and the Mode of its Communication. i6j 
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 tempera- 
ture 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 evident 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 
mirror, 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 ex- 
tremely probable that the total quantity of frigorific rays 
(emitted and reflected) which, coming from the surface of the 
cold mirror, impinge against the surface of the hot mirror, will 
be the same, whatever may be the degree of polish, or reflecting 
power, of the cold mirror. And, if this be the case, we may 
conclude, that the presence of this mirror will have no effect 
whatever on the hot mirror; or, that it will no more expedite its 
cooling than any other body, of any other form, would do, at 
the same distance, and occupying the same space. 
It might perhaps be imagined, that the form of the cold mirror 
might concentrate the rays it emits and reflects, and, by such 
concentration, produce a greater effect on the opposite mirror 
than if its surface were flat, or of any other form ; but a more 
