14  ON  PROTIODIDE  OF  MERCURY. 
behaviour  of  those  formerly  obtained ;  it  also  contained  iodine 
and  mercury. 
Any  of  the  green  product,  when  treated  with  cold  diluted 
nitric  acid,  assumed  a  lighter  color,  and  the  filtrate  contained 
mercury ;  for,  on  oversaturating  it  with  potassa,  a  yellowish  pre- 
cipitate was  thrown  down,  and  hydrosulphuric  acid  produced  a 
black  precipitate.  The  powders,  therefore,  contained  metallic 
mercury  in  minute  division. 
The  conclusion  we  have  come  to,  from  the  above  experiments, 
is,  that  it  is  impossible  to  obtain  pure  protiodide  of  mercury  by 
the  reaction  of  iodide  of  potassium  on  calomel^  no  matter  how  it 
may  be  manipulated. 
In  preparing  the  protiodide  of  mercury  we  have,  therefore,  to 
fall  back  on  the  process  directed,  I  believe,  by  all  Pharmacopoeias, 
of  combining  the  elements  by  trituration.  For  this  case,  how- 
ever, there  is  a  deficiency  in  the  officinal  direction.  Mohr 
asserts  that  some  red  iodide  is  present  in  the  product,  even  if 
less  iodine  be  taken  than  required  for  the  conversion  of  the 
mercury  into  the  protiodide,  and  treating  the  yellowish  green 
powder  with  boiling  hot  alcohol  was  indispensable.  I  have  found, 
however,  that  even  with  this  precaution  a  pure  article  is  not  ob- 
tained. When  the  protiodide,  as  made  by  this  process,  is  left  in 
contact  for  some  time  with  diluted  nitric  acid,  the  presence  of 
mercury  can  be  proved  in  the  filtrate  by  any  of  the  above  men- 
tioned tests.  To  get  rid  of  the  metallic  mercury  with  which  the 
preparation  is  contaminated,  it  must  be  treated  with  dilute  nitric 
acid  and  afterwards  with  hot  alcohol,  to  remove  any  biniodide 
present ;  then  only  is  it  in  the  state  of  purity  in  which  the  physi- 
cian expects  it.  But  how  long  will  it  remain  in  such  a  state  ? 
After  weighing  out  from  the  bottle  several  times,  if  the  opera- 
tion is  not  performed  in  the  dark,  it  must  contain  a  detectable 
quantity  of  mercury,  which  probably  would  not  unfit  it  for  inter- 
nal use,  if  at  the  same  time  another  iodide  was  not  formed.  The 
best  plan,  it  seems  to  me,  to  keep  it  unaltered,  would  be,  after 
careful  preparation,  to  have  it  put  into  black  vials  each  contain- 
ing not  over  one  or  two  drachms. 
