1908] on the Ether of Space. 67 



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 cnt with a sword. 



A spinning disk of paper becomes elastic 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 disk of 

 soft iron. 



A vortex-riug, ejected from an elliptical orifice, oscillates about 

 the stable circular form, as an indiarubber ring would do ; thus fur- 

 nishing a beautiful example of kinetic elasticity, and showing us 

 clearly a fluid displaying some of the properties of a solid. 



A still further example is Lord Kelvin's model of a spring 

 balance, made of nothing but rigid bodies in spinning motion.* 



If the ether can be set spinning, therefore, we may have some 

 hope of making it imitate the properties of matter, or even of con- 

 structing matter by its aid. But how are we to spin the ether ? 

 Matter alone seems to have no grip of it. I have spun steel disks, 

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

 round between them, and tested carefully for the sUghtest effect on 

 the ether. Not the shghtest effect was perceptible. We cannot spin 

 ether mechanically. 



But we can vibrate it electrically ; and every source of radiation 

 does that. An electrified body, in sufficiently rapid vibration, is the 

 only source of ether-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 collision. Not speed, but sudden change of speed, 

 is the necessary condition for generating waves in the ether by 

 electricity. 



We can also infer some kind of rotary motion in the ether ; 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 ether 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. J. J. Thomson's 

 theory makes its moment of momentum exactly equal to e m, the 

 product of charge and pole ; the charge being measured electro- 

 statically and the pole magnetically. 



How can this be shown experimentally ? Suppose we had a spin- 

 ning top enclosed in a case, so that the spin was unrecognisable by 

 ordinary means— it could be detected by its gyrostatic behaviour to 



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



F 2 



