The  time  during  which  distillation  has  to  be  continued  de-
pends on  the  size  of  the  retorts,  but,  as  a  rule,  the  operation
is  so  conducted  that  distillation  is  finished  in  12  hours.  Of
course,  with  the  use  of  very  large  apparatus  in  which  a  great
quantity  of  wood  is  carbonized  at  one  time,  distillation  re-
quires several  days,  since  tlie  heating  of  such  a  large  quantity
of  wood  to  the  temperature  of  decomposition  takes  consider-
able time.  When  the  temperature — about  393°  F. — has  been
reached  at  which  the  wood  begins  to  yield  more  abundant
quantities  of  products  of  distillation,  core  must  be  taken  to
keep  the  fire  under  the  retorts  so  that  the  temperature  increases
gradually  and  reaches  QG2°  F.  only  in  the  last  period  of  dis-
tillation, 80  that  with  a  distilling  time  of  12  hours,  the  tem-
perature in  the  retorts  remains  for  about  ten  hours  below
662°  F.  To  gain  practical  experience  in  regulating  the  tem-
perature in  heating,  which  is  of  special  importance  with  new


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DESTRUCTIVE   DISTILLATION   OF   WOOD.  285

ovena.  it  is  advisable  to  place  a  thermometer  on  one  of  the  re-
torts as  follows  :  In  a  small  aperture  in  the  lid  of  the  retort
is  screwed  an  iron  pipe  closed  below  and  open  at  the  top.  The
length  of  the  pipe  should  be  such  that  the  lower  end  reaches
to  the  center  of  the  retort.  In  this  pipe  is  lowered  by  means
of  a  wire  a  thermometer  graduated  to  GiiO°  F.  (the  boiling
poiut  of  mercury).  By  from  time  to  time  consulting  this
thermometer,  a  conclusion  can  be  drawn  as  to  the  degree  of
heat  prevailing  in  the  retort.  Gxp>erienced  men  can  accurately
judge  of  the  progress  of  distillation  from  the  quantity  of  distil-
late running  off,  and  of  the  gases  escaping  simultaneously.
