306 M. F. Stenger on the Electric 



charge the spectrum shows only the lines of the stratum of 

 gas, and the nature of the electrodes is a matter of indifference. 



(4) In the arc both electrodes become disintegrated 

 although in different degree, whilst with the glow-discharge 

 disintegration takes place only at the kathode. 



§ 1. On the Resistance of the Stratum of Gas. — With the 

 glow-discharge in exhausted gases it is, according to Hittorf *, 

 the first tw T o layers of the kathode-light which oppose 

 resistance to the passage of the electric current, in comparison 

 with which the resistance of the positive brush-discharge is 

 very small. 



Since, as the pressure decreases, both layers continually 

 expand, the resistance of the kathode-light increases simul- 

 taneously. When Hittorf passed, on the other hand, to higher 

 pressures of gas, the thickness of the glow-light decreased 

 and the strength of current increased. With a vacuum-tube 

 filled with nitrogen of about 17 millim. pressure, there 

 appeared, as with high exhaustion, as soon as the circuit 

 was completed, a glow-light of 1 millim. thickness upon the 

 kathode of 17 millim. length, which speedily heated the 

 wire to bright redness, which increased to a white heat 

 sufficient to fuse the thick iridium electrode. If the density 

 of the gas was still further increased, the anode also became 

 white hot, and at the pressure of 53 millim. the anode was 

 even hotter than the kathode. At the same time the strength 

 of the current increased continuously up to 2 amperes, a 

 current-strength not greatly different from that obtained 

 with the arc-light. Certain other observations of Hittorf f 

 and Goldstein J, however, show very clearly that if the 

 experimental conditions are favourable, the resistance of the 

 gas in the glow-light is of the same order as in the Davy 

 arc-light. 



Hittorf employed as kathode a platinum coil heated by 

 means of a strong current passed through it, and found that 

 the resistance remained unaltered as long as the platinum was 

 only red hot, but that the sudden diminution of resistance 

 occurred as soon as it became white hot, which continued as 

 the temperature of the platinum rose. If the experiment was 

 carried out in the same way with a Carre's carbon-rod, it was 

 possible to obtain a discharge from 10 small elements with the 

 electrodes 4 centims. apart. At a distance of 15 centims., 

 40 elements gave a constant current of -^ ampere. When 

 Hittorf employed as kathodes the carbons of the Davy arc, 

 the action was much greater. If, on the other hand, the anode 



* Wied. Ann. xxi. p. 97 (1884). t Ibid. xxi. p. 133 (1884). 

 \ Ibid. xxiv. p. 81 (1885). 



