HEAT. 61 



transmitted through a prism of rock-salt, is found to be une- 

 qually refracted, as is the case with light ; and the rays of 

 heat thus elongated into what is, for the sake of analogy, 

 called a spectrum, are found to possess similar properties to 

 the primary or coloured rays of light. Thus rock-salt is to 

 heat what colourless glass is to light ; it transmits heat of all 

 degrees of refrangibility : alum is to heat as red glass to 

 light ; it transmits the least, and stops the most refrangible 

 rays ; and rock-salt covered with soot represents blue glass, 

 transmitting the most, and stopping the least refrangible rays. 



Certain bodies, again, reflect heat of different refrangi- 

 bility : thus paper, snow, and lime, although perfectly white 

 that is, reflecting light of all degrees of refrangibility, re- 

 flect heat only of certain degrees ; while metals, which are 

 coloured bodies that is, bodies which reflect light only of 

 certain degrees of refrangibility reflect heat of all degrees. 

 Radiant heat incident upon substances which doubly refract 

 light is doubly refracted ; and the emergent rays are polar- 

 ised in planes at right angles to each other, as is the case 

 with light. 



The relation of radiation to absorption also holds good 

 with light as with heat: with the latter it has been long 

 known that the radiating power of different substances is di 

 rectly proportional to their absorptive and inverse to their 

 reflective power ; or rather, that the sum of the heat radia 

 ted and reflected is a constant quantity. So, as has been 

 shown by Mr. Balfour Stewart, the absorption bears the 

 same relation to radiation for heat as to quality as well as 

 quantity. 



Light presents us with similar relations. Coloured glass, 

 when heated so as to be luminous, emits the same light 

 which at ordinary temperatures it absorbs: thus red glass 

 gives out or radiates a greenish light, and green glass a red 

 tint. 



The flame of substances containing sodium yields a yel- 



