TRANSACTIONS OF SECTION A. 705, 
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 Réntgen 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 Réntgen 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 comparison with their valency very high 
refractive indices. 
The conductivity conferred by the rays on a gas is not destroyed by a con- 
siderable 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, de- 
stroyed 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 inclose a very large number of 
molecules. 
Thus Koller found that the specific resistances of petroleum, turpentine, 
and distilled water, when determined from experiments made with very thin 
layers of these substances, were very much larger than when determined from ex- 
periments 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 different according 
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 polarisation for the scattered light is at right angles to the direction of 
the 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, however, confirmed by the experiments made 
by Professor Threlfall 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 conductors. Thus there seems evidence that in the case of con- 
duction through gases, through badly conducting liquids, and through metals, 
_ electric conduction is a process which requires a very considerable space and 
aggregations of large numbers of molecules. I have not been able to find any 
direct experimental evidence as to whether the same is true for electrolytes. 
Experiments on the resistance of thin layers of electrolytes would be of con- 
_ siderable interest, as according to one widely accepted view of electrolysis con- 
duction 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. 
Tn this building it is only fitting that some reference should be made to the 
question of the movement of the ether. You are all doubtless acquainted with the 
heroic attempts made by Professor Lodge to set the ether in motion, and how suc- 
