202 
UNGUENTUM  HYDRARGYRI. 
With  fifty  grains  of  the  ointment  treated  in  this  manner,  I 
obtained'twenty-three  and  a  half  grains  of  mercury. 
Exp.  3.  This  is  performed  by  dissolving  out  the  grease  by 
means  of  highly  rectified  benzine,  and  washing  the  residue  with 
stronger  alcohol ;  the  resulting  black  powder  is  then  to  be  boiled 
with  a  portion  of  ordinary  hydrochloric  acid,  so  that  the  mer- 
cury may  form  a  globule,  which  is  to  be  washed,  dried  and 
weighed. 
With  fifty  grains  of  the  ointment  treated  in  this  manner,  I 
obtained  twenty-three  grains  of  mercury. 
Of  these  three  methods,  the  first  is  by  far  the  best,  yielding  the 
mercury  in  a  pure  state  with  less  trouble  than  the  others,  and 
the  whole  operation  may  be  performed  in  a  few  minutes.  By 
comparing  the  results  of  these  tests,  it  will  be  seen  that  the  first 
will  give  the  largest  amount  of  the  mercury ;  it  is  also,  there- 
fore, the  most  accurate. 
In  no  instance,  however,  did  I  obtain  the  whole  amount  of 
mercury  employed  in  making  the  ointment.  The  bulk  of  the 
mercury  being  so  small,  the  slightest  loss  would  be  perceptible 
when  weighed ;  the  finely-divided  state  of  the  mercury  causes  it 
to  adhere  so  closely  to  the  grease,  that  it  sometimes  requires 
considerable  boiling  to  separate  it,  and  here  we  may  have  a 
slight  loss  by  volatilization,  and  even  with  the  utmost  care,  a 
trace  may  be  washed  away ;  but,  after  all,  with  moderate  care, 
the  results  are  near  enough  for  practical  purposes. 
The  relative  density  of  mercurial  ointment,  or  its  specific 
gravity,  although  not  usually  alluded  to,  may  also  furnish  valu- 
able information,  and,  as  it  is  a  means  often  resorted  to  in 
detecting  adulterations  in  other  preparations,  I  determined  to 
apply  that  test  also  in  the  examination  of  this  ointment,  espe- 
cially as  the  most  usual  cause  of  inferiority  is  occasioned  by  an 
insufficient  quantity  of  mercury  being  employed. 
To  prove  the  delicacy  of  the  test,  I  made  experiments  with 
ointments  of  two  different  strengths ;  the  one  containing  50 
grains  of  mercury  in  100  grains  of  the  preparation  ;  the  other 
containing  49  grains  of  mercury  in  100  of  the  preparation. 
The  specific  gravity  of  each  was  taken,  which,  in  the  first 
instance,  was  1*700,  and  in  the  second,  1-683,  showing  a  differ- 
