826 Prof. Percival Lewis on the Origin of the 



of a molecule, but to the modified electrodynainic constitu- 

 tion of the medium, through which the light travels, causing 

 a variation of the velocity of transmission. The variations 

 here discussed will therefore be closely associated with the 

 variation with the concentration of the ordinary index of 

 refraction e of the solution for the particular light used. In 

 fact, for the case above discussed in detail 



so that 



M=^? <y_i) (>-i + £), 



which gives a relation between [&>] and e. Unfortunately, 

 I have not sufficient information to hand to tell whether 



the variation of the function (e 2 — 1) ( e 2 — 1-f- -) with the 



concentration of the solution is at all similnr to the varia- 

 tions of [ft)], especially in the more complicated cases. I 

 hope, however, that some one will lake up the question 

 experimentally at this point so as to provide the necessary 

 data for a comparison. 



LXXXVI. The Origin of the Bands in the Spectrum of Active 

 Nitrogen. By E. Percival Lewis, Professor of Physics, 

 University of California*. 



[Plate X.] 



IN 1899 the writer observed a new form of afterglow in 

 nitrogen, excited only by a condensed discharge with 

 spark-gap, and having a characteristic band-spectrum f. 

 Further results were published in 1904 J. The approxi- 

 mate wave-lengths of the bands were determined, and the 

 remarkable fact emphasized that the vapours of mercury 

 from the pump and from aluminium, zinc, and platinum 

 electrodes participated in the afterglow some time after the 

 discharge had ceased, their principal spectral lines appearing 

 as long as the glow lasted. Experiments on a larger scale 

 were made by iStrutt §, who showed that other substances 

 were also excited to luminosity by the glowing gas, to which 

 he gave the name "active nitrogen." Strutt and Fowler 

 published the details of the spectrum of the active nitrogen || 

 with greater dispersion than had been available to me. 



* Communicated by the Author. 



t Astrophys. .loarn. xii. p. 8 (1900) ; Ann. dor Phi/s. ii. p. 249 (1900). 



| Astrophys. Journ. xx. p. 49 (1904). 



§ 'Bakerian Lecture,' Proc. Hoy. Soc. Ixxxv. p. 219 (1911). 



j| Proc. Roy. Soc. Ixxxv. p. 777 (1911). 



