Apparent Radiation of Cold. 241 

 the game body in experiment 2, when the pigment is removed. 

 Therefore r'i -f t > r'-f That is to say, the whole amount of 

 calorific rays reflected and radiated from the metallic body, in an 

 atmosphere to which it is relatively cold, will be increased by re- 



diating power ; and a thermometer near it, or at a distance, if re- 

 flectors be employed, will indicate an elevation of temperature, not- 

 withstanding the diminution of the radiating power of the body by 

 this operation. This is the proposition which was to be demon- 

 strated. It is therefore obvious, that the elegant theory of radiant 

 heat proposed by M. Prevost, is rcconcileable with these pheno- 

 mena, and receives additional support from those experiments 

 which some eminent chemists have adduced for its refutation. 



The foregoing investigation suggests a general formula, for ex- 

 pressing the ratio of the reflecting to the absorbent and radiating 

 powers of bodies. The reviewer of Prof. Leslie's Experimental 

 Enquiry on Heat* remarks, that " the reflecting powers of bodies, 

 bear some inverse ratio to their absorbent and projecting (i. e. ra- 

 diating) powers, although so many circumstances unite in modify- 

 ing the proportion, that we are unable, as yet, to express it by one 

 general law." I shall state the general law, according to which 

 these powers vary. Is not this the only sense in which the na- 

 ture of the case can ever admit of a general law 1 



From the term absorption, (which though now sanctioned by 

 general use, appears to me not happily chosen,) we must not be 

 led to suppose that bodies have a positive power of imbibing calo- 

 ric, or of conveying it inwards from their surfaces, the least con- 

 ceivable distance, Independently of conduction. Those incident 

 rays which are not reflected, are necessarily retained at the sur- 

 face on which they impinged. Hence the sum of the refecting and 

 absorbent poicers is a constant quantity ; and we have shown that 

 the sum of the reflecting and radiating powers is constant. From 

 this, and from the fact that a change in the surfaces of neighboring 

 bodies does not disturb their equilibrium of temperature, it may be 

 inferred that the radiating and absorbent powers of any body are 

 equal, and increase or diminish in the same ratio. It is therefore 

 only necessary to express the law by which the radiating and re- 

 flecting powers vary. These being cactcris paribus as the quan- 



• Vide Edinburgh Review. 



31 



