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mental fact, that opaque bodies, generally speaking, radiate the same 

 description of heat at the same temperature. In like manner, the 

 light which they radiate is of the same description at the same tem- 

 perature ; one body is not red while a second is yellow and a third 

 white, but they are all either red or yellow, or white together. 



An analogy has thus been established between radiant heat and 

 light in certain of their properties. Now two opinions have been en- 

 tertained with regard to light : 



1st. Some have regarded it as differing from radiant heat only in 

 wave length. 



2nd. Others have regarded the two as physically distinct, although 

 possessing many properties in common. It has even been thought 

 that some kinds of light have no heating effect on the bodies on 

 which they fall. 



I cannot but think that the facts just stated countenance the 

 former opinion rather than the latter : for Prevost's theory consists 

 of the three following hypotheses : 



1 st. That if an enclosure of any kind be kept at a uniform tem- 

 perature, any body placed within the enclosure, and surrounded by 

 it on all sides, will ultimately attain that temperature. 



2nd. That all bodies are constantly giving out radiant heat, at a 

 rate depending upon their substance and temperature, but independent 

 of the substance or temperature of the bodies that surround them. 



3rd. And, consequently, that when a body is kept at a uniform 

 temperature, it receives back just as much heat as it gives out. 



From these three assumptions may be deduced all the facts that 

 have been stated with regard to radiant heat ; but in the argument 

 it is essential that the rays under consideration shall have the pro- 

 perty of heating the bodies on which they fall, and by which they 

 are absorbed. If this be not granted the argument fails. Now 

 radiant light, or those rays only that affect the retina, have been 

 found to possess properties analogous to those which radiant heat 

 thus possesses in virtue of its departure lowering the temperature of 

 the body which it leaves, and its absorption raising that of the body 

 on which it falls. If, therefore, we suppose all kinds of radiant 

 light to have the property of raising (however little) the temperature 

 of the body by which they are absorbed, the facts that have been 

 stated in this paper regarding light may be shown to be a natural 



