3 to Dr. Herschel's Experiments on the solar, and 



arrangement of the distances. Thus, if the lens be placed at 3-*- 

 inches from a wax-candle, and the thermometer situated, as be- 

 fore, in the secondary focus, we shall be able to draw from 5 to 8 

 degrees of heat, according to the burning of the candle, and the 

 accuracy of the adjustment of the thermometer to the focus. 

 The experiment we have related shews evidently, that rays 

 invested with a power of heating bodies, issue from a candle, 

 and are subject to laws of refraction, nearly the same with 

 those respecting light. 



13^ Experiment. 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 

 9 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.* In 

 the focus of the glass was placed the thermometer No. 3 ; and, 

 when every thing was arranged properly, I covered the lens for 

 five minutes, that the thermometer might assume the tempera- 

 ture of its situation. The result was as follows. 



No. 3. 

 The lens covered o' 64 



Open - - 1 176 



Here, in one minute, the thermometer received 112 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 



• See Plate XIV. . 



