4 
ON  LIQUOR  MAGNESL3E  CITRATIS. 
the  two-thirds  citrate  of  magnesia,  or,  in  other  words,  that  the 
compound  2MgO,  HO,C12H5On,  and  not  the  one  with  3MgO,  is 
the  citrate  retaining  its  solubility  in  water. 
It  would  be  interesting,  from  a  chemical  as  well  as  pharma- 
ceutical view,  to  institute  investigations  to  this  effect  with  pure 
anhydrous  magnesia  or  its  carbonate  of  known  composition,  and 
with  well  determined  citric  acid.  The  two-thirds  citrate,  I  be- 
lieve, has  been  prepared  but  once  (by  Heldt),  and  nothing  is 
known  concerning  it,  except  that  it  is  a  gummy  mass.  Might 
it  not  perhaps  be  obtainable  in  a  dry  state,  and  retain  its  per- 
fect solubility  ? 
But  herewith  the  practical  difficulties  would  not  be  overcome, 
to  prepare  the  solution  of  uniform  quality.  The  method 
suggested  by  Fred.  Stearns,  in  the  Proceedings  of  the  American 
Pharmaceutical  Association,  1857,  page  152,  is  objectionable 
chiefly  on  account  of  the  impossibility  of  preparing  the  solution 
on  the  small  scale  or  at  short  notice,  and  a  loss  of  magnesia  is 
very  likely  to  occur  during  the  filtration  of  the  dissolved  bicar- 
bonate. Although  this  solution  contains  a  large  excess  of  free 
carbonic  acid,  this  is  expelled  by  filtration,  and  the  filtrate  con- 
taining no  uncombined  carbonic  acid,  or  merely  traces  of  it,  is 
apt  to  deposit  neutral  carbonate  unless  used  without  delay. 
The  analyses,  by  Phillips,  Townes,  Otto  and  Lake,  of  com- 
mercial samples  of  subcarbonate  of  magnesia,  agree  very  nearly 
in  the  percentage  of  magnesia,  which  they  found  to  vary  be- 
tween 40*3  and  42 ;  and  with  this  inconsiderable  variation,  it 
might  perhaps  be  advisable  to  return  to  the  use  of  the  carbonate 
in  the  preparation  of  the  liquor  magnesise  citratis.  85  grains 
MgO  are  contained  in  about  210  grains  of  carbonate ;  and  this 
amount  yields,  with  450  grains  of  citric  acid  and  40  grains  bi- 
carbonate of  potassa,  an  agreeable  solution.  I  have  not  tried 
whether  more  carbonate  can  be  dissolved  to  a  permanent  solu- 
tion. 
While  on  this  subject,  it  may  as  well  be  remarked  that  the 
amount  of  syrup  of  citric  acid  (two  fluidounces  to  the  bottle) 
ordered  by  the  Pharmacopoeia  is  entirely  too  much  for  the  large 
doses  requisite  of  this  medicine ;  one-half  the  quantity  is 
amply  sufficient,  producing  a  solution  which  is  pleasantly 
sweetened. 
