12 
ON  PROTIODIDE  OF  MERCURY. 
unwarranted,  and  the  employment  of  cold  water  for  the  com- 
pletion of  the  reaction  between  calomel  and  iodide  of  potassium 
ought  to  furnish  a  purer  product ;  the  iodide,  as  well  as  the 
chloride  of  potassium  being  very  easily  soluble  in  cold  water  ;  the 
reaction  can  take  place  at  once  in  the  finely  powdered  and  well 
mixed  mass  and  the  resulted  chloride  removed  by  filtration  and 
washing.  If  the  iodide  of  potassium  is  not  well  mixed  with  the 
calomel,  a  portion  of  it,  after  the  admixture  of  water,  will  come 
in  contact  with  the  newly  formed  protiodide,  and  decompose  this 
before  all  the  calomel  has  been  decomposed ;  this,  undoubtedly, 
was  the  case  in  Mr.  Bullock's  undertaking,  and  in  this  way  only 
majr  be  accounted  for  the  large  amount  of  the  red  iodide  of  mer- 
cury obtained  from  the  mother  waters. 
In  view  of  the  above  facts  and  inferences  the  following  ex- 
periments were  made  : — 
1.  The  mixed  powder  was  rubbed  with  boiling  hot  water,  im- 
mediately thrown  on  the  filter  and  washed  with  warm  water. 
2.  The  powder  was  treated  with  cold  water,  occasionally 
stirred,  filtered  after  half  an  hour,  and  washed  with  cold  water. 
3.  After  the  rubbing  with  sufficient  cold  water,  the  product 
was  instantly  filtered  and  rapidly  washed  with  cold  water. 
In  all  cases  the  precautions  were  alike,  viz:  the  iodide  of 
potassium  was  free  of  carbonate;  before  weighing  it  was  exposed 
to  a  dull  red  heat  to  destroy  any  iodate  that  might  be  present ; 
it  was  rubbed  to  powder  in  a  mortar  mixed  with  calomel,  and  the 
whole  then  well  triturated  for  ten  or  fifteen  minutes  to  ensure 
an  intimate  mixture.  For  comparison  of  the  results,  the 
quantity  of  the  materials  were  alike  in  the  three  cases  ;  60  grains 
of  calomel  were  used  to  41  grains  of  iodide  of  potassium.  The 
whole  was  then  put  on  a  tared  filter,  the  resulting  iodide  washed 
with  distilled  water  as  above,  until  the  filtrate  no  longer  pre- 
cipitated a  solution  of  nitrate  of  silver  ;  then  well  pressed  between 
bibulous  paper  to  remove  most  of  the  moisture,  and  at  last  dried, 
either  in  a  temperature  of  between  70  and  80  degrees,  or  in  a 
current  of  dry  warm  air.  During  the  whole  of  the  operation, 
light  was  excluded  as  much  as  possible. 
The  reaction  between  calomel  and  iodide  of  potassium  com- 
menced as  soon  as  they  were  mixed  in  the  mortar,  the  powder 
assuming  a  greenish  tinge,  which  was  probably  due  to  atmo- 
