CONDUCTION AND RADIATION. 541 



Sect. 6. Other Laws of Phenomena with respect 

 to Radiation. 



THE laws of radiation as depending upon the sur- 

 face of radiating bodies, and as affecting screens 

 of various kinds interposed between the hot body 

 and the thermometer, were examined by several 

 inquirers. I shall not attempt to give an account 

 of the latter course of research, and of the different 

 laws which luminous and non-luminous heat have 

 been found to follow in reference to bodies, whether 

 transparent or opaque, which intercept them. But 

 there are two or three laws of the phenomena, on 

 the subject of the effects of the surfaces of bodies, 

 which are important. 



1. In the first place, the powers of bodies to 

 emit and to absorb heat, as far as depends upon 

 their surface, appear to be in the same propor- 

 tion. If we blacken the surface of a canister of 

 hot water, it radiates heat more copiously, and in 

 the same measure, it is more readily heated by 

 radiation. 



2. In the next place, as the radiative power in- 

 creases, the power of reflection diminishes, and 

 the contrary. A bright metal vessel reflects much 

 heat ; on this very account it does not emit much ; 

 and hence a hot fluid which such a vessel con- 

 tains, remains hot longer than it does in an un- 

 polished case. 



3. The heat is emitted from every point of the 



