46 CORRELATION OF PHYSICAL FORCES. 



netic needle, the direction of which is parallel to the convolu- 

 tions of the wire. When radiant heat impinges upon the 

 soldered ends of the multiplier, a thermo-electric current is 

 induced in each pair ; and, as all these currents tend to cir- 

 culate in the same direction, the energy of the whole is 

 increased by the co-operating forces : this current, traversing 

 the helix of the galvanometer, deflects the needle from paral- 

 lelism by virtue of the electro-magnetic tangential force, and 

 the degree of this deflection serves as the index of the 

 temperature. 



Bodies examined by these means show a remarkable dif- 

 ference between their transcalescence or power of transmitting 

 heat, and their transparency : thus, perfectly transparent alum 

 arrests more heat than quartz so dark-coloured as to be 

 opaque ; again alum coupled with green glass Melloni found 

 was capable of transmitting a beam of brilliant light, while, 

 with the most delicate thermoscope, he could detect no indi- 

 cations of transmitted heat : on the other hand, rock salt, the 

 most transcalescent body known, may be covered with soot 

 until perfectly opaque, and yet be found capable of trans- 

 mitting a considerable quantity of heat. Radiant heat, when 

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

 equally 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, trans- 

 mitting the most, and stopping the least refrangible rays. 



Certain bodies, again, reflect heat of different refrangibi- 

 lity ; thus, paper, snow, and lime, although perfectly white 

 that is, reflecting light of all degrees of refrangibility, reflect 

 respectively 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 polarised 



