534        griffin's  reverberatory  gas  furnace. 
joined  at  the  top  to  the  iron  body  of  the  furnace,  d.  This  body 
is  4  inches  high  and  4  inches  in  external  diameter.  The  iron 
tube  made  conical  at  the  lower  end,  is  15  inches  long,  2f 
inches  wide  at  the  top,  and  3i  inches  wide  at  the  bottom. 
The  body  of  the  furnace,  is  fitted  with  a  lining  of  fire- 
clay or  plumbago,  the  upper  part  of  which  is  cylindrical,  with 
a  bore  of  2i  inches,  and  the  lower  part  forms  a  massive  diaphragm, 
with  a  central  opening  of  li  inches,  as  shown  by/ in  the  sec- 
tion. Upon  this  diaphragm  is  loosely  placed  a  cast-iron  ring 
■with  three  knife-edges,  represented  in  the  section  and  separate- 
ly by  the  figure  h  and  ^,  the  use  of  which  is  to  support  crucibles. 
By  reversing  this  ring  it  can  be  made  to  suit  crucibles 
of  different  sizes.  In  the  lower  part  of  the  chimney,  e, 
a  reverberatory  dome  of  fire-clay  or  of  plumbago  is  fixed. 
This  serves  to  deflect  the  flame  downwards  upon  the  top  iH|||[|||| 
of  the  crucibles.  The  space  provided  in  the  body  of  Ji 
the  furnace  for  the  reception  of  crucibles  is  2J  inches  wide 
and  2J  inches  high,  and  it  will  take  in  a  crucible  of  nearly  those 
dimensions. 
The  furnace  is  supported  by  a  flanged  iron  ring,  A?,  attached 
to  a  triangular  socket,  which  can  be  screwed  on  the  |  inch  rod 
of  an  ordinary  iron  retort  stand,  as  represented  by  o. 
The  gas,  entering  the  furnace  by  the  supply-pipe,  a,  mixes 
with  the  air,  which  rises  through  the  lower  end  of  the  tube  5, 
and  burns  with  a  smokeless  flame  w^hen  lighted  at  the  upper  end 
of  that  tube.  This  flame  is  then  supplied  with  additional  air  by 
the  lower  end  of  the  tube  a  great  draught  being  produced  by 
the  action  of  the  tall  and  wide  chimney,  e.  When  the  pres- 
sure of  the  gas  is  strong,  the  flame  reaches  from  the  tube  h 
to  the  top  of  the  chimney  being  in  that  case  two  feet  long. 
The  point  of  greatest  heat  is  at  8  or  9  inches  above  the  upper 
end  of  the  tube  5,  and  the  arrangement  of  the  furnace  is  such  as 
to  fix  the  crucible  in  the  focus  of  the  heat.  The  condition  of 
the  crucible  can  at  any  time  be  seen  on  lifting  the  dome  and 
chimney  e,  by  means  of  the  handles  I. 
The  amount  of  gas  supplied  to  the  furnace  is  regulated  by 
the  stop-cock  a,  from  which  a  flexible  pipe  should  lead  to  a  sec- 
ond stop-cock,  attached  to  a  fixed  gas-pipe.  The  latter  should 
be  opened  wider  than  is  necessary  to  supply  the  furnace  with 
