AmAp°r^x876a.rm'}  So-Called  Tasteless  Iron  Compounds.  173 
iodine  made  its  appearance  in  the  diffusate,  the  writer  undertook  the 
experiment.  The  compound  was  prepared,  but  it  was  observed  that, 
owing  to  the  carbonaceous  character  of  the  iron  filings,  a  small  quan- 
tity of  iodine  had  been  converted  into  iodhydric  acid,  which  caused 
effervescence  long  after  the  iodine  color  had  disappeared.  By  now 
adding  the  remaining  third  of  the  iodine,  a  liquid  resulted  which,  by 
reason  of  part  of  its  iodine  being  united  to  hydrogen,  instead  of  iron 
yielded  a  mixture  of  ferrous  iodide,  free  iodine  and  iodhydric  acid,  con- 
taining less  iron  than  is  required  to  form  Fe2I6,  by  an  amount  equiv- 
alent to  the  iodhydric  acid  that  was  formed.  On  now  treating  this 
solution  with  potassium  nitrate,  that  part  of  the  iodine  only  was  con- 
verted into  potassium  iodide  which  corresponded  to  Fe2I6,  the  rest 
remaining  free.  Hence,  if  such  a  solution  is  dialysed,  free  iodine  must 
necessarily  appear  in  the  diffusate.  Before  dialysing  this  liquid  the 
writer  concentrated  it  by  evaporation,  and  thus  got  rid  of  the  free 
iodine.  The  solution,  on  being  treated  like  that  of  the  chloride, 
yielded,  after  a  dialysis  of  four  hours,  a  diffusate  faintly  yellow  from  a 
trace  of  iron,  but  containing  no  free  iodine.  This  liquid,  on  evapora- 
tion, gave  a  white  crystalline  residue  of  potassium  iodide,  representing 
nearly  f  of  the  iodine  originally  used.  The  second  diffusate  rendered 
up  the  remaining  complement. 
The  officinal  ferric  pyrophosphate  was  next  tried,  but,  although  a 
yellowish  diffusate  appeared,  no  decided  amount  of  residue  was  ob- 
tained ;  accordingly  it  must  be  inferred  that  no  crystalloid  is  present, 
and  therefore  dialysis  cannot  effect  a  separation. 
The  ammonio-citroferric  orthophosphate  was  then  prepared  ("  Phar- 
macist," August,  1 871),  and  a  concentrated  solution  precipitated  by 
alcohol.  The  filtrate  contained  a  trace  of  iron,  and  on  evaporation 
yielded  quite  an  appreciable  syrupy  residue,  which,  on  the  addition  of 
ammonia  in  excess,  congealed  to  a  crystalline  mass  of  triammonic 
phosphate.  Dialysis  of  another  portion  of  the  concentrated  solution 
gave  a  yellow  diffusate,  from  which  a  small  amount  of  saline  residue 
was  obtained.  This  result  also  points  to  a  colloidal  condition  of  the 
mixed  compounds. 
The  writer  in  his  paper  on  Ferric  Iodide  ("  Laboratory,"  February, 
1876)  proposed  to  show  that  the  officinal  ferric  pyrophosphate  could 
be  more  conveniently  and  economically  prepared  by  mixing  certain 
proportions  of  ferric  citrate  and  ammonium  pyrophosphate.     On  the 
