Discharge of Electricity through Gases. 327 



there is an essential difference in the thickness of the layer within 

 which we must imagine that condenser action to take place. In the 

 liquids that thickness must be very small, as is shown by the intensity 

 of the observed polarisation currents. The positively electrified matter 

 in every case is kept against the negative surface by a joint action of 

 electrical and chemical forces, for it has been shown by Helmholtz 

 that only thus can we explain a difference of potential between two 

 bodies. It is the chemical forces which keep the two electricities 

 asunder. The gaseous molecules or atoms, however, subject to their 

 mutual encounters, and always having certain velocities, will tend to 

 leave the surface. They are kept near the surface by the electrical 

 forces. I do not, of coarse, mean to imply that the positive part of 

 the condenser is always made up of the same molecules, but only 

 that the time during which each positively electrified particle forms 

 part of the condenser is large compared to the time during which it 

 would be in the neighbourhood of the electrode if they were both 

 unelectrified. 



Suppose, now, that a positive electrode is placed near such a con- 

 denser. The resistance of the gas is so much greater than that of 

 the metal electrode that we shall assume the whole electrode to be of 

 the same potential. The lines of force will then cut the surface at 

 right angles, and could we assume the condenser to be infinitely thin, 

 there would only be a normal force acting on its particles ; but as the 

 lines of force are curved, the particles not in immediate contact with 

 the surface are acted on by a tangential force which will tend to drive 

 them away from the positive electrode. As a steady state will only 

 be possible when the total force is normal throughout the condenser, 

 we arrive at the condition for the steady state that within the con- 

 denser the fall of potential must be the same for equal distances 

 measured along the normal to the surface. 



Some idea of the distribution of potential may be gained by con- 

 sidering a positively charged point near two concentric spheres, the 

 inner one of which is charged with negative electricity. The outer 

 sphere representing the outer coating of the condenser will act as 

 screen ; the distribution on the inner sphere will therefore be uniform. 

 The analogy is, of course, not complete, as we cannot assume the 

 positively electrified particles to be distributed over a surface merely. 



Our experimental evidence speaks in favour of the conclusion to 

 which we have arrived, namely, that the fall of potential is equal for 

 equal distances taken normally away from the negative electrode. 

 This seems to follow at least from the fact that the boundary of the 

 dark space is, whenever that is possible, equidistant from the elec- 

 trode, for we know that the width of the dark space depends on the 

 intensity of current. When the electrode is of such a shape that the 

 normal drawn outwards from it meets its surface again, or whenever 



