ON THE RADIATION OF CALORIC. 183 



ened surface will be 3 times as sensible to cold as the polished 

 one. 



Thus this theory explains why those surfaces which ra- 

 diate the most caloric, produce the greatest cold when cooled, 

 and are most sensible to the impressions either of heat or cold 

 from surrounding bodies. It is equally applicable to the appa- 

 rent reflection of cold. In Fig, 1, 2, 3, let DEFG be the sec- 

 tion of a room. In Fig. 1. let the thermometer T be near a 

 plane mirror AB, This prevents the radiations from the wall 

 DE from reaching T. But this is compensated by its reflec- 

 ting on T a part of the rays which proceed from the space 

 HGFI. Of these the oold body C intercepts the portion KL, 

 and therefore the plane mirror will increase the cold, but in a 

 very minute degree, in the proportion of KL to HGFI. 

 Fig. 2., AB is a concave mirror. This also intercepts the same 

 radiation DE, and, if the thermometer be in the focus of pa- 

 rallel rays, the mirror reflects on it rays proceeding from HI 

 and these are rendered equivalent to those from HGFI in the 

 former case, because the plane reflects only some of the rays 

 on T, while the concave mirror concentrates at its focus all 

 the rays from HI. But when the cold body is introduced, it 

 intercepts the same space KL as before. The degree of cold 

 therefore, from one concave mirror, should be as KL to HI 

 that is, evidently much greater than in the last case. This is 

 supposing the thermometer to be placed exactly in the focus 

 of parallel rays. But if it be placed so that all rays proceed- 

 ing from C would be converged upon it, then only such rays 

 as MCB, which pass through C, will be reflected on T. When 

 the cold body is introduced at C, it will therefore intercept all 

 these rays, and a much greater cold will be produced. 



Fig. 3. represents the action of 2 concave mirrors. Here 

 AB intercepts the radiation from DE, and KL that from HI, 



and 



