292  ON  ALKALINE  IODIDES  AND  BROMIDES, 
"When  instead  of  using  a  solution  of  the  carbonated  alkali,  the 
latter  is  added  to  the  water  in  crystals  of  the  bicarbonate  of 
potassa  or  carbonate  of  soda,  the  reaction  takes  place  as  before ; 
but  the  heat  evolved  by  the  combination  of  the  iodine  and  iron 
will  be  reabsorbed  for  the  solution  of  the  crystals,  and  thus 
there  will  be  no  great  increase  of  temperature,  if  any. 
The  action  of  ferric  iodide  on  carbonate  of  lime  and  of  lithia 
is  the  same  as  the  other  fixed  alkaline  carbonates,  while  that  of 
ferrous  iodide  is  but  slow  and  incomplete.  For  the  formation 
of  iodide  of  calcium  a  fourth  equivalent  of  iodine  may  be  added 
in  the  form  of  ferrous  iodide,  and  a  fourth  equivalent  of  alkali 
in  the  form  of  milk  of  lime.  In  the  case  of  salt  of  lithium  the 
proportion  must  remain  three  equivalents  of  carbonate  of  lithia, 
three  of  iodine,  and  two  of  iron,  the  change  of  the  ferric  oxyd  to 
be  brought  about  afterward  by  boiling  with  iron  filings. 
As  it  is  understood  that  the  new  Pharmacopoeia  has  adopted 
the  bicarbonate  of  potassa  for  all  such  purposes,  I  have  intro- 
duced it  into  the  formulas ;  but  in  its  place  the  purified  carbon- 
ate can  be  used  on  a  larger  scale,  or  the  sulphate  of  potassa,  to 
precipitate  the  iodide  of  calcium  in  the  same  manner  as  directed 
by  Liebig.  For  iodide  of  sodium  we  require  nothing  better  than 
the  crystallized  carbonate  of  soda. 
Iodide  of  Potassium. 
FIRST  METHOD. 
Take  of  Bicarbonate  of  Potassa       400  parts. 
Iodine  508  « 
Iron  in  filings  112  " 
Water  a  sufficient  quantity. 
Place  the  bicarbonate  and  iodine  with  three  times  their  weight 
of  water  into  a  flask  capable  of  holding  twice  the  quantity ;  then 
add  one  half  of  the  iron  in  portions,  and  when  all  the  salt  is  dis- 
solved, add  the  remainder  of  the  iron,  shake  up,  and  boil  until 
the  precipitate  has  become  dense.  Then  test  whether  the  solu- 
tion be  free  from  iron  by  applying  a  drop  of  it  to  the  uppermost 
of  two  small  slips  of  filtering  paper,  one  placed  above  the  other, 
and  touch  what  has  filtered  on  the  lower  slip  with  a  drop  of  so- 
lution of  ferrocyanide  of  potassium.  If  this  produce  a  blue 
stain,  a  little  carbonate  or  bicarbonate  of  potassa  must  be  added 
