CONDUCTION AND RADIATION. 549 



of bodies, is very distinct from their power of trans- 

 mitting heat, which has been called diathermancy 

 by M. Melloni. Thus both a plate of alum and 

 a plate of rock-salt transmit nearly the whole light; 

 but while the first stops nearly the whole heat, the 

 second stops very little of it ; and a plate of opake 

 quartz, nearly impenetrable by light, allows a large 

 portion of the heat to pass. By passing the rays 

 through various media, the heat may be, as it were 

 sifted from the light which accompanies it (PA). 



Moreover, besides the laws of conduction and 

 radiation, many other laws of the phenomena of 

 heat have been discovered by philosophers; and 

 these must be taken into account in judging any 

 theory of heat. To these other laws we must now 

 turn our attention. 



