474 



NATURE 



[September 17, 1806 



however, was too coarse to polarise visible light. Thus, though 

 the structure of the tourmaline is fine enough to ]iolarise the 

 \'isible rays, it may be much too coarse to polarise the Rontgen 

 rays if these have exceedingly small wave-lengths. As far as 

 our knowledge of these rays extends, I think we may say that 

 though there is no direct evidence that they are a kind of light, 

 there are no properties of the rays which are not possessed by 

 some variety of light. _ 



It is clear that if the Rontgen rays are light rays, their wave- 

 lengths are of an entirely different order to those of visible light. 

 It is perhaps worth notice that on the electro-magnetic theory of 

 light we might expect two different types of vibration if we 

 suppose that the atoms in the molecule of the vibrating substance 

 carried electrical charges. One set of vibrations would be due 

 to the oscillations of the bodies carrying the charges, the other 

 set to the oscillation of the charges on these bodies. The wave- 

 length of the second set of vibrations would be commensurate 

 with molecular dimensions ; Can these vibrations be the Rontgen 

 rays ? If so, we should expect them to be damped with such 

 rapidity as to resemble electrical impulses rather than sustained 

 vibrations. 



If we turn from the rays themselves to the effect they produce, 

 we find that the rays alter the properties of the substances 

 through which they are passing. This change is most apparent in 

 the effects produced on the electrical properties of the substances. 

 A gas, for example, while transmitting these rays is a conductor 

 of electricity. It retains its conducting properties for some little 

 time after the rays have ceased to pass through it, but Mr. 

 Rutherford and I have lately found that the conductivity is 

 destroyed if a current of electricity is sent through the Rontgen- 

 ised gas. The gas in this state behaves in this respect like a 

 very dilute solution of an electrolyte. Such a solution would 

 cease to conduct after enough electricity had been sent through 

 it to electrolyse all the molecules of the electrolyte. When 

 a current is passing through a gas exposed to the rays, 

 the current destroys and the rays produce the structure which 

 gives conductivity to the gas ; when things have reached a steady 

 state the rate of destruction by the current must equal the rate 

 of production by the rays. The current can thus not exceed a 

 definite value, otherwise more of the conducting gas would be 

 destroyed than is produced. 



This explains the very characteristic feature that in the passage 

 of electricity through gases exposed to Rontgen rays, the current, 

 though at first proportional to the electromotive force, soon 

 reaches a value where it is almost constant and independent of 

 the electromotive force, and we get to a state when a tenfold 

 increase in the electromotive force only increases the current by 

 a few per cent. The conductivity under the Rontgen rays varies 

 greatly from one gas to another, the halogens and their gaseous 

 compounds, the compounds of sulphur, and mercury vapour, are 

 among the best conductors. It is worthy of note that those 

 gases which are the best conductors when exposed to the rays are 

 either elements, or compounds of elements, which have in com- 

 parison with their valency very high refractive indices. 



The conductivity conferred by the rays on a gas is not de- 

 stroyed by a considerable rise in temperature ; it is, for example, 

 not destroyed if it be sucked through metal tubing raised to a 

 red heat. The conductivity is, however, destroyed if the gas is 

 made to bubble through water, it is also destroyed if the gas is 

 forced through a plug of glass wool. This last effect seems to 

 indicate that the structure which confers conductivity on the gas 

 is of a very coarse kind, and we get confirmation of this from 

 the fact that a very thin layer of gas exposed to the Rontgen 

 rays does not conduct nearly so well as a thicker one. I think 

 we have evidence from other sources that electrical conduction is 

 a process that requires a considerable space — a space large 

 enough to enclose a very large number of molecules. 



Thus KoUer found that the specific resistances of petroleum, 

 turpentine, and distilled water, when determined from experi- 

 ments made with very thin layers of these substances, was very 

 much larger than when determined from experiments with 

 thicker layers. Even in the case of metals there is evidence 

 that the metal has to be of appreciable size if it is to conduct 

 electricity. The theory of the scattering of light by small 

 particles shows that, if we assume the truth of the electro- 

 magnetic theory of light, the effects should be diffeient accord- 

 ing as the small particles are insulators or conductors. When 

 the small particles are non-conductors, theory and experiment 

 concur in showing that the direction of complete polari.sation 

 for the scattered light is at right angles to the direction of the 



NO. 1403, VOL. 54] 



