^"^Nov.'i'S^^™'}    Permanent  Syrup  of  Ferrous  Iodide.  549 
water  bath  for  the  solution  of  the  gkicose  is  ordered,  in  view  of  the 
facility  with  which  solutions  of  solid  glucose,  when  strongly  heated, 
become  yellow. 
The  chemical  decomposition  which  syrup  of  ferrous  iodide  under- 
goes, on  exposure  to  air,  has  always  been  a  matter  of  great  interest  and 
the  subject  of  repeated  studies.  Prof.  J.  M.  Maisch  most  admirably 
describes  them  when  he  states^  that  ^'  on  exposure  to  air  the  color  of  the 
syrup  slowly  changes  to  yellow  and  afterward  to  brown,  the  change 
of  color  proceeding  from  the  exposed  surface  downward.  Diffused 
daylight  seems  to  somewhat  accelerate  the  decomposition,  but  exposure 
to  direct  sunlight  entirely  prevents  the  change,  or,  if  it  has  taken  place, 
restores  the  original  color,  and  finally  renders  the  syrup  colorless. 
The  effects  of  oxidation  become  manifest  first  by  the  production  of  a 
ferric  compound,  and  soon  afterward  by  the  liberation  of  iodine,  recog- 
nized by  the  blue  color  produced  with  starch  paste;  subsequently 
hydrate  of  iron  containing  variable  quantities  of  iodine  is  precipitated 
and  after  a  short  time  this  precipitate  is  not  wholly  redissolved  again 
under  the  influence  of  sunlight,  though  the  solution  becomes  colorless, 
perhaps  from  the  formation  of  hydriodic  acid." 
^N^ow,  what  do  these  facts  indicate  ?  They  show  us  that  the  chemi- 
cal change  is  essentially  one  of  oxidation,  with  the  formation,  first,  of 
a  ferric  compound,  and  a  yellow  or  brown  color,  through  the  libera- 
tion of  iodine ;  and,  secondly,  the  precipitation  of  ferric  hydrate  and 
iodine. 
If  the  oxidized  iron  compound  is  a  ferric  one,  and  that  is  admitted, 
it  cannot  be  normal  ferric  iodide  (FeaTg),  if  such  a  compound  does 
exist,  which  is  disputed,  since  that  compound,  for  its  formation,  would 
require  more  iodine  than  could  be  obtained  from  two  molecules  of  fer- 
rous iodide  (Fel2),  and  hence  if  a  ferric  salt  is  formed,  it  seems  prob- 
able that  it  is  an  oxysalt ;  that  is,  ferric  iodide,  whose  iodine  atoms 
have  been  partially  replaced  by  oxygen  atoms. 
The  change  in  color  to  a  yellow  or  a  brown,  through  the  production 
of  free  iodine,  most  probably  indicates  the  formation  of  hydriodic 
acid  whose  presence  was  first  claimed  by  Mr.  Richard  Phillips,  Jr.,^ 
with  its  subsequent  decomposition  by  the  air  into  free  iodine  and 
water. 
That  there  is  an  acid  formed  prior  to  the  liberation  of  iodine  is  evi- 
1  National  Dispensatory  (3d  Ed.)  p.  1473. 
2  U.  S.  Dispensatory  (15th  Edt.  j  p.  673. 
