COLORING  MATTERS   FROM  COAL  TAR. 
43 
At  the  beginning  of  the  distillation  the  tar  should  not  be 
allowed  to  boil  too  fast.  Some  distillers  at  this  period  pass  a 
current  of  steam  at  110°  or  120*^  through  the  tar,  to  assist  the 
disengagement  of  the  more  volatile  oils.  These  in  condensing 
form  a  limpid  very  fluid  liquid,  having  the  density  -780,  which 
gradually  rises  to  -850 ;  the  mean  density  of  all  the  products 
united  is  about  -830.  It  is  this  which  constitutes  the  benzine  of 
commerce.  It  contains  a  great  variety  of  compounds  whose 
boiling  points  range  from  60*^  to  200°.  They  belong  princi- 
pally to  the  following  series : — CnHn  e.  g.  Amylene  C^H. ; 
Hexylene  (Oleene,  Caproylene),  CgHg;  Heptylene  (Oenenthy- 
lene),  C^H^,  &c.  CnHn-f  3  e.  g.  Propyle  O^^J^i^ ;  Butyle  C^gH^g ; 
Amyle  C20H22,  &c.  Cn  Hn — 6  e.  g.  Benzine,  Ci^fi^^  &c. 
When  the  density  of  the  products  exceeds  -850*^,  the  current 
of  steam  is  stopped  and  the  heat  is  increased.  As  soon  as  the 
temperature  of  the  tar  has  risen  to  200°— 220*^,  the  distillation 
recommences,  and  the  oil  condensed  is  found  to  have  the  sp.  gr. 
.860 — 900,  the  mean  being  from  -880  to  -885.  This  product 
constitutes  the  heavy  oil  of  tar,  and  contains  phenol,  creosote, 
and  aniline. 
Lastly,  the  ultimate  products  of  the  distillation,  which  |on 
cooling  become  a  buttery  mass  (or  crystalline,  if  they  contain 
much  naphthaline), are  set  aside  for  the  preparation  of  paraffine. 
They  are  placed  in  vats,  which  are  cooled,  in  order  that  the 
solid  matters  may  separate  by  crystallisation. 
According  to  Payen,  2000  parts  of  rough  oil  or  tar  obtained 
by  the  distillation  of  Boghead  coal  furnish  on  rectification  : — 
1208  parts  light  oil,  density^ -825 
200    "    heavy  oil,      "     =  .860 
400  pitch. 
The  loss  of  200  parts  represents  the  gases,  and  the  vapors 
and  oils  which  have  escaped.    2900  parts  of  tar  from  gas  works 
using  Boghead  coal,  distilled  in  a  similar  manner,  yielded : — 
Water,  slightly  ammoniacal    .       .        .      168  parts. 
Light  hydrocarbons,  mean  density  .       -820  480 
Heavy  hydrocarbons        "           .       •863  883 
Fatty  pitch,  solid  when  cold,  liquid  at  150°  1195 
Loss  6  per  cent  174 
2900 
