288  Manufacture  of  Sulphuric  Acid.  {^jfme^oi™ 
removed,  as  was  also  the  sulphuric  acid  which  was  formed  in  the 
furnace  and  which  before  had  acted  on  the  iron  and  the  arsenic 
compounds  by  spraying  the  gases  after  they  issued  from  the  fur- 
nace where  the  pyrite  was  burned. 
In  the  technical  preparation  of  the  fuming  acid  very  little  atten- 
tion had  been  paid  to  the  heat  evolved  when  sulphur  dioxide  and 
oxygen  combine,  although  it  amounted  to  22,600  calories.  It  was 
shown  that  the  commonly  accepted  idea  of  the  necessity  of  heating 
the  contact  mass  very  high,  in  order  to  produce  the  combination 
when  the  diluted  furnace  gases  were  used,  was  incorrect,  and  that 
both  the  yield  and  the  life  of  the  mass  could  be  increased  if  the 
tubes  containing  it  were  cooled  in  order  to  carry  off  some  of  the 
heat  generated  in  the  chemical  combination  of  the  two  gases.  A 
form  of  furnace  was  devised,  something  like  a  tubular  boiler  placed 
on  end,  and  the  contact  mass  arranged  in  the  upright  tubes  of  the 
furnace  in  such  a  way  that  the  pressure  and  amount  of  surface  of 
the  mass  exposed  in  each  tube  was  the  same.  Under  these  condi- 
tions the  process  is  a  continuous  one  and  the  mass  retains  its  full 
power  for  a  year  or  more. 
The  ordinary  method  of  absorbing  gases  by  passing  through  a 
series  of  vessels  containing  water  or  dilute  acid  would  not  entirely 
remove  the  sulphur  trioxide  even  when  a  number  of  the  absorbing 
flasks  were  used ;  but  one  vessel  containing  acid  of  97  per  cent,  to 
98  per  cent,  sulphuric  acid  absorbs  it  instantly  and  entirely. 
In  order  to  keep  the  concentration  at  this  point  the  excess  of 
sulphur  trioxide  is  removed  from  time  to  time  or  water  is  added. 
By  the  method  just  described  the  experimenters  were  able  to  obtain 
acid  of  any  concentration  and  mixtures  of  the  acid  and  sulphur  tri- 
oxide in  all  proportions. 
Although  the  amount  of  product  formed  is  not  directly  dependent 
upon  the  nature  of  the  contact  mass,  yet  the  latter  must  be  one 
which  will  give  the  greatest  efficiency  at  about  4500  C.  All  sub- 
stances which  reach  their  highest  efficiency  above  this  temperature 
will  never  cause  a  quantitative  yield,  no  matter  how  long  the  gases 
may  be  kept  in  contact,  as  they  will  be  partly  regenerated  if  they 
have  first  combined.  The  only  substance  which  fulfils  these  con- 
ditions is  platinum,  even  members  of  the  same  family  not  showing 
an  equal  efficiency.  The  introduction  of  this  method  has  enabled 
the  Badische  Anilin-  und  Soda-Fabrik  to  increase  the  yield  of  the 
anhydride  from  18,500  tons  in  1888  to  116,000  tons  in  1900. 
