Chemistry and Physics. 363 



the arc burns quietly along the common axis of the tube and 

 helix. The light emitted by such arcs in various gases has been 

 investigated spectroscopically by W. Groteian and C. Rungb, 

 and a very important result has been obtained. These scientists 

 found that the emission bands which have had their origin 

 referred to a compound of carbon and nitrogen (commonly called 

 cyanogen for brevity), by all other spectroscopists, are really due 

 to nitrogen alone. The importance of the result justifies the pre- 

 sentation of the details given below. 



The arc lamps actually used in this investigation were of 

 smaller dimensions than the ones employed in the technical fur- 

 naces and they were run on direct instead of alternating current. 

 The uni-directional current has the obvious advantage of differen- 

 tiating the radiations from the anode and cathode. With 5,000 

 volts and 3 amperes an arc in air one meter long was obtained. 

 The success of the work depended primarily upon the following 

 essential difference between the ordinary low potential arcs with 

 relatively large amperage and the high potential arcs with small 

 current values. In the former the current is carried by the ions of 

 the metallic vapors as well as by the gaseous medium in which 

 the electrodes are placed, so that the metallic and other " impur- 

 ity " lines usually stretch across the entire gap between the poles. 

 On the other hand, in the case of the high potential arcs the 

 spectral lines pertaining to the constituent elements of the elec- 

 trodes only jut out a short distance from the anode and cathode, 

 while the lines of the gaseous atmosphere span the entire length 

 of the arc. That the current in the center of the arc is carried 

 only by the ions of the atmosphere used is also shown by the 

 experimental fact that the form of the " characteristic curve " of 

 the arc is independent of the nature of the solid electrodes. The 

 authors state that under certain conditions (not specified) they 

 have obtained pure glow discharges, at atmospheric pressure and 

 "high " amperage, which did not show a trace of the spectrum of 

 either electrode. Furthermore, the high potential arcs have a 

 distinct advantage over the ordinary discharges in partially 

 evacuated tubes in that small traces of impurities have a negligi- 

 ble influence on the spectrum of the gas under investigation 

 because this gas is let into the lamp at high pressure, from a 

 bomb, and the impurities are swept out immediately by the vortex. 



As stated above, the spectroscopic investigation of the nitro- 

 gen arc showed that the four groups of bands, whose wave- 

 lengths, expressed in round numbers, are 3360, 3590, 3884, and 

 4216, are due to nitrogen alone and not to cyanogen. The nitro- 

 gen arc attained a length of 30 cms with a current of 1 ampere. 

 The discharge starts with a bright rose color at a point-like spot 

 of the anode and then passes over to the arc proper, which con- 

 sists of a reddish-violet band. When the current of gas is strong 

 no aureole or " glory " can be distinguished. A broader, bluish 

 spot is formed at the cathode which is analogous to the negative 

 glow in a vacuum tube and which radiates the so-called negative 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, So. 226.— October, 1914. 

 25 



