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AmNo°vU,rx8P7h7?rm*}  Iron  and  its  Constituents.  531 
portion  is  left  in  the  black  insoluble  residue.  Iron  phosphide  is  simi- 
larly decomposed,  particularly  on  heating  the  solution,  forming  phos- 
phoretted  hydrogen.  Silicon  partially  may  undergo  the  same  reaction, 
the  larger  quantity  of  this  element,  however,  will  be  found  in  the  black 
carbonaceous  residue.  Sulphur,  if  not  combined  with  copper  and 
arsenic,  will  be  entirely  eliminated  as  gaseous  combinations,  while 
manganese  and  iron  remain  in  solution  as  chlorides.  If  copper  is 
present  among  the  impurities  of  iron,  it  will  combine  with  the  sulphur, 
forming  copper  sulphide,  which  will  be  found  in  the  insoluble  residue. 
Silicic  acid  from  slag  particles,  pre-existing  in  the  material  used,  may 
be  detected  in  minute  quantities,  on  oxidation  and  evaporation  of 
the  resulting  solution  to  dryness,  etc.  If  steel  has  been  used,  slag 
particles  are  absent.  It  is  evident  that  from  a  pure  iron  these  impuri- 
ties are  of  no  significance  j  when,  however,  pig  iron  or  other  impure 
brands  are  used,  they  may  cause  precipitates  in  a  concentrated  solution. 
Drillings  of  soft  steel,  containing  99  per  cent,  of  iron  (Fe),  combine 
at  the  same  time  convenient  shape  with  the  highest  practical  purity. 
I  have  often  had  samples  which  dissolved  perfectly  in  dilute  hydro- 
chloric acid  without  the  application  of  heat.  The  product  of  this 
reaction  is  an  aqueous  solution  of  ferrous  chloride,  Fe2Cl4,  which  is 
filtered,  and  finally  converted  into  ferric  chloride,  Fe2Cl6,  by  the  addi- 
tion of  the  necessary  quantities  of  hydrochloric  and  nitric  acids,  when 
the  following  exchange  of  molecules  will  take  place  :  Fe2Cl4-f-2HCl 
+ 2  H  N  03=Fe2Cl6+ 2N  Oa+  2  H20 . 
The  action  of  sulphuric  acid  on  iron  is  similar  to  the  foregoing,  most 
of  the  impurities  being  carried  ofF  as  gaseous  combinations,  while  the 
graphitic  carbon  is  left  as  a  black  insoluble  residue,  together  with  some 
silicon.  Highly  concentrated  sulphuric  acid  hardly  acts  on  iron  in  the 
cold  ;  but  on  heating  sulphurous  anhydrid,  S02  is  formed,  while  the 
dilute  acid  rapidly  dissolves  it  to  ferrous  sulphate,  liberating  hydrogen. 
The  behavior  of  iron  to  nitric  acid  is  essentially  dependent  on  the 
concentration  of  the  latter.  Highly  concentrated  aqua  fortis  converts 
iron  into  it.s  passive  state,  thus  preventing  any  further  reaction.  When 
in  this  peculiar  modification  it  will  neither  be  acted  on  by  weaker  acid, 
unless  touched  with  another  clean  piece  of  metallic  iron,  when  lique- 
faction and  evolution  of  gas  will  begin  at  once.  Medium  strong 
acid,  of  about  1*25  specific  gravity,  forms  ferric  nitrate,  Fe2N6018,  on 
evolution  of  nitric  oxide,  NO,  which,  in  contact  with  atmospheric 
