INCOMPATIBILTIES  OF  IODIDE  OF  POTASSIUM. 
223 
found  in  solution,  and  green  protiodide  of  mercury  remains  inso- 
luble. "When  they  are  treated  with  cold  water,  the  same  change 
occurs  more  slowly.    HgCl  and  KI,  becoming  Hgl  and  KC1. 
2d,  If,  however,  the  iodide  of  potassium  is  in  excess,  the  pro- 
tiodide of  mercury  is  converted  into  biniodide  and  metallic 
mercury,  the  former  immediately  combining  with  a  part  of  the 
excess  of  iodide  to  form  soluble  iodohydrargyrate  of  potassium. 
2HgI  and  2KI  yield  2KI,HgI2,  and  Hg. 
3d.  When  black  oxide  of  mercury  is  added  to  an  excess  of  iodide 
of  potassium,  it  is  converted  first  into  protiodide,  and  this  subse- 
quently decomposed  into  biniodide  and  metallic  mercury,  part  of 
the  iodide  of  potassium  suffering  decomposition  to  yield  the  iodine 
which  is  replaced  by  oxygen,  so  as  to  form  free  potassa ;  thus, 
2HgO  and  2KI,  become  2HgI  and  2KO.  Then  the  protiodide  and 
excess  of  iodide  of  potassium  react  as  above,  to  produce  free  mer- 
cury and  iodohydrargyrate  of  potassium. 
4th.  When  red  oxide  of  mercury  is  added  to  an  excess  of  iodide  of 
potassium,  they  are  first,  by  double  decomposition,  converted  into 
biniodide  of  mercury  and  potassa,  and  the  former  then  into  iodo- 
hydrargyrate of  potassium,  whilst  free  potassa  is  found  in  the 
clear  solution  which  is  strongly  alkaline ;  thus  HgO2  and  4KI 
become  2KI,  Hgl2  and  2KO. 
5th.  When  subsulphate  of  mercury  (turpeth  mineral)  is  mixed 
with  an  excess  of  iodide  of  potassium,  in  strong  solution,  the  yel- 
low salt  is  instantly  turned  red  by  the  formation  of  biniodide, 
which  is  gradually  dissolved — the  change  may  be  represented 
by  the  following  equation  :  3Hg02,  2S03  and  12KI  yield  3  (2KI 
HgI2)-l-2(KO,  SO3)  +4KO,  the  solution  being  strongly  alkaline. 
6th.  When  white  precipitate  is  mixed  with  an  excess  of  iodide 
of  potassium  it  soon  dissolves — the  solution  smells  of  ammonia. 
7th.  When  blue  mass  is  digested  with  a  solution  of  iodide  of 
potassium,  the  filtered  liquid  has  an  alkaline  reaction,  and  con- 
tains mercury,  as  evidenced  by  its  precipitation  when  a  drop  is 
placed  on  a  bright  copper. 
Lastly,  When  mercury  is  boiled  with  a  strong  solution  of 
iodide  of  potassium,  and  filtered,  the  solution  contains  mercury, 
and  is  decidedly  alkaline.  It  follows,  therefore,  that  iodide  of 
potassium  is  decomposed  with  the  formation  of  biniodide  of  mer- 
cury and  potassa,  yet  the  rationale  is  not  very  clear,  inasmuch 
