704, REPORT— 1896, 
molecules may be greater than the positive part due to the ether, so that the 
specific inductive capacity of the mixture of molecules and ether would be negative ; 
no waves of this period could then travel through the medium—they would he. 
totally reflected from the surface. 
As the frequency of the force gets greater and greater, its effect in making the 
molecules set will get less and less, but the waves will continue to be totally re- 
flected until the negative part of the specific inductive capacity due to the mole- 
cules is just equal to the positive part due to the ether, Here the refractive 
index of the mixture is zero. As the frequency of the force increases, its effect on 
the molecules gets less and less, so that the specific inductive capacity continually 
approaches that due to the ether alone, and practically coincides with it as soon as 
the frequency of the force is a considerable multiple of that of the molecules. In 
this case both the specific inductive capacity and the refractive index of the medium 
are the same as that of the ether, and there is consequently no refraction, Thus 
the absence of refraction, instead of being in contradiction to the Réntgen rays, 
being a kind of light, is exactly what we should expect if the wave-length of the 
light were exceedingly small. 
The other objection to these rays being a kind of light is, that there is no very 
conclusive evidence of the existence of polarisation. Numerous experiments have 
been made on the difference between the absorption of these rays by a pair of 
tourmaline plates when their axes are crossed or parallel. Many observers have 
failed to observe any difference at all between the absorption in the two cases. 
Prince Galitzine and M. de Karnojitsky, by a kind of cumulative method, have 
obtained photographs which seem to show that there is a slightly greater absorption 
when the axes are crossed than there is when the axes are parallel. There can, 
however, be no question that the effect, if it exists at all, is exceedingly small 
compared with the corresponding effect for visible light. Analogy, however, leads 
us to expect that to get polarisation effects we must use, in the case of short 
waves, polarisers of a much finer structure than would be necessary for long ones. 
Thus a wire bird-cage will polarise long electrical waves, but will have no effect 
on visible light. Rubens and Du Bois made an instrument which would polarise 
the infra red rays by winding very fine wires very close together on a framework ; 
this arrangement, however, was too coarse to polarise visible light. Thus, though 
the structure of the tourmaline is fine enough to polarise the visible rays, it may 
be much too coarse to polarise the Réntgen 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 Réntgen rays are light rays, their wave-lengths are of an 
entirely different order from 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 com- 
mensurate 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 effects 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 con- 
ductor 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 Rontgenised 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, 
