180 ON THE RADIATION OF CALORIC. 



rived from the apparent reflection of cold by concave mirrors ; 

 and from the circumstance, that a blackened surface, which, 

 when heated, produces more heat than a polished one, when 

 cooled, produces more cold, though in the latter case it must 

 be supposed to radiate most caloric. In Thomson's An- 

 nals, vols. v. — viii. are given various papers by M. Prevost and 

 Mr Davenport, which appear to me to afford a complete ex- 

 planation of these facts ; but, from the conciseness of their 

 statements, and from their not being illustrated by calculations 

 or diagrams, they are not easily comprehended, and appear 

 not to have been generally understood. Mr Davenport's 

 statement is, " Fill a canister, of which one side is polished, 

 and the other black, with a freezing mixture. Has not the 

 black surface lost a part of its intensity of radiation ? This 

 cannot be denied. Has the polished surface lost its power of 

 reflection ? The answer must be in the negative. It follows 

 then, that unequal diminutions have been imposed on those 

 powers of returning heat to the thermometer, which before 

 were equal. The radiating surface has lost more than the 

 reflecting one, and its thermometer receives less return, than 

 that on the reflecting side." This appears to me a satisfactory 

 explanation ; but from the difficulty of comprehending the rea- 

 soning, without reducing it to calculation, it seems not to have 

 been generally understood. Dr Murray, after admitting both 

 the premises, objects to the concluion. He says, " The clear 

 surface has also had its intensity of radiation proportionally re- 

 duced. And at any temperature, the blackened surface ra- 

 diates more caloric than the clear surface does at the same 

 temperature. The former, therefore, returns more caloric to 

 the thermometer than the latter, or at least, allowing all the 

 effect that can be ascribed to difference of reflection, no cause 

 is assigned why it produces a greater degree of cold." 



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