420  Development  of  the  Chemical  Arts,  {^"^illZ'S^t'"^' 
of  from  8  to  10  c.  m.  in  height.  In  this  manner  reduction  and  oxidation 
alternate  at  intervals  of  five  minutes.  Not  until  six  hours  have  elapsed 
does  it  become  necessary  for  a  more  complete  revivification  of  the  mass 
to  pass  atmospheric  air  over  it  for  an  hour,  for  in  five  to  six  hours  the 
yield  of  oxygen  sinks  from  its  original  quantity  dow^n  to  the  half,  or 
even  the  third.  The  cocks  are  set  at  Vienna  by  a  self-acting  move- 
ment. The  longer  v^atery  vapor  is  introduced,  and  the  retorts  thus 
freed  from  atmospheric  air  before  opening  the  communication  with  the 
gasometer,  the  purer  is  the  oxygen.  Half  a  minute  suffices  to  bring 
Aovjn  the  nitrogen  to  15  per  cent,  if  the  useless  space  in  the  retorts  is 
kept  as  small  as  possible.  If,  as  is  easily  practicable,  the  nitrogen  is 
brought  dovi^n  to  4  per  cent.,  there  is  a  greater  waste  of  oxygen.  To 
be  certain  that  the  amount  of  nitrogen  remains  within  the  limits  of  from 
10  to  15  per  cent.,  samples  are  taken  from  the  gasometer  in  graduated 
tubes,  and  the  oxygen  is  absorbed  by  means  of  known  quantities  of 
potash  and  pyrogallic  acid,  a  reaction  which,  even  in  inexperienced 
hands,  gives  quick  and  accurate  results. 
As  any  cooling  of  the  retorts  below  a  dark-red  heat  diminishes  the 
yield,  care  is  taken  to  heat  both  the  air  and  steam  to  about  300°  C. 
At  Pantin,  where  there  are  several  groups  of  ten  retorts  each,  two  of 
them  are  filled  with  pumice-stone,  and  serve  for  heating  the  air  and  the 
steam.  The  composition  of  the  mass  is  2  molecules  of  NaOH,  i 
molecule  of  MnOg,  and  the  fifth  of  a  molecule  of  oxide  of  copper, 
which  merely  serves  to  separate  the  other  ingredients  and  render  them 
more  accessible  to  the  influence  of  steam  and  air.  At  Comines,  the 
black  oxide  of  manganese  is  regenerated  in  the  ordinary  manner  from 
chlorine  residues,  and  is  almost  pure  ;  its  price  is  2  francs  per  kilo. 
The  great  cost  of  this  fundamental  article  is  not  of  importance,  since  it 
can  be  used  the  longer  the  more  carefully  the  air  is  freed  from  carbonic 
acid.  If,  in  consequence  of  some  inevitable  interruption  of  the  pro- 
cess, the  mass  absorbs  atmospheric  carbonic  acid,  it  is  simply  requisite 
to  heat  to  redness,  and  to  pass  a  current  of  steam  over  it  till  the  escap- 
ing vapors  cease  to  render  lime-water  turbid.  The  temperature  is  then 
raised  and  air  passed  over  the  mass,  when  it  regains  its  original  efficacy. 
The  average  duration  of  a  retort  is  one  year. 
Tessie  du  Motay's  process  yields  oxygen  at  90  per  cent.,  at  the  cost 
of  15  to  30  centimes  per  cubic  metre,*  or,  according  to  the  experiments 
of  Kuppelwieser  in  Vienna,f  3  florins  per  1,000  cubic  feet,  a  price 
*  Phillip's  "  Der  SauerstofF,"  18. 
t  Kuppelwieser,  "Berg,  und  Hiitten  Ztg.,"  1873,  354. 
