312  Red  Mercuric  Oxide.  {AmjiZr,\maTm' 
Another  firm  of  manufacturing  chemists,  equally  as  prominent, 
writes  :  "  Our  analysis  shows  that  red  mercuric  oxide  contains  99-7 
per  cent."HgO,  and  0-3  per  cent,  of  Si02.  The  silica  is,  no  doubt, 
derived  from  the  vessels  in  which  the  mercuric  oxide  is  manufac- 
tured. Commercial  metallic  mercury  varies  between  99  and  100 
percent.,  while  the  distilled  mercury  is  pure." 
Another  prominent  chemical  firm  writes  :  "  We  would  state  that 
our  levigated  mercuric  oxide  conforms  to  all  the  requirements  of  the 
U.S. P.  1890,  save  as  regards  absolute  freedom  from  HNOs;  it  con- 
tains very  small  traces  of  the  latter.  It  conforms  strictly  to  all  the 
requirements  of  the  Ph.G.  iii,  also,  in  regard  to  HNOs.  There  is  no 
doubt  that  the  U.S.P.  is  hypercritical  in  the  case  of  red  mercuric 
oxide,  as  it  is  also  in  several  other  instances ;  for  the  faint  traces  of 
HNO3  that  our  levigated  grade  contains  are  not  known  to  interfere 
with  any  of  the  chemical  applications  of  the  preparation,  and  cannot 
possibly  have  any  influence  on  the  therapeutic  action  of  the  medi- 
cament. To  provide  an  oxide  absolutely  free  from  HN03,  is  practi- 
cally impossible  commercially,  and  there  is  no  necessity  for  the 
preparation." 
Now,  the  point  the  writer  would  make  is  this :  If  the  statements 
of  three  of  the  leading  chemical  firms  of  the  country,  regarding 
the  purity  of  their  red  mercuric  oxide,  be  true,  then  there  should  be 
no  difficulty  whatever  in  the  pharmacist  procuring  an  oxide  that 
could  be  used  as  a  substitute,  if  necessary,  for  metallic  mercury  in 
the  making  of  ointment  of  mercuric  nitrate. 
Further,  the  writer  recommended  the  addition  of  glycerin  to  the 
finished  ointment,  not  to  prevent  sponginess — that  is  a  condition  due 
to  the  application  of  insufficient  heat — but  to  prevent  the  hardening 
and  ultimately  friable  condition  that  obtains  in  the  ointment  on 
long  standing.  Regarding  the  criticism  that  the  addition  of  50 
grammes  of  glycerin  to  1,000  grammes  of  ointment  of  official 
strength  reduces  the  percentage  of  mercuric  nitrate  below  that 
required  by  the  U.S.P.,  there  is  this  to  say  :  that  the  resulting  differ- 
ence in  strength  is  of  no  practical  moment  therapeutically ,  as  phy- 
sicians almost  invariably — save  in  those  cases  requiring  great  stimu- 
lation— dilute  the  ointment  they  prescribe — oftentimes  in  equal  pro- 
portions— with  lard  or  other  fatty  diluent. 
The  final  criticism  of  increased  cost  is  hardly  worth  consideration. 
The  alternative  use  of  75-5  grammes  of  mercuric  oxide  (costing 
