PREPARATION  OF  IODIDE  OF  POTASSIUM. 
439 
phosphorus,  but  one  ounce  of  this  body  is  sufficient  for  the  pro- 
duction of  one  pound  and  a  half  of  iodide  of  potassium,  so  that 
it  only  costs  about  one  penny  for  every  pound  of  iodide  manu- 
factured by  this  process. 
Proto-iodide  of  Mercury, 
The  Pharmacopoeia  directs,  for  the  preparation  of  this  com- 
pound, that  eight  parts  of  mercury  should  be  rubbed  with  five 
of  iodine,  with  the  addition  of  a  little  spirits  ;  the  powder  thus 
produced  is  of  a  dull  olive  color,  and  consists  of  pure  iodide  of 
mercury.  There  is  found  in  commerce  a  powder  of  a  beautiful 
brilliant  yellow  color,  which  sails  under  the  name  of  iodide  of 
mercury,  and  which  is  often  preferred  to  the  pharmacopoeial 
preparation,  on  account  of  the  beauty  of  its  color.  This  bril- 
liant powder  is  prepared  by  precipitation  from  the  proto-nitrate 
of  mercury,  but  as  this  salt  is  insoluble  unless  a  certain  propor- 
tion of  nitric  acid  is  present,  the  free  nitric  acid  so  acts  upon 
the  iodide  at  the  moment  of  its  precipitation  that  the  product  is 
contaminated  with  a  certain  proportion  of  biniodide. 
The  presence  of  this  last  is  conveniently  proved  by  heating  a 
sample  of  the  yellow  iodide  with  a  little  aniline,  which  possesses 
the  property  of  producing  the  well-known  magenta  color  when 
treated  with  a  persalt  of  mercury.  The  pharmacopoeial  prepara- 
tion, if  no  excess  of  iodine  has  been  used,  does  not  exhibit  this 
reaction.  As  the  biniodide  of  mercury  is  so  much  more  power- 
ful in  its  action  than  the  protiodide,  the  substitution  of  the  yellow 
for  the  pharmacopoeial  preparation  is  highly  objectionable  and 
might  be  attended  with  danger. 
Per  iodide  of  Iron. 
It  would  have  been  hardly  worth  while  to  bring  this  subject 
under  the  notice  of  the  Society,  if  it  were  not  according  to  the 
handbooks  of  chemistry  still  a  moot  point,  whether  the  periodide 
of  iron  exists  or  not. 
To  determine  this  question,  a  quantity  of  iodine  was  divided 
into  three  parts  ;  two  of  these  were  combined  with  iron  so  as  to 
form  a  pale-green  liquid,  the  third  portion  was  then  added,  and 
the  whole  boiled  so  as  to  promote  combination.  After  the  dark- 
brown  solution  had  cooled,  it  was  shaken  up  with  about  three 
