218 
ON  PULVIS  EERRI — IRON  BY  HYDROGEN. 
as  sold  in  Edinburgh,  and  had  it  analysed  by  Dr.  Gregory,  Dr. 
Stenhouse,  Prof.  Williamson  and  Dr.  Garrod,  all  of  whom  pro- 
nounced it  to  be  magnetic  oxide  of  iron,  and  published  the  whole 
affair  in  the  Pharmaceutical  Journal  of  January.  In  the  next 
number,  Mr.  Heathfield  pursues  the  subject  by  bringing  forward 
microscopical  evidence,  and  vindicates  the  black  iron  from  the 
charge  of  impurity,  attributing  its  color  to  minuteness  of  division, 
rather  than  to  magnetic  oxide,  and  endeavors  to  throw  doubt  on 
the  correctness  of  the  four  analyses  obtained  by  Mr.  Morson.  And 
finally  Dr.  George  Wilson  appears  in  the  March  number,  reasserts 
the  correctness  of  his  analyses,  and  claims  for  specimens  of  the 
preparation  of  Mr.  Heathfield,  since  examined,  the  strength  of  91 
and  98i  per  cent,  of  metallic  iron. 
By  one  who  is  practically  familiar  with  what  takes  place  in 
the  process  of  reducing  iron  by  hydrogen,  the  causes  of  the  con- 
troversy above  noticed,  are  easily  explained.  The  reduction  tube, 
when  ready  for  the  operation,  is  nearly  filled  with  red  oxide  of  iron, 
obtained  by  calcining  the  sub-carbonate  of  iron  of  the  Pharmaco- 
poeia^© free  it  from  its  water  of  hydration.  After  the  current  of 
hydrogen  is  established  and  the  fire  is  kindled,  the  operator  is 
made  aware  of  the  commencement  of  the  reduction  by  the  ap- 
pearance of  aqueous  vapors  at  the  exit  pipe ;  and  when  it  is  con- 
tinued for  a  length  of  time  and  stopped,  the  contents  of  the 
tube  will  vary  considerably  in  appearance.  Near  the  point \)f 
ingress  of  the  gaseous  current,  if  the  heat  has  been  sufficient,  the 
red  powder  will  have  assumed  an  iron  gray  color,  whilst  further 
along,  the  color  will  be  blacker,  and  finally  perfectly  black.  If 
the  tube  and  contents  have  not  been  heated  to  a  dull  red,  the  red- 
dish color  of  the  oxide  will  be  retained  at  such  point,  and  it  some- 
times happens  in  the  same  operation,  that  the  unreduced  oxide 
and  all  the  grades  of  reduction  to  the  perfect  metallic  state,  may 
be  observed.  It  never  happens  in  practice,  that  all  the  oxide  is 
reduced,  as  the  fire  would  have  to  be  continued  too  long,  to  the 
injury  of  the  reduced  portion,  and  it  is  usual  to  employ  the  par- 
tially reduced  oxide  of  one  operation  for  the  next.  The  line  of 
complete  reduction  is  distinctly  marked,  so  that  the  chemist  can 
by  means  of  a  spatula  separate  readily  the  perfectly  reduced 
iron ;  yet  between  this  and  the  black  oxide  there  is  a  portion 
consisting  of  metallic  iron  and  oxide  in  variable  proportion. 
