1889.] of the various Species of Heavenly Bodies. 243 



which is seen in the " bright-line stars," and it appears to be the 

 most visible of the carbon bands from the same reason in both cases, 

 namely, the absence of continuous spectrum in the blue. 



The last stage in the spectrum of lightning seems to be that in 

 which the brightest lines in the spectrum consist entirely of lines of 

 nitrogen. Such a spectrum has been observed by Col. John Herschel, 

 the following lines being recorded : 



5697 

 500-9 

 463-6 



These are the three strongest lines of nitrogen, the wave-lengths of 

 which, according to Thale'n, are 



567-8 



566-6 



500- 



500- 



463-1 



-5 1 

 -2 J 



We have, therefore, an almost complete sequence of electrical dis- 

 charges through our atmosphere, from discharges so feeble that we 

 only see the 500 fluting of magnesium, or the 1st fluting of manga- 

 nese in their spectra, to those in which the brightest lines of nitrogen, 

 characteristic of intense discharges, are the brightest lines visible. 

 It is important to note that in the latter case we have to deal with 

 discharges through the lower and denser portions of the atmosphere. 

 The conditions of the two extreme cases are therefore very different, 

 and the spectra differ accordingly. In one case the discharges pass 

 through rarefied air charged with meteoric dust, whilst in the other 

 they pass through dense air which is comparatively free from such 

 dust. 



In experiments with large air vacuum-tubes the lines of nitrogen 

 are never seen, and it is extremely improbable, therefore, that they 

 would occur in weak discharges through a space which is much less 

 confined. Hence, when the line at 500 is seen in conjunction with 

 the fluting of manganese, it is in all probability due to magnesium 

 and not to nitrogen. 



The forked lightning discharge can be imitated by a jar spark, or 

 by the spark from an electrical machine, and the brightest lines in 

 the spectra, as we have seen, are identical. 



Fig. 20 shows the various spectra of air charged with meteoric 

 dust when illuminated by electrical discharges of gradually increasing 

 intensities. The lowest temperature of all gives the Mn fluting at 

 558. With the first increase in intensity the iron fluting (615) is at 

 times momentarily added, then magnesium, lead, thallium, and carbon 



