A™eiZer%ao5m-}      Inorganic  Chemistry  of  the  U.S.P.  559 
like  calx  chlorinata,  owes  its  value  to  available  chlorine.  It  is  a 
mixture  principally  of  sodium  chloride  and  sodium  hypochlorite  and 
is  most  frequently  represented  by  the  formula  iSiaCl  -f-  NaQO.  It  is 
a  clear,  very  pale  greenish  liquid,  having  a  faint  odor  of  chlorine  and 
a  disagreeable  alkaline  taste.  It  at  first  colors  red  litmus  paper  blue 
and  then  bleaches  it.  The  assay  process  consists  in  liberating  avail- 
able chlorine  by  the  use  of  hydrochloric  acid,  then  after  the  addition 
of  potassium  iodide  from  which  a  quantity  of  iodine  (equivalent  to  the 
available  chlorine)  is  set  free,  the  mixture  is  titrated  with  sodium 
thiosulphate  (tenth-normal  V.  S.),  the  end  reaction  being  the  dis- 
charge of  the  iodine  color.    No  starch  is  used  as  indicator. 
Magnesia  is  now  Magnesii  Oxidum. — It  should  contain,  after  igni- 
tion, not  less  than  96  per  cent,  of  pure  magnesium  oxide  (MgO). 
It  should  be  kept  in  well-closed  vessels  because  of  its  proneness  to 
absorb  moisture  and  carbon  dioxide  on  long  exposure  to  air.  This 
article  has  been  official  under  the  name  of  magnesia  for  over  half  a 
century,  and  processes  for  its  manufacture  given  until  1890.  It  is 
official  also  in  the  German  Pharmacopoeia  under  the  name  of  mag- 
nesia usta.  It  is  prepared  by  heating  light  magnesium  carbonate  in 
a  crucible  to  a  low  red  heat  until  all  water  and  carbon  dioxide  are 
expelled,  i.  until  a  small  portion  taken  from  the  centre  of  the 
crucible  does  not  effervesce  with  diluted  sulphuric  acid. 
(MgC03)4Mg(OH)2  +  5H20  =  5  MgO  +  4C02  +  6H20  magne- 
sium  oxide  is  almost  insoluble  in  water,  requiring  for  solution  over 
5,000  parts  of  water  at  the  ordinary  temperature  and  about  36,000 
parts  of  boiling  water.  When  moistened  with  water  the  mixture  is 
faintly  alkaline  in  reaction  to  litmus.  The  tests  for  sulphate  and 
chloride  are  omitted,  the  test  for  metallic  impurities  by  means  of 
sulphide  test  has  been  replaced  by  the  time-limit  test.  In  the  old 
Pharmacopoeia  not  more  than  5  per  cent,  of  its  weight  should  be 
lost  on  ignition,  the  new  U.S.P.  not  more  than  15  per  cent,  (limit  of 
water  hydration).  An  assay  method  is  introduced  ;  this  consists  in 
dissolving  a  definite  weight  recently  ignited  magnesium  oxide  in  a 
measured  excess  of  normal  sulphuric  acid  V.  S.,  and  then  retitrating 
with  normal  potassium  hydroxide,  methyl  orange  being  employed 
as  the  indicator.    This  is  a  very  satisfactory  process. 
Magnesii  Oxidum  Ponderosum,  heavy  magnesium  oxide,  was 
formerly  Magnesia  Ponderosa. — This  is  of  exactly  the  same  chemical 
composition  as  the  foregoing,  but  is  a  very  dense  powder,  whilst  the 
