DECOMPOSITION  OF  IODIDE  OF  STARCH,  ETC. 
263 
blood)  be  added  to  gii  of  iodide  of  starch,  the  mixture  loses  its 
blue  color  immediately.     It  is  brought  back  by  nitric  acid. 
6  If  gi  of  healthy  human  urine  be  added  to  ,5i  of  iodide  of 
starch,  the  mixture  is  decolorized  in  five  seconds.  The  blue  co- 
lor is  restored  by  nitric  acid. 
The  above  e fleet  is  produced  whether  the  urine  be  acid  or 
slightly  alkaline  in  reaction. 
^i.  of  urine  mixed  with  of  iodine  water  effectually  prevents 
the  reaction  between  the  iodine  and  starch. 
7.  The  gastric  juiee  acts  differently,  in  this  respect,  accord- 
ing to  circumstances.  If  taken  from  the  stomach  of  the  fasting 
animal  (by  irritation,  through  a  gastric  fistula,  with  a  metallic 
catheter),  it  is  clear  and  colorless,  and  does  not  interfere  at  all 
with  the  reaction  of  starch  and  iodine.  But  that  which  is  col- 
lected during  the  first  half  hour  after  feeding  the  animal  on 
cooked  meat,  and  which  is  slightly  yellowish  in  color,  and  con- 
tains a  little  albuminose  in  solution,  interferes  with  the  reaction 
very  perfectly.  3i  of  such  gastric  juice  decolorizes  an  equal 
quantity  of  iodide  of  starch  in  lesss  than  a  minute. 
A  specimen  of  Buch  gastric  juice,  however,  containing  a  little 
albuminose,  which  had  been  kept  for  some  months,  was  found  to 
have  lost  almost  entirely  its  power  of  preventing  the  union  be- 
tween starch  and  iodine. 
If  the  colorless  gastric  juice,  taken  from  the  fasting  animal, 
which  has  no  effect  on  the  iodide  of  starch,  be  digested  for  an 
hour  at  the  temperature  of  I00p  E.,  with  finely  cut  boiled  meat, 
and  then  filtered,  the  filtered  fluid  interferes  with  the  reaction 
of  iodine  and  starch,  like  the  other  animal  fiuids. 
The  iodine,  in  these  cases,  probably  combines,  as  already  inti- 
mated, directly  with  the  organic  (albuminoid)  matters  of  the  an. 
imal  fluids,  and  is  detected  by  starch  only  after  these  organic 
matters  have  been  destroyed  by  nitric  acid.  It  is  not,  however, 
easy  to  demonstrate  this.  If  the  serum  of  the  blood,  for  exam- 
ple, be  boiled  with  an  excess  of  sulphate  of  soda,  by  which  all 
of  the  albumen,  and  most  of  the  other  organic  substances  are  co- 
agulated, the  clear  filtered  fluid  interferes  with  the  reaction  of 
starch  and  iodine  as  before.  It  might  be  supposed  that  the  io- 
dine, on  the  contrary,  was  converted  into  an  iodide  of  potassium 
or  sodium,  under  the  influence  of  alkaline  carbonates,  and  that 
