700 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



Exper. 13. Refraction of the Heat that accompanies the Coloured part of the 

 Prismatic Spectrum. — I covered a burning lens of Mr. Dollond's, which is nearly 

 Q inches in diameter, and very highly polished, with a piece of pasteboard, in 

 which there was an opening of a sufficient size to admit all the coloured part of 

 the prismatic spectrum, fig. 7. In the focus of the glass was placed the ther- 

 mometer N° 3 ; and, when every thing was arranged properly, 

 I covered the lens for 5 minutes, that the thermometer might Lens covered 64, 

 assume the temperature of its situation. The result was °P en l 176 



as annexed. Here, in 1 minute, the thermometer received 1 1 2 degrees of heat, 

 which came with the coloured part of the solar spectrum, and were refracted to a 

 focus; so that, if the coloured rays themselves are not of a heat-making nature, 

 they are at least accompanied with rays that have a power of heating bodies, and 

 are subject to certain laws of refraction, which cannot differ much from those 

 affecting light. 



Exper. 14. Refraction of the Heat of a Chimney Fire. — I placed Mr. Dollond's 

 lens before the clear fire of a large grate, fig. 8. Its distance from the bars of 

 the grate was 3 feet; and, in the secondary focus of it was placed the thermometer 

 N° 1 . N° 4 was stationed, by way of standard, at 84 inches from the former, and 

 at an equal distance from the fire. Before the thermometers was a slip of mahogany, 

 which had 3 holes in it, -^ of an inch in diameter each. Behind the centre of 

 the 1st hole, -§- of an inch from the back, was placed the thermometer N° 1 ; and 

 between the 2d and 3d hole, guarded from the direct rays of the fire by the par- 

 tition, at the same distance from the back, was put N° 4. 

 Things being thus arranged, the situation of the ap- 

 paratus which carried the thermometers, and that where 

 the lens was fixed, were marked. Then the thermome- 

 ters, having been taken away to be cooled, were re- 

 stored to their places again, and their progress marked 

 as annexed. Here, in 9% the rays coming from the fire, through the burning 

 glass, gave 9f degrees of heat more to the thermometer N° 1, than N° 4, from 

 change of temperature, had received behind the screen. Now to determine whether 

 this was owing merely to a transmission of heat through 

 the glass, or to a condensation of the rays, by the re- 

 fraction of the burning lens, I took away the lens, as 

 soon as the last observation of the thermometers was 

 written down, and continued to take down their progress as 

 thus. Here the direct rays of the fire, we see, could not 

 keep up the thermometer N° 1 ; which lost 2^- degrees of 

 heat, though the lens intercepted no longer any of them. I 

 now restored the burning glass, and continued. Here 

 again, the lens acted as a condenser of heat, and gave 14 

 degrees of it to the thermometer N° 1. I now once 

 more took away the lens, and continued the experiment. 



