146 
ON  THE  MANUFACTURE  OF  OXYGEN. 
the  following  manner : — By  calcining  it  alone  in  an  earthen  ves- 
sel it  is  transformed  into  a  light,  white  oxide,  which,  when 
the  sulphate  is  pure,  can  be  used  in  painting  ;  into  sulphur- 
ous acid,  which  is  collected  in  a  concentrated  solution,  or  as  a 
sulphite,  which  is  now  applied  to  numerous  purposes,  and,  lastly, 
into  pure  oxygen.  The  complete  decomposition  of  sulphate  of 
zinc  does  not  require  a  much  higher  temperature  than  for  the 
decomposition  of  manganese  ;  it  can  be  completely  transformed 
into  oxide  of  zinc  and  into  a  mixture  of  water,  sulphurous  acid, 
and  oxygen.  These  are  separated  by  the  process  which  will 
presently  be  described  for  the  preparation  of  oxygen  by  sul- 
phuric acid. 
Sulphuric  acid  decomposes  at  a  red  heat  into  sulphurous 
acid,  water,  and  oxygen,  in  a  very  simple  apparatus;  a  *small 
retort,  holding  five  litres  filled  with  tbin  leaves  of  platinum,*  or, 
better  still,  a  worm  of  platinum  filled  with  sponge  of  this  metal 
and  made  red  hot.  Introduce  into  it  by  an  S-tube  a  small  stream 
of  sulphuric  acid,  issuing  from  a  vessel  at  a  constant  level. 
The  gases  which  escape  pass  first  through  a  refrigerator,  which 
separates  the  water  from  them,  and  then  into  a  washer,  of  a 
special  form,  which  it  is  here  unnecessary  to  describe.  Pure 
inodorous  oxygen  and  a  saturated  solution  of  sulphurous  acid 
are  the  constant  results.  If  the  v/ashing- water  is  replaced  by 
lixivium  of  soda,  bisulphite  of  soda,  saturated  with  sulphurous 
acid,  is  obtained,  which  can  be  neutralised  by  carbonate  of 
soda,  and  transformed  into  neutral  sulphite  or  hyposulphite. 
If  the  water  clmrged  with  sulphurous  acid  is  passed  into  the 
generator  which  feeds  the  lead-chambers  in  a  sulphuric  acid 
factory,  the  sulphurous  acid  will  be  transformed  into  sulphuric 
acid  at  the  expense  of  the  atmospheric  oxygen.  We  have  cal- 
culated that  it  would  suffice  to  burn  in  the  sulphur  furnace  of  a 
sulphuric  acid  apparatus  double  the  quantity  of  sulphur  con- 
tained in  the  concentrated  solution  of  sulphurous  acid  in  order 
to  entirely  utilise  the  latter  gas,  so  that  with  one  apparatus, 
without  sensibly  augmenting  the  expense,  we  could  appropriate 
a  third  part  of  the  sulphuric  acid  which  it  produces  to  the  pre- 
*  In  large  yesaeh  the  leaves  of  platioum  may  be  replaced  by  pieces  of 
brick. 
