Am  Jour.  Pharm. 
Nov  ,  1877. 
Iron  and  its  Constituents. 
533 
of  iodine.  As  to  the  carbonaceous  residue,  it  may  be  stated  that  it 
contains  chemically  combined  iodine,  and  was  found  by  Eggertz  to 
have  the  following  composition,  when  dried  at  2I2°F. 
Carbon,  C,  .  .  .  .  .  .      59*69  per  cent. 
Water,  H20,       .  .  .  .  .  22*50 
Iodine,  I,  .  .  .  .  .      16  00 
leaving,  on  ignition,  some  siliceous  ash. 
It  is  really  interesting  to  observe  what  an  important  influence  carbon 
exerts  upon  the  metal  iron,  the  commercial  brands  of  which  should  be, 
scientifically,  called  carburets  of  iron.  "  Spiegeleisen,"  a  white  crys- 
tallized and  highly  manganiferous  cast  iron,  which  is  used  in  large 
quantities  in  the  Bessemer  process,  corresponds  to  the  formula  Fe4Cc  ; 
it  contains  its  total  carbon  (about  5  per  cent.)  in  combined  condition. 
In  grey  pig,  the  larger  portion  of  this  element  exists  in  the  graphitic 
modification,  sometimes  approximately  corresponding  to  the  formula 
Fe8Cc  -f-  Cgr.  Among  the  numerous  interesting  phenomena  which 
these  combinations  exhibit,  I  cannot  help  to  mention  one  which, 
although  being  very  frequently  observed,  still  lacks  a  sufficient  explanation 
of  its  causes.  I  refer  to  the  process  of  hardening  steel,  as  practised 
daily  by  every  blacksmith.  If  a  piece  of  steel  at  red  heat  is  dipped  into 
cold  water  an  entire  change  of  its  structure  takes  place ;  its  grain  becomes 
finer  and  denser,  its  tensile  strength  almost  double  to  what  it  was 
before,  while  its  hardness  nearly  reaches  that  of  diamond.  Acids  will 
hardly  attack  it  in  this  state,  no  edge  tool  will  produce  an  impression 
on  the  bar,  which  before  being  subjected  to  this  simple  treatment, 
easily  could  be  drilled  or  filed.  These  facts  become  still  more  inter- 
esting if  we  know  that  at  the  same  time  the  volume  of  the  bar  has 
become  larger  and  its  specific  gravity  decreased.  Salt  water  or  mercury 
will  produce  a  still  higher  degree  of  hardness  and  a  larger  expansion  of 
the  hardened  bar,  while  a  soap  solution  has  no  hardening  effect  on  steel. 
None  of  these  phenomena  will  take  place  in  iron  free  from  carbon, 
while  hardness  and  tensile  strength  will  proportionately  increase  with 
the  latter,  reaching  their  "practical  maximum"  at  1*2  per  cent,  com- 
bined C.  This  can  by  no  means  be  called  a  mere  physical  change, 
but  seems  to  be  the  result  of  a  chemical  reaction  between  the  iron  and  its 
other  constituents.  I  have  previously  mentioned  that  on  dissolving  iron 
in  hydrochloric  acid  the  combined  carbon  is  carried  off  as  carburetted 
hydrogen.    If,  for  instance,  the  gases  that  form,  on  treatment  of 
