SECT. XXVII. HEAT IN SOLAR SPECTRUM. 263 



subject to the same laws of reflection and refraction as rays of 

 light. The index of refraction from a prism of rock-salt, deter- 

 mined experimentally, is nearly the same for light and heat. 



Liquids, the various kinds of glass, and probably all sub- 

 stances, whether solid or liquid, that do not crystallize regularly, 

 are more pervious to the calorific rays according as they possess 

 a greater refractive power. For example, the chloride of sulphur, 

 which has a high refractive power, transmits more of the calorific 

 rays than the oils, which have a less refractive power : oils 

 transmit more radiant heat than the acids ; the acids more than 

 aqueous solutions ; and the latter more than pure water, which 

 of all the series has the least refractive power, and is the least 

 pervious to heat. M. Melloni observed also that each ray of the 

 solar spectrum follows the same law of action with that of ter- 

 restrial rays having their origin in sources of different tempera- 

 tures ; 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 tempe- 

 rature. 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 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 spec- 

 trum ; 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 



