TEANSACTIONS OF SECTION A. 561 



produce sparks. By its means, however, Hertz has beer able to ohserve the inter- 

 ference between waves incident on a wall and the reflected waves. He placed his 

 generating vibrator several wave-lengths away from a wall, and placed the re- 

 ceiving resonant circuit between the generator and the wall, and in this air-space 

 he was able to observe that at some points there were hardly any induced sparks, 

 but at other and greater distances from his generator they reappeared to disappear 

 again in regular succession at equal intervals between his generator and the wall. 

 It is exactly the same phenomenon as what are known as Lloyd's bands in optics, 

 which are due to the interference between a direct and a reflected wave. It follows 

 hence that, just as Young's and Fresnell's researches on the Interference of Light 

 prove the undulatory theory of optics, so Hertz's experiment proves the ethereal 

 theory of electro-magnetism. It is a splendid result. Henceforth I hope no 

 learner will fail to be impressed with the theory — hypothesis no longer — that electro- 

 magnetic actions are due to a medium pervading all known space, and that it is 

 the same medium as the one by which light is propagated, that non-conductors 

 can, and probably always do, as Professor Poynting has taught us, transmit electro- 

 magnetic energy. By means of variable currents energy is propagated into space 

 with the velocity of light. The rotation of the earth is being slowly stopped by 

 the diurnal rotation of its magnetic poles. This seems a hopeful direction in 

 which to look for an explanation of the secular precession of terrestrial mag- 

 netism. It is quite diiferent from Edlund's curious hypothesis that free space 

 is a perfect conductor. If this were true, there would be a pair of great 

 antipoles outside the air, and terrestrial magnetism would not be much like 

 what it is, and I think the earth would have stopped rotating long ago. With 

 alternating currents we do propagate energy through non-conductors. It seems 

 almost as if our future telegraph cables would be pipes. Just as the long sound 

 waves in speaking tubes go round corners, so these electro-magnetic waves go round 

 corners if they are not too sharp. Professor Lodge will probably have something 

 to tell us on this point in connection with lightning conductors. The silvered glass- 

 bars used by surgeons to conduct light are exactly what I am describing. They 

 are a glass, a non-conducting, and therefore transparent, bar surrounded by a con- 

 ducting, and therefore opaque, silver sheath, and they transmit the rapidly alter- 

 nating currents we call light. There would not be the same difficulty in utilising the 

 energy of these electro-magnetic waves as in utilising radiant heat. Having all 

 the vibrations of the same period we might utilise Hertz's resonating circuits, and 

 in any case the second law of thermodynamics would not trouble us when we 

 could practically attain to the absolute zero of these, as compared with heat, long 

 period vibrations. 



We seem to be approaching a theory as to the structure of the ether. There- 

 are difficulties from diffusion in the simple theory that it is a fluid full of motion, a 

 sort of vortex sponge. There were similar difficulties in the wave theory of light 

 owing to wave propagation round corners, and there is as great a difficulty in the 

 jelly theory of the ether arising from the freedom of motion of matter through it. 

 It may be found that there is diffusion or it may be found that there are polarised 

 distributions of fluid kinetic energy which are not unstable when the surfaces are 

 fixed ; more than one such is known. Osborne Reynolds has pointed out another, 

 though in my opinion less hopeful, direction in which to look for a theory of the 

 ether. Hard particles are abominations. Perhaps the impenetrability of a vortex 

 would suffice. Oliver Lodge speaks confidently of a sort of chemical union of two 

 opposite kinds of elements forming the ether. The opposite sides of a vortex ring 

 might perchance suit, or maybe the ether, after all, is laut an atmosphere of some 

 infra-hydrogen element : these two latter hypotheses may both come to the same 

 thing. Anyway we are learning daily what sort of properties the ether must have. 

 It must be the means of propagation of light ; it must be the menus by which 

 electric and magnetic forces exist; it should explain cbemical actions and, if 

 possible, gravity. 



On the vortex-sponge theory of the ether there is no real difficulty by reason of 

 complexity why it should not explain chemical actions. In fact, there is every 

 reason to expect that very much more complex actions would take place at distances 

 1858. 



