255.  Metallic  Copper.  Cu.  —  Found  native  crystallized  in
forms  of  the  isometric  system.  Has  a  brilliant  lustre,  and  a
familiar  reddish  color.  Has  great  hardness  and  tenacity.  Is
very  ductile  and  malleable,  and  one  of  the  best  conductors  of
heat  and  electricity.  Sp.  Gr.  8.8.  Fuses  at  about  780°.  Vol-
atilizes only  at  a  very  high  temperature.  Its  vapor  burns  with
a  beautiful  green  flame,  which  shows  in  the  spectroscope  char-
acteristic bands.  Under  ordinary  conditions  copper  undergoes
no  change  in  the  atmosphere,  but  if  heated  to  redness  in  the  air
it  is  rapidly  oxidized.  In  presence  of  acids  or  solutions  of  chlo-
rides, like  sea-water,  copper  absorbs  oxygen  from  the  air  at  the
ordinary  temperature,  and  is  more  or  less  rapidly  corroded.  A
similar  effect  is  also  produced  by  aqua  ammonia  and  solutions
of  ammonia  salts.  Out  of  contact  with  the  air,  dilute  hydro-
chloric or  sulphuric  acids  have  but  little  action  upon  metallic


§257.]  COPPER.  329

copper.  If  boiled  with  strong  hydrochloric  acid,  it  very  slowly
dissolves  with  the  evolution  of  hydrogen  gas.  Under  the  same
conditions  sulphuric  acid,  if  not  too  dilute,  is  decomposed  by  it,
cupric  sulphate  is  formed,  sulphurous  acid  is  evolved,  and  the
reaction  is  similar  to  [265].  Nitric  acid  is  the  best  solvent,
but,  singularly,  the  strongest  acid  has  no  action  on  the  metal.
When  diluted  with  water,  however,  the  action  is  very  violent;
cupric  nitrate  is  formed,  and  a  gas  is  evolved  which  is  generally
NO  ;  but  when  the  acid  is  very  dilute  this  product  is  more  or
less  mixed  with  N20.
