HEAT. 47 



in planes at right angles to each other, as is the case with 

 light. 



The relation of radiation to absorption also holds good 

 with light as with heat : with the latter it has been long known 

 that the radiating power of different substances is directly 

 proportional to their absorptive and inverse to their reflective 

 power ; or rather, that the sum of the heat radiated and re- 

 flected is a constant quantity. Further, it has been shown 

 by Mr. Balfour Stewart, that the absorption of heat is propor- 

 tional to its radiation, both as to quality and quantity. 



Light presents us with similar relations. Coloured glass, 

 when heated so as to be luminous, emits the same light 

 which at ordinary temperatures it absorbs : thus red glass gives 

 out or radiates a greenish light, and green glass a red tint. 



The flame of substances containing sodium yields a yellow 

 light of such purity that other colours exposed to it appear 

 black a phenomenon shown by the familiar experiment of 

 exposing a picture of bright colours, other than yellow, to the 

 flame of spirits of wine with which common salt is mixed : the 

 picture loses its colours, and appears to be black and white. 

 When the prismatic spectrum of such a flame is examined, it 

 is found to exhibit two bright yellow lines at a certain fixed 

 position. If a source of light be employed which gives no 

 lines in its spectrum, and the light of this incandescent sub- 

 stance be made to pass through the sodium flame, two dark 

 lines will appear in the spectrum precisely coincident in posi- 

 tion with the yellow lines which were given by the sodium 

 flame itself. The same relation of absorption to radiation is 

 therefore shown here : the substance absorbs that light which 

 it yields when it is itself the source of light, The same is 

 true of other substances, the spectra of which exhibit respec- 

 tively lines of peculiar colour and position. Now, the solar 

 prismatic spectrum is traversed by a great number of dark 

 lines ; and Kirchhoff has deduced from considerations such as 

 those which I have shortly stated, that these dark lines in the 

 solar spectrum are due to metals or other substances existing 

 in an atmosphere around the sun which absorb the light from 

 a central incandescent nucleus, each metal absorbing that 



