SECT, xxv.] MAXIMUM OF HEAT IN THE SPECTEUM. 247 



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 refraugibility 

 is less. The rays of heat that are mixed with the blue or 

 violet light pass in great abundance, while those in the ob- 

 scure part which follows the red light are almost totally 

 intercepted. 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 with 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 ; but, 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 to- 

 wards 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, the crown and flint glass 

 prisms transfer that point respectively to the red and to its 

 limit. M. Melloni, observing that the maximum point of heat 

 is transferred farther and farther towards the red end of the 



