550  Permanent  Syrup  of  Ferrous  Iodide.  {^^'-iov^'Sl^^'^' 
denced  by  the  fact  that  the  officinal  syrup,  although  neutral  in  reaction 
when  first  made,  gives,  in  the  writer's  experience,  after  a  time  an  acid 
reaction  with  litmus,  even  when  colorless. 
There  is  no  other  iodine-derived  acid  which  rapidly  decomposes 
with  the  liberation  of  iodine  than  hydriodic  acid,  and  it  seems  proba- 
ble that  the  following  chemical  change  may  be  the  primary  one : 
2Fe^a2+0+H20=:Fe^^^JA+2HI. 
In  other  words,  ferrous  iodide,  in  the  presence  of  oxygen  and  wa- 
ter, is  decomposed  to  yield,  not  ferric  iodide,  Fcole,  since  that  is  im- 
possible, but  ferric  iodide,  four  of  whose  univalent  iodine  atoms  have 
been  replaced  by  two  bivalent  oxygen  ones,  to  form  an  oxyiodide, 
Fe2l202,  and  then  hydriodic  acid,  through  a  combination  of  the  dis- 
placed iodine  with  the  freed  hydrogen  atoms  of  the  water.  The  prob- 
ability of  the  existence  of  this  oxyiodide  is  strengthened  in  the  fact 
that  Pettenkofer^  has  obtained  an  oxy chloride  with  ferric  chloride  and 
ferric  hydrate,  having  an  analogous  formula,  FeaClgOa. 
It  is  admitted  that  in  the  absence  of  its  quantitative  estimation,  as 
a  distinct  chemical  compound,  the  existence  of  this  ferric  oxyiodide 
(Fe2l202)  is  purely  theoretical.  In  view  of  the  fact  that  the  oxidation 
is  a  gradual  one,  the  syrup  being  always,  after  oxidation  commences,  a 
variable  mixture  of  ferrous  iodide,  a  ferric  oxysalt,  free  hydriodic  acid, 
and  iodine  from  decomposed  hydriodic  acid,  it  seems  impossible  to  say 
just  when  all  the  ferrous  salt  has  been  oxydized  into  this  ferric  oxy- 
salt if  it  all  has  been,  prior  to  the  formation  of  ferric  hydrate,  which 
is  doubtful ;  and  it  seems  impossible,  then,  to  frame  a  method 
whereby  its  formula  can  be  determined. 
We  know  that,  primarily,  there  is  formed  a  free  acid,  which  most 
probably  is  hydriodic  acid,  and  that  the  formation  of  this  acid  is  co-in- 
cident with  the  production  of  this  ferric  oxysalt,  but  we  cannot  assert 
that  this  oxysalt  is  Fe2T202  until  it  is  proven  such.  It  may  be  that 
there  are  possible  by  substitution  only  two  ferric  oxyiodides, 
Fe2l40  and  Fe2l202.  In  that  case  the  formation  of  the  first  one  in 
this  instance  could  not  take  place,  because  we  know  that  the  forma- 
tion of  the  hydriodic  acid  is,  as  before  mentioned,  coincident  with  the 
production  of  the  ferric  oxysalt,  and  we  would  then  be  compelled  to 
receive  as  more  probable  the  other,  Fe2l202,  with  the  assertion  that 
if,  in  order  to  form  the  hydriodic  acid,  we  must  have  sufficient  iodine 
1  Gmelin-Kraut,  "  Anorganische  Chemie,"  vol.  3,  p.  360. 
