246 MELLONl's EXPERIMENTS. [SECT. xxv. 



properties of the heat therefore which issues from alum 

 nearly approach to those of light and solar heat. 



Radiant heat in traversing various media is not only ren- 

 dered more or less capable of being transmitted a second 

 time, but, according to the experiments of Professor Powell, 

 it becomes more or less susceptible of being absorbed in dif- 

 ferent quantities by black or white surfaces. 



M. Melloni has proved that solar heat contains rays 

 which are affected by different substances in the same way 

 as if the heat proceeded from a terrestrial source ; whence 

 he concludes that the difference observed between the 

 transmission of terrestrial and solar heat arises from the 

 circumstances of solar heat containing all kinds of caloric, 

 whilst in other sources some of the kinds are wanting. 



Radiant heat, from sources of any temperature whatever, 

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

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

 determined 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 crystallise regu- 

 larly, 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 ob- 

 served also that each ray of the solar spectrum follows the 

 same law of action with that of terrestrial rays having 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 



