316 M. F. Stenger on the Electric 



o^ 



from the carbon by ignition in a current of chlorine, or 

 treatment with molten potash, or by soaking in hydrofluoric 

 acid. As, however, Liveing and Dewar* have shown, these 

 methods are not successful in completely removing iron, 

 magnesium, sodium, and calcium. 



§ 9. Whilst electric discharges in air, hydrogen, nitrogen, 

 or carbon dioxide require a large electromotive force, because 

 they have to overcome a very high resistance, it is possible 

 in the presence of good conducting metallic vapours to obtain 

 the arc-discharge with comparatively few elements. Thus 

 Hittorf f , with cylinders of retort-carbon, the ends of which at 

 a distance of 3 to 4 millim. were brought into a Bunsen 

 flame, in which a bead of potassium-salt was heated, obtained 

 the arc-light with 80 of his elements, without its being 

 necessary to first make contact between the carbons. For 

 this it was of no importance whether both points or only 

 the kathode was surrounded by potassium vapour, because, 

 as with the glow-discharge in exhausted gases, the neighbour- 

 hood of the kathode offered a much greater resistance also 

 in the gases of the flame. 



Further, Gassiot, employing a battery of 400 Grove's ele- 

 ments, observed that upon making contact, the discharge 

 between balls of carbon or of metal was discontinuous, but 

 very quickly passed into the continuous arc-discharge, 

 evidently because metallic vapours had been formed by the 

 sparks which leapt across, which by their small resistance 

 permitted the establishment of a constant current of great 

 intensity. 



Lastly, Herschel's artifice is well known, of producing the 

 arc, not by contact of the electrodes, but by means of a spark 

 allowed to pass between them. 



§ 10. I have already had occasion to mention observations 

 of Hittorf $, in which he obtained in nitrogen of 53 millim. 

 pressure currents of considerable strength by means of his 

 battery of 1600 elements. 



The considerable decrease in the resistance of the stratum 

 of gas, as I have already mentioned, was accompanied by 

 intense white heat of the electrodes, so that here also we may 

 very well regard the abundant formation of metallic vapours 

 as the cause of the phenomenon. 



The influence of the white heat of the electrodes is seen still 

 more distinctly in later researches of Hittorf § and Goldstein ||, 

 which were made with very small pressure of gas. The older 



* Proc. Roy. Soc. xxx. p. 155 (1880) ; xxxiii. p. 406 (1882). 

 t Comptes Rcndiis, lxxxiv. p. 846 (1877). 



\ Wied. Ann. xxiv. p. 81 (1885). § Ibid. xxi. p. 133 (1884). 



|| Ibid. xxi. p. Ill (1884). 



