288      On  the  Heat-  Spectrum  of  the  Sun  and  the  Lime-Light, 
tigated,  some  could  be  perceived  even  in  the  blue,  although  this 
spectrum  was  no  brighter  than  that  obtained  with  the  flint-glass 
prism.  Moreover  absorption- experiments  with  a  plate  of  flint 
glass  and  with  water  gave  a  very  distinct  apparent  absorption  of 
the  luminous  heat.  This  result  (which,  if  correct,  shows  that 
luminous  heat  is  not  absorbed  by  transparent  bodies  in  the  same 
proportion  as  light)  induced  me  to  investigate  very  carefully  the 
absorption  of  luminous  heat  by  flint  glass,  using  for  the  purpose 
the  above-mentioned  method  with  two  rock-salt  prisms.  Having 
in  this  way  separated  certain  homogeneous  rays  of  sunlight,  I 
compared  their  heat-effect  before  and  after  the  insertion  of  a 
plane- parallel  plate  of  flint  glass  so  arranged  that  the  rays  fell 
on  it  perpendicularly. 
Such  experiments  were  made  for  all  the  colours  of  the  solar 
spectrum  ;  and  the  absorption  of  heat  amounted  to  : — 
for  red      .     . 
12  per  cent. 
„  orange 
„  yellow      . 
.     10        „ 
•       7        „ 
„  green  .     . 
„  blue    .     . 
■       6        „ 
•       5        „ 
From  this  amount  of  absorbed  heat  it  is  necessary  to  deduct  the 
perpendicularly  reflected  heat,  which,  for  a  flint-glass  plate,  is 
equal  to  5  per  cent,  of  the  total  incident  heat. 
It  still  remains  a  desideratum  to  try  photometrically  whether 
the  coloured  light  is  not  absorbed  in  the  same  proportion  ;  other- 
wise we  must  suppose  that  in  our  experiments  diffused  heat  was 
not  entirely  excluded. 
According  to  these  experiments,  then,  it  must  be  admitted 
that  the  strong  heat- effect  observed  in  the  luminous  part  of  the 
lime-light  spectrum  in  the  above-mentioned  experiments,  arose 
chiefly  from  heat-rays  which,  in  the  rock-salt  prisms,  as  in  turbid 
media,  were  deflected  by  diffuse  reflection. 
Experiments  were  also  made  on  the  absorption  of  the  ultra-red 
rays  by  transparent  bodies,  such  as  water,  glass,  mica,  quartz, 
and  calc-spar.  From  the  end  of  the  visible  red  to  the  place  in 
the  ultra-red  part  where  thermal  action  quite  ceased  or  at  least 
was  very  feeble,  the  heat-effect  was  compared  before  and  after 
the  insertion  of  the  bodies  mentioned.  It  now  appeared  that, 
as  Melloni  (Pogg.  Ann.  1832)  had  previously  found  with  respect 
to  water,  the  ultima-red  rays  in  passing  through  transparent  bodies 
suffer  a  greater  loss  the  less  their  refrangibility.  In  regard  to  the 
displacement  of  the  maximum  in  the  spectrum  of  the  lime-light 
after  the  insertion  of  the  above-mentioned  bodies,  it  depends  on 
the  thickness  of  the  body  inserted.  For  example,  a  layer  of 
water  of  2  millims.  thickness  caused  no  displacement  of  the 
