AmjS,imarm'}      Manufacture  of  Sulphuric  Acid.  287 
sulphur  dioxide,  and  condensed  the  water.  The  oxygen  and  sulphur 
dioxide,  in  the  presence  of  the  contact  mass,  then  united  to  form 
sulphur  trioxide.  It  was,  however,  not  considered  possible  to  use 
furnace  gases  direct,  and  for  the  manufacture  of  dilute  acid  it  could 
not  of  course  compete  with  the  chamber  process. 
The  Badische  Anilin  and  Soda  Fabrik,  however,  took  up  the  effort 
to  carry  out  the  reaction,  utilizing  the  sulphur  furnace  gases,  and 
found  that  the  dilution  of  these  gases  with  nitrogen,  contrary  to 
Winkler's  view,  did  not  interfere  with  the  reaction.  They  found 
that  the  presence  of  small  quantities  of  solid  impurities  in  the  gases 
did  interfere  and  hence  the  mixed  gases  had  to  be  led  through  cool- 
ing and  condensing  tubes  for  quite  a  distance  before  allowing  them 
to  pass  over  the  contact  mass.  In  fact  the  greatest  care  had  to  be 
taken  to  eliminate  a  variety  of  impurities,  which  if  present  speedily 
rendered  the  contact  mass  inactive. 
An  examination  of  the  furnace  gases  showed  that,  while  the 
action  on  the  contact  mass  was  due  partly  to  the  presence  of  anti- 
mony, bismuth,  lead,  zinc  and  other  substances  in  small  quantities, 
the  most  injurious  substance  was  arsenic,  which  was  able,  when 
present  only  to  the  extent  of  1  to  2  per  cent.,  to  poison  the  mass 
and  render  it  entirely  inactive.  The  removal  of  the  small  amount 
of  arsenic  trioxide  present  in  the  gases  as  a  mist  was  a  problem 
which  had  been  studied  by  many  chemists,  but  had  never  been  suc- 
cessfully solved. 
After  the  expenditure  of  an  immense  amount  of  time,  patience 
and  money  a  method  was  devised  by  which,  through  cooling  and 
washing  and  other  processes,  the  exact  details  of  which  are  not 
given,  the  gases  were  absolutely  freed  of  all  impurities,  especially 
those  in  the  solid  condition.  It  was  found  that  the  ease  with  which 
the  solid  particles  could  be  precipitated  depended  largely  upon  the 
rate  of  cooling,  slow  cooling  greatly  facilitating  it.  Although  it 
was  supposed  that  acid  of  a  concentration  of  90  per  cent,  could  not 
act  on  iron,  or,  if  so,  would  form  sulphur  dioxide,  the  decrease  in 
power  of  the  contact  mass,  which  only  began  to  appear  after  weeks 
or  months,  was  shown  to  be  due  to  the  formation  of  hydrogen 
from  the  iron  and  sulphuric  acid  and  the  action  of  this  on  an  arsenic 
compound  to  form  arsine.  Even  the  trace  of  arsenic  contained  in 
the  small  amount  of  sulphur,  which  passed  through  unprecipitated,. 
was  sufficient  to  injure  the  contact  mass.    But  this  was  easily 
