ON  THE  DETECTION  OF  IODINE.  445 
As  to  the  delicacy  of  this  test,  the  following  observations  were 
made  : 
With  solutions  of  iodide  of  potassium  up  to  one  hundred  thou- 
sandth (1 :  100,000)  the  precipitate  was  abundant,  becoming  less 
blue  and  more  tawny  as  the  dilution  increased.  Beyond  this 
point  the  distinctness  rapidly  fell  off.  The  indications  were  ob- 
servable at  one-four-hundred-thousandth.  With  a  solution  of 
one-eight-hundred-thousandth  it  was  doubtful  whether  any  effect 
was  evident,  though  still  it  was  thought  that  a  darkening  was 
produced. 
The  experiment  can  be  made  in  two  ways,  according  to  the 
result  desired. 
If  it  is  wished  to  observe  the  effect  of  the  chromic  acid  in  in- 
creasing the  delicacy  of  the  indication,  add  the  acid  and  starch 
to  the  very  dilute  solution  of  iodide,  and  then  when  the  extreme 
dilution  is  such  that  no  reaction  appears,  a  drop  of  solution  of 
bichromate  instantly  produces  it. 
But  in  employing  the  reagent  in  the  search  for  iodine,  add 
the  starch  to  the  liquid  to  be  tested,  stir  it  up,  add  a  drop  of  di- 
lute solution  of  bichromate,  enough  to  communicate  a  pale  yellow 
color,  and  finally  add  a  few  drops  of  chlorhydric  acid.  The 
test  is  then  the  production  of  the  characteristic  precipitate,  or  in 
case  of  great  dilution,  approaching  to  a  half-millionth,  merely  a 
tawny  shade  given  to  the  solution. 
It  seems  scarcely  necessary  to  say  that  if  a  very  great  excess 
of  acid  is  used,  and  too  much  bichromate,  the  starch  may  be  made 
to  reduce  the  bichromate.  Even  this,  however,  cannot  deceive 
for  a  bluish-green  solution  is  thereby  produced,  whereas  the  in- 
dications of  iodide  are  in  the  order  of  their  strength :  blue  pre- 
cipitate, tawny  precipitate,  tawny  solution.  Unless  in  the  case 
of  very  exceptional  dilution  above  spoken  of,  a  well  marked  blue 
precipitate  is  always  obtained. 
The  examination  of  the  delicacy  of  the  reaction  with  very  di- 
lute solutions  was  made  at  a  temperature  of  65°  F.  or  thereabouts. 
This  fact  requires  to  be  taken  into  account,  as  according  to  some 
experiments  of  Fresenius  to  be  found  in  the  Jahresbericht  for 
1857,  the  delicacy  of  the  starch  test  increases  as  the  temperature 
falls,  so  that  at  0°  C.  a  fainter  trace  can  be  rendered  evident 
