CHAPTER FIVE 



RADIANT HEAT AND LIGHT 



SECTION 21. How heat gets here from the sun; why 

 things glow when they become very Jwt. 



If we were to go back to our imaginary switchboard 

 we should find a switch, between the heat and the light 

 switches, labeled RADIATION. Suppose we turn it off: 



Instantly the whole world becomes pitch dark ; so does 

 the sky. We cannot see the sun or a star; no electric 

 lights shine ; and although we can " light " a match, it 

 gives no light. The air above the burning match is 

 hot, and we can burn our fingers in the invisible flame, 

 but we can see nothing whatever. 



Yet the world does not get cold. If we leave the 

 switch off for years, while the earth remains in dark- 

 ness and we all live like blind people, it never gets cold. 

 Winter and summer are alike, day and night are just 

 the same. Gradually, after many ages, the ice and snow 

 in the north and in the far south begin to melt as the 

 warmth from the rest of the world is conducted to the 

 polar regions. And the heat from the interior of the 

 earth makes all the parts of the earth's surface 

 warmer. Winds almost stop blowing. Ocean currents 

 stop flowing. The land receives less rainfall, 'until 

 finally everything turns to a desert ; almost the only 

 rain is on the ocean. Animals die even before the rivers 

 dry up, for the flesh eaters are not able to see their prey, 

 and since, without light, all green things die, the animals 

 that live on plants soon starve. Men have to learn to 

 live on mushrooms, which grow in the dark. The world 

 is plunged into an eternal warm, pitch-black night. 



