^"^^Jjur.Pharm.j  jTerriG  Citvo-Phosphate  ctncl  Oitro-PyropJiosphate.  167 
that  matter  following  the  line  of  least  resistance  is  deposited  by  the 
composition  and  interference  of  movements  in  characteristic  and  defi- 
nite forms.  Physical  illustrations  of  this  action  are  found  in  the  so- 
called  nodal  lines,  and  the  formation  of  columnar  basalt. 
The  very  pertinent  and  apt  expression  Aq.  was  long  ago  adopted  to 
distinguish  the  hydrous  water  of  the  hydrites  from  that  which  is  more 
firmly  held  by  the  characteristic  part  of  the  compound,  or  the  mole- 
cular nucleus.  The  hydrous  water  itself  is  combined  in  all  degrees  of 
tenacity  from  the  hydrites,  which  lose  the  whole  by  simple  exposure 
to  such  that  part  with  the  last  portions  only  at  a  red  heat,  or  total 
destruction  of  the  compound.  In  the  application  of  these  hydrites  it 
therefore  becomes  necessary  to  employ  definite  forms,  or  convert  them 
into  known  conditions. 
Ferric  citrate  was  long  considered  by  pharmacists  to  be  anhydrous, 
but  an  appropriate  degree  of  heat  diminishes  its  weight  by  ten  per 
cent.,  which  loss  is  not  augmented  by  an  increased  heat,  short  of  the 
production  of  empyreuma.  The  hydrous  water  is  therefore  IJ  Aq. 
The  new  Pharmacopoeia,  however,  erroneously  states  it  to  be  3  Aq. 
Such  a  deviation  from  the  true  formula  must  naturally  vitiate  the  pro- 
ducts into  which  it  enters,  and  this  has  occurred  to  all  the  processes  in 
which  the  citrate  is  employed. 
The  formerly  official  pyrophosphate  of  iron  is  prepared  by  dis- 
solving one  molecule  or  m.  of  ferric  pyrophosphate,  ¥gJ[I*2^7)3>  3 
ms.  of  triammonic  citrate.  The  process  in  all  its  details,  is,  however, 
altogether  too  circumstantial  for  pharmacists  to  undertake.  In  view 
of  this  fact,  the  writer  proposed  {"  American  Journal  of  Pharmacy,^^ 
April,  1876),  an  analogous  compound  to  be  produced  by  means  of  an 
inverse  method  whereby  ferric  citrate  was  made  to  react  upon  amnionic 
pyrophosphate.  The  proposition  also  embodied  the  substitution  of 
sodic  pyrophosphate  in  place  of  the  amnionic  salt.  But  the  nearest 
approach  to  the  then  official  compound  contained  an  additional  ni.  of  tri- 
sodic  citrate ;  that  is,  the  result  was  represented  by  one  m.  of  ferric  pyro- 
phosphate and  4  ms.  of  the  citrate.  This  ^product  was  derived  from 
the  mixture  of  4  ms.  of  ferric  citrate  and  3  ms.  of  sodic  pyrophos- 
phate. But  the  writer  especially  recommended  a  much  simpler  and 
richer  iron  compound,  formed  by  mixing  two  ms.  of  ferric  citrate  and 
one  m.  of  sodic  pyrophosphate.  This  combination  is  closely  analogous 
to  the  official  salt,  and  has  the  strong  probability  in  its  favor  of  being 
a  definite  sodio-ferric  citro-pyrophosphate.  272  grains  of  ferric  citrate 
