of view of the Kinetic Theory of Matter. 37 



ishes in cross-sectional area much more rapidly with pressure, 

 in tubes whose walls lie very near the cathode, than in those 

 whose walls are far removed from the cathode. It ought also 

 to be found that the form of the discharge stream from the 

 cathode is modified by the form of the glass walls near the 

 cathode. 



The return stream from the walls of the tube to the cathode, 

 indicated in the foregoing paragraphs, can be observed, during 

 the process of exhaustion, in almost any tube. When the pres- 

 sure of the gas is such that the discharge stream from the 

 cathode no longer covers the whole area of the electrode, 

 there can clearly be seen a convergent band, or collar of light, 

 beginning near the walls, of the tube in the vicinity of the 

 cathode and extending to the base of the discharge stream. 

 The general appearance of this return stream to the cathode is 

 shown in fig. 11 for a spherical electrode. For clearness only 



li. 



the meridian section of the streams is shown. As the pressure 

 in the tube is decreased, the return stream appears to spring 

 from points on the walls more remote from the cathode ; the 

 pink light, by which the return stream is distinguished, be- 

 comes fainter, and long before the disappearance of the dis- 

 charge streams from the cathode, all visible evidence of this 

 return stream has vanished. 



For the purpose of illustrating the variation of the action of 

 the walls of a tube with their distance from the cathode, in 

 decreasing the cross-sectional area of the discharge stream from 

 the cathode, the tube illustrated in fig. 12 was constructed. 



12. 



^^ 



As will be seen from the figure, it is formed of two cylindrical 

 glass tubes of very different diameters. The large tube is 

 about 4:-6 cm , and the smaller l'85 cm in diameter. The two 

 spherical aluminum electrodes are each l*8 cm diameter, and 

 2 5 cm radius of curvature. The distance between these elec- 

 trodes is about 20 cm . When the pressure in the tube is not 



