SOLUTION  OF  PYROPHOSPHATE  OF  SODA  AND  IRON.  389 
acid  thus  obtained  the  name  of  pyrophosphoric  acid.  Stromeyer* 
proved  that  the  two  phosphoric  acids  have  the  same  composition, 
and  that  the  precipitates  by  pyrophosphate  of  soda  in  solutions 
of  the  metallic  oxides  are  redissolved  by  an  excess  of  the  pre- 
cipitant ;  exceptions  are  mercuric  and  chromic  oxides,  baryta, 
strontia  and  lime,  which  are  soluble  only  in  small  proportions. 
Stromeyer  observed  also  that  by  heating  with  nitric  acid,  also 
with  sulphuric,  muriatic,  acetic  and  ordinary  phosphoric  acids, 
the  pyrophosphoric  acid  and  pyrophosphate  of  soda  are  converted 
into  the  ordinary  acid  and  phosphate. 
Graham f  published,  in  1833,  the  results  of  his  beautiful  in- 
vestigations on  the  constitution  of  phosphoric  acid  and  the 
phosphates,  and  added  a  third  one — metaphosphoric  acid — to 
those  previously  known. 
BoussingaultJ  found  that  insoluble  phosphates  cannot  be  con- 
verted into  pyrophosphates  by  heating  to  redness. 
Peligot§  analyzed  recently  prepared  glacial  phosphoric  acid, 
and  found  it  to  be  the  monohydrate  ;  after  it  had  partly  liquefied 
by  absorption  of  moisture  and  crystallized  again,  the  lower  layer 
of  crystals  consisted,  of  the  deutohydrate,  and  the  upper  layer  of 
the  terhydrate. 
Persoz||  observed  that  the  solutions  of  several  double  salts  of 
pyrophosphates  possess  a  peculiar  behaviour  towards  various  re- 
agents, which  are  useful  for  detecting  metallic  oxides,  but  do  not 
disturb  their  solution  in  the  form  of  a  double  pyrophosphate.  It 
occurred  to  him  that  a  soluble  double  pyrophosphate  of  iron 
might  be  a  valuable  curative  agent,  for  obvious  reasons  ;  and  he 
prepared  a  solution  of  the  ferric  soda  salt  by  converting  32*5 
grm.  crystallized  protosulphate  of  iron  into  the  tersulphate, 
adding  enough  water  to  make  one  litre  of  solution,  and  mixing 
this  with  one  litre  of  solution  of  crystallized  pyrophosphate  of 
soda  prepared  from  110  grammes  with  sufficient  water.  This 
solution  contains  in  each  litre  3"27  grammes  of  iron  —  4°68  grm. 
*  Getting,  gelelirte  Anzeigen  1830,  No.  12. 
f  Philosoph.  Transact.  1833,  ii.  253. 
%  Ann.  de  Chim.  et  de  Ph.  1834,  fevr.  185. 
\  Ann.  de  Chim.  et  de  Ph.  lxxiii.  286. 
U  Ibid,  ixviii.  163. 
