SKCT. XXV. MELLONl'S EXPERIMENTS. 213 



By reversing the experiment, and exposing different 

 substances to caloric that had already passed through 

 alum, M. Melloni found that the heat emerging from 

 alum is almost totally intercepted by opaque substances, 

 and is abundantly transmitted by all such as are trans- 

 parent and colorless, and that it suffers no appreciable 

 loss when the thickness of the plate is varied within 

 certain limits. The 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 

 rendered more or less capable of being transmitted a 

 second time, but, according to the experiments of Pro- 

 fessor Powell, it becomes more or less susceptible of 

 being absorbed in different quantities by black or v 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 be- 

 tween the transmission of terrestrial and solar heat 

 arises from the circumstances of solar heat containing all 

 kinds of caloric, while in other sources some of the kinds 

 are wanting. 



Radiant heat, from sources of any temperature what- 

 ever, is subject to the same laws of reflection and re- 

 fraction 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 

 substances, whether solid or liquid, that do not crystal- 

 ize 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 trans- 

 mit 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 terrestrial rays hav- 