incident light, while if the small particles are conductors, theory 

 indicates that the direction of complete polarisation makes an 

 angle of 60° with the incident light. This result is not, how- 

 ever, confirmed by the experiments in.ade by Prof. Threlfali on 

 the scattering of light by very small particles of gold. He 

 found that the gold scattered the light in just the same way as 

 a non-conductor, giving complete polarisation at right angles 

 to the incident light. This would seem to indicate that those 

 very finely divided metallic particles no longer acted as con- 

 ductors. Thus there seems evidence that in the case of con- 

 duction through gases, through badly conducting liquids and 

 through metals, electric conduction is a jjrocess which requires 

 a very considerable space and aggregations of large numbers 

 of molecules. I have not been able to find any direct experi- 

 mental evidence as to whether the same is true for electrolytes. 

 Experiments on the resistance of thin layers of electrolytes 

 would be of considerable interest, as according to one widely- 

 accepted view of electrolysis conduction through electrolytes, 

 so far from being effected by aggregations of molecules, takes 

 place by means of the ion, a structure simpler than that of the 

 molecule, so that if this represents the process of electrolytic 

 conduction, there would not seem room for the occurrence of 

 an effect which occurs with every other kind of conduction. 



In this building it is only fitting that some reference should 

 be made to the question of the movement of the ether. V'ou 

 are all doubtless acquainted with the heroic attempts made by 

 Prof. Lodge to set the ether in motion, and how successfully 

 the ether resisted them. It seems to be conclusively proved 

 that a solid body in motion does not set in motion the ether at 

 an appreciable distance outside it ; so that if the ether is dis- 

 turbed at all in such a case, the disturbance is not comparable 

 with that produced by a solid moving through an incompressible 

 fluid, but must be more analogous to that which would be pro- 

 duced by the motion through the liquid of a body of very open 

 structure, such as a piece of wire netting, where the motion of 

 the fluid only extends to a distance comparable with the diameter 

 of the wire, and not with that of the piece of netting. There is 

 another class of phenomena relating to the movement of the 

 ether which is, I think, deserving of consideration, and that is 

 the effect of a varying electro-magnetic field in setting the ether 

 in motion. I do not remember to have seen it ]iointed out that 

 the electro-magnetic theory of light implicitly assumes that the 

 ether is not set in motion even when acted on by mechanical 

 forces. On the electro-magnetic theory of light such forces do 

 exist, and the equations used are only applicable when the ether 

 is at rest. Consider, for example, the case of a plane electric 

 wave travelling through the ether. We have parallel to the 

 wave-front a varying electric polarisation, which on the theory is 

 equivalent to a current ; at right-angles to this, and also in the 

 wave-front, we have a magnetic force. Now, when a current 

 flows through a medium in a magnetic field there is a force act- 

 ing on the medium at right-angles to the ]>lane, which is parallel 

 both to the current and to the magnetic force ; there will thus be 

 a mechanical force acting on each unit volume of the ether when 

 transmitting an electric wave, and since this force is at right- 

 angles to the current and to the magnetic force, it will be in the 

 direction in which the wave is propagated. In the electro- 

 magnetic theory of light, however, we assume that this force 

 does not set the ether in motion, as unless we made this assump- 

 tion we should have to modify our equations, as the electro- 

 magnetic equations are not the same in a moving field as in 

 a field at rest. In fact, a complete discussion of the trans- 

 mission of electro-magnetic disturbances requires a know- 

 ledge of the constitution of the ether which we do not 

 possess. We now assume that the ether is not set in motion 

 by an electro-magnetic wave. If we do not make this assump- 

 tion, we must introduce into our equation quantities represent- 

 ing the components of the velocity of the ether, and unless we 

 know the constitution of the ether, so as to be able to deduce 

 these velocities from the forces acting on it, there will be in 

 the equations of the electro-magnetic field more unknown 

 quantities than we have equations to determine. It is, there- 

 fore, a very es.sential point in electro-magnetic theory to in- 

 vestigate whether or not there is any motion of the ether in a 

 varying electro-magnetic field. We have at the Cavendish 

 Laboratory, using Prof. Lodge's arrangement of interference 

 fringes, made some experiments to see if we could detect any 

 movement of the ether in the neighbourhood of an electric 

 vibrator, using the spark which starts the vibrations as the 

 source of light. The movement of the ether, if it exists, will 



