PRESERVATION  OF  SYRUP  OF   IODIDE  OF  IRON.  267 
Before  adopting  this  expedient  I  used  to  seal  up  my  stock  in 
six-ounce  bottles,  which  I  completely  filled  with  the  hot  syrup 
and  then  tied  over  with  strong  and  well-soaked  bladder-skin. 
This,  after  several  trials,  I  found  the  best  mode  of  excluding  the 
atmosphere. 
It  is  obviously  necessary  to  observe  one  precaution, — don't 
add  the  acid  to*  the  syrup  before  it  has  completely  cooled.  Not 
observing  this,  I  on  one  occasion  found  a  batch  of  syrup  go 
utterly  wrong  from  the  formation  and  crystallization  of  glucose, 
which,  singularly  enough,  did  not  make  its  appearance  until  the 
bottles  of  syrup  had  been  uncorked  for  a  day  or  two. 
I  was  quite  at  a  loss  to  explain  the  modus  operandi  of  the 
phosphoric  acid,  until  it  occurred  to  me  to  ascertain  how  the 
syrup  so  additioned  would  bear  dilution  with  water,  and  how 
keep  when  so  diluted.  Mere  dilution  did  not  affect  its  trans- 
parency when  the  proportion  of  water  to  syrup  was  as  seven  to 
one  even,  but  on  exposing  the  mixture  in  an  open  vessel  to  the 
air,  I  observed  the  fluid  to  gradually  deposit  a  white  sediment 
of  perphosphate  of  iron,  until  about  half  the  water  had  sponta- 
neously evaporated  ;  then,  and  not  till  then,  did  it  begin  to  as- 
sume the  tint  of  oxidizing  iodide  of  iron. 
The  coincidence  of  the  disappearance  of  the  phosphoric  acid, 
and  the  evolution  of  free  iodine,  seemed  to  prove  the  preservative 
effect  of  the  acid  to  be  due  to  the  fact  that  the  iron,  as  soon  as  it 
arrives  at  the  state  of  peroxidation,  is  at  once  seized  by  the 
phosphoric  acid,  and  the  compound  thus  formed  being  insoluble 
in  any  of  the  fluids  present,  the  reduction  of  the  hydriodic  acid 
by  the  peroxide  is  prevented.  We  have,  of  course,  still  the  free 
hydriodic  acid,  but  in  the  presence  of  syrup  it  is  oxidized  very 
slowly,  in  fact  almost  imperceptibly. 
An  experiment  will  at  once  prove  my  hypothesis  correct.  To 
a  solution  of  any  iodide  add  a  little  persalt  of  iron, — an  immedi- 
ate evolution  of  free  iodine  will  be  the  result,  according  to  the 
following  equation : — 
Fe  I  +  Fe2C]3=  3  Fe  Cl+I. 
That  the  iodine  is  "  free  "  is  proved  by  the  readiness  with 
which  it  is  removed  from  solution  when  shaken  with  ether, — an 
observation  first  made  by  Dr.  Squire,  I  believe. 
