FORMATION  OF  ARSENICAL  ACID.
423
Mr.  Pelletier,  however,  fays  that  we  succeed  equally  well  by  using
the  nitric  acid  alone,  in  the  proportion  of  six  parts  of  the  acid  to  one
of  the  white  arsenic.
The  acid  comes  off  in  red  fumes  of  nitrous  gas,
and  the  white  arsenic  afsumes  the  true  characters  of  the  arsenical  acid
(to  it  is  now  called).  It  muſt  be  kept  a  good  while  in  a  ſtrong  heat,
to  expel  all  the  redundant  nitrous  acid.  (Fourcroy  II.  507.  Ed.  1786.)
This  is  an  exact  enough  account  of  the  phenomena,  and  perfectly
instructive  in  the  nature  of  the  operation;  but  having  practised  both
methods,  I  agree  with  Dr.  Scheele,  that  his  previous  solution  in  muriatic ¬
  acid  enables  the  nitric  acid  to  act  on  a  much  greater  quantity
of  the  arsenical  oxyd.
The  manner  in  which  this  process  produces  its  effect  is  sufficiently
evident.  White  arsenic  muſt  be  considered  as  a  metallic  oxyd,  containing ¬
  a  very  moderate  quantity  of  oxygen,  and  capable  of  a  higher  degree -
  of  oxydation.  I  always  viewed  it  in  this  light;  and  on  this
principle,  I  explained  Mr.  Macquer’s  experiments,  and  the  effects
which  it  produces  with  nitre;  a  part  of  which  is  the  change  of  the  nitric
acid  into  nitrous  acid,  in  consequence  of  the  abstraction  from  it  of  a
part  of  its  oxygen,  attracted  by  the  arsenic.  In  Mr.  Scheele’s  process,
the  nitric  acid  alone  supplies  oxygen  to  the  arsenic,  and  thus  oxydates
it  to  the  greatest  degree  of  which  it  is  capable;  in  which  high  state  of
oxydation,  the  abundance  of  oxygen  which  it  contains  gives  it  the
qualities  of  an  acid,  and  deprives  it  of  attraction  for  other  acids,  but
dispoſes  it  to  unite  ſtrongly  with  alkalis.  This  is  the  proper  explication ¬
  of  the  process  according  to  the  principles  of  the  new  theory;
and  there  is  an  experiment  described  by  Mr.  Scheele,  which  gives
great  support  to  the  French  explication  of  the  phenomena,  and  establiſhes ¬
  it  without  a  doubt.  This  experiment  is  made  with  the  acid
of  arsenic.  If  some  of  this  acid  be  put  into  a  retort  by  itself,  and  exposed ¬
  to  the  action  of  heat  alone,  it  endures  a  low  red  heat  without
change,  or  is  only  melted.  But  if  the  heat  be  increased,  and  continued, -
  the  greater  part  of  the  acid  arises  gradually  into  the  neck  of