282     magnesium:  its  preparation  and  properties. 
Clair  Deville  and  Caron  is  due  the  productive  process  at  present 
in  use  in  this  country  under  a  patent  granted  to  Mr.  Sonstadt, 
of  Manchester,  a  gentleman  to  whom  great  praise  must  be 
awarded  for  the  energy  and  perseverance  with  which  he  has 
overcome  the  difficulties  which  lay  in  the  way  of  the  preparation 
of  magnesium  on  a  large  scale.  At  the  present  time,  when  so 
many  are  engaged  in  discussing  the  value  and  best  modes  of 
applying  the  magnesium  light  to  photographic  purposes,  it  may 
be  of  interest  to  give  a  succinct  account  of  the  preparation  and 
properties  of  this  remarkable  metal  in  so  far  as  they  may  be  of 
interest  to  our  readers. 
The  process  of  manufacturing  magnesium  may  be  most  con- 
veniently described  as  consisting  of  three  stages : — 1.  The 
preparation  of  chloride  of  magnesium.    2.  The  reduction  of  the 
metal.    And  3.  Purification  by  distillation.    Each  of  these- 
operations  will  now  be  considered  separately. 
I. — Preparation  of  Chloride  of  Magnesium, 
The  manufacture  of  chloride  of  magnesium  is  far  from  being 
so  simple  as  it  would  seem  to  be.  It  is  true  that  it  is  very  easy 
to  obtain  it  in  solution  by  dissolving  magnesia,  or  its  carbonate, 
as  commonly  sold,  in  hydrochloric  acid;  but  if  we  evaporate 
this  solution  to  dryness  in  order  to  obtain  the  salt  in  the  solid 
state,  we  find  that  as  the  last  portions  of  water  are  being  given 
off  they  decompose  some  of  the  chloride,  thereby  reforming 
magnesia  and  setting  free  hydrochloric  acid,  consequently  the 
residue  left  by  evaporation  contains  a  considerable  amount  of 
infusible  magnesia.  This  difficulty,  however,  is  well  known  to 
be  overcome  by  the  addition  of  either  common  salt  or  the  chlo- 
rides of  potassium  or  ammonium  to  the  solution  of  chloride  of 
magnesium  ;  no  decomposition,  then,  occurs  on  rapidly  evapora- 
ting the  solution  to  dryness,  in  consequence  of  the  formation  of 
a  stable  and  fusible  double  chloride  of  magnesium  with  chloride 
of  sodium,  potassium,  or  ammonium,  as  the  case  may  be.  This 
fact  has  been  taken  advantage  of  by  Deville  and  Caron  and  by 
Sonstadt. 
Mr.  Sonstadt,  in  the  specification  of  his  patent  (dated  No- 
vember 8th,  1862),  when  describing  his  mode  of  manufacturing 
chloride  of  magnesium,  lays  particular  stress  on  the  necessity 
