ON  ALKALINE  IODIDES  AND  BROMIDES. 
291 
takes  place  in  an  alkaline  solution  of  a  hypo-iodite,  or  iodate, 
but  little  hydrogen  is  set  free  until  the  oxysalts  have  been  re- 
duced to  iodides. 
On  adding  three  equivalents  of  iodine  to  two  of  metallic  iron 
under  water,  they  form  a  brown  solution  of  ferric  iodide.  That 
the  third  equivalent  of  iodine  is  held  in  chemical  combination  is 
proved  by  the  fact,  that  on  the  addition  of  a  caustic  or  carbon- 
ated alkali  to  this  solution  no  hypo-iodous  acid  is  formed,  but  a 
precipitation  of  ferric  oxyd  takes  place,  exactly  as  when  adding 
a  solution  of  alkali  to  one  of  ferric  chloride. 
When  to  this  precipitated  ferric  oxyd  more  alkali  is  added, 
and  then  a  solution  of  ferrous  salt  or  iodide,  ferrous  oxyd  is  pre- 
cipitated and  forms  with  the  other  the  black  oxyd.  If  lime  was 
the  alkali,  ferrous  sulphate  will  form  sulphate  and  iodide  of  cal- 
cium with  magnetic  oxyd. 
If  instead  of  adding  a  ferrous  salt,  the  brown  mixture  is 
heated  with  excess  of  iron  filings,  hydrogen  gas  is  evolved  and 
black  oxyd  is  formed. 
It  has  been  shown  above  that  the  oxyacids  of  iodine  are  re- 
duced during  this  reaction.  When  therefore  we  mix  four  equiva- 
lents of  iodine  with  two  of  metallic  iron  under  water,  and  then 
add  four  equivalents  of  caustic  alkali,  or  part  carbonated  and 
part  caustic  alkali,  there  results  a  mixture  of  ferric  oxyd  and 
hypo-iodite  of  the  alkali,  and  when  this  is  heated  with  iron  filings, 
the  result  is  simply  magnetic  oxyd  and  alkaline  iodide. 
The  action  of  iodine  on  iron  and  water  also  takes  place  in  a 
solution,  or  in  the  presence  of  a  carbonated  alkali.  The  iodide  of 
iron  as  it  forms  is  at  once  decomposed  by  the  carbonate;  car- 
bonic acid  escapes,  and  oxyd  of  iron  and  iodide  of  the  alkaline 
metal  remain. 
By  adding  three  equivalents  of  iodine  to  two  equivalents  of  iron 
contained  in  a  solution  of  four  equivalents  of  carbonate  of  the 
alkali,  and  afterwards  a  solution  of  an  equivalent  of  ferrous  iodide, 
the  products  are  black  oxyd,  iodide  of  the  alkaline  metal,  and 
carbonic  acid,  which  is  given  off. 
By  adding  four  equivalents  of  iodine  to  two  of  iron  contained 
in  a  solution  of  four  equivalents  of  carbonated  alkali,  and  after- 
wards two  equivalents  or  more  of  iron,  the  result  is  the  same, 
but  with  this  advantage,  that  the  black  oxyd  is  less  hydrated 
and  becomes  denser  on  boiling. 
