308 DISCUSSIONS IN COSMOLOGY. 



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 elements 

 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 law." But the heat 

 which produced this dissociation previous to the forma- 

 tion of the planets could not have been derived from 

 the condensation of the nebula ; for the quantity so 

 derived prior to the existence of the outermost planet 

 must have been infinitesimal indeed. The heat exist- 

 ing in the nebula previous to condensation must have 

 come from some other source ; and we can conceive of 

 no other save that which we have been considering. 



The Gaseous State the first Condition of a Nebula. — 

 If the foregoing be the true explanation of the origin 

 of nebulae, it will follow that the gaseous state will in 

 most cases be the first or original condition, and that 

 a nebula giving a continuous spectrum will only be 

 found after it has condensed to a considerable extent. 



The irresolvable nebulae which exhibit bright lines, 

 in all probability consist, as Mr. Huggins maintains, of 

 glowing gas without anything solid in them. In short, 

 they are nebulae in their first stage of development, 

 and have not as yet condensed sufficiently to become 

 possessed of nuclei. If we adopt the generally accepted 



