142 BELL SYSTEM TECHMC.IL JOURNAL 



is that it can siir\ ive even if tlie voltage between anode and cathode 

 is far below the resonance-potential of the atoms of the gas, which 

 seems impossible. A year ago it seemed that this effect could always 

 be ascribed to high-voltage high-frequency oscillations generated in 

 the arc. This explanation was presently confirmed in some cases and 

 disfjualificd in others, and now it appears that when there are no 

 oscillations an astonishingly strong potential-ma.ximum de\elops 

 within the ionized gas. Potential-maximum and oscillations alike 

 are pr(»l)ably to be regarded as manifestations of space-charge. 



The Glow in a rarefied ^as is a magnificent sight when the gas is 

 rarefied to the proper degree, not too little and not too much; divided 

 into luminous clouds of di\'crse brightnesses and diverse colors, re- 

 calling Tennyson's "fluid haze of light," yet almost rigidly fi.xed 

 in their distances and their proportions, it is one of the most theatrical 

 spectacles in the repertoire of the physical laboratory-. The grand 

 divisions of the completely-de\eloped discharge are four in number, 

 two relalix'eK' flim and two bright; beginning from the cathode end. 



ODD 



Fig. 8— The Crookcs dark space between the cathode (thin line at left) and the 

 negative glow. See footnote ". (.Aston, Proeerdings of the Royal Society) 



they are the Oookcs dark space, the negative glow, the I'aiada\ dark 

 space, and the positive column. .Additional gradations of color and 

 brightness can often be seen very close to the cathode and \-ery close 

 to the an(Kje. Photographs of the glow which give anything ap- 

 proaching a true idea of its appearance to the e\e are hard to find. 

 I repHKluce in Fig. 7 some photographs taken nearly fifty years ago by 

 de la Rue, which have re.ip|RMred in many a text; they show chiefly 

 the striking flocculent cloudlets into which the positive column some- 

 limesdivides itself. In Fig. 1) there are tw.) sketches made by Graham. 



