1888.] of the various Species of Heavenly Bodies. 



23 



constituerait une difference physique bien plus distincte entre le III 

 et le IV type qu' entre le II et le III ; et le IV type representerait 

 une phase bien distincte, la derniere peut-etre avant rextinction 

 totale. Le role physique du IV type est done encore si mysterieux, 

 que j'ai cru pouvoir encore me conformer a 1'exemple de d' Arrest, en 

 suivant la classification formelle de Secchi." — C. F. Pechiile, ' Expedi- 

 tion Danoise pour l'Observation du Passage de Venus, 1882,' p. 25, 

 (Copenhagen, J. H. Schultz, 1883). 



II. Proposed New Grouping of all Celestial Bodies according 

 to Temperature. 



Having, then, gone over the various classifications of stars accord- 

 ing to their spectra, I now proceed to consider the question of the 

 classification of celestial bodies from a more advanced point of view. 

 I pointed out in the year 1886 that the time had arrived when stars 

 with increasing temperatures would require to be fundamentally 

 distinguished from those with decreasing temperatures, but I did not 

 then know that this was so easy to accomplish as it now appears to be 

 ('Nature,' vol. 34, p. 228); and, as I have already stated, when we 

 consider the question of classification at all, it is neither necessary 

 nor desirable that we should limit ourselves to the stars ; we must 

 include the nebulae and comets as well. Stellar variability should 

 not introduce any difficulties, seeing that as a rule in its extremest 

 form it is the passage from one spectrum to another, even if of a 

 different type, owing to sudden changes of temperature. 



In the first classification on these lines, which is certain to be modi- 

 fied as our knowledge gets more exact, it is desirable to keep the 

 groups as small in number as possible ; the groups being subsequently 

 broken up into sub-groups, or, even into species, as the various 

 minute changes in spectra brought about by variations of temperature 

 are better made out. 



For the purpose of making clear what follows, I here introduce 

 from my paper of November 17th, the "temperature curve," on which 

 is shown the distribution of nebulae, comets, and of stars as divided 

 into classes by Vogel, on the two arms of the curve. 



On one arm of this we have those stages in the various heavenly 

 bodies in which in each case the temperature is increasing, while on 

 the other arm we have that other condition in which we get first 

 vaporous combination, and then ultimately the formation of a crust 

 due to the gradual cooling of the mass, in dark bodies like, say, the 

 companion to Sirius. At the top we of course have that condition in 

 which the highest temperature must be assumed to exist. 



To begin, then, a more general classification with the lowest tempe- 

 ratures, it is known that the nebulae and comets are distinguished 



