CITRINE  OINTMENT  OF  THE  BRITISH  PHARMACOPEIA.  297 
of  it  was  lard,  with  one  pound  of  which  an  ounce  of  mercury 
dissolved  in  nitric  acid  was  incorporated  ;  it  was  found  very 
effectual.  This  is  the  same  formula  that  was  introduced  into 
the  Dublin  Pharmacopoeia  of  1807  ;  but  it  was  so  hard  that  it 
could  not  be  mixed  with  other  ointments  so  as  to  be  smooth, 
without  the  greatest  trouble,and  melting  could  not  be  used  without 
changing  its  chemical  constitution.  With  a  view  of  remedying 
this  defect,  the  Colleges  introduced  oil,  forgetting  that  nitrate  of 
mercury  solidifies  olive-oil,  and  renders  it  even  hard.  Changes  in 
the  ratio  of  nitric  acid,  in  its  strength,  in  the  temperature  at  which 
the  solution  of  the  mercury  was  effected,  and  at  which  the  so- 
lution was  presented  to  the  fatty  matter,  were  made,  but  failed 
in  attaining  and  preserving  the  qualities  required. 
I  believe  that  the  following  is  the  theory  of  these  changes. 
As  there  are  two  oxides  of  mercury,  so  there  are  two  nitrates, 
which  by  certain  agencies  become  basic.  When  mercury  is 
dissolved  in  nitric  acid  in  the  ratio  directed  by  the  Colleges, 
the  solution  will  be  found  to  contain  both  the  protoxide  and  per- 
oxide, one  or  other  predominating,  according  to  the  temperature 
at  which  the  solution  was  effected.  The  two  salts  soon  separate ; 
the  protonitrate  crystallizes,  the  pernitrate  remains  in  solution, 
retaining,  however,  a  little  of  the  former.  If  either  of  these 
nitrates,  or  its  solution,  be  mixed  gradually  with  water,  its 
basic  salt  will  sooner  or  later  be  precipitated  :  sooner,  the  less 
free  acid  it  contains ;  later,  as  it  contains  much  ;  but  in  every 
case  the  basic  salt  at  length  will  be  precipitated.  If  the  pro- 
tonitrate has  been  decomposed  by  a  small  quantity  of  water,  the 
resulting  basic  salt  will  be  white  ;  if  by  a  large  quantity,  yel- 
low ;  if  by  a  large  quantity  of  boiling  water,  it  will  be  bluish 
dark-grey.  These  degrees  of  color  are  due  to  the  abstraction 
of  successive  quantities  of  acid.  If  pernitrate  be  decomposed 
by  cold  water,  it  affords  an  orange-brown  salt ;  but  according 
to  Thcnard  and  others,,  if  much,  boiling  water  be  used,  the 
whole  acid  is  washed  away,  and  leaves  pure  red  oxide. 
Much  the  same  series  of  changes  may  be  observed  when,  in- 
stead of  water,  the  mercurial  nitrates  are  exposed  to  the  action 
of  the  fatty  acids  ;  but  owing  to  the  considerable  excess  of 
strong  nitric  acid  and  the  high  temperature  present,  additional 
phenomena  are  induced.    When  the  nitric  acid  holding  the 
