This  statement,  however,  requires  to  be  made  with  some  reservation,  for


PHOSPHORUS.  691

although  it  is  doubtless  true  of  the  aqueous  acid,  and  possibly  of  the  liquid
acid,  it  is  not  true  of  the  gas.  If  acid  potassium  fluoride  be  heated  in  a
platinum  retort,  and  the  pure  gaseous  hydrofluoric  acid  so  produced  be
allowed  whilst  hot  to  blow  upon  a  little  heap  of  crystallized  silicon  sup-
ported on  a  porcelain  crucible  lid,  the  silicon  at  once  takes  fire  and  bums
brilliantly  in  the  gas,  forming  silicon  fluoride  and  hydrogen.  If  the  neck
of  the  retort  be  more  than  an  inch  or  two  in  length,  it  is  necessary  to  heat
it  in  order  to  keep  the  gas  sufficiently  hot.  The  importance  of  this  obser-
vation lies  in  the  fact  that  the  spontaneous  ignition  of  crystallized  silicon  is
generally  regarded  as  in  all  cases  a  sufficient  test  ioi  free  fluorine  ;  but  it  is
evident  that  unless  the  temperature  of  the  gas  is  below  a  certain  point  the
combustion  of  silicon  is  not  a  safe  criterion. — Chem.  News,  Dec.  6,  1895,
278  ;  from  Proc.  Chem.  Soc,  Nov.  7,  1895.
PHOSPHORUS.
