204 
ON  HYDRARGYRI  IODIDUM. 
elements  in  their  atomic  proportions  as  a  substitute,  which  plan 
has  held  ground  till  the  present  time. 
There  have,  nevertheless,  objections  been  urged  against  this 
process— and  not  without  reason — for  it  is  found  that  there  is 
always  some  metallic  mercury  or  red  iodide  present  as  a  con- 
tamination ;  the  latter  may  be  removed  by  washing  with  strong 
alcohol,  but  this  will  not  remedy  the  mercurial  contamination. 
M.  Boutigny,  of  Paris,  proposed  to  escape  these  sources  of 
trouble  by  grinding  the  proto-chloride  of  mercury  with  the  pro- 
per proportion  of  iodide  of  potassium,  to  a  fine  powder  ^  washing 
with  boiling  water  and  drying  in  the  dark,  but  it  has  been  found 
to  fail,  the  product  being  contaminated  with  calomel  and  metal- 
lic mercury. 
From  the  most  careful  examination  of  the  whole  subject,  both 
by  experiment  and  the  sifting  of  all  that  has  appeared  in  every 
authority  accessible  to  me,  I  have  arrived  at  the  conclusion,  that 
the  true  process  is,  the  double  decomposition  of  a  proto-salt  of 
mercury  by  pure  iodide  of  potassium,  and  this  opinion  I  find 
supported  in  Gmelin's  invaluable  Treatise,  vol.  vi.  page  35, 
where  he  states,  that  the  mercurous  acetate  when  ground  to- 
gether with  pure  iodide  of  potassium,  and  permitted  to  remain 
for  some  time  in  contact  and  then  washed,  would  yield  a  proto- 
iodide  of  mercury,  which  is  free  from  the  objections  urged 
against  the  product  of  the  formula  which  have  been  noticed  in 
the  earlier  part  of  this  paper. 
The  most  ready  way  of  preparing  the  mercurous  acetate,  I 
have  found  to  be  the  decomposition  of  the  proto-chloride  of  mer- 
cury, by  equal  quantities  of  caustic  potassa,  dissolved  in  four 
times  its  weight  of  distilled  water,  washing  till  solution  of  nitrate 
of  silver  gave  no  evidence  of  chloride  of  potassium ;  then  heat- 
ing acetic  acid  on  an  excess  of  the  oxide,  filtering  while  hot,  and 
then  drying  the  flaky  crystals,  which  should  be  treated  as  be- 
fore stated. 
The  formula  (see  Watts'  note  in  Gmelin's  Handbook,  vol  vi. 
35)  which  I  offer  for  its  preparation  is — 
Take  of  Proto-Acetate  of  Mercury,  259  grs. 
Pure  Iodide  of  Potassium,  162.5  grs. 
Grind  the  crystals  of  the  mercurous  acetate  to  a  fine  powder, 
then  add  the  iodide  dissolved  in  a  small  quantity  of  water,  tri- 
