TJNGr.  HYDRARGYRI  NITR. 
273 
than  the  unguentum  hydrargyri  nitratis.  The  ingredients 
which  enter  into  the  composition  of  this  ointment  are  lard,  olive 
oil,  mercury,  and  nitric  acid  ;  and  it  is  directed  to  be  prepared 
by  first  dissolving  the  mercury  in  the  nitric  acid,  and  then  mix- 
ing the  solution  while  hot  with  the  lard  and  olive  oil  previously 
melted  together.  When  successfully  made  the  ointment  should 
be,  1,  of  a  pure  pale  yellow  color ;  2,  of  a  soft  and  uniform 
consistence ;  and  3,  should  be  able  to  retain  its  color  and  soft- 
ness unchanged.  When  the  acid  mercurial  solution  is  mixed 
with  the  melted  lard  and  oil,  we  observe  a  chemical  action  ac- 
companied by  a  rise  of  temperature  to  take  place,  which  is  more 
or  less  observable  according — 1,  to  the  state  of  the  mercurial 
solution  ;  2,  to  the  temperature  of  the  lard  and  oil  at  the  time 
the  solution  is  added ;  and  3,  to  the  amount  of  stirring  em- 
ployed to  effect  the  combination.  The  chemical  action  arises 
from  the  influence  of  the  nitrous  acid  in  the  mercurial  solution 
on  the  lard  and  oil.  The  warmer  the  fats,  the  richer  the  nitric 
solution  in  lower  oxides  of  nitrogen,  and  the  more  energetic  the 
stirring,  the  more  sudden  will  the  chemical  action  take  place, 
and  the  higher  will  rise  the  temperature  of  the  mixture  ;  and 
on  the  contrary,  the  cooler  the  fats,  the  poorer  the  mercurial 
solution  in  nitrous  acid,  and  without  stirring,  the  mixture  occa- 
sions scarcely  any  chemical  action  and  rise  of  temperature  at  all. 
As  a  sudden  and  high  temperature  is  always  detrimental  to  the 
ointment,  we  must  especially  guar<;l  against  this,  which  is  per- 
fectly in  our  power.  But  as  we  have  to  assist,  and  to  some  ex- 
tent develope,  chemical  action  to  produce  the  ointment,  we  must 
remember,  1,  that  the  larger  the  amount  of  nitrous  acid  in  the 
solution  the  lower  must  be  the  temperature  of  the  fats  when  the 
mixture  is  made;  and  2,  that  the  poorer  in  nitrous  acid  the  higher 
may  the  temperature  of  the  fats  be.  It  must  always  be  borne 
in  mind,  however,  that  ointments  whose  temperature  in  the 
making  rises  above  212°  Fahr.  will  turn  out  dark  in  color  and 
hard  in  consistence,  even  if  the  color  and  consistence  is  tolera-. 
bly  good  when  first  made;  but  those  which  are  made  at  a  lower 
temperature  will  always  prove  unobjectionable  in  every  respect. 
I  divide  the  preparation  of  this  ointment  into  three  periods. 
I  allow  the  solution  of  the  mercury  in  the  nitric  acid  to  go  on 
18 
