242     OXYGENNESIS,  FOR  THE  PRODUCTION  OF  OXYGEN,  ETC. 
6th.  By  the  action  of  sulphuric  acid  on  bichromate  of  potash. 
Three  parts  bichromate  potash  and  four  parts  sulphuric  acid  are 
heated  together  in  a  capacious  retort.  An  evolution  of  oxygen 
gas  easy  to  regulate  is  the  result.  In  this  experiment  we  might 
congratulate  ourselves  if  the  process  is  conducted  to  the  end 
without  a  fracture  of  the  retort. 
Of  the  six  processes  just  described,  two  only  are  now  used— viz., 
chlorate  of  potash  with  or  without  manganese,  and  manganese 
alone.  More  recently  other  processes  have  been  recommended, 
one  of  which,  by  heating  together  nitrate  of  soda  and  oxide  of 
zinc,  has  been  patented.  From  this  mixture  oxygen  is  said  to 
be  produced  at  a  cheaper  rate  than  by  any  other  method  at 
present  known.  Unfortunately  for  the  value  of  this  discovery 
the  product  is  contaminated  with  a  considerable  per-centage  of 
nitrogen.  M.  Kuhlman,  of  Lille,  discovered  and  published  an 
ingenious  and  beautiful  process  for  the  production  of  oxygen  by 
means  of  baryta.  He  found  that  by  passing  a  current  of  com- 
mon air  through  caustic  baryta  heated  to  dull  redness,  peroxide 
of  barium  was  formed,  which,  on  an  increase  of  temperature,  is 
resolved  into  oxygen  gas  and  caustic  baryta ;  the  latter  ready 
again  to  perform  its  part  in  a  similar  operation.  The  idea  nat- 
urally suggested  itself  that  the  means  were  now  at  hand  for 
getting  oxygen  from  the  atmosphere  in  any  quantity  at  a  small 
cost.  This  method,  although  so  promising,  has  been  for  the 
present  abandoned ;  it  was  found  that  after  a  few  operations, 
either  from  a  molecular  change,  or  from  the  silica  or  other 
impurities,  a  sort  of  glass  or  fusion  resulted  on  the  surface,  the 
baryta  then  refusing  to  act  again. 
We  now  come  to  the  consideration  of  the  new  method  for  the 
generation  of  oxygen  recently  introduced  by  myself.  The  pro- 
cess or  the  compound  employed  in  it  has  been  named  oxygenne- 
sis.  It  will  have  doubtless  been  observed  by  you  that  in  all  the 
processes  hitherto  known,  a  high  temperature  is  necessary,  and 
until  that  point  is  reached,  no  product  whatever  is  obtained ; 
this  fact  we  may  consider  as  the  chief  difficulty  experienced  in 
the  preparation  of  oxygen,  and  more  especially  so  when  sulphu- 
ric acid  is  used.  If,  for  example,  by  the  mere  addition  of  sul- 
phuric acid  to  bichromate  of  potash  in  the  cold,  we  could  get 
the  same  results  which  are  obtained  by  the  application  of  heat, 
