DIATHERMAXOUS AND ATHERMAXOUS BODIES. 811 



substances which allow light to pass freely through 

 them behave differently with regard to heat. Clear 

 glass and water are transparent for light rays; but with 

 regard to thermal rays glass is only diathermanous for 

 such as are at the same time luminous, while it stops a 

 portion of those which are dark. Water is only diather- 

 manous for luminous heat rays. Rock-salt appears to 

 be among solid bodies nearly the only substance which 

 transmits freely all kinds of thermal rays. On the 

 other hand, there are substances which are opaque to 

 light but diathermanous for dark heat ray s ; for example, 

 iodine dissolved in disulphide of carbon, and black glass. 



Atmospheric air is in a high degree diathermanous. 

 The solar rays pass freely through the atmosphere 

 without warming it. If air, instead of being diather- 

 manous, could stop heat rays, its temperature would 

 be raised most where it first comes in contact with solar 

 rays of yet undiminished thermal intensity, that is, at 

 great elevations above the ground. But observations 

 of the temperature of the air at various altitudes prove 

 that the air is warmest near the surface of the earth, and 

 that its temperature becomes gradually lower and lower 

 as we reach greater altitudes. It follows that the air 

 is not warmed by thermal rays which proceed directly 

 from the sun, but by those which are emitted by the 

 earth which has been heated by the solar rays. 



Those substances which do not transmit heat rays 

 c absorb ' a part and c reflect ' a part of the rays which 

 impinge upon them. From the great analogy between 

 radiant heat and light we may infer that smooth, bright, 

 and polished bodies preferably reflect thermal rays, while 

 dark and rough bodies preferably absorb them. 



