352 Prof. Draper on the 'production of Light by Heat. 



inciding with, but overlapping the colours of the other. In 

 such a spectrum there must undoubtedly be a general com- 

 mixture of the rays; but may we not fairly inquire whether, 

 if an elementary prism were used, the same facts would hold 

 good ; or, if the anterior face of the prism were covered by a 

 screen, so as to expose a narrow fissure parallel to the axis of 

 the instrument, would there be found in the spectrum it gave 

 every colour in every part, as in Sir David Brewster's original 

 experiment? M. Melloni has shown how this very consi- 

 deration complicates the phsenomena of radiant heat ; and it 

 would seem a very plausible suggestion that the effect here 

 pointed out must occur in an analogous manner for the phac- 

 nomena of light. 



I proceed now to the third branch of the inquiry, — to ex- 

 amine the relation between the temperatures of self-luminous 

 bodies and the intensity of the light they emit, premising it 

 with the following considerations. 



The close analogy which is traced between the phasnomena 

 of light and radiant heat lends countenance to the supposition, 

 that the law which regulates the escape of caloric from a body 

 will also determine its rate of emission of light. Sir Isaac 

 Newton supposed that whilst the temperature of a body rose 

 in arithmetical progression, the amount of heat escaping from it 

 increased in a geometrical progression. The fallacy of this was 

 subsequently shown by Martin, Erxleben, and Delaroche ; and 

 finally Dulong and Petit gave the true law, "when a body cools 

 in vacuo, surrounded by a medium whose temperature is con- 

 stant, the velocity of cooling for excess of temperature in arith- 

 metical progression increases as the terms of a geometrical 

 progression, diminished by a constant quantity." The intro- 

 duction of this constant depends on the operation of the theory 

 of exchanges of heat ; for a body, when cooling under the 

 circumstances here given, is simultaneously receiving back a 

 constant amount of heat from the medium of constant tempe- 

 rature. 



Whilst Newton's law represents the rate of cooling of bodies, 

 and therefore the quantities of heat they emit, when the range 

 of temperature is limited, and the law of Dulong and Petit 

 holds to a wider extent, there are in our inquiry certain cir- 

 cumstances to be taken into account not contemplated by 

 those philosophers, Dulong and Petit throughout their me- 

 moir regard radiant heat as a homogeneous agent, and look 

 upon the theory of exchanges, which is indeed their starting- 

 point and guide, as a very simple affair. But the progress of 

 this department of knowledge since their times has shown, 

 that precisely the same modifications as are found in the co- 



