460  Elm  Bark,  Flaxseed  and  Mustard.  {J^g*2SSt 
these  results  are  offered  simply  as  voicing  the  result  of  my  personal 
experimentation  on  this  now  important  subject. 
If  a  slab  of  dried  elm  bark  be  split  longitudinally,  and  the  exposed 
edges  moistened  with  very  weak  iodine  tincture,  a  light  purplish- 
blue  color,  more  or  less  prominent,  follows,  which  disappears 
quickly.  If  the  bark  be  previously  moistened  with  dilute  sulphuric 
acid,  the  blue  color  is  quite  permanent.  The  same  is  true  when  it 
is  moistened  with  a  dilute  solution  of  iodine  in  iodide  of  potassium, 
acidulated  with  sulphuric  acid.  Ten  specimens  taken  by  me  at  ran- 
dom from  a  lot  of  Wisconsin  bark,  and  others  from  Kentucky  and 
Ohio,  demonstrated  that  all  of  them  would  respond  to  the  test,  and 
I  have  found  none  that  did  not  do  so,  although  some  specimens  of 
bark  contain  more  starch  than  others.  If  a  thin  shaving,  not  of  the 
inner  side  of  a  slab  of  elm,  be  dropped  into  a  dilute  solution  of 
iodine  in  iodide  of  potassium,  made  acid  with  sulphuric  acid,  the 
shaving  will  usually  turn  blue,  or  become  streaked  with  blue, 
although,  as  the  inner  surface  of  the  bark  is  often  free  from  starch, 
the  shaving  must  be  from  beneath  the  surface.  In  this  connection 
it  should  be  added  that  this  starch-free  layer  is  of  varying  thick- 
ness, sometimes  being  a  mere  film,  again  (seldom)  forming  the  bulk 
of  the  bark.  Under  these  conditions,  it  is  to  be  presumed  that  enough 
starch  must  be  present  in  powdered  elm  to  make  it  respond  to  the 
iodine  test. 
To  Detect  Starch. — Powder  the  elm  bark,  and  thoroughly  mix  I 
gramme  with  100  c.c.  of  water,  by  rubbing  in  a  mortar,  the  water 
being  gradually  added  ;  then  boil. 
To  2  c.c.  iodine  test  solution,  U.  S.  P.,  add  8  c.c.  10  per  cent,  sul- 
phuric acid. 
Mix  I  c.c.  of  the  iodine  mixture  with  8  c.c.  of  the  cold  mucilage; 
a  clear,  quite  permanent  blue  color  will  be  produced.  If  iodine 
tincture  without  acid  be  added  to  the  mucilage,  the  blue  color  will 
be  developed,  but  disappears  quickly.  If  the  mucilage  is  to  be 
made  of  the  whole  bark,  shave  it  transversely  into  thin  shavings, 
put  i  gramme  into  a  capacious  mortar  and  rub  constantly  with 
ioo  c.c.  of  boiling  water,  gradually  added,  until  a  thick  mucilage 
results. 
The  coloration  that  results  in  presence  of  too  much  iodine  is  not 
bright  blue,  for  if  an  excess  of  iodine  is  used,  a  shade  of  purple  or 
brownish  yellow  results.    It  is  essential  that  no  excess  of  iodine  be 
