Jan. V, i8!>0 



NA TURE 



303 



[Here were shown an ignited pUlinuai wire, tlie electric arc ^ 

 l)et\veentwo carbons, an electric machine spark, an induction-coil 

 spark, and a vacuum tabe glow. Also a large nail was magnet- 

 ised by being wrapped in the current, and two helices were 

 suspended and seen to direct and attract each other.] 



To make a m rgnet, then, we only need a current of electricity 

 flowing round and round in a whirl. A vortex or whirlpool of 

 electricity is in fact a magnet ; and vice vers'i. And the.e 

 whirls have the power of directing and attracting other pre- 

 viously e.xistinj; whirls according to certain laws, called the 

 laws of magnetism. And, moreover, they have the power of 

 exciting fresh whirls in neighbouring cmductors, and of repelling 

 them according to the laws of diamagnetism. The theory of the 

 actions is kno\\n ; though the nature of the -whirls, as of the 

 .-imple stream of electricity, is at present unknown. 



[Here was shown a large electro-magnet and an induction-coil 

 vacuum discharge spinning round and round when placed in its 

 field.] 



.So much for what happens when electricity is made to travel 

 alo-ig conductors, i.e. when it travels along like a stream of 

 water in a pipe, or spins round and round like a wdiirlpool. 



But there is another set of phenomena, usually regarded as 

 distinct, and of another order, but which are nit so distinct as 

 they appear, which manifest themselves when you join the pump 

 to a piece of glass or any non-conductor and try to force the 

 electricity through that. You succeed in driving some through, 

 but the flow is no longer like that of water in an open pipe ; it is as 

 if the pipe were completely obstructed by a number of elastic par- 

 titions, or diaphragms. The water cannot move without straining 

 and bending these diaphragms, and if you allow it, these strained 

 partitions will recover themselves and drive the water back again. 

 [Here was explained the process of charging a Leyden j ir.] 

 The essential thing to remember is that we may have electri- 

 cal energy in two forms, the static and the kmetic ; and it is 

 therefore also possible to have the rapid alternati )n from one 

 of the-e forms to the other, called vibration. 



Now we will pass to the second question: What do you 

 mean by light ? And the first and obvious answer is, Everybody 

 knows. And everybody that is not blind does know to a certain 

 extent. We have a special sense organ for appreciating light, 

 whereas we have none for electricity. Nevertheless, we must 

 admit that we really know very little about the intimate nature 

 of light — very little more than about electricity. But we do know 

 this, that light is a for.n of energy ; and, moreover, that it is energy 

 rapidly alternating between the static and the kinetic forms 

 — that it is, in fact, a special kiid of energy of vibration. We are 

 absolutely certain that light is a periodic disturbance in some 

 medium, periodic both in space and time : that is to say, the 

 same appearances regularly recur at certain equal intervals of 

 distance at the sane time, and also present themselves at equal 

 intervals of time at tlie same place ; that in fact it belongs to the 

 class of motions called by mathematicians undulatory or wave 

 motions. The wave motion in this model (Powell's wave ap- 

 paratus) results from the sinple up-and-down motion popularly 

 associated with the term wave. But when a mathematician calls 

 a thi'ig a wave he means that the disturbance is represented by a 

 certain general type of formula, not that it is an up-and-down 

 motion, or that it looks at all like those things on the top of the 

 sea. The motion of the surface of the sea falls within that 

 formula, and hence is a special variety of wave motion, and the 

 term wave has acquired in popular use this signification and 

 nothing else. So that when one speaks ordinarily of a wave or 

 undulatory motion one immediately thinks of something heaving 

 np and down, or even perhaps of something breaking on the 

 shore. But when we assert that the form of energy called light 

 is undulatory, we by no means intend to assert that anything 

 whatever is moving up and down, or tint the motion, if we 

 could see it, would be anything at all like what we are accustomed 

 to in the ocean. The kind of motion is unknown ; we are not 

 even sure that there is anything like motion in the ordinary sense 

 of the word at all. 



Now how much connection between electricity and light have 

 we perceived in this glance into their natures ? Not much truly. 

 It amounts to about this : That on the one hand electrical energy 

 may exist in either of two forms— the static form, when insu- 

 lators are electrically strained by having had electricity driven 

 partially through thecn (as in the Leyden jar), which strain is a 

 form of energy because of the tendency to discharge and do work ; 

 .and the kinetic form, where electricity is moving bodily along 

 through conductors or whirling round and round inside them, 



which motion of electricity is a form of energy, because the con- 

 ductors and whirls can attract or repel each other and thereby 

 do work. 



