and the Mode of its Communication. 1 1 5 



Suppose two concave reflecting mirrors, of highly 

 polished metal, each 18 inches in diameter, and 18 

 inches focal distance, to be placed opposite to each 

 other at the distance of 10 feet, in a large quiet room, 

 in which the air and the walls of the room remain con- 

 stantly at the same temperature (that of freezing water, 

 for instance), without any variation. 



If we suppose the floor, ceiling, walls of the room, 

 and doors and windows, to be lined with a covering of 

 ice, at the temperature of freezing water, we can then, 

 without any difficulty, conceive that the temperature of 

 the room may remain the same, notwithstanding the 

 presence of hotter bodies, which are brought into it for 

 the purpose of making experiments. 



Let us now suppose one of the mirrors to be at the 

 temperature of freezing, and the other at that of boiling 

 water ; and let us see what effects they would produce 

 on each other by their radiations. 



And first, with respect to the hot mirror, it is evi- 

 dent* that it will be cooled, not only by the frigorific 

 rays which proceed from the cold metal of which the 

 opposite mirror is constructed, but also by such of the 

 frigorific rays from the sides of the room as, impinging 

 against the polished reflecting surface of the cold mir- 

 ror, and being reflected by that surface, happen to fall 

 on the surface of the hot mirror without being reflected 

 by it. 



But, as the quantity of rays which the cold mirror 

 reflects is greater in proportion as the reflecting surface 

 is more perfect, while the quantity of rays emitted by 

 this cold mirror is less in proportion as its reflecting 

 surface is more perfect, it is extremely probable that the 

 total quantity of frigorific rays (emitted and reflected) 



