concerning Heat. 141 



ity of cooling might be attributed, I found it in their 

 opacity. 



The rays which cannot penetrate the surface of a 

 body must necessarily be thrown back, or reflected ; 

 and as the rays of light, which have much analogy with 

 the invisible calorific or frigorific rays, easily penetrate 

 glass, though they are reflected, at least for the greatest 

 part, by metallic surfaces, I suspected beforehand the 

 result of the experiment with the two bottles, one of 

 glass, and the other of tinned iron. 



The state of a heated body, or a body which con- 

 tains a certain quantity of caloric, has been compared 

 to that of a sponge which contains a certain quantity of 

 water. Supposing this comparison to be just, we might 

 compare the loss of heat by the emission of the ca- 

 lorific rays to the loss of water by evaporation. Let 

 us try if this comparison can supply us with the means 

 of throwing some light on the interesting subject of our 

 researches. 



Instead of the sponge filled with water, let us substi- 

 tute the earth, and suppose, for a moment, that the 

 earth is everywhere equally heated, and its surface, in 

 all parts, covered with a bed of the same kind of soil, 

 equally moist. 



As a square league in a mountainous country contains 

 more surface, or more superficial acres than a square 

 league situated in the plain, it is evident that more 

 water would be evaporated from the whole surface of 

 the earth in a given time, if the earth were covered with 

 mountains than if its surface were an immense plain, 

 and, consequently, that more caloric ought to be pro- 

 jected from the surface of any solid body broken with 

 asperities, than from the surface of another body, of the 



