702 - REPORT— 1896. 
charge; with-this arrangement, however, the charged particles had to give up their 
charges to'the' disc if the gold leaves of the electroscope were to be affected, and we’ 
know that it is'extremely difficult, if not impossible, to get electricity out of a: 
charged gas mérély by bringing the gas in contact with a metal. Lord Kelvin’s 
electric strainers are an example of this. It is a feature of Perrin’s experiment 
that since it acts by induction the indications of the electroscope are independent 
of the communication of the charges of electricity from the gas to the cylinder, and: 
since the cathode rays fall on the inside of the cylinder the electroscope would 
not be affected, even if there were such an effect as is produced when ultra-violet 
light falls upon the surface of an electro-negative metal when the metal acquires a 
positive charge. Since any such process cannot affect the total amount of electricity 
inside the cylinder, it will not affect the gold leaves of the electroscope ; in fact, 
Perrin’s experiments prove that thecathode rayscarry a charge of negativeelectricity. 
The other view held as to the constitution of the cathode rays is that they are 
waves in the ether. It would seem difficult to account for the result of Perrin’s 
experiment.on this view, and also I think very difficult to account for the magnetic 
deflection of the rays. Let us take the case of a uniform magnetic field: the 
experiments ‘which have been made on the magnetic deflection of these rays seem: 
to make it clear that in a magnetic field which is sensibly uniform, the path of 
these rays is curved ; now if these rays were due to ether waves, the curvature of 
the path would-show thatthe velocity of propagation of these waves varied from 
point to point of the path. That is, the velocity of propagation of these waves is 
not only affected by the magnetic field, it is affected differently at different parts 
of the field.. But in a uniform field what is there to differentiate one part 
from another; ‘so as to account for the variability of the velocity of wave 
propagation in such a field? The curvature of the path in a uniform field 
could not be accounted for by supposing that the velocity of this wave motion 
depended: on the strength of the magnetic field, or that the magnetic field, 
by distorting ‘the shape of the boundary of the negative dark space, changed 
the direction of ‘the wave front, and so produced a deflection of the rays. The 
chief reason for Supposing that the cathode rays are a species of wave motion is 
afforded‘ by Lenard’s discovery, that when the cathode rays in a vacuum tube fall 
on a thin aluminium window in the tube, rays having similar properties are 
observed on the side of the window outside the tube ; this is readily explained on 
the hypothesis that the rays are a species of wave motion to which the window is 
partially transparent, while it is not very likely that particles of the gas in the 
tube could fdrce their way through a piece of metal. This discovery of Lenard’s 
does not, howéver, seem to me incompatible with the view that the cathode rays are 
due to negatively charged particles moving with high velocities. The space outside 
Lenard’s: tube ‘must have heen traversed by Réntgen rays: these would put the 
surrounding gas in a state in which a current would be readily started in the gas if 
any electromotive force acted upon it. Now, though the metal window in Lenard’s 
experiments ‘was connected with the earth, and would, therefore, screen off from the 
outside of the tube any effect arising from slow electrostatic changes in the tube, it 
does not follow that it would be able to screen off the electrostatic effect of charged 
particles moving to and from the tube with very great rapidity. For in order to 
screen off electrostatic effects, there must be a definite distribution of electrification 
over the screen: changes in this distribution, however, take a finite time, which 
depends upon the dimensions of the screen and the electrical conductivity of the 
material of which it is made. If the electrical changes in the tube take place at 
above a certain rate, the distribution of electricity on the screen will not have time 
to adjust itself, and the screen will cease to shield off all electrostatic effects. Thus 
the very rapid electrical changes which would take place if rapidly moving charged 
bodies were striking against the window might give rise to electromotive forces 
in the region outside the window, and produce convection currents in the gas 
which has been made a conductor by the Réntgen rays. The Lenard rays would 
thus be analogous in character to the cathode rays, both being convective currents 
of electricity. Though there are some points in the behaviour of these Lenard rays 
which do not admit of a very ready explanation from this point of view, yet the 
