ON  LIQUOR  MAGNESIA  CITRATIS.  3 
This  amount  deducted  from  the  citric  acid  employed  leaves  450 
— 26-75=423-25  grains  to  combine  with  magnesia.  The  equiva- 
lent weight  of  magnesia  is  20,  or  3  equivalents  =  60.  The  re- 
maining citric  acid  requires,  therefore,  for  complete  neutraliza- 
tion, 201  :  60  :  :  423-25  :  126*34  grains  of  pure  magnesia,  or 
only  6J  grains  more  than  ordered  by  the  Pharmacopoeia. 
Commercial  citric  acid,  however,  mostly  contains,  and  proba- 
bly often  consists  altogether  of  the  bihydrate  ;  the  amount  re- 
quired for  neutralizing  the  bicarbonate  of  potassa  will  then  be 
300-6  :  210  ;  :  40  :  27-94  grains,  leaving  450—27-94=422-06 
grains  citric  acid  to  combine  with  magnesia,  of  which  will  be  re- 
quired for  complete  neutralization  120-65  grains;  for 
210  :  60  ::  422-06  :  120-65. 
This  amount  is  that  of  the  Pharmacopoeia,  within  §  gr.  It  is 
obvious,  therefore,  that  if  pure  magnesia  is  used,  its  quantity  is 
sufficient  to  form  with  almost  the  entire  amount  of  citric  acid, 
the  neutral  citrate  of  magnesia  3MgO,C12H5On+14  aq.?  which 
has  been  shown  by  Richter,  Delabarre  and  Prof.  Procter  to  be 
separated  from  its  aqueous  solution  on  standing,  and  then  to  be 
as  good  as  insoluble  in  water. 
Exposed  to  the  atmosphere,  calcined  magnesia  gradually  ab- 
sorbs and  combines  with  water,  and  afterwards  the  hydrate 
formed  unites  with  carbonic  acid.  A  magnesia  which  effervesces 
with  acids  must  have  previously  become  hydrated  to  a  certain 
extent.  The  English  heavy  calcined  magnesia  appears  to  with- 
stand hydration  and  carbonation  much  better  than  the  light 
article ;  I  have  observed  that  it  may  become  quite  solid  by  the 
absorption  of  water  and  still  be  free,  or  contain  mere  traces  of 
carbonic  acid.  If  a  good  quality  of  this  magnesia  is  used  in  the 
proportion  of  100  grains  to  450  grains  of  citric  acid,  a  clear 
solution  is  not  obtained,  and,  on  standing,  a  considerable  pre- 
cipitation takes  place.  Repeated  experiments  made  with  this 
magnesia  have  shown  that  between  85  and  90  grains  of  it  are 
sufficient  for  the  citric  acid.  85  grains  is  about  two-thirds  of 
126J  grains,  the  quantity  calculated  above  as  furnishing  neutral 
citrate  of  magnesia. 
These  facts  appear  to  me  to  demonstrate  very  clearly  that  the 
solution,  instead  of  containing  the  neutral  citrate,  really  contains 
