PROBABLE ORIGIN OF NEBVL.K. 307 



When our sun was in the nebulous condition it no 

 doubt was self-luminous like other nebulae, and doubt- 

 less 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 essentially 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 nruhsim of a 

 thermal unit for each cubic foot; and the quantity of 

 light given out could not possibly have rendered the 

 mass visible. Consequently the heat and light pos- 

 sessed 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 produce 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, assuming the central position, 

 and the metalloids keeping to the outside. The denser 



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



