532  Liquor  Magnesii  Bromidi.  {Am^Z'im&Tm' 
1  OHBr + 4MgC03.MgH202. 5H20= 5MgBr2 + 1 1 H20 + 4C02 
808  484  918  198  176 
Now  as  each  fluidounce  of  water  weighs  455.66  grains,  the  same 
volume  of  dilute  hydrobromic  acid  of  the  pharmacopoeia,  having  a 
specific  gravity  of  1*077,  should  weigh  490.75  grains;  and  as  the 
solution  contains  10  per  cent,  of  absolute  acid,  one  fluidounce  then 
contains  49.075  grains  of  available  HBr. 
Then  if  808  parts  of  anhydrous  HBr,  as  shown  in  the  equation 
above,  yield,  on  neutralization  with  magnesium  carbonate,  918  parts 
of  magnesium  bromide,  49.075  parts  of  anhydrous  HBr  should  give 
55*76  parts  of  magnesium  bromide  (or  exactly  6*97  grains  to  each 
fluidrachm). 
If  808  parts  of  anhydrous  HBr  require  for  neutralization  484 
grains  of  officinal  carbonate  of  magnesium,  then  49.075  parts  should 
require  less  or  29*4  parts  (or  1  pint  of  officinal  hydrobromic  acid 
would  require  470*4  grains  of  officinal  magnesium  carbonate). 
The  usefulness  of  magnesium  bromide,  in  its  acceptability  and 
decisive  action,  seems  to  reside  in  the  fact  that  it  contains  a  larger 
percentage  of  combined  bromine  than  any  other  bromide,  with  the 
exception  of  the  lithium  compound,  as  the  following  table  of  percent- 
ages of  combined  bromine,  in  various  bromides,  will  show : 
Formula         Mol.  Wt.    Per  Cent. 
Nickel  bromide  NiBr23H20  271*6  5876 
Potassium  "   KBr.  118.8  67.17 
Zinc          "   ZnBr2  224*5  71*09 
Iron          "   FeBr2  215  74*23 
Sodium      "   NaBr  102-8  77*62 
Calcium     "   CaBr2  199*6  79*95 
Ammonium  bromide  NH4Br  97*8  81*59 
Magnesium      "   MgBr2  183*6  86*92 
Lithium           "   LiBr  86*8  91.93 
The  main  advantage  of  preparing  the  solution  by  this  method  over 
others,  is  the  constancy  in  strength  of  the  resulting  product,  arising 
from  the  fact  that  pure  hydrobromic  acid  of  pharmacopoeial  strength 
is  readily  obtainable.  The  difficulty  of  making  magnesium  bromide 
by  granulation  and  preparing  the  solution  direct,  is  that  on  evapora- 
tion of  the  solution,  a  grayish,  pasty  mass  is  left,  excessively  hygro- 
scopic in  nature,  which,  on  continued  application  of  heat  to  induce 
thorough  dryness,  is,  according  to  Balard,  who  gives  the  hydrated 
compound  the  formula  MgBr26H20,  decomposed  into   HBr  and 
