Foundations of the Universe 277 



Electrons and Light 



The particles even of a piece of cold iron are in a state of 

 vibration. No nerves of ours are able to feel and register the 

 waves they emit, but your cold poker is really radiating, or 

 sending out a series of wave-movements, on every side. After 

 what we saw about the nature of matter, this will surprise none. 

 Put your poker in the fire for a time. The particles of the glow- 

 ing coal, which are violently agitated, communicate some of their 

 energy to the particles of iron in the poker. They move to and 

 fro more rapidly, and the waves which they create are now able 

 to affect your nerves and cause a sensation of heat. Put the 

 poker again in the fire, until its temperature rises to 500 C. It 

 begins to glow with a dull red. Its particles are now moving 

 very violently, and the waves they send out are so short and rapid 

 that they can be picked up by the eye we have visible light. 

 They would still not affect a photographic plate. Heat the iron 

 further, and the crowds of electrons now send out waves of vari- 

 ous lengths which blend into white light. What is happen- 

 ing is the agitated electrons flying round in their orbits at a 

 speed of trillions of times a second. Make the iron "blue hot," 

 and it pours out, in addition to light, the invisible waves 

 which alter the film on the photographic plate. And beyond 

 these there is a long range of still shorter waves, culminating in 

 the X-rays, which will pass between the atoms of flesh or 

 stone. 



Nearly two hundred and fifty years ago it was proved that 

 light travelled at least 600,000 times faster than sound. Jupiter, 

 as we saw, has moons, which circle round it. They pass behind 

 the body of the planet, and reappear at the other side. But it 

 was noticed that, when Jupiter is at its greatest distance from us, 

 the reappearance of the moon from behind it is 16 minutes and 

 36 seconds later than when the planet is nearest to us. Plainly 

 this was because light took so long to cover the additional dis- 

 tance. The distance was then imperfectly known, and the speed 



