PREPARATION  AND  PROPERTIES  OF  MAGNESIUM. 
347 
granular,  or  even  fibrous,  according  to  the  mode  in  which  it  is 
broken ;  in  the  former  case  it  is  silver-white  and  very  shining, 
in  the  latter  more  bluish-gray  and  dull.  Its  hardness  is  less  than 
that  of  calc-spar.  A  moderate  red  heat  is  sufficient  to  fuse  it. 
In  dry  air  it  is  completely  unchanged,  and  does  not  lose  even 
the  lustre  of  its  surface ;  but  in  moist  air  it  is  speedily  covered 
with  a  layer  of  hydrate  of  magnesia.  Heated  to  redness  in  con- 
tact with  the  air,  it  ignites,  and  burns  with  an  intense  dazzling 
white  light,  forming  magnesia.  The  light  evolved  during  com- 
bustion in  oxygen  is  of  unusual  intensity.  A  piece  weighing  0.1 
grm.,  burnt  in  this  gas,  gave  a  light  equal  to  that  of  about  110 
wax  candles.  As  the  surface  of  the  burning  metal  was  but  small, 
that  of  the  effective  portion  of  the  candle-flame  being  six  or  eight 
times  larger,  it  may  be  supposed  that  the  intensity  of  the  light 
of  magnesium  burning  in  oxygen  gas  exceeds  that  of  a  candle- 
flame  more  than  500  times. 
The  metal  decomposes  pure  cold  water  but  slowly  ;  water  con- 
taining acid,  however,  is  rapidly  decomposed  by  it.  Thrown 
upon  dilute  nitric  acid,  it  ignites  instantaneously.  Concentrated 
sulphuric  acid  dissolves  it  with  difficulty.  It  is  not  attacked 
by  a  mixture  of  sulphuric  acid  and  fuming  nitric  acid  in  the  cold. 
It  burns  in  chlorine  gas  when  heated  previously  to  its  introduc- 
tion, and  also  in  vapor  of  bromine,  but  with  more  difficulty.  Its 
combustion  in  the  vapors  of  sulphur  and  iodine  proceeds  with 
greater  liveliness.  Its  spec.  grav.  at  41°  F.  is  1-7430.  If  the 
atomic  value  of  magnesium  be  calculated  from  this,  it  is  found 
to  be  twice  that  of  nickel,  namely  86  instead  of  43.  The  mag- 
nesium obtained  by  electrolysis  may  be  readily  filed,  bored,  sawn, 
and  beaten  out  flat,  but  exhibits  scarcely  more  ductility  than 
zinc  at  ordinary  temperatures,  whilst  that  reduced  by  potassium 
is  very  extensible,  and  may  be  hammered  out  into  thin  plates. 
This  difference  arises  from  the  circumstance  that  the  magnesium 
reduced  by  potassium  retains  a  little  of  that  metal ;  that  obtained 
in  the  electro- chemical  way,  on  the  other  hand,  nearly  always 
contains  a  small  quantity  of  aluminium  and  silicium. 
Barium,  calcium  and  strontium  cannot  be  reduced  in  the  same 
manner. — London  Ohem.  Graz.,  from  Ann.  der  Ckem.  unci 
Pharm. 
