August 26, 1909J 



NA TURE 



253 



sun, and the gifts of the sun are conveyed to us by the 

 aether. It is to the sun that we owe, not merely night 

 and day, springtime and harvest, but it is the energy of 

 ihe sun, stored up in coal, in waterfalls, in food, that 

 practically does all the work of the world. 



How great is the supply the sun lavishes upon us be- 

 . comes clear when we consider that the heat received by 

 the earth under a high sun and a clear sky is equivalent, 

 according to the measurements of Langley, to about 7000 

 horse-power per acre. Though our engineers have not yet 

 ■discovered how to utilise this enormous supply of power, 

 they will, I have not the slightest doubt, ultimately succeed 

 in doing so ; and when coal is exhausted and our water- 

 power inadequate, it may be that this is the source from 

 which we shall derive the energy necessary for the world's 

 work. When that comes about, our centres of industrial 

 activity may perhaps be transferred to the burning deserts 

 of the Sahara, and the value of land determined by its 

 suitability for the reception of traps to catch sunbeams. 



This energy, in the interval between its departure from 

 the sun and its arrival at the earth, must be in the space 

 between them. Thus this space must contain something 

 which, like ordinary matter, can store up energy, which 

 can carry at an enormous pace the energy associated with 

 light and heat, and can, in addition, e.xert the enormous 

 ■stresses necessary to keep the earth circling round the 

 sun and the moon round the earth. 



The study of this all-pervading substance is perhaps the 

 most fascinating and important duty of the physicist. 



On the electromagnetic theory of light, now universally 

 accepted, the energy streaming to the earth travels through 

 the asther in electric waves ; thus practically the whole of 

 the energy at our disposal has at one time or another 

 been electrical energy. The aether must, then, be the 

 seat of electrical and magnetic forces. We know, thanks 

 to the genius of Clerk Maxwell, the founder and inspirer 

 of modern electrical theory, the equations which express 

 the relation between these forces, and although for some 

 purposes these are all we require, yet they do not tell 

 us very much about the nature of the ather. 



The interest inspired by equations, too, in some minds is 

 apt to be somewhat Platonic ; and something more grossly 

 mechanical— a model, for example, is felt by many to be 

 more suggestive and manageable, and for them a more 

 powerful instrument of research, than a purely analytical 

 theory. 



Is the aether dense or rare? Has it a structure? Is 

 it at rest or in motion? are some of the questions which 

 force themselves upon us. 



Let us consider some of the facts known about the 

 asther. When light falls on a body and is absorbed by 

 It, the body is pushed forward in the direction in which 

 the light is travelling, and if the body is free to move it 

 is set in motion by the light. Now it is a fundamental 

 principle of dynamics that when a body is set moving in 

 a certain direction, or, to use the language of dynamics, 

 acquires momentum in that direction, some other mass 

 must lose the same amount of momentum ; in other words, 

 the amount of momentum in the universe is constant. 

 Thus when the body is pushed forward by the light some 

 other system must have lost the momentum the body 

 acquires, and the only other system available is the wave 

 of light falling on the body ; hence we conclude that there 

 must have been momentum in the wave in the direction in 

 which it is travelling. Momentum, however, implies mass 

 in motion. We conclude, then, that in the jether through 

 ■which the wave is moving there is mass moving with 

 the velocity of light. The experiments made on the 

 pressure due to light enable us to calculate this mass, and 

 we find that in a cubic kilometre of jether carrying light 

 as intense as sunlight is at the surface of the earth, the 

 mass moving is only about one-fifty-millionth of a milli- 

 gram. We must be careful not to confuse this with the 

 mass of a cubic kilometre of ^ther ; it is only the mass 

 moved when the light passes through it ; the vast majority 

 of the Eether is left undisturbed by the light. Now, on 

 the electromagnetic theory of light, a wave of light may 

 be regarded as made up of groups of lines of electric 

 force moving with the velocity of light ; and if we take 

 this point of view we can prove that the mass of jether 

 per cubic centimetre carried along is proportional to the 



NO. 2078, VOL. 81] 



energy possessed by these lines of electric force per cubic 

 centimetre, divided by the square of the velocity of light. 

