154       THE  PREPARATION  OF  HYDROGEN  REDUCED  IRON. 
can  appreciate  perfectly  all  the  difficulties  to  be  encountered  in 
purifying  gas.  The  affinities  between  gaseous  substances  and 
re-agents  are  very  limited,  particularly  when  the  latter  are 
liquids  ;  prolonged  shaking  is  frequently  requisite  to  obtain  com- 
plete absorption  ;  and  it  is  scarcely  necessary  to  recall  here  the 
fact  that  oOOO  shakings  are  sometimes  necessary  to  force  sul- 
phuric acid  to  absorb  olefiant  gas.  Then,  to  purify  hydrogen, 
there  must  be  a  slow  disengagement  of  gas,  which  must  be  dis- 
tributed through  porous  bodies,  saturated  with  the  proper  re- 
agents, and  these  bodies  must  be  introduced  into  tubes  vertically 
disposed,  and  the  gas  enter  at  the  superior  extremity  of 
these  tubes.  Thus  hydrogen,  notwithstanding  its  great  light- 
ness, traverses  these  tubes  from  above  downwards,  and  comes  in 
contact  with  the  re-agents,  depositing  its  impurities  and  losing 
all  its  sulphur. 
The  simple  friction  of  the  vulcanised  india-rubber  tubes  gene- 
rally used  for  connecting  the  different  parts  of  apparatus,  is 
another  source  of  sulphur.  A  current  of  pure  hydrogen  or  pure 
carbonic  acid  passed  through  these  tubes  will  produce  in 
the  water  through  which  the  gases  bubble  up  a  deposit  of 
sulphur,  which  can  then  be  transformed  into  sulphuric  acid 
by  the  action  of  nitric  acid,  and  which  can  be  weighed  in 
the  state  of  sulphate  of  baryta.  Therefore,  if  india-rubber 
tubes  are  used,  they  should  be  boiled  in  a  solution  of  potash 
before  they  are  employed  to  join  the  different  parts  of  a  hydro- 
gen apparatus,  when  this  gas  is  employed  to  reduce  the  oxide 
of  iron. 
For  the  preservation  of  reduced  iron  from  oxidation,  it  should 
be  put  into  previously  dried  glass  bottles  ;  and  this  operation 
should  be  effected  in  an  atmosphere  of  hydrogen.  The  iron 
should  be  introduced  by  means  of  glass  measures,  containing 
exactly  a  given  weight  of  iron.  The  bulbs  must  then  be  sealed 
with  the  blowpipe.  Thus  it  appears  that  all  the  reduced  irons 
of  commerce  which  have  been  examined  contain  sulphur;  that 
they  often  deposit  silica  and  blackish  substances  when  treated 
by  weak  acids,  and  that  consequently  they  are  impure.  It 
would  be  well  if  pharmaceutical  chemists  would  themselves  care- 
fully prepare  iron  used  for  medicinal  purposes,  since  manufac- 
turers can  only  supply  it  of  a  relative  purity. — Chem.  Nqws, 
from  Journal  de  Pharmacie  et  de  Chimie,  t.  xxxviii.  p.  275. 
