MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 37 



pointed out must occur in an analogous manner for the 

 phenomena of light. 



I next pass to the third branch of this investigation 

 to examine 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 has been traced between the 

 phenomena of light and radiant heat lends countenance 

 to the supposition that the law which regulates the es- 

 cape of heat from a body will also determine its rate of 

 emission of light. Sir Isaac Newton supposed that while 

 the temperature of a body rose in an arithmetical pro- 

 gression, the amount of heat escaping from it increased 

 in a geometrical progression. The error of this was sub- 

 sequently shown by Martin, Erxleben, and Delaroche, 

 and finally Dulong and Petit gave the true law : " When 

 a hot body cools in vacuo, surrounded by a medium the 

 temperature of which is constant, the velocity of cooling 

 for excess of temperature in arithmetical progression in- 

 creases as the terms of a geometrical progression, dimin- 

 ished by a constant quantity." The introduction of this 

 constant depends on the operation of the theory of the 

 exchanges of heat ; for a body when cooling under the 

 circumstances here supposed is simultaneously receiving 

 back a constant amount of heat from the medium of con- 

 stant temperature. 



While 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 

 the present inquiry certain circumstances to be taken 

 into account not contemplated by those philosophers. 

 Dulong and Petit, throughout their memoir, regard radi- 

 ant heat as a homogeneous agent, and look upon the 

 theory of exchanges, which is indeed their starting-point 



