190 ANNUAL I1EPOET SMITHSONIAN INSTITUTION, 1921. 



is practically zero in the negative glow itself and increases in a 

 linear manner as we move towards the cathode. This means that the 

 density of the positive electrification (excess of positive over negative 

 ions) in the dark space is constant. 



No theory yet put forward can account for these numerical rela- 

 tions ; one can, however, be certain that ionization is going on at all 

 points throughout the dark space, and that it reaches a very high 

 intensity in the negative glow. 21 This ionization is probably caused 

 for the most part by electrons liberated from the surface of the cath- 

 ode (cathode rays). These, when they reach a speed sufficient to 

 ionize by collision, liberate more free electrons which, in their turn, 

 become ionizing agents, so that the intensity of ionization from this 

 cause will tend to increase as we move away from the cathode. 



The liberation of the original electrons from the surface of the 

 cathode is generally regarded as due to the impact of the positive 

 ions (positive rays) generated in the negative glow and the dark 

 space. Even this idea, for which there is a fair amount of definite 

 evidence, is now called in question for Ratner 22 has recently described 

 experiments proving " that the initial discharge of electricity through 

 vacuum tubes is not brought about by the impact of positive ions 

 against the surface of the cathode, and that positive ions impinging 

 upon the cathode with velocities corresponding to a fall through a 

 potential difference up to 2000 volts, are unable to liberate electrons 

 from the surface of the cathode." It must, however, be borne in mind 

 that the nature of the positive ions used in Ratner's experiments is 

 not known with certainty, and that the intensity of bombardment 

 was of an entirely different and smaller order than that usualty asso- 

 ciated with normal discharge. 



During the work on the length of the dark space a very curious 

 and interesting phenomenon was observed in hydrogen and all 

 the gases of the helium group. This consisted of a dark space 

 very small and very dark, immediately in front of the cathode, 

 inside the Crookes' dark space. 23 It can only be clearly seen by look- 

 ing across the face of a large plane cathode when its appearance in 

 pure helium or neon is very striking. Its properties are completely 

 different from those of the Crookes' dark space. Its length, which is 

 usually less than a millimeter, is independent of the pressure and 

 varies roughly inversely as the square root of the current density. 

 As the field close to the cathode varies directly as the square root of 

 the current density this suggested at once that the new dark space 

 represented a definite constant fall of potential. There is now little 



21 Aston : Proc. Roy. Soc, 96A, 200, 1919. 



22 Ratner: Phil. Mag., 40, 795, 1920. 



23 Aston : Proc. Roy. Soc, 80A, 45, 1907. 



