2 
ON  LIQUOR  MAGNESLE  CTTRATIS. 
of  the  pharmaceutical  profession,  I  learned  that  they  met  with 
the  same  difficulty,  and  overcame  it  only  by  adopting  a  formula 
differing  from  that  of  the  Pharmacopoeia  in  the  proportion  of  the 
materials  employed,  some  departing  in  this  instance  from  all  the 
quantities  directed  by  our  national  standard.  I  have  heard  that 
some  apothecaries  employ  only  240  to  300  grains  of  citric  acid 
to  the  bottle,  and  reduce  the  amount  of  magnesia  or  carbonate 
of  magnesia  accordingly.  Prof.  Parrish,  in  the  third  edition  of 
his  Treatise  on  Pharmacy,  gives  360  grains  citric  acid  to  105 
grains  of  magnesia,  which,  calculated  for  the  officinal  quantity 
of  450  grains  of  citric  acid,  would  require  131  grains,  or  11 
grains  more  of  magnesia  than  the  Pharmacopoeia  orders.  This 
proportion  may  suit  very  well  to  a  calcined  magnesia  which  by 
exposure  to  the  air  has  become  considerably  hydrated  and  car- 
bonated, but  it  exceeds  the  amount  of  a  fair  quality  of  calcined 
magnesia  by  about  30  per  cent.,  and  of  pure  magnesia,  free  from 
carbonic  acid  and  water  of  hydration,  by  nearly  50  per  cent.,  as 
I  shall  presently  show. 
What  the  Pharmacopoeia  means  by  the  term  "magnesia"  is 
pretty  clear,  from  the  officinal  process  for  preparing  it.  It  di- 
rects to  expose  carbonate  of  magnesia  to  a  red  heat  for  two 
hours,  or  until  the  carbonic  acid  is  entirely  expelled.  Carbon- 
ate of  magnesia  is  a  hydrated  basic  carbonate ;  on  exposing  it 
to  heat,  the  water  of  hydration  is  given  off  first,  and  afterwards 
the  carbonic  acid,  so  that  if  all  the  carbonic  acid  has  been  ex- 
pelled, the  earth  consists  only  of  MgO. 
Among  the  tests  for  magnesia,  the  Pharmacopoeia  requires 
non-effervescence  with  acids ;  consequently  the  preparations 
wherein  magnesia  is  used  are,  or  ought  to  be,  based  upon  per- 
fectly calcined  magnesia,  of  the  formula  MgO. 
For  the  solution  under  consideration,  our  national  authority 
orders  450  grains  crystallized  citric  acid  to  120  grains  magnesia. 
Citric  acid  has  the  formula  3HO,C12H5On+aq.  weight  of  equiva- 
lent 201,  or  it  contains  2  aq.,  and  has  then  an  equivalent  weight 
of  210.  40  grains  bicarbonate  of  potassa  (equivalent  weight 
100-2,  or  3  equivalents  =  300*6)  require  for  complete  neutrali- 
zation 26*75  grains  monohydrated  citric  acid;  for 
300-6  :  201  :  :  40  :  26*75. 
