World-life; or, Comparative geology . r. They are not firma-ments of suns. They are incandescent cosmical are dust so intensely heated that some or all of it isin a state of vaporization. This is another grand consum-mation. A matured conjecture of Sir William Herschelis confirmed. The world-stuff which Laplace demanded isat hand. Let us see whether the aspects which it presentssustain the idea of progressive world-growtli. Evidences of development seem to be afforded by theforms of the nebulae. Of these we may enumerate thefollowing classes : *It is impossible to say whether the app
World-life; or, Comparative geology . r. They are not firma-ments of suns. They are incandescent cosmical are dust so intensely heated that some or all of it isin a state of vaporization. This is another grand consum-mation. A matured conjecture of Sir William Herschelis confirmed. The world-stuff which Laplace demanded isat hand. Let us see whether the aspects which it presentssustain the idea of progressive world-growtli. Evidences of development seem to be afforded by theforms of the nebulae. Of these we may enumerate thefollowing classes : *It is impossible to say whether the apparently continuous spectra of someof these nebulie are crossed or not by darlc lines. 42 COSMIC A L DUST. 1. Amorphous Nebulm.— Here we may include thegreat nebula in the sword-handle of Orion.* I reproducefor the reader (Figure 6) the careful drawing executed This is one of the brightest of the nebulae;but at the same time it has resisted all efforts at resolu-tion. Its spectrum, accordingly, consists of a small num-. FiG. 6. The Great Nebula in Orion. Central Part. Drawn by ber of bright lines. Here belong also, the two MagellanicClouds, visible to the naked ej^e in the southern hemi-sphere. I am not aware that their spectrum has beenobtained. 2. Spiral NehulcB.—T\\Q nebula No. 3,239 Herschel * Director otto Strnve classes this among spiral nebuloe (^Monthly Notices,Astronomical Society, London, 14 March, 1856, xvi, 139: Gantier, Archives desSciences rhysiques et Naturelles, Geneva, 186-2, translated, Smithsonian Eeport,1863,299). It is possibly beginning to pass into the spiral phase. See also Bond: On the Spiral Structure of the great Nebula in Orion, Monthly No-tices, xxii, 203-7. t Further, on this nebula, see Nature, 22 November, 1877, p. 67, and 18 July,1878, p. 313; Schellen: Spectral Analysis, 534. NEBULA. 43 (Figure 7) presents the form of a sickle or greatly curvedtail of a comet. It seems to be an elongated mass of lightjust
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