Speculations as to an identity of nature between nebulæ and comets are no novelty; they presented themselves, as they could hardly fail to do, to the mind of Sir William Herschel; but some consistency was first given to them by the recent experimental researches of Mr. Lockyer. It is true that the results of light analysis are far from being decisive in their favour. The spectra of the two classes of bodies are fundamentally unlike. No gaseous nebula gives a trace of the carbon-bands which characterise nearly all comets; and no comet has yet furnished any direct evidence of the presence of hydrogen among its constituents. Moreover, nebulæ (apart from the stars contained in them) seem to emit no genuinely continuous light, while cometary nuclei glow in the ordinary manner of white-hot solid and liquid substances. Traces of a spectroscopic analogy can indeed be shown to exist; but they are met with only in the secondary elements of each spectrum. The resemblance seems only incidental; the dissimilarity essential.

This does not, however, detract from the closeness of a physical analogy, the deep import of which cannot be too forcibly dwelt upon. Both comets and nebulæ consist of enormous volumes of gaseous material, controlled by nuclear condensations, whether of the same or of a different nature in the two genera we need not now stop to inquire. Both, there is the strongest reason to believe, shine through the effects of electrical excitement.