224       ACTION  OF  IODIDE  OF  STARCH  ON  VARIOUS  SALTS. 
ing  chloride,  likewise  decolor  iodide  of  starch.  With  arsenic 
acid  there  is  no  decoloration. 
If  we  bring  to  the  maximum  of  oxidation  by  nitric  acid  the 
metals  which  decolor  iodide  of  starch,  silver  and  mercury  are 
the  only  two  which  retain  that  property.  This  decoloring  action 
of  these  two  metals  on  iodide  of  starch  is  easily  explained  in 
another  manner,  their  great  affinity  for  iodine.  Vauquelin  ob- 
served, that  in  triturating  nitrate  of  mercury  with  iodine,  iodide 
of  mercury  was  formed,  and  probably  also  iodate.  It  is  only  neces- 
sary to  place  a  crystal  of  iodine  in  a  solution  of  mercuric  nitrate  to 
see  the  red  iodide  form  on  its  surface,  and  the  same  time  a  still 
lighter  white  powder  appears,  which  is  the  iodate.  I  have  ob- 
served the  same  reaction  with  iodine  and  nitrate  of  silver ;  when 
put  in  contact  with  this  salt,  the  iodine  is  quickly  surrounded 
with  a  yellow  aureola  of  iodide,  the  quantity  of  which  gradually 
increases. 
M.  Deville  has  shown  how  strong  an  affinity  exists  between 
silver  and  iodine,  since  under  the  action  of  hydriodic  acid  the 
chloride  of  silver  itself  is  decomposed.  But  the  most  curious 
thing  is,  that  iodide  of  starch  agitated  with  chloride  of  silver, 
very  readily  yields  its  iodine  to  it,  and  transforms  it  into  iodide. 
Chlorine  is  found  in  the  filtered  liquor,  and  the  blue  color  has 
disappeared. 
Iodide  of  starch  is  the  most  sensitive  reagent  for  ascertaining 
the  presence  of  silver,  should  no  mercury  be  present.  Thus,  in 
100  c.  c.  of  liquor  containing  Ith  of  a  milligramme  of  silver,  a 
cubic  demicentimetre  of  iodide  of  starch  produces  decoloration, 
whereas  the  same  quantity  produces  perceptible  coloration  in 
100  c.  c.  of  pure  water.  In  a  smaller  volume  of  liquor,  we  can 
recognise  the  presence  of  a  ~  of  a  milligramme  of  silver  with 
with  iodide  of  starch. 
We  can  instantly  discover  the  presence  of  silver  in  commer- 
cial lead,  by  dissolving  it  in  nitric  acid,  letting  the  liquor  cool 
and  then  pouring  in  a  little  iodide  of  starch.  The  excess  of  acid 
must  be  previously  saturated  with  carbonate  of  lime,  so  that  it 
should  have  no  action  on  the  iodine.  A  similar  quantity  of  lead, 
containing  no  silver,  becomes  perceptibly  colored  by  the  addition 
of  a  few  drops  of  this  solution. 
In  these  experiments  I  have  used  the  soluble  iodide  of  starch, 
prepared  by  Dr.  Quesneville. — Chemist,  from  Oomptes  Eendus. 
