38 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 



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

 this department of knowledge since their time has shown 

 that precisely the same modifications found in the colors 

 of light occur also for heat; a fact conveniently desig- 

 nated by the phrase " ideal coloration of heat," and, fur- 

 ther, that the wave-length of the heat emitted depends 

 upon the temperature of the radiating source. It is one 

 thing to investigate the phenomena of the exchanges of 

 heat-rays of the same color, and another when the colors 

 are different. A complete theory of the exchanges of 

 heat must include this principle, and, of course, so too 

 must a law of cooling applicable to any temperature. 



There is another fact to some extent considered by 

 Dulong and Petit, but not of such weight in their in- 

 vestigations, where the range of temperature was small, 

 as in these where it rises as high as nearly 3000 Fahr. 

 This is the difference of specific heat of the same body 

 at different temperatures. At the high temperatures 

 herein employed, there cannot be a doubt that the 

 capacity of platinum for heat is far greater than that at 

 a low point. This, therefore, must affect its rate of calo- 

 rific emission, and probably that for light also. 



From these and similar considerations we should be 

 led to expect that as the temperature of an incandescent 

 solid rises, the intensity of the light emitted increases 

 very rapidly. 



I pass now to the experimental proofs which substan- 

 tiate the foregoing reasoning. 



The apparatus employed as the source of the light 

 and measure of the temperature was the same as in the 

 preceding experiments a strip of platinum brought to 

 a known temperature by the passage of a voltaic current 

 of the proper force, and connected with an index which 

 measured its expansion. 



The principle upon which the intensities of the light 



