DECOMPOSITION  OF  IODIDE  OF  STARCH,  ETC. 
265 
It  appears,  then,  that  whenever  iodine  is  set  free  from  its  com- 
bination with  potassium,  in  presence  of  starch  and  an  animal  or- 
ganic substance,  either  living  or  recently  dead,  it  combines  with 
the  organic  substance  in  preference  to  uniting  with  the  starch. 
The  influence  of  the  animal  fluids  in  preventing  the  mutual 
reaction  of  iodine  and  starch  is,  as  might  be  expected,  not  un- 
limited, but  depends  on  the  relative  quantities  of  the  iodine  and 
the  organic  substance  with  which  it  combines.  If  there  be  but 
little  iodine  present,  it  is  all  taken  up  by  the  organic  substance, 
and  shows  no  reaction  with  starch  ;  while  if  present  in  large 
quantity,  it  attacks  the  starch  also,  and  produces  the  character- 
istic blue  color.  Thus,  one  drachm  of  iodine  water,  mixed  with 
one  drachm  of  gastric  juice  holding  albuminose  in  solution,  has 
no  reaction  on  starch;  but  a  mixture  of  two  drachms  of  the  for- 
mer to  one  drachm  of  the  latter,  strikes  with  starch  a  percepti- 
ble blue  color.  A  single  drop  of  the  alcoholic  tincture  of  iodine, 
mixed  with  one  drachm  of  gastric  juice,  is  sufficient  to  produce 
a  blue  color  with  one  drop  of  the  starch  solution.  Other  animal 
fluids,  however,  appropriate  larger  quantities  of  free  iodine. 
Two  drops  of  tincture  of  iodine,  for  example,  are  completely  sat- 
urated by  one  drachm  of  healthy  urine,  so  that  the  mixture  gives 
no  reaction  with  starch  ;  three  drops  give  only  a  purplish  color, 
while  four  drops  strike  a  deep  blue  color  on  the  addition  of 
starch. 
With  the  adoption  of  proper  precautions,  the  reactions  des- 
cribed above  will  not  expose  us  to  any  serious  error  when  endea- 
voring to  ascertain  the  presence  of  either  starch  or  iodine  in  the 
animal  fluids.  In  testing  for  iodine,  we  have  only  to  add  a  little 
nitric  acid  after  dropping  in  the  starch  ;  and  in  testingfor  starch, 
we  should  be  sure  to  add  iodine  enough  to  more  than  saturate 
the  albuminoid  substances  present.  It  is  at  least  evident,  how- 
ever, from  the  foregoing  facts,  that  iodine  cannot  possibly  exist 
in  a  free  state  in  the  animal  fluids,  but  is  always  in  either  a  me- 
tallic or  an  organic  combination. — Am,  Journ.  Med,  Sci.  April, 
1856. 
