416  The  Structure  of  the  Starch  Grain.  {^embefS' 
potassium  iodid  0-5  per  cent.)  and  place  a  piece  of  ordinary  filter 
paper,  which  usually  contains  starch,  over  the  top  of  the  flask,  and 
then  continue  the  heating  until  the  solution  is  decolorized,  which 
takes  place  at  about  8o°  C,  the  filter  paper  will  be  colored  blue, 
showing  that  some  of  the  iodin  has  been  volatilized.  Furthermore, 
on  cooling  the  solution,  as  already  pointed  out,  the  blue  color  begins 
to  return  at  about  65 0  C.  and  increases  in  intensity  until  at  about 
400  C.  it  reaches  the  maximum  ;  though  it  is  less  intense  than  the 
color  produced  when  an  iodin  solution  is  added  to  a  starch  solution 
cooled  to  this  temperature.  These  results  may  be  interpreted  as 
showing  that  iodin  forms  a  compound  with  starch  which  is  disso- 
ciated on  the  application  of  heat;  that  is,  at  a  temperature  between 
60  and  8o°  C,  part  of  the  freed  iodin  is  volatilized  as  such,  and  the 
amount  remaining  in  solution  depends  upon  the  length  of  time  the 
heat  is  applied.  Also  the  loss  of  iodin  in  this  way  accounts  for  the 
decrease  in  color  of  the  starch-iodin  solution  on  cooling. 
An  attempt  was  made  to  remove  the  free  iodin  from  the  hot  color- 
less starch-iodin  solution  by  the  addition  of  chloroform,  but  it  was 
found  difficult  to  handle  the  material,  as  chloroform  boils  at  about 
6o°  C.  Subsequent  experiments,  however,  showed  that  unaltered  po- 
tato starch  grains  as  well  as  a  solution  of  potato  starch  would  remove 
the  iodin  from  a  chloroformic  solution  at  the  ordinary  temperature. 
These  experiments  were  conducted  as  follows  :  5  c.c.  of  an  iodin 
solution,  of  the  strength  already  stated,  were  shaken  in  a  separatory 
funnel  with  25  c.c.  of  chloroform,  and  most  of  the  chloroform  con- 
taining the  iodin  separated.  To  this  were  added  5  grammes  of  dried 
starch.  The  latter,  however,  did  not  take  up  the  iodin  from  the 
solution ;  but  on  the  addition  of  a  small  quantity  of  water  (about 
10  c.c.)  and  after  shaking  the  mixture,  the  starch  and  iodin  com- 
bined, and  almost  all  of  the  iodin  in  the  chloroformic  solution  may 
be  removed  in  this  way.  The  addition  of  2  c.c.  of  potassium  iodid 
solution  (5  per  cent.),  even  with  0-5  gramme  of  starch,  causes  the 
iodin  to  be  taken  up  immediately.  The  starch  in  a  soluble  starch 
solution  also  combines  immediately  with  the  iodin  in  a  chloroformic 
solution. 
In  summing  up  the  observations  on  the  behavior  of  iodin  and 
starch,  it  seems  to  me  that  we  are  dealing  with  a  chemical  com- 
pound of  iodin  and  soluble  starch  ;  but  that  the  combination  is  a 
feeble  one,  being  easily  dissociated  upon  the  application  of  heat, 
