OBSERVATIONS  ON  FERRIC  HYDRATE. 
455 
land  the  ferric  citrates  and  tartrates  used  in  medicine  are  suc- 
cessfully made  in  large  quantities  by  dissolving  ferric  hydrate, 
prepared  from  ferric  sulphate,  in  solutions  of  the  respective  acids 
and  acid-salts.  Secondly,  I  have  frequently  seen  moist  ferric 
hydrate  perfectly  dissolve  in  solutions  of  acids  or  acid-salts,  even 
though  the  precipitate  has  been  washed  with  common  water  con- 
taining sulphate  of  calcium, — a  final  washing  with  distilled  water 
having,  for  various  reasons,  been  neglected.  Thirdly,  I  have 
often  noticed  that  pure  ferric  hydrate,  soluble  when  freshly  pre- 
cipitated, becomes  imperfectly  so  if  long  kept,  moist  or  dry.  It 
is  true  that  when  alkali  is  added  to  solution  of  ferric  sulphate, 
instead  of  the  latter  to  the  former,  an  insoluble  oxysulphate  is 
precipitated,  and  a  similar  compound  may,  possibly,  be  formed 
under  other  circumstances  ;  but  ferric  hydrate,  properly  prepared 
and  fairly  washed,  is  readily  soluble  if  only  it  be  used  in  the 
moist  and  recently-precipitated  condition,  with  a  solution  of  acid 
or  acid-salt  which  is  not  too  weak,  and  the  mixture  be  not  boiled 
or  even  strongly  heated  for  any  considerable  length  of  time.  The 
fact  is  that  ferric  hydrate,  even  though  kept  under  water,  de- 
composes after  a-  time,  or  more  quickly  if  heated,  losing  the 
elements  of  water  and  becoming  an  oxyhydrate,  a  body  insoluble 
in  weak  acids,  and,  also  unlike  ferric  hydrate,  incapable  of  act- 
ing as  an  antidote  to  arsenic,  that  is,  incapable  of  forming  ferrous 
arseniate. 
It  may  be  useful  again  to  draw  attention  to  the  decided  altera- 
tion in  properties  which  ferric  hydrate  spontaneously  undergoes 
when  exposed  beneath  the  surface  of  water*  or  when  boiled  with 
water,  as  evidence  that  this  substance  (Fe26HO)  is  a  true  ana- 
logue of  hydrate  of  sodium  (NaHO),  &c,  and  not  a  hydrous  ferric 
oxide  (Fe203,3H20).  It  is  more  reasonable  to  suppose  that  in 
acquiring  new  properties  ferric  hydrate  becomes  changed  to  new 
compounds  than  to  consider  that  the  changes  result  from  the  loss 
of  a  portion  of  water  already  existing  as  water.  Between  ferric 
hydrate  (Fe26HO)  and  ferric  oxide  (Fe2Os)  there  would  appear  to 
be  several  oxy hydrates,  analyses,  &c,  of  most  of  which  have  al- 
*  [This  fact  was  probably  first  noticed  in  April,  1842,  in  the  American 
Journal  of  Pharmacy,  by  the  Editor,  and  afterwards  by  Wittstein  {Buclx. 
Repert.  xliii,  366.)] 
