and the Mode of its Communication . 125 
appearances might be which indicated that event. But, to return 
from this digression. 
Having found that the intensity of the calorific rays emitted 
by a hot body, at any given temperature, depends much on the 
surface of such body, — that a polished metallic surface, for in- 
stance, throws off much fewer rays than the same surface, at 
the same temperature, would emit, if painted, or blackened in 
the smoke of a lamp or candle, I was desirous of finding out, 
whether the frigorific rays from cold bodies are affected in the 
same manner, by the same means, and in the same degree. 
It was to ascertain that point, that the experiment No. 20 was 
made; and, although the result of that experiment afforded 
abundant reason to conclude, that those substances which, when 
hot, throw off calorific rays in the greatest abundance, actually 
throw off great quantities of frigorific rays, when they are cold ; 
yet, as the relative quantities of these rays could not be exactly 
determined by that experiment, in order to ascertain so import- 
ant a fact, I had recourse to the following simple contrivance. 
Exper. No. 24. Having found, by the result of the last ex- 
periment, (No. 23,) that the calorific emanations of a circular 
disk of polished brass, 3 inches in diameter, at the temperature 
of 112 0 F. were just counterbalanced by the frigorific emana- 
tions of an equal disk of the same polished metal, at the tem- 
perature of 32 0 F. placed opposite to it, so that one of the balls 
of the thermoscope placed between these two disks, at equal 
distances, was just as much heated by the one as it was cooled 
by the other, I now blackened the two disks, by holding them 
over the flame of a wax candle, and repeated the experiment 
with them, so blackened. 
