Amliiuld^vm'}    S°me  Official  and  Non-Official  Iodides.  387 
bined  and  a  light-green  solution  has  resulted.  In  case  the  iron  wire  is 
extremely  fine  add  the  iodine  in  separate  portions  shaking  the  mixture 
each  time  until  the  iodine  color  is  discharged.  Pass  the  solution 
through  a  plain  filter  and  follow  with  water  until  the  whole  filtrate 
measures  eight  fluidounces.  Powder  the  plumbic  acetate,  dissolve  it 
in  four  fluidounces  of  water  and  add  acetic  acid,  drop  by  drop,  until  the 
solution  becomes  limpid.  Pass  the  solution  through  a  plain  filter  and 
follow  with  water  until  the  filtrate  measures  eight  fluidounces.  Now 
pour  this  solution  into  the  ferrous  iodide  solution,  collect  the  yellow 
magma  on  a  plain  filter,  wash  it  with  water  and  dry  the  powder  in 
the  open  air. 
Mercuric  iodide. — This  salt  appears  in  at  least  three  peculiar  states 
of  aggregation.  A  white  extraordinary  amorphous  salt,  usually  having 
but  a  transient  existence  almost  invariably  precedes  the  generation  of 
the  red  ordinary  amorphous  salt.  This  latter  compound  is  the  official 
form  and  its  preparation  offers  no  special  difficulties.  A  crystalline 
variety  is  most  readily  obtained  by  dissolving  the  official  salt  in  a  hot 
concentrated  solution  of  sodium  chloride  or  ammonium  chloride  and 
letting  the  liquor  cool.  The  pharmacopoeia  states  that  the  official  salt 
is  a  crystalline  powder.  That  the  product  as  officially  prepared  is 
largely  contaminated  by  the  crystalline  form,  cannot  be  doubted. 
Such  must  result  from  the  order  of  mixing  the  solutions  as  officially 
directed.  In  this  process  the  first  portion  formed  will  naturally 
dissolve  in  the  excess  of  potassic  iodide,  and  subsequently  separate  in 
crystals  as  the  remainder  of  the  chloride  is  added.  The  pharmacopoeia 
also  states  that  the  iodide  is  soluble  in  solutions  of  mercuric  chloride. 
This  result  is  also  insisted  on  in  various  text -books.  It  is,  however, 
an  exaggeration,  to  say  the  least,  since  the  addition  of  only  a  few  drops 
of  the  iodide  solution  causes  a  permanent  precipitate  with  the  whole 
volume  of  the  chloride  solution.  A  great  advantage  results  from  dis- 
solving the  mercuric  chloride  by  means  of  sodium  or  ammonium 
chloride.  There  is  also  an  advantage  in  the  use  of  ferrous  iodide. 
AYhen  a  solution  of  ferrous  iodide  is  poured  into  an  excess  of  mercuric 
chloride  solution,  containing  sodium  chloride,  the  ordinary  amorphous 
mercuric  iodide  results.  But  in  this  case  a  considerable  proportion  of 
the  iodide  is  retained  in  solution  by  means  of  an  excess  of  mercuric  chlo- 
ride. The  addition  of  potassic  iodide  to  the  filtrate  gives  an  abun- 
dant precipitate  which  exhibits  itself  in  pearly  streaks,  similar  to  plum- 
bic iodide  under  the  same  conditions.    But  it  rapidly  becomes  floccu- 
