ON  PREPARING  RED  OXIDE  OF  MERCURY  OINTMENT.  365 
There  is,  in  fact,  just  the  difference  that  is  seen  between  coarsely- 
pounded  colored  glass  and  the  same  glass  finely  pulverized. 
Under  the  microscope  this  difference  is  yet  more  clearly  per- 
ceptible. An  examination  of  the  precipitated  oxide  shows  that 
none  of  its  particles  exceed  the  thirty-thousandth  of  an  inch  in 
diameter ;  while  the  same  scrutiny  applied  to  the  best  levigated 
nitric  acid  shows  that  although  a  great  number  of  its  particles 
scarcely  exceed  the  size  just  mentioned,  many  of  them  are  as 
large  as  the  five-hundredth  of  an  inch.  The  diameter  of  a  large 
proportion  of  the  particles  of  the  unlevigated  nitric  oxide  is  as 
much  as  the  one-hundredth  of  an  inch. 
The  advantages  of  ointment  made  with  the  precipitated  oxide 
of  mercury  over  that  made  with  the  so-called  nitric  oxide,  are,  in 
the  first  place,  that  supposing  ointments  of  equal  therapeutical 
value  be  used,  greater  economy  is  gained  by  the  use  of  the  pre- 
cipitated oxide,  since  a  less  proportion  of  it  will  suffice  in  the 
same  quantity  of  ointment. 
But  there  is  a  more  serious  objection  to  be  urged  against  the 
use  of  the  nitric  oxide,  and  that  is,  that  the  presence  of  a  quan- 
tity of  gritty  particles  in  an  ointment  which  is  to  be  rubbed  in 
over  a  raw  and  irritable  portion  of  skin,  produces  a  totally  dif- 
ferent action  in  it  to  what  is  sought  for  when  an  ointment  of  the 
red  oxide  of  mercury  is  employed. 
One  of  the  uses  of  the  ointment  of  the  flowers  of  sulphur  in 
the  treatment  of  scabies  is,  that  the  gritty  particles  of  sulphur 
do,  as  it  is  rubbed  in  over  the  skin,  actually  rupture  the  tunnels 
in  the  epidermis  in  which  the  acarus  scabiei  resides,  and  so  lay 
bare  the  itch-mite  to  the  poisonous  influence  of  the  sulphur ;  and 
although  the  particles  of  well-levigated  oxide  are  much  less  coarse 
than  those  of  the  sulphur,  still  it  must  be  remembered  that  they 
are  applied  usually  to  much  more  delicate  surfaces. 
This  disadvantage  in  the  use  of  nitric  oxide  I  have  more  espe- 
cially noticed  in  hospital  and  dispensary  practice,  where  the 
oxide  employed,  except  for  ophthalmic  use,  is  often  by  no  means 
carefully  levigated,  and  where  the  introduction  of  ointment  made 
with  the  precipitated  oxide  would  ensure,  irrespective  of  quality, 
a  fine  division  of  the  particles. 
But  I  have  also  often  had  occasion  to  observe  on  the  skins  of 
