236  Ointment  of  Mercuric  Nitrate.  {AmMa^i?harm' 
ture  of  ioo°  C.  (21 2°  F.) ;  then  withdraw  the  heat,  gradually  add 
70  grammes  of  nitric  acid,  and,  when  the  reaction  moderates,  reap- 
ply the  heat  until  effervescence  ceases."  (It  may  now  be  stirred 
gently  once  or  twice  while  cooling,  but  not  vigorously,  as  has  been 
recommended.)  "  Then  allow  the  mixture  to  cool  to  400  C.  (1040 
F.).  Having  dissolved  the  mercury  in  the  remainder  of  the  nitric 
acid  with  the  aid  of  sufficient  heat  to  prevent  the  solution  from  crys- 
tallizing, add  this  solution  to  the  mixture."  Now  raise  the  temper- 
ature to  6o°  C,  add  the  lard,  which  at  this  temperature  will  melt 
and  permit  of  thorough  incorporation,  and  maintain  the"  tempera- 
ture until  no  further  evolution  of  gas  takes  place,  thereby  obviating 
the  tendency  to  form  a  spongy  mass.  "  When  the  mass  has  become 
entirely  cold,  mix  it  thoroughly  by  trituration,  avoiding  the  use  of  a 
metallic  spatula." 
The  reaction  for  the  production  of  mercuric  nitrate  in  the  process 
is  as  follows : 
3Hg  +  8HN03  =  3Hg(N03)2  +  N202  +  4H20 
6Hg  +  8HNO3  =  3Hg2(NOs)2  +  N2Oz  +  4H20 
If  the  solution  of  the  metal  is  effected  in  contact  with  the  acid  at 
the  ordinary  temperature,1  it  is  positively  certain,  as  seen  by  the  fore- 
going reaction,  that  both  mercuric  and  mercurous  nitrate  form, 
consequently  the  ointment  receives  both  nitrates  from  the  beginning. 
At  the  same  time  there  is  produced  the  colorless  gas  nitrogen 
dioxide  (N202).  When  this  colorless  gas  comes  in  contact  with  air, 
it  unites  with  its  oxygen,  forming  red  fumes  of  the  tetroxide  (N204). 
Now  upon  mixing  and  stirring  this  nitric  acid  solution  of  mercuric 
nitrate  with  the  fat,  this  nitrogen  dioxide  takes  up  oxygen  from  the 
air  that  is  stirred  into  the  ointment  (for  the  more  it  is  stirred  the 
brighter  yellow  will  be  its  color),  and,  whatever  may  be  the  effect  of 
this  gas  as  a  most  energetic  oxidant,  we  are  sure  of  the  production 
of  a  decidedly  more  disagreeable  odor  than  is  given  off  under  certain 
other  conditions  by  the  fatty  base  of  this  ointment. 
1  It  will,  however,  be  found  that  when  the  solution  of  mercury  in  nitric  acid 
is  effected  at  the  temperature  of  a  water-bath  or  higher,  and  maintaining  tem- 
perature about  twenty  minutes,  no  precipitation  or  cloudiness  will  occur  in  the 
solution  on  the  addition  of  water,  or  of  diluted  hydrochloric  acid  (absence  of 
mercurous  salt). 
