80  shows  Page's  apparatus,  in
Vhich  a  feeble  current  of  electri-
city produces  a  rocking  motion  of
the  metallic  masses  resting  on  the
bars  of  brass.  The  best  effects  are
Fig.  80.  produced  between  good  and  bad

conductors  of  heat,  the  former  being  the  hot  bodies.
90.  Heat  is  conducted  in  crystallized  bodies,  in  curves
springing  from  the  sources  of  heat.  In  plates  of  homoge-
neous substances  these  curves  are  circles ;  in  those  of  a  crys-
talline texture,  belonging  to  the  rhombohedral  system,  the
curves  are  ellipses  of  very  exact  form,  whose  longer  axes  are
in  the  direction  of  the  major  crystalline  axis — proving  the
conducting  power  of  such  bodies  to  be  greatest  in  that  direc-
tion. The  mode  of  experimenting  in  such  cases  is  to  cover
the  surface  of  the  crystalline  plate  with  wax,  heat  very  gra-
dually, and  watch  the  lines  of  fusion  on  the  surface.
91.  The  sense  of  touch  gives  us  a  good  idea  of  the  dif-
ferent conducting  power  of  various  solids.  All  the  articles
in  an  apartment  have  nearly  the  same  temperature ;  but  if
we  lay  our  hand  on  a  wooden  table,  the  sensation  is  very  dif-
ferent from  that  which  we  feel  on  touching  the  marble
mantel  or  the  metal  door-knob.  The  carpet  will  give  U3
still  a  different  sensation.  The  marble  feels  cold,  because  it
rapidly  conducts  away  the  heat  from  the  hand  ;  while  the
carpet,  being  a  very  bad  conductor,  retains  and  accumulates
the  heat,  and  thus  feels  warm.  Clothing  is  not  itself  warm,
but,  being  a  bad  conductor,  retains  the  heat  of  the  body.  A
film  of  confined  air,  is  one  of  the  worst  conductors;  loose
clothes  are  therefore  warmer  than  those  which  fit  closely.
