8 Dr. J. Croll on the Origin of Nebulce. 



rat ure. And it is scarcely conceivable that the temperature 

 could have been derived from the condensation of their masses. 

 When our sun was in the nebulous condition it no doubt was 

 self-luminous like other nebulae, and doubtless would have 

 appeared, if seen from one of the fixed stars, pretty much like 

 other nebulae as viewed from our earth. The spectrum would 

 no doubt have revealed in it the presence of incandescent gas. 

 At all events we have no reason to conclude that our nebula 

 was in this respect an exception to the general rule, and es- 

 sentially different from others of the same class. The heat 

 which our nebula could have derived from condensation up to 

 the time that Neptune was formed, no matter how far the 

 outer circumference of the mass may originally have extended 

 beyond the orbit of that planet, could not have amounted to 

 over ? of a thermal unit for each cubic foot ; and the 



7000000 . -tit 



quantity of light given out could not possibly have rendered 

 the mass visible. Consequently the heat and light possessed 

 by the mass must have been derived from some other source 

 than that of gravity. 



We have further evidence that the heat and light of nebulae 

 cannot have been derived from condensation. If there be 

 any truth, as there doubtless is, in Mr. Lockyer's view of the 

 evolution of the planets, then the nebulae out of which these 

 bodies were evolved must have originally possessed a very 

 high temperature — a temperature so high, indeed, as to pro- 

 duce perfect chemical dissociation of the elements. In short, 

 "the temperature of the nebulae," as Mr. Lockyer remarks *, 

 " was then as great as the temperature of the sun is now." 

 Mr. Lockyer's theory is that the metals and the metalloids, 

 owing to excessive temperature, existed in the nebulous mass 

 uncombined — the metals, owing to their greater density, assu- 

 ming the central position, and the metalloids keeping to the 

 outside. The denser the metal the nearer would its position 

 be to the centre of the mass, and the lighter the metalloid the 

 nearer to the outside. As a general rule the dissociated ele- 

 ments would arrange themselves according to their densities ; 

 and it is for this reason, he considers, that the outer planets 

 Neptune, Uranus, Saturn, and Jupiter, are less dense than 

 the inner planets, since they must have been formed chiefly of 

 metalloids, while the inner and more dense planets would 

 consist chiefly of metallic elements. 



" The hypothesis," says Mr. Lockyer, " is almost worthless 

 unless we assume very high temperatures, because unless you 

 have heat enough to give perfect dissociation, you will not 

 have that sorting-out which always seems to follow the same 



* ' Why the Earth's Chemistry is as it is," p. 55, 1877. 



