be  used  for  dyeing  directly  without  fear,  for 
the  tissues  will  not  retain  a  trace  of  the  arsenic  If  necessary, 
the  arsenic  may  be  easily  removed  from  the  coloring  matter  by 
one  of  the  following  processes  : — 
1.  Powder  the  rough  product  and  treat  it  with  strong  hydro- 
chloric acid  ;  then  dilute  with  water,  and  saturate  the  clear 
solution  with  a  slight  excess  of  soda.  The  coloring  matter  is 
precipitated,  while  the  arsenic  remains  in  solution  in  the  alkali, 
and  it  is  only  necessary  to  wash  the  precipitate  once  or  twice 
with  cold  water  to  obtain  the  coloring  matter  quite  pure. 
2.  The  rough  product  dissolved  in  water  is  treated  with  a 
quantity  of  quick  lime  corresponding  to  the  arsenical  compounds 
it  contains,  plus  a  slight  excess.  The  coloring  matter  is  pre- 
cipitated, as  well  as  the  arsenical  compounds, — the  latter  in  the 
form  of  insoluble  calcareous  salts.  The  liquor  and  the  precipi- 
tate (unseparated)  are  now  treated  with  carbonic,  acetic,  or 
tartaric  acid,  either  of  which  will  dissolve  the  coloring  matter 
and  leave  the  arsenic. 
In  this  process  aniline  gives  about  its  own  weight  of  coloring 
matter. 
The  process  of  MM.  Depouilly  and  Lauth,  "for  the  manu- 
facture of  various  colored  products  derived  from  aniline,"  is 
very  similar  to  that  of  Mr.  Perkin.  They  take  a  solution  of  a 
salt  of  aniline  and  treat  it  with  a  solution  of  chloride  of  lime. 
The  first  drops  of  the  chloride  produce  a  violet  coloration,  and, 
on  continuing  the  addition  of  the  re-agent,  a  deep  violet  pre- 
cipitate is  formed,  which  constitutes  the  coloring  matter.  This 
is  collected  and  washed  with  slightly  acidulated  water.  When 
the  washings  are  uncolored,  the  precipitate  is  collected  on  a  fil- 
