concerning Heat. 161 



A was to the density of that at the surface of the reser- 

 voir B as the square of 24 to the square of 6, or as 

 i 6 to i. 



I imagined that if the quantity of heat which a given 

 quantity of light is capable of exciting depended any 

 way on its density, as the densities were so different 

 in this experiment, I could not fail to discover the 

 fact by the difference of time which it would require 

 to raise the two thermometers the same number of 

 degrees. 



Having continued the experiment more than an hour, 

 on a very fine day, when the sun was near the meridian 

 and shone extremely bright, I did not find that one of 

 the reservoirs was heated perceptibly quicker than the 

 other. 



Experiment No. 2. I placed the reservoir of heat 

 A still nearer the focus of the lens, in a situation where 

 the solar spectrum was only 4! lines in diameter, and 

 where blackened paper caught fire in two or three sec- 

 onds ; and I removed the reservoir B still farther from 

 the focus, advancing it forward till the diameter of the 

 spectrum was 2 inches 3 lines. 



The densities of the light at the surfaces of the reser- 

 voirs in this experiment were as 32 to i. 



The temperature of the reservoirs as well as that of 

 the atmosphere, at the beginning of the experiment, was 

 54 F., = 9 f R. 



The reservoir A, after having been exposed to the 

 action of very intense light near the focus of the lens 

 for 24 minutes 40 seconds, was raised to the tempera- 

 ture of 80 F., = 2 if R. 



The reservoir B, which was much farther from the 

 fjcus of its lens, was raised to the same temperature, 



VOL. II. II 



