NA TURE 



[January 14, 19c 9 



writings of Faraday, whose whole life may be said to have 

 been directed towards a better understanding of these 

 cthereous phenomena. Indeed, the statue in your entrance 

 hall may be considered as the statue of the discoverer of 

 the electric and magnetic properties of the lether of space. 



Faraday conjectured that the same medium which is 

 concerned in the propagation of light might also be the 

 agent in electromagnetic phenomena. " For my own 

 part," he says, "considering the relation of a vacuum to 

 the magnetic force, and the general character of magnetic 

 phenomena external to the magnet, I am much more in- 

 clined to the notion that in the transmission of the force 

 there is such an action, external to the magnet, than that 

 the effects are merely attraction and repulsion at a 

 distance. Such an action may be a function of the aether ; 

 for it is not unlilcely that, if' there be an aether, it should 

 have other uses than simply the conveyance of radiation." 



This conjecture has been amply strengthened by sub- 

 sequent investigations. 



One more function is now being discovered; the aether 

 is being found to constitute matter — an immensely interest- 

 ing topic, on which there are many active workers at 

 the present time. I will make a brief quotation from 

 your present professor of natural philosophy (J. J. Thom- 

 son), where he summarises the conclusion which we all 

 see looming before us, though it has not yet been com- 

 pletely attained, and would not by all be' similarly ex- 

 pressed : — 



" The whole mass of any body is just the mass of aether 

 surrounding the body which is carried along by the Fara- 

 day tubes associated wNth the atoms of the body. In fact, 

 all mass is mass of the aether ; all momentum, momentum 

 of the «ther ; and all kinetic energy, kinetic energy of 

 •he ^ther. This view, it should be said, requires the 

 density of the aether to be immensely greater than that 

 of any known substance." 



Yes, far denser — so dense that matter by comparison is 

 like gossamer, or a filmy, imperceptible niist, or a Milkv 

 Way. Not unreal or unimportant — a cobweb is not un- 

 real, nor to certain creatures is it unimportant, but it 

 cannot be said to be massive or dense ; and matter, even 

 platinum, is not dense when compared with the aether. 

 Not until last year, however, did I realise what the density 

 of the aether must really be,' compared with that modifica- 

 tion of it which appeals to our senses as matter, and 

 which for that reason engrosses our attention. If I have 

 time I will return to that before I have finished. 



Is there any other function possessed by the aether 

 which, though not yet discovered, may lie within the 

 bounds of possibility for future discovery? I believe there 

 is, but it is too speculative to refer to, beyond saving that 

 it^ has been urged as probable by the a'uthors 'of " The 

 Unseen Universe," and has been thus tentativelv referred 

 to by Clerk Maxwell : — 



" Whether this vast homogeneous expanse of isotropic 

 matter is fitted not only to be a medium of phvsical inter- 

 action between distant bodies, and to fulfil other phvsical 

 functions of which, perhaps, we have as yet no concep- 

 tion, but also ... to constitute the material organism 

 of beings exercising functions of life and mind as high or 

 higher than ours are at present — is a question far 

 transcending the limits of physical speculation." 



And there, for the present', I leave that aspect of the 

 subject. 



I shall now attempt to illustrate some relations between 

 jether and matter. 



The question is often asked, Is aether material? This 

 is largely a question of words and convenience. Un- 

 doubtedly the a;ther belongs to the material or physical 

 universe, but it is not ordinary matter. I should prefer 

 to say it is not " matter " at' all. It mav be the sub- 

 stance or substratum or material of which matter is com- 

 posed, but it would be confusing and inconvenient not to 

 be able to discriminate between matter, on the one hand, 

 and aether on the other. If you tie a knot on a bit of 

 string, the knot is composed of string, but the string is 

 not composed of knots. If you have a smoke or vortex- 

 ring in the air, the vorte.x-ring is made of air, but the 

 atmosphere is not a vortex-ring, and it would be onlv 

 confusing to say that it was. 



