8.  Why  is  felt  a  better  conductor  of  heat  when  it  is  very  firmly  packed
than  when  loosely  packed  ?


Gekeral  Propekties  of  Magnets


305.  Magnets.  It  lias  been  known  for  many  centuries  that
some  specimens  of  the  ore  known  as  magnetite  (Fe,0,)  have  the
property  of  attracting  small  bits  of  iron  and  steel.  This  ore
probably  received  its  name  from  the  fact  that  it  is  especially
abundant  in  the  province  of  Magnesia,  in  Thessaly,  although
the  Latin  writer  Pliny  saya  that  the  word  "magnet"  ia  derived
from  the  name  of  the  Greek  shepherd  Magnes,  who,  on  the  top
of  Mount  Ida,  observed  the  attraction  of  a  large  stone  for  his
iron  erook.  Pieces  of  this  ore  which  exhibit  this  attractive
property  are  known  as  natural  magjiets.
It  was  also  known  to  the  ancients  that  artificial  magnets  may
be  made  by  stroking  pieces  of  steel  with  natural  magnets,  but
it  was  not  until  about  the  twelfth  century  that  the  discovery
was  made  that  a  suspended  magnet  will  assume  a  north-and-
south  position.  Because  of  tliis  latter  property  natural  magnets
became  known  as  lodestones  (leading  stones),  and  magnets,
either  artificial  or  natural,  began  to  be  used  for  determining
directions.  The  first  mention  of  the  use  of  the  compass  in
Europe  is  in  1190.  It  ia  thought  to  have  been  introduced  from
China.
Magnets  are  now  made  either  by  stroking  bars  of  steel  in
one  direction  with  a  magnet,  or  by  passing  electric  currents
about  the  bars  in  a  manner  to  be  described  later.    The  £orm

>  This  chapter  ahcnild  aiUier  be  aixompatiled  or  preceded  by  laboratory  experi-
ments oil  magnellc  fields  and  on  the  molecular  nutiire  of  magaetlBOi.  Etee,  tor
Bxtunple,  Eiperimeata  25  aud  2G  of  the  authors'  iii»aa&!i.
