Fire and Heat 107 



tion and radiant energy held by scientists to-day. In the 

 first place, scientists prefer to speak not of the " heat," but 

 of the " radiant energy " of the sun. For heat is only one 

 effect of radiant energy ; another effect is light. Every body 

 whether stick, stone, metal, plant, or animal, which has exposed 

 surfaces, is radiating or giving up energy. The lower the 

 temperature of the body, the less is the amount of energy it 

 radiates. The higher its temperature, the greater the amount 

 of energy radiated. But radiation occurs as long as the tem- 

 perature of a body is above what is known as absolute zero, 

 -273 C. 



In the second place, every body, according to this theory, is 

 composed of countless exceedingly minute particles, called 

 molecules. The molecules are unceasingly active ; their activity 

 varies, however, according to the temperature of the body. 

 In a piece of cold iron, for example, the activity of the molecules 

 is very slow in comparison with that of the molecules in a piece 

 of red-hot iron ; while in a mass of molten iron the molecular 

 activity is wonderfully increased above the latter. The activity 

 of the molecules of a body increases with rise in the temperature 

 of the body. 



Now whenever a body is warmer than any surrounding 

 objects, or whenever its molecular vibrations are more rapid 

 than those of the surrounding objects, this body is said to 

 radiate or give up energy more rapidly and freely than the other 

 objects. The radiant energy given up always moves in straight 

 lines. This your experiments make quite clear. If the energy 

 is deflected from its original path, its direction is then de- 

 termined by the angle of the reflecting surface with the direc- 

 tion of the radiant energy before being deflected. This you 

 know by your experiments in reflecting sunlight upon objects 

 at a distance and in changing the direction of the heat from the 

 iron ball. 



The ether as the medium of transfer. Radiant energy is 

 transmitted by radiation and through a medium which science 



