the Occurrence of Enhanced Lines in 'the Arc. 293 



subsequent experiment the carbon was replaced by a rod of 

 graphite. Under otherwise similar conditions as before, the 

 tip of the graphite cathode did not become white hot during 

 arcing and, perhaps as a consequence of this deficiency, the 

 arc burned most unsteadily and was difficult to keep going 

 for any length of time. We concluded that, since evidently 

 fewer electrons were being expelled from the graphite than 

 from the white-hot carbon, the resistance of the medium 

 through which the arcing current passed was greater in the 

 case of the graphite arc than in that of the carbon arc. 

 Consequently, it would necessitate the application of a stronger 

 electric field in order to drive the electric current through a 

 graphite arc and to maintain the latter stable. Now, with a 

 graphite cathode the spark lines of lead became visible in the 

 arc ! This result seemed to us to mean that an increase of 

 the resistance of the medium, or otherwise stated a lowering 

 of the degree of ionization prevailing therein, is one of the 

 conditions for the appearance of spark lines in the arc. In 

 order to test this inference the carbon cathode, which in the 

 first experiment had prevented the emission of enhanced lines, 

 was again mounted in the apparatus, and the resistance of 

 the medium between the carbon tip and the lead anode 

 increased by gently blowing through it with the mouth. 

 This very simple modus operandi produced the looked for 

 effect, for the spark lines of lead were brought out quite 

 plainly ! 



§ 5. Effect of a drop of liquid placed between the electrodes. 



Previous experimenters had obtained the spark lines in the 

 arc by burning the latter under water ; but, unless the liquid 

 be constantly renewed, it soon becomes opaque with colloidal 

 matter, and detailed observations are thereby seriously 

 impeded. We therefore looked for some more convenient 

 method of attaining the same object, and finally adopted the 

 following very simple process of changing the nature of the 

 medium through which the arc was passed, at all events 

 during the early stages of the discharge. The two electrodes, 

 one of which may be a carbon or graphite rod, are so arranged 

 that the lower one (generally the metal and either positive 

 or negative) remains fixed, whereas the upper one can be 

 rapidly moved up and down by means of a vertical rack 

 motion. Both are well rounded off at the ends and present 

 a smooth surface. After the current has been put on the 

 electrodes are first brought to within about two or three 

 millimetres from one another, and by means of a small 



