448  PRODUCTION  OF  VERY  HIGH  TEMPERATURES. 
rates  it  from  the  cavity  below.  The  author  employs  as  fuel, 
cinders  from  the  hearth  of  a  furnace  heated  with  the  dry  coal  of 
Charleroy.  These  cinders  are  found  mixed  with  pieces  of  coal, 
and  are  sifted  upon  a  sieve  with  square  holes  of  2  millimeters  in 
the  side.  What  passes  through  the  sieve  is  rejected.  The  force 
of  the  maximum  temperature  begins  about  2  or  3  centimeters 
above  the  iron  plate,  and  is  only  7  or  8  centimeters  high.  The  coals 
employed  must  vary  from  the  size  of  a  small  pea  to  that  of  a  nut. 
The  crucible  is  placed  in  the  centre  of  the  cylinder  and  surround- 
ed with  kindled  wood,  upon  which  pieces  of  coal  of  the  size  of  a 
nut  are  laid,  and  upon  these  the  proper  fuel  of  the  furnace.  The 
blast  is  then  forced  in  slowly  and  gradually  increased.  The  coals 
above  remain  cold  from  the  transformation  of  the  carbonic  acid 
into  carbonic  oxyd,  which  gas,  in  the  author's  furnace,  burns 
with  a  flame  2  meters  in  height.  The  heat  produced  by  this 
arrangement  is  called  by  the  author  the  "  blue  heat,"  from  its 
peculiar  tint.  In  it  the  best  ordinary  crucibles  run  down  like 
glass.  The  author  uses  three  kinds  of  crucible.  The  first  is  of 
quicklime  and  is  made  of  well  burned  lime,  slightly  hydraulic, 
which  is  cut  with  a  knife  or  saw  into  prisms,  with  a  square  base 
8  or  10  centimeters  in  the  side,  and  12  or  15  centimeters  high. 
The  edges  are  rounded  and  a  hole  is  made  in  one  end  of  conveni- 
ent size.  Sometimes  an  inner  crucible  is  used,  each  having  its 
own  cover.  When  the  substance  to  be  heated  is  very  refractory, 
only  one  crucible  is  used,  and  the  walls  of  this  are  made  3  or  4 
centimeters  thick.  The  base  of  the  crucible  must  be  5  or  6  cen- 
timeters below  the  bottom  of  the  cavity.  The  space  between  the 
crucible  and  the  walls  of  the  cylinder  must  be  5  or  6  centimeters. 
In  using  a  lime  crucible,  charcoal  is  first  to  be  introduced,  little 
by  little,  till  the  crucible  is  covered,  the  heat  is  then  very 
gradually  increased  till  the  crucible  becomes  red,  when  the  coals 
are  removed  to  make  sure  that  the  crucible  is  not  cracked,  after 
which  the  heat  may  be  urged  to  the  utmost.  The  second  kind 
of  crucible  is  of  carbon.  The  author  uses  gas  retort  carbon  and 
fashions  it  on  a  lathe.  To  free  the  material  from  impurities  it 
may  then  be  strongly  heated  in  a  current  of  chlorine,  by  which  pro- 
cess it  loses  weight.  These  crucibles  are  placed  within  crucibles  of 
lime,  the  intervening  space  being  filled  with  calcined  alumina.  The 
third  species  of  crucible  is  made  of  alumina,  obtained  by  calcin- 
