ON  THE  MANUFACTURE  OP  OLEUM  JETHEREUM.  181 
5.  The  temperature  at  6  o'clock;  P.  M. . 
6.  The  yield. 
The  management  of  the  fires  in  the  foregoing  operations  was 
such  that  the  temperatures  would  rise  rapidly  to  about  295°, 
the  draft  door  was  then  closed  until  300°  to  305°  were  reached, 
when  the  fires  were  dampened  by  means  of  ashes.  The  tem- 
perature would  then  generally  rise  from  310°  to  315°.  As  soon 
as  the  temperature  had  fallen  again  to  between  300°  and  305°, 
heat  was  again  applied  until  it  began  to  rise,  when  the  fires 
were  again  dampened ;  the  temperature  would  then  again  rise 
and  reach  sometimes  as  high  as  320°.  The  management  of  the 
heat  requires  great  care  and  constant  attention,  for,  if  the  fire 
is  kept  strong  until  the  temperature  reaches  310°,  the  liquid  in 
the  retort  is  very  apt  to  froth  over.  In  managing  the  fires, 
the  operator  should  be  able  to  raise  the  heat  or  withdraw  it  at 
very  short  notice.  To  this  end  it  is  best  to  work  without  the 
dampers ;  all  that  is  necessary  when  the  fire  is  to  be  damp- 
ened, will  be  to  close  the  draft  door,  open  the  fire  door,  and 
cover  the  live  coal  with  coal  if  the  fire  is  moderate,  or  with 
ashes  if  it  be  very  strong.  The  advantage  of  keeping  the 
damper  open  will  be  obvious  here,  for  a  cold  draft  will  enter 
at  the  fire  door  and  cause  more  rapid  cooling.  The  fire  can 
be  raised  again  at  short  notice  by  opening  the  draft  door  and 
closing  the  fire  door. 
The  formation  of  a  crust  on  the  top  of  the  liquid  in  the  re- 
tort is  considered  favorable  to  the  distillation,  as  it  appears  to 
prevent  the  frothing  up  almost  entirely.  In  one  case  (No.  9, 
table  2)  the  temperature  had  attained  336°  without  causing 
frothing  over,  while  in  other  cases  where  the  crust  had  become 
accidentally  broken  and  the  temperature  was  much  lower,  the 
liquid  frothed  over  like  a  flash,  scarcely  leaving  time  to  save 
the  distillate.  In  all  cases,  except  the  last-mentioned,  the 
frothing  takes  place  slowly,  affording  ample  time  to  make  the 
proper  disconnections.  The  froth  was  generally  allowed  to 
rise  within  one-half  inch  of  the  neck  before  the  connections 
were  loosened,  and  the  retort  was  not  entirely  disconnected 
until  it  began  to  enter  the  neck.    It  was  observed  that  if  the 
