214 MAXIMUM OF HEAT IN SPECTRUM. SECT. XXV. 



ing their origin in sources of different temperatures ; so 

 that the very refrangible rays may be compared to the 

 heat emanating from a focus of high temperature, and 

 the least refrangible to the heat which comes from a 

 source of low temperature. Thus if the calorific rays 

 emerging from a prism be made to pass through a layer 

 of water contained between two plates of glass, it will 

 be found that these rays suffer a loss in passing through 

 the liquid, as much greater as their refrangibility is less. 

 The rays of heat that are mixed with the blue or violet 

 light pass in great abundance, while those in the obscure 

 part which follows the red light are almost totally inter- 

 cepted. The first, therefore, act like the heat of a 

 lamp, and the last like that of boiling water. 



These circumstances explain the phenomena observed 

 by several philosophers will regard to the point of 

 greatest heat in the solar spectrum, which varies with 

 the substance of the prism. Sir William Herschel, 

 who employed a prism of flint glass, found that point to 

 be a little beyond the red extremity of the spectrum : 

 bat according to M. Seebeck, it is found to be upon the 

 yellow, upon the orange, on the red, or at the dark 

 limit of the red, according as the prism consists of 

 water, sulphuric acid, crown or flint glass. If it be 

 recollected that in the spectrum from crown glass, the 

 maximum heat is in the red part, and that the solar 

 rays, in traversing a mass of water, suffer losses inversely 

 as their refrangibility, it will be easy to understand the 

 reason of the phenomenon in question. The solar heat 

 which comes to the anterior face of the prism of water 

 consists of rays of all degrees of refrangibility. Now, 

 the rays possessing the same index of refraction with 

 the red light suffer a greater loss in passing through the 

 prism than the rays possessing the refrangibility of the 

 orange light, and the latter lose less in their passage than 

 the heat of the yellow. Thus the losses, being inversely 

 proportional to the degree of refrangibility of each ray, 

 cause the point of maximum heat to tend from the red 

 toward the violet, and therefore it rests upon the yellow 

 part. The prism of sulphuric acid acting similarly, but 

 with less energy than that of water, throws the point of 

 greatest heat on the orange ; for the same reason, tho 



