298    CITRINE  OINTMENT  OF  THE  BRITISH  PHARMACOPOEIA. 
mercurial  salts  in  solution  is  mixed  with  the  melted  fatty  matter, 
the  chemical  action  induced  is  first  to  form  Fourcroy's  pomatum  ; 
the  uncombined  acid  is  withdrawn  by  decomposition,  and  the 
basic  mercurial  salts,  which  are  now  yellow,  communicate  that 
color  to  the  ointment.  But  the  decomposition  of  the  basic  salts 
does  not  stop  here  ;  the  abstraction  of  nitric  acid  still  slowly 
continues,  until  at  length  the  mercurial  salts  are  resolved  into 
the  two  original  oxides — slate-grey  and  orange-red — by  the  ad- 
mixture of  which  colors  an  ointment  of  a  brownish  hue,  lighter 
or  deeper  according  to  age  and  circumstances,  will  result.  Heat 
will  bring  about  these  changes  more  rapidly  ;  without  heat  a 
very  long  time  will  be  required. 
This  series  of  changes  took  place  in  a  remarkable  manner 
when  I  repeated  the  process  of  the  British  Pharmacopoeia  for  ma- 
king citrine  ointment.  When  the  hot  mercurial  solution  was 
poured  into  the  hot  lard  and  oil,  and  well  stirred,  an  efferves- 
cence was  excited  which  would  soon  have  overflowed,  but  that  the 
vessel  was  capable  (as  the  Pharmacopoeia  directs)  of  containing 
six  times  the  volume  of  the  ointment.  During  this  violent 
effervescence  the  color  of  the  liquid  ointment  became  continually 
darker,  until,  the  effervescence  having  ceased,  the  ointment  was 
found  to  be  of  a  dark  mahogany  color.  When  perfectly  cold, 
it  did  not  solidify,  but  remained  of  the  consistence  of  treacle,  and 
might  be  poured  from  one  vessel  to  another.  The  bottom  of 
the  basin  in  which  it  was  made  was  found  to  be  lined  with  a 
dark  slate  colored  powder,  which  proved  to  be  reduced  mercury. 
In  four  months  after,  the  ointment  had  attained  the  consistence 
of  fresh  butter  in  the  hot  days  of  summer. 
Thus  the  process  of  the  British  Pharmacopoeia  is  liable  to 
failure.  From  all  the  experiments  I  have  made,  and  they  have 
been  numerous,  and  varied  according  to  the  instructions  of  the 
three  Colleges,  I  am  led  to  believe  that  the  degree  of  chemical 
action,  as  evidenced  by  the  activity  of  the  effervescence  when 
the  ingredients  are  mixed,  is  the  main  point  to  be  attended  to. 
If  the  ingredients  be  mixed  cold,  and  the  temperature  main- 
tained at  a  low  degree,  the  ointment  will  be  hard  and  of  a  pale 
yellow  hue.  If  the  ingredients  be  mixed  very  hot,  and  the 
temperature  allowed  to  rise  with  the  chemical  action,  the  result- 
ing ointment  will  be  brown  and  too  soft.    Intermediate  temper- 
