560  Inorganic  Chemistry  of  the  U.S.P.  {ADecimberPhim' 
foregoing  is  very  light  and  bulky.  Its  chief  advantage  lies  in  its 
density,  which  permits  a  decrease  in  the  bulk  of  the  dose  amounting 
to  about  one-fourth.  It  is  prepared  by  calcining  the  heavy  carbonate* 
but  may  also  be  prepared  by  triturating  the  light  variety. 
Mangani  Dioxidum  is  now  Mangani  Dioxidum  Prcecipitatum. — 
The  black  oxide  of  manganese  of  the  old  Pharmacopoeia  was  the 
native  crude  manganese  dioxide,  containing  at  least  66  per  cent,  of 
the  pure  dioxide  (Mn02).  The  same  was  official  in  the  1880  U.S.P. 
under  the  name  of  Mangani  oxidum  nigrum.  Its  principal  use  was 
in  the  manufacture  of  chlorine  water,  for  which  purpose  this  was 
considered  sufficiently  pure.  In  the  present  U.S.P.  aqua  chlori  is 
replaced  by  liquor  chlori  compositus,  in  the  preparation  of  which 
manganese  dioxide  is  not  employed.  The  latter  is,  however,  so  fre- 
quently prescribed  for  internal  administration,  that  it  was  deemed 
advisable  to  retain  it  among  the  official  articles,  but  in  a  much  purer 
state,  hence  mangani  dioxidum  praecipitatum  was  made  official. 
This  is  made  by  reacting  upon  pure  manganese  sulphate  with 
ammonia  water  and  hydrogen  dioxide.  A  precipitate  consisting 
chiefly  of  manganese  dioxide  results,  which  is  thoroughly  washed 
and  dried.  It  contains  also  other  oxides  of  manganese,  but  should 
correspond  to  not  less  than  80  per  cent,  of  manganese  dioxide.  In 
the  old  Pharmacopoeia  a  test  for  organic  impurities  is  described 
which  is  omitted  in  the  new.  The  test  for  metallic  sulphides,  by 
treatment  of  the  dioxide  with  diluted  hydrochloric  acid,  in  which 
attention  is  paid  to  the  gas  evolved,  i.  e.y  if  odor  of  hydrogen  sulphide, 
or  blackening  of  lead  acetate  paper  occurs,  is  omitted.  The  test  for 
antimony  sulphide,  in  which  it  is  directed  to  heat  the  mixture  of 
manganese  dioxide  and  hydrochloric  acid  and  treat  the  filtered  solu- 
tion with  hydrogen  sulphide  T.  S.  and  obtain  an  orange- colored  pre- 
cipitate, is  replaced  by  an  inferior  test,  whose  only  advantage  lies 
in  not  requiring  the  use  of  hydrogen  sulphide.  In  this  test,  if  to 
1  gramme  of  the  dioxide  and  2  grammes  of  oxalic  acid  20  c.c.  of 
water  be  added  followed  by  3  c.c.  of  sulphuric  acid  and  the  mixture 
digested  for  three  hours  in  a  water  bath,  complete  solution  should 
be  effected.  This  test  also  detects  other  (insoluble)  substances. 
The  gravimetric  assay  method  of  the  old  Pharmacopoeia,  in  which 
1  gramme  of  the  dioxide  treated  with  hydrochloric  was  required 
to  oxidize  a  definite  weight  of  ferrous  sulphate,  is  replaced  by  a 
fairly  satisfactory  volumetric  method.    In  this  0-2  gramme  of  the 
