364  Development  of  the  Chemical  Arts.     { '''^iZ:^;^^"' 
The  trifling  value  of  the  residual  sesquioxide  which  contains  iron, 
and  is  therefore  useless  in  the  glass  manufacture,  is  not  taken  into  ac- 
count. This  calculation  dates  from  the  time  when  the  re-oxidation  of 
manganese  was  still  an  unsolved  problem.  If  the  price  of  oxygen 
obtained  from  manganese  ranges  from  3*45  to  5*98  francs  it  is  cheaper 
by  more  than  one-half  than  that  procured  from  chlorate  of  potash, 
which  Dupre*  calculates  10  francs. 
Deville  and  Debrayf  found  a  much  cheaper  source  in  sulphuric  acid, 
which,  at  elevated  temperatures,  is  resolved  into  oxygen,  sulphurous 
acid  and  water.  Retorts  containing  five  litres  of  very  mfusible  glass 
were  partially  filled  with  platinum  foil,  or  with  fragments  of  brick,  and 
heated  to  redness,  whilst  sulphuric  acid  was  allowed  to  flow  in  in  a 
slender  stream.  The  escaping  gases  are  led  through  a  cooling  appara- 
tus in  order  to  condense  sulphuric  acid,  and  into  water  to  remove 
sulphurous  acid.  By  this  process  2*436  kilos,  of  sulphuric  acid  of  the 
sp.  gr.  1*827  yielded  240  litres  of  oxygen  at  the  expense  of  one  franc 
per  cubic  metre.  On  its  application  the  cost  of  smelting  platinum 
was  from  20  to  30  centimes  per  kilo. 
According  to  a  paragraph  by  MoignoJ  the  firm  of  Jose  de  Susine 
&  Co.,  of  Paris,  prepared  by  this  process  oxygen  at  0*85  franc  per  cubic 
metre,  reconverting  the  sulphurous  acid  into  sulphuric  acid. 
Instead  of  the  free  acid,  Deville  and  Debray  propose  the  use  of  sul- 
phate of  zinc  ;^ioo  kilos,  of  the  anhydrous  salt  yielded  in  their  ex- 
periments 6*8  cubic  metres  of  oxygen — far  more  than  the  best  black 
oxide  of  manganese — 22  kilos,  sulphurous  acid  gas,  and  51  kilos,  oxide 
of  zinc. 
Wagner's  statement||  must  be  noted  that,  in  the  year  1867  both 
these  methods  were  not  carried  out  in  Deville's  laboratory,  perhaps 
because  the  development  of  sulphurous  acid  complicated  their  execu- 
tion ;  in  fact,  they  have  both  been  left  in  the  background  in  industrial 
practice.  As  an  attempt  in  that  direction,  we  must  notice  the  pro- 
cedure of  Archereau,§  who  employed  sulphuric  acid  in  its  cheapest 
combination,  gypsum.  He  maintained  that,  by  heating  ground  gypsum 
with  sand,  he  could  obtain  silicate  of  lime,  whilst  sulphurous  acid  was 
"  Dupre,  "  Compt.  Rend.,"  Iv,  736. 
t  Deville  and  Debray,  "  Compt.  Rend.,"  li,  822. 
J"Les  Mondes,"  1867,  p.  494. 
II  Wagner  *' Jahresberichte,"  1867,  216. 
^  Archeieau,  "  Dingler's  Polyt.  Journ.,"  clxxviii,  57. 
