When  any  of  these  oxides  is  heated  with  an  acid,  a  manganous
salt  is  obtained.  Salts  of  this  class  are,  in  fact,  the  only  stable  sub-
stances in  which  manganese  is  combined  with  an  acid  radical.  In
this  action  the  oxides  containing  more  oxygen  than  does  MnO  give
off  oxygen,  or  oxidize  the  acid  (cf.  p.  112).  When  the  oxides  are
heated  with  bases,  in  the  presence  of  air,  manganates  are  always
formed.  In  this  case,  with  oxides  containing  a  smaller  proportion
of  oxygen  than  MnO3,  oxygen  is  taken  from  the  air.
Manganous  Compounds* — The  manganous  salts  are  formed  by
the  action  of  acids  upon  the  carbonate  or  any  of  the  oxides.  Thus
the  chloride  MnCl2,4H2O  is  obtained  in  pale-pink  crystals  from  a
solution  made  by  treating  the  dioxide  with  hydrochloric  acid  and
driving  off  the  chlorine  liberated  by  oxidation  (p.  112).  The  hydrox-
ide Mn(OH)2  is  formed  as  a  white  precipitate  when  a  soluble  base  is
added  to  a  solution  of  a  manganous  salt.  This  body  passes  into
solution  when  ammonium  salts  are  added,  and  cannot  be  precipitated
in  their  presence  on  account  of  the  formation  of  molecular  ammonium
hydroxide  and  the  suppression  of  hydroxide-ion  (cf.  magnesium
hydroxide,  p.  429).  The  hydroxide  quickly  darkens  when  exposed
to  the  air  and  passes  over  into  hydrated  manganic  oxide  MnO  (OH).
Manganous  sulphate  gives  pink  crystals  of  a  hydrate.  Below  6°
the  solution  deposits  MnSO4,7H2O,  which  is  a  vitriol  (p.  433).
Between  7°  and  20°  the  product  is  MnSO4,5H20,  asymmetric  and
resembling  CuSO4,5H2O.  Above  25°  monosymmetric  prisms  of
MnSO4,4H2O  are  obtained.  These  hydrates  have  different  aqueous
tensions  and  may  be  formed  from  one  another  by  lowering  or  raising
the  pressure  of  water  vapor  around  the  substance  (p.  83).
