Conductivity of Gases. 317 



observations do not show the decrease in resistance so 

 strikingly, no doubt because, whilst the resistance at the 

 kathode is greatly diminished by the formation of metallic 

 vapour, the resistance of the positive light is increased by the 

 higher pressure of gas. 



But if by suitable means we heat the kathode in exhausted 

 gas to the point of volatilization, we may obtain powerful 

 currents even with small electromotive forces. In fact, 

 Hittorf and Goldstein have observed these phenomena ; I 

 will only mention of their results, that with a distance between 

 the electrodes of 6 centim., one small element produced a 

 constant current with high exhaustion and heating of the 

 kathode to white heat. 



§ 11. Warren DeLaRue and Miiller*, in a comprehensive re- 

 search, have endeavoured to show that glow-discharge and arc- 

 discharge pass into each other in consequence only of change 

 in the density of the gas. I wish here to express again my 

 doubt (referring also to the criticism of Goldstein f) whether 

 the authors did in fact have arc-light ; their assumption is 

 also in direct contradiction with the previously published 

 observation of Gassiot, on the passage of glow-discharge into 

 arc-discharge at unaltered pressure. 



Moreover, in accordance with the preceding consideration, 

 the pressure is of small importance ; but everything depends 

 upon whether the space between the electrodes is filled with 

 incandescent metallic vapours or with ordinary gas. I have 

 further had opportunity in my experiments to observe the 

 passage of an arc-discharge into a glow-discharge, which is 

 easily explained on the same principle. 



If with the apparatus figured in fig. 2, at a pressure of 

 10 millim. the arc was lengthened until the resistance increased 

 to the point of extinction, there appeared immediately in front 

 of the anode a stratum of blue glow-light about 1 millim. thick 

 and 10 millim. long, which, after a few seconds, disappeared 

 with the light between the carbons. 



If, then,the resistance became too great for the arc-discharge 

 to continue, the conductivity of the still glowing metallic 

 vapours was still sufficient for a short time to permit a much 

 feebler current in the form of glow-discharge. 



§ 12. In this concluding section, I should like to make 

 some remarks on the luminosity of gases and vapours, which 

 connect themselves with the experiments, already several 

 times mentioned, of Hittorf. If, in electric glow- discharges, 

 the electrodes came to an intense white-heat, the blue glow- 



* Phil. Trans, clxxi. p. 65 (1879). 



t Goldstein, Fortschr. der Physik, 1880. 



