OBTAINING  SPIRITS,  ETC.,  FROM  WOOD.  531 
tion  is  so  rapid  that  without  the  use  of  superheated  steam  it  is 
difficult  to  keep  up  sufficient  heat  without  charring  the  wood  in 
some  parts,  which  must  be  avoided  as  long  as  possible,  as  the 
quality  of  the  spirits  is  very  poor  after  charring  commences  until 
they  have  passed  through  difficult  processes  of  refining.  While 
this  process  is  going  on  rosin  is  formed  and  runs  down  to  the 
bottom  of  the  retort,  and  is  drawn  out  before  the  charring  com- 
mences. 
In  using  the  vacuum  or  partial  vacuum  in  the  distillation,  it 
will  be  found  advantageous  to  shut  off  the  connections  of  pump 
and  condensers  with  the  retort  occasionally,  to  allow  pressure  of 
steam  in  the  retort,  which  rapidly  forces  the  heat  through  the 
wood  again  without  charring  it,  unless  the  temperature  of  the 
steam  is  too  high.  The  pressure  of  steam  in  the  retort  is  not 
generally  allowed  to  reach  more  than  three  or  four  atmospheres 
before  it  is  again  reduced.  As  soon  as  the  yield  of  the  spirits  of 
turpentine  becomes  inconsiderable,  the  heat  is  raised  and  the 
charring  commences,  and  as  soon  as  this  progresses  rapidly,  the 
steam  may  be  nearly  or  quite  shut  off,  and  the  acid  formed  will 
be  much  stronger  than  if  the  supply  of  steam  be  kept  up  during 
the  entire  distillation.  The  use  of  the  vacuum  or  partial  vacuum 
may  be  resorted  to  frequently  throughout  the  entire  distillation, 
the  products  coming  over  purer  and  at  a  much  lower  heat  than 
they  otherwise  would.  While  the  charring  of  the  wood  takes 
place,  the  vapors  passing  off  consist  of  acid,  wood  naphtha, 
spirits,  oil,  tar,  incondensable  vapors,  &c.  A  very  advantageous 
way  of  separating  the  tar  from  the  other  vapors  is  to  surround 
the  condenser  with  a  bath,  heated  to  such  a  degree  as  to  enable 
the  vapors  of  the  naphtha,  acid,  and  spirits,  together  with  the 
light  oils,  to  pass  through  it  without  condensation,  but  cool 
enough  to  condense  the  tar,  and  so  placed  that  the  tar  will  run 
back  into  the  retort  while  the  vapors  of  the  acid,  naphtha,  spirits, 
&c,  pass  on  through  another  condenser,  which  may  be  cold,  and 
separate  the  gas  from  the  other  products,  or  it  may  be  heated 
sufficiently  to  carry  the  wood  naphtha  over  with  the  gas  into 
another  condenser,  where  it  is  finally  separated  from  the  gas. 
The  distillation  is  continued  until  the  wood  is  thoroughly  charred 
and  the  tar  converted  into  a  beautiful  black  lustrous  pitch,  the 
