ArASi^8£!m"}    S°me  Official  and  Son-Official  Iodides.  389 
In  either  case  the  generated  iodide  or  oxide  surrounds  the  imper- 
vious calomel  fragments,  and  thus  terminates  the  progress  of  the  reac- 
tion. The  pharmacopoeia  states  that  calomel  is  an  impalpable  pow- 
der. The  writer  finds  that  the  English  calomel,  which  is  alleged  to 
be  superior  to  other  kinds,  feels  decidedly  granular,  and  with  a  half- 
inch  power  exhibits  a  large  proportion  of  transparent  crystalline 
particles.  That  such  must  be  the  case  becomes  evident  from  the  fact 
that  sublimed  calomel  has  a  crystalline  structure,  and  hence  any 
degree  of  attrition,  however  extreme,  cannot  conceal  the  nature  of  its 
pedigree.  AYhen  precipitated  calomel,  which  is  usually  amorphous, 
is  employed  under  these  circumstances,  perfect  double  decomposition 
results,  owing  to  the  pervious  character  of  the  molecular  aggregation. 
It  seems  that  the  only  practically  available  source  of  amorphously 
precipitated  calomel  is  mercurous  nitrate.  This  salt  is  itself  rather 
difficult  to  prepare  in  a  pure  state  and  of  definite  composition.  Its 
composition  is  variously  given  as  Hg  X03  Aq,  and  Hg  X03.  The 
writer  prefers  employing  it  in  proportion  to  the  molecular  weight  of 
the  latter  formula,  as  then  no  objectionable  excess  would  result  were 
it  after  all  composed  in  this  manner. 
Mercurous  nitrate  is  prepared  by  the  rather  tedious  process  of  dis- 
solving mercury  in  an  excess  of  moderately  dilute  nitric  acid.  It 
then  forms  in  crystals  which  decompose  in  pure  water,  but  readily 
dissolve  in  water  very  slightly  acidified  with  nitric  acid.  Doubtless 
a  definite  amount  of  freshly  precipitated  amorphous  mercurous  chlo- 
ride prepared  by  any  convenient  method  would  be  equally  suitable 
in  place  of  the  mercurous  nitrate  of  the  following  formula  : 
Mercuric  chloride   68  grains. 
Mercurous  nitrate   262  " 
Potassium  iodide  ,   166  " 
Sodium  chloride   500 
Nitric  acid  diluted. 
"Water  of  each  sufficient. 
Mix  the  mercuric  chloride,  sodium  chloride  and  potassium  iodide 
and  stir  the  mixture  with  sufficient  water,  gradually  added,  until  the 
salts  are  all  dissolved.  Pass  this  solution  through  a  plain  filter  and 
follow  with  water  until  the  whole  filtrate  measures  six  fluid-ounces. 
Powder  the  mercurous  nitrate,  add  four  fluid-ounces  of  water  and 
sufficient  diluted  nitric  acid,  drop  by  drop,  until,  with  constant  stirring, 
a  clear  solution  is  obtained.  "Warm  the  first  solution  and  gradually 
add  the  second  whilst  stirring  the  mixture,  or  mix  the  two  solutions 
