42 
COLORING  MATTERS  FROM  COAL  TAR. 
they  are  all  submitted  to  distillation  in  order  to  isolate  the  prin- 
ciples capable  of  industrial  application.  But  first  of  all  it  is 
necessary  to  separate  the  tar,  as  far  as  possible,  from  the  ammo- 
niacal  liquor  which  is  found  with  it.  For  this  purpose  it  is 
heated  for  some  hours  to  80°  or  100^  C,  by  which  it  is  rendered 
more  liquid^  and  then  the  water  separates  more  easily.  It  is 
then  allowed  to  cool  very  slowly,  and  the  water  is  drawn  off  by 
a  tap  placed  at  the  lower  part  of  the  boiler.  A  certain  quan- 
tity of  tar  obstinately  retains  the  water,  constituting  a  buttery 
matter,  which  may  be  allowed  to  run  away  with  the  water,  to 
be  added  afterwards  to  another  quantity  of  tar,  to  be  dehydrated 
by  a  fresh  operation. 
Experience  seems  to  have  demonstrated  that  the  most  simple 
process,  that  is  to  say,  distillation  over  a  naked  fire  at  the  ordi- 
nary pressure,  is  still  the  most  practicable  and  advantageous. 
As  the  volatile  products  have  but  little  latent  heat,  the-  height 
of  the  still  should  be  somewhat  less  than  the  diameter ;  for  the 
same  reason  the  head  must  be  carefully  protected  from  cold, 
and  it  is  well  to  furnish  the  inside  with  a  circular  gutter,  in 
which  the  products  condensed  in  the  head  may  be  collected  and 
run  into  the  refrigerator.  By  this  means  the  products  are 
prevented  from  flowing  back  into  the  boiling  tar  and  being  de- 
composed by  coming  in  contact  with  the  sides  of  the  still,  which, 
especially  towards  the  end  of  the  operation,  become  very  hot. 
In  condensing  the  vapors  it  is  necessary  to  observe  certain 
precautions.  At  the  beginning  of  the  operation,  when  the 
lighter  and  more  volatile  oils  are  passing,  the  worm  must  be 
well  cooled  to  make  quite  sure  of  the  condensation.  Later, 
when  the  heavier  and  less  volatile  products  are  coming  over, 
the  water  in  the  refrigerator  may  be  allowed  to  get  heated  to 
30°  or  40*^,  and  at  last  when  the  matters  capable  of  solidifying, 
such  as  naphthaline  and  paraffine,  pass,  the  temperature  of  the 
refrigerator  should  never  be  under  40^^,  and  it  may  be  allowed 
without  inconvenience  to  rise  to  60^^  or  10^.  At  this  tempera- 
ture the  products  condense  perfectly,  but  remain  liquid  and  run 
with  ease.  If  the  refrigerator  were  kept  quite  cold  during  the 
whole  process,  it  might  happen  towards  the  end  that  the  conden- 
sing tube  would  become  blocked  up  by  the  solidified  products, 
and  a  dangerous  explosion  might  ensue. 