 But though lines of electric force carry some of the aether 

 along with them as they move, the amount so carried, 

 even in the strongest electric fields we can produce, is but 

 a minute fraction of the sether in their neighbourhood. 



This is proved by an experiment made by Sir Oliver 

 Lodge in which light was made to travel through an 

 electric field in rapid motion. If the electric field had 

 carried the whole of the aether with it, the velocity of 

 the light would have been increased by the velocity of 

 the electric field. As a matter of fact, no increase what- 

 ever could be detected, though it would have been registered 

 if it had amounted to one-thousandth part of that of the 

 field. 



The Eether carried along by a wave of light must be an 

 exceedingly small part of the volume through v;;hich the 

 wave is spread. Parts of this volume are in motion, but 

 by far the greater part is at rest ; thus in the wave front 

 there cannot be uniformity, at some parts the iether is 

 moving, at others it is at rest — in other words, the wave 

 front must be more analogous to bright specks on a dark 

 ground than to a uniformly illuminated surface. 



The place where the density of the aether carried along 

 by an electric field rises to its highest value is close to a 

 corpuscle, for round the corpuscles are by far the strongest 

 electric fields of which we have any knowledge. We know 

 the mass of the corpuscle, we know from Kaufmann's 

 experiments that this arises entirely from the electric 

 charge, and is therefore due to the sether carried along 

 with the corpuscle by the lines of force attached to it. 



.'\ simple calculation shows that one-half of this mass 

 is contained in a volume seven times that of a corpuscle. 

 Since we know the volume of the corpuscle as well as the 

 mass, we can calculate the density of the aether attached 

 to the corpuscle ; doing so, we find it amounts to the 

 prodigious value of about 5x10", or about 2000 million 

 times that of lead. Sir Oliver Lodge, by somewhat 

 different considerations, has arrived at a value of the same 

 order of magnitude. 



Thus around the corpuscle aether must have an extrava- 

 gant density : whether the density is as great as this in 

 other places depends upon whether the gather is com- 

 pressible or not. If it is compressible, then it may be 

 condensed round the corpuscles, and there have an 

 abnormally great density ; if it is not compressible, then 

 the density in free space cannot be less than the number 

 I have just mentioned. 



With respect to this point we must remember that the 

 forces acting on the aether close to the corpuscle are pro- 

 digious. If the aether were, for example, an ideal gas 

 the density of which increased in proportion to the pressure, 

 however great the pressure might be, then if, when ex- 

 posed to the pressures which exist in some directions close 

 to the corpuscle, it had the density stated above, its density 

 under atmospheric pressure would only be about 8x10-^^, 

 or a cubic kilometre would have a mass less than a gram ; 

 so that instead of being almost incomparably denser than 

 lead, it would be almost incomparably rarer than the 

 lightest gas. 



I do not know at present of any effect which would 

 enable us to determine whether aether is compressible or 

 not. And although at first sight the idea that we are 

 immersed in a medium almost infinitely denser than lead 

 might seem inconceivable, it is not so if we remember 

 that in all probability matter is composed mainly of holes. 

 We may, in fact, regard matter as possessing a bird-cage 

 kind of structure in which the volume of the Eether dis- 

 turbed by the wires when the structure is moved is in- 

 finitesimal in comparison with the volume enclosed by 

 them. If we do this, no difficulty arises from the great 

 density of the aether ; all we have to do is to increase the 

 distance between the wires in proportion as we increase 

 the density of the Eether. 



Let us now consider how much aether is carried along 

 by ordinary matter, and what effects this might be ex- 

 pected to produce. 



The simplest electrical system we know, an electrified 

 sphere, has attached to it a mass of EEther proportional to 

 its potential energy, and such that if the mass were to 

 move with the velocity of light its kinetic energy would 



