*  Experiments  show  that  with  artificial  heat  the  molecular  condition
of  the  surface  varies  radiation  as  well  as  reflection.  In  fact,  white  lead  is
as  good  a  radiator  as  lamp-black.— On  one  side  of  a  sheet  of  paper  paste  let-
ters of  gold-leaf.  Spread  over  the  opposite  side  a  thin  coating  of  scarlet
iodide  of  mercury— a  salt  which  turns  yellow  on  the  application  of  heat.
Turn  the  scarlet  side  down.  Hold  over  the  paper  a  red-hot  iron.  The  gold-
leaf  will  reflect  the  heat,  but  the  paper  spaces  between  the  letters  will
absorb  it,  and  on  turning  the  paper  over,  the  gilt  letters  will  be  found
traced  in  scarlet  on  a  yellow  background.
THE     STEAM-ENGINE.
261


FIG.  186.
IV.    THE   STEAM-ENGINE.
WHEN  steam  rises  from  water  at  a  temperature
of  212°,  it  has  an  elastic  force  of  nearly  15  Ibs.  per
square  inch.  If  the  steam  be
confined  and  the  temperature
raised,  the  elastic  force  will  be
rapidly  increased.
1.  The  Steam-engine  is  a
machine  for  using  the  elastic
force  of  steam  as  a  motive
power.  There  are  two  classes,
high-^pressure  and  low-pressure.
In  the  former,  the  steam,  after
it  has  done  its  work,  is  forced

OUt    into    the     air  ;     in    the    latter,       Steam-chest  and  Cylinder  of

it    is    condensed    in    a    separate

chamber  by  a  spray  of  cold  water.  As  the  steam
is  condensed  in  the  low-pressure  engine,  a  vacuum
is  formed  behind  the  piston  ;  while  the  piston  of  the
high-pressure  engine  acts  against  the  pressure  of  the
air.  The  elastic  force  of  the  steam  must  be  15  Ibs.
