Discharge of Electricity through Gases. 331 



inside the glow readily take up the potential of the surrounding space. 

 A screen of glass, therefore, in front of an electrode will become 

 covered with electricity until the potential in its neighbourhood does 

 not vary in a normal direction, and if the ray from a negative 

 electrode passes tangentially, there can be no deflective force on the 

 particles which make up the ray, the piece of glass will act as an 

 electrical screen. We shall never be able to show for the same reason 

 any deflecting force due to a statically charged body, for if the body 

 is simply placed inside the vacuum, near enough to infldence the 

 current, it will discharge itself, and if it is covered with some non- 

 conductor, that non-conductor will become charged on its outside until 

 its effect on the lines of force has been counterbalanced. 



A serious objection against Mr. Crookes' view that particles are 

 projected from the negative electrode with great velocity has been 

 first urged by Mr. E. Wiedemaim, I believe, and was afterwards 

 repeated by Goldstein. It would apply with equal force against the 

 theory which I am defending. Particles moving with great velocity 

 in one direction should send out light, the wave-length of which is 

 affected by the motion. ISTo such effect can be observed, although 

 Goldstein has undoubtedly proved that the velocity must be sufficiently 

 large to make it perceptible. But the particles moving with great 

 velocity are not themselves luminous, that is proved by the existence 

 of the dark space. It is only when their velocity has become 

 sufficiently reduced and the energy is distributed in all directions, 

 that the particles are luminous ; but then we do not get a one-sided 

 displacement, but only a very small widening due to the motion of 

 the particles in all directions. 



Mr. Crookes has given reasons why we should consider the region 

 of the dark space as one in which directed motion prevails, and 

 although Hittorf has raised serious objections against the arguments 

 drawn from his radiometer experiments, which seem to be explained 

 by secondary temperature effects, the general conclusion which he 

 has drawn from his experiments is not thereby invalidated, for the 

 rise of temperature itself requires explanation. 



Proposed Test of the Theory. 



The most conclusive proof of our theory would be the demonstra- 

 tion of the fact that each particle of matter carries with it the same 

 amount of electricity. We shall not, of course, be able to prove this 

 for each single particle, but I propose to show how we can decide the 

 point experimentally as far as the average amount is concerned. 

 Suppose a small straight beam is cut out of the glow and placed in a 

 field of uniform force, the lines of which cut the rays of the glow at 

 right angles. The force being everywhere normal to the rays, these 

 will curl up in a circle. This has been shown to be true experimentally 



VOL. xxxvii. z 



