^pfembef^-}     The  Structure  of  the  Starch  Grain.  415 
with  sand  and  then  with  iodin,  would  seem  to  confirm  the  view  that 
a  true  chemical  compound  is  produced  by  iodin  and  soluble  starch, 
whether  in  the  grain  or  in  solution. 
That  the  amount  of  soluble  starch  varies  in  the  starch  grains  of 
different  origin  can  be  demonstrated  by  the  treatment  of  definite 
amounts  of  the  starches  with  definite  quantities  of  iodin  solution. 
On  treating  05  gramme  of  the  several  commercial  starches  with 
2  c.c.  of  iodin  solution  (o-i  per  cent,  of  iodin  and  0-5  per  cent,  of 
potassium  iodid),  it  will  be  found  that  there  is  a  marked  difference 
in  the  intensity  of  the  color  in  the  grains,  those  of  potato  and 
maranta  being  colored  deep  blue,  while  those  of  corn  and  wheat  are 
scarcely  at  all  colored,  the  mixtures  being  somewhat  of  a  purplish 
color.  These  observations  seem  to  accord  with  the  experiments  of 
Rouvier  (13),  who  found  that  the  different  classes  of  starch  took  up 
varying  amounts  of  iodin,  and  are  contrary  to  the  statement  by 
Stocks  (9)  that  the  shade  of  color  varies  with  the  amount  of  iodin 
but  not  with  the  different  starches.  In  other  words,  the  shade  and 
intensity  of  color  not  only  vary  according  to  the  strength  of  iodin 
solution,  but  also  according  to  the  kind  of  starch  used. 
It  is  well  known  that  if  a  starch  solution  be  treated  with  iodin  in 
the  cold,  a  blue  coloration  will  be  the  result,  and  that  if  this  solu- 
tion be  then  heated  the  blue  color  will  disappear,  but  will  return  on 
cooling  the  solution,  though  less  intensely  than  before.  This 
peculiar  behavior  of  starch  and  iodin  has  never  been  satisfactorily 
explained.  Nageli  and  Schwendener  (14)  consider  that  the  loss  of 
color  on  the  application  of  heat  is  due  to  the  production  of  hydri- 
odic  acid ;  but  this  does  not  explain  the  reappearance  of  the  color 
on  cooling ;  and  so  far  as  I  am  aware  no  one  has  ever  demonstrated 
the  presence  of  this  acid.  If  this  acid  were  present,  then  according 
to  Mylius  (7)  and  Meineke  (8),  the  blue  color  would  remain  on 
heating  the  solution,  since  they  claim,  as  already  pointed  out,  that 
the  blue  color  is  due  to  the  presence  of  this  acid.  It  is  thus  apparent 
that  there  is  considerable  contradiction  in  the  arguments  that  have 
been  presented  on  this  subject. 
The  following  experiments  tend  to  explain  the  phenomenon:  If 
we  take  0-5  gramme  of  potato  starch  and  mix  it  with  50  c.c.  of 
water  and  heat  the  mixture  in  an  Erlenmeyer  flask  of  about  iooc.c. 
capacity  on  a  water  bath  to  a  temperature  of  about  8o°  C.  for  one 
hour,  and  then  add  5  c.c.  of  iodin  solution  (iodin  01  per  cent,  and 
