§44 Mr. G-. A. Hemsalech and the Comte de Gramont on 



A remarkable effect upon the character of the spectrum of 

 magnesium was observed with an arc burning in nitrogen. 

 The flame line X 4571 and the triplets at X 3838 and X 5184 

 show the usual development as given by an arc in air. The 

 band at X5007 is absent, requiring probably the presence of 

 oxygen for its emission. The Rydberg series, on the other 

 hand, is far more developed than in air : the individual lines 

 are much better defined and, instead of being winged towards 

 the red, they appear only very slightly widened symmetrically, 

 The spark line X4481, although relatively feeble, passes 

 right across the space between the electrodes and appears 

 quite sharp without any sign of haziness or broadening. 

 Since the arc in this case was burning quite steadily, the 

 emission of the spark line under these conditions cannot be 

 attributed to the action of strong electric fields, for these 

 were evidently feeble — in fact, a voltmeter indicated the 



existence of an average gradient of only about 150 - 7 — . 



But it is possible that both the establishment of a stable arc 

 between magnesium poles in nitrogen and the peculiarity of 

 its spectrum are caused by some special chemical reaction 

 between this gas and the electrode metal. As is well known, 

 magnesium forms a nitride at high temperatures : it may 

 therefore be that the continuous emission of A, 4481 as a 

 narrow line and the sharpening of the lines of the Rydberg 

 nebulous series are an optical manifestation of the process 

 involved in the formation of magnesium nitride.. In this 

 connexion it is useful to recall the important, fact observed 

 by Dr. de Watteville that in the air-coal gas flame the series 

 triplets of magnesium are emitted only in the explosion 

 region * and, as one of us has shown, the excitation of the 

 characteristic cone lines is most probably associated with 

 the formation of nitrides |. No trace of the spark line 

 X 4481 or of any line of the Rydberg series has, however, 

 been observed in the air-coal gas cone, and their emission by 

 the nitrogen arc may possibly be brought about with the 

 additional help of the electric current. When the obser- 

 vations of the arc in nitrogen are made at the instant of 

 separation of the electrodes the character of the spectrum 

 is similar to that of the first phase spectrum in air — namely, 

 the lines of the Rydberg series are winged on the red side 

 and the spark line X 4481 is symmetrically widened. It is 

 therefore probable that during the initial stages the light 



* C. de Watteville, Phil. Trans. Roy. Soc. A. vol. 204. p. 151 (1904). 

 t G. A. Hemsalech, Phil. Mag. vol. xxxiv. p. 229 (1917). 



