UNGUENTUM  HYDRARGYRI. 
203 
ence  of  -017,  where  the  difference  of  the  mercury  is  but  one 
grain  in  a  hundred. 
From  actual  trials  with  ointments  made  of  various  proportions 
of  mercury  and  grease,  the  following  table  was  formed : — 
1  part  Mercury  to  10  parts  Grease  =  Sp.  Grav.  "981 
2 
<< 
a 
tt 
10  " 
il 
u 
1-065 
3 
«< 
il 
ti 
10  " 
ti 
<< 
1-147 
4 
«< 
it 
a 
10  " 
a 
u 
1-229 
5 
a 
it 
u 
10  " 
tt 
ti 
1-311 
6 
a 
tt 
tt 
10  " 
(< 
tt 
1-393 
7 
(t 
II 
a 
10  " 
u 
tt 
1-471 
8 
tt 
il 
il 
10  " 
ti 
(« 
1-548 
9 
a 
it 
ti 
10  " 
a 
a 
1-625 
10 
it 
ti 
10  " 
a 
  it 
ti 
1-700 
In  taking  the  specific  gravity  of  the  ointment,  the  following 
precautions  are  necessary : 
It  should  be  carefully  introduced  into  a  1000  grain  bottle,  so 
that  the  neck  may  not  be  soiled  ;  about.  100  grains  will  answer. 
After  being  warmed  gently  so  as  to  melt  the  ointment,  it  is  set 
aside  to  cool,  by  which  it  becomes  solid  and  free  from  air.  The 
exact  amount  of  the  ointment  is  then  ascertained,  the  bottle  be- 
ing previously  tared.  After  filling  the  bottle  with  water  at 
60°  F.,  the  weight  of  the  contents  is  observed  and  the  specific 
gravity  calculated  in  the  usual  manner.  As  an  example,  sup- 
pose the  ointment  introduced  is  80*5  grains,  and,  after  filling 
the  bottle  with  water,  the  total  weight  is  1028-5  grains.  The 
water  alone  would  then  weigh  1028*5 — 80*5=948  grains ;  con- 
sequently the  difference  between  this  number  and  1000  is  the 
amount  of  water  displaced,  which  is  52  grains ;  and  the  specific 
gravity  is  found  by  dividing  the  weight  of  the  water  into  the 
weight  of  the  ointment.  80.5-^52=1*548  is  the  specific  grav- 
ity, and  by  reference  to  the  table,  we  see  it  would  contain  eight 
parts  of  mercury  to  ten  parts  of  grease. 
I  may  here  remark  that,  even  in  an  ointment  properly  made, 
a  slight  discrepancy  from  the  table  may  occur  from  using  differ- 
ent proportions  of  lard  and  suet ;  but  the  variations  from  this 
cause  are  but  slight. 
In  the  course  of  the  foregoing  experiments,  I  had  procured 
samples  of  mercurial  ointment  from  various  sources,  both  from 
