156 
Calomel. 
(  Am.  Jour.  Pbarm. 
t     April  1,  1871. 
Applying  these  preliminaries,  we  will  soon  arrive  at  a  working  for- 
mula for  calomel,  which,  from  experience,  I  do  not  hesitate  to  recom- 
mend as  preferable  to  the  process  of  the  U.  S.  Ph. 
One  pound  of  pure  nitric  acid  (spec.  grav.  1"25)  is  put  in  a  mat- 
rass, heated  by  means  of  a  sand-bath,  and  one  and  a  half  pounds  of 
mercury  (or  an  excessive  quantity)  is  added.  They  are  warmed  to- 
gether first  until  no  more  red  fumes  are  evolved,  and  afterwards  the 
digestion  is  continued  for  one  hour  more,  or  until  the  liquid  begins  to 
become  discolored  or  yellowish,  when  the  matrass  must  be  at  once  re- 
moved, and  the  contents  transferred  to  a  capacious  evaporating-dish. 
The  solution  now  contains  a  basic  proto-nitrate.  (The  digestion  must 
not  be  interrupted,  because  crystals  may  deposit  which  are  afterwards 
very  difficult  to  re-dissolve.) 
To  the  warm  concentrated  solution  in  the  evaporating-dish  is  now 
added,  without  delay  and  during  constant  agitation,  a  boiling  hot 
mixture  of  twelve  pounds  of  distilled  water  and  half  pound  (more)  of 
the  nitric  acid.  This  free  acid  is  added,  in  order  to  prevent  subse- 
quent precipitation  of  basic  salt  when  water  is  mixed  with  the  solu- 
tion. The  mixture,  thus  diluted  and  made  sour,  should  now  be  tested, 
to  ascertain  that  it  can  be  further  diluted  with  water  without  decom- 
position, because,  otherwise,  basic  salt  may  be  thrown  down  with  the 
calomel  when  the  solution  is  finally  added  to  the  dilute  hydrochloric 
acid.  The  solution  is  next  filtered,  to  free  it  perfectly  from  all  basic 
salt  which  may  be  undissolved.  (This  operation  is,  of  course,  not 
performed  until  the  liquid  hns  first  become  cold.) 
The  precipitation  now  folloAvs  by  pouring  the  clear,  cold,  filtered 
solution  of  basic  protonitrate  of  mercury  into  a  mixture  of  one  and 
three-quarter  pounds  of  hydrochloric  acid,  (spec.  grav.  1*18,)  and 
twenty  pounds  of  water,  stirring  uninterruptedly.  The  precipitate 
is  washed  by  decantation,  and  the  wasliing  afterwards  finished  on  a 
fi-lter.  It  is  necessary  that  the  mercurial  solution  should  be  added 
to  the  dilute  acid,  and  not  vice  versa,  in  order  to  prevent  the  possible 
contamination  of  the  preparation  with  protonitrate. 
The  preparation  obtained  in  this  manner  is  a  snow-white  impalpable 
powder,  which,  if  it  has  been  well  washed,  is  as  perfectly  free  from 
bichloride  and  basic  nitrate  of  mercury  as  Jewell's  hydrosublimate  of 
mercury. 
