309.  Metallic  Magnesium,  Mg,  is  readily  obtained  by  decom-
posing the  anhydrous  chloride  with  metallic  sodium,  also  by
electrolysis.     It  is  a  silver-white  metal,  melting  at  a  red  heat,
and  volatilizing  at  a  high  temperature  in  an  atmosphere  of  hy-
drogen.    It  is  malleable  and  ductile,  is  susceptible  of  a  high
polish,  and  does  not  tarnish  in  dry  air.     Heated  in  the  air  it
takes  fire  and  burns  with  great  splendor  [59],  and  it  is  now
much  used  as  a  source  of  pure  white  light  when  great  bril-
liancy is  required.     Boiling  water  acts  upon  the  metal  quite
rapidly,  but  it  decomposes  cold  water  only  very  slowly.
310.  Magnesic  Oxide  {Calcined  Magnesia),  MgO,  is  obtained
when  the  metal  is  burnt  in  air.     It  can  also  be  obtained  by  cal-
cining the  carbonate  or  the  nitrate.     It  is  a  bulky  white  powder,
wholly  infusible,  and  emitting  a  bright  white  light  when  heated
before  the  blow-pipe.     Intensely  heated,  it  appears  to  volatilize
unchanged.     When  mixed  with  water  it  slowly  unites  with  it
to  form  a  hydrate.     The  oxide  obtained  by  calcining  the  nitrate
is  much  denser  than  that  made  from  the  carbonate,  and  possesses
remarkable  hydraulic  qualities.     When  mixed   with  water,  it
soon  sets  forming  a  hard  compact  mass  resembling  marble.     If
the  oxide  is  heated  to  a  very  high  temperature,  it  loses  its  power
of  uniting  with  water,  and  dissolves  only  slowly  even  in  the  strong-
est acids.     Crystallized  Mg  0  (Figs.  5  to  7),  Periclase,  has  been
found  in  small  grains  imbedded  in  a  limestone  rock  ejected  from
Vesuvius,  but  otherwise  it  does  not  occur  uncombined  in  nature.
