ON HEAT. 



73 



whatever may be the metal employed to conftruct the veflel ; 

 provided always that the external furface of the veflel be very 

 clean, and the temperature of the air the fame. 



In order that the cooling fliall be effected in the fame time, The furface only 

 nothing more is required than that the external furface of the 

 veflel be truly metallic, and not covered with oxi4e or other 

 foreign bodies. 



On the enquiry, what quality all the metals might have in This arifes from 

 common, and poflefs in the fame degree, to which this re- ^g^ 801 ^ ot 

 markable equality of their fufceptibility of cooling might be 

 attributed, I found it in their opacify. 



The rays which cannot pene rate the furface of a body, by which the 

 muft neceflarily be thrown back or reflected ; and as the rays ea lsre e 

 of light, which have much analogy with the inviflble calo- 

 rific or frigorific rays, eafily penetrate glafs, though they 

 are reflected, at lead for the greateit part, by metallic fur- 

 faces, I fufpecled beforehand the refult of the experiment 

 with the two bottles, one of glafs and the other of tinned 

 iron. A^ 



The ftate of a heated body, or a body which contains a Ufual comparl- 

 certain quantity of caloric, has been compared to that of 'a. ^" ° r .^ to 

 fponge which contains a certain quantity of water. Sup- fponge. 

 poling this comparifon to be juft, we might compare the lofs 

 of heat by the emiffion of the calorific rays, to the lofs of 

 water by evaporation. Let us try if this comparifon can fup- 

 ply us with the means of throwing fome light on the interefting 

 iubject of our researches. 



Inftead of the fponge filled with w^ater, let us fubititute The fame am- 

 the earth, and fuppofe for a moment that the earth is every pi 

 where equally heated, and its furface in all parts covered with 

 a bed of the fame kind of foil equally moid. 



As a fquare league in a mountainous country contains more If it were true, 

 furface or more fuperlicial acres than a fquare league fitualed wou?d emhmoxe 

 in the plain, it is evident that more water would be evaporated heat than a 

 from the whole furface of the earth in a given time^f the earth fmoochone « 

 were covered with mountains, than if its furface were an im- 

 menfe plain, and coniequently, that more caloric ought to be 

 projected from the furface of any folid body broken with afpe- 

 rites, than from the furface of another body of the fame form 

 and dimeniions, which is fmooth or well polilhed. 



This 



