352 
DETECTION  OF  ARSENIC  BY  COPPER. 
will  be  reduced,  and  the  precipitate  takes  place.  Arsenious  acid, 
however,  is  precipitated  by  copper  even  from  faintly  acidulated 
solutions. 
It  is  difficult  to  conceive  why,  in  the  above  case,  the  hydro- 
chloric solution  of  arsenious  acid  was  treated  with  chlorate  of 
potassa.  I  have  made  about  twenty  experiments  with  arsenious 
and  arsenic  acid  dissolved  in  muriatic  acid,  which,  when  treated 
with  very  small  quantities  of  chlorate  of  potassa,  gave  some- 
times no  reaction  whatever,  with  copper.  I  cannot  explain  this, 
except  by  assuming  that  thereby  a  very  volatile  superchloride 
of  arsenic  is  formed,  which  is  readily  volatilized  on  boiling. 
The  use  of  chlorate  of  potassa  must,  therefore,  be  strictly 
avoided. 
Attention  must  be  directed  to  still  another  precaution,  namely, 
to  treat  only  lukewarm  with  muriatic  acid,  the  stomach  and 
other  substances  to  be  tested  for  arsenic.— otherwise  chloride 
of  arsenic  will  be  volatilized.  It  appears  advisable  to  extract 
the  substances  in  a  flask  with  muriatic  acid,  which  may  be  di- 
luted with  an  equal  bulk  of  water,  and  to  collect  the  vapors  in  a 
separate  vessel,  so  as  to  lose  no  trace  of  the  arsenic.  If 
arsenious  acid  is  heated  gently  in  a  retort  with  muriatic  acid,  with- 
out boiling  the  liquid,  oily  drops  of  terchloride  of  arsenic  and 
very  strong  muriatic  acid  are  distilled  over;  the  former,  it  ap- 
pears, volatilizes  readily  in  connection  with  gaseous  muriatic 
acid. 
In  the  beginning  of  these  experiments  I  was  of  the  opinion 
that  arsenic  acid  could  not  be  precipitated  by  copper,  and  I 
endeavored  to  reduce  it  previously  to  arsenious  acid,  which  is 
easily  accomplished  by  a  current  of  sulphurous  acid,  and  heat- 
ing subsequently  to  boiling.  A  concentrated  solution  of  arsenic 
acid  thus  heated,  deposits  on  cooling  arsenious  acid  in  fine  octohe- 
drons,  and  contains  no  arsenic  acid. 
Lippert  states  (Am.  J.  Ph.  1861,  408)  that  the  deposit  upon 
copper  consists  of  Cu5  As.  Though  I  had  at  first  supposed 
it  to  be  pure  arsenic,  I  have  subsequently  proven  that  it  retains 
copper,  and  succeeded  afterwards  to  separate  it  from  copper  by 
means  of  ammonia.  I  must  suggest,  however,  whether  this 
precipitate  from  arsenious  aoid  might  not  more  likely  be  As  Cu3, 
and  the  precipitate  from  the  arsenic  acid  solution  As  Cu., 
