64 
Lecture  Experiments. 
5  Am.  Jour.  Pharjl 
\     Feb.  1,1871. 
Under  the  circumstances  mentioned  five  centimeters  of  fluid  cyano- 
gen are  obtained.  On  the  opening  of  the  cock  the  gas  streams  out 
with  great  violence.  But  the  large  peach-colored  flame  soon  dimin- 
ishes in  consequence  of  the  cooling  of  the  liquid.  This  refrigeration 
is  also  shown  by  the  coating  of  ice  upon  the  exterior  of  the  tube. 
The  condensed  cyanogen  remains  unchanged  for  weeks. 
6.  Alternate  reduction  and  oxidation. 
Whoever  is  accustomed  to  heat  the  oxide  of  copper  for  combustions-^ 
in  a  copper  crucible  over  a  gas  flame,  has  undoubtedly  observed  the 
beautiful  phenomena,  which,  in  rapid  alternation,  take  place  upon  the- 
metallic  surface.  Ignited  in  the  quiet  flame,  the  under  portion  of  the- 
crucible  exhibits  the  full  copper  color  ;  but  when  a  gentle  draft  of  air- 
draws  the  flame  upon  one  side,  it  rushes  through  all  the  colors  of  the 
rainbow  and  is  blackened  in  a  moment.  As  soon  as  it  is  enveloped 
in  the  reducing  flame  it  acquires  its  primitive  metallic  lustre. 
The  thought  suggested  itself  that  herein  lay  the  germ  of  an  experi- 
ment appropriate  to  the  lecture-table.  After  many  trials  the  follow- 
ing form  has  appeared  best  adapted  to  the  end  in  view : 
An  iron  ring  is  placed  upon  a  triangle,  in  the  middle  of  which  a 
small  brightly  polished  copper  bell  is  placed.  A  powerful  gas-burner, 
the  flame  of  which  touches  the  inside  of  the  bell,  quickly  brings  the 
metal  to  a  glow.  After  some  moments  it  is  blackened.  A  strong 
stream  of  hydrogen  is  led  by  means  of  an  india-rubber  tube  into  a 
glass  funnel,  which  is  just  of  the  proper  size  to  cover  the  bell.  At  the 
moment  when  the  hydrogen  comes  into  contact  with  the  metal,  the 
oxidised  crust  is  removed  and  the  copper  acquires  again  its  primitive 
metallic  lustre.  If  the  funnel  with  its  atmosphere  of  hydrogen  be 
now  removed,  the  incoming  air  immediately  produces  oxidation,  the 
progress  of  which  can  be  noted  by  the  succession  of  the  colors.  When 
the  bell  is  at  glow-heat  the  phenomena  of  oxidation  and  reduction  can 
be  repeated  ad  libitum. 
I  was  at  first  fearful  that  the  superposition  and  removal  of  the 
hydrogen  funnel  might  cause  detonations,  but  I  have  never  noticed 
the  slightest  disturbance. 
The  hydrogen  which  is  employed  in  these  experiments  must  be 
pure.  The  presence  of  sulphuretted  or  arseniuretted  hydrogen  alters 
the  surface  of  the  copper.  The  combinations  thus  formed  are  not 
decomposed  by  water,  and  the  copper  does  not  acquire  again  its  beau- 
tiful metallic  lustre  until  it  is  rubbed  off  with  sandpaper. 