And, on the other hand, that light is the rapid alternation o f 

 energy from one of these forms to the other — the static form where 

 the medium is strained, to the kinetic form when it moves. It is 

 ju.t conceivable then that the static form of the energy of light 

 is rfcc/ro-static, that is, that the medium is electrically strained, 

 and that the kinetic form of the energy of light is elect ro-VvatWz, 

 that is, that the motion is not ordinary motion, but electrical 

 motion — in fact that light is an electrical vibration, not a material 

 one. 



On November 5 la-t year there died at Cambridge a man in 

 the full vigour of his faculties— such faculties as do not appear 

 many times in a century — whose chief work has been the establish- 

 ment of this very fact, the discovery of the link connecting light 

 and electricity ; and the proof — for I believe it amounts to a proof 

 — that they are different manifestations of one and the same class 

 of phenomena — that light is, in fact, an electro-magnetic disturb- 

 ance. The premature death of James Clerk Maxwell is a loss to 

 science which appears at present utterly irreparable, for he was 

 engaged ia re earches that no other man can hope as yet .ade- 

 quately to grasp and follow out ; but fortunately it did not occur 

 till he had pu'ilished his book on "Electricity and Magnetism," 

 one of thoie immortal productions which exalt one's idea of the 

 mind of man, and which has been mentioned by competent critics 

 in the same breath as the " Principia " itself. 



But it is not perfect like the "Principia " ; much of it is rough- 

 hewn, and requires to be thoroughly worked out. It contains 

 numerous misprints and errata, and part of the second volume is 

 so difficult as to be almost unintelligible. Some, in fact, consists 

 of notes written f or private use, and not intended for publication. 

 It seems next to impossible now to mature a work silently for 

 twenty or thirty years, as was done by Newton two and a half 

 centuries ago. But a second edition was preparing, and much 

 might have been improved in form if life had been spared to the 

 illustrious author. 



The main proof of the electro magnetic theory of light is 

 this. The rate at which light travels has been measured many 

 times, and is iiretty well known. The rate at which an electro- 

 magnetic wave disturbance "would travel if such could be gene- 

 rated (and Mr. Fitzgerald of Dublin thinks he has proved that it 

 cannot be generated directly by any known electrical means) can 

 be also determined by calculation from electrical measurements. 

 The two velocities agree exactly. This is the great physical 

 constant known as the ratio V, which so many physicists have 

 been measuring, and are likely to be measuring for some time to 

 come. 



Many and brilliant as were Maxwell's discoveries, not only in 

 electricity, but also in the theory of the nature of gases, and in 

 molecular science generally, I cannot help thinking that if one of 

 them is more striking and more full of future significance than 

 the rest, it is the one I have just mentioned — the theory that light 

 is an electrical phenomenon. 



The fir-t glimpse of this splendid generalisation was caught 

 in 1845, five and thirty years ago, by that prince of pure 

 experimentalists, Michael Faraday. His reasons for suspect- 

 ing some connection between electricity and light are not 

 clear to us — in fact they could not have been clear to him ; 

 but he see us to have felt a conviction that if he only tried 

 long enough, and sent all kinds of rays of light in all pos- 

 sible directions across electric and magnetic fields in all sorts 

 of media, he must ultimately hit upon something. Well, this 

 is very nearly what he did. With a sublime patience and 

 perseverance which remind one of the way Kepler hunted down 

 guess after guess in a different field of research, Faraday com- 

 bined electricity, or magnetism, and light in all manner of ways, 

 and at la->t he was rewarded with a result. And a most out-of-the- 

 way result it seemed. First you have to get a most powerful 

 magnet and very strongly excite it ; then you have to pierce its 

 two poles with holes, in order that a beam of light may travel 

 from one to the other along the lines of force ; then, as ordinary 

 light is no good, y on must get a beam of plane polarised light and 

 send it between the poles. But still no result is obtained until, 

 finally, you interpose a piece of a rare and out-of-the-way 

 material which Faraday had him-elf discovered and made, a 

 kind of glass which contains borate of lead, and which is 

 very heavy, or dense, and wdiich must be perfectly annealed. 



And now, when all these arrangements are completed, what is 



