Dr. J. Croll on the Origin of Nebula. 9 



law." But the heat which produced this dissociation previous 

 to the formation of the planets could not have been derived 

 from the condensation of the nebula ; for the quantity so de- 

 rived prior to the existence of the outermost planet must have 

 been infinitesimal indeed. The heat existing in the nebula 

 previous to condensation must have come from some 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 nebulas, it 

 will follow that the gaseous state will in most cases be the 

 first or original condition , and that a nebula giving a con- 

 tinuous spectrum mil only be found after it has condensed to 

 a considerable extent. 



The irresolvable nebulas which exhibit bright lines, in all 

 probability consist, as Mr. Huggins maintains, of glowing gas 

 without any thing solid in them. In short they are nebulas in 

 their first stage of development, and have not as yet condensed 

 sufficiently to become possessed of nuclei. If we adopt the 

 generally accepted nebular hypothesis, I cannot understand 

 how we can consistently deny the existence of gaseous nebulae; 

 for, according to the nebular hypothesis, the central nucleus 

 which constitutes a sun or star, and which exhibits a con- 

 tinuous spectrum, was formed by condensation as surely as the 

 planets or the satellites have been. Were we to go back suf- 

 ficiently far in the past, we should come to a time when not 

 only our globe but the sun himself consisted of gaseous matter 

 only. If we admit this, then why not also admit that there 

 may be nebulas at the present time in a condition similar to 

 what our sun must formerly have been. 



The gaseous condition of nebulas seems to follow as a con- 

 sequence from Mr. Lockyer's theory. For in order that the 

 materials in the formation of a sun or star may arrange them- 

 selves according to their densities, dissociation is requisite ; 

 but there can be no dissociation except in the gaseous con- 

 dition. 



Star- Clusters. — The wide-spread and irregular manner in 

 which the materials would in many cases be distributed through 

 space after collision, would prevent a nebula from condensing 

 into a single mass. Subordinate centres of attraction, as was 

 long ago shown by Sir William Herschel (in his famous memoir 

 on the formation of stars*), would be established, around which 

 the gaseous particles would arrange themselves and gradually 

 condense into separate stars, which would finally assume the 

 condition of a cluster. 



* Phil. Trans, for 1811. 



