474  Intelligence  and  Miscellaneous  Articles. 
cient  thickness  of  the  vapours  raised  to  a  suitable  temperature.  By 
the  experiments  the  results  of  which  I  am  about  to  indicate,  eight 
substances  are  added  to  the  list  of  vapours  which  produce  an  ab- 
sorption-spectrum. 
Selenium,  heated  to  about  700°  C,  gives  a  vapour  of  which  a  few 
centimetres  thickness  is  reddish,  and  the  tint  becomes  more  red  as 
the  thickness  increases.  A  stratum  25  centims.  thick  absorbs  all 
the  rays  of  the  spectrum  as  far  as  the  red  region  near  the  place  oc- 
cupied by  the  line  c  of  the  solar  spectrum.  When  we  make  the 
experiment  with  a  porcelain  tube  closed  at  its  two  ends  by  parallel 
plates  of  glass,  and  gradually  heated  by  a  row  of  gas-jets,  during  the 
whole  period  of  the  heating  we  observe  only  a  progressive  extinction 
of  all  the  regions  of  the  spectrum,  starting  from  the  most  refran- 
gible rays,  as  far  as  the  red,  without  any  trace  of  dark  lines  ;  but  if 
we  continue  to  raise  the  temperature,  the  tint  of  the  more  expanded 
vapour  brightens,  and  the  different  regions  of  the  spectrum  reap- 
pear, furrowed  with  groups  of  black  bands  in  the  blue  and  the  violet. 
The  appearance  has  a  certain  resemblance  to  the  absorption-spectrum 
of  selenious  acid  which  I  recently  described ;  but  it  is  not  due  to 
the  accidental  production  of  that  substance,  as  I  have  assured  my- 
self by  always  heating  the  selenium  in  an  atmosphere  of  carbonic  acid 
carefully  dried,  which  does  not  produce  any  visible  trace  of  selenious 
acid. 
Protochloride  of  selenium,  obtained  by  bringing  dry  chlorine  upon 
an  excess  of  selenium,  is  a  brown  limpid  liquid,  the  vapour  of  which 
furrows  the  spectrum  with  lines  which  commence  at  the  boundary 
between  the  green  and  the  blue,  and  extend  as  far  as  the  extremity 
of  the  violet. 
Bromide  of  selenium  exercises  its  absorbent  properties  in  a  different 
region  of  the  spectrum.  It  produces  systems  of  nearly  equidistant 
lines  when  observed,  like  the  protochloride,  in  a  thickness  of  10 
centims. 
Tellurium  is  more  favourable  than  the  preceding  substances  for 
the  observation  of  the  phenomenon.  Heated  in  a  tube  of  green 
glass  of  2  or  3  centims.  diameter,  previously  filled  with  dry  carbonic 
acid,  at  a  temperature  near  that  at  which  the  glass  begins  to  melt,  it 
emits  a  golden-yellow  vapour,  which  produces  a  very  brilliant  ab- 
sorption-spectrum, much  more  extended  towards  the  red  than  those 
of  sulphur  and  selenium,  and  composed  of  systems  of  fine  lines 
spreading  out  from  the  yellow  as  far  as  into  the  violet. 
Protochloride  of  tellurium  was  prepared  by  the  action  of  dry  chlo- 
rine on  tellurium  contained  in  a  narrow  tube.  It  forms  a  black 
mass  fusible  into  a  red  liquid,  which  is  reduced  into  a  yellow  vapour 
that  acts  very  vigorously  upon  light.  One  centim.  thickness  is 
sufficient  for  the  observation  of  the  absorption-spectrum  of  this  sub- 
stance, which  is  peculiarly  developed  in  the  orange  and  the  green. 
Protobromide  of  tellurium  is  obtained  easily  by  the  action  of  bro- 
mine on  an  excess  of  tellurium.  It  is  a  crystallized  substance  which, 
by  the  action  of  heat,  emits  a  violet  vapour  that  gives  an  absorption- 
spectrum  the  most  remarkable  lines  of  which  are  in  the  red  and  the 
yellow. 
