SECT. II.] GENERAL NOTIONS. 33 



If within an enclosure closed in all directions, and maintained 

 by some external cause at a fixed temperature a, we suppose dif 

 ferent bodies to be placed without touching any part of the bound 

 ary, different effects will be observed according as the bodies, 

 introduced into this space free from air, are more or less heated. 

 If, in the first instance, we insert only one of these bodies, at the 

 same temperature as the enclosure, it will send from all points of 

 its surface as much heat as it receives from the solid which sur 

 rounds it, and is maintained in its original state by this exchange 

 of equal quantities. 



If we insert a second body whose temperature 6 is less than a, 

 it will at first receive from the surfaces which surround it on 

 all sides without touching it, a quantity of heat greater than that 

 which it gives out : it will be heated more and more and will 

 absorb through its surface more heat than in the first instance. 



The initial temperature b continually rising, will approach with 

 out ceasing the fixed temperature , so that after a certain time 

 the difference will be almost insensible. The effect would be op 

 posite if we placed within the same enclosure a third body whose 

 temperature was greater than a. 



41. All bodies have the property of emitting heat through 

 their surface; the hotter they are the more they emit; the 

 intensity of the emitted rays changes very considerably with the 

 state of the surface. 



42. Every surface which receives rays of heat from surround 

 ing bodies reflects part and admits the rest : the heat which is not 

 reflected, but introduced through the surface, accumulates within 

 the solid; and so long as it exceeds the quantity dissipated by 

 irradiation, the temperature rises. 



43. The rays which tend to go out of heated bodies are 

 arrested at the surface by a force which reflects part of them into 

 the interior of the mass. The cause which hinders the incident 

 rays from traversing the surface, and which divides these rays into 

 two parts, of which one is reflected and the other admitted, acts in 

 the same manner on the rays which are directed from the interior 

 of the body towards external space. 



F. H. 3 



