34 



NATURE 



[May 14, 1891 



with atoms. And yet the wonder is that atoms vibrate 

 so slowly. If a Hertzian generator were, say, lo"^ cm. 

 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 probably 

 complicated Hertzian generators. By making a com- 

 plicated shape, as, for example, a Leyden jar, a small 

 object may have a slow period of vibration. All that 

 is required is that the capacity and self-induction may 

 be large in comparison with the size of the con- 

 ductor. We saw that these rapidly vibrating generators 

 have but little energy in them : they rapidly give out 

 their energy to the ether 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 suddenly radiated 

 off their energy. The reason why atoms in the air, for 

 instance, do not radiate away their energy like this is 

 because all their neighbours are sending them waves. 

 Each molecule is a generator, but it is a detector as well. 

 It is kept vibrating by its neighbours : it occupies a part 

 of the ether 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 neighbourhood it finds itself 

 in ; and as the waves of light are very long compared 

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

 bourhood are probably, independently of their motions to 

 and fro, each vibrating in the same way. It is interest- 

 ing to calculate how much of the energy in the air is in 

 the form of vibrations of the ether between the molecules 

 of air. A rough calculation shows that in air at the 

 ordinary density and temperature only a minute fraction 

 of the total energy in a cubic centimetre 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 

 densities, there may be more energy in the ether than in 

 the matter within each cubic centimetre. All this shows 

 how wide-reaching are the results 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, consist- 

 ing of two conductors, A and B, sparking into one 

 another. Before each spark, and while 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 

 B, and converge on B. Where they are narrow, the ether 

 is much strained ; where wide, the ether is but little 

 strained. Each tube must be looked upon as a tube 

 of unit strain. The nature of the strain of the ether is 

 not known ; it is, most probably, some increased motion 

 in a perfect liquid. We must not be surprised at the 

 nature of the strain being unknown. We do not know 

 the nature of the change in a piece of 'r.dia-rubber when 

 it is strained, nor indeed in any solid, and though the 

 ether is much simpler in structure than india-rubber, it 

 can hardly be wondered at that we have not yet dis- 

 covered its structure, for it is only within the present 

 century that the existence of the ether was demonstrated, 

 while men have known solids and studied their properties 

 and structure for thousands of years. Any way, there is 

 no doubt that the ether is strained in these tubes of force 

 when A and B are oppositely charged, and that the 

 energy per cubic centimetre of unstrained ether is less 

 than that of strained ether, and that the work done in 

 what is called charging A and B is really done in strain- 

 ing the ether all round them. When the air gap breaks 

 down, and an electric spark takes its place, there is quite 

 a new series of phenomena produced. Suddenly, the 

 strained ether relieves itself, and, in doing so, sets up new 



NO. II 24, VOL. 44] 



motions in itself. The strained state was probably a 

 peculiar state of motion, and in changing back to ordin- 

 ary ether a new and quite distinct state of motion is set 

 up. This new state of motion all round the conductors 

 is most intense near the spark, and is usually described 

 as an electric current in the conductors and across the 

 spark, or as a rushing of the electric charge from one con- 

 ductor to the other. The electric current is accompanied 

 by magnetic force in circles round it, and the tubes of 

 magnetic force define the nature of the new movement 

 in the ether as far as we know it. Hitherto, for the sake 

 of simplicity, the existence of this magnetic force has 

 been unnoticed. It is due to a peculiar motion in the 

 ether all round what are called electric currents. The 

 current in fact consists of little else than a line, all round 

 which this movement is going on ; like the movement 

 surrounding an electrified body, but also unlike it. When- 

 ever electric forces are changing, or electrified bodies 

 moving, or electric currents running, there this other 

 peculiar motion exists. We have every reason for think- 

 ing that this, which may be called the magnetic strain in 

 the ether, as the movement all round electrified bodies 

 was called the electric strain — that this magnetic strain 

 only exists in these three cases : (i) when the electric strain 

 is changing ; (2) when electrified bodies are moving ; and 

 (3) when electric currents are running. These three may 

 be all cases of one action : certainly the magnetic strain 

 that accompanies each is the same, and it seems most 

 likely that the electric change is only another aspect of 

 the magnetic strain. There are analogies to this in the 

 motion of matter that partly help and partly annoy, 

 because they partly agree and partly will not agree with 

 the etherial phenomena. Take the case described in a 

 former article of a chain transmitting waves. Attention 

 was drawn to the displacement of a link and to its rota- 

 tion. Now for the analogy : to seem at all satisfactory 

 the first thing that would strike one would be to pay 

 attention to two motions^ to the velocity of displacement 

 of the link and to its rotation. This would lead to in- 

 terminable difficulties in carrying out the analogy. We 

 cannot liken electric strain to a velocity in this direct and 

 simple way, because, what are we to do with a change in 

 the strain which produces the same eflects as a continuous 

 current "i A change in the strain is all very well, it would 

 be like a change in the velocity, but what about a con- 

 tinuous change in the velocity : we can hardly suppose a 

 velocity continually increasing for ever : we are evidently 

 landed in immediate difficulties. It is better therefore to 

 be content to liken the electric strain to a displacement 

 of the chain link. It seems most likely that it really is a 

 peculiar motion in the ether, but we must be content for 

 the present with the analogy. If we want to drive it 

 further, we must suppose stress in the chain that draws 

 the link back to be due to a motion in the chain or of 

 things fastened to it, and then the changed motions pro- 

 duced by a displacement of the chain might be analogous 

 to the peculiar motions accompanying electric strain. It 

 would lead us too far to work out this analogy. Return- 

 ing to the simpler case of the displacement of the link 

 representing electric strain, and the velocity of its rotation 

 representing magnetic strain, see how the actions near a 

 Hertzian generator may be likened to what takes place 

 when a wave is being sent along a chain. While the 

 conductors are being slowly charged we must suppose 

 electric strain to be produced in all the surrounding space. 

 This is a comparatively slow action, and as the rate of 

 propagation is very rapid, the electric strain will rise 

 practically simultaneously in the whole neighbourhood, 

 and that it does so is a most important fact to be taken 

 account of in all our deductions from these experiments. 

 This slow charging must be represented by a slow raising 

 of one end of the chain, which raises the rest of it to a 

 great distance apparently simultaneously if the raising be 

 done slowly. Suddenly the air gap breaks. This might 



