^"^■jun^'e'ifs^^''"'"}    Finely  Divided  Metals  and  Ferric  Salts.  295 
which  can  easily  be  obtained  absolutely  free  from  iron,  which  are  not 
oxidized  by  water,  and  which  do  not  evolve  hydrogen  with  dilute  acids, 
separation  from  the  finely-divided  metal  mits^  always  precede  titration  ; 
for  even  silver  and  aluminium  are  attacked  in  feebly  acid  solutions 
by  permanganate. 
7.  Mitscherlich  (Zeit.  anal.  Chem.,  11,  72)  has  stated  that  in  the  re- 
duction of  ferric  solutions  it  is  absolutely  necessary  that  the  whole 
of  the  zinc  should  be  dissolved  before  titration ;  the  reason  adduced 
being  that  iron  is  precipitated  on  the  surface  of  the  zinc,  and  does  not 
dissolve  until  the  last  traces  of  the  zinc  themselves  disappear.  If  this 
statement  be  accurate,  objection  may  be  taken  to  my  method  detailed 
above ;  but  I  much  doubt  its  accuracy.  Experiment  showed  that  the 
titre  of  an  acidulated  iron  solution  was  independent  of  the  time  it  had 
remained  in  contact  with  the  zinc-dust.  This  might  be  explained  in 
this  special  case  by  supposing  the  finely  divided  zinc  to  be  practically 
enveloped  in  a  protecting  layer  of  hydrogen,  but  other  experiments 
would  lead  me  to  believe  that  such  a  supposition  is  unnecessary.  Ex- 
amination of  pieces  of  granulated  zinc  free  from  iron  removed  either 
before  or  after  complete  reduction  of  both  hot  and  cold  ferric  solutions, 
always  failed  to  give  evidence  of  iron.  Beebe's  method  of  reducing 
ferric  solutions  (CAem..  News,  LIU,  269)  would  also  be  untrustworthy 
were  Mitscherlich's  statements  correct.  Without  doubt  the  zinc  in  all 
cases  becomes  coated  with  a  black  deposit,  which  as  Rodwell,  Yogel, 
and  others  have  shown,  contains  in  addition  to  the  iron  present  origi- 
nally in  the  impure  zinc  itself,  zinc  combined  with  lead,  sulphur  and 
carbon. 
8.  After  the  work  of  which  this  paper  is  a  short  account  was  fin- 
ished, I  casually  came  across  a  reference  in  Fr6my^s  Encycloptdie  to  a 
paper  by  Brown,  on  the  reduction  of  ferric  compounds  by  zinc.  I 
have  procured  the  paper  referred  to  {Iron,  1878,  361),  and  find  that 
Brown's  method  consists  in  reducing  iron  ores  directly  by  fusion  with 
pulverized  zinc  (Hobson  and  Sylvester  had  shown  that  at  a  temperature 
of  205°  zinc  becomes  so  brittle  that  it  may  be  powdered  in  a  mortar). 
Brown  has  also  used  this  pulverized  zinc  to  reduce  ferric  salts  in  acid 
solutions ;  but  that  his  method  is  founded,  as  is  the  current  one,  on 
the  reducing  action  of  nascent  hydrogen,  and  not  on  the  direct  reducing 
powers  of  the  zinc,  is  obvious  from  the  following  quotation : — There 
should  be  but  a  very  small  excess  of  sulphuric  acid  present^  so  that  at 
the  end  of  an  hour  or  two  only  about  half  the  zinc  will  be  dissolved.^' 