1 See Lodge, Phil. Mag., .^pril, 1507. 

 NO. 2046, VOL. 79] 



The essential distinction between matter and a;thcr is 

 that matter moiies, in the sense that it has the properly 

 of locomotion and can effect impact and bombardment, 

 while aether is strained, and has the property of exerting 

 stress and recoil. All potential energy exists in the iethcr. 

 It may vibrate, and it may rotate, but as regards loco- 

 motion it is stationary — the most stationary body we 

 know — absolutely stationary, so to speak ; our standard of 

 rest. 



-Ml that we ourselves can effect, in the material universe, 

 is to alter the motion and configuration of masses of 

 matter ; we can move matter by our muscles, and that 

 is all we can do directly ; everything else is indirect. 



But now comes the question. How is it possible for 

 matter to be composed of tether? How is it possible for 

 a solid to be made out of fluid? A solid possesses the 

 properties of ;-igidity, impenetrability, elasticity, and such 

 like ; how can these be imitated by a perfect fluid such 

 as the ajther must be? The answer is, they can be 

 imitated by a fluid in motion, a statement which we make 

 with confidence as the result of a great part of Lord 

 Kelvin's work. 



It may be illustrated by a few experiments. 



A wheel of spokes, transparent or permeable when 

 stationary, becomes opaque when revolving, so that a ball 

 thrown against it does not go through, but rebounds. The 

 motion only affects permeability to matter ; transparency 

 to light is unaffected, until something near the speed of 

 light itself is reached. 



A silk cord hanging from a pulley becomes rigid and 

 viscous when put into rapid motion, and pulses or waves 

 which may be generated on the cord travel along it with 

 a speed equal to its own velocity, whatever that velocity 

 may be, so that they appear to stand still. This is a case 

 of kinetic rigidity, and the fact that the wave-transmission 

 velocity is equal to the rotatory speed of the material is 

 typical and important, for in all cases of kinetic elasticity 

 these two velocities are of the same order of magnitude. 



A flexible chain, set spinning, can stand up on end while 

 the motion continues. 



A jet of water at sufficient speed can be struck with 

 a hammer, and resists being cut with a sword. 



A spinning disc of paper becomes clastic, like flexible 

 metal, and can act like a circular saw. Sir William White 

 tells me that in naval construction steel plates are cut by 

 a rapidly revolving disc of soft iron. 



A vortex-ring, ejected from an elliptical orifice, oscillates 

 about the stable circular form, as an india-rubber ring 

 would do, thus furnishing a beautiful example of kinetic 

 elasticity, and showing us clearly a fluid displaying some 

 of the properties of a solid. 



.■\ still further example is Lord Kelvin's model of a 

 spring balance, made of nothing but rigid bodies in 

 spinning motion.' 



If the aether can be set spinning, therefore, we may 

 have some hope of making it imitate the properties of 

 matter, or even of constructing matter by its aid. But 

 how are we to snin the aether? Matter alone seems to 

 have no grip of it. I have spun steel discs, a yard in 

 diameter, 4000 times a minute, have sent light round and 

 round between them, and tested carefully for the slightest 

 effect on the aether. Not the slightest effect was per- 

 ceptible. We cannot spin aether mechanically. 



But we can vibrate it electricilly, and every source of 

 radiation does that. An electrified body, in sufficiently 

 rapid vibration, is the only source of aether-waves that we 

 know, and if an electric charge is suddenly stopped it 

 generates the pulses known as X-rays, as the result of 

 the colPision. Not speed, but sudden change of speed, is 

 the necessary condition for generating waves in the aether 

 by electricity. 



We can also infer some kind of rotary motion in the 

 aether, though we have no such obvious means of detecting 

 the spin as is furnished by vision for detecting some kinds 

 of vibration. It is supposed to exist whenever we put a 

 charge into the neighbourhood of a magnetic pole. Round 

 the line joining the two the aether is spinning like a top. 

 I do not say it is spinning fast : that is a question of its 

 density ; it is, in fact, spinning with excessive slowness, 

 but it is spinning with a definite moment of momentum. 

 I Address to Section A of British Association at Montreal, 1884. 



