and the Mode of its Communication. 73 



tain so important a fact I had recourse to the following 

 simple contrivance. 



Experiment No. 24. Having found, by the result 

 of the last experiment (No. 23), that the calorific ema- 

 nations of a circular disk of polished brass, 3 inches in 

 diameter, at the temperature of 112 F., were just 

 counterbalanced by the frigorific emanations of an equal 

 disk of the same polished metal, at the temperature of 

 32 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 blacke.ned. 



I knew, from the results of former experiments, that 

 the intensity of the calorific radiations from the hot 

 disk would be very much increased, in consequence of 

 its surface being blackened ; and I was certain that, if 

 the intensity of the frigorific radiations of the cold disk 

 should not be increased in exactly the same degree, the 

 ball of the thermoscope, exposed to the simultaneous 

 actions of these two disks, could not possibly remain at 

 the same constant temperature, that of 72. 



The result of the experiment was very decisive ; the 

 bubble of spirit of wine remained at rest, which proved 

 that the intensities of the rays emitted by the two disks 

 still continued to be equal at the surface of the ball 

 of the thermoscope, which, at equal distances, was ex- 

 posed to their simultaneous action. 



Hence we may conclude, that those circurnstances 

 which are favourable to the copious emission of calorific 

 rays from the surfaces of hot bodies are equally favour- 

 able to a copious emission of frigorific rays from similar 

 bodies when they are cold. 



