ON  PULVIS  FERRI — IRON  BY  HYDROGEN. 
219 
Hence  there  is  no  reason  why  imperfectly  reduced  iron  should  be 
sold  except  either  through  ignorance  of  the  process,  carelessness 
or  fraud.  Mr.  Heathfield,  therefore,  in  alleging  the  action  of 
hydrogen  in  a  hot  tube  as  a  reason  for  purity,  is  only  condition- 
ally correct. 
It  sometimes  happens  in  operating  in  a  4  inch  tube,  that  the 
interior  particles  in  a  part  of  the  oxide  are  black,  and  the  exterior 
iron  grey.  Now  in  reference  to  color,  it  may  be  said  that  no 
metallic  iron  is  perfectly  black ;  but  when  the  oxide  is  reduced 
at  the  lowest  heat  possible,  so  that  its  particles  do  not  con- 
tract or  weld  together  by  excess  of  heat,  after  reduction,  it  is 
of  a  dark  iron  grey  hue,  and  will  dissolve,  with  rapid  effer- 
vescence of  hydrogen  in  dilute  sulphuric,  acid  like  a  carbonate ; 
when,  however,  the  heat  has  been  allowed  to  become  cherry  red, 
after  the  reduction,  the  particles  contract  more  and  more,  until, 
if  the  heat  has  been  continued  long  enough,  a  metallic  mass  is 
obtained,  difficult  to  powder,  and  requiring  the  avoidance  of  blows 
of  the  pestle  to  prevent  its  condensation  into  solid  shining  pebble- 
like masses.  The  color  of  over-heated  iron  is  light  iron  grey, 
with  a  peculiar  lustrous  appearance,  derived  from  the  trituration 
necessary  to  pulverize  it. 
Now  when  the  operator  opens  his  tube  and  finds  that  he  has 
but  a  partial  product,  and  that  the  interior  part  of  the  oxide  is 
not  thoroughly  reduced,  he  is  tempted  to  overlook  the  presence 
of  the  latter,  and  convert  all  to  powder.  This  may  be  occasional 
and  accidental,  or  habitual,  according  to  the  conscientiousness  or 
knowledge  of  the  manufacturer,  and  whilst  this  is  the  case,  iron 
by  hydrogen  will  constantly  vary  in  composition ;  we  see  by  the 
analyses  of  Dr.  Wilson  admitted  by  Mr.  Heathfield,  that  his  own 
preparation  varies  from  2  to  22  per  cent  of  foreign  matter. 
Undoubtedly,  the  best  condition  of  this  preparation,  when  re- 
moved from  the  tube,  is  that  of  a  light  spongy  mass,  like  light 
carbonate  of  magnesia,  yet  more  compressible,  and  of  a  dull, 
rather  dark  iron  grey  color.  When  a  fragment  is  struck  on  a 
bright  anvil  with  a  smooth  hammer,  a  thin  brilliant  lamina  of 
metal  is  obtained,  and  when  the  powder  is  strongly  pressed  with 
a  bright  spatula,  with  traction,  the  eompressed  surface  exhibits  a 
metallic  lustre. 
It  may  be  well  to  say  a  word  in  reference  to  the  sulphur 
