388  Some  Official  aud  Non- Official  Iodides.  {^^5™' 
lent  and  ultimately  deposits  more  compactly.  With  a  half-inch 
power  thh  precipitate  is  seen  to  be  composed  almost  entirely  of  pris- 
matic needles.  The  writer  believes  that  an  amorphous  mercuric 
iodide  is  the  appropriate  medicinal  form.  The  following  process  is 
suggested  to  obtain  such  a  product. 
Iron  in  fine  wire   240  grains. 
Iodine   508  " 
Mercuric  chloride...,   541  " 
Sodium  chloride   240  " 
Water,  sufficient. 
Pour  three  fluid-ounces  of  water  on  the  iron  wire,  add  the  iodine 
at  intervals,  if  necessary,  shake  the  mixture  occasionally,  until  the 
brown  color  is  discharged,  and  pass  the  light  green  solution  of  ferrous 
iodide  through  a  plain  filter.  Then  follow  with  water  until  the  whole 
filtrate  measures  eight  fluid-ounces.  Powder  the  mercuric  chloride 
add  the  sodium  chloride,  and  pour  on  four  fluid-ounces  of  water,  stir 
the  mixture  until  the  salts  are  dissolved,  and  pass  the  solution  through  a 
plain  filter,  adding  water  until  the  whole  filtrate  measures  eight  fluid- 
ounces.  Now  gradually  pour  the  iron  solution  into  this,  stirring  the 
mixture  meanwhile.  Let  the  precipitate  subside,  and  decant  the  super- 
natant liquor.  Wash  the  residuary  precipitate  three  or  four  times  in 
succession,  each  time  with  six  to  eight  fluid-ounces  of  water,  then  pour 
it  into  a  plain  filter,  and  expose  the  drained  precipitate  in  the  open  air 
to  dry. 
Mercurous  iodide. — It  is  very  difficult  to  prepare  this  salt  in  a 
suitable  medicinal  form,  although  the  official  process  gives  no  such  in- 
dication ;  bat  the  process  of  the  pharmacopoeia  does  not  yield  a  de- 
sirable product.  Elsewhere,  November,  1884,  the  writer  suggested  a 
method  for  preparing  mercurous  iodide  by  double  decomposition  be- 
tween potassio-mercuric  iodide  and  mercurous  chloride.  The  method 
is  founded  on  a  correct  principle,  but  the  practical  difficulties  pre- 
sented by  the  rather  coarse  crystalline  structure  of  commercial  calomel 
interfere  with  a  necessarily  complete  interchange  of  elements.  Fownes' 
Chemistry  states  that  mercurous  oxide  is  readily  obtained  by  treating 
calomel  with  solution  of  potash.  The  writer,  however,  found  that 
the  structural  character  also  prevents  a  thorough  decomposition  in  this 
case.  No  less  than  half  a  dozen  repetitions  of  the  process  of  treatment 
with  potash  and  nitric  acid  will  suffice  for  the  practically  complete 
conversion  of  the  calomel  into  mercurous  nitrate. 
