DETECTION  OF  ARSENIC  BY  COPPER. 
351 
present  in  considerable  quantity.  Therefore  the  coating  of  cop- 
per with  a  grey  mirror,  is  the  property  of  arsenious  acid  only,  if 
but  a  minute  quantity  be  present."  The  reporter  W.  now  men- 
tions several  experiments,  showing  that  arsenic  acid  is  not,  or 
but  incompletely,  precipitated  by  metallic  copper,  and  that  this 
indifference  must  operate  against  the  employment  of  copper  for 
the  detection  of  arsenic. 
These  statements  induced  me  to  institute  experiments  on  the 
behaviour  of  metallic  copper  to  arsenic  acid,  reports  of  which  I 
had  the  honor  to  lay  before  the  meeting  of  the  German  Apothe- 
caries' Association  at  Coburg.  But  I  had  not  concluded  my 
experiments,  and  several  objections  were  made,  which  I  then 
could  not  answer,  for  want  of  experiments,  but  propose  to  do 
now. 
Regarding  the  behaviour  of  arsenic  acid  to  metallic  copper, 
the  statements  of  the  above  chemists  are  evidently  incorrect, 
though  I  admit  that  heretofore  I  had  not  experimented  with 
arsenic  acid,  because  my  test  was,  first  of  all,  intended  for  cases 
of  poisoning  with  arsenious  acid,  as  arsenic  acid  does  not  occur 
in  commerce,  and  T  know  not  of  a  single  case  of  poisoning  by  it. 
A  very  dilute  aqueous  solution  of  arsenic  acid  was  mixed 
with  half  its  volume  of  pure  muriatic  acid,  and  boiled  with 
bright  copper  wire.      Not  the  slightest  precipitate  of  arsenic 
occurred,  and  the  copper  remained  perfectly  bright.    No  altera- 
tion of  the  copper  could  be  observed,  after  increasing  the 
muriatic  acid  to  one  volume.    But  after  taking  to  one  volume  of 
the  solution  two  volumes  of  concentrated  muriatic  acid  and  boiling, 
the  copper  was  instantly  covered  with  the  characteristic  grey  layer 
of  arsenic.    This  reduction  takes  place  even  in  very  dilute  solu- 
tion, and  the  reaction  of  arsenic  acid  is  not  less  delicate  than 
that  of  arsenious  acid,  if  calculated  for  the  proportion  of  me- 
tallic arsenic,  a  solution  of  which,  containing  1-100000,  yields 
still  good  results,  and  then   the  dilution  with  muriatic  acid  has 
not  yet  been  calculated     It  seems,  therefore,  as  if  the  English 
chemist  failed  with  the  reaction  of  arsenic  acid  simply  from  not 
having  acidulated  enough  with  muriatic  acid.     A  liquid  not 
very  acid  does  not  attack  metallic  arsenic,  or  but  very  slightly, 
and  therefore  no  arsenic  can  be  precipitated;  but  if  the  solu- 
tion is  sufficiently  acid  to  dissolve  the  copper,  the  arsenic  acid 
