392 
VARIETIES. 
the  gas-producer  to  the  bottom  of  the  regenerator  to  prevent  air  entering 
and  mingling  with  the  fuel  before  it  is  burnt ;  but  from  the  furnace,  down- 
ward through  the  -regenerators,  the  advance  of  the  heated  medium  is 
governed  mainly  by  the  draught  in  the  tall  stack,  or  chimney. 
"  Great  facility  is  afforded  in  the  management  of  these  furnaces.  If, 
whilst  glass  is  in  the  course  of  manufacture,  an  intense  heat  is  required, 
an  abundant  supply  of  gas  and  air  is  given ;  when  the  glass  is  made,  and 
the  condition  has  to  be  reduced  to  working  temperature,  the  quantity  of 
fuel  and  air  is  reduced.  If  the  combustion  in  the  furnace  is  required  to 
be  gradual  from  end  to  end,  the  inlets  of  air  and  gas  are  placed  more  or 
less  apart  the  one  from  the  other.  The  gas  is  lighter  than  the  air;  and  if 
a  rapid  evolution  of  heat  is  required,  as  in  a  short  puddling  furnace,  the 
mouth  of  the  gas  inlet  is  placed  below  that  of  the  air  inlet ;  if  the  reverse 
is  required,  as  in  the  long  tube- welding  furnace,  the  contrary  arrangement 
is  used.  Sometimes,  as  in  the  enameller's  furnace,  which  is  a  long  muffle, 
it  is  requisite  that  the  heat  be  greater  at  the  door  end  of  the  muffle  and 
furnace,  because  the  goods,  being  put  in  and  taken  out  at  the  same  end, 
those  which  enter  last,  and  are  withdrawn  first,  remain,  of  course,  for  a 
shorter  time  in  the  heat  at  that  end;  and  though  the  fuel  and  air  enters 
first  at  one  end  and  then  at  the  other,  alternately,  still  the  necessary  dif- 
ference of  temperature  is  preserved  by  the  adjustment  of  the  apertures  at 
those  ends. 
"  Not  merely  can  the  supply  of  gas  and  air  to  the  furnace  be  governed 
by  valves  in  the  passages,  but  the  very  manufacture  of  the  gas-fuel  itself 
can  be  diminished  or  even  stopped,  by  cutting  off  the  supply  of  air  to  the 
grate  of  the  gas-producer  ;  and  this  is  important,  inasmuch  as  there  is  no 
gasometer  to  receive  and  preserve  the  aeriform  fuel,  for  it  proceeds  at  once 
to  the  furnaces. 
"  Some  of  the  furnaces  have  their  contents  open  to  the  fuel  and  com- 
bustion, as  in  the  puddling  and  metal-melting  arrangements;  others  are 
enclosed,  as  in  the  muffle  furnaces  and  flint-glass  furnaces, 
"  The  economy  in  the  fuel  is  esteemed  practically  as  one-half,  even  when 
the  same  kind  of  coal  is  used  either  directly  for  the  furnace  or  for  the  gas- 
producer  ;  but,  as  in  the  latter  case,  the  most  worthless  kind  can  be  em- 
ployed, such  as  slack,  &c,  which  can  be  converted  into  a  clean  gaseous 
fuel  at  a  distance  from  the  place  of  the  furnace,  so,  many  advantages  seem 
to  present  themselves  in  this  part  of  the  arrangement." 
Faraday  concludes  his  lecture  with  the  following  conclusive  figures: — 
"  Carbon,  burnt  perfectly  into  carbonic  acid  in  a  gas-producer,  would 
evolve  about  4000°  of  heat ;  but,  if  burnt  into  carbonic  oxyd,  it  would  only 
evolve  1200°.  The  carbonic  oxyd,  in  its  fuel  form,  carries  on  with  it  the 
2800°  in  chemical  force,  which  it  evolves  when  burning  in  the  real  furnace 
with  a  sufficient  supply  of  air.  The  remaining  1200°  are  employed  in  the 
gas-producer  in  distilling  hydrocarbons,  decomposing  water,  &c.  The 
whole  mixed  gaseous  fuel  can  evolve  about  4000°  in  the  furnace,  to  which 
the  regenerator  can  return  about  3000°more." — Amer.  Jour,  of  Sci.  and  Arts. 
