concerning Heat. 159 



communicated or excited must be, in all cases, as the 

 quantity of light absorbed. 



I have just made some experiments which appear to 

 me to establish this fact beyond question. 



Having procured from the optician Lerebours two 

 lenses perfectly equal, and of the same kind of glass, 

 4 inches in diameter, and of n^ focus, I exposed them 

 at the same time to the sun, side by side, about noon, 

 when the sky was very clear ; and by means of two 

 thermometers, or reservoirs of heat, of a peculiar con- 

 struction, I determined the relative quantities of heat 

 that were excited in given times by the solar rays at dif- 

 ferent distances from the foci of the lenses. 



The two reservoirs of heat are a sort of flat boxes of 

 brass filled with water. Each of these reservoirs is 

 3 inches io| lines in diameter, and 6 lines thick, well 

 polished externally on all sides except one of its two 

 flat faces, which was blackened by the smoke of a 

 candle. On this face the solar rays were received in 

 the experiments. 



Each of these reservoirs of heat weighs when empty 

 6850 grains, poids de marc (near a pound troy), and 

 contains 1210 grains of water (about 2 oz. 2 dwts.). 



Taking the capacity of brass for heat to be to that 

 of water as i to n, it appears that the capacity of 

 the metallic box weighing 6850 grains is equal to the 

 capacity of 622 grains of water; and, adding this quan- 

 tity of water to that contained in the box, we shall have 

 the capacity of the reservoir prepared for the experi- 

 ments equal to that of 1832 grains of water. 



Each reservoir is kept in its place by a cylinder of 

 dry wood, one of the extremities of the cylinder being 

 fixed in a socket in the centre of the interior face of the 



