400 Celestial Chemistry. 



103 Nebulas, and increased them to 2500, throwing out at the 

 same time the supposition that although he had resolved nearly- 

 all those of Messier' s list into clusters of stars, and discovered 

 hundreds of other clusters, yet that there were numerous 

 nebulas which, from their being easily visible, and yet resist- 

 ing every attempt by increased aperture or power to resolve 

 them, must really consist, not of stars or separate masses of 

 condensed solid matter, but of gaseous or vaporous fluid. He 

 was thence led to those beautiful speculations which, enlarged 

 and illustrated by Laplace, who showed the mathematical pos- 

 sibility of such an action of condensation as Sir W. Herschel 

 required, have become widely known as the Nebular hypothesis, 

 and. as to the validity of which so much controversy has been 

 excited. We all know, too, that of late years the tendency of 

 telescopic observation, as conducted by the magnificent instru- 

 ments of Lord Eosse, and corroborated by the splendid achro- 

 matic of Harvard University, Cambridge, U. S., has been to 

 reduce the numbers of the irresolvable nebulas ; and in parti- 

 cular that, after the great nebula of Orion had, in parts at 

 least, been resolved, the general impression was that the 

 nebular hypothesis had lost all substantial evidence in its 

 favour, and though it still might be contended that the exist- 

 ing systems had such an origin, yet that examples of matter 

 in the nebulous form were not to be found in the heavens. 

 Still, it must not be forgotten that Lord Eosse, while re- 

 solving many of the nebulas, had discovered others which 

 resisted his instrumental powers, and that to many clusters 

 there were fantastic wisps of nebulous light appended, and 

 diffuse patches of light attached, which defied resolution, though 

 they were evidently connected with the objects he pronounced 

 to be starry in construction. 



Amongst the most wonderful of the nebulous bodies are 

 those called by Sir W. Herschel planetary nebula. These 

 present the appearance of small discs of uniform light, usually 

 circular, and generally blue, or bluish green in colour. They 

 are few in number and bright, looking, in the telescope with a 

 low power, very much like stars out of focus. Now it was 

 apparent to Sir W. Herschel that bodies like these, having no 

 central condensation of light nor stellar nucleus, could not be 

 globular clusters of stars, which necessarily would be brighter 

 in the centre than at the edges. A cylinder form of stars 

 seen endways might certainly present such an appearance, or a 

 flat disc of stars placed at right angles to our line of sight 

 would also look like a planetary nebula ; but such forms are 

 too improbable to detain us a moment. He, therefore, came 

 to the conclusion that these were masses of truly nebulous or 

 vapoury matter in the primal form, and presenting various 



