ON  HYDRARGYRI  IODIDUM  VIRIDE,  B.  P. 
337 
ON  HYDRARGYRI  IODIDUM  YIRIDE,  B.  P. 
By  0.  H.  Wood,  F.O.S, 
The  "  Green  Iodide  of  Mercury  "  of  the  British  Pharmaco- 
poeia is  clearly  intended  to  be  identical  with  the  "  Iodide  of 
Mercury  "  of  the  London  Pharmacopoeia.  This  is  expressly 
implied  by  the  "  Synonym  Hydrargyri  Iodidum  Lond." 
given  in  the  description  of  the  preparation  in  the  last  edition  of  the 
British  Pharmacopoeia.  In  both  works  the  chemical  compound 
mercurous  iodide  is  obviously  meant,  and  the  British  Pharmaco- 
pseia  defines  this  by  giving  the  formulae  "  Hg2I  or  Hgl."  But 
the  London  Pharmacopoeia,  in  describing  this  compound,  states, 
that  "  when  freshly  prepared  it  is  dingy  yellow  ;"  while  the 
British  Pharmacopoeia  names  it  "  Green  Iodide,"  and  describes 
it  as  "  a  dull  green  powder.''  This  discrepancy  occurring  in  the 
description  of  a  definite  chemical  body  is  a  little  remarkable, 
and  requires  some  explanation.  The  processes  and  the  propor- 
tions of  ingredients  given  in  the  two  Pharmacopoeias  are  almost 
identical;  yet  according  to  one  authority  we  obtain  a  yellow, 
and  according  to  the  other  a  green  product. 
I  have  frequently  made  the  iodide  of  mercury  according  to  the 
London  Pharmacopoeia,  and  have  always  found  that  it  possessed 
a  dull  yellow  color,  certainly  with  a  shade  of  green  upon  it,  but 
nevertheless  the  yellow  vastly  preponderating.  Yet  chemical 
authorities  generally  indicate  that  mercurous  iodide  is  a  green 
substance.  In  Gmelin's  '  Chemistry  '  mercurous  iodide  is  va- 
riously described  as  "  a  green,"  "  a  dark  green,"  and  "  a  yel- 
lowish-green ;"  but  no  mention  is  made  of  its  being  yellow. 
The  British  Pharmacopoeias  of  1864  and  1867  are  therefore  in 
accord  with  the  leading  chemical  manual. 
Being  desirous  of  understanding  this  matter  more  perfectly,  I 
made  a  few  experiments  to  elucidate  the  point.  Upon  repeating 
the  process  as  given  in  the  British  Pharmacopoeia,  I  found  that 
the  first  effect  of  triturating  the  mercury  and  iodine  with  the 
spirit  was  to  obtain  a  red  color  ;  due,  no  doubt,  firstly  to  the 
solution  of  the  iodine,  and  secondly  to  the  formation  of  the  red 
iodide  of  mercury  ;  but  after  continuing  the  trituration  for  some 
time  the  mass  became  a  decided  dark  green.    This  then  was 
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