COMMUNICATION  OF  HEAT.
90
portion  to  the  quantity  of  matter,  and  consequently  to  the  quantity  of
heat,  which  is  to  be  either  communicated  or  taken  away,  that  it  does  in
the  larger,  the  quantity  of  matter  being  as  the  cube,  and  the  surface
only  as  the  square  of  the  diameter.  And,  accordingly,  some  experiments ¬
  made  by  Sir  Iſaac  Newton,  and  by  Dr.  Martin,  agree  sufficiently -
  with  this  general  rule,  which  applies  equally  to  all  masses  of  unequal
size,  but  of  the  fame  matter,  and  having  the  fame  form,  whatever  that
be.
This  enables  us  to  understand  how  it  happens,  that  in  thoſe  arts  in
which  much  heat  and  fuel  are  employed,  as  in  the  extraction  of  metals
from  their  ores,  founderies,  distilleries,  and  the  like,  experience  has
ſhewn,  that  the  larger  they  make  their  furnaces  and  stills,  and  other
such  utensils,  they  do  their  work  with  so  much  the  lefs  fuel  ;  for,  in
the  larger  furnaces  and  stills,  and  other  such  utensils,  the  extent  of  surface, ¬
  communicating  with  the  air,  bears  a  smaller  proportion  to  the
contents  of  the  furnace  or  still,  and  to  the  quantity  of  fuel  employed,
or  of  heat  produced,  than  in  smaller  utensils  of  the  fame  nature  ;  and
the  waſte  of  heat  is  therefore  leſs.
We  can  alfo  explain,  by  the  fame  general  principle,  the  flowness
with  which  heat  is  communicated  from  one  part  to  another  of  those
bodies,  or  afsemblages  of  bodies,  which  are  uncommonly  rare  or
spongy,  through  which  it  paſses  much  more  flowly  than  through  the
dense  and  heavy.  A  thick  rod  of  iron,  for  example,  or  copper,  if  one
end  of  it  be  put  into  the  fire,  and  made  red  hot,  transmits  the  heat  so
quickly,  and  in  such  quantity,  to  the  other  end,  that  it  cannot  be
touched  without  burning  the  hand;  while  a  stick  of  wood  of  the  same
size  can  easily  be  held  by  the  one  end,  while  the  other  is  burning.
A  piece  of  cork  transmits  heat  through  it  still  more  flowly  than  the
wood,  or  is  a  worse  conductor  of  heat  ;  and  a  mass  of  wool,  or  feathers,
or  furs,  excel  even  the  cork  in  the  flowness  with  which  heat  is  conducted ¬
  through  them.  Such  materials  compose  a  mass  so  spongy  and
rare,  and  between  the  parts  of  which  there  is  so  little  closeness  of  con¬