224 hertz's experiments. 



can study a great deal about their structure. They are truly very 

 long waves of light. Atoms are Hertzian generators whose period 

 of vibration is hundreds of millions of millions per second. A Hert- 

 zian generator may vibrate rapidly, but it is miserably slow compared 

 with atoms. And yet the wonder is that atoms vibrate so slowly. H* 

 a Hertzian generator were, say, 1()~' *^^'" long, about the size of a good 

 big atom, its period of vibration would be some hundreds of times too 

 rapid to produce ordinary light. Atoms are j)robably com])licated 

 Hertzian generators. By making a complicated shape, as, for exam- 

 ple, a Leyden jar, a small object may have a slow period of vibraticm. 

 All that is re<]uii'ed is that the capacity and self-induction may be 

 large in comparison with the size of the conductor. We saw that these 

 rapidly vibrating generators have but little energy in them; they rap- 

 idly give out their energy to the aether near them. This is also the 

 case with atoms. These, when free to radiate, give up their energy 

 with wonderful rapidity. How short a time a flash of lightning lasts! 

 It is hardly there but it is gone: the heated air molecules have so sud- 

 denly radiated otf their energy. The reason ^xhy atoms in the air, for 

 instance, do not radiate away their energy like this is because all their 

 neighbors are sending them waves. Each molecule is a generator, but 

 it is a detector as Avell. It is kept vibrating by its neighbors: it occu- 

 pies a part of the a?ther that is in continual vibration, and so the atom 

 itself vibrates. As each atom can radiate so rapidly, it must be a good 

 detector; its own vibrations must be very much controlled by the 

 neighborhood it finds itself in; and as the waves of light are very long- 

 compared with the distances apart of molecules, those in any neigh- 

 borhood are probably, independently of their motions to and fro, each 

 vibrating in the same way. 



It is interesting to calculate how much of the energy in the air is in 

 the form of vibrations of the {lether between the molecules of air. A 

 rough calculation shows that in air at the ordinary density and tem- 

 perature only a minute fraction of the total energy in a cubic centi- 

 meter is in the ;ether; but when we deal with high temperatures, such 

 as exist in lightning flashes, and near the sun, and with very small den- 

 sities, there may be more energy in the aether than in the matter within 

 each cubic centimeter. All this shows how wide-reaching are the re- 

 sults of Hertz's experiments. They teach us the nature of waves of 

 light. We can learn much by considering how the waves are generated. 

 Let us consider what goes on near the generator, consisting of two con- 

 ductors, A and B, sparking into one another. Before each spark, and 

 Avhile A and B are being comparatively slowly what is called charged 

 with electricity, the ;ether around and between them is being strained. 

 The lines of strain are the familiar tubes of electric force. If A be 

 positive, these tubes diverge from all points of A, and most from the 

 knob between it and /i, and converge on 7^. Where they are narrow. 



