METALLIC  COPPER  A  TEST  FOR  SULPHURIC  ACID.  359 
ence  of  traces  of  sulphurous  acid,  the  wire  loses  its  brightness, 
and  turns  after  some  time  faint  reddish,  afterwards  grey  or 
brown. 
When  testing  for  arsenic  it  is  necessary  to  bear  in  rnind  this 
reaction  of  sulphurous  acid  in  the  presence  of  muriatic  acid. 
Sulphuric  and  muriatic  acid  frequently  contain  traces  of  sul- 
phurous acid,  and  may  therefore  lead  to  mistakes  with  arsenic. 
All  the  sulphur  acids  containing  less  oxygen  than  sulphurous 
acid,  will  necessarily  produce  the  same  reaction.  Bat  while  the 
precipitate  by  arsenic  adheres  to  the  copper,  and  does  not  stain 
the  fingers,  the  deposit  produced  by  the  sulphurous  acid  colors 
the  skin  black;  the  former,  on  being  shaken  with  a  few  drops  of 
ammonia,  separates  in  fine  scales  and  leaves  the  copper  bright, 
which  is  not  the  case  with  the  latter. 
The  most  apparent  difference  in  the  behaviour  of  the  two  pre- 
cipitates is  that  towards  a  boiling  mixture  of  equal  volumes  of 
muriatic  acid  and  water,  which  dissolves  the  former  with  evolu- 
tion of  hydrogen,  but  not  the  latter,  which  will  retain  its  black 
color.  The  precipitate  by  arsenic  on  being  thus  treated,  yields 
at  first  a  white  crystalline  ring  of  arsenious  acid,  and  the  copper 
appears  silver-white  from  a  coating  of  arsenico-copper  ;  on  con- 
tinuing the  heating  the  whole  of  the  arsenic  will  be  volatilized 
and  the  copper  regains  its  original  color. 
Water  containing  sulphurous  acid  does  not  react  with  copper 
either  on  boiling  or  at  ordinary  temperature  ;  on  the  addition  of 
muriatic  acid,  the  reaction  takes  place.  Sulphites  behave  in  the 
same  way.  It  is  remarkable  that  this  reaction  is  produced  only 
by  muriatic  acid  (probably  also  by  the  hydracids  of  the  other 
halogens.)  If  sulphuric  acid  be  used  no  such  reaction  takes 
place,  but  after  the  addition  of  a  grain  of  table  salt,  the  copper 
becomes  dim  and  soon  reddish.  It  seems  probable,  therefore, 
that  under  certain  conditions  this  reaction  might  be  useful  for 
the  detection  of  chlorides ;  it  will  at  least  assist  in  explaining 
these  phenomena. 
This  method  of  recognizing  sulphurous  acid  by  copper,  is 
useful  for  detecting  sulphurated  hops.  One  strobile  is  macerated 
with  water  for  half  an  hour,  the  infusion  is  mixed  with  an  equal 
volume  of  muriatic  acid,  and  the  mixture  boiled  with  copper 
wire  ;  in  a  few  minutes,  a  faint  but  evident  reaction  was  ob- 
