1888. J of the various Species of Heavenly Bodies. 



far brighter at that temperature than almost any other spectral line 

 or fluting of any element whatever. 



I suggested that the association or non-association of hydrogen 

 lines with the lines due to the olivine constituents of the meteorites 

 might be an indication of the greater or less sparseness of the swarm, 

 the greatest sparseness being the condition denning fewest colli- 

 sions, and therefore one least likely to show hydrogen. This sugges- 

 tion was made partly because observations of comets and laboratory 

 work have abundantly shown that great liability to collision in the 

 one case, and increase of temperature in the other, are accompanied 

 by the appearance of the carbon spectrum instead of the hydrogen 

 spectrum. 



The now demonstrated meteoric origin of these celestial bodies 

 renders it needful to discuss the question in somewhat greater detail, 

 with a view to classification ; and to do this thoroughly it is requisite 

 that we should study the rich store of facts which chiefly Sir William 

 Herschel's labours have placed before us regarding the various forms 

 of nebulas, in order to ascertain what light, if any, the new view 

 throws on their development. 



To do this the treatment must be vastly different from that — the 

 only one we can pursue — utilised in the case of the stars, the images 

 of all, or nearly all, of which appear to us as points of light more or 

 less minute ; while, in the case of the nebulae, forms of the most 

 definite and, in many cases, of the most fantastic kind, have been long- 

 recognised as among their chief characteristics. 



It will at once be evident that since the luminosity of the meteorites 

 depends upon collisions, the light from them, and from the glow of 

 the gases produced from them, can only come from those parts of a 

 meteor-swarm in which collisions are going on. Visibility is not the 

 only criterion of the existence of matter in space ; dark bodies maj 

 exist in all parts of space, but visibility in any part of the heavens 

 means, not only matter, but collisions, or the radiation of a mass of 

 vapour produced at some time or other by collisions. The appear- 

 ances which these bodies present to us may bear little relation to 

 their actual form, but may represent merely surfaces, or loci of 

 •disturbance. 



It seemed proper, then, that I should seek to determine whether 

 the view I have put forward explains the phenomena as satisfactorily 

 as they have been explained by old ones, and, whether, indeed, it can 

 go further and make some points clear which before were dark. 



To do this it is not necessary in the present paper to dwell at any 

 great length either on those appearances which were termed nebu- 

 losities by Sir William Herschel or on irregular nebulas generally ; 

 but it must be remarked that the very great extension of the former 

 — which there is little reason to doubt will be vastly increased by 



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