OBTAINING  OIL  FROM  BITUMINOUS  COAL. 
553 
order  to  obtain  the  largest  quantity  of  crude  parafine  oil  from 
coals  by  means  of  this  process,  and  produce  the  smallest  quantity 
of  permanent  gas  by  the  action  of  the  heat  employed,  whatever 
may  be  the  apparatus  used,  care  must  be  taken  to  heat  the  coals 
gradually,  and  to  apply  the  lowest  temperature  necessary  to  com- 
plete the  operation.  During  the  distillation  or  driving  off,  a 
permanent  gas  will  be  produced,  and  this  gas  may  either  be 
collected  or  suffered  to  escape,  as  may  be  thought  expedient. 
The  crude  oil  obtained,  as  already  described,  is  purified  in  the 
following  manner  :— I  put  the  oil  into  a  cistern,  and  heat  it  (by 
a  steam  pipe  or  other  means)  to  a  temperature  of  about  150° 
Fahr.  When  thus  treated,  water  and  undissolved  impurities 
contained  in  the  oil  will  separate  more  readily  from  it  than  when 
cold,  and  the  oil  being  left  in  a  state  of  rest  and  kept  warm  for 
about  a  day,  many  of  those  impurities  will  fall  to  the  bottom  of 
the  cistern,  and  the  oil  may  then  be  run  off  into  another  vessel, 
leaving  the  residuum  behind.  For  distilling  the  oil  the  inventor 
prefers  to  use  an  iron  still  with  a  worm-pipe  connected  to  it, 
passing  through  a  refrigeratory  apparatus,  which  is  kept  at  or 
about  the  temperature  of  55°  Fahr.,  as  already  mentioned.  The 
still  is  heated  by  a  fire  underneath  it,  which  is  kept  up  until  the 
whole  of  the  oil  has  been  distilled  over,  and  it  will  then  be  found 
that  the  still  contains  some  dry  carbonaceous  residuum,  which 
should  be  taken  out  before  the  still  is  again  used.  The  oil  is  to 
be  run  from  the  condensing  apparatus  as  it  distils  over  into  a 
leaden  vessel,  where,  to  each  100  gallons  there  are  gradually 
added  10  gallons  of  the  oil  of  vitriol  of  commerce.  After  this 
mixture  has  been  well  stirred  for  about  an  hour,  it  is  to  remain 
at  rest  for  about  twelve  hours,  so  that  the  oil  of  vitriol  and  the 
impurities  with  which  it  has  combined  may  settle  at  the  bottom. 
The  supernatant  oil  is  then  drawn  off  into  anir  on  vessel,  and 
four  gallons  of  a  solution  of  caustic  soda,  of  a  specific  gravity 
1.300  (water  being  1.000,)  added  to  each  100  gallons.  The  soda 
and  oil  are  stirred  together  for  about  an  hour,  so  as  to  neutralize 
any  acid  which  may  remain  in  the  oil,  and  also  take  up  any  im- 
purities capable  of  combining  with  it,  after  which  the  -contents 
of  the  vessel  are  allowed  to  remain  at  rest  for  about  six  or  eight 
hours,  so  that  the  solution  of  soda  may  subside,  and  then  the 
supernatant  oil  is  to  be  drawn  off,  and  again  distilled  in  the  same 
