MODES OF DIFFUSING HEAT. 349 



551. Incident Rays. When radiant heat falls 

 upon a surface it maybe transmitted, absorbed or reflected. 

 If transmitted, it may be refracted. Rock salt crystal 

 transmits nearly all, reflects very little, and absorbs hardly 

 any. Lampblack absorbs nearly all, reflects very little, and 

 transmits none. Polished silver reflects nearly all, absorbs 

 a little, and transmits none. 



552. Diathermancy. Bodies that transmit ra- 

 diant heat freely are called diathermanous; those 

 that do not are called athermanous. These terms 

 are to heat, what transparent and opaque are to light. 

 Eock salt is the most diathermanous substance known. 

 Heat that is radiated from a non-luminous source, as from 

 a ball heated below redness, is called obscure heat ; while 

 part of that radiated from a luminous source, as from the 

 sun or from a ball heated to redness, is called luminous 

 heat. Heat from a luminous source is generally composed 

 of both luminous and obscure rays. ( 652.) 



553. Selective Absorption. The power of any 

 given substance to transmit heat varies with the nature of 

 the heat or of its source. For example, glass, water or 

 alum allows the sun's luminous heat rays to pass, while 

 absorbing nearly all of the heat rays from a vessel filled 

 with boiling water. In other words, these substances are 

 diathermanous for luminous rays, but athermanous for 

 obscure rays. The physical difference between luminous 

 and obscure heat rays will subsequently be explained. 



(a.) A solution of iodine in carbon bi-sulphide transmits obscure 

 rays but absorbs luminous rays. By means of these substances, 

 luminous and obscure rays may be sifted or separated from each 

 other. Dry air is highly diathermanous ; watery vapor is highly 

 athermanous for obscure rays. 



