from  mixing  in  case  of   back-pressure  and  from  going
down  into  the  receiver.     The  oxygen  finally  passes  up


through  a  jar  D  of  about  two  liters  capacity,  filled
with  pieces  of  solid  potassium  hydrate.  This  arrange-
ment effects  a  very  durable  and  reliable  purification


14  NOTES    ON    THE    CHEMISTRY    OF    IRON.
of  the  oxygen,  freeing  it  from  the  last  traces  of  moist-
ure and  carbonic  acid.  An  exactly  similar  arrange-
ment (not  shown  on  Fig.  IV.)  is  had  for  the  air  which
is  pressed  through  the  apparatus  after  each  combustion
to  expel  the  oxygen.  A  three-way  glass  tube  connects
the  combustion-tube  with  the  two  potash-jars,  for  each
of  which  there  is  a  glass  stop-cock;  thus  the  oxygen
and  air  purifiers  can  be  shut  off  and  connected  alter-
nately at  will.  Where  blast  is  available  it  can  be
conveniently  used  to  press  air  through  the  apparatus.
