ON  HYDRARGYRT  IODIDUM  YIRIDE,  B.  P. 
339 
It  thus  became  evident  that  the  green  product  was  far  from 
what  it  should  be,  according  to  the  formula  given  by  the  Pharma- 
copoeia. 
I  then  prepared  some  of  the  iodide,  continuing  the  trituration 
until  the  green  color  had  passed  to  the  dingy  yellow,  which, 
according  to  the  London  Pharmacopoeia  is  characteristic  of  the 
product.  A  portion  of  this  compound  was  then  analysed,  and 
the  following  results  obtained: — 
Mercuric  Iodide,  ....  5-0 
Mercurous  Iodide,  ....  92*8 
Metallic  Mercury,       ....  2-2 
100-0 
On  comparing  these  numbers  with  those  preceding  it  will  be 
seen  that  by  continuing  the  trituration  the  red  iodide  had  to  a 
great  extent  disappeared,  and  so  had  the  free  mercury.  No 
doubt  these  two  substances  had  entered  into  combination  and 
formed  a  further  portion  of  the  mercurous  iodide.  The  results 
so  far,  then,  seem  to  admit  of  an  easy  explanation.  By  the 
constant  trituration  the  last  portions  of  the  metallic  mercury 
reach  a  state  of  extremely  fine  divisions.  In  this  state  the 
metal  possesses  somewhat  of  a  blue  color  (well  known  to  us  in 
blue  pill,  etc.,)  which,  mixed  with  the  yellow  of  the  mercurous 
iodide  already  formed,  would  produce  a  green.  According  to 
this  view,  therefore,  the  green  of  the  iodide  is  due  to  a  mechani- 
cal mixture  of  mercurous  iodide  and  mercury.  It  equally  fol- 
lows from  this  that  the  true  color  of  pure  mercurous  iodide  must 
be  yellow.  This  explanation  derives  additional  support  from  the 
fact  that  the  green  powder  cannot  be  obtained  and  kept  of  any 
uniform  tint.  As  first  got  it  is  a  decided  grass-green,  but  even 
while  it  is  drying,  however  carefully  the  light  be  excluded,  the 
color  changes,  approaching  more  and  more  to  yellow.  At  the 
same  time  that  this  change  goes  on  the  proportion  of  mercuric 
iodide  diminishes,  and  consequently  the  proportion  of  metal- 
lic mercury  must  diminish  also,  thereby  lessening  the  green  and 
augmenting  the  yellow  color. 
In  Gmelin's  'Handbook'  three  iodides  of  mercury  are  de- 
scribed ;  namely,  the  mercurous  iodide,  the  mercuroso-mercuric 
