AmkJP°Xx876arm  }  So-Called  Tasteless  Iron  Compounds.  177 
citric  acid  or  an  acid  monad  citrate.  Mixtures  of  ferric  chloride,  sul- 
phate, iodide,  citrate,  etc.,  with  sufficient  monad  citrate  acquire  a  uni- 
form green  color.  The  officinal  ferric  pyrophosphate  has  a  green  color 
of  a  different  shade,  due  to  an  excess  of  ferric  citrate  ;  addition  of 
triammonic  citrate  changes  it  to  the  green  of  the  other  mixtures. 
Ammonio-citroferric  orthophosphate  is'green  only  by  reflected  light  and 
red  (derived  from  the  presence  of  free*ferric  hydrate,)  by  transmitted  ; 
addition  of  either  di-  or  monammonic  citrate  causes  a  change  to  the 
green  of  the  other  mixtures. 
If  ferrous  salts  have  a  marked  affinity  for  nitrogen  dioxide,  much 
more  have  the  ferric  salts  for  nitrogen  trioxide  and  tetroxide.  Ferric 
salts  retain  these  gases  so  firmly  that  prolonged  boiling  of  their  solutions 
fails  to  expel  them,  and  even  the  dried  compounds  must  be  maintained 
at  a  comparatively  high  temperature  Jfor  a  considerable  time  before 
complete  separation  is  effected.  In  consideration  of  this  fact  the  use 
of  potassium  chlorate  in  place  of  nitric  acid  is  greatly  preferable. 
Furthermore,  the  writer  has  ascertained  the  important  fact  ("  Phar- 
macist," October,  1872,)  that  there  is  a  chemical  difference  between 
ferric  salts  produced  by  nitric  acid,  and  those  generated  by  means  of 
potassium  chlorate. 
From  the  chemical  and  physical  differences  of  the  ammonio-ferric 
sulphates,  obtained  from  the  two  modifications  of  ferric  sulphate,  the 
writer  infers  that  through  the  action  of  nitric  acid,  probably  aided  by 
heat,  the  constitution  of  the  ferric  molecule  itself  is  altered.  For  this 
reason  the  writer  proposes  to  designate  the  normal  condition  of  mole- 
cule as  orthoferric,  and  that  in  the  modified  state  as  metaferric.  All 
the  orthoferric  salts  are  distinguished  by  their  lighter  colors.  Ammo- 
nio-orthoferric  sulphate  is  beautifully  yellow  and  of  greater  stability  than 
the  ammonio-metaferric  sulphate,  which  has  a  violet  color.  Formerly, 
tincture  of  ferric  chloride  was  prepared  from  a  ferric  hydrate,  obtained 
by  exposing  ferrous  carbonate  to  the  air  until  the  conversion  was 
effected.  The  reputation  of  tincture  of  ferric  chloride  was  built  upon 
this  preparation,  which  had  not  sustained  the  action  of  nitric  acid  or 
heat.  Neither  did  this  tincture  possess  the  etherial  odor  which 
characterizes  the  new  officinal  tincture,  in  which  the  presence  of 
nitrogen  oxides  generates  ethyl  nitrite,  upon  which  this  odor  depends. 
Tincture  of  ferric  chloride,  recently  prepared  and  treated  with  potas- 
sium citrate  so  as  to  form  the  so-called  tasteless  tincture,  will  also 
12 
