ON  PROTIODIDE  OF  MERCURY.  13 
spheric  moisture.  60  grains  of  calomel  must  produce  83  grains 
protiodide  of  mercury,  41  grains  of  iodide  of  potassium  ought  to 
decompose  58J  grains  of  calomel,  consequently,  by  the  above 
proportions,  If  grains  calomel  would  be  left  unaltered,  and  the 
obtained  product  ought  to  weigh  but  82  grains.  The  practical 
result,  however,  was,  for  the  above  three  cases,  49,  56  and  50  grs. 
respectively.  What,  then,  has  become  of  the  difference,  which 
in  all  cases  is  about  and  even  more  than  one  third  of  the  cal- 
culated result?  The  obtained  powder  was  of  an  olive  green 
color,  almost  without  a  yellow  tinge,  one  of  a  very  dark  almost 
black  hue,  while,  according  to  all  authorities,  the  protiodide  has 
a  yellowish  green  color,  which  on  exposure  to  light  darkens 
to  olive  green,  and  at  last  black  (Mohr).  It  was  obvious  that 
the  product  could  not  be  protiodide,  but  that  a  decomposition 
had  taken  place,  which  made  it  probable  that  both  iodine  and 
mercury  might  be  found  in  the  filtrate. 
On  a  clean  copper  plate  a  drop  of  tbe  solution  did  not  pro- 
duce a  silvery  stain,  and  iodide  of  potassium  no  precipitate  ;  proto- 
chloride  of  tin  at  first  produced  a  white  precipitate,  changing  to 
a  grayish  color  on  further  addition ;  the  precipitate  formed  by 
gradual  addition  of  sulphuretted  hydrogen  showed  the  charac- 
teristic changes  of  color,  yellowish,  orange,  brown  and  black. 
Thus  the  presence  of  mercury  in  the  filtrate  was  established. 
Sulphate  of  copper  when  added  to  the  solution  produced  a 
whitish  precipitate,  leaving  the  supernatant  liquor  of  a  reddish 
brown  color,  which  disappeared  on  the  further  addition  of  sul- 
phate of  copper  and  a  larger  quantity  of  protosulphate  of  iron  ; 
the  precipitate  was  iodide  of  copper,  and  proves  the  presence  of 
iodine,  which  also  manifested  itself  by  giving  a  scarlet  red  pre- 
cipitate with  corrosive  sublimate. 
The  same  reactions  were  obtained  with  the  filtrates  of  the 
three  preparations.  Another  experiment  was  then  made  by 
taking  calomel  in  considerable  excess,  to  see  whether,  with  a  part 
of  it,  iodide  of  potassium  would  produce  protiodide  of  mercury 
without  subjecting  the  latter  to  the  decomposing  influence  of 
some  unaltered  iodide  of  potassium.  Accordingly  60  grains  of 
calomel  were  well  mixed  with  but  35  grains  of  iodide  of  potassium, 
and  this  powder  gradually  added  to  water  and  triturated  with 
the  same.    A  green  powder  resulted,  and  the  filtrate  showed  the 
