It  is  most  easily  procured  for  purposes  of  experiment
from  the  atmosphere,  by  withdrawing  the  oxygen  of  the  air


327t  What  of  selenious  acid  ?    328.  "What  of  tellurium  ?    329.  Give  ihe
history  of  nitrogen.
200


NON-METALLIC   ELEMENTS.
by  phospborus.  Tbis  is  easily
done  by  burning  some  pbos-
pborus  in  a  floating  capsule,
under  an  air-jar,  upon  the  pneu-
matic cistern,  (fig.  248.)  Tbt
strong  affinity  of  pbosphorus  foi
oxygen  enables  it  to  witbdra-w
every  trace  of  this  element,
leaving  behind  nitrogen  nearly
pure,  containing  about  4^0^^  ^^
phosphorus.  The  water  soon  ab-
Fig.  248.  sorbs  the  snow-white  phosphoric

acid.  The  first  combustion  of  the  phosphorus  expels  a
portion  of  the  air  by  expansion  ;  but  as  the  combustion  pro-
ceeds, the  water  rises  in  the  jar,  until  it 'occupies  about
Jth  of  its  space.  When  this  experiment  is  performed  over
mercury,  the  white  phosphoric  acid  remains  unchanged.
