1889.] of the various Specie* of Heavenly Bodies. 245 



until there is a complete spectrum. The next stage of increasing 

 intensity is that observed by Schuster in which magnesium is repre- 

 sented by 6. Then comes Vogel's spectrum, entirely without man- 

 ganese, but with Z>, Tl (535), H (F, 486), C band (468474), and 

 N(500). Schuster did not make observations beyond 500, so that the 

 continuity in that region is apparently broken. It is possible that the 

 broad band in the blue observed by Vogel was the group of nitrogen 

 lines, the brightest of which is at 463 ; but in that case it is difficult 

 to understand why a decided maximum was not recorded. Finally, 

 we have the spectrum observed by Col. Herschel, in which those 

 nitrogen lines appear brighter than all the rest, exactly as they 

 appear in an intense spark discharge in our laboratories. 



The question will probably arise in some minds how it is that if we 

 assume that the luminosity of nebulae and auroras both proceed from 

 meteoric dust, that in the case of the nebulae we have to deal chiefly 

 with the magnesium fluting at 500, whereas in the case of the aurorae 

 the line most constantly seen by itself is the manganese line at 558 ? 

 The importance of this question becomes evident when we remember 

 that the line 558 is seen for hours without the interference of any 

 other line whatever, and seen under conditions which indicate that 

 the higher reaches of the atmosphere are so full of the glowing stuff 

 which produces the line that the light is sufficiently intense to be 

 reflected by the particles lower down. It may be that in this difference 

 we have an important piece of evidence regarding the origin of the 

 luminosity in the two cases in question. 



In the case of the nebulas, the light of which I have attributed to 

 collisions, it is obvious that the collisions which produce the lowest 

 temperature will always be greatest in number, that is to say, there 

 will be more grazes than smashes. In any case, however, where the 

 luminosity is produced in this way there will be sufficient tempera- 

 ture brought about by impacts to volatilise the constituents of the 

 meteorites. Considering the meteorites merely from what we know 

 about their composition from those which have fallen on the earth, 

 we must assume that the largest constituent of meteorites is olivine. 



Where, therefore, we are dealing with collisions merely, we should 

 expect to get the spectrum of olivine produced say 10,000 times, while 

 ihe spectrum of the other substances would only be produced once in 

 consequence of more extensive collisions. But when we pass from 

 the nebulae to the meteoric dust in our air we are no longer dealing 

 with collisions ; we are dealing with luminosity brought about by 

 electrical discharges ; and it requires no long argument to show that 

 these electric discharges would be more likely to travel along and to 

 render luminous the metallic constituents of the dusfc rather than 

 the silicates of magnesium or of any other metal. 



In this way, then, we should expect to get electrically exhibited the 



