ELECTRICITY AND LIGHT 297 



he knows its universal law, and he might profitably occupy your attention for many 

 evenings in tracing the consequences of that grand but simple statement of physical 

 fact. Think, for instance, of the many apparently altogether incompatible results to 

 which it leads : planetary perturbations, combined with stability of the solar system ; 

 precession and nutation of the earth's axis, yet permanence of the seasons ; and 

 constant tidal agitation of the sea, yet no consequent submersion of continents. 

 He might next trace the growth of stars, with their attendant planets, from an 

 original chaotic distribution of matter calculate even the temperature which each 

 would acquire during its growth all by the aid of this recognised law, of whose 

 own explanation he yet remains absolutely ignorant. 



So it is with the splendid phenomenon of optics. No experimenter has yet, to 

 his certain knowledge, verified by direct proof the existence of that wonderful 

 elastic jelly which we know must fill all space without offering perceptible resistance 

 to bodies moving through it that jelly whose inconceivably rapid quivering is the 

 mechanism by which not only do we see all objects, from the sun to the most 

 minute of stars, but by which comes to us continually from the sun that supply 

 of energy without which vegetable life, and with it that of animals, must at once 

 cease from the earth. Yet a lecturer could never be at a loss on matters connected 

 with light and colour, or with radiation in any of its varied forms. And all the 

 time he is consciously in total ignorance of the nature of this extraordinary lumini- 

 ferous ether upon which they all entirely depend. 



A few years ago no qualified physicist would have ventured an opinion as to 

 the nature of electricity. Magnetism had been (to a certain extent, at least) cleared 

 up by an assumption that it depended on electric currents ; and from Orsted and 

 Ampere to Faraday and Thomson, a host of brilliant experimenters and mathematicians 

 had grouped together in mutual interdependence the various branches of electro- 

 dynamics. But still the fundamental question remained unsolved, What is Electricity ? 

 I remember Sir W. Thomson, eighteen years ago, saying to me, " Tell me what 

 electricity is, and I'll tell you everything else." Well, strange as it may appear to 

 you, I may now call upon him to fulfil his promise. And for good reason, as you 

 shall see. 



Science and Scotland have lately lost in Clerk Maxwell one of their greatest 

 sons. He was, however, much better known to science than to Scotland. In 

 scientific ranks true merit is almost always certain to be recognised in popular 

 ranks modest merit scarcely ever is. His was both true and exceptionally modest 

 merit. One grand object which he kept before him through his whole scientific 

 life was to reduce electric and magnetic phenomena to mere stresses and motions 

 of the ethereal jelly. And there can be little doubt that he has securely laid the 

 foundation of an electric theory like the undulatory theory of light admirably 

 simple in its fundamental assumptions, but, like it, requiring for its full development 

 the utmost resources of mathematical analysis. It cannot but seem strange to the 

 majority of you to be told that we know probably as much about the secret 

 mechanism of electricity as we do about that of light, and that it is more than 

 exceedingly probable that a ray of light is propagated by electric and electro- 

 magnetic disturbances. It can be but a small minority of you who have been at 



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