REPORT ON RADIANT HEAT. 
293 
Invisible rays. Red rays. 
Flint glass. 000 143 
Coach glass ....... 143.200 
Crown glass. 182 294 
Dark red glass. 000 692 
5.) Sir D. Brewster has lately been engaged in some re¬ 
searches on this subject, an abstract of which he has kindly 
communicated in manuscript for this Report. Agreeably to 
the view he has established of the solar prismatic spectrum as 
consisting of spectra of three primary colours superposed , and 
having their maxima at different points, he regards the heating 
power as due, in like manner, to another primary spectrum 
superposed in the same way; and similarly the chemical rays. He 
makes the following statements with respect to the heating rays. 
1st, There is no proof whatever of the existence of invisible 
rays of any kind beyond the red or the blue extremity of the 
spectrum. Sir W. Herschel’s experiments prove the existence 
of heat beyond the visible extremity of the spectrum which he 
used ; but Sir D. Brewster has succeeded in rendering the 
spectrum visible at every point where any heat was produced. 
By particular processes he has traced the light at that end 
greatly beyond the place where Frauenhofer makes the spec¬ 
trum terminate. 
The same he considers established in regard to the blue end 
of the spectrum and of the deoxidizing rays. He thinks it ex¬ 
tremely probable that the heating and illuminating rays are 
different rays ; but they have never yet been found in a state 
of complete separation. 
2ndly, Until it is proved, therefore, or rendered probable, that 
the same intensity of light of different colours, as it proceeds 
directly from the sun, is accompanied with different degrees of 
heat, we must assume it as true that the heating power is pro¬ 
portional to the illuminating power of the different rays of 
solar light. 
3rdly, It appears from Dr. Seebeck’s experiments on the 
water spectrum, that this relation holds generally in it, as he 
found the maximum of heat to be in the yellow rays, or coinci¬ 
dent with the maximum of light. Hence Sir D. Brewster draws 
the important conclusion, that water has the same degree of 
transparency for the solar heating rays that it has for light , 
which is the same as all colourless transparent media have for 
light; that is, water absorbs equally all the different rays of 
solar heat, in the same manner as it does all the different rays 
of solar light. 
